Study On The Seepage-stress Coupling Model Of Hydraulic Fracturing SRV Region In Shale Gas Reservoir

The exploration and development of shale gas is related to the country's energy security and is of great strategic significance.Hydraulic fracturing is the main technology of shale gas mining.The size and transformation effect of SRV(Stimulated Reservoir Volume)caused by hydraulic fracturing is one of the core scientific problems in the development of shale gas hydraulic fracturing technology.Based on a large number of literature investigations,the effects of micro-cracks on shale permeability are studied by theoretical analysis,experimental research and numerical simulation.The mathematical model of dual-media seepage-stress coupling in SRV region is established.Numerical simulations reveal the seepage-stress coupling effects during shale reservoir development.The main conclusions of this paper are as follows:(1)The fluid-solid coupling seepage experiments of intact rock samples and micro-fractured rock samples were carried out,and the variation law of permeability with confining pressure and pore pressure was obtained.The experimental results show that the permeability of fractured rock samples is five orders of magnitude larger than that of intact rock samples.The permeability decreases as the effective pressure increases,and the rate of decrease gradually decreases.(2)According to the response surface analysis of the influencing factors of shale permeability,it is found that the shale permeability is more sensitive to confining pressure than the pore pressure.When the Terzaghi effective stress formula is used to analyze the relationship between permeability and effective stress,the permeability is underestimated.The stress sensitivity of intact rock samples is extremely strong,and the stress sensitivity of fractured rock samples is strong.(3)Based on the modified matchstick model,according to the Carman-Kozeney equation,the mathematical expression of the porosity and anisotropy permeability of the SRV region with the effective stress under the uniaxial strain condition is established,combined with the seepage field and the stress field.The governing equations are used to establish a seepage-stress coupled equations for shale reservoirs.(4)According to the actual production data and laboratory experimental data,the permeability model of SRV region is fitted,and the fitting degree is good.Comparing the model with the classical S-D model,it is found that the model has better applicability when the effective stress is small.(5)Using COMSOL numerical simulation software,the single well pressure decay process in shale gas mining process was simulated.The simulation results show that the pressure of the fractured reservoir decreases rapidly at the initial stage of mining;after that,the pressure gradient and gas production rate gradually decrease;the main crack determines the expansion direction of the SRV region during the fracturing process,and the SRV region size varies with the number of cracks.The angle between the secondary crack and the main crack increases,and the smaller the angle,the greater the pressure drop of the reservoir with the smaller angle between the secondary crack and the main crack,and the faster the falling speed;As the angle increases,the pressure drop of the reservoir on both sides of the secondary crack tends to be symmetrical.For randomly distributed cracks,the more intersecting and intersecting points,the greater the range of reservoir pressure drop.