GAS OIL CAPILLARY-PRESSURE OF CHALK AT ELEVATED PRESSURES

Accurate capillary pressure curves are essential for studying the reco very 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 deter mined for a methane/n-pentane system. Measurements on a 5-md outcrop c halk core were made at pressures of 70, 105, and 130 bar, with corresp onding 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 l inear 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 LFT of abo ut 5 mN/m. According to the results of this study, the recovery of oil by gravity drainage in naturally fractured chalk reservoirs may be si gnificantly underestimated if standard laboratory capillary pressure c urves are scaled by IFT only. However, general conclusions cannot be m ade on the basis of only this series of experiments on one chalk core.