1. Introduction
Nowadays, most active large reservoirs are at the tertiary development stage. Different enhanced oil recovery (EOR) approaches are designed to improve oil production at this stage. The main aim of EOR techniques is to reduce the residual oil saturation after primary recovery or secondary recovery methods such as waterflooding. It is estimated that about 45% of OOIP is the objective of enhanced oil recovery techniques [
1]. Besides well-known, classical EOR methods such as chemical EOR, gas flooding, and thermal methods, some new ones are gaining attention lately. One of them is Low Salinity Water Injection (LSWI), which is one of the promising techniques of tertiary recovery and is simple from the point of technical implementation. In general, it is the same as a waterflooding process, but with the optimized water, in the case of salinity and ionic composition. The low salinity water (LSW) is prepared by seawater dilutions and adjusting ions compositions in the injected brine, also called engineered water (EW).
LSW injection (LSWI) impacts the primary crude oil-brine-rock (CBR) system mainly by disturbing the system equilibrium by altering the ion composition of water [
2]. Changing the interactions in the CBR system modifies the wettability, enhances the microscopic sweep efficiency and affects the capillary pressure and other CBR parameters such as rock surface properties. The performance of LSW and EW flooding in carbonate and sandstone formations was studied by different experimental and modeling approaches. They prove the effectiveness of this technique in reducing the residual oil in various cases [
3,
4]. This method is less investigated in carbonate rocks than in sandstones [
5]. It was believed that the presence of clays is required for a successful LSW injection, but the recently promising implementations of LSW in carbonates are reported in papers [
6,
7].
Different mechanisms are active during the oil recovery by LSWI in carbonates and sandstones [
8,
9,
10]. For carbonates, there are different proposed mechanisms that affect the rock-fluid and fluid-fluid interactions. The rock-fluid interactions are recognized as the reason for wettability alteration. Mechanisms such as multi-ion exchange, mineral dissolution, change in surface charge, and the adhesion energy of CBR are the main mechanisms observed in the literature [
2,
11,
12]. The Multi-Ion exchange mechanism is due to the presence of
,
and
ions and their interactions with oil and rock surface. The negatively charged
approaches the positively charged carbonate rock surface and wetting water phase; while,
reacts with negatively charged crude oil components and separates them from the rock surface, resulting in the wettability alteration [
13]. For sandstone reservoirs, the polar compounds of oil (including asphaltene and resin content) with multivalent cations form a bond on a clay surface [
2]. This statement cannot be confirmed for the carbonate case because of the lack of clay content. Another mechanism, mineral dissolution, occurs due to the difference in the ions composition in the injected water and the rock, which results in the movement of cations from the rock surface to the water, which leads to oil detachment and wettability alteration. This process is recognized as one of the independent wettability alteration mechanisms [
14]. All listed mechanisms lead to the wettability alteration to a more water-wet state, resulting in the oil’s detachment from rock surfaces.
The success of this method depends on factors such as the injected water properties, the rock type, oil characteristics, and injection scenarios. All these parameters directly or indirectly affect the wettability change process. This topic is well covered by Austad et al. [
15]. Carbonates wetting state depends on the chemical properties (carboxylic material) of crude oil, reservoir conditions, and the reservoir rock composition. Hence, the wetting can be altered by affecting the CBR interactions, such as the salinity and the concentration of reactive ions in the injected water, also called potential determining ions (PDIs).
Most of the review papers highlighted the critical criteria for successful LSWI implementation. In the paper of Tetteh et al. [
6], the authors went through several key parameters that affect the performance of LSWI in carbonates. The effect of temperature, injected water salinity and composition, the composition of oil sample (acid and base numbers), rock structure specifications, and aging duration were analyzed. The authors also proposed different mechanisms based on the media length scale. Another screening criteria was also discussed by Chavan et al. [
16]. The salinity of injected brines, oil compositions, the wettability state of rock, and the connate water properties were considered for screening and predicting a favorable condition for LSWI performance in carbonates. They suggested that high clay content, optimum brine salinity, basic environment, presence of polar components in the oil, oil-wet state of the rock, and brackish connate water are required to achieve a successful LSWI.
With the help of machine learning methods, other screening criteria were developed by analyzing available data of different successful and unsuccessful experimental studies reported in the literature. Wang et al. reported such an approach to screening required criteria for LSWI in sandstone reservoirs [
17]. The authors mentioned that a combination of criteria is needed to achieve the wettability alteration, and the effect of a single parameter is not enough to evaluate the method’s performance. A similar data-driven analysis was reported by Salimova et al. for carbonate formations [
18]. Data from different sources were analyzed to investigate the effect of rock and fluid properties on the active mechanisms of LSWI in carbonates and the success of the approach. They found that parameters such as pH, water salinity, and ion composition are strong parameters to forecast the possible outcome of the LSWI implementation.
The effect of oil properties as a screening parameter on the performance of LSWI was also discussed in other sources such as Tetteh et al. [
6] and Hao et al. [
7]. It is widely reported that the initial state of wettability is strongly affected by the crude oil properties. Presence of polar components in oil results in a more oil-wet state due to their adsorption to the rock surface. Parameters such as the acid and the base number impact carbonates wettability and, consequently, recovery due to the wettability alteration by LSWI. The higher the acid number, the more oil-wet rock is, which affects the oil extraction by LSWI [
19]. Base number variation was also studied by Puntervold et al. (2008) [
20]. Hence, acid and base numbers are other oil properties that should be considered for a successful selection and design of LSWI [
21]. The effect of other properties, such as the fraction of heavy polar components in oil, was also investigated by Tang et al. [
22,
23].
Even though considering the oil parameters are significant in the LSWI screening and implementation, the effect of physical oil parameters such as oil viscosity and API on the success of LSWI has not been investigated yet. [
16]. For example, in 160 core flooding tests analyzed by Salimova et al. [
18], the viscosity of only 3% of oil samples was higher than 15 cp, which shows the tendency to apply LSWI for light oils. Hence, there is still a question on the performance of LSWI in heavier oil, where other parameters such as acid number are favorable.
The application of LSWI in the combination of thermal EOR methods for heavy oil was reported in a few papers in the literature. Abbas et al. [
23] conducted oil displacement experiments with different oil samples with 1700, 1000, and 700 cp viscosity. Hot water was used in their tests, which combined thermal EOR and LSWI. Their results showed a low recovery for oil samples with 1700 cp viscosity. Al-Saedi et al. [
24,
25] conducted a similar approach with LSWI and thermal steam injection to a core saturated with a 600 cp oil sample. The core flooding was conducted on a cycled injection scheme. Despite the steam effect here, the injection of LSWI resulted in higher recovery than the standalone steam flooding, which shows the possible activity of LSWI mechanisms for heavy oil. Sekerbayeva et al. [
26] and Shakeel et al. [
27] also used LSWI for 170–190 cp oil combined with chemical EOR methods. Due to the high temperature and presence of chemicals, still, the effect of LSWI is not clear. Similar recovery tests for heavy oil were reported in the papers [
28,
29,
30,
31,
32,
33,
34].
Hence, still, the applicability of LSWI as a standalone EOR approach for heavy oil is not clear. Oil viscosity should be another criterion for screening LSWI as an EOR approach. In this study, we will answer these questions by conducting laboratory experiments and collecting data reported in the literature.