Analysis of fluid flow in naturally fractured reservoirs (NFRs), as a highly heterogeneous and complex system, requires a detailed study of the fracture-matrix interactions. The main process of fluid movement between the fracture and matrix is spontaneous imbibition (SI), which can occur in co/countercurrent fluid flow states. In addition, most carbonate rocks are fractured and non-water-wet, which can lead to low oil recovery. Wettability greatly affects the performance of the SI process. Injection of water or chemicals can be insufficient because fluids mostly pass through highly permeable fractures and lead to early breakthrough. Therefore, the wettability alteration mechanism should be applied in NFRs, and low-salinity water (LSW) injection is considered an effective enhanced oil recovery (EOR) approach. In this review, experimental and numerical studies of co/counter-imbibition are analyzed to show the importance of investigating the fracture-matrix interactions. In addition, the review shows the wettability effect on imbibition in fractured rocks. The review of experimental studies of LSW imbibition in fractured carbonates shows the possibilities for implementing an EOR method. However, the wettability alteration process during SI using LSW has not yet been studied, and no simulation models of co/countercurrent flows have yet been provided. Based on this review, more experimental studies are recommended to duplicate co/countercurrent imbibition using LSW. Advanced techniques such as CT scanning, MRI, and NTI can be used to reveal fluid distribution. Using experimental data, numerical models can be developed to characterize dynamic wettability alteration during co/countercurrent imbibition.

The results of many previous studies on low salinity/controlled ions water (CIW) flooding suggest that future laboratory and modeling investigations are required to comprehensively understand and interpret the achieved observations. In this work, the aim is co-optimization of the length of the injected slug and soaking time in the CIW flooding process. Furthermore, the possibility of the occurrence of several governing mechanisms is studied. Therefore, the experimental results were utilized to develop a compositional model, using CMG GEM software, in order to obtain the relative permeability curves by history matching. It was concluded that CIW slug injection, concentrated in the potential-determining ion, can increase oil recovery under a multi ion exchange (MIE) mechanism. The wettability of the carbonate rocks was changed from a mixed or oil wet state toward more water wetness. However, there is a CIW slug length, beyond which extending the length does not significantly improve the rock wettability, and consequently, the oil production, which is known as the optimum slug size. This implies that the optimization of the injection process, by minimizing the slug size, can decrease the need for the CIW supply, therefore lowering the process expenditure. Moreover, if the exposure time of the rock and CIW is increased (soaking), a higher level of ion substitution is probable, leading to more oil detachment and production. Rock dissolution/precipitation (leading to a pH change) was found to have a negligible contribution.

The application of low salinity water and ion management of the injected water affects the oil recovery in carbonate formations. Different forced and spontaneous imbibition experiments have been practiced on the carbonate core samples to show the performance of this smart water injection. Various experimental and modeling approaches have been applied by different researchers to optimize the smart waterflooding process. To achieve more practical conditions, the injection time of the smart water should be reduced to control the preparation cost on the field scale. In this paper, we present findings from different modeling/experimental studies to improve the performance of smart water flooding in carbonate formations by the idea of shock/soaking. Different researches showed that the presence of active ions such as Mg2+ and SO42- in the injection water alters the wettability of carbonates to more water-wet state and also reduces the IFT between the oil and the injected brine. Hence, spiking active ions concentration in the low salinity water improves oil recovery from carbonate formation. In this work, the optimized smart brine was used for injection with novel injection scheme. The optimized brine was set to be injected as the shock slug between two slugs of high salinity water. This smart water shock flooding was designed to reduce the pore volume of low salinity water flooding. The effect of the slug on relative permeability curves was modeled and analyzed in the core and sector scales. Also we experimentally studied the effect of soaking time after the shock on wettability alteration and improvement in recovery by re-injection of high salinity normal brine. Characterization tests such as contact angle measurement confirmed the effect of shock/soaking on alteration of governing mechanisms such as multi-ion exchange which leads to wettability alteration in the process. Our core flooding experiments showed that the shock injection at the best design can improve the tertiary recovery up to 7.8%. Also, modeling at the reservoir sector shows noticeable incremental oil recovery during the shock injection and high salinity water injection after it. Our modeling/experimental studies clearly illuminated a new approach to improve the performance of low salinity water flooding in an efficient and cheaper way. By this approach, higher oil recovery can be achieved by the application of less amount of diluted water which is beneficial for the oil industry.

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