Confinement effect on CO2 and CH4 permeability in shale

Gas flow in shales follows a number of physical mechanisms that include Knudsen diffusion, Darcy flow, and adsorption in the matrix and micro pores. The aim of the study is to resolve the interplay of gas transport in these media at increased effective stress as well as net pore pressure. In this research, we investigated the nature of gas transport in the matrix of shale by sending He, CH4 and CO2 gases through a transient upstream pressure pulse decay instrument. A series of experiments were conducted at constant pore pressures and a gradually increasing confining pressure. The same study was done in three different scenarios, injecting He, CO2 and CH4. At a constant pore pressure, gas permeability appears to decrease with an increasing confining pressure and effective stress. With increasing effective stress, the slip factor also decreases along with the permeability. The decrease in slip could be attributed to prestressing, that is likely to create new fractures. Among the three purged gases, permeability of shale to CH4 is the highest, and that to CO2 is the lowest owing to its high adsorption. Higher permeability of CH4 against He, could be attributed to the dual transport mechanism.