A shale apparent gas diffusion model under the influence of adsorption effects to analyze microscopic gas diffusion behavior

Abstract

A considerable amount of shale gas is present in an adsorbed state, with the diffusion process, under the influence of adsorption effects, playing a crucial role in the production of shale gas in wells. In this paper, a supercritical adsorption model, including micropore-filling and monolayer adsorption, was first established based on a multiscale pore structure. Furthermore, a shale apparent gas diffusion model, influenced by a real gas effect, multiple adsorption mechanisms, effective stress, adsorption expansion, and temperature, was constructed. The new model has been verified by using isothermal adsorption tests and isobaric diffusion tests in respect of shale. Change mechanisms relating to different types of diffusion coefficients, with respect to temperature and pressure, were also examined. The findings indicated that adsorption effects had certain controlling influence on surface diffusion and total gas diffusion. Moreover, it was thought important that the impact of multiple adsorption mechanisms on the diffusion process should also be considered. The influence of the real gas effect, effective stress, adsorption deformation, and surface diffusion on shale gas diffusion was analyzed. In addition, the scale effect relating to shale gas diffusion was analyzed, from which it was concluded that the diffusion coefficient when cylindrical shale was applied was closer to the environment existing in in situ reservoirs. The results provide new insights into the behaviors of gas diffusion in shale, which is important for gas recovery and production prediction in shale reservoirs.