Ion Composition Effect on Spontaneous Imbibition in Limestone Cores

Abstract

Studies on low salinity oil recovery have accelerated in recent years. Detailed focus has been put on understanding competing underlying mechanisms behind observed improved oil recovery. The main aim in this work is to tune the ionic composition of imbibing brine to maximize the oil recovery from spontaneous imbibition experiments on limestone outcrop cores, as an analogue for an offshore Brazilian carbonate reservoir. Improved spontaneous imbibition experiments with smooth natural brine exchange between primary, secondary, and tertiary fluid imbibition cycles at high temperatures were accomplished, reducing systematic errors in the recovery data. Contact angle, zeta potential, and interfacial tension measurements completed the data set. It was observed through improved oil recovery from tertiary imbibition that holistic dilution of synthetic seawater (10 times) had limited the impact on improved oil recovery on our limestone cores. However, selective dilution of synthetic seawater with respect to the NaCl content resulted in significantly improved oil recovery. In line with this observation, brines depleted in NaCl and enriched in Mg 2+ and SO 4 2− ions were tested and resulted in improved oil recovery in tertiary modes. A positive test on the role played by the SO 4 2− ion was recorded when synthetic seawater was enriched 2 and 4 times in SO 4 2− ion concentration; however, substantial recovery increases required an increased Mg 2+ content. Contact angle measurements on polished rock chips, cut and shaped from the same rock material and aged in brines at 96 °C, confirmed the observed results, where zeta potential measurements at both 25 and 70 °C showed inconclusive results.