Capillary pressure-saturation relations in quartz and carbonate sands: Limitations for correlating capillary and wettability influences on air, oil, and supercritical CO2 trapping

Capillary pressure (P-c)-saturation (S-w) relations are essential for predicting equilibrium and flow of immiscible fluid pairs in soils and deeper geologic formations. In systems that are difficult to measure, behavior is often estimated based on capillary scaling of easily measured P-c-S-w relations (e.g., air-water, and oil-water), yet the reliability of such approximations needs to be examined. In this study, 17 sets of brine drainage and imbibition curves were measured with air-brine, decane-brine, and supercritical (sc) CO2-brine in homogeneous quartz and carbonate sands, using porous plate systems under ambient (0.1 MPa, 23 degrees C) and reservoir (12.0 MPa, 45 degrees C) conditions. Comparisons between these measurements showed significant differences in residual nonwetting phase saturation, S-nw,S-r. Through applying capillary scaling, changes in interfacial properties were indicated, particularly wettability. With respect to the residual trapping of the nonwetting phases, S-nwr,S- CO2>S-nwr,S- decane>S-nwr,S- air. Decane-brine and scCO(2)-brine P-c-S-w curves deviated significantly from predictions assuming hydrophilic interactions. Moreover, neither the scaled capillary behavior nor S-nw,S-r for scCO(2)-brine were well represented by decane-brine, apparently because of differences in wettability and viscosities, indicating limitations for using decane (and other organic liquids) as a surrogate fluid in studies intended to apply to geological carbon sequestration. Thus, challenges remain in applying scaling for predicting capillary trapping and multiphase displacement processes across such diverse fields as vadose zone hydrology, enhanced oil recovery, and geologic carbon sequestration.