Fluid-Structure Coupling Seepage Mechanism And Theory For Fractured Network Of Shale Gas Reservoirs

Shale gas is a new type of energy that is self-generated and stored in ultra-low permeability shale reservoirs.Natural fractures,artificial induced fractures and artificial main fractures are the main seepage channels of shale gas reservoirs.It has many seepage mechanisms such as adsorption gas desorption,diffusion,slippage,stress sensitivity,darcy and high speed non-darcy law,which make production dynamics and capacity prediction extremely complicated.Therefore,it is of great significance to study the fluid-solid coupling seepage mechanism and theory of shale gas reservoir network.Based on the microscopic deformation mechanism of porous media,combined with the force analysis and experimental tests,the stress-strain mechanism of particle volume deformation,particle shape deformation and inter-particle deformation caused by the combination of net stress and fluid pressure is proposed.The equation of effective stress of particle volume,effective stress of particle shape and effective stress between particles was established.The combined effect of three effective stresses is consistent with Skempton's effective stress.The effective stress equations of total volume,matrix volume,pore volume,porosity and permeability are derived.And the non-uniform coefficient and stress concentration factor are introduced.The generalized effective stress equation is established and the effective stress equation suitable for matrix-fracture medium is obtained.The microdeformation mechanism and effective stress theory are proved by strain experiment of three kinds of porous media.Moreover,the new theory is used to explain the classical experimental results which cannot be explained by the past theories.Studies have shown that the effective stresses of the total volume,the matrix volume,the pore volume,the porosity and the permeability are different and the effective stresses under different boundary conditions and stress paths are also different.It is not possible to carry out equivalent simulation of the fluid pressure drop process by increasing confining pressure.We can only approximate the permeability effective stress coefficient by the fracture compression coefficient in the matrix-fracture system with fracture-dominated permeability.Based on the porous microscopic deformation mechanism,the exact and approximate expression of porosity compressibility coefficient,pore compressibility coefficient,pore permeability index and permeability stress sensitivity coefficient are derived.The stress sensitivity of in-situ closed fractures,self-supported fractures and fracturing agent supported fractures at different scales was tested.The exact expressions and approximate expressions of porosity compressibility,pore compressibility,porosity sensitivity exponent and permeability stress sensitivity coefficient are derived.Based on the above theoretical and experimental studies,the permeability stress sensitive equation for shale gas reservoirs is established.The results indicate that the permeability of shale gas reservoirs is dominated by fractures.It is more convenient to use fracture porosity and fracture compressibility coefficient instead of total porosity and total pore compressibility coefficient when calculating the stress sensitivity coefficient of permeability by using the pore permeability index theory.Combined with the new permeability stress sensitivity equation,Klinbenberg gas slip equation and Langmuir isotherm adsorption model,the fluid-solid coupling seepage equation of shale gas reservoir is derived.Based on this equation and the triple-medium and three-linear flow model,productivity model of volume fracturing horizontal well of shale gas reservoir are derived.The influencing factors are analyzed.Co MPared with the previous model,the productivity model of this paper does not have the optimal well length and the optimal crack spacing,which is consistent with the fact that the more reservoir fracture is broken,the horizontal section is longer,and the shale gas reservoir productivity is higher.Interstitial spacing or crack density,crack width,and original formation pressure are the main controlling factors.Since the stress-sensitive effect has the opposite effect on the slippage effect,desorption and diffusion effects,the shale production capacity may increase monotonously or have an inflection point with the decrease of the bottomhole flow pressure.