Adsorption Characteristics of CH4 and CO2 in Shale at High Pressure and Temperature

In this study, the adsorption patterns of CH4 and CO2 on Longmaxi shale under high-pressure and temperature conditions were examined. Differences in their adsorption behavior, specifically under varying pressures and temperatures, were assessed in the context of monolayer adsorption, multilayer adsorption, and micropore filling theories. A key parameter, the absolute adsorption capacity (Vabs) ratio of CO2 to CH4 (αCO2/CH4), was used to understand the preferential selection of CO2 over CH4.

Our results demonstrated that CH4's excess adsorption curves remained relatively consistent, regardless of the experimental temperature. In contrast, CO2's excess adsorption curves showed a pronounced "sharp peak" at lower temperatures (30 and 55 °C) near the critical pressure, a characteristic absent at higher temperatures (80 and 100 °C). Modeling the data revealed that both the Dubinin–Astakhov and Brunauer–Emmett–Teller (BET) models provided an excellent fit across all experimental conditions, but the Langmuir model was more suited for the curves at 80 and 100 °C, while BET was more fitting for 30 and 55 °C. In terms of adsorption affinity, CO2 demonstrated a stronger affinity than CH4, with the αCO2/CH4 value ranging from 2.47 to 12.16. This affinity heightened with increasing pressure but was moderated by higher temperatures, with minimal effects observed beyond 80 °C. The data also suggested potential applications, such as enhanced gas recovery processes in shallow reservoirs and CO2 capture and storage in deeper reservoirs. These findings provide crucial insights into the CH4 and CO2 adsorption dynamics on shale at elevated pressures and temperatures, with implications for future EGR–CCS processes.