Measurement of the surface diffusion coefficient for adsorbed gas in the fine mesopores and micropores of shale organic matter

• Tests were conducted with shale core plugs under in-situ stress and temperature.
• The surface diffusion stage was determined according to material balance.
• The surface diffusion coefficient was 10−16 cm2/s, increasing with the growth of gas pressure.
• Measured BET surface area of the organic matter with oxidation treatments.
• Calculated the diffusion distance by referring the BET surface area and pore networks.

In shale plays, the surface diffusion is of great significance in the mass transport of adsorbed gas, and obtaining its diffusion coefficient is crucial for reservoir evaluation as well as field production forecast. This paper presented a pressure decay method to measure the surface diffusion coefficient of adsorbed gas (methane) with shale core plugs under in-situ conditions. Based on the gas storage and flow mechanisms, pore size distribution and material balance, the bulk transport (non-surface diffusion) data was excluded. The data which represented the surface diffusion in the fine mesopores and micropores of organic matter was dealt with a spherical diffusion model. Unlike previous works which estimated the diffusion distance via sample size, the diffusing distance (114.5–190.4 nm) was calculated by combining a simplified geometric model and the BET surface area of the organic matter (4.08–9.52 m2/g) which was tested by N2-adsorption after oxidation treatments. The measured surface diffusion coefficient was about 10−16 cm2/s, and increased with the growth of gas pressure, in accord with the surface diffusion theories. Ultimately, although the diffusion coefficient was small, considering the short diffusing distance and large surface area, surface diffusion could transport the adsorbed gas to big flow channels, promoting the output of adsorbed gas in shale reservoirs.