Impact of clays on CO₂ adsorption and enhanced gas recovery in sandstone reservoirs

Carbon dioxide enhanced gas recovery (EGR) is a promising technique to sequester CO₂ and boost natural gas recovery from conventional depleted and unconventional tight gas reservoirs. Clay minerals are usually present in sandstone reservoirs and their influence on the efficiency of CO₂-EGR is yet to be examined. In this study, the impact of clays on CO₂ adsorption was evaluated for different sandstone rocks with various amounts and types of clays in the temperature range from 50–100 °C and pressures up 20 bars. The results showed that the adsorption of CO₂ on sandstone rocks depends on the clay type, amount, and distribution. Clay-rich sandstone rocks, which have swellable clays such as illite, showed the highest CO₂ uptake at a temperature of 50 °C and a pressure of 20 bars with total CO₂ uptake of 4.6 and 2.6 mg/g for Kentucky and Scioto rocks, respectively. In contrast, sandstone samples with low clay content and a considerable percentage of carbonates showed CO₂ uptake just above 1.5 mg/g for Bandera sandstone and 1.1 mg/ gm for Berea sandstone at similar conditions. Moreover, raising the temperature to 75 °C decreased the CO₂ uptake on sandstones. However, the alteration of clays crystallinity at a temperature of 100 °C improves the CO₂ adsorption. Adsorption isotherm analysis revealed that at the CO₂ adsorption is monolayer at low temperature (50 °C) and pressure of 20 bars; whereas multilayer adsorption at 75 and 100 °C is predicted by Freundlich isotherm model. The thermodynamic analysis illustrated that the adsorption of CO₂ on sandstone rocks is physisorption and exothermic on Kentucky, Scioto, and Berea sandstones and endothermic on Bandera sandstone. Core flooding experiments at 100 °C revealed the potential of CO₂-EGR for clay-rich sandstone and highlighted the role of clays distribution.