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Organically crosslinked gels have been used to control water production in high temperature applications. These chemical systems are based on the crosslinking of a polyacrylamide-based polymer/ copolymer with an organic crosslinker. Polyethyleneimine (PEI) has been used as an organic crosslinker for polyacrylamide-based copolymers to provide thermally stable gels.Literature reported that PEI can form aqueous gels with polyacrylamide (PAM) at room temperature. In this paper, we show for the first time the possibility of crosslinking polyacrylamide with PEI at temperatures up to 140°C (285°F) and pressures up to 30 bars (435 psi). This paper reports data both in bulk and in porous media.The gelation time of the PAM crosslinked with PEI at high temperatures up to 140°C (285°F) and pressures up to 435 psi (30 bars) was measured. The effects of polymer concentration, crosslinker concentration, temperature, salinity, initial pH value, and the initial degree of hydrolysis of the polymer on the gelation time were examined in detail. All measurements were conducted in the steady shear mode. 13 C Nuclear Magnetic Resonance Spectroscopy ( 13 C NMR) was used to relate the gelation time to changes in the structure of the polymer and hence explain the variation in the gelation time in terms of the gelling system chemistry.In bulk, thermally stable gels were obtained by crosslinking PAM with PEI at 130°C (266°F) for at least 8 weeks. The performance of the PAM/PEI system in sandstone cores at a temperature of 90°C (194°F) and pressure drops of 68.95 bars (1,000 psi) was examined. The system was found to be stable for 3 weeks, where the permeability was reduced by a factor of 100%.

Literature review shows that improved oil recovery (IOR) by lowsalinity waterflooding could be attributed to several mechanisms, such as sweep-efficiency improvement, interfacial-tension (IFT) reduction, multicomponent ionic exchange, and electrical-doublelayer (EDL) expansion. Although these mechanisms might contribute to IOR by low-salinity water, they may not be the primary mechanism. Therefore, the main objective of this study is to investigate if the mechanism of EDL expansion could be the principal reason for IOR during low-salinity waterflooding.Low-salinity water results in a thicker EDL when compared to high-salinity water, so we tried to eliminate the effect of low-salinity brines on double-layer expansion to show to what extent IOR is related to EDL expansion caused by low-salinity water. The double-layer expansion is dependent on the electric surface charge, which is a function of the pH of brine; therefore, the pH levels of low-salinity brines were decreased in this study to provide low-salinity brines that can produce a thinner EDL, similar to high-salinity brines. f-potential measurements were performed on both rock/brine and oil/brine interfaces to demonstrate the effect of brine pH and salinity on EDL. Contact angle and coreflood experiments were conducted to test different brine salinities at different pH values, which could assess the effect of water salinity and pH on rock wettability and oil recovery, and hence involvement of EDL expansion in the IOR process.f-potential results in this study showed that decreasing the pH of low-salinity brines makes the electrical charges at both oil/ brine and brine/rock interfaces slightly negative, which reduces the double-layer expansion caused by low-salinity brine. As a result, the rock becomes more oil-wet, which was confirmed by contact-angle measurements. Moreover, coreflood experiments indicated that injecting low-salinity brine at lower pH values recovered smaller amounts of oil when compared to the original pH because of the elimination of the low-salinity-water effect on the thickness of the double layer. In conclusion, this study demonstrates that expansion of the double layer is a dominant mechanism of oil-recovery improvement by low-salinity waterflooding.

The ionic strength of injection water can have a major impact on oil recovery resulting from the use of low-salinity brines. Understanding how the water and oil chemistry affects the final recovery from a physicochemical point of view is necessary in order to optimize low-salinity waterflooding. It is clear from the literature that wettability is a key factor in achieving the low-salinity effect. Optimum ionic strength and conditions for low-salinity flooding with respect to wettability are still uncertain.In this paper, we studied fluid/rock interactions at different salinity levels and elevated temperature conditions in terms of wettability and surface charge. Wettability is determined by a high-temperature/high-pressure (HT/HP) contact-angle method and zeta-potential technique. Outcrop rocks and stock-tank crudeoil samples were used in all experiments. Synthetic formation brine water, aquifer water, and seawater were evaluated under highpressure conditions. Zeta potential of sandstone rocks and selected clay minerals was measured as a function of ionic strength.Wettability of oil/brine/sandstone systems depends on salinity, temperature, and rock mineralogy. Using aquifer water in Berea sandstone improved the wettability toward a water-wet condition. The same aquifer water behaved in a different way when a different sandstone surface was tested. In Scioto sandstone, aquifer water changed the wettability to a neutral state. Low-salinity water expanded the double-layer thickness and eventually increased the zeta-potential magnitude. As a result of this expansion, it provides a greater opportunity to alter the wettability and enhance oil recovery. This study indicates that clay content in sandstone rocks can significantly alter the wettability either toward water-wet or intermediate. On the basis of the results obtained from this study, it is clear that low-salinity waterflooding can improve oil recovery in the field.

Several mechanisms which could contribute to improved oil recovery (IOR) by low-salinity waterflooding have been proposed. However, the main mechanism is still debated. Nasralla et al. (2011a) showed that low-salinity water has an impact on the electrical charges at oil/brine and rock/brine interfaces, and it causes expansion of the double layer, which is the main reason behind wettability alteration and high oil recovery. Nevertheless, double-layer expansion by low-salinity water might contribute, but not be the primary mechanism. Therefore, this paper investigates this mechanism and shows how it is involved in the process of IOR. Electrical surface charge is a function of brine's pH, so the injected brine's pH was manipulated to show how oil recovery during low-salinity waterflooding depends on changes in electrical charges. Coreflood experiments were conducted using Berea sandstone cores 20 in. long; different brines (TDS varies from 0.5 to 55 kmg/l) were injected. Furthermore, zeta potential at rock/brine and oil/brine interfaces was measured for the brines used at different pHs. The contact angle technique was used to evaluate the alteration of rock wettability by the same brine at a different pH. Zeta potential results showed that decreasing low-salinity brines’ pH makes the electrical charges at both oil/brine and brine/rock interfaces weakly negative, which reduces the repulsion forces between oil and rock, and as a result the rock becomes more oil-wet, which was confirmed by contact angle measurements. Moreover, coreflood experiments indicated that injecting low-salinity brine at lower pH recovered less amount of oil compared to the original pH because of the elimination of the low-salinity water effect on electrical charges. In conclusion, expansion of the double layer is a dominant mechanism of oil recovery improvement by low-salinity waterflooding.

High-salinity water such as seawater, or formation brines, is frequently injected in carbonate reservoirs. Ion interactions between injection water, reservoir fluids, and rock surface are quite complex. It has recently come to be believed that the chemistry of injection water can significantly enhance oil recovery. Several reaction mechanisms were suggested, including rock dissolution, change of surface charge, and/or sulfate precipitation.This study attempts to characterize the electrokinetics of limestone and dolomite suspensions at 25 and 50°C. In addition, reaction mechanisms at the water/rock interface were established. Synthetic formation brine, seawater, and aquifer water were chosen from Middle East reservoirs. Carbonate particles were soaked in high-and low-salinity water. A phase-analysis-light-scattering (PALS) technique was used to determine the zeta potential (surface charge) of carbonate particles over a wide range of pH, ionic strength, and temperature.Zeta potential of limestone particles was significantly affected by calcium ion. Low-salinity water created more negative charges on limestone and dolomite particles by expanding the thickness of the diffuse double layer. Individual divalent cations decreased the zeta potential of limestone particles in sodium chloride solutions, while sulfate ions showed a negligible effect. Limestone particles in high-salinity water had decreased zeta potential. The solubility of calcium ions increased as temperature was increased and thus created additional negative charges. The absence of sulfate in aquifer water strongly influenced the dolomite surface charge. In summary, surface-charge adjustment from positive to negative can alter the wettability of carbonate rock from preferentially oil-wet to waterwet. As a result, residual-oil saturation should be decreased. IntroductionInterfacial phenomena at carbonate/water interfaces are controlled by the electrical-double-layer 1 (EDL) forces. Therefore, it is necessary to understand the behavior of the ions' interactions with the rock surface. Charged species 2 are transferred across any solid/liquid interface only until reaching equilibrium. The interface can be visualized as a semimembrane 3 that allows the common charged species between solid and solution to pass through. These species are called potential-determining ions. As a result of the relative motion between the charged dispersed phase and the bulk liquid, the EDL is sheared. The potential, at this shear plane, is commonly called electrokinetic or zeta potential (). Various methods are applied to measure the potential at the shear plane. However, the most commonly used technique is the electrophoresis method (Pierre et al. 1990).

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Optimization of any oil-recovery process is based on understanding of the recovery mechanisms, whereas the underlying mechanisms of improving oil recovery by low-salinity-water injection are still debated. Wettability alteration is one of the mechanisms suggested to be the primary mechanism of low-salinity water. Therefore, wettability alteration by low-salinity water in sandstone reservoirs is examined by using the contact-angle technique. The effect of changing ionic strength on the electrokinetic charges is investigated by use of the zeta-potential technique to explain the causes of wettability alteration by low-salinity water. Moreover, coreflood experiments were performed in order to correlate between the wettability alteration and oil-recovery improvement caused by low-salinity water.Mica sheets were used for the contact-angle measurements to represent sandstone rock. The effect of water salinity on wettability was studied by using synthetic water over a wide range of salinities (from 0 to 174 000 mg/L), with two different crude oils at different conditions of pressure and temperature. Zeta-potential tests were conducted to measure the electrokinetic charges for combinations of each of the two crude oils or Berea sandstone and different brines. Berea sandstone cores were used for waterflooding experiments by injecting the same brines tested in the wettability measurements.Low-salinity water showed lower contact angles compared with high-salinity water for the two types of crude oil, which demonstrates the ability of low-salinity water to alter the rock wettability to more water-wet. In addition, low-salinity water made the surface charges at rock/brine and oil/brine interfaces strongly negative. As a result, the repulsive forces between rock and oil surfaces increase, which leads to expansion of the double-layer and, consequently, wettability alteration and oil-recovery improvement as confirmed by coreflood experiments.