Selectivity of adsorption Isotherms based on fluid and rock types: Insights into carbon sequestration

Modeling of adsorption performance is necessary for optimum selection of adsorption mechanism, design, and capacity of the system. For reservoir development, adsorption isotherm plays a key role in determining the ultimate recovery from unconventional medium. Comparatively, isotherm models have not been described for different unconventional medium to delineate gas adsorption potential. This study aims to model three adsorption isotherm models, namely: Langmuir, Brunner-Emmett-Teller (BET) and Toth utilizing carbon dioxide (CO₂) and methane (CH₄) gases for tight sandstone, shale, and coal formations. Experimental adsorption data is used comprising tight sandstone at 190^oF, shale at 104^oF, coal formation at 131^oF. The applicability of three adsorption isotherm models is compared and discussed to study the adsorption characteristics and behavior of CO₂ and CH₄. Models are generated in the commercial software using pressure and adsorption data for CO₂ and CH₄. Adsorption isotherms are used and adsorption data fitting with the experimental adsorption data of three models at constant temperature and varying pressures is performed and their model parameters are noted. The isotherm suitability for single gas species is determined and correlation coefficient (R²) is calculated. The obtained results showed that Langmuir, BET and Toth models align well with experimental CH4/CO₂-sandstone, CH4/CO₂-shale and CH₄-coal, while all models poorly matched for CO₂-coal system. Using three isotherm models the best fit is shown by the BET model for all samples. This ability of a model to match experimental data can be attributed to fulfilling the model's assumptions, the complexity of the system being studied, the accuracy of model parameterization, and the reliability of the experimental measurements. The findings from this study provide a deeper insight into the selection of adsorption isotherms based on the behavior of unconventional reservoirs for CO₂ and CH₄ gases.