Oil recovery enhancement in carbonate reservoirs via low saline water flooding in presence of low concentration active ions; A case study

Different research reported that the main mechanism for low salinity water flooding in carbonate reservoir is multi ion exchange (MIE) which occurs when the concentration of active ions such as sulfate and calcium is high and the temperature of the reservoir is in the range of 90-110°C. In many cases, due to the reservoir condition and availability of the injection brine, meeting criteria for an effective ion exchange mechanism is not possible. In this study, the feasibility of LSW flooding for oil recovery enhancement from carbonated reservoir at low concentration of active ions was investigated. Rock and fluid samples from an Omani oilfield were used for the experiments. Different experiments were conducted to study the rock/brine/oil interactions during the LSW process and the recovery improvement. Core flooding experiments were conducted to study the impact of different dilutions on the tertiary recovery. To investigate the effect of salinity changes on fluid-fluid interactions, IFT measurements between crude oil and different brine salinities were carried out at reservoir temperature. Also, effects of LSW (different dilutions) on the wettability alteration were examined by performing contact angle measurement tests as well as spontaneous imbibition tests. The oil recovery results by core flooding experiments showed a noticeable improvement by switching to the diluted brines. Results of wettability study showed that, due to the lack of sufficient active ions and relatively low reservoir temperature, wettability alteration towards more water wet condition did not occur in this case. But, Interfacial tension measurements showed that brine dilutions reduced oil/brine IFT appreciably. Therefore, in this special case, observations proved that the IFT reduction is the main mechanism contributing oil recovery enhancement. Hence, low salinity water in carbonate reservoirs is recommended for similar cases even when the criteria of MIE mechanism are not met.