A Coupled Diffusion Model for Gas Transport in Tight Unconventional Reservoirs

Abstract

Abstract Tight unconventional reservoirs are being developed in the global natural gas market. Gas production from unconventional tight reservoirs such as coalbeds or shales is a multi-physics process where diffusion and sorption are coupled. Gas diffusion, the complex flow mechanism in the tight unconventional reservoirs, is crucial during gas production, of which the mechanisms have not been well understood. Gas diffusion in rocks is a complex process, especially when gas is adsorbable to rock surfaces, where surface diffusion cannot be ignored. This paper aims to analyze the sensitivity of rock and gas parameters during gas diffusion and provide insights into the physics of gas transport in tight rocks. We develop a dual-diffusivity model for tight rocks at the continuum scale. We study the impact of two types of diffusions under different conditions and include the adsorption-induced swelling within this model. Also, the free and adsorbed gas concentration fields are simulated in a coal core. The simulation results show that compared with surface diffusion, free gas diffusion is more likely to dominate the diffusion process at relatively low pressure, high temperature and high porosity. In addition, the results indicate the importance of the level of coal swelling on the diffusion process in coal.