Pore-Scale Imaging to Investigate Wettability and Recovery Mechanism for Surfactant/Polymer Flooding

Abstract

Surfactant-polymer (SP) flooding is a promising chemical enhanced oil recovery method in carbonates with high salinity brines especially where an existing waterflooding infrastructure is in place. SP flooding improves both the microscopic and macroscopic sweep efficiencies. The polymer viscosifies the injection water to control mobility, while the surfactant reduces the oil-water interfacial tension, and possibly alter wettability, to mobilize trapped oil. In this work, we investigate SP flooding recovery mechanisms in carbonates at the pore-scale. Two polymers were used in this work: a Thermo-Viscosifying Polymer (TVP) and an Acrylamido tertiary butyl sulfonate (ATBS) with an acrylamide (AM) copolymer. Surfactants utilized were different grades of an amphoteric carboxybetaine. A set of four coreflooding experiments were performed at 90°C with a high-salinity injection brine (57,000 ppm). The first two corefloods investigated the independent performance of the chemicals (i.e. polymer flooding and surfactant flooding). The remaining two investigated the combined effects of SP flooding. The effects of different aging times were studied, too. Furthermore, micro-CT imaging were conducted at different steps both before and after flooding to evaluate saturation distributions and the chemicals effects on recovery. The images were also processed to segment the different phases which allows further evaluation of recovery mechanisms at the pore-scale. Based on fluid occupancy maps, samples aged for one week exhibited strong water-wetness while samples aged for two weeks exhibited moderate water-wetness. In samples aged for one week, the final fluid occupancy was (i.e. remaining oil existed as) buckets of oil surrounded by water. While in samples aged for two weeks, the remaining oil was weekly attached to the surface of the pores. In terms of the recovery performance of the independent processes, the polymer flood showed better recovery than the surfactant flood. This supports the conclusion that in water-wet media the effect of surfactant alone is not significant if not coupled with mobility-control. In general, this work also demonstrates the utility of Micro-CT scanning in providing better understanding of recovery mechanisms at the pore-scale. With more accurate segmentation of the different phases, it can also provide a mean for in-situ contact angle measurements.