Numerical Simulation On Shale Gas Migration With The Consideration Of Solid Deformation And Multiple Flow Mechanisms

Shale gas transportation in the micro/nanoscopic pores and fractures of shale reservoirs is a complex process of multi-physics coupling.To correctly understand the shale gas migration is of great significance for the high-efficiency exploitation of shale gas.Combining shale deformation with multiple flow mechanisms including viscous flow,Knudsen diffusion and surface sorptive diffusion,numerical simulation and parametric analysis on sh ale gas migration is conducted in this work by using the software of COMSOL Multiphysics.The focus is put on the impacts of surface sorptive diffusion and characteristic parameters of shale reservoirs on the shale gas mobility when shale deformation is taken into account.This work consists of the following parts:(1)Considering the coupling of shale deformation and multiple flow mechanisms,i.e.,stress-seepage coupling,the fluid-solid coupling equations of the single porosity model and the dual porosity model of shale reservoir are derived based on the Biot's theory for the isotropic elastic porous materials,respectively.Such equations include the equilibrium equations of shale deformation,the control equations of shale gas transportation,the equation of porosity evolution and the apparent permeability equation.(2)Based on the single porosity model of shale reservoirs,numerical simulations on the shale gas migration under the uniaxial strain conditions and the stress-constrained conditions are carried out.We also quantitativley examined the influence of surface sorptive diffusion and characteristic parameters of shale reservoirs on the intrinsic permeability,the apparent permeability and Knudsen number which represent the features of shale gas transportation.The results show that shale gas migration depends on the competitive factors such as pore pressure,desorption,Knudsen diffusion and surface sorptive diffusion.Surface sorptive diffusion and Knudsen diffusion are independent of each other and are critical for shale gas mobility.In particular,the effect of surface sorptive diffusion is more distinct with the decrease of pore pressure.Neglecting surface sorptive diffusion may give lower apparent permeability,i.e.,weaker shale gas m obility.Compared to surface sorptive diffusion and Knudsen diffusion,shale deformation can only give rise to very small change of the permeabilities and Knudsen number.Moreover,it is found that parameters such as initial porosity,intrinsic permeabilit y,pore bulk modulus and boundary pressure have important effects on Knudsen number.The apparent permeability depends on the initial porosity,intrinsic permeability,shale Young's modulus,pore bulk modulus,Langmuir pressure constants,Langmuir volume strain,boundary pressure and constrained stress while the intrinsic permeability is affected by the parameters such as shale Young's modulus,bulk modulus of shale grain,pore bulk modulus,Langmuir pressure constants,Langmuir volume strain,the ratio of pore sorptive strain to shale sorptive strain,boundary pressure and constrained stress.Furthermore,shale gas mobility under the stress-constrained conditions is generally weaker than that under the uniaxial strain conditions.The influence of the above parameters shows similar trend under two kind of conditions.(3)Based on the dual porosity models of shale reservoirs,the shale gas migration under the uniaxial strain conditions is numerically simulated and analyzed.It is concluded from the parametric analysis that shale gas transportation in the porous matrix is evidently accelerated in the presence of natural fractures.For a given pore pressure,however,the flowing capacity of shale gas in the porous matrix is hardly improved.Due to the high permeability of fractures system,surface sorptive diffusion of fractures system has negligible effect on both the flow rate and the permeability of shale gas migration in the pores.Fractures system imposes on the permeability of the pores system only when fractures system changes the deformation of the porous matrix which is mainly affected by Langmuir pressure constant,Langmuir volume strain constant and the Young's modulus of fractures system.