Residual Supercritical CO2 Saturation in an Oil-wet Sandstone: a Pore-scale Analysis

Abstract:

Residual supercritical CO₂ (scCO₂) in an oil-wet Bentheimer sandstone was imaged at high 3D resolution (3.4μm)³ with an x-ray micro-computed tomograph (μCT). The residual saturation measured (S_CO2,r = 12%) was significantly lower than in an analogue strongly water-wet plug (S_CO2,r = 35%). The residual CO₂ was split into many small disconnected clusters, and the cluster size distributions followed a power law correlation, similar to those reported for water-wet rock. However, the CO₂ was more frequently located in smaller pores than in the analogue water-wet case. On the μCT images we were able to measure scCO₂-water interfacial areas and capillary pressures of each CO₂ bubble in-situ. These capillary pressures (P_c) showed a distribution function which ranged from -1 MPa to +1 MPa, and peaked at P_c = 0. This variation in P_c will influence the mass transfer process (of CO₂ into water) as it changes the chemical potential; but it is clear that the interfacial areas are large and thus provide a good basis for dissolution trapping.

Overall we conclude that oil-wet storage rock has a significantly lower capillary trapping capacity, although we still observed residual CO₂ at the pore-scale.