Tortuosity of kerogen pore structure to gas diffusion at molecular- and nano-scales: A molecular dynamics simulation

Abstract

The tortuosity of kerogen pore structures is a vital parameter in the quantification of the gas diffusion ability and production of shale reservoirs. In this study, we perform a molecular dynamics simulation on the gas diffusion in the molecular- and nano-scale pores of shale kerogen to evaluate the tortuosity of the kerogen pore structure to gas diffusion. A novel diffusion model is proposed to evaluate the tortuosity by considering gas adsorption affinity, transportation regime, and probe gas atomic size. The results indicate that the tortuosity of the kerogen pore structure to gas diffusion is not a constant; instead, it highly depends on the probe gas atomic size and gas adsorption ability. The diffusive tortuosity is overestimated when the effect of gas adsorption ability is ignored. We find that the electrical tortuosity is lower than the diffusive tortuosity. This confirms that the tortuosity is underestimated when considering a purely geometric effect.