Analysis of Gas Diffusion Mechanisms in Shale Matrices during Gas Injection and Production: Model Match and Insights

Understanding the physical mechanisms of exploitation of such sources that occurs in the shale matrix in the middle and late stages is critical in the world's energy supply. However, there is a current lack of research on the elusive relationship between mechanisms governing gas diffusion in shale matrices and the efficiency of gas production. In addition, various pores exist in a shale matrix within different diffusion mechanisms, affecting the mass transfer. In this work, we establish a microscopic model that considers the explicit interactions among various pore systems in the gas diffusion processes. The model was first verified with reported stress-dependent diffusion experimental data and then extended to the field scale. A sensitivity analysis was finally conducted to investigate the gas diffusion mechanism in gas production. The evolutions of the gas diffusion coefficient depended on the competition among the interactions, adsorption strain, and effective stress. "Production sensitive range" in which the shale gas production rate could be improved explicitly exists. Larger initial macroscopic and microscopic pore diffusivities can improve the early stage and overall gas-production efficiencies, respectively. Gas depletion is highly sensitive to extraction pressure in the middle and late production stages; as a result, adjusting the extraction pressure in a timely manner can improve the gas yield. In the deformable range, shale reservoirs with a large pore bulk modulus have better gas production rates in the middle stage. This work provides new insights into improving the gas production performance in the field.