Carbon dioxide adsorption to 40 MPa on extracted shale from Sichuan Basin, southwestern China

CO2-enhanced shale gas recovery is a promising technology for promoting shale gas production and ideally for CO2 geological sequestration through sorption. Despite this, research studies to date have solely been performed at pressures and temperatures substantially lower than those that actually occur in shale formations. The thrust of this work is to measure and model the carbon dioxide adsorption behavior for the full spectrum of in situ pressure range up to 40 MPa with an intention to provide the baseline values of carbon sequestration planning for the Longmaxi shale formation, Sichuan China. In this study, CO2 adsorption isotherms were measured up to 40 MPa for one sample for five different temperatures (40, 60, 80, 100, and 120 degrees C) by using the gravimetric method. In the experimental program, we uniquely treated the shale with supercritical CO2 to extract the extractable components to eliminate the high-pressure CO2 extraction effect on adsorption. This pre-extraction was found to significantly reduce the specific surface area of the shale. All excess adsorption trends increased rapidly to maxima followed by the decline to a plateau. The negative excess (Gibbs) adsorptions were observed for two samples at 60 degrees C and at 40 and 60 degrees C. At 60 degrees C, maxima were observed near the critical pressure of CO2 (7.38 MPa) for all samples, which is attributed to the rapid increase of CO2 density near the critical pressure. Adsorption data were fit using several models and the Dubinin-Radushkevich model modified with k times the gas density (MDR k model) was found to most closely describe the experimental results above 30 MPa. The model-fitted carbon-dioxide maximum absolute adsorption capacities (n(0)) were 0.150-0.272 mmol/g at 60 degrees C. Further, n(0) decreased with increasing temperature. Thermodynamic data such as heats of adsorption were calculated and used to evaluate CO2 adsorption properties under various conditions. The data also showed that the total organic carbon content of the shale was the primary factor determining CO2 adsorption. The results of this work provide useful knowledge regarding the adsorption characteristics of actual shale formations and are expected to assist in future gas extraction and sequestration projects.

Automated summary

This study measured and modeled the adsorption behavior of CO2 in shale formations, providing baseline values for carbon sequestration planning. The experimental program involved extracting extractable components from shale using supercritical CO2. The results showed that the total organic carbon content of the shale was the primary factor determining CO2 adsorption and provided useful knowledge for future gas extraction and sequestration projects.