Numerical Simulation Of Shale Gas Transport Considering Anisotropy Of Reservoir Solid Deformation And Multiple Flow Mechanisms

Shale reservoirs are unconventional oil-gas reservoirs.Migration capacity and recovery efficiency of shale gas are affected by reservoir deformation,multiple flow mechanisms and their coupling effect.Experimental research has shown that mechanical properties of shale reservoirs,such as the elastic constants,the permeability and the surface adsorption diffusion coefficients,are remarkably anisotropic.Consequently,reservoir deformation and gas flow are also featured with anisotropy.However,current research on shale gas transport based on apparent permeability models often neglects or partially considers the anisotropy of reservoir deformation and flow mechanisms.Migration ability and production capacity of shale gas in the reservoirs can therefore not be accurately evaluated.Using theoretical modelling and numerical simulation methods and considering the anisotropy of both shale reservoir deformation and multiple gas flow mechanisms,transport process and production capacity of shale gas were systematically investigated in this paper.The impact of anisotropic features is quantitatively examined so as to provide theoretical references to high-efficient exploration of shale gas.The major research contents and outcomes of this work include:(1)Based on the physical model of Biot linear elastic porous media,a set of fluid-solid coupling control equations of shale gas transport which take into account for the effects of anisotropic mechanical properties of shale reservoirs on solid deformation,viscous flow,dynamic effects and surface adsorptive diffusion are established,including porosity model,apparent permeability model,balance equations of reservoir deformation and control equations of gas flow.Among which,anisotropy of the elastic constants and the adsorption strain is accounted for the reservoir deformation,and anisotropy of the orientation distribution of pores,the initial intrinsic permeability,Knudsen number and the initial surface adsorption diffusion coefficients are considered in multiple flow mechanisms.(2)Based on the anisotropic transport model of shale gas,numerical simulation was conducted on the shale gas migration process in two anisotropic reservoirs and two isotropic reservoirs under the stress constrained boundary conditions.The comprehensive effects of reservoir anisotropy was analyzed through comparing the variations of porosity,intrinsic permeability,apparent permeability,pore pressure gradient and flow flux with the pore pressure and the migration time in four different reservoirs.Research results show that reservoir anisotropy does not change the mechanisms involved in reservoir deformation and gas migration,but has significant impact on quantitative evaluation of migration capacity and production ability of shale gas.Not only is shale gas migration in anisotropic reservoirs is obviously different from that in isotropic reservoirs,but also remarkable discrepancy can be found in shale gas migration for different anisotropic reservoirs.(3)For the selected anisotropic shale reservoirs,parametric analysis on the mechanical properties which affect the anisotropy of reservoir deformation and gas flow was systematically carried out and the influence of individual mechanical property on the reservoir porosity or shale gas production capacity was revealed.Research results show that the anisotropy of the elastic modulus,Poisson’s ratio and the adsorption strain can have significant impact on the porosity variation during the migration process while the effect of shear modulus anisotropy is negligible.Moreover,when the anisotropy of elastic moduli,Poisson’s ratio and adsorption strain coefficients remains unchanged,reservoir porosity increases with the overall increase of the above elastic constants.Anisotropy of the intrinsic permeability and the surface adsorption diffusion which depends on the initial intrinsic permeability and the initial surface adsorption diffusion coefficients as well as their orientation distribution functions can greatly change the start time of shale gas migration and the gas flux in the reservoirs.Increasing the intrinsic permeability and the surface adsorption diffusion coefficients can promote the gas flux when anisotropy of the intrinsic permeability and the surface adsorption diffusion coefficients remains constant.Flow flux and shale gas migration start time under anisotropy of dynamic effects are obviously different from those under isotropy of dynamic effects when the intrinsic permeability is low.Such difference diminishes with the increase of the intrinsic permeability or the decrease of dynamic effect.The effect of anisotropy of dynamic effects can be neglected in the cases of large intrinsic permeability.