Research On Seepage-stress Coupling Mechanism Of Shale Gas Reservoir Based On Dual Media Model

China is rich in shale gas resources and has huge development potential.The shale gas in the actual reservoir exists in the organic shale and its interlayer in the adsorption and free state and is in the state of triaxial stress.The development process of shale gas is actually a dynamic process of coupled action of shale gas transport flow and matrix deformation.Changes in reservoir pressure and in-situ stress will cause changes in shale reservoir permeability.Therefore,revealing the percolation-stress coupling mechanism of shale rock mass under stratigraphic conditions and establishing the evolution equation of shale gas reservoir permeability will be more in line with development practice,and have important theoretical significance and application value for shale gas reservoir productivity evaluation and development program design.Based on the analysis of the reservoir mechanism and flow mechanism of shale gas reservoir,the pressure pulse attenuation method was used to carry out the pressure sensitive permeability experiment of shale gas reservoir.Based on the spring system model,the double pore permeability model of shale gas is derived under the influence of various factors,such as adsorption layer,diffision,matrix deformation and slippage effect,etc.,and the effects of effective stress,pore pressure,stress sensitivity coefficient,pore radius,initial fracture opening,adsorption layer thickness and slip coefficient on permeability are discussed.Based on the proposed permeability model,the fluid-solid coupling mathematical model of shale reservoir considering the interaction between seepage field and stress field is established.The shale core numerical simulation is carried out,and the relationship between the shale pressure and stress with time when the permeability does not change and when the permeability changes are taken into account is analyzed.Finally,numerical simulation of shale gas dual media horizontal well production is carried out,and the effects of matrix initial porosity,initial fracture opening,fracture length and fracture spacing on average fracture pressure and daily gas volume of shale reservoir are studied.The conclusions are as follows:(1)The results of shale fluid-solid coupling test show that the core permeability is obviously higher when the pore pressure is about 2MPa,and the maximum permeability is 4.603×10-5mD.When the confining pressure is between 10MPa-15MPa,the permeability decreases rapidly.When the confining pressure exceeds 15MPa,the permeability decreases slowly.Core permeability decreases with the increase of confining pressure,pore pressure and effective stress.The lower the reservoir pressure,the stronger the permeability stress sensitivity of shale.The larger the pore pressure,the smaller the effective stress range with strong stress sensitivity.(2)The mathematical model of shale gas dual medium permeability based on the spring system model has clear physical significance and can consider the influence of adsorption,diffusion,slippage effect,stress sensitivity and other factors.Numerical experiments based on this model show that:the influence of stress sensitivity coefficient,pore radius,crack opening and slip coefficient under stress free condition on permeability is significant;the permeability decreases with the increase of the thickness of the adsorption layer and the stress sensitivity coefficient,and increases with the increase of the pore radius,crack opening and slip coefficient under the stress free condition.(3)Numerical simulation results of small shale cores show that:the average stress of shale matrix decreases with time.The average stress of fracture zone increases with time.When the permeability is considered,the average stress in the fracture zone is lower and rises more slowly than when the permeability is constant.The average core pressure decreases with time.The pressure changes in the fracture area are significant.When the permeability changes are taken into account,the average pressure in the fracture zone drops more slowly than when the permeability changes,and the average pressure in the core is higher than when the permeability does not change.(4)The shale gas development simulation of segmented fracturing horizontal well shows that:shale gas horizontal well has a high production capacity at the initial stage of production,with a daily output of up to 10,000 cubic meters.After 200 days,the horizontal well tends to be stable and the daily output is 1,000 cubic meters.The average fracture pressure decreases rapidly in the early stage of production.Pressure propagation is slow in reservoir matrix area.The average fracture pressure of shale gas reservoirs decreases with the increase of the width of fracture region and the length of fracture system,and increases with the increase of the spacing between fracture regions.The daily gas production increases with the increase of the width of fracture zone and the length of fracture system,up to 200,000 cubic meters.The daily gas production decreases with the increase of fracture zone spacing,and when the crack spacing is 5m,the maximum is about 200,000 cubic meters.However,the initial porosity of matrix has little influence on the average fracture pressure and daily gas production of shale reservoirs,and the difference in daily production is the largest in the middle production period,which is about 1000 cubic meters.