Reservoir Connate Water Chemical Composition Variations Effect on Low-Salinity Waterflooding

Abstract

Abstract Extensive experimental work has indicated that Low-Salinity Waterflooding (LSW) is an enhanced oil recovery technique that improves oil recovery by lowering and optimizing the salinity of that injected water. Most of the LSW studies focused on the injection of brine salinity and composition. The question remains how does the salinity and composition of reservoir connate water affects the LSW performance. Therefore, in this paper different connate water compositions were used (TDS varies from 1550 up to 174156 ppm) to investigate the role of reservoir connate water on the performance of LSW. Nine Spontaneous Imbibition (SI) experiments and six coreflood experiments were performed. Two sandstone types (Bandera and Buff Berea) with different clays contents and stock-tank crude oil samples were used in all the experiments. This work describes experimental studies of SI of oil by low-salinity and high salinity brines using 20″ long outcrop sandstone samples. This study focused on the effect of connate water composition and temperature, 77 and 160°F, on the performance of LSW. The coreflood experiments have been conducted using 6 in. outcrop Buff Berea sandstone cores at 150°F and 500 psi. Oil recovery and pressure drop were observed and analyzed after each coreflooding experiment to examine the effect of the connate water composition (Na+, Ca+2, and Mg+2) on the performance of the LSW in secondary recovery mode. Reservoir connate water composition and salinity have a dominant influence on the oil recovery rate. The Ca2+, Mg2+, and Na+ ions play a key role in oil mobilization in different sandstone rocks. Reservoir cores saturated with connate water contained divalent cations of Ca+2 and Mg+2 showed higher oil recovery for cores saturated with monovalent cations Na+. Low-salinity waterflooding showed a high potential to improve oil recovery in the SI experiments at different temperature levels. For high permeability Buff Berea cores, the SI oil recovery ranged from 38 to 69% OOIP, while oil recovery of the low permeability Bandera cores ranged from 17 to 45% OOIP at 77°F and 14.7 psia. As the temperature increased from 77 to 160°F, an additional oil recovery up to 4.2% of OOIP was observed by SI for Buff Berea cores. From the coreflood experiments, low-salinity brine had a significant positive effect on oil recovery for sandstone cores saturated with divalent cations (Ca+2 and Mg+2). The magnitude of incremental oil recovery increased from 51.9 to 58.9% OOIP when the reservoir connate water salinity increased from 3,420 to 36,350 ppm. On the other hand, increasing the monovalent cations (Na+) from 1,550 to 137,670 ppm resulted in slight increase in oil recovery (1.2% OOIP). The oil recovery from the coreflood runs for the Buff Berea cores ranged from 58.9 to 35.9% OOIP. The oil recovery decreased when the salinity of reservoir connate water decreased.