Carbon dioxide EGR and sequestration in mature and immature shale: Adsorption study

Recently, CO2 sequestration in different shale formations has become an attractive option owing to the abundance of shale basins. However, these formations have different maturity levels and contain a variety of minerals such as carbonates and clays. The amount of CO2 in shale formations is related to the adsorption capacity of these rocks. In this work, adsorption of CH4 and CO2 on three different shales (mature and immature) is studied at different temperatures (50 °C–150 °C). Increasing the percentage of CO2 in the gas mixture raised the adsorption capacity. The different shale samples behave differently at different temperatures. Two of the investigated shale samples (low and moderate total organic carbon (TOC)) showed an endothermic response from 50 °C to 100 °C with a tremendous increase in the adsorption capacity followed by an exothermic response and significant reduction in the capacity at 150 °C. The third sample, which had the highest TOC, had no CH4 adsorption at 50 °C but showed an endothermic response at high temperature with a significant increase in CH4 adsorption. This temperature dependent adsorption behavior is linked to thermally induced alterations in the crystallinity of clay minerals and the organic material. The thermodynamic analysis reveals that the investigated shales have a high affinity to CO2, while adsorption entropy and enthalpy were less compared to CH4. The adsorption isotherms reveal that adsorption of CH4 and CO2 on the shale best modeled by Freundlich isotherm due to surface heterogeneity of the shale and by BET isotherms due to pore filling at high pressure values.