Effect of Wettability and CO2 on Asphaltene Precipitation in Shale Oil Reservoir

In order to fully study the influencing factors of asphaltene precipitation in CO2 injection into shale reservoirs, this study is the first to use the modified Peng-Robinson equation of state (PR-EOS) considering fluid-solid interaction to study the effect of wettability on asphaltene precipitation. In this work, first, the PR-EOS is modified by adding a fluid-wall interaction term to consider the wettability effect, and critical properties of fluids in nanopores are incorporated. Then, the fugacity coefficients of fluid and gas calculated by the modified PR equation are coupled into the thermodynamic model of asphaltene precipitation in the shale reservoir. Subsequently, the proposed thermodynamic model is utilized to investigate the effects of wettability and CO2 on asphaltene precipitation in a shale oil reservoir. The results reveal the following: (a) As the contact angle decreases, the asphaltene onset pressure will increase, and the amount of asphaltene precipitation will also increase. (b) The equilibrium system can be transformed from a three-phase system to a two-phase system by the wettability effect and confinement effect. (c) The wettability and CO2 have a coupling effect on asphaltene precipitation of shale oil. When the contact angle is 120 degrees, the mole fraction of CO2 is 20%, and the lower asphaltene onset pressure (L-AOP) is 622.30 psi, which is close to the L-AOP without considering wettability and CO2 (618.43 psi).