Analysis\nof Gas Diffusion Mechanisms in Shale Matrices\nduring Gas Injection and Production: Model Match and Insights

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