Low salinity water flooding has been shown to improve both secondary and tertiary recovery in core floods (Yildiz and Morrow 1996; Lager et al. 2008). Low salinity water-flooding is an enhanced oil recovery technique that is especially attractive due to its relatively low cost, its simple operational design and its low environmental risk. It has been observed by these researchers that injection of brine with a different composition than formation water or seawater, may give an increase in recovery and acts as secondary or tertiary injection fluid.
Several mechanisms were proposed which helps in improving oil recovery by injection of low salinity water. The debate about the primary mechanism of IOR by low-salinity water creates some uncertainties about the success and the optimum conditions of the application of low-salinity water-flooding on the field scale (Ramez A. Nasralla and Hisham A. Nasr-El-Din, 2014). The main proposed low salinity mechanisms are:
2.6.1. Fines Migration
An attempt was made by Tang and Morrow (1999) to explain migration of fines in low salinity flooding. They observed that migration of fines and wettability alteration is interlinked. Previous work has also shown that water flooding works in case of water-wet conditions. In presence of high salinity brine, clay minerals remain undisturbed, which makes rock oil-wet and results in poorer sweep efficiency. On the other hand, they found removal of fines from the surface in case of low salinity brines. They concluded that fines mobilization resulted in exposure of underlying surfaces, which increased water-wetness of the system.27 Removal of particles block the pore throats and diverts the flow to the un-swept area thus improves the sweep efficiency (Figure 7).
The mechanism of fines migration was explained by the Deryaguin-Landau-Verwey-Overbeek (DLVO) theory of colloids (Deryaguin and Landau, 1941; Verwey and Overbeek, 1948). The permeability reduction occurs if the ionic strength of the injected brine is equal to or less than, the critical flocculation concentration (CFC), which is strongly dependent on the relative con centration of divalent cations such as Ca2+ and Mg2+ (Khilar et al., 1990). Divalent cations have been known to stabilize the clay by lowering the zeta potential resulting in the lowering of the repulsive force.
16 Tang and Morrow, 1999, has shown that fine migration is likely to occur during the low salinity flooding. However, enhancement in oil recovery has also observed without any fine migrations.
Figure 7: Fine migration mechanism (Tang and Morrow, 1999)
2.6.2. pH Increase
Some studies have shown a rise in pH during low salinity production laboratory experiments (Figure 8). There are mainly two reasons for increase in pH are: 1) dissolution of minerals, 2) exchange of cations.
Dissolution of carbonate results high amount of hydroxyl ions (OH-) and cation exchange between the rock and the brine, which could explain the pH increase.
Dissolution reactions expressed by Lager et al, 2008:
𝐶𝑎𝐶𝑂3 ↔ 𝐶𝑎+2+ 𝐶𝑂3−2 (6) 𝐶𝑂3−2+ 𝐻2𝑂 ↔ 𝐻𝐶𝑂3−+ 𝑂𝐻− (7)
17 Figure 8: pH variation during a low salinity flood (Lager et al, 2008)
However, in cation exchange process, the mineral surface will exchange H+ ion in the brine.
This decreases H+ concentrations in the brine, resulting increases in pH. Speed of reactions depend on the amount of calcite mineral present.
Lager et al, 2008, stated that if a pH of above 9 was observed inside the reservoir then it would consider equivalent to alkaline flooding. They added that the increase in pH level allows the reaction of some of the oil compounds that result in generation of in-situ surfactants. Hence, the oil recovery could also be increased from the production of surfactant and interfacial tension reduction, by increase in pH.
2.6.3. Multicomponent Ion Exchange
Sposito38 stated that Vander Waals interactions, ligand exchange and cation bridging are some of the dominant adsorption mechanisms. According to DLVO theory, at high ionic strengths Vander Waals forces allow particles to be located close to each other. Ligand exchange occurs when carboxylate groups substitute hydroxyl group38. In cation bridging, a cation acts as a bridge between a negative charged surface and negatively charged functional groups (Arnarson and Keil, 2000).
Lager25 observed that presence of divalent ions is essential in order to achieve multi-component ion exchange (MIE), hence, increase in recovery by injecting a low salinity brine. Second
18 observation was made that decrease in Mg+2 concentration in effluent indicates adsorption of magnesium ion.
Low concentration of divalent ions in low salinity brine causes MIE to take place between adsorbed crude oil components, cations in brine and clay surfaces37. The result is that organic polar compounds and organic-metallic complexes are removed from the surface, causing increase in water-wetness and an increase in oil recovery.
Figure 9: Adsorption mechanisms (Webb et al., 2008) (adapted from37)
2.6.4. Double layer Effects
An electrical double layer is a thin surface layer of spatially separated opposite electrical charges, formed at the interface of two phases (Figure 10). According to Ligthelm, in case of low salinity brine injection, cations have the lower ability to take down the negative charges of the oil and clay. Which increase the repulsive forces between the oil/water and rock surface. Hence, helps in releasing of oil components and increase in oil recovery. In practical terms, in a high saline environment, polar components of oil form organo-metallic complexes by adsorbing on surface (Zhang et al, 2006). This changes wettability of rock to the less water-wetness.
19 When low salinity brine is injected, the ability of cations to screen off the negative charges between oil and rock is reduced; electrical double layer expands and repulsive forces increases.
This scrape off the oil molecules from surface and increase in oil recovery.
Figure 10: Double layer structure near the negatively charged surface (Ramez A. Nasralla and Hisham A. Nasr-El-Din, 2014).
20 3.
Experimental Work
Flooding experiments were performed in order to evaluate the EOR methods. This way we will be able to suggest an effective brine composition which leads to a significant increase in oil recovery. In this section we will discuss mainly about the experimental procedure, apparatus, and materials used.
This section includes core details, fluid properties, operating conditions, brine composition, preparation & execution of experiments and analysis after.