Gas/Oil Capillary Pressure of Chalk at Elevated Pressures

Abstract

Summary Accurate capillary pressure curves are essential for studying the recovery of oil by gas injection in naturally fractured chalk reservoirs. A simple and fast method to determine high-pressure drainage capillary pressure curves has been developed. The effect of gas/oil interfacial tension (IFT) on the capillary pressure of chalk cores has been determined for a methane/n-pentane system. Measurements on a 5-md outcrop chalk core were made at pressures of 70, 105, and 130 bar, with corresponding IFT's of 6.3, 3.2, and 1.5 mN/m. The results were both accurate and reproducible. The measured capillary pressure curves were not a linear function of IFT when compared with low-pressure centrifuge data. Measured capillary pressures were considerably lower than IFT-scaled centrifuge data. It appears that the deviation starts at an ΓFT of about 5 mN/m. According to the results of this study, the recovery of oil by gravity drainage in naturally fractured chalk reservoirs may be significantly underestimated if standard laboratory capillary pressure curves are scaled by EFT only. However, general conclusions cannot be made on the basis of only this series of experiments on one chalk core.