| With the reducing of conventional oil and gas,shale gas is more and more important in the energy industry.There are organic matter and inorganic matter in the shale gas matrix,and their transport capacity varies huge.The content of organic matter and its micro-scale heterogeneity may significantly affect the flow diffusion capacity of shale gas in the matrix and further affect the shale gas production of horizontal wells.In this paper,heterogeneous shale gas reservoirs are the main research objects.The theoretical analysis and numerical calculation methods are used comprehensively to study the microscopic migration laws and flow mechanisms of shale gas.The main research results and innovations are as follows:(1)Based on the heterogeneous characteristics of the matrix in the shale reservoir,in the shale organic matter pores,the slip flow of free gas,the diffusion of gas in the kerogen,the adsorption and desorption of the surface of the nanopore wall and the adsorption gas are considered.In the inorganic matter,the slip flow of the free gas and the evaporation of the water film are considered.The basic governing equations of shale gas in organic and inorganic pores at nanometer scale were obtained.(2)Based on the seepage and diffusion model of shale gas in organic and inorganic pores at the nanometer scale,the effectiveness of the shale gas seepage diffusion model at nanometer scale is verified by numerical simulation and experimental data.By comparing different model parameters—initial pressure in organic pores,pressure gradient and kerogen thickness,initial humidity in inorganic pores,and water-methane diffusion coefficient,it was found that higher pressure gradients and kerogen thickness can increase the model’s mass flow rate,higher initial pressure will result in lower mass flow rate.In inorganic pores,the evaporation of water film significantly affects the mass flow rate of the model.The initial relative humidity has influence on the mass flow at the initial moment,but has no effec for steady-state mass flow rate.The water-methane diffusion coefficient has little effect on the mass flow rate under steady state,but has a great influence on the time to reach the steady state.(3)Based on the heterogeneity of shale matrix,a shale gas diffusion model in the matrix at micrometer scale was established.By comparing the parameters in the three models—the TOM content,the initial pressure,and the initial relative humidity—the higher TOM content is found to enhance the molar flow of shale gas diffusion in the matrix,and the lower initial pressure enhances shale gas.The molar flow produced by diffusion in the matrix,the lower relative humidity also enhances the molar flow rate of shale gas diffusion in the matrix.(4)The shale gas diffusion model in micro-fractures is established.The influence of TOM content on the flow velocity,the pressure of the fluid in the fracture and the contribution rate of the fracture to the mass flow are discussed.The flow velocity of the fluid in the fracture increases with the increase of the TOM content,and the diffusion contribution rate of the fracture to the matrix decreases with the increase of the TOM content.Different fracture shapes have a large impact on the mass flow rate of shale matrix diffusion models with micro-fractures.Different aspect ratios have an effect on the mass flow of the fracture model,and the longer the length of the fracture in the diffusion direction,the more obvious the effect. |
