The environmental concerns to reduce carbon dioxide emissions have led to huge advancements in carbon capture and sequestration in different geological formations in the past two decades. Due to the high abundance of shale basins around the world, in recent years, CO2 sequestration potential in these formations has become an attractive option. However, most of the shale formations are considered immature to become shale oil or shale gas. These types of shale usually contain high organic matter (kerogen) generally expressed as total organic carbon (TOC) compared to mature shale reservoirs. The poor petrophysical properties of shale rocks in terms of porosity and permeability the adsorption capacity became a very significant factor that controls the amount of CO2 these formations can store. Previous experimental studies showed that high TOC indicates high adsorption capacity. Here adsorption of CO2, CH4 and 10%CO2/90%CH4, adsorption isotherms and adsorption thermodynamics were studied on intact three shales SH1, SH2 (immature) and SH3 (mature) from Qusaiba field in Southern Saudi Arabia at 50, 100 and 150°C. We attested that as CO2 amount increased, the adsorption capacity of the three shales increased accordingly justifying the good potential for using CO2 for enhanced improved methane recovery. Furthermore, the three shales exhibited diverse responses to the studied temperatures. SH1 and SH3 showed an endothermic trend from 50 to 100°C with a huge increase in the adsorption followed by an exothermic significant decline at 150°C. SH2 had no CH4 adsorption at 50°C and exhibited endothermic adsorption behavior; increasing the adsorption at elevated temperatures. These variations in adsorption comportment resulted from thermally induced alterations in the clay minerals crystallinity confirmed by x-ray diffraction at 50 and 150°C. Thermodynamics analysis of Gibbs free energy, adsorption enthalpy and entropy concluded that all three shales has high affinity to CO2 and lower adsorption enthalpy and entropy compared to CH4. That because of the strong electrostatic attraction between partially negative CO2 molecules and shale clay minerals positive surface cations. Contrary to many studies in the literature, adsorption isotherms fitting demonstrated that adsorption of CO2, CH4 and 10%CO2/90%CH4 on the three shales is multi-layered and best modeled by Freundlich and BET isotherms rather than Langmuir mono- layer isotherm.

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