Evaluation of Water-Based Enhanced Oil Recovery (EOR) by Wettability Alteration in a Low-Permeable Fractured Limestone Oil Reservoir

Abstract

This paper introduces a comprehensive method on how to evaluate wetting properties and oil recovery potential by spontaneous imbibition of “smart water” into a low-permeable limestone reservoir (≈1 mD). The reservoir temperature was 110 °C, and the salinity of the formation water was high (∼208 000 ppm). Crude oils from different possible source rocks were characterized for acid and base numbers, asphaltene, viscosity, and density. The potential of water-based enhanced oil recovery (EOR) was evaluated on the basis of the following studies: (1) The wetting potential of crude oils toward carbonate surface was investigated. (2) The presence of capillary forces in the core material was tested by spontaneous imbibition. (3) The initial wetting condition was determined by the chromatographic wettability test on mildly cleaned cores. (4) The surface reactivity of limestone core toward Ca2+, Mg2+, and SO42− was evaluated, when exposed to seawater at different temperatures (70−150 °C). (5) The presence of anhydrite, CaSO4(s), in the limestone core material was confirmed, which will affect the initial wetting conditions. (6) The potential of ”smart water” to enhance oil recovery from limestone cores containing crude oil and formation water at 110 °C was evaluated by imbibition of seawater and modified seawater. Because of the low content of acidic material in the crude oils and the presence of anhydrite in core material, the limestone cores were expected to be preferentially water-wet, which was confirmed by the chromatographic wettability test. About 40% of original oil in place (OOIP) was recovered in a spontaneous imbibition process at 110 °C using formation water. In a tertiary imbibition process with seawater and modified seawater, the oil recovery increased to ∼50% of OOIP.