The influence of CO2 and CH4 mixture on water wettability in organic rich shale nanopore

CO2 has been considered as an effective fluid to replace CH4 in shale rock. Wetting behavior of water in CO2 / CH4 mixtures in organic nanopores is a key parameter influencing CO2 based enhanced gas recovery processes in shale. However, there is lack of fundamental understanding of water wetting in such systems. We perform Molecular Dynamics (MD) simulations of nanoscopic liquid water drops on a graphite substrate mimicking an organic-rich shale pore in the presence of CH4/CO2 mixtures at temperatures in the range 300 K to 400 K. The equilibrium contact angle of the water droplets on graphite is a pronounced function of CH4 as well as CO2 pressure with the water droplet lifting-off, that is reaching a 180o contact angle, at a threshold pressure of 12 MPa for CO2 and 78 MPa for CH4. The contact angles recovered from simulations match well with
experimental literature via the modified Young’s equation. The line tension in our studied systems does not show a specific dependence on temperature. As compared to methane, CO2 exhibits a stronger interaction with organic-rich surfaces, which has strong impact on wettability. For CH4/CO2 mixtures, this translates in an approximately linear increase of the contact angle with the
CO2 mole fraction.