Experimental Study of Adsorption Effects on Shale Permeability

CH4 adsorption plays an important role in the permeability evolution of unconventional gas reservoirs. In this paper, an experimental method for simultaneous measurement of rock adsorption and permeability has been developed. For this experimental method, the CH4 adsorption amounts were obtained using a volumetric method. The permeability was measured by considering gas diffusion from the reference chamber to the core sample, under the pressure difference. A set of adsorption-permeability experiments were conducted on shale samples from lower Silurian Longmaxi Formation in the Sichuan Basin. The experimental results show that both the adsorption and swelling behavior of shale can be well described by the Langmuir equation. The effects of adsorption on permeability are influenced by two factors: (1) adsorption-induced storage, which causes an incremental in apparent porosity, leading to a significant error in permeability measurement if true porosity is used; and (2) adsorption-induced swelling, which potentially closes the existing natural fractures and reduces the intrinsic permeability. The adsorption storage effects are more significant at low pressure and are influenced by the experimental configurations (ratio of chamber volume to pore volume). With the increase in adsorption-induced swelling strain, the permeability declines by a cubic function during the adsorption process. Since swelling strain is linearly proportional to the amount of CH4 adsorbed, the behaviors of permeability and the amount of adsorbing gas follow similar trends.