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As an effective approach to evaluate shale gas production, macroscale numerical simulations have been conducted; however, multiscale storage space and complex gas–water two-phase transport mechanisms are not systematically considered. To deal with this, a rigorous cross-scale gas–water two-phase transport simulation method that considers the pore, core, and field scale is proposed herein. First, to investigate the fluid storage state in different pore types during the gas–water two-phase transport process, a shale pore network gas–water transport simulation is conducted. Subsequently, a core-scale model that considers the distribution of organic matter is constructed, and the gas–water two-phase transport behaviors are upscaled from the pore to the core by incorporating nanomicroscale effects. Next, a shale gas–water two-phase macroscale numerical simulation is implemented based on the core-scale simulation results. Multiple interacting continua are used to describe the shale matrix and microfractures, whereas the embedded discrete fracture method is employed to represent hydraulic fractures. Finally, the manner by which nanomicroscale effects, stress sensitivity, and SRV area affect the production of shale gas reservoirs is discussed comprehensively. This study provides a practical method to estimate shale gas production by rigorously considering gas–water nanomicroscale effects to reduce uncertainties during productivity evaluation. [ABSTRACT FROM AUTHOR]

This paper presents an integrated study from fracture propagation modeling to gas flow modeling and a correlation analysis to explore the key controlling factors of intensive volume fracturing. The fracture propagation model takes into account the interaction between hydraulic fracture and natural fracture by means of the displacement discontinuity method (DDM) and the Picard iterative method. The shale gas flow considers multiple transport mechanisms, and the flow in the fracture network is handled by the embedded discrete fracture model (EDFM). A series of numerical simulations are conducted to analyze the effects of the cluster number, stage spacing, stress difference coefficient, and natural fracture distribution on the stimulated fracture area, fractal dimension, and cumulative gas production, and their correlation coefficients are obtained. The results show that the most influential factors to the stimulated fracture area are the stress difference ratio, stage spacing, and natural fracture density, while those to the cumulative gas production are the stress difference ratio, natural fracture density, and cluster number. This indicates that the stress condition dominates the gas production, and employing intensive volume fracturing (by properly increasing the cluster number) is beneficial for improving the final cumulative gas production. [ABSTRACT FROM AUTHOR]

• A 3D hydro-mechanical coupled model is presented to study hydraulic fracturing interaction with bedding planes. • Cohesive zone method is employed to represent the bedding planes explicitly. • Fracture propagation behaviors are analyzed under different patterns. Bedding planes are abundant in the continental shale oil reservoirs, but their impact on hydraulic fracturing is still not fully understood. To investigate the effects of stress condition, bedding plane's properties and layer lithology on hydraulic fracture propagation in layered formation, a 3D hydro-mechanical coupled model is presented. The bedding planes are explicitly represented by cohesive elements with degraded properties compared to the rock. The results show that increasing the stress difference ratio of the vertical principal stress to horizontal principal stress promotes the hydraulic fracture propagating across bedding planes, and the critical value of vertical principal stress required for hydraulic fracture crossing rises with the decrease of bedding plane's critical traction. Furthermore, when the critical traction ratio of bedding plane to rock drops below 0.7, the fracture propagation pattern changes from crossing to deflecting. Additionally, the extension pressure increases slightly, the fracture length decreases and the area of opening bedding planes rises. As the permeability of bedding plane increases, the fracture cannot cross the bedding planes and even stops extension. The layer lithology contrast also affects fracture propagation patterns. This study examines the fracture propagation behaviors under different patterns, including the opening morphology of bedding plane and the evolution of injection pressure and strain energy. The simulation results may provide useful insights for hydraulic fracturing design in layered formations. [ABSTRACT FROM AUTHOR]

Fracture penetration or deflection may occur when the hydraulic fracture approaches weak interface in multi-layered formation. However, the intrinsic mechanism of fracture propagation pattern remains unclear. A model was presented to investigate hydraulic fracture propagation in multi-layered formation via the phase field method. The interface is explicitly represented by a weak thin layer. Fluid flow in the fractured media was modeled by using the indicator function of phase field. The iterative algorithm was described and validated against the benchmark cases. Simulation results show that the evolution of injection pressure is more complex when the fracture deflects than that when the fracture penetrates the interface. The fracture is prone to penetrate the softer layer the perforated layer while deflects to the interface if the adjacent layer is stiffer. Decreasing the modulus ratio of interface to layers promotes the fracture deflecting to the interface, while increasing the stress ratio contributes to the fracture penetrating the interface. The stacking mode of layers greatly affects the fracture propagation morphology in three or more layered formations. Rock heterogeneity doesn't alter the fracture propagation pattern as a whole, but it affects the local fracture morphology. The results provide a cloud chart of the relationship between fracture propagation pattern and controlling factors for the potential of field application. [ABSTRACT FROM AUTHOR]

• An improved thermo-hydro-mechanical coupling model is developed. • Effects of shear dilation and thermal stress on heat extraction are investigated. • Effects of fracture properties and deviatoric stress on THM processes are studied. • Practically useful guidelines for developing sustainable EGSs are proposed. Shear dilation of fractures has been recognized as a main mechanism of permeability enhancement by hydraulic stimulation in Enhanced Geothermal Systems (EGSs); however, the interactive role of fracture shear dilation and thermo-hydro-mechanical (THM) coupling processes in long-term heat extraction performance of EGSs remains unclear. In this study, we develop a novel THM coupling model based on the discrete fracture network approach, which can realistically capture important processes including hybrid normal-shear deformation of fractures, thermal expansion of rocks, fluid flow in both fractures and rocks, and heat convection/conduction as well as local thermal non-equilibrium effect and changes in physical parameters of fluid. We quantitatively investigate the effects of fracture network geometries and geomechanical boundary constraints on fracture shear dilatancy, and the resulting heat transfer characteristics of EGSs. Numerical results reveal that shear dilation of fractures can be triggered by transient pore pressurization and thermal stress under anisotropic in-situ stress condition, and would severely engender flow channeling as well as anisotropic heat transfer, which strongly impact the heat extraction performance. The production temperature tends to be overestimated while the thermal production rate may be underestimated, if the shear dilatational behavior is not incorporated. Increased in-situ stress ratio and injection/production pressure would magnify the effects of shear dilation, and lead to considerable enhancement of fracture permeability, eventually resulting in much earlier and quicker temperature drop. Excessive increase of fracture density and the location of injection-production wells parallel to potential channelized flow paths, formed by intersected fractures preferentially oriented for shear sliding, tend to form short circulating flow paths and reduce the heat extraction performance. Our study demonstrates the importance of considering fracture shear dilation and fully-coupled THM behaviors when evaluating the long-term performance and efficiency of heat extraction in EGSs. [ABSTRACT FROM AUTHOR]