Engineered Low Salinity Waterflood in Carbonate Reservoirs: Part-2-Boosting Fluid-Fluid Interaction and Oil Recovery by Cost Effective Additives
Authors: Yonebayashi Hideharu , Hiraiwa Takeshi , Tange Masaaki , Yamada Tatsuya , Terajima Kohei , Yamamoto Kohei , Khlaifi Khuloud Tareq , Bahri Sami , Hoeth Laurien Wichers , Stalker Robert , Graham Gordon 
Published: September 2024

Abstract
The academia and industry have paid more attention to low salinity water (LSW) enhanced oil recovery (EOR) in carbonate reservoirs with a variety of mechanisms. Instead of widely-discussed mechanism of rock-fluid interaction (i.e., wettability alteration), recent research has focused on fluid-fluid interaction (FFI) which forms a micro-dispersion phase as an interfacial reaction of oil-water mixing, and now considered to be a key driver of oil improving mechanism. This paper, focused on the FFI mechanism between oil and LSW, demonstrates efficiently boosting the FFI using small amount of additives to improve the value of LSW EOR in a targeted offshore giant carbonate field. Four stock tank oil (STO) samples were collected from multiple sub-layers (L1, L2, L3, and U) in two main producing reservoirs. Brines were prepared as formation water (FW), sea water (SW), and pure LSW (diluted to 1-20% SW). The FFI tests measured micro-dispersion ratios (MDR), which represent water content increment by the oil-water interfacial reactions, to screen the most LSW-reactive oil which is defined as "positive oil". To boost the FFI, three additives to LSW: diethyl ketone (DEK), dimethyl ketone (DMK), and copper chloride were independently evaluated using the pre-screened most positive oil. The boosting effectiveness was investigated by MDR improvement and core floods.
The FFI tests revealed higher MDR as switching from FW and SW to more diluted LSW. The test screened STO-L2 as the most positive oil because of the highest MDR. The salinity sensitivity test showed there were two-staged MDR increase when diluting. The MDR jumps were indicated at the threshold salinities which existed between 4-5% SW and 2-3% SW for triggering the radical oil-water interfacial reaction. Based on the screening and basic sensitivity results, the subsequent tests used 1%SW as a base LSW. Through the compatibility analysis, DEK and DMK dosing ratio to LSW was designed as 2.0 wt%. The additive-sensitivity FFI tests revealed a noticeable MDR increase against the pure LSW case. The boosting potential of oil recovery was evaluated by four core flood tests (CF01-CF04). The CF01-CF03 used the identical reservoir core (i.e., reused after solvent cleaning and aged again for avoiding possible core-nature uncertainties). Thus, common secondary SW injection achieved similar oil recovery among three tests. The tertiary untreated LSW injection revealed +3% IOIP additional oil recovery after the secondary SW injection (CF01). The largest tertiary oil recovery increment against the secondary SW injection was achieved by the LSW-plus-DEK injection (CF02) as +10.7 %IOIP (three-times higher than that of pure LSW case). The LSW-plus-DMK (CF04) and LSW-plus-CUCl2 (CF03) tertiary injections showed +8.7% and +5% increments against the secondary SW injection, respectively. Through all the tests, we found a linear correlation between MDR and oil recovery increment.