Effect of strain on gas adsorption in tight gas carbonates: A DFT study

The geometrical properties of the reservoir rocks are usually affected by natural thermodynamics or environmental changes. These factors may modify the distribution and the amount of gas in place in the reservoir. To address these properties, we conduct density functional theory calculations to study the effect of strain on the adsorption of natural gas components, such as CH₄, CO₂, C₂H₆, and N₂ in tight-gas carbonate reservoirs, which are represented by calcite (104). The simulation results show that, regardless of the strain value (-3% to 3%), all considered gas species are physiosorbed on the surface of a carbonate reservoir with the largest the adsorption energy, (E_ads) for CO₂ molecules. In addition to their weak interaction with the surface, CH₄ molecules show no particular trend in terms of adsorption for the considered values of the applied strain. The effect of strain becomes more pronounced in the case of CO₂ and C₂H₆ molecules. For example, depending on the concentration of the molecules, the E_ads per molecule can be increased by more than 25% by applying tensile strain. These findings can be useful for determining the estimated ultimate recovery in carbonaceous tight gas reservoirs by quantifying the geomechanical effects on the adsorbed gas.