Study On Percolation Mechanism Coupling Adsorbed-free Phases And Productivity Evaluation For Deep Shale Gas

The Wufeng-Longmaxi shale gas reservoirs with buried depths between 3500 m and4500 m in Sichuan Basin have great potential for development.Due to the extremely high temperature and pressure of the original reservoirs,however,the occurrence of shale gas remains to be clarified,and the complex flow of shale gas in micro-and nano-pores is not clearly understood.Therefore,it is necessary to clarify the behavior of shale gas adsorption under the condition of deep reservoirs,establish a percolation model suitable for deep shale gas,and establish a reasonable model for productivity evaluation.In this paper,molecular models of illite,quartz and organic matter pores are established,and molecular simulation techniques are used to reveal the adsorption behavior of shale gas in the pores under the condition of deep reservoirs.Based on the adsorption thermodynamics,a theoretical model for monolayer/multilayers adsorption of deep shale gas is established,with its applicability analyzed by fitting the adsorption isotherms of shale gas.The results show that the shale gas in the mesopore（size 2～50 nm）forms strong and weak adsorbed layers,which are dominated by the strong layers.The size of the mesopore has little effect on the adsorbed gas density,but the adsorbed gas densities in the micropores（size<2 nm）are obviously different from those in mesopores.The maximum adsorbing capacity of the pore surface is mainly affected by the surface roughness and surface cavity.Under the condition of deep reservoirs,the theoretical model for double-layer adsorption cannot characterize the adsorbed gas density reasonably,while the Langmuir equation based on single-layer adsorption is more suitable for the adsorption of deep shale gas.Based on the Navier-Stokes equation and theories of gas adsorption and slippage,models coupling the flows of adsorbed and free gas in pores are established,which explain the results for gas flow in pores from molecular simulation.Furthermore,the capillary bundle pore model is applied to establish the model for shale apparent permeability coupling the percolation of adsorbed and free gas.The model is verified by the measurements of the gas permeabilities of shales under the deep temperature and different gas pressures.Then the effects of the gas pressure,pore structure,property of pore surface,gas adsorption and effective stress on apparent permeability are analyzed,as well as the fluidity of free and adsorbed gas.The results show that there is a linear relationship between the apparent permeability coupling the flow of adsorbed and free gas and the reciprocal of the free gas density;for low pressure,small tangential momentum adjustment coefficient（TMAC）of the pore wall and small pore size,the contribution of adsorbed gas to the flux increases;ignoring gas adsorption overestimates the apparent permeability;with the increase of TMAC and pore size,the apparent permeability tends to the Klinkenberg permeability,and the influence of gas adsorption on percolation decreases;considering the effective stress,the apparent permeability with mainly small pores increases with the decreasing pore pressure for pore pressure less than10～20 MPa due to the enhancement of slippage effect.Based on the apparent permeability model,the numerical model for unsteady gas production is established.Through the proposed model and simulative experiment of gas production from fractured shale under constant pressure,the characteristics of deep shale gas production are analyzed.Based on the numerically simulated results of gas production from cubic shale matrix,the analytical model for the transfer of deep shale gas is optimized.The results show that the natural depletion of deep shale gas generally has a short period with high production and a long period with low production;the shale permeability during the natural depletion is as low as 10^-10 m D;the contribution of adsorbed gas to the production increases obviously under low pressure.By coupling the gas transfer from shale matrix,percolation in fracture network and percolation in main fracture,and considering the changing permeability caused by the changing effective stress,the productivity model for the fractured horizontal well is established with the production supplied by the fractured zone.Then,the effects of bottomhole pressure,geological factors and fracturing factors on the productivity of deep shale are analyzed.The results show that the small width of fractures in network or large fracture compressibility cause the serious close of fractures under low bottomhole pressure,further results in the decreasing production with the increasing drawdown pressure.Under high bottomhole pressure in early stage,the increasing matrix permeability,density of fractures in network and width of the main fracture significantly increase the productivity;the decreasing distance between main fractures does not significantly reduce the productivity;the initial permeability of the main fracture hardly affects the productivity.This paper clarifies the characteristics of deep shale gas adsorption in different minerals and organic matter pores,reveals the nature of shale gas percolation in micro-and nano-pores,and establishes the productivity evaluation model for deep fractured horizontal wells.The results provide theoretical reference for the effective development of deep shale gas.