A study examining gas diffusion mechanisms in shale reservoirs under stress: Involving experimental observations and model development

Gas diffusion in the matrix system is the primary factor limiting shale gas productivity during later-stage extraction. Accurately evaluating the diffusion coefficient is essential for productivity prediction and extraction optimization. However, current understanding of gas diffusion mechanisms under varying stress conditions remains inadequate, and few diffusion models can accurately capture the fractal structure of shale reservoirs. This study determined the diffusion coefficients of gases with different adsorption capacities in cylindrical shale samples under varying stress conditions. The pore fractal structure was characterized using scanning electron microscopy and low-temperature nitrogen adsorption. Based on these characterizations, a new effective gas diffusion model was developed using fractal theory, integrating stress sensitivity, adsorption effects, adsorption layer thickness, and multiple diffusion mechanisms. Experimental results showed a complex, nonlinear relationship between gas diffusion coefficients and stress. The diffusion coefficient of non-adsorbing gas (He) was significantly higher than that of adsorbing gas (CO 2 ). The proposed model agreed well with experimental data. Further analysis revealed that adsorbing gas diffusion was dominated by surface diffusion, and its diffusion coefficient decreased due to pore constriction caused by adsorption-induced swelling and thickening of the adsorption layer, highlighting the significant roles of adsorption and surface diffusion. Furthermore, maximum pore size and fractal dimension significantly influenced diffusion coefficients, confirming the critical control of pore heterogeneity on gas diffusion.