Effect of temperature and pore size on the hydraulic properties and flow of a hydrocarbon oil in the subsurface

Capillary pressure relationships in a porous medium determine the distribution of immiscible fluids under static conditions and can largely influence the movement of the fluids when the system is not at equilibrium. Theory predicts that for a given porous medium, the effect of different fluid properties or changes in temperature on capillary pressure are due to changes in interfacial tension and contact angle of the system. The capillary pressure-saturation curves measured here for hydrocarbon oil-water do not have a constant capillary pressure ratio with saturation when compared to the water-air system, and changes in ratio were found when comparing the water-air curves measured at different temperatures. Thus, the scaling theory based on interfacial tensions and contact angles does not adequately account for differences in capillary pressures due to different fluid pairs or temperatures. Also, the curves show the residual wetting and nonwetting phase saturations are greatly affected by temperature and sometimes by the fluid pair, which is not accounted for in the scaling theory and cannot be predicted. When the capillary pressure-saturation relationship is extended to the calculation of relative permeabilities and the prediction of fluid flow in the subsurface, the differences in measured and scaled capillary pressure curves lead to differences in the predicted flow rates and saturations of the immiscible fluids in the subsurface. Thus, care must be used when applying capillary pressure-saturation data from one fluid system to that of another, or when applying it to different conditions.