Spontaneous Imbibition Study: Effect of Connate Water Composition on Low-Salinity Waterflooding in Sandstone Reservoirs

Abstract

Abstract Extensive experimental work has indicated that low-salinity waterflooding is an enhanced oil recovery technique that improves oil recovery by lowering and optimizing the salinity of the injected water. Most of the low-salinity waterflooding studies focused on the injection brine salinity and composition. The question remains, how does the salinity and composition of the reservoir connate water affect the low-salinity waterflooding performance? Therefore, in this work different connate water compositions were used to investigate the role of reservoir connate water on the performance of low-salinity waterflooding. In this paper, nine spontaneous imbibitions experiments were performed. Two sandstone types (Bandera and Buff Berea) with different clay contents were used. The mineralogy of the rock samples was assessed by X-ray powder diffraction, scanning electron microscopy, and X-ray fluorescence. This work describes the experimental studies of the spontaneous imbibition of oil by low-salinity and high-salinity brines using 20 in. length outcrop samples. The main objectives of the spontaneous imbibition study was to investigate the role of the composition of the reservoir connate water (Na+, Ca2+, and Mg2+), the effect of rock permeability, and test the effect of temperature (77 and 150°F) on the performance of the low-salinity waterflooding recovery. The volume of the produced oil was monitored and recorded against time on a daily basis. Imbibition brine samples were analyzed at the end of each experiment. Results demonstrate that the spontaneous imbibition oil recovery ranged from 38 to 69% OOIP for high permeability Buff Berea cores (164-207.7 md), while oil recovery of the low permeability Bandera cores (31.1-39.2 md) ranged from 20 to 51.5% OOIP at 77°F and 14.7 psia. The oil recovery decreased when the average pore-throat radius decreased. The reservoir connate water composition has a dominant influence on the oil recovery rate. The changes in the ion composition of reservoir connate water (Ca2+, Mg2+, and Na+) showed a measurable change in the oil production trend. Reservoir cores saturated with connate water containing divalent cations of Ca+2 and Mg+2 showed higher oil recovery than for cores saturated with monovalent cations Na+. In all cases, a measurable ion exchange was observed, while there was no significant change in the pH of the imbibition brine during the experiment. The ions exchange effect was more pronounced than the pH effect in the low-salinity waterflooding performance for Buff Berea and Bandera sandstone. As the temperature increased from 77 to 150°F, an additional oil recovery up to 15.4% of OOIP was observed by spontaneous imbibition for Buff Berea cores.