Hydro-Mechanical Coupling Simulation And Its Application To Shale Gas Reservoirs

Shale gas resource has wide distribution and large reserve.However,hydraulic fracturing should be used to achieve the economic production due to its ultra-low permeability.After hydraulic fracturing,shale gas reservoirs usually possess multiscale fractures and become more stress-sensitive.Besides,fluid flow in shale gas reservoirs is subject to some nonlinear and coupled processes,such as adsorption/desorption,molecular diffusion,Knudsen diffusion and phase change.In order to accurately predict the production of a fractured shale gas reservoir,it is necessary to consider different flow mechanisms within different regions and the influence of hydro-mechanical coupling.In this paper,the main research contents and methods are listed as follows:(1)some improved hydro-mechanical coupling models of fractured porous media(i.e.,equivalent continuum model,double porosity model and embedded discrete fracture model)are developed,the improved equivalent continuum model is appropriate for multiple isolated fractures,the improved double porosity model can accurately simulate the matrix-fracture fluid exchange during entire transient period,and the embedded discrete fracture model incorporates the effect of each fracture explicitly based on the dimensionality reduction without requiring the simulation mesh to conform to the fracture geometry;(2)an adaptive hybrid model is used to simulate the hydro-mechanical coupling in multi-scale fractured shale gas reservoirs,in which hydraulic fractures are modeled explicitly by using the embedded discrete fracture model,and numerous micro-fractures are modeled by using the continuum models,we consider phase change,adsorption/desorption,molecular diffusion,Knudsen diffusion in general multi-phase,multi-component system,and fluid and formation parameters can be arbitrary nonlinear functions of the primary variables.The displacement discontinuity at hydraulic fracture interfaces and the effect of non-linear proppants can be simulated accurately;(3)the stabilized extended finite element method is developed to avoid the displacement oscillation on hydraulic fracture interfaces,and a modified fixed-stress sequential implicit method is applied to solve the hybrid model,in which a mixed space discretization,i.e.,finite volume method for fluid flow and stabilized extended finite element method for geomechanics,is used,and then a robust numerical simulator is developed for isothermal,multi-phase,multi-component flows in deformable shale gas reservoirs,and its accuracy and advantages are demonstrated through several numerical examples;(4)the depletion production and injection gas production of shale gas reservoirs are simulated,then several key factors for gas exploitation are investigated,and results show that enhanced gas recovery by CO2 injection is feasible.In addition,the depletion production of a condensate gas is simulated to illustrate that our simulator can be applied to simulate the coupled multiphase,multi-component fluid and geomechanics in practical fractured reservoirs.In summary,an efficient multi-scale simulation theory and method for hydro-mechanical coupling in shale gas reservoirs is developed in this paper,which provides the stable theory and technology guidance for effective development of shale gas reservoirs.