Pursuing Upside Potential of Hybrid Low Salinity Waterflood EOR in Carbonate Reservoirs

Abstract In our previous LSW EOR research focused on fluid-fluid interaction (FFI) forming micro-dispersion (MD) as an interfacial reaction between oil and LSW (sea water, SW, diluted to 1%), we identified diethyl ketone (DEK) as an effective additive that can accelerate the interfacial reaction and improve oil recovery. The tertiary mode core flood of hybrid LSW EOR (containing 2wt% DEK) achieved +11% IOIP and +8% IOIP compared with secondary sea water injection and pure LSW injection, respectively. This paper pursued further upside potential of hybrid LSW EOR by exploring more effective additives to assist FFI. The exploration was carried out based on a simple evaluation of micro-dispersion ratio (MDR) that was defined as the increase in water-in-oil content resulting from oil-water interfacial reactions. The MDR, measured by FFI tests statically mixing oils and LSW, represented a key performance indicator highly correlated with oil recovery improvement because our previous work revealed a linear correlation between MDR and oil recovery increment. For this study, a total number of ten stock tank oils (STOs) were collected from multiple locations of sub-reservoirs in a target offshore carbonate field via two sampling campaigns. The first campaign collected four samples of STO-L1/L2/L3/U from mid-dip sections of lower (L) and upper (U) reservoirs in the field while the second campaign collected six samples of STO-L5/L6/L7/L8/L9/L10 from mainly down-dip sections of the L reservoir. The multiple locational sampling was deployed based on our previous findings in which a molecular-level fluid analysis using Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR MS) detected involvement of heavy polar components into MD formation in the FFI mechanism. Therefore, deeper location oils containing rich heavy-ends were expected to demonstrate higher MDR because of compositional gradients in large structural depth variation in the target field. In each campaign, the best positive oils showing the highest MDR (i.e., 20 v/v by STO-L2 and 199 v/v by STO-L5) at pure LSW were screened as reference for further hybrid LSW EOR potential survey. This clearly demonstrated that MDR can be higher in down-dip oils. Additive candidates were CuCl2 and ketones such as dimethyl ketone (DMK), diethyl ketone (DEK), and methyl ethyl ketone (MEK). In addition, dimethyl sulfoxide (DMSO) was selected from the perspective of polar substances due to having larger dipole moments than the ketone candidates. Furthermore, a sensitivity analysis of additive concentration was conducted for techno-economic optimization. The reference MDR of mid-dip STO-L2 increased to 53 v/v at 2wt% DEK while the reference of down-dip STO-L5 increased to 300 v/v at 0.5 wt% DEK, 273 v/v at 1.0 wt% DMSO, and 330 v/v at 0.1 wt% MEK. The exploration with concentration sensitivity showed optimum concentration ranges that improve techno-economic potential (e.g., six times MDR with one quarter DEK concentration). Our new efforts of more-affordable additive designs demonstrated increasing value of hybrid LSW EOR option.