Study On Flow Mechanism And Mathematical Model Of Shale Gas Considering Multiple Transport Mechanisms

Shale gas is an important unconventional natural gas resource.However,shale gas reservoirs have extremely low porosity and permeability.The adsorbed gas and free gas are coexistence in shale gas reservoirs.Additionally,the development method of shale gas is the horizontal well with multiple stage fracturing.These characteristics of shale gas are the vital problems that need to be solved urgently in exploitation.There are several methods explaining the occurrence and transport characteristics of shale gas in this research,such as intermolecular interaction potential simulation,simplified local density theory(SLD),and numerical simulation.Initially,methane has a strong storage capacity in the shale reservoir.The adsorptivity of the organic matter in shale is compared by analyzing the experimental data and simulating the intermolecular interaction potential.The adsorption layers for methane in shale are exactly calculated.In the second part,this paper contrasts several high-pressure isothermal adsorption models and improve the methane property calculation method,especially the adsorption function.The SLD-RK method is established to calculate the physical property of supercritical methane in the micro-nano pore by revising the RK and SLD equation of state.The Langmuir-Freundlich(LF)and Dubinin-Radushkevich(DR)adsorption equations are combined to establish the high-pressure isothermal adsorption and desorption model.Moreover,the gas microscopic transport model in the micro-nano pore of the matrix is updated to explore the macro effect of multiple micro transport mechanism of shale gas by considering the variation of gas and reservoir physical properties.Based on the aforementioned models,the new shale gas flow model is proposed by adding the natural and hydraulic fracture influencing factors.The embedded discrete fracture is adopted into the single medium,either.What’s more,the impact of multiple transport mechanisms on shale gas production capacity is concluded comprehensively.The results illustrate that:(1)The order of the adsorption capacity for methane in different shale organic matter structures is surface<microfracture<micropore.Meanwhile,the adsorption layer of methane is related to the shale pore size.The multilayer adsorption occurs in microfractures with a width of 1.44 nm～1.59 nm and micropores with an inner diameter of 1.49 nm～1.83 nm,whereas the surface structure is the monolayer adsorption.(2)The coexistence of monolayer adsorption and multilayer adsorption for methane adsorption mechanisms are characterized by combining LF and DR functions.The new high-pressure isothermal methane adsorption and desorption model combine the advantage of SLD-RK method,which can fit experimental data accurately.The excess adsorption experimental data of 18 shale samples R2 is 0.9624,and the R2 of desorption hysteresis experimental data in 5 groups is 0.9494.(3)The contribution order of transport mechanism to productivity in micronano pores of the matrix is shown as follows:slippage flow>surface diffusion>Knudsen diffusion.(4)With the decrease of reservoir pressure,the matrix permeability reduces gradually,and the contribution of each microscopic transport mechanism to productivity changes dynamically.In the vicinity of the horizontal wellbore,the variation of matrix permeability and transport mechanisms is maximum.The extent of variation near the hydraulic fracture is less than that near the horizontal wellbore.With the distance from wellbore and fractures increasing,the range of variation decreases gradually.The above results in this research further enhance the shale gas flow mechanism and model,which can provide a more reasonable theoretical basis for the development of the shale gas reservoir.