Molecular simulation of competitive adsorption of CO₂ and short-chain alkanes under water containing conditions in tight oil reservoirs

Tight reservoirs have low permeability, small porosity, and pervasive micro-nano pores, so water flooding has poor development effects. The use of CCUS-EOR (Carbon Capture, Utilization, and Storage-Enhanced Oil Recovery) technology can realize the geological sequestration of CO₂ in the reservoir while improving the crude oil recovery efficiency. Currently, research on CO₂ sequestration mechanisms focuses mainly on saline aquifer sequestration, with less emphasis on adsorption and sequestration during CO₂ flooding in tight oil reservoirs under water containing conditions. To address the above problems, we established a pore wall model for tight reservoirs using hydroxylated quartz cells based on molecular simulation methods, in which the fluid component models of CO₂, crude oil short-chain alkanes, and water were contained; and investigated the competitive adsorption characteristics of CO₂ and crude oil short-chain alkanes under water containing conditions. Results show that under water containing conditions, the adsorption isotherms of each component during the competitive adsorption of CO₂ and CH₄, CO₂ and C₂H₆ were in accordance with the class I adsorption isotherm, and the absolute adsorption amount, excess adsorption amount, and heat adsorption of CO₂ were larger than those of CH₄ and C₂H₆. The adsorption of CO₂ and crude oil short-chain alkanes on the quartz wall was physical. Under simulation conditions, the number of water molecules had a significant impact on the adsorption amount of CO₂ and a relatively small impact on the adsorption amount of CH₄. The increase in the proportion of CO₂ increased CO₂ adsorption amount but decreased CH₄ adsorption amount. The increase of temperature reduced the absolute adsorption amount of CO₂ and CH₄. The increase of pore size increased the absolute and excess adsorption amounts of both CO₂ and CH₄. The type of wall mineral showed a significant impact on the adsorption capacity of CO₂ and CH₄.