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Occurrence and mobility of trace elements during oxidant stimulation of shales in Yichang, Hubei province of China
Abstract
Oxidant stimulation is a promising technology for shale permeability enhancement, but it is still face with the problem of produced water with high total dissolved solids. For the consideration of environmental protection and water reuse, the mobility of trace elements (TEs) should be evaluated before the in-situ application of this technology. In this study, carbonate-rich shale and silicate-rich shale, collected in Yichang, Hubei province of China, were used to react with sodium persulfate (Na2S2O8) at different experimental conditions. The sequential chemical extraction was used to analyze the occurrence of TEs in shales and their mobilization mechanism. Influence factors including oxidant concentration, solid:liquid ratio, initial pH and temperature were systematically investigated to explore their effect on the mobility of TEs. Results showed that both two shales had a similar affinity between TEs and fractionation phases. Tough the TEs associated with residual fraction was most obvious in shales, various associations between TEs and other extractable fractions were also apparent, such as Co (61.7% on average) and Cu (59.2% on average) in organic matter-bound fraction, and Sr (53.2% on average) in carbonates-bound fraction. The occurrence of TEs determined their extent of mobilization during oxidant stimulation, and the complex interactions including acidization, oxidation, adsorption and precipitation influence the mobility of TEs as well. The carbonate minerals and pyrite were both critical minerals for decreasing the mobility of TEs. Carbonate minerals in shale could effectively buffer the pH of the system to mitigate the acidization reaction. The near-neutral environment was also beneficial for the Fe(OH)3 precipitation then resulted in the fixation of TEs through adsorption. Besides, the generated gypsum from carbonate dissolution was also benefit for the incorporation of TEs. Meanwhile, sulfate generated from pyrite oxidation and persulfate hydrolysis also directly precipitated with TEs such as Ba and Sr to reduce their dissolved content in the reaction system. Influence factors analysis showed that the mobility of TEs was strongly depended on the oxidant concentration and solid: liquid ratio and to a lesser extent on the temperature and initial pH. Considering the environmental risk and water management cost, systematic investigation on shale composition and associated TEs, and optimization of engineering parameters should be carried out before the in-situ application of this technology.
... Redox potential has a strong influence on the mobility of trace metals in aquatic ecosystems [11]. Indeed, under oxidising conditions, Fe and Mn (oxy)hydroxides are formed and lead to the formation of insoluble precipitates with the other trace metals. ...
... Indeed, their shallow depths can favour the resuspension of sedimentary particles under the effect of strong waves and currents. These particulates are capable of adsorbing dissolved metal cations under basic and reducing conditions or releasing them under acidic and oxidising conditions [11]. ...
... However, the partial dissolution of organic matter, illustrated by the high TOC content of these waters in this season, would have partially inhibited the presence of these metals in soluble form in these waters in this season. This would have been the case of the shallow depth of this ecosystem conducive to the resuspension of particulate matter, which can adsorb the dissolved form of these trace metal in these basic conditions [11]. In this season, the favourable effects of temperature, transparency, basic character and salinity of these waters on the relatively high presence of Cd, Cr, Cu, Fe, Hg, Ni, Pb and Zn in them were shown by the results of NPCA. ...
This study was carried out over the period from June 2019 to May 2020. Its main objective was to monitor the seasonal dynamics of As, Cd, Cr, Cu, Fe, Mn, Hg, Pb, Ni and Zn in the waters of the lagoon area II of Ébrié system. To this end, the study of the seasonal dynamics of the temperature, pH, salinity, conductivity, transparency, dissolved oxygen content and TOC content of these waters, as well as the mean depth of this ecosystem, was also studied with the aim of approaching their implication in this process. With the exception of the TOC content determined according to NF EN 1484 standard and the trace metals contents obtained according to MA 203-Met ICP-MSMS standard in the lab, the other physical and chemical parameters were determined in situ in the water samples collected. The trace metals contents of these waters decrease from its dry season to its flood season. The marine water inputs would favour a relatively high presence of these trace metals in them in their dry and rainy seasons, due to their basic character and their relatively high transparency and salinity in these seasons. The waters of Comoé river would partially disadvantage this process in its flood season, due to the slightly acidic character and low transparency and salinity of these waters in this season. Over all the study period, the relatively high temperature, conductivity, and oxygen content of these waters would have been conducive to the presence of these trace metals in dissolved form in them through the decomposition of organic matter. The relatively important organic matter content of these waters and the relatively shallow depth of this ecosystem would have disadvantaged this process over all the study period.
... Redox potential has a strong influence on the mobility of trace metals in aquatic ecosystems [11]. Indeed, under oxidising conditions, Fe and Mn (oxy)hydroxides are formed and lead to the formation of insoluble precipitates with the other trace metals. ...
... Indeed, their shallow depths can favour the resuspension of sedimentary particles under the effect of strong waves and currents. These particulates are capable of adsorbing dissolved metal cations under basic and reducing conditions or releasing them under acidic and oxidising conditions [11]. ...
... However, the partial dissolution of organic matter, illustrated by the high TOC content of these waters in this season, would have partially inhibited the presence of these metals in soluble form in these waters in this season. This would have been the case of the shallow depth of this ecosystem conducive to the resuspension of particulate matter, which can adsorb the dissolved form of these trace metal in these basic conditions [11]. In this season, the favourable effects of temperature, transparency, basic character and salinity of these waters on the relatively high presence of Cd, Cr, Cu, Fe, Hg, Ni, Pb and Zn in them were shown by the results of NPCA. ...
... Following complete reaction, the samples were washed three times with ultrapure water, and the supernatants (~ 3 mL extraction acid and 3 portions of ~ 3 mL washing solution) were collected in 15 mL centrifuge tubes. Acetic acid buffer solution (NaOAc-HOAc), dilute acetic acid (HOAc), and dilute hydrochloric acid (HCl) are commonly used as leaching acids in the extraction of carbonate components from soils and sediments, and their leaching abilities are enhanced in the order described based on previous studies (Tessier et al. 1979;Rauret et al. 1999;Yang et al. 2021). ...
Purpose
Carbonate components in soils and sediments play a crucial role in the study of surface processes and the reconstruction of paleoclimates. The extraction of carbonate components from soils and sediments is often carried out using different types of acids. However, which reagents can effectively extract the carbonate components while minimizing the destruction of non-carbonate minerals in soils and sediments has not been fully explored yet.
Methods
The present study conducted a series of conditional leaching experiments on six standard soil and sediment samples with diverse geological backgrounds and varying carbonate contents.
Results
The results revealed that dilute acetic acid (HOAc, 0.5 mol/L) performed better than acid solution (HCl) and acetic acid buffer solution (NaOAc-HOAc) in promoting the complete dissolution of carbonate minerals and avoiding the dissolution of clay minerals. It was also observed that pre-treating the samples with neutral ammonium acetate buffer solution (NH4OAc) or potassium chloride buffer solution (KCl) was necessary to remove exchangeable components. The Mg/Ca ratios of the silicate components after leaching carbonate in the standard soil and sediment samples we used exhibited significant differences (ranging from 0.41 to 1.60).
Conclusion
The dilute acetic acid (HOAc, 0.5 mol/L) is advisable for the extraction of carbonate components from soils and sediments, and the pre-treatment of removing exchangeable components is essential for samples with low carbonate content. It is more reasonable to use the element ratios of the dilute acid insoluble fraction or the average compositions of minerals in soils and sediments for individual correction for the contribution of the silicate components.
... Organic matter, pyrite, siderite, and chlorite in black shale were usually preserved in a reducing depositional environment (Macquaker et al., 2014;Liao et al., 2020). Such redox-sensitive compositions commonly exhibit high chemical reactivity under oxic conditions (Jin et al., 2013;Li et al., 2020a, b, c;Yang et al., 2020;Cheng et al., 2021a, b;Yang et al., 2021;Wang et al., 2022). For example, by means of mercury injection, nitrogen adsorption, and scanning electron microscopy, it was observed that H 2 O 2 -treated shale samples have higher porosity and better nanoscale pore network connectivity Zhou et al., 2018), which may improve matrix diffusion capacity. ...
A relatively large number of pH-sensitive minerals (e.g., calcite, dolomite, siderite) and redox-sensitive compositions (e.g., organic matter, siderite, pyrite, and chlorite) are present in gas-bearing shale formations. During hydraulic fracturing, some of these compositions can be dissolved under acidic or oxic condition and may generate critical transport pathways to bypass water blockage within the fracture-matrix interface. This paper reports the effects of 5% HCl or 15% H2O2 reactive fluids on transport pathway and methane diffusion capacity of Longmaxi shale samples. Experiments show that these sensitive minerals are distributed evenly throughout the shale matrix as dispersed or isolated grains. Reactivity differences between sensitive compositions and non-sensitive minerals cause heterogeneous dissolution at micrometer scale and generate substantial new pore space in the reaction zones, giving an overall pore volume increase by 4.6–6.6 vol % calculated using μCT data. Apart from dissolution pores, many oxidation-related microfractures with a spacing less than 10 μm are also observed in H2O2-treated shale. Due to resolution limitations of SEM and μCT imaging, these new transport pathways appearing as separated void spaces are actually connected by much smaller pores at nanoscale, leading to a greater than 3.7-fold increase in macropore diffusivity. Despite more water imbibition into the H2O2-treated shales, the largest increase in diffusivity is also observed post oxidizing treatment. Our study clearly demonstrates that the newly formed transport pathways can reduce the diffusion pathway length and tortuosity of nanopore systems, thereby significantly improving wet shales’ diffusivity. These observed changes in transport pathway and diffusivity indicate that acidizing or oxidizing treatment may be a possible method for mitigating water blockage within fracture-matrix interface of hydraulically fractured Longmaxi shale gas formation.
The heavy metals in soils developed from the black rock series originate from the parent rock, but their sources and enrichment mechanisms in the parent rock remain unclear. This study explores the enrichment mechanisms, occurrence forms, and ecological environmental effects of cadmium (Cd) and chromium (Cr) in the black rock series. Results revealed average concentrations of 1.15 mg/kg for Cd and 193.08 mg/kg for Cr. Cd showed moderate enrichment (CdEF=31.03), while Cr had slight enrichment (CrEF=4.42). Both metals were mainly in the residual fraction (44.22 % for Cd, 69.02 % for Cr), followed by the Fe-Mn oxide-bound fraction (24.07 % for Cd, 18.51 % for Cr). The Risk Assessment Code (RAC) indicated moderate risk for Cd (10 %≤RAC<30 %) and low risk for Cr (1 %≤RAC<10 %). The Secondary Phase to Primary Phase ratio (RSP) suggested mild Cd pollution (1 <RSP≤2) and no Cr pollution (RSP≤1). Cd poses a significant release risk, necessitating attention to its ecological safety. Hydrothermal and volcanic activities were primary sources of Cd, while TOC and volcanic activities influenced Cr. Anoxic conditions in the Lower Cambrian Ocean facilitated Cd and Cr enrichment. This study enhances understanding of heavy metal enrichment in black rock series and guides pollution management.
Copper (Cu) is an indispensable micronutrient for plants, animals, and microorganisms and plays a vital role in different physiological processes. However, excessive Cu accumulation in agricultural soil, often through anthropogenic action, poses a potential risk to plant health and crop productivity. This review article provided a comprehensive overview of the available information regarding Cu dynamics in agricultural soils, major sources of Cu contamination, factors influencing its mobility and bioavailability, and mechanisms of Cu uptake and translocation in rice plants. This review examined the impact of Cu toxicity on the germination, growth, and photosynthesis of rice plants. It also highlighted molecular mechanisms underlying Cu stress signaling and the plant defense strategy, involving chelation, compartmentalization, and antioxidant responses. This review also identified significant areas that need further research, such as Cu uptake mechanism in rice, Cu signaling process, and the assessment of Cu-polluted paddy soil and rice toxicity under diverse environmental conditions. The development of rice varieties with reduced Cu accumulation through comprehensive breeding programs is also necessary. Regulatory measures, fungicide management, plant selection, soil and environmental investigation are recommended to prevent Cu buildup in agricultural lands to achieve sustainable agricultural goals.
Deep subsurface stimulation processes often promote fluid-rock interactions that can lead to the formation of small colloidal particles that are suspected to migrate through the rock matrix, partially or fully clog pores and microfractures, and promote the mobilization of contaminants. Thus, the goal of this work is to understand the geochemical changes of the host rock in response to reservoir stimulation that promote the formation and migration of colloids. Two different carbonate-rich shales were exposed to different solution pHs (pH = 2 and 7). Iron and other mineral transformations at the shale-fluid interface were first characterized by synchrotron-based XRF mapping. Then, colloids that were able to migrate from the shale into the bulk fluid were characterized by synchrotron-based extended X-ray absorption structure (EXAFS), scanning electron microscopy (SEM), and single-particle inductively coupled plasma time-of-flight mass spectrometry (sp-icpTOF-MS). When exposed to the pH = 2 solution, extensive mineral dissolution and secondary precipitation was observed; iron-(oxyhydr)oxide colloids colocated with silicates were observed by SEM at the fluid-shale interfaces, and the mobilization of chromium and nickel with these iron colloids into the bulk fluid was detected by sp-icpTOF-MS. Iron EXAFS spectra of the solution at the shale-fluid interface suggests the rapid (within minutes) formation of ferrihydrite-like nanoparticles. Thus, we demonstrate that the pH neutralization promotes the mobilization of existing silicate minerals and the rapid formation of new iron colloids. These Fe colloids have the potential to migrate through the shale matrix and mobilize other heavy metals (such as Cr and Ni, in this study) and impacting groundwater quality, as well produced waters from these hydraulic fracturing operations.
Multistaged hydraulic fracturing of a horizontal well may significantly improve the permeability of the calcareous shale reservoir, but hard to improve the transport capacity of gas in nanopores, resulting in insufficient gas supply from the shale matrix to fractures and rapid production decline after the stimulation. Nevertheless, the seepage capacity and production will increase due to the water-rock interaction during the soaking process after fracturing. Our research proposes to utilize the reaction between persulfate and calcareous shale to strengthen the water-rock interaction, achieving oxidative dissolution and swelling. Ammonium persulfate was selected as the fracturing fluid additive to analyze the effect of oxidative dissolution and swelling on the macroscopic and microscopic seepage capacity of calcareous shale through technical means such as stress sensitivity test, NMR, and SEM. We conclude that persulfate's oxidative dissolution and swelling will greatly promote fractures and increase permeability in calcareous shale reservoirs to reconstruct the seepage channels. The findings of this study have implications for determining and optimizing the fracturing fluid's performance and the postfracturing production management system to develop calcareous shale reservoirs in a green and efficient manner.
Although multistage hydraulic fracturing is routinely performed for the extraction of hydrocarbon resources from low permeability reservoirs, the downhole geochemical processes linked to the interaction
of fracturing fluids with formation brine and reservoir mineralogy remain poorly understood. We present
a geochemical dataset of flowback and produced water samples from a hydraulically fractured reservoir in
the Montney Formation, Canada, analyzed for major and trace elements and stable isotopes. The dataset
consists in 25 samples of flowback and produced waters from a single well, as well as produced water samples from 16 other different producing wells collected in the same field. Additionally, persulfate breaker
samples as well as anhydrite and pyrite from cores were also analyzed. The objectives of this study were to
understand the geochemical interactions between formation and fracturing fluids and their consequences
in the context of tight gas exploitation. The analysis of this dataset allowed for a comprehensive understanding of the coupled downhole geochemical processes, linked in particular to the action of the oxidative
breaker. Flowback fluid chemistries were determined to be the result of mixing of formation brine with
the hydraulic fracturing fluids as well as coupled geochemical reactions with the reservoir rock such as
dissolution of anhydrite and dolomite; pyrite and organic matter oxidation; and calcite, barite, celestite,
iron oxides and possibly calcium sulfate scaling. In particular, excess sulfate in the collected samples was
found to be mainly derived from anhydrite dissolution, and not from persulfate breaker or pyrite oxidation.
The release of heavy metals from the oxidation activity of the breaker was detectable but concentrations of
heavy metals in produced fluids remained below the World Health Organization guidelines for drinking
water and are therefore of no concern. This is due in part to the co-precipitation of heavy metals with iron
oxides and possibly sulfate minerals.
Knowledge of geochemical processes that occur during hydraulic fracturing of shale resources for natural gas recovery can provide insight into in situ mineral precipitation and dissolution reactions that affect shale pore and fracture networks, and ultimately the ability to recover natural gas. Measurement of dissolved chemical species in produced waters, which serve as indicators of water-rock reactions in the reservoir, is one approach for monitoring subsurface geochemistry. However, an ability to distinguish effects of water-rock reactions versus reservoir fluid mixing on chemical concentration and speciation in produced waters requires experimental investigation to better understand natural geochemical tracer behavior.
In this study, we explored Li, Sr, and U as geochemical tracers of ion−exchange and mineral dissolution reactions occurring during a series of flow-through laboratory experiments designed to evaluate mineral reactions occurring in shale reacted with hydraulic fracturing fluid (HFF). The experiments, reported in detail in a separate study (Paukert Vankeuren et al., 2017), were designed to emulate shut-in conditions (period after hydraulic fracturing but before production when HFF remains in the reservoir to allow for opening of new fractures for hydrocarbon recovery) for Marcellus Shale undergoing hydraulic fracturing in southwestern Pennsylvania, USA. In the companion study presented here, effluent samples from the core flood experiments were analyzed for changes in Li isotopes (δ⁷Li), radiogenic Sr isotopes (εSrsw), and U concentration as possible unique geochemical tracers to indicate water-rock reaction. Experimental and reactive transport modeling results demonstrated that the peak of U in experimental effluents is derived from dissolution of U-containing calcite, which may explain the pulse of U observed in produced water collected from hydraulically fractured Marcellus Shale on the first day of flowback. Experimental results also showed that changes in δ⁷Li values and εSrsw from the flow-through experiments differ from values measured in a time series of produced waters from Marcellus Shale gas wells. Thus, although ion−exchange and mineral dissolution and precipitation reactions result in an observable change in Sr isotope signals in laboratory experiments, this change may not be identifiable in produced waters in the field due to complications associated with geologic heterogeneity and overprinting of other signals such as fluid mixing and transport. While U shows some promise as a tracer of HFF-shale reactions, particularly carbonate dissolution, it may be necessary to continue searching for other tracers present in produced waters to aid in elucidating in situ water-rock reactions of interest.
We investigate sediment sources, depositional conditions and diagenetic processes affecting the Middle Devonian Marcellus Shale in the Appalachian Basin, eastern USA, a major target of natural gas exploration. Multiple proxies, including trace metal contents, rare earth elements (REE), the Sm-Nd and Rb-Sr isotope systems, and U isotopes were applied to whole rock digestions and sequentially extracted fractions of the Marcellus shale and adjacent units from two locations in the Appalachian Basin. The narrow range of εNd values (from −7.8 to −6.4 at 390 Ma) is consistent with derivation of the clastic sedimentary component of the Marcellus Shale from a well-mixed source of fluvial and eolian material of the Grenville orogenic belt, and indicate minimal post-depositional alteration of the Sm-Nd system. While silicate minerals host >80% of the REE in the shale, data from sequentially extracted fractions reflect post-depositional modifications at the mineralogical scale, which is not observed in whole rock REE patterns.
Limestone units thought to have formed under open ocean (oxic) conditions have δ²³⁸U values and REE patterns consistent with modern seawater. The δ²³⁸U values in whole rock shale and authigenic phases are greater than those of modern seawater and the upper crust. The δ²³⁸U values of reduced phases (the oxidizable fraction consisting of organics and sulfide minerals) are ∼0.6‰ greater than that of modern seawater. Bulk shale and carbonate cement extracted from the shale have similar δ²³⁸U values, and are greater than δ²³⁸U values of adjacent limestone units. We suggest these trends are due to the accumulation of chemically and, more likely, biologically reduced U from anoxic to euxinic bottom water as well as the influence of diagenetic reactions between pore fluids and surrounding sediment and organic matter during diagenesis and catagenesis.
The study reported here evaluates the degree to which metals, salt anions and organic compounds are released from shales by exposure to water, either in its pure form or mixed with additives commonly employed during shale gas exploitation. The experimental conditions used here were not intended to simulate the exploitation process itself, but nevertheless provided important insights into the effects additives have on solute partition behaviour under oxic to sub-oxic redox conditions.In order to investigate the mobility of major (e.g. Ca, Fe) and trace (e.g. As, Cd, Co, Mo, Pb, U) elements and selected organic compounds, we performed leaching tests with black shale samples from Bornholm, Denmark and Lower Saxony, Germany. Short-term experiments (24 h) were carried out at ambient pressure and temperatures of 100 °C using five different lab-made stimulation fluids. Two long-term experiments under elevated pressure and temperature conditions at 100 °C/100 bar were performed lasting 6 and 2 months, respectively, using a stimulation fluid containing commercially-available biocide, surfactant, friction reducer and clay stabilizer.Our results show that the amount of dissolved constituents at the end of the experiment is independent of the pH of the stimulation fluid but highly dependent on the composition of the black shale and the buffering capacity of specific components, namely pyrite and carbonates. Shales containing carbonates buffer the solution at pH 7-8. Sulphide minerals (e.g. pyrite) become oxidized and generate sulphuric acid leading to a pH of 2-3. This low pH is responsible for the overall much larger amount of cations dissolved from shales containing pyrite but little to no carbonate. The amount of elements released into the fluid is also dependent on the residence time, since as much as half of the measured 23 elements show highest concentrations within four days. Afterwards, the concentration of most of the elemental species decreased pointing to secondary precipitations. Generally, in our experiments less than 15% of each analysed element contained in the black shale was mobilised into the fluid.
Hydraulically fractured shales are becoming an increasingly important source of natural gas production in the United States. This process has been known to create up to 420 gallons of produced water (PW) per day, but the volume varies depending on the formation, and the characteristics of individual hydraulic fracture. PW from hydraulic fracturing of shales are comprised of injected fracturing fluids and natural formation waters in proportions that change over time. Across the state of Pennsylvania, shale gas production is booming; therefore, it is important to assess the variability in PW chemistry and microbiology across this geographical span. We quantified the inorganic and organic chemical composition and microbial communities in PW samples from 13 shale gas wells in north central Pennsylvania. Microbial abundance was generally low (66 to 9400 cells/mL). Non-volatile dissolved organic carbon (NVDOC) was high (7-31 mg/L) relative to typical shallow groundwater, and the presence of organic acid anions (e.g., acetate, formate, and pyruvate) indicated microbial activity. Volatile organic compounds (VOCs) were detected in four samples (∼1-11.7 μg/L): benzene and toluene in the Burket sample, toluene in two Marcellus samples, and tetrachloroethylene (PCE) in one Marcellus sample. VOCs can be either naturally occurring or from industrial activity, making the source of VOCs unclear. Despite the addition of biocides during hydraulic fracturing, H2S-producing, fermenting, and methanogenic bacteria were cultured from PW samples. The presence of culturable bacteria was not associated with salinity or location; although organic compound concentrations and time in production were correlated with microbial activity. Interestingly, we found that unlike the inorganic chemistry, PW organic chemistry and microbial viability was highly variable across the 13 wells sampled, which can have important implications for the reuse and handling of these fluids.
Development of unconventional shale gas wells can generate significant quantities of drilling waste, including trace metal-rich black shale from the lateral portion of the drillhole. We carried out sequential extractions on 15 samples of dry-drilled cuttings and core material from the gas-producing Middle Devonian Marcellus Shale and surrounding units to identify the host phases and evaluate the mobility of selected trace elements during cuttings disposal. Maximum whole rock concentrations of uranium (U), arsenic (As), and barium (Ba) were 47, 90, and 3333 mg kg-1, respectively. Sequential chemical extractions suggest that although silicate minerals are the primary host for U, as much as 20% can be present in carbonate minerals. Up to 74% of the Ba in shale was extracted from exchangeable sites in the shale, while As is primarily associated with organic matter and sulfide minerals that could be mobilized by oxidation. For comparison, U and As concentrations were also measured in 43 produced water samples returned from Marcellus Shale gas wells. Low U concentrations in produced water (< 0.084 to 3.26 μg L-1) are consistent with low-oxygen conditions in the wellbore, in which U would be in its reduced, immobile form. Arsenic was below detection in all produced water samples, which is also consistent with reducing conditions in the wellbore minimizing oxidation of As-bearing sulfide minerals.
Recent increases in the use of hydraulic fracturing (HF) to aid extraction of oil and gas from black shales have raised concerns regarding potential environmental effects associated with predictions of upward migration of HF fluid and brine. Some recent studies have suggested that such upward migration can be large and that timescales for migration can be as short as a few years. In this article, we discuss the physical constraints on upward fluid migration from black shales (e.g., the Marcellus, Bakken, and Eagle Ford) to shallow aquifers, taking into account the potential changes to the subsurface brought about by HF. Our review of the literature indicates that HF affects a very limited portion of the entire thickness of the overlying bedrock and therefore, is unable to create direct hydraulic communication between black shales and shallow aquifers via induced fractures. As a result, upward migration of HF fluid and brine is controlled by preexisting hydraulic gradients and bedrock permeability. We show that in cases where there is an upward gradient, permeability is low, upward flow rates are low, and mean travel times are long (often >10(6) years). Consequently, the recently proposed rapid upward migration of brine and HF fluid, predicted to occur as a result of increased HF activity, does not appear to be physically plausible. Unrealistically high estimates of upward flow are the result of invalid assumptions about HF and the hydrogeology of sedimentary basins.
Uranium and other radionuclides are prominent in many unconventional oil/gas shales and is a potential contaminant in flowback/produced waters due to the large volumes/types of chemicals injected into the subsurface during stimulation. To understand the stability of U before and after stimulation, a geochemical study of U speciation was carried out on three shales (Marcellus, Green River, and Barnett). Two types of samples for each shale were subjected to sequential chemical extractions: unreacted and shale reacted with synthetic hydraulic fracture fluid. A significant proportion of the total U (20-57%) was released from these three shales after reaction with fracture fluid, indicating that U is readily leachable. The total U released exceeds labile water soluble and exchangeable fractions in unreacted samples, indicating that fluids leach more recalcitrant phases in the shale. Radiographic analysis of unreacted Marcellus shale thin sections shows U associated with detrital quartz and the clay matrix in the shale. Detrital zircon and TiO2 identified by electron microprobe could account for the hotspots. This study shows that significant proportions of U in three shales are mobile upon stimulation. In addition, the extent of mobilization of U depends on the U species in these rocks.
The coexistence of calcium (Ca²⁺), sulfate (SO4²⁻) with trace metal cations (M(II)) can possibly lead to M(II)-gypsum coprecipitation and solid solution formation. However, gypsum's role in the fixation of M(II) is still largely unknown. This study investigated the precipitation of Ca²⁺ and SO4²⁻ in the presence of M(II) (i.e., Cu²⁺, Zn²⁺, or Cd²⁺) and the incorporation of the metal cations into the gypsum structure at different environmental conditions. Trace metals in two natural gypsum samples (Yunnan and Neimeng, China) and one hydrometallurgical byproduct gypsum sample from a Cu refinery were also assessed. X-ray diffraction, scanning electron microscopy-energy dispersive X-ray spectroscopy, and Fourier transform infrared spectroscopy were employed to characterize the solid samples. The lab-scale results showed that the M(II) content in the coprecipitates increased with pH and the initial M(II) concentration. At lower pH, Zn²⁺ and Cd²⁺ were dominantly fixed through isomorphic substitution for Ca²⁺ in the gypsum structure, while Cu²⁺ was likely incorporated into the channel structure of the gypsum where the water molecules reside. In contrast, at higher pH, thermodynamic modeling results indicated that M(II)-hydroxides and base metal sulfates (Mx(OH)y(SO4)z) were formed and constituted an appreciable fraction of M(II)-bearing species in the host solids. Our study further showed that trace amounts of Cu, Zn, and Cd were also substitutianly incorporated in the two natural and one hydrometallurgical gypsum samples. The present study may have important implications for understanding the geochemical cycle of trace metals in Ca²⁺ and SO4²⁻ coexisting systems, as well as possible attenuation mechanisms that may occur along their supersaturation and coprecipitation.
Oxidation stimulation of gas-bearing shale is promising in enhancing shale gas recovery by improving shale matrix conductivity and promoting gas desorption. However, this technology is still in its infancy. Shale-oxidant interaction which is critical for the efficiency, safety and environment friendly of shale oxidation stimulation is still needed to be explored. In this study, carbonate-rich shale and silicate-rich shale, collected from Shuijingtuo Formation in Yichang, Hubei province of China, were used to conduct oxidative dissolution experiments with hydrogen peroxide (H2O2), sodium hypochlorite (NaClO) and sodium persulfate (Na2S2O8) at formation temperature. Combined with the analysis of water chemistry of reacted solutions, as well as the mineral composition, micromorphology, TOC content and mass change of shales, the conclusion that silicate-rich shale is more suitable for oxidation stimulation can be obtained. Na2S2O8 is more superior than H2O2 and NaClO, while the optimal OM removal oxidant is NaClO. The contribution of mineral-oxidants reaction is more profound than the organic matter removal to shale oxidative dissolution. Carbonates and pyrite are both critical minerals. Carbonates dissolution can buffer the solution pH which impedes the acid corrosion of other minerals and also releases Ca to participate in the precipitation of gypsum, which brings threaten to the effectiveness of shale oxidative stimulation by clogging pores throat and fracture or may help to induce spontaneous microfracture propagation caused by crystal growth. What's more, the dissolution of carbonates can accelerate the oxidation of organic matter, however the precipitation of gypsum can inversely hinder its oxidation. Iron-bearing mineral precipitation and other aluminosilicate minerals precipitation at alkaline condition is another important negative factor for shale oxidation stimulation. In order to accurately evaluate the effect of oxidation stimulation on shale gas recovery, the flow-through experiments should be conducted in the future.
The drastic decline of shale gas production after fracturing depresses the development of this unconventional gas resource. Although shale oxidant stimulation can dissolve unstable composition to enhance permeability, the shale-fluids interaction during this stimulation process is still not yet clear. In this study, the organic-rich shale collected from the Cambrian Shuijingtuo Formation of Yichang, Hubei province, China was selected to react with three oxidants, hydrogen peroxide (H2O2), sodium hypochlorite (NaClO) and sodium persulfate (Na2S2O8) at formation temperature. Variation of water chemistry, mineral composition and micromorphology were analyzed to reveal the mechanism of shale oxidative dissolution and evaluate its influence on shale permeability enhancement. Results showed that pyrite and OM can be discrepantly oxidized with different oxidants. At the same oxidation duration, the acidic environment was beneficial for carbonate dissolution, while the alkaline environment was favorable to the dissolution of dolomite and tectosilicate minerals such as quartz, albite, illite and chlorite. Nevertheless, the serious thermal decomposition of H2O2, precipitation of gypsum and ferric hydroxide (Fe(OH)3) occurred during shale-fluids interaction at formation temperature might impede the enhancement of shale permeability. The oxidative dissolution of shale also brought about the release of trace elements, which might result in groundwater pollution. For the in-situ application of shale oxidative dissolution, difficulties such as thermal decomposition, secondary minerals precipitation and possible groundwater pollution should be considered further in the future.
Due to high salinity and complex components including varied inorganic salts, organic agents, released heavy metals, radioactive elements and clastic particles, environmental and economical treatments of a huge amount of flowback wastewater from shale gas wells have attracted both public concerns and industrial interests. Unlike the existing methods such as deep well reinjection and recycling, this paper explores the feasibility of directly reserving an injected hydraulic fracturing (HF) fluid in a shale formation by utilizing its strong capillary imbibition and permanent sequestration capacity. Fluid imbibition-flowback and nuclear magnetic resonance experiments are conducted to check the microscopic reliability: once being imbibed into shale, only less than 20% of the total fluid is driven out even under a pressure gradient of 22.1~62.6 MPa/m. Macroscopic flowback data from Longmaxi shale gas wells in Sichuan Basin further demonstrates that adding 10~30 days to current shut-in operations the flowback rates of a HF fluid decrease approximately 5%~15%, meaning a reduction of 2000~6000m3 wastewater per single well without extra technical procedures or additives. Moreover, fractured sample displacement experiments, simulation and flied data statistics confirm that extending shut-in time has an overall improvement on well productivity and flowback fluid quality.
Million tons of drill cuttings generated from shale gas development are currently disposed of in landfills, buried in-situ, or reused as road fill. Cuttings, core samples and operating drilling mud from the Marcellus Shale Energy and Environmental Laboratory in WV were studied to better characterize drill cuttings and to evaluate trace metal mobility in various disposal environments. Results showed that physical and chemical properties of drill cuttings are impacted by the host-rock formation, calcium sources and the residual drilling-mud. Barite in residual drilling mud forms a coating around rock cuttings and influences the mobility of Sr in drill cuttings. Trace metals in drill cuttings are primarily associated with pyrite and total organic carbon (TOC) phases in the shale. Based on sequential extraction results, various trace metal mobilities are controlled by additional solid fractions in drill cuttings, such as pyrite, TOC, calcite, barite and exchangeable clays in the shale.
While hydraulic fracturing activities in the lower 48 states continue to increase, even doubling the rig count since 2016, the increased production from unconventional source shale has positioned the USA as a top oil producer in 2018. Although oil and gas production from shale formations has proven to be economical, it remains very challenging in part due to the presence of the ductile, polymer nature of the hydrocarbon source material, kerogen. This organic matter is intertwined among silicate, aluminosilicate and other minerals as fine laminae that weave among the shale rock fabric, adding soft cohesion to the material. A potential solution has been developed, a new type of reactive fracturing fluid composed of strong oxidizers such as bromate (BrO3-), which could mitigate the adverse effects of the kerogen on the hydraulic fracturing operation and enhance subsequent overall fracture conductivity. High resolution SEM imaging of shale samples before and after fluid treatment demonstrate notable porosity enhancement in kerogen-rich shale samples (KRS) with varying maturities and chemical composition. The stability of the new fluid formulation at elevated reservoir temperatures in addition to the demonstrated results, suggest measureable improvements in fracture conductivity and hence in the ultimate oil and gas recovery from future hydraulic fracturing operations.
Due to the sintering phenomena of coal ash (CA), uranium is hardly leached even under high-concentration H2SO4 solution. It is difficult to collect and take advantage of those potential nuclear sources. In this study, the coal with high content of uranium (112.56 ug/g) was collected from Menwang, Yunan Province, Southwestern China. Sequential chemical extraction and leaching experiment were conducted. Based on X-ray diffraction and scanning electronic microscopy analysis, elements (U, V, Cs, Rb, Ga, and Sr) were surrounded by minerals during coal combustion when the temperature was over 700 °C. Therefore, thermochemical treatment with (NH4)2SO4 was used to activate the amorphous Al in CA, and led to the release of element at the following water leaching. The leaching contents of U, Cs, Rb, and Sr, under the thermochemical condition of the CA/(NH4)2SO4 = 2:4, 400 °C, and 1 h, were 1.89, 2.31, 8.39, and 1.74 times higher than directly leaching CA with 0.4 M H2SO4. Moreover, uranium was extensively leached by deionized water at the condition of solid to liquid ratio of 1:10, 60 °C, and 2 h, which was characterized by the advantages of the environment friendly, saving resources, and reducing equipment maintenance investment.
There are many advantages to using supercritical carbon dioxide (ScCO2) fracturing technology to exploit shale gas reservoirs in China, including minimal damage to the environment or formation, and displacing methane (CH4) in the adsorbed state. When ScCO2 enters fractures in the formation, ScCO2-water-shale interactions may affect the physicochemical properties of shale. In this study, a high-pressure reaction system was adopted to simulate ScCO2-water-shale interactions under ScCO2 stimulation conditions. The element mobilization and pore structure before and after the reaction were measured using ICP-MS, XRF. The results show that the major elements, including Ca, Mg, Na, K, and Al, exhibit varying degrees of mobilization after the interactions because of dissolution of carbonate and silicate minerals in shale samples. Compared with the major elements, trace elements have a lower mobility, quantified as <13.97%. The specific surface areas and pore volumes of two shale samples increase at different degrees after the reaction. The interactions have a more significant influence on the micropores. In addition, fractal features of the shale pore structure were analyzed. The fractal dimensions of the shale samples increase after the reaction, indicating that pore surface roughness increases, and pore structure morphology gradually transforms from regular to complex.
Chemical oxidation is proposed as an effective means to react and dissolve small regions of coal in the near wellbore region, thereby raising permeability for gas flow. In this study, we investigated the effect of sodium hypochlorite (NaClO) treatment on the structure of bituminous coal (Coal B) and subbituminous coal (Coal S) separately from the Bowen and Surat basins in Queensland, Australia. Swelling and leaching tests showed that both coals swelled, dissolved and broke in 5%wt. aqueous solutions of NaClO. Coal S reacted more vigorously in 5% NaClO, with 49% mass loss and 3840 mg/L of dissolved organic carbon (DOC) measured in the oxidant filtrate, than Coal B. The Coal B mass loss in 5% NaClO was 4.5% with 430 mg/L DOC measured in the filtrate. After NaClO treatment the total accessible pore volume of Coal S particles increased from 4.6% to 6.1%, and the porosity of Coal B increased from 8.6% to 8.9%. Pore size distributions determined from mercury intrusion porosimetry (MIP) indicated that oxidation enlarged the pores in Coal S more significantly than Coal B. Scanning electron microscopy (SEM) confirmed oxygen generated large pores on the surface of Coal S particles, but there were no significant changes on Coal B. We used a microfluidic cleat flow cell (CFC) to inject NaClO into artificial channels scribed in polished samples of Coal S and Coal B, and measured an increase in the widths of the channels after NaClO treatment. The increase in channel width observed in the CFC indicated that coal solubilisation was a more dominant mechanism than coal swelling. Similarly, the channel aperture of Coal S increased more than Coal B. CFC results also showed that NaClO etched dull coal bands (inertinite-rich) more significantly than bright coal bands (vitrinite-rich), and we proposed this result was due to the greater porosity in semi-fusinite, which allowed greater penetration of NaClO in dull coal bands than in bright coal bands. The low coal rank sample (Coal S) with higher liptinite content and more oxygen content was more susceptible to oxidisation by NaClO than Coal B.
a large of fracturing fluid enters in a well of shale gas reservoir to create a fracture network, but the recovery of fracturing fluid is generally less than 30%. Fracturing fluid from the hydraulic fractures usually invades the microfractures and matrix by spontaneous imbibition during the shut–in. Recent studies show that the water–rock interaction may induce shale structure failures, which can significantly affect imbibition rate. Due to the presence of oxidizable compositions (e.g., pyrite and organic matter (OM)), oxidation easily induced the structure failures and dissolution pores. However, its effects on imbibition of water into the shale is poorly understood. In this study, the imbibition experiments of deionized water (DIolution could lead to a high porosity and good connectivity of nanoscale pores networks in shale cubes. Moreover, oxidative dissolution decreased the barriers of microfractures propagation according to the decrease of zeta potential between shale–water system, and meanwhile accelerated the release of clay hydration forces to induce microfractures. The results indicate that the coordinative effect between spontaneous imbibition and oxidative dissolution may play a significant role in increasing the gas supply ability of nanoscale pores and microfractures, thus achieving the oxidizing stimulation of shale formation to enhance shale gas recovery. water) and oxidative fluid under no confining pressure conditions were conducted to determine the imbibition characteristics; shale cubes(1×1×1cm) and crushed samples(380–830μm) were treated by DI water and oxidative fluid for revelation of the change in the composition and the associated dissolution structures, and explanation of the imbibition characteristic of oxidative fluid in shale. The results show that final amount of oxidative fluid imbibed is higher than that of DI water; oxidation–induced microfractures during the imbibition lead to a “phase step” of normalized imbibed volume vs. time curve, and “S” characteristic of normalized imbibed volume vs. square root of time (sqrt time) curve. These differences are mainly caused from the improvement of imbibition pathway and the increase of water retention space by oxidation. After the oxidation treatment of crushed shale samples for 48 h, lots of oxidation-induced microfractures and dissolution pores were observed by field–emission scanning electron microscopy. Combining the analysis of X–ray diffraction (XRD) and atomic absorption spectroscopy (AAS) found that the dissolution pores seemed to strongly attribute on the loss of calcite, dolomite, and pyrite. Results from mercury injection capillary pressure analysis showed that the oxidative diss
Spontaneous microfracture propagation caused by mineral crystallization or growth has been demonstrated in a variety of volume-increasing mineral replacement reactions. This brings a new look on the way microfractures may be generated in the shale formation. Because carbonates and pyrite are highly reactive minerals during shale-fluid reactions and may be the most common sources of replacement reaction, 10 wt. % sulfuric acid (H2SO4) and 10 wt. % ammonium persulfate ((NH4)2S2O8) solutions were used to react with the cm- and mm-sized shale samples, which have a reactive mineral composition of 2.2-4.7 wt. % calcite (CaCO3) and 4.3-4.8 wt. % dolomite (CaMg(CO3)2), and 1.8-2.7 wt. % pyrite (FeS2). A deionized water experiment was performed as a replacement-free control. We monitored the reaction-induced fractures using X-ray tomography and scanning electron microscopy imaging. The related mineral dissolution and new mineral precipitation were also examined. Experiments showed that reactions of the unconfined shale samples with H2SO4 and (NH4)2S2O8 solution have a great potential for generating chemically-induced fractures due to the replacement of carbonate minerals by gypsum (CaSO4·2H2O) crystal. The replacement process was supposed to occur through interface-coupled dissolution-precipitation reaction. It allows the gypsum precipitation in the immediate vicinity of the dissolving carbonate mineral surfaces. Because gypsum has a higher molar volume (74.4 cm3/mol) than calcite (36.9 cm3/mol) and dolomite (64.3 cm3/mol), the local replacement reactions can generate internal swelling stress that drives fracturing of the surrounding shale matrix. The reaction induced stress is on the grain scale and derived from the crystallization pressure. Based on the calculation from the degree of supersaturation of CaSO4 solution, the crystallization pressure can easily exceed 30 MPa that may provide a sufficient local swelling stress to cause intensive shale microfracturing. This implies that the replacement of calcite and dolomite grains by calcium sulfate crystals could provide an additional driving force to generate microfractures during shale hydraulic fracturing.
The recent increase in unconventional oil and gas exploration and production has prompted a large amount of research on hydraulic fracturing, but the majority of chemical reactions between shale minerals and organic matter with fracturing fluids are not well understood. Organic matter, primarily in the form of kerogen, dominates the transport pathways for oil and gas; thus any alteration of kerogen (both physical and chemical properties) upon exposure to fracturing fluid may impact hydrocarbon extraction. In addition, kerogen is enriched in metals, making it a potential source of heavy metal contaminants to produced waters. In this study, we reacted two different kerogen isolates of contrasting type and maturity (derived from Green River and Marcellus shales) with a synthetic hydraulic fracturing fluid for two weeks in order to determine the effect of fracturing fluids on both shale organic matter and closely associated minerals. ATR-FTIR results show that the functional group compositions of the kerogen isolates were in fact altered, although by apparently different mechanisms. In particular, hydrophobic functional groups decreased in the Marcellus kerogen, which suggests the wettability of shale organic matter may be susceptible to alteration during hydraulic fracturing operations. About 1% of organic carbon in the more immature and Type I Green River kerogen isolate was solubilized when it was exposed to fracturing fluid, and the released organic compounds significantly impacted Fe oxidation. Based on the alteration observed in both kerogen isolates, it should not be assumed that kerogenic pores are chemically inert over the timeframe of hydraulic fracturing operations. Shifts in functional group composition and loss of hydrophobicity have the potential to degrade transport and storage parameters such as wettability, which could alter hydrocarbon and fracturing fluid transport through shale. Additionally, reaction of Green River and Marcellus kerogen isolates with low pH solutions (full fracturing fluid, which contains hydrochloric acid, or pH 2 water) mobilized potential trace metal(loid) contaminants, primarily S, Fe, Co, Ni, Zn, and Pb. The source of trace metal(loid)s varied between the two kerogen isolates, with metals in the Marcellus shale largely sourced from pyrite impurities, whereas metals in the Green River shale were sourced from a combination of accessory minerals and kerogen.
This work reports the chemical speciation and leaching characteristics of seven hazardous trace elements (HTEs, including Hg, As, Cr, Cd, Ba, Mn, and Pb) in the coal and fly ash samples collected from four coal-fired power plants in China. The physical structure and chemical composition of the fly ash were characterized by the scanning electron microscope (SEM) and the energy dispersive X-ray spectrometer (EDX). The chemical speciation of HTEs in the coal and fly ash was measured by the modified three-step sequential extraction method, proposed by the European Community Bureau of Reference (BCR). Leaching characteristics of HTEs in the fly ash were investigated by a single batch leaching test. The concentration of HTEs in solid and liquid samples was determined by the direct mercury analyzer DMA 80 (Hg in solid) and the inductively coupled plasma-mass spectrometry (ICP-MS). Results show that for the coal, concentration of Hg, As, Cr, Cd, Ba, Mn, and Pb is 0.06–0.22, 0.63–4.01, 8.91–13.09, 0.06–0.15, 108.67–229.21, 49.94–100.24 and 6.74–26.38 mg/kg, respectively. Mercury is mainly in the residual form while Cd and Ba are primarily in reducible form. Compared with other HTEs, manganese in water/acid soluble and exchangeable fraction has the large percentage. For the fly ash, the concentration of Hg, As, Cr, Cd, Ba, Mn and Pb is 0.17–1.26, 5.15–25.74, 43.25–64.61, 0.56–0.70, 777.05–970.70, 163.83–831.47 and 28.94–119.57 mg/kg, respectively. Mercury and chromium are mainly in the residual speciation. Arsenic and manganese in water/acid soluble and exchangeable form have high ratio with value of 7.27–58.60% and 6.14–62.27%, respectively. Cadmium and barium are primarily in the reducible form. Based on the risk assessment code, manganese in the coal can pose high or very high risk on the environment. Leaching concentration of Cr in some fly ash is higher than permissible limits and the pH value of leaching solution for the fly ash is alkaline. Considering huge fly ash production from coal combustion and complex landfill conditions, some suitable disposal measures to minimize the risk on the environment are needed.
This study investigated heavy metal chemical speciation and leaching behavior from a board-type spent selective catalytic reduction (SCR) catalyst containing high concentrations of vanadium, chromium, nickel, copper, zinc, and lead. A three-step sequential extraction method, standard toxicity characteristic leaching procedure (TCLP), and leaching characteristic tests have been performed. It was found that the mobility of six heavy metals in the spent SCR catalyst was significantly different. The mobility of the six heavy metals exhibited the following order: Ni > Zn > V > Cr > As > Cu. Meanwhile, TCLP test results revealed relatively high Zn and Cr leaching rate of 83.20% and 10.35%, respectively. It was found that leaching rate was positively correlated with available contents (sum of acid soluble, reducible and oxidizable fractions). Leaching characteristics tests indicated that pH substantially affected the leaching of these heavy metals. In particular, the leaching of Cr, Ni, Cu, and Zn was positively influenced by strong acid, while V and As were easily released in the presence of strong acid and strong alkali (pH < 3 or pH > 11). In terms of kinetics, the leaching of Cr, Ni, Cu, Zn, and As within the spent catalyst was dominated by erosion and dissolution processes, which were rapid reaction processes. V was released in large amounts within 1 h, but its leaching amount sharply decreased with time due to readsorption.
Trace elements were commonly used as additives to facilitate anaerobic digestion. However, their addition is often blind because of the complexity of reaction conditions, which has impeded their widespread application. Therefore, this study was conducted to evaluate deficiencies in trace elements during anaerobic digestion by establishing relationships between changes in trace element bioavailability (the degree to which elements are available for interaction with biological systems) and digestion performance. To accomplish this, two batch experiments were conducted. In the first, sequential extraction was used to detect changes in trace element fractions and then to evaluate trace element bioavailability in the whole digestion cycle. In the second batch experiment, trace elements (Co, Fe, Cu, Zn, Mn, Mo and Se) were added to the reaction system at three concentrations (low, medium and high) and their effects were monitored. The results showed that sequential extraction was a suitable method for assessment of the bioavailability of trace elements (appropriate coefficient of variation and recovery rate). The results revealed that Se had the highest (44.2%–70.9%) bioavailability, while Fe had the lowest (1.7%–3.0%). A lack of trace elements was not directly related to their absolute bioavailability, but was instead associated with changes in their bioavailability throughout the digestion cycle. Trace elements were insufficient when their bioavailability was steady or increased over the digestion cycle. These results indicate that changes in trace element bioavailability during the digestion cycle can be used to predict their deficiency.
The migration of trace elements during the pyrolysis of Mongolian oil shale is studied at the macroscopic (the porosity change) and microscopic (the occurrence state) levels. Four groups of oil shale (904, 1054, 10513 and 10807) in the Dalai Bulang mining area of Mongolia were studied. Oil shale samples were heated in a tube furnace in argon atmosphere. Final temperatures of pyrolysis varied from 300 °C to 900 °C resulting in different char. The content of eight kinds of heavy metal elements in the oil shale and the pyrolysis semi-coke were determined by inductively coupled plasma mass spectometry (ICP-MS), through which the distribution and the migration of these elements were analyzed. The specific surface area and pore distribution of different semi-coke were detected by specific surface area analyzer. The influence of surface area and pore distribution on heavy metal element migration was investigated. The sequential chemical extraction (SEC) was carried out on the original oil shale samples. Trace elements in different occurrence models were also detected by ICP-MS. Then the relationship between occurrence models and trace element migration during pyrolysis was also analyzed. The results show that: 1) The concentrations of heavy metal elements in four kinds of oil shale groups are similar, and the occurrence models of the same kind of heavy metal elements in four samples are also similar. 2) The migration amount of Zn and Cu is small at the temperature below 500 °C, while both of the elements escaped rapidly at the temperature above 700 °C. Because Zn and Cu are mainly present in the carbonate which decomposed rapidly at the temperature above 600 °C. 3) The mobility of most of the elements in the sample shows a downward trend during 500–600 °C and 800–900 °C. During these temperature ranges the specific surface area of the semi-coke was increased, because the elements was reabsorbed by the semi-coke.
Low permeability renders a significant fraction of coal seam gas (CSG) resources sub-economic. An effective permeability enhancement strategy is thereby crucial in monetising a large proportion of low permeability CSG resources. This paper introduces the concept of using oxidants for permeability enhancement, describes a practical screening method to evaluate potential oxidants and provides knowledge about the coal behaviour in oxidants. A test based on time-lapse photography and image analysis of coal particles immersed in liquid oxidants was used to assess the extent and rate of change of coal particle size. Complementary leaching tests determined the extent of coal solubilisation by quantifying the change in coal mass and leachate organic content. The swelling profiles of coal particles hand-picked from a low permeability CSG coal core (Bowen Basin, Australia) were first examined in solutions of potassium chloride, and then pyridine for the purpose of method development and validation. Finally, the swelling ratio, SR, and rate of swelling Sr (% area change per 6 h), of coal particles immersed in oxidising solutions of sodium hypochlorite (0.1%, 1% and 10% NaClO), potassium permanganate (0.015%, 0.03%, 0.1%, 1%, 3% and 5% KMnO4), hydrogen peroxide (1%, 3%, 10% and 30% H2O2) and potassium persulfate (1% and 3% K2S2O8) were examined.
Results provide evidence for coal solubilisation (maximum mass loss = 15%) and the propensity to swell (maximum particle size increase = 15%) in all the candidate oxidant stimulants as well as coal breakage in specific oxidants and at specific concentrations (1% NaClO and 3%, 5% KMnO4). The swelling and solubilisation of the coals used in this study tends to increase with higher oxidant concentrations. Anisotropic swelling was also clearly observed in 1% NaClO. Coal reacted vigorously with NaClO and KMnO4, but only slightly with K2S2O8 and H2O2. Massive coal solubilisation occurs in NaClO and KMnO4, but negligible in K2S2O8 and H2O2. In terms of coal oxidation to enhance permeability, NaClO and KMnO4 seem to be more promising than K2S2O8 and H2O2.
For the situation of in situ application, it remains unclear if the net effect of coal swelling and coal solubilisation will increase or decrease permeability. In addition, coal breakage may lead to void space or new cracks in the coal matrix, which could have the potential to increase the coal permeability. Confined core-flooding tests that simulate in situ conditions are required to elucidate this behaviour.
Gas extraction from shale has been challenging due to extremely low permeability in shale formations. Recently, high temperature treatment methods were introduced to remove organic matters from shale to increase shale pore diameter and reservoir permeability. In this study, hydrogen peroxide was used as oxidizer to react with organic matters in shale samples for 6, 12, 24, and 48 h, respectively. X-ray diffraction (XRD) tests were employed to study the mineral compositions. Then, thermal decomposition experiments of shale samples were also conducted by the combined Thermogravimetry-Fourier transform infrared spectroscopy (TG-FTIR) technique to analyze the weight loss behaviors and gas emission in N2 environment during pyrolysis. Afterwards, the effect of pyrolysis and reaction duration on pore diameter were investigated by liquid nitrogen adsorption and desorption tests. Finally, scanning electron microscopy (SEM) images of the shale samples were analyzed to reveal pore structure change on the surface of shale samples. From the experiments, it was found that the organic matters could react with H2O2 at ambient temperature and pore diameter increased with reaction time. The mean pore diameter increased by 40% when samples were treated with H2O2 for 48 h. More larger pores were observed on the surface of both pyrolyzed and H2O2 treated shale samples from the SEM images. Moreover, the pyrolyzed samples have larger pores compared to the H2O2 treated samples due to the decomposition of minerals at high temperature.
Geochemical interactions between shale and hydraulic fracturing fluid may a ffect produced-water chemistry and rock properties. It is important to investigate the rock-water reactions to understand the impacts. Eight autoclave experiments reacting Marcellus and Eagle Ford Shale samples with synthetic brines and a friction reducer were conducted for more than 21 days. To better determine mineral dissolution and precipitation at the rock-water interface, the shale samples were ion milled to create extremely smooth surfaces that were characterized before and after the autoclave experiments using scanning electron microscopy (SEM). This method provides an unprecedented level of detail and the ability to directly compare the same mineral particles before and after the reaction experiments. Dissolution area was quantified by tracing and measuring the geometry of newly formed pores. Changes in porosity and permeability were also measured by mercury intrusion capillary pressure (MICP) tests. Aqueous chemistry and SEM observations show that dissolution of calcite, dolomite, and feldspar and pyrite oxidation are the primary mineral reactions that control the concentrations of Ca, Mg, Sr, Mn, K, Si, and SO4 in aqueous solutions. Porosity measured by MICP also increased up to 95%, which would exert significant influence on fluid flow in the matrix along the fractures. Mineral dissolution was enhanced and precipitation was reduced in solutions with higher salinity. The addition of Polyacrylamide (a friction reducer) to the reaction solutions had small and mixedeffects on mineral reactions, probably by plugging small pores and restricting mineral precipitation. The results suggest that rockwater interactions during hydraulic fracturing likely improve porosity and permeability in the matrix along the fractures by mineral dissolution. The extent of the geochemical reactions is controlled by the salinity of the fluids, with higher salinity enhancing mineral dissolution. © 2017. The American Association of Petroleum Geologists. All rights reserved.
Hydraulic fracturing for gas production is now ubiquitous in shale plays, but relatively little is known about shale-hydraulic fracturing fluid (HFF) reactions within the reservoir. To investigate reactions during the shut-in period of hydraulic fracturing, experiments were conducted flowing different HFFs through fractured Marcellus Shale cores at reservoir temperature and pressure (66oC, 20 MPa) for one week. Results indicate HFFs with hydrochloric acid cause substantial dissolution of carbonate minerals, as expected, increasing effective fracture volume (fracture volume + near-fracture matrix porosity) by 56-65%. HFFs with reused produced water composition cause precipitation of secondary minerals, particularly barite, decreasing effective fracture volume by 1-3%. Barite precipitation occurs despite the presence of antiscalants in experiments with and without shale contact, and is driven in part by addition of dissolved sulfate from the decomposition of persulfate breakers in HFF at reservoir conditions. The overall effect of mineral changes on the reservoir has yet to be quantified, but the significant amount of barite scale formed by HFFs with reused produced water composition could reduce effective fracture volume. Further study is required to extrapolate experimental results to reservoir-scale, and to explore the effect that mineral changes from HFF interaction with shale might have on gas production.
We developed an integrated framework of combined batch experiments and reactive transport simulations to quantify water-rock-CO2 interactions and arsenic (As) mobilization responses to CO2 and/or saline water leakage into USDWs. Experimental and simulation results suggest that when CO2 is introduced, pH drops immediately that initiates release of As from clay minerals. Calcite dissolution can increase pH slightly and cause As re-adsorption. Thus, the mineralogy of the USDW is ultimately a determining factor of arsenic fate and transport. Salient results suggest that: (1) As desorption/adsorption from/onto clay minerals is the major reaction controlling its mobilization, and clay minerals could mitigate As mobilization with surface complexation reactions; (2) dissolution of available calcite plays a critical role in buffering pH; (3) high salinity in general hinders As release from minerals; and (4) the magnitude and quantitative uncertainty of As mobilization are predicated on the values of reaction rates and surface area of calcite, adsorption surface areas and equilibrium constants of clay minerals, and cation exchange capacity. Results of this study are intended to improve ability to quantify risks associated with potential leakage of reservoir fluids into shallow aquifers, in particular the possible environmental impacts of As mobilization at carbon sequestration sites.
The use of hydraulic fracturing techniques to extract oil and gas from low permeability shale reservoirs has increased significantly in recent years. During hydraulic fracturing, large volumes of water, often acidic and oxic, are injected into shale formations. This drives fluid-rock interaction that can release metal contaminants (e.g., U, Pb) and alter the permeability of the rock, impacting the transport and recovery of water, hydrocarbons, and contaminants. To identify the key geochemical processes that occur upon exposure of shales to hydraulic fracturing fluid, we investigated the chemical interaction of hydraulic fracturing fluids with a variety of shales of different mineralogical texture and composition. Batch reactor experiments revealed that the dissolution of both pyrite and carbonate minerals occurred rapidly, releasing metal contaminants and generating porosity. Oxidation of pyrite and aqueous Fe drove precipitation of Fe(III)-(oxy)hydroxides that attenuated the release of these contaminants via co-precipitation and/or adsorption. The precipitation of these (oxy)hydroxides appeared to limit the extent of pyrite reaction. Enhanced removal of metals and contaminants in reactors with higher fluid pH was inferred to reflect increased Fe-(oxy)hydroxide precipitation associated with more rapid aqueous Fe(II) oxidation. The precipitation of both Al- and Fe-bearing phases revealed the potential for the occlusion of pores and fracture apertures, whereas the selective dissolution of calcite generated porosity. These pore-scale alterations of shale texture and the cycling of contaminants indicate that chemical interactions between shales and hydraulic fracturing fluids may exert an important control on the efficiency of hydraulic fracturing operations and the quality of water recovered at the surface.
Hydraulic fracturing of unconventional hydrocarbon reservoirs is critical to the United States energy portfolio; however, hydrocarbon production from newly fractured wells generally declines rapidly over the initial months of production. One possible reason for this decrease, especially over time scales of several months, is the mineralization and clogging of microfracture networks and pores proximal to propped fractures. One important but relatively unexplored class of reactions that could contribute to these problems is oxidation of Fe(II) derived from Fe(II)-bearing phases (primarily pyrite, siderite, and Fe(II) bound directly to organic matter) by the oxic fracture fluid and subsequent precipitation of Fe(III)-(oxy)hydroxides. The extent to which such reactions occur, their rates, mineral products, and physical locations within shale pore spaces are unknown. To develop a foundational understanding of potential impacts of shale iron chemistry on hydraulic stimulation, we reacted sand-sized (150-250 μm) and whole rock chips (mm-scale) of shales from four different formations (Marcellus Fm., New York; Barnett Fm., Central Texas; Eagle Ford Fm., Southern Texas; and Green River Fm., Colorado) at 80°C with synthetic fracture fluid, with and without HCl acid. These four shales contain variable abundances of clays, carbonates, and total organic carbon (TOC). We monitored Fe concentration in solution and evaluated changes in Fe speciation in the solid phase using synchrotron-based techniques. Solution pH was the most important factor affecting the release of Fe into solution. For reactors with an initial solution pH of 2.0 and low carbonate content in the initial shale, the sand-sized shale showed an initial release of Fe into solution during the first 96 hours of reaction, followed by a plateau or significant drop in solution Fe concentration, indicating that mineral precipitation occurred. In contrast, in reactors with high pH buffering capacity, little to no Fe was detected in solution throughout the course of the experiments. In reactors that contained no added acid (initial pH = 7.1), there was no detectable Fe release into solution. The carbonate-poor whole rock samples showed a steady increase, then a plateau in Fe concentration during 3 weeks of reaction, indicating slower Fe release and subsequently slower Fe precipitation. Synchrotron-based x-ray fluorescence mapping coupled with x-ray absorption spectroscopy (both bulk and micro) showed that when solution pH was above 3.25, Fe(III)-bearing phases precipitated in the shale matrix. Initially, ferrihydrite precipitated on and in the shale, but as experimental time increased, the ferrihydrite transformed to either goethite (at pH 2.0) or hematite (pH > 6.5). Additionally, not all of the released Fe(II) was oxidized to Fe(III), resulting in precipitation of mixed-valence phases such as magnetite. Idealized systems containing synthetic fracture fluid and dissolved ferrous chloride but no shale showed that in reactors open to the atmosphere at low pH (< 3.0), Fe(II) oxidation is inhibited. Surprisingly, the addition of bitumen, which is often extracted by organic compounds in the fracture fluid, can override this inhibition of Fe(II) oxidation caused by low pH. Nonetheless, O2 in the system is still the most important factor controlling Fe(II) oxidation. These results indicate that Fe redox cycling is an important and complex part of hydraulic fracturing and provide evidence that Fe(III)-bearing precipitates derived from oxidation of Fe(II)-bearing phases could negatively impact hydrocarbon production by inhibiting transport.
Elution of sodium persulfate on spinel-type LiMn2O4 were investigated based on the research of hydrolytic process of sodium persulfate solution at different conditions. The hydrolysis of sodium persulfate followed first order kinetics, and the activation energy and pre-exponential factor were 125.41 kJ . mol(-1) and 1.09 x 10(18) h(-1), respectively. The stoichiometric ratio of S2O82- /H+/SO42- was 1:2:2. The SO4-center dot free radicals were generated during the hydrolysis of sodium persulfate and the hydrolysis rate was determined by the formation rate of SO4-center dot. With sodium persulfate solution as eluent, the ratios of both the lithium extraction and manganese loss were low below 35 degrees C However, lithium ions were nearly extracted thoroughly with little manganese loss above 75 degrees C, because the SO4-center dot generated in hydrolysis of sodium persulfate had an inhibitory effect on manganese loss. After sodium persulfate solution was hydrolyzed for 72 h at 75 degrees C, lithium ions were nearly extracted thoroughly, but manganese loss was serious. The products were characterized by scanning electron microscopy (SEM), thermogravimetry (TG), X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FT4R) and X-ray photoelectron spectroscopy (XPS). The results showed that the morphology and spinel-type structure were hardly changed after extraction of lithium, and the amount of trivalent manganese decreased with the process of elution. The elution, on the basis of hydrolysis of sodium persulfate, was found a reaction process which involved redox reaction and ion exchange reaction.
Owing to the presence of chemically unstable compositions (e.g., clay minerals, carbonate, pyrite, and organic matter (OM)) which are closely related to shale structure failures or dissolution pores, the methods to increase gas well long-term productivity from tight shale matrix need to take into account the chemical interaction of shale with injected fluid. In this study, black shale samples obtained from Lower Silurian Longmaxi formation in Sichuan basin, China were treated with 15 wt% hydrogen peroxide (H2O2) for the comprehensive understanding of the change in shale composition and the associated dissolution structures. The measurements of mass loss, total organic carbon content, and mineralogical composition showed that carbonate, reductive inorganic minerals containing ferrous iron (e.g., pyrite and chlorite), and OM in samples exhibited strong dissolution; however, the other minerals behaved in a non-reactive manner at the experimental time scale. After the oxidative treatment for 240 h, a large amount of oxidation-induced fractures and dissolution pores were observed by field-emission scanning electron microscopy. The fractures mainly oriented parallel to lamination were attributed to the dissolution of OM and structural alteration of clay minerals. All the dissolution pores seemed to be strongly dependent on the loss of dolomite, pyrite, and OM. Results from high-pressure mercury intrusion and low-pressure nitrogen adsorption analysis showed that these dissolution pores ranging from 10 to 500 nm in diameter exhibited a significant increase in pore volume due to the removal of interconnected pore-filling OM, while the volume of pores > 1 μm in size exhibited a minor increase because the micrometer-size dissolved particles appeared to be discrete or unconnected. Thus the oxidative dissolution could lead to the higher porosity and better connectivity of nanometer-size pore networks in shale samples. The induced fractures reduced the size of diffusion dominant zones in shale matrix, and the dissolution pores increased the size of gas transport pathways into fractures. These results indicate that the injection of H2O2 may play an important role in shale matrix stimulation by oxidative dissolution which is likely to improve matrix diffusivity.
The production rate of a typical shale gas well generally has steep decline trend at the initial stage but small declines at later times. Some empirical relationships have been proposed to describe the declining production rates and thus forecast the final cumulative production of a shale gas well. However, these empirical relationships can hardly elucidate the mechanisms that cause the special shale gas production trend. In this study, a novel two-part Hooke's model (TPHM) for the permeability and effective stress relationship is developed and incorporated into the hydro-mechanical COMSOL solver to determine the production rate of shale gas wells against time. The TPHM conceptualizes shale rock into soft part and hard part, which comply with the natural-strain-based and engineering-strain-based Hooke's laws, respectively , and contribute differently to the decreasing permeability with increasing effective stresses. The simulation results are analyzed and compared with those for which the permeability change effect is not considered. The analysis indicates that the decrease in stress-induced permeability plays a non-negligible part in the decline of the production rate.
The effect of pH changes on leachability of light and heavy metals from shale drill cuttings generated from unconventional shale gas production was investigated. Cuttings, being the primary byproduct generated from drilling operations, belong to the potentially hazardous type of wastes due to presence of heavy and radioactive elements and remains of drilling fluid. In this regard, assessment of potentially dangerous components (PDCs) from rock waste materials was performed by application of batch leaching tests, which has provided information on the sensitivity of leaching under externally imposed changes in pH (natural or caused by treatment) in specific scenarios. The description of shale rocks mineralogical and chemical properties was performed by means of X-ray fluorescence spectroscopy, diffractometry as well as scintillation spectrometry. The concentrations of released constituents due to the leaching tests were measured by atomic absorption spectrophotometry. Results were compared and discussed accordingly with the waste acceptable criteria of elution limits.
Analysis of the substrate revealed that the elemental composition was dominated by light elements, whereas heavy metals were present in trace amounts. However, noticeable release of barium (2.0–4.6%) was also recorded, which has originated from not only rock material but also drill mud. Minor mobility was observed for transition elements such as Cr, Co, Fe, Mn, Ni, Zn, Cu and Pb. Results revealed that drill cuttings follow the requirements for other than hazardous and municipal type of deposition, with exception for barium. Moreover, content of radioactive isotopes fulfill the requirements range of acceptable concentrations.
Hydraulic fracturing is an important technological advance in the extraction of natural gas and petroleum from black shales, but water injected into shale formations in the fracturing process returns with extraordinarily high total-dissolved-solids (TDS) and high concentrations of barium, Ba. It is generally assumed that high TDS comes from the mixing of surface water (injected fluid) with Na–Ca–Cl formation brines containing elevated Ba, but the mechanisms by which such mixing might occur are disputed. Here we show that Ba in water co-produced with gas could originate from water-rock reactions, with Ba levels observed in produced waters reached on a time scale relevant to hydraulic fracturing operations. We examined samples from three drill cores from the Marcellus Shale in Pennsylvania and New York to determine the possible water-rock reactions that release barium during hydraulic fracturing. Two samples, one containing microcrystalline barite (BaSO4) and one without barite, contain elevated concentrations of Ba relative to the crustal average for shale rocks. A third sample is slightly depleted in Ba relative to the crustal average. Micro-XRF measurements and SEM/EDS analysis combined with chemical sequential extraction methods reveal that a majority of the Ba in all samples (55–77 wt.%) is present in clays and can only be leached from the rock by dissolution in hydrofluoric acid. Thus, a majority of barium in our samples is relatively inaccessible to leaching under hydraulic fracturing conditions. However, the balance of Ba in the rocks is contained in phases that are potentially leachable during hydraulic fracturing (e.g., soluble salts, exchangeable sites on clays, carbonates, barite, organics).
In this study, the mobilization, redistribution, and fractionation of trace and rare earth elements (REE) during chemical weathering in mid-ridge (A), near mountaintop (B), and valley (C) profiles (weak, weak to moderate, and moderate to intense chemical weathering stage, respectively), are characterized. Among the trace elements, U and V were depleted in the regolith in all three profiles, Sr, Nb, Ta, Zr, and Hf displayed slight gains or losses, and Th, Rb, Cs, and Sc remained immobile. Mn, Ba, Zn, Cu, and Cr were enriched at the regolith in profiles A and B, but depleted in profile C. Mn, Pb, and Co were also depleted in the saprock and fractured shale zones in profiles A and B and enriched in profile C. REEs were enriched in the regolith and depleted at the saprock zone in profiles A and B and depleted along profile C. Mobility of trace and REEs increased with increasing weathering intensity. Normalized REE patterns based on the parent shale revealed light REE (LREE) enrichment, middle REE (MREE), and heavy REE (HREE) depletion patterns. LREEs were less mobile compared with MREEs and HREEs, and this differentiation increased with increasing weathering degree. Positive Ce anomalies were higher in profile C than in profiles A and B. The Ce fractionated from other REE showed that Ce changed from trivalent to tetravalent (as CeO2) under oxidizing conditions. Minimal REE fractionation was observed in the saprock zone in profiles A and B. In contrast, more intense weathering in profile C resulted in preferential retention of LREE (especially Ce), leading to considerable LREE/MREE and LREE/HREE fractionation. (La/Yb)N and (La/Sm)N ratios displayed maximum values in the saprock zone within low pH values. Findings demonstrate that acidic solutions can mobilize REEs and result in leaching of REEs out of the highly acidic portions of the saprock material and transport downward into fractured shale. The overall behavior of elements in the three profiles suggests that solution pH, as well as the presence of primary and secondary minerals, play important roles in the mobilization and redistribution of trace elements and REEs during black shale chemical weathering.
The Eh-pH diagrams for arsenic species are shown in Figs. 9 and 10. The thermodynamic data for important arsenic species are given in Table 9.
Rapid expansion of hydraulic fracturing operations for natural gas and oil production can impact water quality. Water that flows back to the surface as part of the hydraulic fracturing process and during well production can contain trace elements, including regulated metals and metalloids, mobilized by interactions of the fracturing fluid with the formation. The rate and extent of mobilization depend on the geochemistry of the formation, the composition of the fracturing fluid, and the contact time. Laboratory experiments detailed here examined the influence of water chemistry on element mobilization from core samples taken from the Eagle Ford Formation, which is currently producing natural gas from hydraulically fractured zones. Fluid properties were varied with regard to pH, oxidant level, and solid:water ratio. Release of elements (Ca, Mg, Fe, Ba, As) from the Eagle Ford samples strongly depended on pH, which in turn was primarily controlled by calcite dissolution. Presence of oxygen and the addition of H2O2 led to pyrite oxidation and resulted in an elevated amount of sulfate. Barium concentrations were largely controlled by the amount of sulfate present through solubility equilibrium of barite that formed as a secondary phase. The effect of increasing solid:water ratio on the extent of mobilization varied widely for different elements. Taken together, these findings demonstrate the need to understand both the aqueous and geochemistries of a hydraulically fractured formation with regard to elemental mobilization in produced and flowback waters.
A survey on the (palaeo-) diversity of the Lower and Middle Ordovician conodont faunas from the Yichang Region, Hubei Province, Central China is presented. Analysis of the number of species through time shows the species richness is cyclical developed in the succession - varying from low to high. One major conodont diversity peak is recorded in the mid to late Floian Stage (Oepikodus evae Zone, stage slice FI 2, Lower Ordovician), where 33 species are recorded in the lower unit of the Dawan Formation. A smaller but second high diversity is found in the Darriwilian Stage (Lenodus variabilis Zone; stage slice Da 2, Middle Ordovician) with a maximum of 23 species. Secondary, smaller diversification maxima are present in the early Tremadocian, late Tremadocian, early Dapingian and late Darriwilian stages. Current information on acritarchs, brachiopods, and trilobites shows that the highest diversity maximum of conodonts corresponds to a similar high in brachiopod diversity, whereas acritarch and trilobite maximum diversity appear after the conodonts/brachiopods in the region.
Radium occurs in flowback and produced waters from hydraulic fracturing for unconventional gas extraction along with high concentrations of barium and strontium and elevated salinity. Radium is often removed from this wastewater by co-precipitation with barium or other alkaline earth metals. Distribution equation for Ra in the precipitate is derived from the equilibrium of the lattice replacement reaction (inclusion) between Ra2+ ion and the carrier ions (e.g., Ba(2+), Sr(2+)) in aqueous and solid phases and is often applied to describe the fate of radium in these systems. Although the theoretical distribution coefficient for Ra-SrSO4 (Kd=237) is much larger than for Ra-BaSO4 (Kd=1.54), previous studies have focused on Ra-BaSO4 equilibrium. This study evaluates the equilibria and kinetics of co-precipitation reactions in Ra-Ba-SO4 and Ra-Sr-SO4 binary systems and in Ra-Ba-Sr-SO4 ternary system under varying ionic strength (IS) conditions that are representative of brines generated during unconventional gas extraction. Results show that radium removal generally follows theoretical distribution law in binary systems and is enhanced in Ra-Ba-SO4 system and restrained in Ra-Sr-SO4 system by high ionic strength. However, experimental distribution coefficient (Kd(')) varies over a wide range and cannot be described by the distribution equation that does not account for radium removal by adsorption. Radium removal in ternary system is controlled by the co-precipitation of Ra-Ba-SO4, which is attributed to rapid BaSO4 nucleation rate and closer ionic radii of Ra(2+) with Ba(2+) than with Sr(2+). Carrier (i.e., barite) recycling during water treatment was shown to be remarkably effective in enhancing radium removal even after co-precipitation was completed. Calculations based on experimental results show that Ra levels in the precipitate generated in centralized waste treatment facilities far exceed regulatory limits for disposal in municipal sanitary landfills and require careful monitoring of allowed source term loading (ASTL) for technically enhanced naturally occurring materials (TENORM) in these landfills. Several alternatives for sustainable management of TENORM are discussed.
This study focuses on the abundance and mobility of Ca, Fe, S and trace elements (As, Cd, Co, Cr, Cu, Mn, Mo, Ni, Pb, U, V and Zn) in black shale (alum shale) in SE, Sweden. Samples of non-weathered, weathered and burnt black shale were chemically characterized and the potential element release from them was assessed by standard water-based leaching tests and pH/redox-regulated availability tests. Sequential chemical extractions provided further information on the phases in which the elements are bound. Results show that the shale is very rich in As (88-122 ppm), Cd (0.4-4.6 ppm), Mo (61-176 ppm), U (27-71 ppm) and V (496-1560 ppm). Cadmium and Mo, bound mainly in sulphides or organic matter, are very mobile in the non-burnt shale, with mobilization rates of up to 19% (190 mu g/kg) and 25% (16 mg/kg), respectively, using only water as extraction medium. The non-weathered shale is also relatively rich in Cu (113 ppm), Ni (100 ppm) and Zn (304 ppm), the latter two in particular showing behaviour similar to that of Cd, but with lower mobilization rates. In all samples U and V arc found mainly in weathering-resistant mineral phases and thus have a lower mobility, but due to the high abundance in the material, significant amounts of U can be released on longer time scales (up to 6 mg/kg, as indicated by the pH/redox-regulated test). Less than 1% of the As is released in all the leaching tests, indicating that upon oxidation it is retained in the solid phase. The overall conclusion is that this material has a high potential for releasing Cd, Mo, Ni, U and Zn (luring weathering.
The 13 C values of dissolved HCO 3 - in 75 water samples from 15 oil and gas fields (San Joaquin Valley, Calif., and the Houston-Galveston and Corpus Christi areas of Texas) were determined to study the sources of CO 2 of the dissolved species and carbonate cements that modify the porosity and permeability of many petroleum reservoir rocks. The reservoir rocks are sandstones which range in age from Eocene through Miocene. The 13 C values of total HCO 3 - indicate that the carbon in the dissolved carbonate species and carbonate cements is mainly of organic origin. The range of 13 C values for the HCO 3 - of these waters is -20-28 per mil relative to PDB. This wide range of 13 C values is explained by three mechanisms. Microbiological degradation of organic matter appears to be the dominant process controlling the extremely low and high 13 C values of HCO 3 - in the shallow production zones where the subsurface temperatures are less than 80°C. The extremely low 13 C values (< -10 per mil) are obtained in waters where concentrations of SO 4 2- are more than 25 mg/l and probably result from the degradation of organic acid anions by sulfate-reducing bacteria ( SO 4 2- + CH 3 COO - 2 HCO 3 - + HS - ). The high 13 C values probably result from the degradation of these anions by methanogenic bacteria ( CH 3 COO - + H 2 O ai HCO 3 - + CH 4 ). Thermal decarboxylation of short-chain aliphatic acid anions (principally acetate) to produce CO 2 and CH 4 is probably the major source of CO 2 for production zones with subsurface temperatures greater than 80°C. The 13 C values of HCO 3 - for waters from zones with temperatures greater than 100°C result from isotopic equilibration between CO 2 and CH 4 . At these high temperatures, 13 C values of HCO 3 - decrease with increasing temperatures and decreasing concentrations of these acid anions.