Experimental investigation on fluid/fluid and rock/fluid interactions in enhanced oil recovery by low salinity water flooding for carbonate reservoirs

The low salinity water flooding (LSWF) technique has established itself as cost-effective and environmentfriendly in nature but its mechanisms are still unclear. Recovery due to LSWF mainly depends on two types of interaction, viz. fluid/fluid and rock/fluid. Despite the significant investigation, there is still no consensus on which interaction mainly governs oil recovery. This study intends to provide essential answers to the aforementioned issues, as well as additional significant insights into the dilution behaviour of Sea Water (SW) to be injected into a carbonate formation with equivalent salinity. A series of experiments were designed to assess both fluid/fluid and rock/fluid interaction thoroughly viz. compatibility test, IFT, interfacial rheology, zeta potential, and contact angle. A comprehensive number of individual and sequential coreflooding were conducted on outcrop cores to quantify the oil recovery potential of optimum salinities and gain more insight into wettability shifts due to LSWF. Rock/fluid and fluid/fluid interfaces showed different optimal conditions for improved oil recovery. Investigation of fluid/fluid interaction showed an optimal salinity of 2dSW as optimal salinity due to IFT reduction and increased viscoelastic properties. Rock/fluid interaction showed an optimal salinity of 10dSW due to enhanced zeta potential and calcite dissolution. Rock/fluid interaction showed an incremental recovery of 14% and fluid/fluid interaction showed an incremental recovery of 7% over simple SW injection. Sequential flooding based on rock/fluid interaction showed an incremental recovery of 16% over simple SW. Oil recovery based on rock/fluid interaction showed the highest recovery as compared to fluid/fluid interaction. Increased interfacial elasticity and calcite dissolution lead to wettability alteration and increased oil recovery due to LSWF. The effectiveness of LSWF depends upon optimum salinity and wettability alteration capability of low salinity slug. Results of this study also indicate that considering only one interaction either rock/fluid or fluid/fluid can lead to erroneous interpretation of LSWF data. Rock/fluid interaction and rock geochemical reactions must be considered while designing the LSWF recipe and low salinity slug should be injected right from the start to reap the benefits of LSWF.