Gas/Oil Capillary Pressure: Measurements at Reservoir Conditions and Effect on Gas-Gravity Drainage

Abstract

ABSTRACT Predictions of oil recovery by immiscible gas injection require accurate laboratory determination of gas-oil capillary pressures. Two series of drainage capillary pressure measurements were performed on sandstone core samples with and without connate water saturation. Binary mixtures of methane/n-heptane at different equilibrium pressures were used to obtain wide variations in the gas- liquid interfacial tensions (IFT) values. Measurements have shown that the gas-oil capillary pressures determined at different levels of IFT and without connate water can be transformed into one capillary pressure function (Pc/IFT) down to IFT = 3.7 mN/m. Below this value, the capillary pressures measured were significantly lower than those estimated by the Young- Laplace equation. In the presence of connate water, the gas-oil capillary pressures were always higher than the ones measured without water saturation. The impact using measured instead of scaled capillary pressures from mercury-air data was evaluated through numerical simulations of free gas-gravity drainage on a one meter-long block of a fractured reservoir. Simulations have shown that the kinetics and the level of oil recovery are very sensitive to the capillary pressure curves used as input data. For low IFT values and without connate water saturation the oil recovery is underestimated by about 6% PV using scaled capillary pressures from mercury-air data. An opposite variation is observed in presence of connate water saturation.