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

Tight reservoirs are poorly developed by water flooding due to low permeability, small porosity, and extensive development of micron and nano pores. The use of CCUS-EOR technology can realize the geological sequestration of CO₂ in the reservoir while improving the crude oil recovery efficiency. Currently, research on CO₂ sequestration mechanisms primarily focuses 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, this paper constructs a pore wall model for tight reservoirs using hydroxylated quartz cells based on molecular simulation methods, establishes the fluid components of CO₂, crude oil short chain alkanes and water, respectively, and investigates the competitive adsorption characteristics of CO₂ and crude oil short chain alkanes under water containing conditions. The results show that under water containing conditions, the adsorption isotherms of each component during the competitive adsorption of CO₂ and CH₄, CO₂ and C₂H₆ are in accordance with the class I adsorption isotherm, and the absolute adsorption amount, excess adsorption amount and heat of adsorption of CO₂ are larger than those of CH₄ and C₂H₆. The adsorption of CO₂ and crude oil short chain alkanes on the quartz wall is physical adsorption. Under simulated conditions, the number of water molecules has a significant impact on the adsorption amount of CO₂ and a relatively small impact on the adsorption amount of CH₄. An increase in the proportion of CO₂ will lead to an increase in its adsorption amount and a decrease in CH₄ adsorption amount. An increase in temperature will reduce the absolute adsorption amount of CO₂ and CH₄. An increase in pore size also increases the absolute and excess adsorption amounts of both CO₂ and CH₄. The type of wall mineral has a significant impact on the adsorption capacity of CO₂ and CH₄.