Abstract
Low salinity water (LSW) enhanced oil recovery (EOR) has gained more attention in carbonate reservoirs with variety of mechanism hypothesis. Recent research focused on fluid-fluid interaction, forming micro-dispersion as an interfacial reaction of oil-water mixing, can reflect a main driver of oil improving mechanism. This paper focuses on the mechanism and demonstrates further boosting the fluid-fluid interactions using small amount of additives to improve LSW EOR value in an objective offshore giant carbonate field. Four stock tank oil (STO) samples were collected from multiple sub-layers (L1, L2, L3, and U). Brines were prepared as formation water (FW), sea water (SW), and LSW (diluted to 1-20%SW). The fluid-fluid interaction tests measured micro-dispersion ratios (MDR), which represent water content increment by the oil-water interfacial reactions, to screen the most positive oil for low-salinity effect. To boost the reaction in LSW, three additives: diethyl ketone (DEK), dimethyl ketone (DMK), and copper chloride were independently evaluated using the screened positive oil. The boosting effectiveness was investigated by MDR improvement and core flood tests.
The larger micro-dispersion phase was formed as switching from FW and SW to more diluted LSW in the fluid-fluid interaction tests. The STO-L2 was screened as the most positive oil because of the highest MDR. The two-staged MDR increase indicated the existence of threshold salinity (i.e., between 4-5%SW and 2-3%SW) for triggering the radical oil-water interfacial reaction. To clearly capture booster's impacts, the subsequent tests used 1%SW as a base LSW. The compatibility analysis determined 2.0 wt% of DEK and DMK as dosing ratio to LSW. The additive boosting impacts on fluid-fluid interaction revealed a noticeable MDR increase compared to that of pure LSW case. The boosting potential of oil recovery was evaluated by three core flood tests using the identical reservoir core (i.e., reused after solvent cleaning and aged again for avoiding a common 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. The largest oil recovery boosting occurred by the tertiary LSW+DEK injection as +10.7 %IOIP (three-times higher than that of pure LSW case). Finally, we found a linear correlation between MDR and oil recovery increment.