Experimental investigation on the contribution of gas molecular diffusion to gas mass flux in micro-nano pores

ABSTRACT The mass transfer from the matrix to the fracture face is driven by both concentration and pressure differences. In this work, high-temperature high-pressure (HPHT) systems for diffusion experiments with only concentration differences were used to determine the diffusion coefficient, and flow experiments with only pressure differences were also conducted; and the magnitude of gas molecular diffusion and its contribution to production were analyzed in this study. The results show as follows: (1) Gas flow from the matrix to the fracture system is driven by the combined effect of gas molecular diffusion and seepage. The pore structure characteristics of the reservoir and the contribution of the diffusion to the yield can vary greatly. (2) In tight reservoirs with an average permeability of 0.3067 mD, the contribution of gas molecular diffusion to the total gas mass flux is only 0.08%, while in shale reservoirs, the average permeability is 0.0015 mD; the contribution of diffusion to the total gas mass flux could be as large as 1%. (3) The contribution of molecular diffusion to gas production is closely related to the pore sizes of the porous medium. The smaller the pore sizes are, the greater the contribution of molecular diffusion to gas production.