Microfluidic Evaluation of Hybrid Low Salinity Waterflood EOR in Carbonate Reservoirs

Abstract One of dominant reactions in low salinity waterflood (LSW) enhanced oil recovery (EOR) mechanism is a fluid-fluid interaction (FFI) that forms micro-dispersion (MD) at the interface of oil and LSW in contact. The MD ratio (MDR), defined as an increase of water content in MD phase compared with that of original water content in oil, is considered to be linearly associated with an oil recovery improvement in tertiary mode LSW core floods. For further understanding of the role of MD formation, we applied microfluidics technology to visualize displacement process with the FFI mechanism to increase oil recovery. Referred to a series of 2D CT-image stack scanned from the target carbonate core pieces, a representative microfluidic chip was manufactured with reproducing typical pore throat paths and carbonate-corresponding wettability. The reference pieces were collected from a vicinity of locations where plug-cores were taken for core floods so that the microfluidic tests could use sister core information for a fair comparison purpose with the previous core flood study. A total of four microfluidic flood experiments were performed evaluating LSW injection (diluted to 1% sea water (SW), TDS 430 ppm) or hybrid LSW (1% SW containing 2wt% diethyl ketone) injection compared to SW injection (TDS 43,000 ppm). In the past tertiary mode core flood tests, the highest MDR oil, STO-L2, showed noticeable oil recovery increment (+11% IOIP) with hybrid LSW while a clear increment (+3% IOIP) with pure LSW. Thus, two series of microfluidic tests were performed. The first series of runs (i.e., continuous secondary mode injection of SW vs LSW) evaluated pure LSW injection using a synthetic pattern shape of porous media that represented relatively larger spaces than the actual ones so that FFI reaction can be promoted due to more contact opportunity between LSW and oil. The tests captured clear snapshots of emulsification with increasing trend of differential pressure during the LSW injection stage. The second series of runs evaluated hybrid LSW injection (i.e., after SW injection, tertiary mode injection of hybrid LSW or followed by chase SW) using the realistic microfluidic chip that has smaller pore throat than the first chip. To secure sufficient MD forming reaction time, a 7-day ageing period was set during the hybrid LSW injection stage. Consequently, the post-ageing flood showed a clear increase in differential pressure with additional oil recovery (+8.4% IOIP) while the pre-ageing stage reached plateau oil recovery. The similar oil recovery increment (+8.6% IOIP) was confirmed by switching from hybrid LSW to chase SW injection. During both pure and hybrid LSW injection stages, we observed a similar behavior that increased differential pressure. Those common trends imply varying viscosity and/or interfacial viscoelasticity of LSW/hybrid LSW contacted oils.