Effect of CO2 Supercritical State on Its Performance in Porous Rock

CO2 has been used for enhanced oil/gas recovery, the production of geothermal power, and the production of water from CO2 storage in saline formations. The processes are a displacement in porous rocks. The dynamics of the displacement is controlled by CO2 phase (gas/liquid/supercritical), the wetting behaviour of CO2-fluid in porous rocks, and pore structure. This paper will report our recent findings on the effect of CO2 phase, particularly its supercritical state, on it pore wetting behaviour and its displacement in sand stone core samples. CO2 wetting behaviour in a pore is very different from its behaviour on a flat surface in an open space, in which CO2 pore contact angle is much higher than that measured from a flat surface. The CO2 phase significantly affects the CO2-fluid contact angle in an oil-wet pore. Supercritical CO2-fluid contact angles are larger than gas CO2-fluid contact angles, but are smaller than liquid CO2-fluid contact angles. Salinity has a significant effect on the CO2-brine-glass pore contact angle in a water-wet pore, θbrine＞θwater.
Our investigation on gas CO2-water, liquid CO2-water and supercritical CO2-water displacements in sand stone core samples indicates that CO2 phase significantly affect the capillary pressure-saturation curve, water production behavior and relative permeability. For the gas CO2-water system, the cumulative volume of water production is significantly smaller than the cumulative volume of CO2 injection. Liquid CO2-water drainage gives a linear relationship between water production volume and time. The cumulative volume of water production almost equals the cumulative volume of CO2 injection. For the supercritical CO2-water system, an irregular water production curve is obtained.