Pursuing Upside Potential of Low Salinity Waterflood EOR in Carbonate Reservoirs

Abstract As an option for water-based enhanced oil recovery (EOR) in carbonate reservoirs, the industry and academia have focused attention on low salinity water (LSW) EOR. While a variety of mechanisms, but mainly rock-fluid interaction (i.e., wettability alteration), have been discussed, several groups have focused on fluid-fluid interaction (FFI) forming micro-dispersion (MD) phase as an interfacial reaction of oil-LSW mixing. Our previous research, molecular level analysis applying petroleomics technology to compare components in pre/post-reacted oil and MD fluid, identified key molecules contributing to MD formation as being heavier polar components in LSW-reactive oil. The FFI is considered acting as an oil recovery improving driver in the LSW EOR process since a monotonic correlation is observed between micro-dispersion ratio (MDR) and oil recovery improvement in tertiary mode LSW core floods. Based on these findings, this paper pursues the potential LSW-EOR upside by exploring more FFI reactive (richer polar-heavier-ends) oils. Taking into account compositional gradients due to gravity segregation in a target offshore carbonate field with large structural depth difference, a total of ten stock tank oils (STOs) were collected from multiple locations of sub-reservoirs via two sampling campaigns. Four samples of STO-L1/L2/L3/U were collected from mid-dip sections of lower (L) and upper (U) reservoirs in the first campaign. In the second campaign, six samples of STO-L5/L6/L7/L8/L9/L10 were collected from mainly down-dip sections of the L reservoir. Screening of better LSW-reactive oils was carried out based on a simple evaluation of MDR that was defined as the increase in water-in-oil content resulting from FFI tests. The FFI tests statically mixed oils and brines (i.e., formation water or sea water, diluted to 1-20% sea water as LSW). Oils showing high MDR were defined as "positive oils" while those showing low MDR were defined as "negative oils". Higher MDRs were measured by switching from FW and/or SW to more diluted LSW. The STO-L2 and STO-L5 with 1% SW were identified as the most positive oils in each campaign due to the highest MDR of 20.3 v/v and 199 v/v, respectively. The latter in down-dip region showed 10 times higher MDR than the former in mid-dip. An overall trend showed increasing MDR when moving to down-dip region. This proved the hypothesis that the down-dip region contained higher MDR oils due to more polar heavy-end components involved in the MD formation mechanism. There was MDR dependency on locations from the perspectives of not only vertically but also horizontally. Over 20 times higher MDR was observed from STOs in west and south areas (i.e., MDR 199 v/v by STO-L5 and MDR 172 v/v by STO-L8) compared with north area (i.e., 8 v/v by STO-L3). The areal trend was aligned with a general fault trend from north-west to south-east direction that can separate the west and south areas and the north area. The upside-potential exploration, based on the strategic plan of geochemical-perspective location selection, revealed clear location-dependent MDR behavior. The findings are practically worthy for increasing values of LSW EOR option, aiding in selection of targets with greatest potential, and facilitating more effective field applications.