Microfluidic Evaluation of Hybrid Low-Salinity Waterflood Enhanced Oil Recovery in Carbonate Reservoirs Driven by Fluid/Fluid Interaction Mechanism

Summary

One of the dominant reactions in low-salinity waterflood (LSW) enhanced oil recovery (EOR) mechanisms is a fluid/fluid interaction (FFI) that forms microdispersion (MD) at the interface of oil and LSW in contact. The MD ratio (MDR), defined as an increase of water content in the MD phase compared with that of the original water content in oil, is considered to be linearly associated with an oil recovery improvement in tertiary-mode LSW corefloods. To further understand the role of MD formation, we applied microfluidics technology to visualize the displacement process using the FFI mechanism to increase oil recovery. Referred to a series of 2D computed tomography (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 corefloods so that the microfluidic tests could use sister core information for a fair comparison purpose with the previous coreflood study. A total of four microfluidic flood experiments were performed evaluating LSW injection [diluted to 1% seawater (SW), total dissolved solids (TDS) 430 ppm] or hybrid LSW [1% SW containing 2 wt% diethyl ketone (DEK)] injection compared with SW injection (TDS 43,000 ppm).

In the past tertiary-mode coreflood tests, the highest MDR stock-tank oil (STO)-L2 showed a noticeable oil recovery increment of +11% initial oil in place (IOIP) with hybrid LSW while a clear increment (+3% IOIP) with pure LSW. Thus, two series of microfluidic tests were performed using the same STO-L2. 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 an increasing trend of differential pressure (dP) during the LSW injection stage. The second series of runs evaluated hybrid LSW injection [i.e., after seawater (SW) injection, tertiary-mode injection of hybrid LSW or followed by chase SW] using the realistic microfluidic chip that has smaller pore throats than the first chip. To secure sufficient MD forming reaction time, a 7-day aging period was set during the hybrid LSW injection stage. Consequently, the tertiary flood showed a clear increase in dP with additional oil recovery (+8.5 to 11.1% IOIP ultimately), even though the preaging stage reached plateau oil recovery with a minor oil recovery increase. Regarding tertiary-stage production behaviors, while the continuous injection sequence in corefloods allowed gradual oil production, the microfluidics floods, including the aging step, could allow quicker oil production by resuming injection. Specifically, the aging step worked effectively, reaching an equilibrium of FFI interaction. During both pure and hybrid LSW injection stages, we observed a similar behavior that increased dP with emulsion formation in the oil phase. Those observations imply varying viscosity and/or interfacial viscoelasticity of LSW/hybrid LSW-contacted oils.