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Laboratory Strategies for Hydrate Formation in Fine-Grained Sediments
Journal of Geophysical Research: Solid Earth
Abstract
Fine-grained sediments limit hydrate nucleation, shift the phase boundary, and hinder gas supply. Laboratory experiments in this study explore different strategies to overcome these challenges, including the use of a more soluble guest molecule rather than methane, grain-scale gas-storage within porous diatoms, ice-to-hydrate transformation to grow lenses at predefined locations, forced gas injection into water saturated sediments, and long-term guest molecule transport. Tomographic images and thermal and pressure data provide rich information on hydrate formation and morphology. Results show that hydrate formation is inherently displacive in fine-grained sediments; lenses are thicker and closer to each other in compressible, high specific surface area sediments subjected to low effective stress. Temperature and pressure trajectories follow a shifted phase boundary that is consistent with capillary effects. Exo-pore growth results in freshly formed hydrate with a striped and porous structure; this open structure becomes an effective pathway for gas transport to the growing hydrate front. Ice-to-hydrate transformation goes through a liquid stage at premelt temperatures; then, capillarity and cryogenic suction compete, and some water becomes imbibed into the sediment faster than hydrate reformation. The geometry of hydrate lenses and the internal hydrate structure continue evolving long after the exothermal response to hydrate formation has completely decayed. Multiple time-dependent processes occur during hydrate formation, including gas, water and heat transport, sediment compressibility, reaction rate, and the stochastic nucleation process. Hydrate formation strategies conceived for this study highlight the inherent difficulties in emulating hydrate formation in fine-grained sediments within the relatively short time scale available for laboratory experiments.
... Li et al., 2018;Ye et al., 2020). The formation and enrichment of gas hydrates in fine-grained sediments differ significantly from those in coarsegrained sediments (Bai et al., 2022;Lei & Santamarina, 2018;Ren et al., 2020). High-saturation gas hydrates have been reported in fine-grained sediments (e.g., clayey silt or silt) in the northern South China Sea (SCS) (J. ...
... However, many drilling results show that high biological component intervals do not correspond to high gas-hydrate saturation intervals (Bai et al., 2022;Su et al., 2021). Based on laboratory simulations, gas hydrates in fine-grained sediments have two models: (a) pore-filling and (b) displacement particles (Lei & Santamarina, 2018;Lei et al., 2019;Ren et al., 2020). Pore-filling and displacement particle models can form high-saturation gas hydrates (Mi et al., 2022;Ren et al., 2020). ...
... Pore-filled gas hydrates are more common under natural conditions (Mohammadmoradi & Kantzas, 2018). Displaced-particle gas hydrates may be related to massive, vein-like, and tuberculate forms of gas hydrates in fine-grained sediments (Lei & Santamarina, 2018;Lei et al., 2019). However, the laboratory samples that induce hydrate formation differ significantly from natural sediments (Ren et al., 2020). ...
The particle size of sediments below the seabed is a crucial factor affecting the formation and enrichment of gas hydrates. Apart from the formation and enrichment law of gas hydrate in coarse‐grained sediments (dominated by a sandy‐sized fraction), in the fine‐grained sediments (<62.5 μm) which accounts for more than 90% of offshore gas hydrate resources globally, the control effect of sediment particle size on gas hydrate is still unclear. Therefore, understanding the relationship between the fine‐grained sediment particle size and gas hydrate enrichment is essential for revealing the global distribution and dynamic evolution of gas hydrates. Here, we analyzed the vertical gas hydrate saturation, particle size parameters of sediments, whole‐rock minerals, and clay mineral components based on drilling data and sediment samples from fine‐grained gas hydrate reservoirs (GHRs) in the Shenhu area of the northern South China Sea. The results show that in fine‐grained sediments, the coarse particles cannot improve the reservoir quality or enrich the gas hydrate because many fine particles fill the intergranular pores formed by the coarse particles. Meanwhile, the fine particles were dominated by clay minerals, especially in the illite/smectite mixed layer, which significantly reduced the permeability of the sediment layer and was not conducive to the enrichment of gas hydrates. Moreover, sedimentary processes directly control the sediment particle size and mineral composition, which play an essential role in controlling GHRs at the macroscale. In the fine‐grained sediments, very fine sediments (<8 μm) have a more significant negative impact on gas hydrate enrichment.
... Methane hydrate preferentially grows at the gas-liquid interface and accumulates in the space occupied by the gas phase ( Fig. 4a-2 and Fig. 4a-3) , creating a loosely high porous structure. Carbon dioxide, due to its higher solubility in water, is often used as a substitute for methane to synthesize hydrate in sediments (Lei and Santamarina, 2018). Results show that carbon dioxide hydrate is preferentially formed at the gasliquid interface with a small part directly touching particles. ...
... Results show that carbon dioxide hydrate is preferentially formed at the gasliquid interface with a small part directly touching particles. Synthesized carbon dioxide hydrate is loose and has a highly porous structure, which can provide effective paths for gas migration (Fig. 4b) (Lei, 2017;Lei and Santamarina, 2018). ...
... The case of high hydrate saturation (i.e., hydrate saturation S h > 0.6) needs to be furtherly clarified. In addition, many experiments have used THF as a proxy for methane to avoid guest molecule limitation, due to the mutual solubility of THF and water (Lei and Santamarina, 2018;Zhang et al., 2022a). THF hydrate saturation can be controlled accurately by adjusting the mass ratio of THF to water. ...
Commercial development of natural gas hydrate, over 90% of which is found in fine-grained sediments worldwide, is of significance to energy structure, energy security, and global climate. One of the most critical physical parameters for hydrate exploitation is reservoir permeability, which directly affects the efficiency and economic feasibility of gas production. In this paper, hydrate morphologies and pore habits are identified in both natural cores and synthesized fine-grained specimens. Laboratory and field tests of permeability are summarized, and the dependence of permeability on various influencing factors is comprehensively discussed. Results show that the hydrate morphologies and pore habits are predominantly influenced by particle size, stress state, and geological conditions. They exist in the form of lenses, nodules, chunks, veins, and others within fine-grained sediments. The effective permeability is typically measured as larger than 1 millidarcy (mD) in laboratory tests, however, field tests have shown that it can range from 0.01 to 1 mD. There is a lack of effective permeability models that can be used in numerical simulations to predict gas production capacity. Furtherly, challenges to current research are analyzed and future research prospects are proposed. Developing new measurement techniques, bridging the gaps of different methods and scales, as well as establishing appropriate permeability models are needed for reservoir simulators to accurately predict gas production. Collaborative and comparative studies are needed to develop agreeable measurement methods and testing protocols to address the challenges of better understanding the permeability in hydrate-bearing fine-grained sediments.
... Hydrate growth in sediments reflects the competition between capillarity and skeletal forces (Clennell et al., 1999;Dai et al., 2012;Lei and Santamarina, 2018;. Hydrate growth is grain-displacive in fine-grained sediments (and low effective stress), resulting in segregated lenses, veins, and chunks ( Figure 3). ...
... Pore-invasive hydrate in coarse sediments,: (a) lab-formed hydrate under excess-gas condition (Lei et al., 2019a); (b) Natural hydrate-bearing silt pressure cores retrieved from Gulf of Mexico (Lei et al., 2022). Grain-displacive hydrate in fine-grained sediments: (c) lab-formed CO2 hydrate in a gas-driven fracture across a water-saturated kaolinite specimen (Lei and Santamarina, 2018); (d) Natural hydrate lenses in a pressure core retrieved from NGHP 01 project (National Energy Technology Laboratory -See also Yun et al., 2011). ...
... Reproducing methane-limited hydrate formation in nature is particularly challenging within typical laboratory time scales, i.e., weeks to months. Because of high advection, laboratory studies of hydrate formation are much simpler in coarse-grained sediments than in fine-grained deposits (Spangenberg et al., 2005;Lei and Santamarina, 2018). Hydrate can form by forcing free gas invasion into water-saturated sediment within the stability field; in this case, a hydrate film/crust forms around the invading gas body (Lei et al., 2019a&b). ...
The geo-science and engineering fields have critical roles to play towards a sustainable energy future. This state-of-the-art review focusses on five areas where the geotechnical community has been involved the most: the oil and gas sector with emphasis on methane hydrates, carbon geological storage, geothermal, energy geo-storage, and nuclear waste storage. Extensive tables in the appendix identify potential geotechnical contributions to all energy resources (mining is critically needed to supply raw materials for the energy sector, yet is not addressed in this review). Energy-related applications involve a wide range of time and length scales, coupled thermo-hydro-chemo-mechanical processes, multi-phase fluids, high pressure-temperature-stress conditions, fines migration, reactive fluid transport and phase transformations, and multi-physics repetitive loads. Analysis and design require careful experimentation (including field-scale studies) and advanced numerical simulations. Educational programs must address the knowledge needs in energy geo-engineering.
... Moreover, this liquid-like layer could also transport guest molecules, such as tetrahydrofuran (THF), since guest molecules in water must migrate toward the hydrate formation front to facilitate the formation of hydrate nodules [31]. On the other hand, the hydrate liquid-like layer can combine different hydrate-hydrate particles and hydrate-sediment particles, which plays a great role in influencing mechanical properties [31][32][33][34]. This may be one of the crucial reasons why the stiffness and strength of hydrate-bearing sediments are usually higher than those of hydrate-free sediments. ...
... However, some researchers have proposed that the micrometer-thick liquid films at the hydrate-sediment interface may be artifacts in CT images. This concern arises from the possibility of the X-ray diffraction to generate spurious layers with varying electron densities at material boundaries [34,38], thus suggesting a significantly smaller actual liquid film thickness. Thus, the debate regarding whether the surface nano-characteristics of hydrates can be precisely obtained from X-ray CT tests remains unresolved. ...
... However, recent studies have suggested that the initial gas/water distribution is not always consistent with the final hydrate distribution [35][36][37]. Likewise, Lei et al. [38] has observed the gas-water interface redistribution during hydrate formation in clay sediments, indicating that the location of reaction surface (gas-liquid interface) changes during the overall process of hydrate formation. The time-variant location of the reaction surface (gas-water interface) further leads to the changeable surface area (A TS ). ...
... The theoretical hypothesis that water migration during hydrate formation is the key influencing factor of HBS heterogeneity is firstly proposed according to previous experimental findings [17,38]. As illustrated by Fig. 2, the water migration is considered to involve two typical processes: (i) the water diffusion through the hydrate layer and (ii) the water transportation along the solid surface. ...
Lack of knowledge on the formation mechanism of heterogeneous hydrate-bearing sediments (HBS) leads to difficulties of forming nature-like HBS, further resulting in unknown implications of HBS heterogeneity to the hydrate field exploitation. This work aims to reveal the unaddressed formation mechanism of heterogeneous HBS by theoretical hypothesis, model derivation, and experimental verification. The hypothesis that the water migration (including water diffusion and transportation) during hydrate formation plays a significant role in controlling HBS heterogeneity is firstly proposed. Since the dual effects of water migration on hydrate formation rate is non-negligible, a modified hydrate formation kinetic model is established by integrating the effects of water diffusion and water transportation on dynamic reaction area. Furthermore, three typical nature-like HBS (lumpy, layered, and homogeneous HBS) with the same hydrate saturation of 0.673 but different heterogeneities are formed by changing the initial water distribution. The proposed kinetic model successfully predicts that the average formation rate of lumpy HBS is 16 times faster than that of homogeneous HBS. This model solves the problem in describing the variations of formation rates of homogeneous and heterogeneous HBS. The hydrate distribution and morphology are also captured by X-ray Computed Tomography (X-CT) scans. The X-CT results further confirm that the spontaneous water transportation against gravity results in heterogeneous hydrate distribution and various hydrate morphologies in pores. Hence, the newly proposed formation mechanism and formation method of heterogeneous HBS in this work provide theoretical support and research method for future studies on efficient exploitation of naturally occurring heterogeneous HBS.
... Although some researchers use heavier gas (Xenon) for the gashydrate formation to enhance contrast levels between different materials [30,31], physical properties become different from the desired methane gas hydrate [32]. Alternatively, pore brine can be formed with heavier salts (NaBr, KI) instead of natural NaCl [33]. This method is more suitable for modeling gas hydrates in porous media, and it provides good contrast between the brine and hydrate. ...
... The use of salt brine is common in microcomputed tomography imaging of methane gas hydrates because the brine acts as an increasing phase contrast agent to separate the hydrate and water in images. [33] demonstrated efficient phase contrast-enhancing methods based on using NaBr, KI and NaCl brines as hydrate-forming fluids, as opposed to the regular formation with distilled water. These methods also simulate the natural scenario of hydrate formation -formation in the bottom sediments of the seas where water has some salinity. ...
Fast multi-phase processes in methane hydrate-bearing samples are challenging for micro-CT quantitative study because of complex tomographic data analysis involving time-consuming segmentation procedures. This is due to the sample multi-scale structure changing in time, low X-ray attenuation and phase contrast between solid and fluid materials, as well as large amount of data acquired during dynamic processes. We propose a hybrid approach for automatic segmentation of tomographic data from time-resolved imaging of methane gas-hydrate formation in sandy granular media. First, we use an optimized 3D U-net neural network to perform segmentation of mineral grains that are characterized by low contrast to the surrounding pore brine-saturated phases. Then, we perform statistical clustering based on the Gaussian mixture model for separating the pore-space phases that are characterized by gray-level instabilities caused by dynamic processes during hydrate formation. The proposed approach was used for segmenting several hundred gigabytes of data acquired during an in-situ tomographic experiment at a synchrotron. Automatic segmentation allowed for studying properties of the hydrate growth in pores, as well as dynamic processes such as the incremental pore-brine flow and redistribution.
... However, the relationships between pore structures, hydrate content and distribution, and HBS permeability have not been accurately explained in the literature, largely due to experimental challenges in permeability measurements. These challenges include maintaining the phase equilibrium conditions of hydrates, preventing mass transfer and blockage of sediment particles, avoiding hydrate formation in unexpected locations, accurately determining hydrate content within the sediments, and minimizing permeability changes caused by pressure differences (Lei and Santamarina, 2018). Consequently, the permeability results of HBS reported in the literature exhibit multiple discrepancies. ...
... As a new clean energy in the 21st century, NGH is characterized by abundant reserves and high energy density, which organic carbon is twice the total carbon of existing oil, natural gas and coal [1][2][3]. The exploration and development will help optimize the energy structure and reduce carbon emissions. ...
The shallow hydrate reservoir in the Shenhu Sea area is mainly composed of clayey silt. Clay mineral has an important impact on the mechanical properties, and the hydrate decomposition aggravates this impact. Therefore, the composition and geological conditions of shallow clay hydrate-bearing sediment in Shenhu sea area are fully considered, hydrate-bearing sediment samples with similar physical properties are synthesized in situ. Then, indoor triaxial mechanical experiments are carried out, and the effect of clay minerals on the mechanical property is analyzed. The results show that the clay content and clay type have an important impact on the mechanical properties of unconsolidated hydrate-bearing sediment. With the increase of clay content, the strain hardening characteristics are prominent, the yield stage is longer, and the plasticity is enhanced. Hydrate-bearing sediment with different clay content shows similar mechanical laws under the influence of hydrate saturation and effective confining pressure. The peak strength, elastic modulus and Poisson’s ratio all show a downward trend, but the peak strength and elastic modulus change more obviously. The peak strength changes linearly with hydrate saturation, while nonlinearly with effective confining pressure, especially 0–3 MPa. This is the comprehesive result of clay particle’s movement and fragmentation, clay hydration and expansion, affecting hydrate formation and sediment cementation. When the content ratio of montmorillonite/illite decreases, the peak strength and elastic modulus show an increasing trend. Because the frictional resistance and connection strength of illite crystal layer are larger with bigger particle size, weaker hydration and thinner water film. The research can provide reference for drilling and production engineering of natural gas hydrate (NGH) reservoir in the Shenhu sea area.
... Hyodo et al. [11] (2013) carried out stiffness experiments on samples of water and gas-saturated hydrate deposits and found that the stiffness and strength of hydrate deposits saturated with gas were significantly higher than those saturated with water. Through experiments, Lei et al. [17] reached the findings that in the formation of argillaceous siltstone, hydrate particles, when formed, would occupy the original space of the rock particles and be distributed in uneven micro-fractures in a belt or lenticular zone. This finding is very similar to the results of laboratory X-CT scanning and field logging. ...
The South China Sea has abundant reserves of natural gas hydrates, and if developed effectively, it can greatly alleviate the pressure on the energy supply in China. But the hydrate reservoirs in the sea area are loose, shallow, porous, and have poor mechanical properties. During the drilling process, the invasion of drilling fluid into this kind of reservoir is likely to induce mass decomposition of gas hydrate and, in turn, a significant reduction in mechanical strength around the wellbore as well as instability of the wellbore. In this study, in light of the engineering background of exploratory wells at the South China Sea, a temperature and pressure field model in a gas hydrate reservoir at sea during open circuit drilling was established, and then, based on this model, a comprehensive model for the stability analysis of the well drilled in the hydrate reservoir at sea was constructed, both of them with errors of less than 10%. With these two models, the effects of different drilling parameters on wellbore stability were investigated. The gas and liquid produced by the decomposition of hydrates in the formation will increase the pore pressure in the formation, thereby reducing the effective stress in the formation. The closer the formation is to the wellbore, the more thorough the decomposition of hydrates in the formation and the greater the effective plastic strain. Keeping all other conditions constant, the increase in drilling fluid invasion pressure and temperature, as well as reservoir permeability, will lead to a decrease in the mechanical strength of the formation around the wellbore and an expansion of the wellbore yield zone. The results can provide a theoretical reference for the stability analysis at sea.
... The pore-scale properties of host sediments are key factors affecting the formation of hydrates Zhang et al., 2021). Sediments with coarse grains, well-connected pores are conducive to hydrate formation, whereas fine-grained, clay-rich sediments with lower permeability impede hydrate formation due to their restricted pore spaces (Lei and Santamarina, 2018;Tréhu et al., 2004;Li et al., 2021a). ...
... During the hydrate growth process, when effective stress is larger than the capillarity, hydrates will grow in the pore or invade adjacent pores (Dai et al., 2012), thus ω will increase. However, when effective stress is smaller than the capillarity, the growth of hydrates will be affected by water-mineral interactions and particle-displacive in gas hydrate sediments (Lei and Santamarina, 2018), which leads to the complex change of ω. ...
The threshold pressure is critical to characterizing multi-phase nonlinear flow through tight porous media under effective stress. Due to the
complex and heterogeneous pore structures, the essential controls on the threshold pressure of tight porous media are not determined. In
this study, based on the fractal theory, a theoretical model for the threshold pressure of gas–water two-phase flow through tight porous media
is proposed. The derived model considers the effective stress, pore structures, gas–water capillary pressure, and boundary layer. The predicted
threshold pressure from the developed model is in good agreement with the available experimental results, which validates the model.
Moreover, based on the derived model, the effects of relevant parameters (e.g., gas–water surface tension, contact angle, initial porosity, and
elastic modulus) on the threshold pressure are studied. Under a given effective stress, threshold pressure decreases as the initial porosity (or
elastic modulus) increases. However, threshold pressure increases with the increase in gas–water surface tension (or contact angle). In addition,
a positive relationship exists between threshold pressure and water saturation in tight porous media. From a practical standpoint, this
model is of great significance in predicting threshold pressure and researching on the gas–water two-phase flow mechanism in tight sandstone
gas reservoirs.
... Moreover, most of natural gas hydrate deposits are hosted by silty and clayey sediments (Boswell & Collett, 2011;Boswell et al., 2020) and the mechanisms of hydrate formation in such fine-grained sediments are still poorly investigated at the lab scale. This is partly due to the difficulty in forming hydrate in poorly permeable clays (Chuvilin et al., 2002;Lei & Santamarina, 2018). From a mineralogy point of view, clays are hydrated aluminosilicates with a complex chemical and crystalline structure characterized by a stack of layers forming thin plate-like particles (Grim, 1962), whereas from a geotechnical ones, they represent sediment particles with sizes smaller than 2 or 4 μm (Das, 2008). ...
On Earth, natural hydrates are mostly encountered in clay‐rich sediments. Yet their formation processes in such matrices remain poorly understood. Achieving an in‐depth understanding of how methane hydrates accumulate on continental margins is key to accurately assess (a) their role in sustaining the development of some chemosynthetic communities at cold seeps, (b) their potential in terms of energy resources and geohazards, and (c) the fate of the methane releases, a powerful greenhouse gas, in this changing climate. This study investigated the formation of methane hydrates and their gas storage capacity (GSC) in clay‐rich sediments. A set of hydrate experiments were performed in matrices composed of sand, illite‐rich clay, and montmorillonite‐rich clay at different proportions aiming to determine the role of mineralogy on hydrate formation processes. The experiments demonstrate that a clay content of 10% in a partially water saturated sand/clay mixture increases the induction time by ∼60%, irrespective of the nature of the clay used. The increase in water saturation in the two matrices promotes hydrate formation. Micro‐Raman spectroscopic analyses reveal that increasing the clay content leads to a decrease in the hydrate small‐cage occupancy, with an impact on the storage capacity. Finally, the analyses of collected natural samples from the Black Sea (off Romania) enable us to estimate the GSC of the deposit. Our estimates is different from previous ones, and supports the importance of coupling multiscale properties, from the microscale to the geological scale, to accurately assess the total amount of methane hosts in hydrate deposits worldwide.
... Under local thermal stimulation, the gas production behavior of gas HBS can be affected by the combined effects of thermal transfer characteristics, hydrate occurrence modes, permeability, hydrate self-protection effects, and hydrate regeneration. For HBS (S1), hydrates preferentially form in the pores and exist in a pattern of pore filling in the sediments [Lei and Santamarina, 2018]. Under thermal stimulation, hydrates on the surface of sediment particles dissociate first, while hydrates in the pores dissociate more slowly due to thermal transfer and self-protection effects [Wu et al., 2020;Belosludov et al., 2007]. ...
... Gas hydrate is an ice-like crystalline solid composed of water and natural gas which is stable under high-pressure and low-temperature conditions, and naturally occurs in permafrost and offshore areas [1][2][3], of which marine gas hydrate reserves account for more than 90% of the total reserves [4,5]. With the intensification of global warming and the frequent occurrence of extreme weather events, especially the carbon dioxide emission value and the need for carbon neutrality, global energy consumption is facing a huge adjustment. ...
With the continuous growth in global energy demand, the exploration and development of hydrates has been the focus of increasing attention, and the accurate evaluation of the mechanical properties of hydrate layers has become particularly important. In this study, using a self-developed hydrate sample preparation device and hydrate triaxial seepage test platform, triaxial shear tests were carried out using the in situ synthesis method for hydrate sediment in the laboratory, and the stress–strain curves of hydrate sediment with different levels of saturation were obtained. By analyzing the stress–strain curve, the mechanical parameters of hydrate sediment were calculated and simulated using ABAQUS (2021, Dassault systemes, Vélizy Villacoublay France) finite element software. Several p-y curves were calculated and compared with the simulation results, and the p-y curve correction method of the hydrate layer in a shallow seabed was obtained. It was found that the strength of the hydrate sediment increased with an increase in saturation. At the same time, an increase in confining pressure and a decrease in temperature also increased the strength of hydrate deposits. Through comparison with the existing API (American Petroleum Institute) standard p-y curve, it was found that its strength is low because the existence of the hydrate improves the formation strength.
... In addition, it is difficult for gas to enter inside matrix pores due to the ultra-high capillary force caused by both microcapillary interstice and the water film surrounding clay particles. On the other hand, hydrate formation continuously extracts water from the matrix to the previously formed hydrates (microfracture-filling and foraminiferafilling in this study) and the matrix becomes compacted at the meanwhile [88]. Thus, although the direct verification is limited by the resolution of micro-CT, it should be hard to form dispersed pore-filling (embedded) hydrates in such capillary and microcapillary interstice of clay-dominated silty matrix. ...
... Overpressure (ΔP) and/or subcooling (ΔT) have certainly been defined as driving forces for formation/dissociation of hydrate. In effect, further deepening of researches has also revealed the deeper driving forces of hydrategas concentration in solution [15][16][17]. Put another way, from this deeper perspective, stability of hydrate (EP point, scheme 1 in Fig. 1) is maintained by a high gas concentration under appropriate conditions of P and T. Accordingly, other than still parameters P and T, another process of continuous inflow and outflow of some liquid water can also regulate local heat environment and then gas concentration (scheme 2 in Fig. 1) [18], implying possible local non-stabilities of gas hydrate even within stable regions profiled by P and T [19,20]. ...
... However, more than 90% of the global total NGH occurs in clayey−silty or silty sediments on the sea floor. 28,29 The research on the mechanical properties of clayey−silty sediments is of great value for the exploration and development of the hydrate. ...
It is of great significance to study the mechanical properties of hydrate-bearing sediments for safe and efficient exploitation of hydrate resources. Considering the lack of studies on clay-bearing fine-grained reservoirs, submarine clay taken from the Shenhu Sea in the South China Sea and quartz sand were used to synthesize hydrate-bearing clayey-silty sediments (HBCS). A series of consolidated-drained tests were carried out to investigate the effects of hydrate saturation, effective confining pressure, and their coupling on the mechanical properties of HBCS. The results show that with the increase in hydrate saturation and the decrease in effective confining pressure, the stress−strain curve of the HBCS shows a trend from strain hardening to strain softening. The peak strength and residual strength increase with the increase in effective confining pressure and hydrate saturation. The secant modulus E 50 increases with the increase in hydrate saturation and shows an irregular change trend with the increase in effective confining pressure. The contact area between particles determines the change rule of Secant modulus E 50. With the increase in hydrate saturation, the cohesion first increased rapidly and then increased slowly. The internal friction angle increases with the increase in hydrate saturation, but the increase is small. This indicates that hydrate has a great influence on the cohesion of sediments, but a small influence on the internal friction angle.
... Consequently, it is easy to form a water lock at the narrow throat [15], which will reduce the channel connectivity and decrease the seepage. However, the formation process and the molecular mechanism of the water lock is far away from clear because the spatial scale of conventional macroscopic experiments is limited in tens to hundreds of microns [16]. Evidently, to elucidate the underlying mechanism of this process, one need to investigate the water/gas two-phase flow in the nanopore at the molecular level, with additionally considering that there exist a large number of nanoscale pores in hydrate bearing sediments besides microscale pores [17]. ...
The water/gas two-phase flow is a frequently encountered question in the percolation field, and is especially important for the exploitation of natural gas hydrates because their decomposition products are exactly liquid water and natural gas. We studied the water/methane two-phase flow in a hydrophilic cylindrical nanopore by performing molecular simulations, and obtained high-quality nanoflows under different water saturation (Sw) thanks to the newly established nano-manometer method to control pressure difference accurately. With increasing Sw, the methane flow decreases almost linearly until a sudden stop when Sw ≥ 0.52. The formation of the water lock accounting for this phenomenon is observed clearly, and the larger Sw, the earlier formation of the water lock as well as the longer water lock. Based on careful data analysis, a water lock model and its formation mechanism are newly proposed with two pieces of strong evidence — the continuous reduction of the surface area of the water/gas interface when the water lock forms and the existence of maximum thickness of water film for different Sw. Thus, the competition between the surface tension of the water/gas interface and the adsorption of the water/wall interface controls the development of the water lock. These findings are very helpful for understanding the two-phase percolation and optimizing the gas production and water removal schemes during hydrate exploitation. In addition, the nano-manometer can be widely used in other nanoflow simulations for measuring the local pressure accurately.
... Cementing hydrate often occurs at the contact surface of two soil particles. In such a case, a small amount of hydrate can significantly improve the strength and stiffness of MHBS (Helgerud et al. 1999;Yun et al. 2007;Dvorkin et al. 1999;Lei and Santamarina 2018;Lijith et al. 2019). To analyze the influence of soil particle size on mechanical behavior of MHBS, Luo et al. (2018) performed triaxial compression tests, and their results showed that particles with a large size can significantly improve the strength of MHBS. ...
Geomechanical behavior of methane hydrate-bearing sediment (MHBS) plays a major role in evaluation of the stability of a hydrate reservoir. A series of triaxial compression tests were conducted on MHBS to study the influence of temperature and pore pressure conditions on its mechanical behaviors. The experimental results show that temperature and pore pressure have a significant effect on the stress–strain curve, stiffness, and strength of MHBS. As temperature decreases and/or pore pressure increases, the stress–strain curve manifests an enhanced strain-softening characteristic, stiffness, and strength. Furthermore, MHBS cohesion also tends to exhibit a significant increase, but its internal friction angle almost remains constant with decreasing temperature and/or increasing pore pressure. These findings imply that the change in temperature and pore pressure affects the strength of MHBS, which occurs predominately due to change in its cohesiveness. To describe these impacts, a phase state parameter is introduced to characterize the temperature and pore pressure conditions. Meanwhile, three empirical formulas for relating the secant modulus, strength and cohesiveness to phase state parameter are presented. Good agreement between simulation and measured data indicates that the phase state parameter can effectively describe temperature and pore pressure conditions. The proposed empirical formulas are able to address the influences of temperature and pore pressure conditions on geomechanical characteristics of MHBS.
... • Porosity and hydrate saturation are varied to analyze grain cementation (Lei and Santamarina, 2018) CO 2 Six sediments Deionized water • When the mineral is hydrophilic, the pore habit is not pore filling but cementing (Lei et al., Note: CO 2 = carbon dioxide; NaCl = sodium chloride; NaBr = sodium bromide. ...
Natural gas hydrates (NGHs) are efficient and promising energy resources because of their high energy density. In addition, NGH occurs in sediments under certain pressure and temperature conditions and has the potential to meet the increasing global energy demand. However, efficient exploitation of NGH requires a precise characterization and understanding of the hydrate formation, accumulation, and dissociation mechanisms. In this context, the microstructural characterization of gas hydrate is essential and requires specialized methods and equipment. While traditional imaging and characterization tools offer fundamental microstructural analysis, x-ray microcomputed tomography (µCT) has gained recent attention in producing high-resolution three-dimensional images of the pore structure and habits of hydrate-bearing sediments and providing the spatial distribution and morphology of gas hydrate. Further, µCT offers the direct visualization of the hydrate structure and growth habits at a high resolution ranging from the macro to micro-metric scale; therefore, it is extensively used in NGH characterization. This review summarizes the theoretical basis of µCT imaging spanning the setup of the experimental apparatus and visualization techniques. The applications of µCT in NGH reservoir characterization, such as hydrate types and their constituents, physical and chemical properties, occurrence, and accumulation, are presented. Hydrate characterization using µCT imaging is explicitly discussed, including a general understanding of hydrate pore-habit prediction, saturation and percolation behavior, seepage and permeability, and the influence of hydrate saturation on the mechanical properties of hydrate-bearing sediments. Last, conclusions and recommendations for future research are provided. This review offers a reference for understanding the application of µCT to evaluate gas hydrates, which contributes to exploiting these energy resources.
... Overpressure (ΔP) and/or subcooling (ΔT) have certainly been defined as driving forces for formation/dissociation of hydrate. In effect, further deepening of researches has also revealed the deeper driving forces of hydrategas concentration in solution [15][16][17]. Put another way, from this deeper perspective, stability of hydrate (EP point, scheme 1 in Fig. 1) is maintained by a high gas concentration under appropriate conditions of P and T. Accordingly, other than still parameters P and T, another process of continuous inflow and outflow of some liquid water can also regulate local heat environment and then gas concentration (scheme 2 in Fig. 1) [18], implying possible local non-stabilities of gas hydrate even within stable regions profiled by P and T [19,20]. ...
... In coarse-grained soil, the following three hydrate morphologies are commonly assumed: pore-filling, load-bearing and cementation [1]. As hydrate formation in sediments depends on many factors, such as the geological conditions and the fluid conductivity of the sediments, the transportation mechanism of gas, etc., many complex hydrate morphologies can exist including segregated veins, nodules and lenses in fine-grained soil and heterogeneous hydrate patches in coarse-grained soil under high effective stress [2,3]. Moreover, it is a significantly difficult task to identify the occurrence of paleo-gas hydrate in fossil sediments. ...
The maximum shear modulus (Gmax) is an important factor determining soil deformation, and it is closely related to engineering safety and seafloor stability. In this study, a series of bender element tests was carried out to investigate the Gmax of a hydrate-bearing carbonate sand (CS)–silt mixture. The soil mixture adopted a CS:silt ratio of 1:4 by weight to mimic the fine-grained deposit of the South China Sea (SCS). Tetrahydrofuran (THF) was used to form the hydrate. Special specimen preparation procedures were adopted to form THF hydrate inside the intraparticle voids of the CS. The test results indicate that hydrate contributed to a significant part of the skeletal stiffness of the hydrate-bearing CS–silt mixture, and its Gmax at 5% hydrate saturation (Sh) was 4–6 times that of the host soil mixture. Such stiffness enhancement at a low Sh may be related to the cementation hydrate morphology. However, the Gmax of the hydrate-bearing CS–silt mixture was also sensitive to the effective stress for an Sh ranging between 5% and 31%, implying that the frame-supporting hydrate morphology also plays a key role in the skeletal stiffness of the soil mixture. Neither the existing cementation models nor the theoretical frame-supporting (i.e., Biot–Gassmann theory by Lee (BGTL)), could alone provide a satisfactory prediction of the test results. Thus, further theoretical study involving a combination of cementation and frame-supporting models is essential to understand the effects of complicated hydrate morphologies on the stiffness of soil with a substantial amount of intraparticle voids.
... When the pore gas pressure is lower than the pore water pressure, hydrate skin may collapse and fail due to the pressure difference in gas and water, which allows water to flow into the gas-filled pore and contributes to the formation of a new hydrate skin ( Fig. 4b) (Meyer et al. 2018b). Water redistribution may occur as hydrate grows faster at the place where heat dissipation is quicker (Lei and Santamarina 2018;. ...
Natural gas hydrate is a promising energy resource in the future because of its little contamination and huge reserve. However, gas exploitation may induce large deformation and failure of the seabed due to a reduction in the stiffness and strength of the hydrate-bearing sediment (HBS). Therefore, it is essential to investigate the mechanical behavior of the HBS for safe and efficient gas exploitation. Additionally, it is widely acknowledged that the hydrate morphology inherently affects the mechanical behavior of the HBS. This paper aims to critically synthesize the information on the hydrate morphology and mechanical behavior of the HBS available in the literature to facilitate the application of the research result into engineering practice and provide guidance for future investigation. Hydrate morphology is identified firstly both in natural and synthesized HBS. The similarities and differences of the hydrate morphology in the HBS synthesized using the excess-gas and excess-water methods are highlighted. The available experimental data on the small-strain stiffness, strength, and stress–strain behavior are critically selected and grouped into two categories based on the synthesizing methods. It has been creatively discovered that most mechanical parameters (e.g., bulk modulus, shear modulus, cohesion, dilation angle) share a concave power relationship with the hydrate saturation SH for the HBS synthesized using the excess-water method. While it is a convex power relationship for the bulk modulus, shear modulus, and dilation angle, and a linear relationship for the cohesion c when the HBS is synthesized using the excess-gas method. These observations contribute to establishing the conceptual model reflecting the particle-level failure mechanism of the HBS synthesized using different methods. Afterward, the creep behavior of the HBS, the reported constitutive models, the associated advantages and limitations of each model, and the mechanical response during hydrate dissociation (e.g., depressurization, thermal stimulation, carbon dioxide replacement), are summarized and discussed. It is expected that the state-of-the-art review can deepen our understanding of the mechanical behavior of the HBS and assist in the design of gas extraction programs without triggering potential geohazards.
Article highlights
Similarities and differences in the hydrate morphology of the HBS synthesized using the excess gas and excess water methods are clarified.
The experimental data on stiffness, strength and stress strain in the literature are critically selected and synthesized.
The influence of hydrate morphology on the mechanical behavior of the HBS is comprehensively analyzed.
The creep behavior and mechanical response to hydrate dissociation are summarized.
The constitutive equations on stiffness, strength and stress strain are summarized and evaluated.
... It is hypothesized that fracture-filling NGH is a result of high-flux gas-bearing fluids seepage, whereas the pore-filling NGH indices distributed low-flux diffusion environment [5]. It is also proved that fracture-filling NGH tends to present discontinuous and inhomogeneous characteristics, and usually develops in the forms of nodular, veins, layers or massive pure gas hydrate [20,28]. The accumulation mechanisms of fracture-filling NGH are distinguished from that of pore-filling NGH, and the presence of fractures would inevitably lead to discrepancies in physical and mechanical properties. ...
Fracture-filling natural gas hydrates (NGH) are widely distributed in marine shallow clayey-silt sediment, and usually develop in forms of nodular, veins, layers, and massive pure hydrate. Interactions between these hydrate individuals and their surrounding sediment are crucial to evaluate potential submarine geohazards related to fracture-filling NGH systems. In this paper, we develop a novel apparatus to detect interfacial strength between hydrate individual and its host sediment. Shear strengths of three typical substances (i.e., massive pure hydrate, hydrate-saturated sediment, and hydrate-to-sediment interfaces) in fracture-filling NGH systems are examined and compared comprehensively by taking ice as substitute. Possible failure patterns of the fracture-filling NGH systems are discussed, and a tree-like model is proposed to describe formation mode and failure patterns of the fracture-filling NGH reservoirs. The results indicate that the load–displacement curves and shear surface characteristics of the ice-sediment interface differs significantly from those of the pure ice and ice-bearing sediment. The load–displacement curves of the ice-sediment interfacial specimens show double-peak loading characteristics, with multiple interlaced cracks on the shear surface. Additionally, the strength of the ice-sediment interfacial specimens is sensitive to shear rate, and decreases with increasing shear rate. These double-peak failure behaviors and rambling cracks in the shear surface are attributed to the nonlinear contact interfaces and two transitional sub-systems (i.e., silt-bearing ice and ice-bearing silt) in the specimens.
... Limited by exploitation technology, it was believed that the exploitation of hydrates in sandy sediment reservoirs with higher porosities and permeabilities was the preferred scheme, and testing was mainly based on HBSs synthesized from sandy sediment [19]. However, a lot of hydrates are found in fine-grained sediments on the sea floor [20]. In sandy sediments, hydrates occur in the form of pore distributions and are approximately homogeneously distributed macroscopically. ...
Natural gas hydrates are a strategic energy resource in China. The China Geological Survey has discovered segregated hydrate mass formations under the seepage mechanism in the South China Sea through exploration, and gas hydrates occur in nodular, massive, and vein formations in silty clay sediment. Previous work has focused on the analysis of sediment mechanical properties with respect to the uniform distribution of natural gas hydrates in pore spaces, but the mechanical properties of hydrate-bearing sediments containing segregated hydrate masses are not well understood. Spherical hydrates are used to characterize nodular hydrates, a method is proposed for the preparation of sediment samples containing segregated hydrates masses, and a series of triaxial compression tests are carried out on the samples containing spherical hydrates with two kinds of particle sizes at a certain volume fraction. The paper presents triaxial stress–strain curves for the samples containing spherical hydrates. A model for predicting elastic modulus is established. The results present two distinct stages in the triaxial compression tests of silty clay sediments containing spherical hydrates; they also show that the elastic moduli predicted by the model are in good agreement with the experimental results when the model parameters are set at α = 0.5 and β = −0.21. These results provide fundamental mechanical parameters for the safety evaluation of strata containing segregated gas hydrates.
... Permeability of natural hydrate-bearing samples is known as hard to be measured under in-situ conditions due to the difficulties in preserving and transporting natural samples (Waite et al., 2008). Most published experiments are completed by using artificially synthesized samples (Lei and Santamarina, 2018;Waite et al., 2009). In the laboratory, the main challenge in measuring the permeability of artificially synthesized samples is maintaining the phase-equilibrium condition of hydrate, which should be able to prevent hydrate dissociation or reformation while preventing the formation of ice converted from water. ...
Permeability governs the fluid flow of hydrate-bearing sediment and affects the efficiency of natural gas production from hydrate reservoirs. The permeability in hydrate-bearings sediments is estimated empirically and appears to vary widely for sediments. This study focused on the sandy hydrate-bearing sediments and intended to elucidate the evolution of effective permeability by the mean of pore network modeling. A hydrate kinetics theory-based pore network model (KT-PNM) has been developed, in which the simulation of hydrate formation in porous media is implemented by employing two sub-processes, i.e., hydrate nucleation and hydrate growth. This KT-PNM has been applied to simulate hydrate formation in different pore networks. The permeability reduction exponent (N) for sandy hydrate-bearing sediments is determined to fall in the range of 3–4 based on the simulations of seven sandy samples. An empirical equation used for predicting the effective permeability in sandy hydrate-bearing sediments has been further proposed. The developed model and simulations are hoped to provide valuable insights of pore-space effects on hydrate into permeability prediction for studying the fluid seepage in hydrate-bearing sediments and facilitate the numerical simulation of gas production from the hydrate reservoirs.
... As an example, the presence of internal pores and pore space formed from enmeshed diatoms ( Fig. 1) provides gas hydrate nucleation surfaces (Bahk et al., 2013a.;Dai et al., 2014) and pore spaces that are larger than what is typically available in diatom-free fine-grained sediment (Lei and Santamarina, 2018;Kraemer et al., 2000). As an example, Kraemer et al. (2000) observed that hydrate was present in diatom-rich layers and absent from intervening, diatom-poor layers along the Blake Ridge, offshore eastern USA. ...
The Ulleung Basin Gas Hydrate field expeditions in 2007 (UBGH1) and 2010 (UBGH2) sought to assess the Basin's gas hydrate resource potential. Coring operations in both expeditions recovered evidence of gas hydrate, primarily as fracture-filling (or vein type) morphologies in mainly silt-sized, fine-grained sediment, but also as pore-occupying hydrate in the coarser-grained layers of interbedded sand and fine-grained systems. A commonality across many of these occurrences is the presence of diatoms in the fine-grained sediment. Here we tested fine-grained sediment (median grain size <12.5 μm) associated with hydrate occurrences at four UBGH2 sites (UBGH2-2-2, UBGH2-3, UBGH2-6 and UBGH2-11) to investigate potential impacts of diatoms on efforts to extract methane from hydrate, or to tap hydrocarbon reservoirs beneath hydrate-bearing sediment. Two key considerations are: the extent to which diatoms control sediment mechanical properties, and the extent to which pore-water freshening, which occurs as gas hydrate breaks down during resource extraction, alters the diatom control on sediment mechanical properties. We conducted experiments to measure sediment index properties, sedimentation behavior and compressibility to address these considerations. We relied on scanning electron microscope (SEM) imagery and X-ray powder diffraction (XRD) to characterize the sediment mineralogy. Our high-level findings are that at the ∼20–45% (by volume) diatom concentrations observed at these UBGH2 sites, sediment compressibility increases with diatom content, but diatoms only appear to increase porosity and permeability at the highest diatom concentration (∼45%). Our measurements suggest in situ compression indices of 0.35–0.55 and permeabilities on the order of 0.01milliDarcies (1 × 10⁻¹⁷ m²) can be anticipated at these sites. Importantly, these properties are not expected to vary significantly upon pore water freshening that accompanies gas hydrate dissociation during production.
Clarifying the mechanical response of hydrate-bearing silty-clayey sediments (HBSCSs) concerning various mining factors is crucial for safety and consecutive hydrate production. Comprehension of the impact of compaction patterns on the sediment structure within gas hydrate reservoirs, influenced by the geological age and setting, remains limited in the field. Two types of HBSCS samples were thus prepared, differentiated by the consolidation sequence: the CG sample, consolidated before hydrate formation, and the GC sample, consolidated after hydrate formation, corresponding to compaction patterns in this study, with their mechanical behaviors analyzed considering different influencing factors. The conclusions indicate that an initial shearing dilatancy peak occurs in volumetric strain curves of CG samples with 40% hydrate saturation. In addition, the shearing dilatancy trend of CG samples caused by strong particle exclusion and hydrate accumulation promotes the increase in strength and stiffness as well as the decrease in final volumetric strain of HBSCSs, when compared with GC samples. The frictional property of HBSCSs prepared by the CG method is stronger than that of the GC method due to hydrate displacing sediment particles, although weaker cemented property; there are increases of 9.54° and 0.46 MPa in the internal friction angle and cohesion of CG samples as the hydrate saturation increases from 0 to 40%, while the variation in internal friction angle of GC samples is inconspicuous. The results in this study provide potential theoretical support for predicting the compaction patterns on the mechanical behaviors of hydrate silty-clayey reservoirs.
Natural gas hydrate reservoirs at sea areas are shallow, loose and porous, and poor in mechanical properties in general, so wellbore instability is likely to occur after the drilling fluid infiltrates into the reservoir during drilling. In this study, effects of different factors on the stability of horizontal wellbore have been analyzed by establishing a coupling model of multi-physical fields of the drilled wellbore and reservoir of marine natural gas hydrate. The results show that when horizontal well is drilled into a gas hydrate reservoir, the plastic yield zone of the wellbore is not uniform in distribution and no longer a regular fan ring shape due to the different stress distribution around the wellbore from vertical well. Meanwhile, the closer the formation to the wellbore is, the longer the exposure of the formation to drilling fluid is, so the more thorough the hydrate decomposition, the greater the effective plastic strain, and the greater the risk of wellbore instability is. With the increase of pressure and temperature of drilling fluid and reservoir permeability, the range of mechanical strength drop around the wellbore would expand, making the yield zone of the wellbore and thus the risk of wellbore instability rise. But with the increase of reservoir porosity, the yield zone of wellbore decreases gradually, which is good for the wellbore stability during drilling, but once hydrate around the wellbore decomposes completely during long term production, the risk of wellbore instability may rise sharply. The results of this study can provide a theoretical reference for the stability analysis of horizontal well drilled, the safety pressure control during wellbore drilling and the evaluation of limit extension length of horizontal section in hydrate reservoir at sea areas.
Natural gas hydrate (NGH) has the characteristics of low pollution and large reserves, which is considered as a very promising energy source. The extraction of NGH would cause the strength and stiffness reduced in hydrate-bearing sediment (HBS) reservoirs, which could lead to geological disasters. The mechanical properties of HBS have been studied by a large number of scholars, but it has not been sorted out and summarized for HBS under the influence of multiple factors. In this paper, the hydrate types, synthesis methods, and test conditions are summarized. The effects of hydrate saturation, loading rate, temperature, pore pressure, confining pressure, fine content, and clay content on mechanical properties and deformation behavior of HBS are outlined. The microstructure of the synthesis, decomposition, and shear processes of HBS specimens is also analyzed based on the CT triaxial shear test system, and the microscopic mechanism of HBS specimens in the shearing process is summarized. Finally, the shortcomings of current research on the mechanical properties of HBS are discussed, and provided corresponding suggestions to promote the development of the mechanical properties of HBS.
Natural gas hydrate widely distributed in marine sediments and permafrost has brought great attention due to its large reserves. Unlike conventional reservoirs, the effective pore structures vary from time and space due to hydrate dissociation and secondary formation in the development, which produces significant impacts on gas flow and production. Therefore, figuring out the evolution of dynamic pore structures is of great importance for the efficient development of hydrate deposits.
In this work, excess-water hydrate formation method was combined with micro-computed tomography to study hydrate transition effects on the evolution of dynamic pore structures. Gas state equation and chemical reaction dynamics were combined for separating the representative 3D images at different stages of hydrate formation into four phases, which are respectively hydrate, water, gas and solid skeleton. Hydrate pore habit evolution, formation characteristics, spatial distribution heterogeneity and its effect on the effective porosity variation were studied in detail. Afterwards, a modified maximal ball method was employed to extract hydrate-bearing pore networks at different stages of hydrate phase transition. Hydrate phase transition effects on the effective pore and throat radii distributions, pore and throat cross-sections, throat lengths and distance among connected pore bodies, as well as pore topology were further investigated based on the extracted networks.
Results show that hydrate pore habit varies in porous media during hydrate formation with the main pore habit of pore filling mode. Hydrate spatial distribution exhibits some heterogeneity, causing diverse hydrate saturation at different layers during hydrate phase transition. Hydrate disrupted pore integrity to some extent, resulting in more extracted pore bodies and throats with increased hydrate saturation. In addition, hydrate phase transition reduces pore-throat radii and distribution regularity to different degrees, and results in more irregular pore-throat morphology, decrease of throat length and distance among connected pore bodies as well as poorer connectivity at the same time. This study provides a novel insight in better understanding the evolution of dynamic pore structures and lays a good foundation for the effective development of natural gas hydrate deposits.
Fast multi-phase processes in methane hydrate bearing samples pose a challenge for quantitative micro-computed tomography study and experiment steering due to complex tomographic data analysis involving time-consuming segmentation procedures. This is because of the sample’s multi-scale structure, which changes over time, low contrast between solid and fluid materials, and the large amount of data acquired during dynamic processes. Here, a hybrid approach is proposed for the automatic segmentation of tomographic data from time-resolved imaging of methane gas-hydrate formation in sandy granular media, which includes a deep-learning 3D U-Net model. To prepare a training dataset for the 3D U-Net, a technique to automate data labeling based on sample-specific information about the mineral matrix immobility and occasional fluid movement in pores is proposed. Automatic segmentation allowed for studying properties of the hydrate growth in pores, as well as dynamic processes such as incremental flow and redistribution of pore brine. Results of the quantitative analysis showed that for typical gas-hydrate stability parameters (100 bar methane pressure, 7°C temperature) the rate of formation is slow (less than 1% per hour), after which the surface area of contact between brine and gas increases, resulting in faster formation (2.5% per hour). Hydrate growth reaches the saturation point after 11 h of the experiment. Finally, the efficacy of the proposed segmentation scheme in on-the-fly automatic data analysis and experiment steering with zooming to regions of interest is demonstrated.
Les hydrates de gaz sont abordés dans une grande variété de sujets scientifiques, notamment les géosciences et les procédés industriels tels que la production et le stockage du froid. L’étude des mécanismes de formation et de dissociation des hydrates de gaz en milieux poreux est primordiale pour mieux comprendre la dynamique des hydrates de gaz présents dans les sédiments des marges continentales et optimiser les procédés de stockage du froid. L’objectif de cette étude est de comprendre l’effet de paramètres « clés » comme le degré de saturation en eau, la taille de particules, le débit d’injection du gaz et la morphologie du milieu poreux, sur la cinétique de formation des hydrates et leur capacité de stockage en milieux poreux. Pour cela, deux dispositifs expérimentaux, ont été utilisé dans le but de croiser les connaissances et les méthodologies développées dans les deux disciplines ; Géosciences et Génie des procédés, afin de générer des données complémentaires de cinétique et de thermodynamique, et ainsi mieux caractériser le processus de formation des hydrates en milieu poreux. Le premier dispositif est une cellule haute pression, le deuxième est une cellule d’analyse thermique différentielle. Les résultats obtenus ont montré une tendance décroissante du temps d’induction de la formation des hydrates avec l’augmentation du débit de gaz. Cette tendance n’a pas été observée pour le degré de saturation en eau, la taille de particules, et la morphologie du milieu poreux. Ces paramètres ont montré une faible influence sur le temps d’induction. Les valeurs obtenues confirment le caractère stochastique de la nucléation des hydrates. Une distribution hétérogène des hydrates dans le milieu poreux a été observé dans la trajectoire de la pression au cours des expériences dans la cellule haute pression. Cette distribution des hydrates semble aussi stochastique que le temps d’induction, empêchant ainsi une consommation totale de l’eau même dans des conditions d’excès de gaz. A travers l’ensemble des expériences, il a été montré que la quantité d’hydrates formée dépend fortement du contact entre les deux phase gaz et liquide. Ce dernier est lié à la distribution spatiale de ces deux phases dans le milieu poreux. La présence de méso-pores dans le milieu poreux favorise le contact gaz-liquide ce qui s’est traduit par des performances plus élevée en matière de quantité d’hydrates formée comparé à un milieu poreux sans volumes de pores internes. Il a été montré en comparant les résultats des deux dispositifs expérimentaux que la quantité d’hydrates formée dépend également du système et de l’approche expérimentale utilisés.
Low production rate, limited recovery range, and short period of stabilized production of natural gas hydrate (NGH) field trials indicate a large gap to commercial production. To improve the efficiency of gas recovery from NGH reservoir, a large number of studies on reservoir stimulation have been reported. In this review, various aspects of reservoir fracability, including brittleness, fracture toughness, and fracability index model are comprehensively analyzed to provide an overall view on the generation of fractures in marine NGH reservoir. The main reservoir stimulation methods are summarized into four aspects: hydraulic fracturing, jet breaking, overlying layer modification and split grouting, of which the working principle, advantages, and limitations are highlighted. It indicates split grouting is the most promising method for achieving two goals of both reservoir skeleton reinforcement and permeability enhancement. Besides, the production enhancement by reservoir stimulation of non-diagenetic hydrate reservoir should be further evaluated considering the evolution of artificial seepage channels during hydrate decomposition. Finally, further developments are suggested to obtain reliable results of the fracability of NGH reservoir, bridge the gap of reservoir stimulation between concept design and practical application, and highlight the combination between reservoir stimulation and NGH reservoir stability for safe and efficient gas production.
Knowledge of the mechanical properties of natural gas hydrate reservoirs is fundamental to the safe and commercial extraction of natural gas hydrate. In our work, according to the characteristics of marine sediments in the South China Sea, gas hydrate samples with matrices containing 0%, 10%, 20%, and 30% montmorillonite or illite were prepared based on the saturated gas method. Under effective confining pressures of 2, 3 and 4 MPa, drained compression tests were performed on the samples. The results show that the clay type and clay content affect the failure strength and deformation of clayey silt hydrate sediments. The presence of clay causes the clayey silt hydrate samples to exhibit strain hardening behavior accompanied by shear shrinkage, and the failure strength and stiffness decrease with increasing clay content, as does the internal friction angle. The strength, stiffness, and Poisson's ratio of samples containing illite are generally greater than those containing montmorillonite. In addition, due to the strong bound water between particles, the cohesion of hydrate samples containing montmorillonite with similar hydrate saturations is higher than that of samples containing illite, while the internal friction angle is lower. These results are valuable for production well siting assessment in clayey silt hydrate reservoir and provide requisite theoretical basis for wellbore safety design.
Given the complexity of the thermo-hydro-chemically coupled phase transition process of hydrates, real-time in-situ observations are required. Thermometry maps are particularly essential in analyzing the heat transfer process during the growth and dissociation of crystal hydrates. In this study, we present the temporally and spatially resolved thermometry of the formation of tetrahydrofuran hydrates based on the temperature dependence of the chemical shift of the water proton. Images of temperature changes were synchronously obtained using a 9.4 T 1H magnetic resonance imaging (MRI) system to predict the saturation level of the aqueous solution, phases of the solid hydrates, and the positive temperature anomaly of the exothermic reaction. It was observed that variations in the MRI signal decreased while the temperature rise differed significantly in space and time. The results predicted in this study could have significant implications in optimizing the phase transition process of gas hydrates.
Offshore western Svalbard plumes of gas bubbles rise from the seafloor at the landward limit of the gas hydrate stability zone (LLGHSZ; ∼400 m water depth). It is hypothesized that this methane may, in part, come from dissociation of gas hydrate in the underlying sediments in response to recent warming of ocean bottom waters. To evaluate the potential role of gas hydrate in the supply of methane to the shallow subsurface sediments, and the role of anaerobic oxidation in regulating methane fluxes across the sediment-seawater interface, we have characterised the chemical and isotopic compositions of the gases and sediment pore waters. The molecular and isotopic signatures of gas in the bubble plumes (C1/C2+ = 1 × 10⁴; δ¹³C-CH4 = -55 to -51 ‰; δD-CH4 = -187 to -184 ‰) are similar to gas hydrate recovered from within sediments ∼30 km away from the LLGHSZ. Modelling of pore water sulphate profiles indicates that subsurface methane fluxes are largely at steady state in the vicinity of the LLGHSZ, providing no evidence for any recent change in methane supply due to gas hydrate dissociation. However, at greater water depths, within the GHSZ, there is some evidence that the supply of methane to the shallow sediments has recently increased, which is consistent with downslope retreat of the GHSZ due to bottom water warming although other explanations are possible. We estimate that the upward diffusive methane flux into shallow subsurface sediments close to the LLGHSZ is 30550 mmol m⁻² yr⁻¹, but it is < 20 mmol m² yr⁻¹ in sediments further away from the seafloor bubble plumes. While anaerobic oxidation within the sediments prevents significant transport of dissolved methane into ocean bottom waters this amounts to less than 10% of the total methane flux (dissolved + gas) into the shallow subsurface sediments, most of which escapes AOM as it is transported in the gas phase.
An unusual ice type, called hair ice, grows on the surface of dead wood of
broad-leaf trees at temperatures slightly below 0 °C. We describe
this phenomenon and present physical, chemical, and biological
investigations to gain insight in the properties and processes related to
hair ice. Tests revealed that the biological activity of a winter-active
fungus is required in the wood for enabling the growth of hair ice. We
confirmed the fungus hypothesis originally suggested by Wegener (1918) by
reproducing hair ice on wood samples. Treatment by heat and fungicide suppresses the formation of hair ice. Fruiting bodies of Asco-
and Basidiomycota are identified on hair-ice-carrying wood. One species,
Exidiopsis effusa (Ee), was present on all investigated samples.
Both hair-ice-producing wood samples and those with killed fungus show
essentially the same temperature variation, indicating that the heat
produced by fungal metabolism is very small, that the freezing rate is not
influenced by the fungus activity, and that ice segregation is the common
mechanism of ice growth on the wood surface. The fungus plays the role of
shaping the ice hairs and preventing them from recrystallisation. Melted
hair ice indicates the presence of organic matter. Chemical analyses show a
complex mixture of several thousand CHO(N,S) compounds similar to fulvic
acids in dissolved organic matter (DOM). The evaluation reveals decomposed
lignin as being the main constituent. Further work is needed to clarify its role
in hair-ice growth and to identify the recrystallisation inhibitor.
Hydrates may occur where thermodynamic conditions permit and where methane concentration in the water exceeds a threshold level, but they will only concentrate where gas flow is focused. Existing models of submarine gas hydrate occurrence encapsulate the system of transport and reactions into a one dimensional model (e. g. Rempel and Buffet 1998, Zatsepina and Buffett 1998, Xu and Ruppel 1999). With this simplification we can constrain key parameters, but it is difficult to capture the geological complexity of real systems. To predict the spatial distribution of hydrates we need to account for the range of mechanisms by which methane can move though the sediments.
Natural and artificial gas hydrates with internal pores of nano- to centimeters and weak grain-cementation have been widely reported, while the detailed formation process of grain-cementing hydrates remains poorly identified. Pore-scale morphology of carbon dioxide (CO2) hydrate formed in a partially brine-saturated porous medium was investigated via X-ray computed microtomography (X-ray CMT). Emphasis is placed on the pore-scale growth patterns of gas hydrate, including the growth of dendritic hydrate crystals on pre-formed hydrate and water-wetted grains, porous nature of the hydrate phase, volume expansion of more than 200% during the water-to-hydrate phase transformation, preference of unfrozen water wetting hydrophilic minerals, and the relevance to a weak cementation effect on macro-scale physical properties. The presented pore-scale morphology and growth patterns of gas hydrate are expected in natural sediment settings where free gas is available for hydrate formation, such as active gas vents, gas seeps, mud volcanoes, permafrost gas hydrate provinces, and CO2 injected formation for the sake of geologic carbon storage; and in laboratory hydrate samples synthesized from partially brine-saturated sediments or formed from water-gas interfaces. This article is protected by copyright. All rights reserved.
The use of high resolution tomographic techniques has allowed for unprecedented observations and a renewed understanding of geomaterials and processes. A laboratory x-ray scanner is used to explore the potential of the technology in the context of complex geotechnical systems. Tests benefit from the fast and non-destructive nature of x-ray measurements and the micron-scale resolution that is attainable. Several first-time observations are reported in this manuscript. It is shown that subsurface volume loss in sandy soils can cause the formation of sharply-defined low-density pipes; cryogenic suction consolidates sediments next to ice lenses during ground freezing; root growth involves transverse expansion, and the stress relaxation at the tip facilitates further longitudinal invasion; blade insertion causes successive shear localizations; the incipient formation of desiccation cracks is not necessarily along a planar front, in fact, the fracture plane may split as it encounters heterogeneities at the tip. Finally, it is shown that x-rays can be used to monitor chemical processes that cause coupled mechanical effects, such as osmotic consolidation induced by ionic diffusion and mineral dissolution. While short-time events may not be tomographically imaged, single x-ray radiographs can be analysed and compared to gain extensive process information.
The physical properties of gas hydrate-bearing sediments depend on the
volume fraction and spatial distribution of the hydrate phase. The host
sediment grain size and the state of effective stress determine the
hydrate morphology in sediments; this information can be used to
significantly constrain estimates of the physical properties of
hydrate-bearing sediments, including the coarse-grained sands subjected
to high effective stress that are of interest as potential energy
resources. Reported data and physical analyses suggest hydrate-bearing
sands contain a heterogeneous, patchy hydrate distribution, whereby
zones with 100% pore-space hydrate saturation are embedded in
hydrate-free sand. Accounting for patchy rather than homogeneous hydrate
distribution yields more tightly constrained estimates of physical
properties in hydrate-bearing sands and captures observed
physical-property dependencies on hydrate saturation. For example,
numerical modeling results of sands with patchy saturation agree with
experimental observation, showing a transition in stiffness starting
near the series bound at low hydrate saturations but moving toward the
parallel bound at high hydrate saturations. The hydrate-patch size
itself impacts the physical properties of hydrate-bearing sediments; for
example, at constant hydrate saturation, we find that conductivity
(electrical, hydraulic and thermal) increases as the number of
hydrate-saturated patches increases. This increase reflects the larger
number of conductive flow paths that exist in specimens with many small
hydrate-saturated patches in comparison to specimens in which a few
large hydrate saturated patches can block flow over a significant
cross-section of the specimen.
The injection of carbon dioxide, CO2, into methane hydrate-bearing sediments causes the release of methane, CH4, and the formation of carbon dioxide hydrate, even if global pressure-temperature conditions remain within the CH4 hydrate stability field. This phenomenon, known as CH4-CO2 exchange or CH4-CO2 replacement, creates a unique opportunity to recover an energy resource, methane, while entrapping a greenhouse gas, carbon dioxide. Multiple coexisting processes are involved during CH4-CO2 replacement, including heat liberation, mass transport, volume change, and gas production among others. Therefore, the comprehensive analysis of CH4-CO2 related phenomena involves physico-chemical parameters such as diffusivities, mutual solubilities, thermal properties, and pressure- and temperature-dependent phase conditions. We combine new experimental results with published studies to generate a data set we use to evaluate reaction rates, to analyze underlying phenomena, to explore the pressure-temperature region for optimal exchange, and to anticipate potential geomechanical implications for CH4-CO2 replacement in hydrate-bearing sediments.
The behavior of fine-grained mineral systems is dependent on pore-fluid characteristics. The systematic analysis of previously published studies supports the development of a fabric map in the pH and ionic concentration space as a working hypothesis. This conceptual study is complemented with an extensive battery of tests where surface charge and particle interactions are controlled through pore-fluid characteristics. The macro-scale tests include sedimentation, viscosity and liquid limit, and involve a wide range of solid volume fractions (suspension to sediment) and strain levels. Experimental results permit the development of an updated fabric map on the pH-ionic concentration space which takes into consideration all experimental results. The fabric map is structured around a critical pH level and a threshold ionic concentration beyond which van der Waals attraction prevails.
Hydrate-bearing sediments may destabilize spontaneously as part of
geological processes, unavoidably during petroleum drilling/production
operations or intentionally as part of gas extraction from the hydrate
itself. In all cases, high pore fluid pressure generation is anticipated
during hydrate dissociation. A comprehensive formulation is derived for
the prediction of fluid pressure evolution in hydrate-bearing sediments
subjected to thermal stimulation without mass transfer. The formulation
considers pressure- and temperature-dependent volume changes in all
phases, effective stress-controlled sediment compressibility,
capillarity, and the relative solubilities of fluids. Salient
implications are explored through parametric studies. The model properly
reproduces experimental data, including the PT evolution along the phase
boundary during dissociation and the effect of capillarity. Pore fluid
pressure generation is proportional to the initial hydrate fraction and
the sediment bulk stiffness; is inversely proportional to the initial
gas fraction and gas solubility; and is limited by changes in effective
stress that cause the failure of the sediment. When the sediment
stiffness is high, the generated pore pressure reflects thermal and
pressure changes in water, hydrate, and mineral densities. Comparative
analyses for CO2 and CH4 highlight the role of gas
solubility in excess pore fluid pressure generation. Dissociation in
small pores experiences melting point depression due to changes in water
activity, and lower pore fluid pressure generation due to the higher gas
pressure in small gas bubbles. Capillarity effects may be disregarded in
silts and sands, when hydrates are present in nodules and lenses and
when the sediment experiences hydraulic fracture.
The stability of submarine gas hydrates is largely dictated by pressure
and temperature, gas composition, and pore water salinity. However, the
physical properties and surface chemistry of deep marine sediments may
also affect the thermodynamic state, growth kinetics, spatial
distributions, and growth forms of clathrates. Our conceptual model
presumes that gas hydrate behaves in a way analogous to ice in a
freezing soil. Hydrate growth is inhibited within finegrained sediments
by a combination of reduced pore water activity in the vicinity of
hydrophilic mineral surfaces, and the excess internal energy of small
crystals confined in pores. The excess energy can be thought of as a
``capillary pressure'' in the hydrate crystal, related to the pore size
distribution and the state of stress in the sediment framework. The base
of gas hydrate stability in a sequence of fine sediments is predicted by
our model to occur at a lower temperature (nearer to the seabed) than
would be calculated from bulk thermodynamic equilibrium. Capillary
effects or a build up of salt in the system can expand the phase
boundary between hydrate and free gas into a divariant field extending
over a finite depth range dictated by total methane content and
pore-size distribution. Hysteresis between the temperatures of
crystallization and dissociation of the clathrate is also predicted.
Growth forms commonly observed in hydrate samples recovered from marine
sediments (nodules, and lenses in muds; cements in sands) can largely be
explained by capillary effects, but kinetics of nucleation and growth
are also important. The formation of concentrated gas hydrates in a
partially closed system with respect to material transport, or where gas
can flush through the system, may lead to water depletion in the host
sediment. This ``freeze-drying'' may be detectable through physical
changes to the sediment (low water content and overconsolidation) and/or
chemical anomalies in the pore waters and metastable presence of free
gas within the normal zone of hydrate stability.
Indian National Gas Hydrate Program Expedition 01 has established that clay-rich marine sediment from the Krishna-Godavari (KG) basin in the eastern Indian margin hosts one of the richest gas hydrate deposits in the world. Resistivity at-bit images and pressure cores reveal that the gas hydrate morphology in clay-rich sediment varies from complex vein structures (grain displacing) to invisible pore filling. Existing rock physics models, which relate acoustic data to in situ gas hydrate concentrations, generally assume isotropic pore-filling gas hydrate, which yields misleading concentration estimates for fractured fine-grained sediments. The anisotropic KG basin sediment presents additional complications. Here we apply differential effective medium theory to incorporate grain-displacing morphologies by which gas hydrate is included as vertical ellipsoids with aspect ratios ranging from those of thin veins up to those of nodules in an elastic anisotropic background. We have estimated gas hydrate concentrations from sonic velocities at hole 10D in the KG basin considering three basic gas hydrate morphologies: (i) pore filling, (ii) grain displacing, and (iii) a combination of grain displacing and pore filling. Average gas hydrate saturations for these three cases are 35–42%, 27–30%, and 33–41% of the total porosity, respectively, in the depth range 60–140 m below seafloor (mbsf). Saturation is highest at ∼67 mbsf for any morphology but the values differ between morphologies. For the pore-filling morphology, the maximum gas hydrate saturation of 56% is 18–22% higher than the grain-displacing morphology and 2–9% higher than the combined morphology. Estimates differ by ±6% of the sediment volume with rotations of gas hydrate veins from vertical to horizontal.
Studies of geologic and geophysical data from the offshore of India have revealed two geologically distinct areas with inferred gas hydrate occurrences: the passive continental margins of the Indian Peninsula and along the Andaman convergent margin. The Indian National Gas Hydrate Program (NGHP) Expedition 01 was designed to study the occurrence of gas hydrate off the Indian Peninsula and along the Andaman convergent margin with special emphasis on understanding the geologic and geochemical controls on the occurrence of gas hydrate in these two diverse settings. NGHP Expedition 01 established the presence of gas hydrates in KrishnaGodavari, Mahanadi and Andaman basins. The expedition discovered one of the richest gas hydrate accumulations yet documented (Site 10 in the Krishna-Godavari Basin), documented the thickest and deepest gas hydrate stability zone yet known (Site 17 in Andaman Sea), and established the existence of a fully-developed gas hydrate system in the Mahanadi Basin (Site 19).
The stability conditions of submarine gas hydrates (methane clathrates) are largely dictated by pressure, temperature, gas composition, and pore water salinity. However, the physical properties and surface chemistry of the host sediments also affect the thermodynamic state, growth kinetics, spatial distributions, and growth forms of clathrates. Our model presumes that gas hydrate behaves in a way analogous to ice in the pores of a freezing soil, where capillary forces influence the energy balance. Hydrate growth is inhibited within fine-grained sediments because of the excess internal phase pressure of small crystals with high surface curvature that coexist with liquid water in small pores. Therefore, the base of gas hydrate stability in a sequence of fine sediments is predicted by our model to occur at a lower temperature, and so nearer to the seabed than would be calculated from bulk thermodynamic equilibrium. The growth forms commonly observed in hydrate samples recovered from marine sediments (nodules, sheets, and lenses in muds; cements in sand and ash layers) can be explained by a requirement to minimize the excess of mechanical and surface energy in the system.
High-angle, open mode fractures control the presence of natural gas hydrate in water-saturated clays at the Keathley Canyon 151 site in the northern Gulf of Mexico, which was investigated for gas hydrates as part of the Chevron Joint Industry Project drilling in 2005. We analyze logging-while-drilling resistivity images and infer that gas hydrate accumulated in situ in two modes: filling fractures and saturating permeable beds. High-angle hydrate-filled fractures are the most common mode for gas hydrate occurrence at this site, with most of these fractures dipping at angles of more than 40° and occurring between 220 and 300 m below seafloor. These fractures all strike approximately N–S, which agrees with the 165°SE–345°NW maximum horizontal stress direction determined from borehole breakouts and which aligns with local bathymetric contours. In one interval of hydrate-filled fractures, porosity increases with increasing hydrate saturation. We suggest that high pore pressure may have dilated sediments during fracture formation, causing this increase in porosity. Furthermore, the formation of gas hydrate may have heaved fractures apart, also increasing the formation porosity in this interval.
1] The interaction among water molecules, guest gas molecules, salts, and mineral particles determines the nucleation and growth behavior of gas hydrates in natural sediments. Hydrate of tetrahydrofuran (THF) has long been used for laboratory studies of gas hydrate-bearing sediments to provide close control on hydrate concentrations and to overcome the long formation history of methane hydrate from aqueous phase methane in sediments. Yet differences in the polarizability of THF (polar molecule) compared to methane (nonpolar molecule) raise questions about the suitability of THF as a proxy for methane in the study of hydrate-bearing sediments. From existing data and simple macroscale experiments, we show that despite its polar nature, THF's large molecular size results in low permittivity, prevents it from dissolving precipitated salts, and hinders the solvation of ions on dry mineral surfaces. In addition, the interfacial tension between water and THF hydrate is similar to that between water and methane hydrate. The processes that researchers choose for forming hydrate in sediments in laboratory settings (e.g., from gas, liquid, or ice) and the pore-scale distribution of the hydrate that is produced by each of these processes likely have a more pronounced effect on the measured macroscale properties of hydrate-bearing sediments than do differences between THF and methane hydrates themselves. Components: 6330 words, 6 figures, 1 table. (2007), Observations related to tetrahydrofuran and methane hydrates for laboratory studies of hydrate-bearing sediments, Geochem. Geophys. Geosyst., 8, Q06003, doi:10.1029/2006GC001531.
Marine controlled source electromagnetic (CSEM) data have been collected to investigate methane seep sites and associated gas hydrate deposits at Opouawe Bank on the southern tip of the Hikurangi Margin, New Zealand. The bank is located in about 1000 m water depth within the gas hydrate stability field. The seep sites are characterized by active venting and typical methane seep fauna accompanied with patchy carbonate outcrops at the seafloor. Below the seeps gas migration pathways reach from below the bottom simulating reflector (at around 380 m sediment depth) towards the seafloor, indicating free gas transport into the shallow hydrate stability field. The CSEM data have been acquired with a seafloor-towed, electric multi-dipole system measuring the inline component of the electric field. CSEM data from three profiles have been analyzed using 1D and 2D inversion techniques. High resolution 2D and 3D multichannel seismic data have been collected in the same area. The electrical resistivity models show several zones of highly anomalous resistivities (>50 Ωm) which correlate with high amplitude reflections located on top of narrow vertical gas conduits, indicating the coexistence of free gas and gas hydrates within the hydrate stability zone. Away from the seeps the CSEM models show normal background resistivities between ~1 and 2 Ωm. Archie's Law has been applied to estimate gas/gas hydrate saturation below the seeps. At intermediate depths between 50 and 200 m below seafloor saturations are between 40 and 80%, and gas hydrate may be the dominating pore filling constituent. At shallow depths from 10 m to the seafloor, free gas dominates as seismic data and gas plumes suggest.
The 75-μm particle size is used to discriminate between fine and coarse grains. Further analysis of fine grains is typically based on the plasticity chart. Whereas pore-fluid-chemistry-dependent soil response is a salient and distinguishing characteristic of fine grains, pore-fluid chemistry is not addressed in current classification systems. Liquid limits obtained with electrically contrasting pore fluids (deionized water, 2-M NaCl brine, and kerosene) are combined to define the soil "electrical sensitivity." Liquid limit and electrical sensitivity can be effectively used to classify fine grains according to their fluid-soil response into no-, low-, intermediate-, or high-plasticity fine grains of low, intermediate, or high electrical sensitivity. The proposed methodology benefits from the accumulated experience with liquid limit in the field and addresses the needs of a broader range of geotechnical engineering problems.
TOKYO SKYTREE is the tallest free-standing antenna tower; whose height is 634 m. However, the number of lightning strikes to a tall structure increases in proportion to the structure height. Therefore, it is important to establish lightning protection methods for facilities in and around such tall antenna tower. There is a possibility that comparatively large magnetic fields occur inside an observatory by the current flowing through steel columns composing the TOKYO SKYTREE. This paper presents simulation results of lightning current distribution and magnetic fields around the observatory. Simulated results are from the finite-difference time-domain (FDTD) method which is one of the most widely used numerical electromagnetic analysis methods. The knowledge of these current distribution characteristics and magnetic fields would be very useful for lightning protection of TOKYO SKYTREE.
The paper shows that cooling of fairly dry soil, with freezing nucleated at one spot, causes certain pores to fill abruptly with ice, thus depleting the water content of the surrounding soil. This conclusion agrees with available data. It is suggested that the ice pressure is slightly greater than atmospheric pressure when pores fill. A solution model of secondary heaving is used to illustrate an increase of ice pressure from the freezing front to the base of the ice lens.
Frost heave results from ice segregation and ice lens formation and growth as a soil freezes. This formation is initiated by cracking of the soil in the frozen fringe. Therefore, evaluation of the ice lens initiation requires the determination of the crack initiation condition in the frozen fringe. A new fundamental approach is proposed to determine the ice lens initiation condition using the soil freezing characteristics curve (SFCC). It is demonstrated that an ice lens initiates close to the so-called ice-entry value defined using the SFCC. Ice lens initiation conditions for different boundary conditions were determined in a laboratory using the SFCC and were then compared with the ice lens initiation conditions from a one-dimensional open system frost heave tests. The results using the SFCC showed good agreement with the values determined experimentally. It was therefore concluded that the SFCC derived information can be used as an input parameter in existing frost heave models to establish the segregation temperature.
Gas hydrates consist of guest gas molecules encaged in water cages. Methane hydrate forms in marine and permafrost sediments. In this study, we use optical, mechanical and electrical measurements to monitor hydrate formation and growth in small pores to better understand the hydrate pore habit in hydrate-bearing sediments. Hydrate formation in capillary tubes exposes the complex and dynamic interactions between nucleation, gas diffusion and gas solubility. The observation of hydrate growth in a droplet between transparent plates shows that the hydrate shell does not grow homogeneously but advances in the form of lobes that invade the water phase; in fact, the hydrate shell must be discontinuous and possibly cracked to justify the relatively fast growth rates observed in these experiments. Volume expansion during hydrate formation causes water to flow out of menisci; expelled water either spreads on the surface of water-wet substrates and forms a thin hydrate sheet, or remains next to menisci when substrates are oil-wet. Hydrate formation is accompanied by ion exclusion, yet, there is an overall increase in electrical resistance during hydrate formation. Hydrate growth may become salt-limited in trapped water conditions; in this case, aqueous brine and gas CH4 may be separated by hydrate and the three-phase system remains stable within the pore space of sediments.
For the past three decades, discussion of naturally-occurring gas hydrates has been framed by a series of assessments that indicate enormous global volumes of methane present within gas hydrate accumulations. At present, these estimates continue to range over several orders of magnitude, creating great uncertainty in assessing those two gas hydrate issues that relate most directly to resource volumes – gas hydrate’s potential as an energy resource and its possible role in ongoing climate change. However, a series of recent field expeditions have provided new insights into the nature of gas hydrate occurrence; perhaps most notably, the understanding that gas hydrates occur in a wide variety of geologic settings and modes of occurrence. These fundamental differences - which include gas hydrate concentration, host lithology, distribution within the sediment matrix, burial depth, water depth, and many others - can now be incorporated into evaluations of gas hydrate energy resource and environmental issues. With regard to energy supply potential, field data combined with advanced numerical simulation have identified gas-hydrate-bearing sands as the most feasible initial targets for energy recovery. The first assessments of potential technically-recoverable resources are now occurring, enabling a preliminary estimate of ultimate global recoverable volumes on the order of ~3 × 1014 m3 (1016 ft3; 150 GtC). Other occurrences, such as gas hydrate-filled fractures in clay-dominated reservoirs, may also become potential energy production targets in the future; but as yet, no production concept has been demonstrated. With regard to the climate implications of gas hydrate, an analogous partitioning of global resources to determine that portion most prone to dissociation during specific future warming scenarios is needed. At present, it appears that these two portions of total gas hydrate resources (those that are the most likely targets for gas extraction and those that are the most likely to respond in a meaningful way to climate change) will be largely exclusive, as those deposits that are the most amenable to production (the more deeply buried and localized accumulations) are also those that are the most poorly coupled to oceanic and atmospheric conditions.
Gas hydrates are solid crystalline compounds of water and methane that are similar to snow. They form in and block gas wells and lines and cause fouling of heat exchangers when the gas is cooled. Here, finally, is long-awaited information detailing the successful methods for not only removing gas hydrates but also for preventing their reforming. This book addresses methods of hydrate removal and, most importantly, prevention of hydrate build-up. New topics of using hydrate properties for new technologies and production of gas from natural gas hydrate deposits are also discussed. Information specific to hydrate formation in gas includes: conditions and area of hydrate-carrying rock; estimating gas amounts in gas hydrate deposits; the role of natural gas hydrates in global changes; the physical principles and models of gas hydrate deposit development; results of the Messoyakhi gas hydrate field development; and hydrates in space. Contents includes: Physical-chemical aspects; Properties of hydrates; Mechanism of hydrate formation; Technological appellations of hydrates; Natural hydrates of gases; Conclusion; and Bibliography.
Gas hydrates are a vast energy resource with global distribution in the permafrost and in the oceans. Even if conservative estimates are considered and only a small fraction is recoverable, the sheer size of the resource is so large that it demands evaluation as a potential energy source. In this review paper, we discuss the distribution of natural gas hydrate accumulations, the status of the primary international R&D programs, and the remaining science and technological challenges facing commercialization of production. After a brief examination of gas hydrate accumulations that are well characterized and appear to be models for future development and gas production, we analyze the role of numerical simulation in the assessment of the hydrate production potential, identify the data needs for reliable predictions, evaluate the status of knowledge with regard to these needs, discuss knowledge gaps and their impact, and reach the conclusion that the numerical simulation capabilities are quite advanced and that the related gaps are either not significant or are being addressed. We review the current body of literature relevant to potential productivity from different types of gas hydrate deposits, and determine that there are consistent indications of a large production potential at high rates over long periods from a wide variety of hydrate deposits. Finally, we identify (a) features, conditions, geology and techniques that are desirable in potential production targets, (b) methods to maximize production, and (c) some of the conditions and characteristics that render certain gas hydrate deposits undesirable for production.
The enthalpies of the reactions in which carbon dioxide hydrate is dissociated to carbon dioxide vapor and either water or ice are determined by an analysis with the Clapeyron equation. The most important feature of the new analysis is the direct use of the Clapeyron equation rather than the Clausius-Clapeyron equation. The analysis takes into account the finite volumes of the condensed phases, the nonideality of the vapor phase, and the solubility of carbon dioxide in water. New data for the solubility in the vicinity of the (water+hydrate+vapor) coexistence curve are employed. The enthalpy change of the reaction CO2·nH2O(s)=CO2(g)+nH2O(l) is found to vary from (63.6±1.8) kJ·mol−1 to (57.7±1.8) kJ·mol−1 between quadruple points Q1 and Q2, and the hydration number varies from (6.6±0.3) to (5.6±0.3) over the same range. The results are compared to values reported in the literature that were determined by various techniques.
Sediment particles affect the phase behavior of gas hydrates, both by increasing the surface energy where pore geometry forces hydrate crystals to attain high curvatures and through wetting interactions that cause aqueous films to coat particle surfaces. These effects produce only slight changes to the gas solubility through most of the hydrate stability zone, so the particle size has only a modest influence on the rate of hydrate accumulation when the sediments are homogeneous. In hydrate reservoirs, however, discontinuous changes in sediment properties are common and such stratigraphic boundaries often coincide with hydrate anomalies. These anomalies are a natural consequence of variations in subsurface sediment properties. By accounting for sediment-hydrate interactions, I show how compositional diffusion supplies the growth of hydrate spikes in coarse-grained sediments immediately adjacent to hydrate-free regions (HFRs) in more fine-grained sediments where the solubility is slightly elevated. Over timescales comparable with Milankovitch cycles, hydrate spikes are typically less than a meter in width and contain essentially all of the hydrate that would have otherwise occupied the much larger adjacent HFR if sediment heterogeneities were absent. Hydrate can form in the more fine-grained sediments only once the spike achieves a sufficiently high saturation level (often >90% of pore volume) that the solubility is continuous across the stratigraphic boundary. The wetting interactions that stabilize much of the residual liquid when hydrate forms an interconnected skeleton spanning many pore diameters can also partially unload sediment particle contacts, and lead to the growth of segregated hydrate nodules and lenses.
The Indian National Gas Hydrate Program (NGHP) Expedition 1, of 2006, cored through several methane gas hydrate deposits on the continental shelf around the coast of India. The pressure coring techniques utilized during the expedition (HYACINTH and PCS) enabled recovery of gas hydrate bearing, fine-grained, sediment cores to the surface. After initial characterization core sections were rapidly depressurized and submerged in liquid nitrogen, preserving the structure and form of the hydrate within the host sediment. Once on shore, high resolution X-ray CT scanning was employed to obtain detailed three-dimensional images of the internal structure of the gas hydrate. Using a resolution of 80 μm the detailed structure of the hydrate veins present in each core could be observed, and allowed for an in depth analysis of orientation, width and persistence of each vein. Hydrate saturation estimates could also be made and saturations of 20–30% were found to be the average across the core section with some portions showing highs of almost 60% saturation. The majority of hydrate veins in each core section were found to be orientated between 50 and 80° to the horizontal. Analysis of the strikes of the veins suggested a slight preferential orientation in individual sample sections, although correlation between individual sections was not possible due to the initial orientation of the sections being lost during the sampling stage. The preferred vein orientation within sample sections coupled with several geometric features identified in individual veins, suggest that hydraulic fracturing by upward advecting pore fluids is the main formation mechanism for the veined hydrate deposits in the K–G Basin.
The physical characteristics of hydrate-bearing sediments sampled by pressure coring from the Ulleung Basin in the Sea of Japan (East Sea) were investigated using an instrumented chamber capable of testing recovered natural sediments that have never left the methane hydrate stability field. The heterogeneous distribution of segregated hydrate veins and lens structures in sediments results in highly variable geophysical and geomechanical properties. The scaled production test was conducted by controlled depressurization of pressure cores while dissociation and gas production were concurrently monitored using various sensors in the instrumented chamber. The hydrate saturation was estimated to be ∼19.5% in the pore space. Data show a sharp reduction in sediment shear and bulk stiffnesses during hydrate dissociation. Relatively fast gas migration was observed, probably along high-conduction planes left behind as hydrate veins dissociated. The spatial distribution of hydrates in sediment was analyzed based on 3-D image processing. The phenomena relevant to the production test and sampling effects during pressure coring are discussed.
An experimental study was performed to visually observe the driving force dependence of hydrate growth in a porous medium filled with either liquid water and dissolved CO2 or liquid water and gaseous CO2. The given system subcooling, DeltaTsub, i.e. the deficiency of the system temperature from the triple CO2-hydrate-water equilibrium temperature under a given pressure, ranged from 1.7 K to 7.3 K. The fine dendrites initially formed at DeltaTsub = 7.3 K changed quickly into particulate crystals. For DeltaTsub = 1.7 K, faceted hydrate crystals grew and the subsequent morphological change was hardly identified for an eight-day observation period. These results indicate that the physical bonding between hydrate crystals and skeletal materials becomes stronger with decreasing driving force, suggesting that the fluid dynamic and mechanical properties of hydrate-bearing sediments vary depending on the hydrate crystal growth process.
On 17 May 2009, the Kongsberg EM302 multibeam echo sounder on board the U.S. National Oceanic and Atmospheric Administration's (NOAA) Okeanos Explorer was collecting bathymetry and water column acoustic data offshore of northern California when it suddenly imaged a previously undiscovered 1400-meter-high plume (Figure 1) rising from the seafloor at 40°32.13'N, 124°47.01'W. The ship was mapping in water depths of approximately 1830 meters and heading east up the northern California continental margin 20 kilometers north of the Gorda escarpment. The continental shelf in this area is known to have subsurface and water column thermogenic and methane gas, although no plumes from this area previously have been reported from deeper than the continental shelf. The plume, which rises vertically 1000 meters before being deflected to the north, was recorded for approximately 5 minutes before it disappeared from the data. The recording was made at night, so the ship's bridge watch was not able to see any surface manifestations of the plume at that time. The plume is composed of individual streams of acoustic reflectors, best seen in a video assembled from the water column data (http://ccom.unh.edu/NOAA_oceanexploration). The digital terrain model created from the multibeam bathymetry shows that the plume rises from the base of a large, previously unknown, amphitheater-like failure.
A numerical model is explored which simulates frost heave in saturated,
granular, air-free, solute-free soil. It is based on equations developed
from fundamental thermomechanical considerations and previous laboratory
investigations. Although adequate data are lacking for strict
experimental verification of the model, we note that simulations produce
an overall course of events together with significant specific features
which are familiar from laboratory experience. Simulated heave histories
show proper sensitivities in the shapes and orders of magnitude of
output responses and in the relations between crucial factors such as
heave rate, freezing rate, and overburden.
The old theory that frost heaving is due to change in volume of water frozen was based on experiments with closed systems. Field observations and recent experiments indicate that soils, when subjected to freezing under normal conditions, usually behave as open systems. When the freezing of saturated soils results in little or no heaving, part of the water is forced through the soil voids below the zone of freezing, compressing or expelling air. Excessive heaving results when water is pulled through the soil to build up layers of segregated ice. These ice layers grow in thickness because water molecules are pulled into the thin film that separates the growing ice crystals from underlying soil particles. Since heavy surface loads may be heaved and much force is required to pull water through impervious clay, the water is put under high tension. Heaving is limited by the tensile stress that may be developed in the water and by downward growth of ice crystals in soil voids. These two factors also probably explain the rhythmic banding due to alternating layers of ice and clay. In well-consolidated clays the surface uplift equals the total thickness of the ice layers, the water content of the clay between the ice layers remaining approximately constant; but heaving is continuous and regular instead of intermittent. Clay is soft near the lowest ice layer because much of the water is unfrozen, the hardness increasing higher up where the temperature is lower and freezing has gone on for a longer time. Additional evidence has been obtained by freezing in open systems other liquids than water.
Gas hydrates represent an unconventional methane resource and a production/safety risk to traditional oil and gas flowlines. In both systems, hydrate may share interfaces with both aqueous and hydrocarbon fluids. To accurately model macroscopic properties, such as relative permeability in unconventional systems or dispersion viscosity in traditional systems, knowledge of hydrate interfacial properties is required. This work presents hydrate cohesive force results measured on a micromechanical force apparatus, and complementary water-hydrocarbon interfacial tension data. By combining a revised cohesive force model with experimental data, two interfacial properties of cyclopentane hydrate were estimated: hydrate-water and hydrate-cyclopentane interfacial tension values at 0.32±0.05 mN/m and 47±5 mN/m, respectively. These fundamental physiochemical properties have not been estimated or measured for cyclopentane hydrate to date. The addition of surfactants in the cyclopentane phase significantly reduced the cyclopentane hydrate cohesive force; we hypothesize this behavior to be the result of surfactant adsorption on the hydrate-oil interface. Surface excess quantities were estimated for hydrate-oil and water-oil interfaces using four carboxylic and sulfonic acids. The results suggest the density of adsorbed surfactant may be 2× larger for the hydrate-oil interface than the water-oil interface. Additionally, hydrate-oil interfacial tension was observed to begin decreasing from the baseline value at significantly lower surfactant concentrations (one to three orders of magnitude) than for the water-oil interfacial tension.
The growth of Xe clathrate hydrate in a Xe−ice system was observed in situ by microfocus X-ray computed tomography (CT). In the initial stage of pressurization with Xe gas, quasi-two-dimensional hydrate growth on the ice surface was observed at the Xe−ice interface. With time, Xe hydrate grain grew not only along the ice surface (cover growth), but also toward the gas phase (outer growth), and inside the ice phase (inner growth). A lens-shaped Xe hydrate was formed at the initial interface between the gas phase and ice. Anisotropic hydrate growth resulted in a lens-shaped Xe hydrate. Xe hydrate growth at the gas−ice interface is strongly influenced by the supply of H2O molecules. The inner growth in the lens-shaped Xe hydrate changed in shape from spherical to elliptical. Variations in the contrast in cross-sectional CT images show that the inside of the Xe hydrate region became condensed as time elapsed. Inner growth would result from the formation of a condensed hydrate region, which may lead to a change in the Xe gas supply through the hydrate layer. Observations of Xe hydrate growth by microfocus X-ray CT can be applied to a discussion of the hydrate growth process in a gas−ice system.
In this review, we summarize the available experimental data from recently developed molecular level techniques on the surface structure, surface premelting layer thickness, and friction of ice. We conclude that surface premelting of ice is responsible for the unique surface properties of the important substance.
We present experimental structure-I clathrate hydrate (methane, carbon dioxide, and methane−carbon dioxide) equilibrium and ice-melting data for mesoporous silica glass. In both cases, high capillary pressures result in depressed solid decomposition temperatures (clathrate dissociation and ice melting), as a function of pore diameter. Clathrate dissociation data show a significant improvement over existing literature data, which is attributed to the improved experimental techniques and interpretative methods used. Through application of a melting (or clathrate dissociation) modified Gibbs−Thomson relationship to experimental data, we determine similar values of 32 ± 2, 32 ± 3, and 30 ± 3 mJ/m2 for ice−water, methane clathrate−water, and carbon dioxide clathrate−water interfacial tensions, respectively. The data are important for the accurate thermodynamic modeling of clathrate systems, particularly with respect to subsea sedimentary environments, and should prove useful in the simulation of potential methane hydrate exploitation and carbon dioxide sequestration schemes.
Equilibrium conditions of CH4, CO2, and C3H8 hydrates confined in small pores of porous glass were determined. The dissociation temperature of each hydrate at a given pressure shifted lower than that for bulk hydrate; the largest shift for CH4 hydrate was −12.3 K ± 0.2 K for 4-nm-diameter pores and the shift decreased to only −0.5 K for 100-nm pores. CH4 hydrate experiments at temperatures lower than the quadruple point of 270.6 K in 30-nm porous glass showed no shift of the equilibrium line. All temperature shifts were fitted by the Gibbs-Thomson equation; the best fits for CH4, CO2, and C3H8 hydrates predicted hydrate−water interfacial energies of 1.7(3) × 10-2 J/m2, 1.4(3) × 10-2 J/m2, and 2.5(1) × 10-2 J/m2, respectively. Both type-I hydrates of CH4 and CO2 had interfacial energies within 20% of each other but significantly smaller than the type-II hydrate of C3H8. Ice formation in the same porous glass fit the Gibbs−Thomson relation with an interfacial energy of 2.9(6) × 10-2 J/m2, which is in good agreement with established values. The estimated interfacial tensions between gas hydrates and water were found to be only weakly affected by the kinds of gas. This indicated that the pore effect on the phase equilibrium was mainly due to the water activity change. The wide range of experiments on pore size, temperature, and the kind of gas allowed us to evaluate the validity of previous model predictions for pore effects on gas hydrate stability.
Equilibrium pressures for the dissociation of propane hydrate confined in silica gel pores of nominal radii 7.5, 5.0, 3.0, and, 2.0 nm were measured across a wide temperature range, and were higher than those for bulk propane hydrate. The upward shift from the bulk pressures, which depended on the pore size, was 200−300% for the smallest pore size. In each of the pressure−temperature profiles, the temperature at which the four phases (hydrate, ice, liquid water, and gas) apparently coexisted was identified, and this temperature was a linear function of the reciprocal pore size. The enthalpy of dissociation for the equilibrium involving hydrate, liquid water, and gas was estimated from the data. The change with pore size in this apparent enthalpy appeared to be analogous to the variation in the heat of melting of pore ice as sometimes reported.
The pressure-temperature profiles for the hydrate-ice-gas and hydrate-liquid water-gas equilibria were measured for methane and propane hydrates in 70-â«-radius silica gel pores. In both cases, the equilibrium pressures were 20-100% higher than those for the bulk hydrates. The dissociation characteristics of the gas hydrates in pores were also studied calorimetrically by heating the hydrates under about zero pressure from 100 K to room temperature. It was found that after the initial dissociation into ice and gas the hydrate became totally encapsulated among the pore walls and the ice caps formed at the pore openings. The hydrate thus trapped in the interior of the pore remained stable up to the melting point of pore ice. These results are similar to those obtained in previous studies on the bulk hydrates which are also stabilized by a shielding layer of ice. However, the apparent increase in the stability of the pore hydrates was found to be much larger than that of the bulk hydrates. The composition of methane hydrate in 70-â« pores was determined to be CHâ·5.94HâO, and its heat of dissociation into pore water and gas, obtained calorimetrically, was 45.92 kJ molâ»Â¹; The corresponding values in the bulk phase are 6.00 and 54.19 kJ molâ»Â¹, respectively. 30 refs., 4 figs., 2 tabs.
The solubility of CO2 in liquid water inside the stability field of CO2-hydrate is determined experimentally. Equilibrium between liquid and hydrate phases is achieved in a porous medium to ensure that the stable phases are isolated from the vapor phase (which is used to maintain pressure). Changes in the solubility with temperature are monitored using measurements of electrical resistance. The experiments indicate that the solubility decreases as temperature is lowered into the stability field of CO2-hydrate.
a b s t r a c t The synthesis of available geological information and surface temperature evolution in the Alaska North Slope region suggests that: biogenic and deeper thermogenic gases migrated through fault networks and preferentially invaded coarse-grained layers that have relatively high hydraulic conductivity and low gas entry pressures; hydrate started forming before the beginning of the permafrost; eventually, the permafrost deepened and any remaining free water froze so that ice and hydrate may coexist at some elevations. The single tested specimen (depth 620.47e620.62 m) from the D unit consists of uncemented quartzitic fine sand with a high fraction of fines (56% by mass finer than sieve #200). The as-received specimen shows no evidence of gas present. The surface texture of sediment grains is compatible with a fluvial-deltaic sedimentation environment and shows no signs of glacial entrainment. Tests conducted on sediments with and without THF hydrates show that effective stress, porosity, and hydrate saturation are the major controls on the mechanical and geophysical properties. Previously derived relationships between these variables and mechanical/geophysical parameters properly fit the measurements gath-ered with Mount Elbert specimens at different hydrate saturations and effective stress levels. We show that these measurements can be combined with index properties and empirical geomechanical rela-tionships to estimate engineering design parameters. Volumetric strains measured during hydrate dissociation vanish at 2e4 MPa; therefore, minimal volumetric strains are anticipated during gas production at the Mount Elbert well. However, volume changes could increase if extensive grain crushing takes place during depressurization-driven production strategies, if the sediment has unexpectedly high in situ porosity associated to the formation history, or if fines migration and clogging cause a situation of sustained sand production.
The formation of segregated ice is of fundamental importance to a broad range of permafrost and periglacial features and phenomena. Models have been developed to account for the microscopic interactions that drive water migration, and predict key macroscopic characteristics of ice lenses, such as their spacings and thicknesses. For a given set of sediment properties, the temperature difference between the growing and incipient lenses is shown here to depend primarily on the ratio between the effective stress and the temperature deviation from bulk melting at the farthest extent of pore ice. This suggests that observed spacing between ice lenses in frozen soils, or traces of lenses in soils that once contained segregated ice, might be used to constrain the combinations of effective stress and temperature gradient that were present near the time and location at which the lower lens in each pair was initiated. The thickness of each lens has the potential to contain even more information since it depends additionally on the rate of temperature change and the permeability of the sediment at the onset of freezing. However, these complicating factors make it more difficult to interpret thickness data in terms of current or former soil conditions.
Micellar solutions were found to increase gas hydrate formation rate and alter formation mechanism for ethane and natural gas hydrates. A critical micellar concentration (CMC) of sodium dodecyl sulfate water solution was found to be 242 ppm at hydrate-forming conditions, where CMC was best determined by hydrate induction time. At surfactant concentrations above the CMC, hydrate formation rates in a quiescent system increased by a factor greater than 700. Above the CMC, hydrates initiated subsurface around the micelle-solubilized hydrocarbon gas. Developing hydrate particles migrated and adsorbed on the water-wet cell walls at the water–gas interface, where interstitial water–surfactant solution of the packed hydrate particles continued to react at the high rate. The development overcomes major limitations of future hydrate use in an industrial-scale natural gas storage process.