A State-Of-The-Art Low Salinity Water EOR in Carbonate Reservoir-Boosting Oil Recovery by Exploring Additives
Authors: Yonebayashi Hideharu , Hiraiwa Takeshi , Tange Masaaki , Sawata Masaru 
Published: April 2024

The industry and academia have continued efforts for aiming more oil recovery by focusing on certain ions in low salinity water (LSW) brine composition with variety of mechanism hypotheses. This paper reviews past history, findings, and the latest focus for exploring upside potentials of LSW EOR in carbonate reservoirs. Based on recent research focused on fluid-fluid interaction (FFI), forming micro-dispersion (MD) as an interfacial reaction of oil-water mixing, we highlight useful additives which can boost the interactions and improve LSW EOR gains. The majority of past LSW EOR investigation have been focusing on rock-fluid interaction, therefore, the subsequent engineered LSW investigation have been conducted from the perspective of reducing positive-charged carbonate rock surface by adding negative-charged materials such as sulfate ions. However, this approach is not practical from operational and/or reservoir management viewpoints due to scaling and reservoir souring risks. Together with sulfate ions, certain positive-charged cations have been also investigated to prevent oil droplets absorbing on rock surface by connecting with negative-charged components in oil. In recent, beyond the common investigated ions, new ions such as copper and chromium were reported to improve LSW EOR effect. Those efforts should be admired; however, it should point out uncertainties in the rock-fluid-interaction-oriented investigation in case of carbonates, because inconsistencies are often observed between screening stage results such as spontaneous imbibition tests and oil recovery evaluation results in core floods.

In parallel with the engineered LSW optimization based on the rock-fluid interaction, the different approach was established based on the FFI as highly practical and consistent evaluation. The approach relied on micro-dispersion ratios (MDR) representing water content increment by the oil-water interfacial reactions because the oil recovery improving magnitude was demonstrated well correlating with MDR. Namely, aiming further oil recovery is equivalent to boost the FFI between LSW and positive oils. In the most recent research, three additives for pure LSW (diluted to 1% sea water): diethyl ketone (DEK), dimethyl ketone (DMK), and copper chloride were investigated as a sensitivity analysis from the perspective of MDR improvement and core flood tests. The ketones revealed noticeable increase of MDR more than double of that in pure LSW case. The tertiary oil recovery after sea water injection stage consistently increased +3%, +5%, and +11% IOIP for pure LSW, LSW+Cu(II), and LSW+DEK cases, respectively. A monotonous correlation was observed between those additional oil increment and MDR. Subsequently, a molecular-level fluid analysis using Fourier-transform ion cyclotron resonance mass spectrometer elucidated certain components taking main roles of FFI mechanism. Based on the functional components, further optimization of engineered LSW designs is expected as way forward.