Secure storage of carbon dioxide in underground reservoirs has been an increasingly interesting topic for the researches in the recent decade. The literature includes great works covering the idea of storing CO2 in depleted oil and gas fields or deep saline aquifer. In this study, long-term adsorption of CO2 and hydrocarbon gases is discussed as another opportunity to permanently store greenhouse gases and reduce the amount of the carbon dioxide that enters atmosphere.

The simplified local-density (SLD) theory was used for matching the experimental data and providing predictions of high-pressure supercritical adsorption isotherms of CO2 and hydrocarbon gases. The SLD model is able to capture the contributions from the fluid-fluid and fluid-solid interactions and, despite the typical assumption of uniform bulk phase density, the model plots the variable density profile of the fluids in nanopore. An extensive set of adsorption measurements available in the literature is used in this evaluation. The slit and cylindrical pore geometry are assessed and the effect of the pressure, temperature, pore size and fluid composition is also discussed in detail.

The results show that the SLD-PR model can predict absolute adsorption of the CO2 and hydrocarbon gases within the experimental uncertainty range. For heavier components (C3+), the model illustrates a thicker adsorbed wall close to the pore surface, the adsorption is enhanced while the pressure is increased to a certain point. This observation is in line with the pore-fluid interaction energy which shows a positive trend in terms of molecular size and pressure. In addition, it is concluded that the pore size lower than 2 nm show an exponentially high interest in adsorbing the gas molecules at supercritical conditions. Finally, the results recommend that the simplified local density model gives promising estimates for converting excess adsorption data to absolute adsorption data and calculating the storage capacity of the reservoir rock.

Using the outcomes of this study, millions of tons of carbon dioxide can be safely stored in carefully selected high-organic content rocks. The proposed method can also have some applications in predicting the hydrocarbon-in-place and production behavior in shale reservoirs with high organic carbon content.

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