High-pressure adsorption of gases on shales: Measurements and modeling

Abstract

Abstract In recent years, shales have attracted increased attention as a potential source of natural gas. In the U.S., recovery of natural gas from shales is expected to grow significantly in the coming years. Shale gas reservoirs may also offer the potential for CO2 sequestration. Since a large proportion of gas in shale reservoirs is in an adsorbed state, knowledge of gas adsorption behavior on shales is required for design of optimal gas recovery and sequestration processes. Only limited measurements have been reported in the literature for high-pressure gas adsorption on shales. In this work, adsorption isotherms of methane, nitrogen and CO2 were measured on a New Albany shale sample from the Illinois basin. As-received samples were used for measurements at 328.2 K and pressures to 12.4 MPa. At about 7 MPa pressure, the excess adsorptions on New Albany shale for N2, CH4 and CO2 are in the ratio 1:3.2:9.3. This N2:CH4 ratio is similar to that for gas adsorption on coals and activated carbons, while the adsorption ratios of CO2:CH4 and CO2:N2 are much higher than those typically seen for coals. Further, the amounts adsorbed on this shale are 10 to 30 times lower than adsorption on coals of varying rank. The low levels of total organic carbon content (5.5%) and higher ash content of the shale (90%) play a role in reducing the gas adsorption capacity of the shale compared to coal. The newly acquired data yielded average experimental uncertainties of about 0.0095, 0.0081 and 0.024 mmol/g for the methane, nitrogen and CO2 adsorption isotherms, respectively. At a pressure of about 7 MPa, the excess adsorption of methane, nitrogen and CO2 are 0.0374, 0.0116 and 0.1085 mmol/g, respectively. A simplified local-density (SLD) model was applied to describe the adsorption data of this study and was found to represent these data within the expected experimental uncertainties. Further, the model was applied to data for methane and CO2 on 34 shale samples reported in the literature. These data were represented with average absolute percentage deviations (%AAD) of about 4 and 9 for methane and CO2, respectively. In these predictions, the SLD model required one gas-specific parameter plus two adsorbent-specific parameters that are independent of the adsorbing gas species.