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Modeling of fracture opening by explosive products
Abstract
In traditional hydraulic fracturing stimulation, the effective conductivity of low permeability rock is increased by generating/activating fractures through injection of pressurized fluid. One possible extension of traditional hydraulic fracturing is to increase the loading rate that the driving fluid applies on the formation. Methods that use dynamic loading potentially extend stimulation to previously inaccessible geological resources. In contrast to pump-driven hydraulic fracturing methods, dynamic stimulation can generate stresses in the source region that may be significantly larger than the in-situ stress, which helps to create fractures that may grow in directions other than that of the minimum in-situ stress. Furthermore, the increased loading rate has potential to generate fractures in relatively ductile formations. With dynamic loading, fractures are initially generated by the diverging stress wave propagating from the energy release zone. It has been shown that the crack area and final extent depend on the ability of explosive products to flow into cracks after wave propagation. Here we investigate the potential for crack generation from stress waves, and the opening of those cracks via the flow of explosive product gases into the cracks by developing a coupled Finite Element (FE) and Godunov scheme for fluid-solid interactions.
... In these blasting models, rock fracture and exfoliation due to dynamic blast impacts were considered, but the action of explosion-generated gas driving fracture propagation was ignored. Settgast et al. (2017) established a deflagration model considering fluid-solid interactions through FEM and Godunov scheme coupling, and revealed the potential of stress waves generating cracks and explosion-generated gas opening these cracks. Fakhimi and Lanari (2014) successfully simulated fracture propagation in rock blasting by combining the DEM to calculate rock fracture and smoothed particle hydrodynamics to describe explosion gas flow. ...
... However, this model did not consider the impact of the deflagration stress wave. Settgast et al. 27 investigated the potential for fracture generation from stress waves and the opening of those fractures via the flow of blasting product gases into the cracks by developing a coupled finite element and Godunov scheme for fluid-solid interactions. Tian 28 established a numerical model of shale reservoir deflagration fracturing by using the finite element method to simulate the fracture propagation process by making the appropriate units fail. ...
Methane deflagration fracturing is a new reservoir stimulation method that serves the efficient development of shale gas reservoirs. However, the propagation law of deflagration fractures is still unclear. In this paper, a numerical model considering the effect of stress wave impact and gas drive of deflagration fracturing was established based on the continuum-discontinuum element method (CDEM). The correctness of the numerical model was verified by comparing it with a laboratory experiment, the steady and unsteady analytical solutions of gas flow, and the approximate solution of fracture propagation. Then, numerical simulations of methane deflagration fracturing in vertical wells and horizontal wells under different factors were carried out to analyze the fracture mechanism. The results indicate that deflagration fracturing in vertical wells can break through the stress concentration around the borehole; the initial radial fractures are formed under the action of stress wave impact and then propagate substantially under the driving action of high-pressure gas. The in-situ stress difference affects the deflagration fracture propagation and makes the half-fracture length in the direction of maximum principal stress larger than that in the direction of minimum principal stress. The more significant the stress difference is, the more noticeable this deviation will be. When the deflagration peak pressure is high, the reservoir burst degree is large, which is conducive to enlarging the stimulation range of deflagration fracturing. Staged deflagration fracturing in horizontal wells can form 5-8 obvious fractures perpendicular to the horizontal borehole in each explosion section. A large cluster spacing and explosion section length are conducive to expanding the stimulation scope. Moreover, the propagation of deflagration fractures will be induced by the natural fractures, and the natural fracture with a considerable length or a slight angle between the dip angle and the propagation direction of deflagration fractures is more likely to be activated.
... Sheng et al. proposed and solved flow behavior analysis of a complexflow model for the explosive fracturing of a well in an unconventional reservoir (Sheng et al., 2015). Settgast et al. developed a new computational model for predicting the late-time gas driven fracturing associated with the use of propellants and high explosives (Settgast et al., 2017). ...
Well stimulation methods such as hydraulic fracturing are often utilized in order to increase the productivity index and permeability of the reservoir's rock. On the other hand, explosion operation in the wellbore can stimulate the reservoir by generating and propagating cracks around the drilled portion of the rock mass. In this study, cracks initiation and propagation due to an explosion in the rock around a wellbore has been numerically simulated using a coupled finite difference-boundary element method. This approach includes two separate stages, firstly, modeling of the radial cracks initiation due to shock wave propagation using the dynamic finite difference method and, secondly, modeling of the cracks propagation due to gas expansion using the quasi-static boundary element method. After each stage of the explosion, the numerical results have been presented and discussed. These results mainly showed that this approach can model the initiation and propagation of several explosion-induced fractures around a wellbore which can consequently predict an increase in the well productivity of an oil field.
Methane deflagration fracturing is a kind of reservoir stimulation technology for shale gas reservoirs. A 3D numerical model is established based on the continuum–discontinuum element method (CDEM) to study the fracture propagation laws of methane deflagration fracturing fracture. The linear elastic constitutive is applied to the block element, the tensile-shear composite constitutive considering strain softening is applied to the interface element to simulate rock fracture, and the Landau model is applied to describe the deflagration pressure change. The correctness of the model is verified by comparing it with the blast fracturing experiment of a cement rock sample. Then, the 3D numerical simulations are carried out to study the deflagration fracture mechanism. The results indicate that methane deflagration fracturing forms complex fractures around the borehole, with shear fracture dominating and a few tensile radial fractures. Influenced by the stress difference, the fracture range in the direction σH is larger than that in the direction σh, forming an approximate elliptic cylinder deflagration fracture zone. The effects of rock elastic modulus, Poisson's ratio, peak pressure, and pressurization rate on the reservoir crack and fracture range are positively correlated, while the influence of initial minimum principal stress, stress difference, reservoir tensile strength, and cohesion on deflagration fracturing is negatively correlated. Among them, the most sensitive influencing factors are the deflagration peak pressure and the initial minimum principal stress, and the influences of the reservoir rock physical and mechanical parameters are not significant.
INTRODUCTION
The efficient development of shale gas reservoirs greatly depends on the progress of hydraulic fracturing technology, and in recent years, technical systems such as volume fracturing, factory fracturing of the horizontal well group, and temporary plugging re-fracturing have been gradually formed (Guo et al., 2018; Guo et al., 2014; Li et al., 2020; Liu et al., 2020; Qu et al., 2021; Zhou et al., 2019). However, for deep–ultra-deep marine shale gas reservoirs and continental–marine-continental transitional shale gas reservoirs, the application of traditional hydraulic fracturing technology has encountered bottlenecks due to large in-situ stress or severe hydration damage (Dong et al., 2021; He et al., 2020; Zhao et al., 2021). There is a need to develop reservoir enhancement techniques with high peak pressures and little or no water, such as deflagration fracturing (Middleton et al., 2015; Wei et al., 2018; Zhai et al., 2018). Deflagration fracturing (called high-energy gas fracturing), is first developed and tested by Sandia National Laboratories in the late 1970s (Schmidt et al., 1980). A propellant gas generator is used to control the deflagration process, generating a suitable peak pressure and action time. Compared to hydraulic fracturing, the peak pressure is higher and the action time is shorter.
Fiber-optic-based distributed acoustic sensors (DAS) are a new technology that can be deployed in a well and are continuously interrogated during operations. These sensors measure the strain (or strain rate) at all points along the fiber and have been used extensively to monitor hydraulic stimulations. The data from these sensors indicate that they are sensitive to high-frequency signals associated with microseismicity and low-frequency signals associated with fracture growth. We have developed a set of idealized models to simulate these signals and to identify interpretation methods that may be used to estimate fracture location, geometry, and extent. We use a multiphysics code that includes rock physics, fluid flow, and elastic-wave propagation to generate synthetic DAS measurements from a set of simple models that mimic hydraulic fracturing. We then relate the signals observed in the synthetic DAS to specific features in the model such as fracture height, width, and aperture. Our results demonstrate that the synthetic DAS measurements may be used to interpret field DAS measurements and to optimize the design of future sensor deployments for sensitivity to fracture attributes.
The effect of the medium damage on seismic waves generated by underground chemical explosions in hard rock is investigated. An explosion experiment conducted in New Hampshire in 2016 has demonstrated that the amount of explosive gas products released in the cavity improves seismic coupling, which is manifested in higher seismic amplitudes. It has also shown that detonating explosions in water-filled boreholes has similar effects on the spectra as increasing the amount of the gaseous detonation products by changing the explosive type. A postexplosion well logging survey revealed that using explosives releasing higher amount of gaseous products results in an increase in length and aperture of the explosion generated macrofractures. Placing the charges into water-filled rather than drained boreholes results in a similar increase in fracture dimensions. Thus, the extent and intensity of postexplosion macrofracturing correlates with improved seismic coupling expressed as P and Rg amplitude increase, particularly in the low-frequency range. The radiation patterns of the P waves are different for thewaveforms bandpassed in a low-frequency range (1–15 Hz) and a high-frequency range (15–100 Hz). The symmetry of the radiation patterns indicates the presence of nonzero terms associated with the off-diagonal moment tensor terms (Mxz and Myz). The amplitude of the seismic component attributed to the off-diagonal moment tensor elements is significant and can be as large as 15%–16% of the isotropic moment. The observed P-wave radiation pattern is consistent with either sliding along the pre-existing fractures and zones of weakness or shear failure along the high-angle borehole parallel fractures during the explosions.
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