Low Salinity Waterflooding Effect Research Articles

Predicting the electrokinetic properties of the crude oil/brine interface for enhanced oil recovery in low salinity water flooding

The low-salinity waterflooding (LSWF) technique during enhanced oil recovery has received increasing attention over the last decade. Several studies have attempted to understand the effects of LSWF through both experiments and modelling, but their results are inconsistent due to a lack of understanding of the crude oil/brine and brine/rock interfaces. In this paper, the crude oil/brine interface was studied by developing a triple-layer surface complexation model. The carboxyl groups (–COOH) were attributed to the surface charge and electrical triple-layer development of the crude oil in LSWF. The zeta potentials of the emulsion at various pH levels and the calcium and magnesium concentrations were measured to examine the interface. These data were then directly fitted to the simulated zeta potentials to determine the surface site density of –COOH and the associated equilibrium constants for the dissociation and adsorption of calcium and magnesium. The –COOH site density was determined by fitting the pH-independent zeta potential, while the equilibrium constant values were estimated from the variations in the zeta potential with the changes in pH and the concentrations of calcium and magnesium. The determined surface complexation parameters were validated by comparing the experimental zeta potential data from different ionic solutions. The developed surface complexation model was used along with the estimated parameters to predict the interface of crude oil in seawater, formation water, and their dilutions. The simulated zeta potential results agreed well with the experimental data, demonstrating that the model is applicable to understand the crude oil/brine interface in LSWF. Finally, the importance of the prediction of the surface and zeta potentials in the evaluation of the interface and the estimation of electrostatic forces, and thus the wettability alteration, was discussed.

Combining preformed particle gel and low salinity waterflooding to improve conformance control in fractured reservoirs

The recovery from fractured reservoirs is usually low. The areal heterogeneity is one result of the fractured reservoir. Low salinity waterflooding (LSWF) and preformed particle gel (PPG) have recently drawn great interest from the oil industry. LSWF can only increase displacement efficiency, and it has little or no effect on sweep efficiency whereas PPG can plug fractures and improve sweep efficiency, but they have little effect on displacement efficiency. The coupled method bypasses the limitations of each method when used individually and improves both displacement and sweep efficiency.The main objective of this study was to determine whether the combining technologies can improve conformance control in fractured sandstone reservoirs. Before the study was conducted, the effects of low salinity waterflooding, number of fractures, and PPG strength were studied. The PPG was injected into the fracture at a flow rate 2.0 ml/min. Brine was injected at a different flow rate after PPG placement to test the effect of flow rate on the PPG’s plugging efficiency. Laboratory experiments showed that the oil recovery factor and the Frrw increased when the concentration of injected brine changed from conventional salinity to low salinity and the areal sweep efficiency was improved. However, the PPG extruded pressure decreased when the PPG swelled in a low-brine concentration. At a high-flow rate, there was no significant effect on the Frrw. Combining two different EOR technologies can improve displacement and sweep efficiency and, in turn, enhance conformance control.

Effects of Formation Brine Compositions on Effectiveness of Low Salinity Brine Injection in Carbonate Reservoir

Nowadays, Low Salinity Waterflooding (LSWF) is considered as one of the most effective techniques to enhance oil recovery. Essential mechanism is a Multi-Ion Exchange (MIE). In carbonate formation, collaboration of Potential Determining Ions (PDI) with rock surface and crude oil results in adjustment of surface equilibrium. These ions in injected brine are thoroughly investigated but effects from formation brine are not well documented. In this study, ratio of divalent ions including calcium and magnesium ions in formation brine is studied. Results from imbibition test shows that formation brine containing calcium ion to magnesium ion ratio of 1:1 yields response to LSWF. Oppositely, formation brines with high ratios of 10:1 and 15:1 do not respond to LSWF. As calcium ion in connate water diffuses to injected water, high calcium ion will exhibit dissolution mechanism of rock surface which is part of oil recovery mechanisms. Formation brine with high calcium ion is therefore not recommended for LSWF. High portion of magnesium ion in formation brine does not affect effectiveness of LSWF. Dissolution of magnesium from rock may be obstructed but oil can still be liberated from calcium carboxylate complex and dissolution of calcium. Coreflood test shows similar result which confirms previous explanation.

Contribution of osmotic transport on oil recovery from rock matrix in unconventional reservoirs

The fluid transport mechanisms in fracture and rock matrix play a critical role in economic viability of hydrocarbon recovery from unconventional reservoirs. Several transport mechanisms, including advection, diffusion, and convection, have varying degrees of contribution to hydrocarbon recovery depending on the transport properties of the formation. The contribution of the concentration gradient driven diffusion, or osmotic transport on hydrocarbon recovery has been numerically investigated in this study. The main objective of this study is to determine when the osmotic transport plays an important role on the mass exchange between the fractures and the rock matrix.In this study, a model for the mass transport between the rock matrix and the fractures has been formulated and validated with the experimental data. The numerical results showed that osmosis is an important force imbibing water into low permeability rock matrix and enhancing the effectiveness of low salinity waterflooding on oil recovery. The imbibition of water into oil-wetted shale matrix is mainly driven by the osmotic transport and wettability alteration. The contribution of the osmotic transport on oil recovery continues a long period of time, typically in a few years. This contribution increases if the membrane efficiency is high and the matrix block size is small. However, shale membrane efficiency is typically less than 10% considerably reducing the oil recovery factor by osmosis to less than 5%. Higher membrane efficiency and lower diffusion coefficient of dissolved ions increase the contribution of osmosis on the oil recovery from shale matrix.

Effects of low-salinity waterflooding on capillary pressure hysteresis

In this study, low-salinity waterflooding enhanced-oil recovery mechanisms linked to the water–crude oil interfacial viscoelastic response are investigated considering a distinct interfacial effect, namely the water-crude oil interfacial viscoelasticity response. In contrast, wettability alteration is interpreted through analysis of capillary hysteresis. A capillary pressure experimental setup used in our previous research, is utilized here to capture capillary hysteresis. Moreover, new improvements over the traditional quasi-static porous plate method have been implemented to accelerate measurements. The dynamic-static method, i.e. a combination of continuous injection in drainage and stepwise quasi-static method in imbibition on short (<1″ long) core samples, was found to capture the correct envelopes of the capillary pressure curves in a much shorter time span. Two pairs of experiments were conducted to investigate the interfacial visco-elasticity and wettability alteration effects on capillary hysteresis. One pair is conducted on Minnelusa formation rock samples and diluted TC crude oil at 30°C and without any significant aging to minimize wettability alteration. Two core plugs were flooded with high-salinity and low-salinity brines, separately. Results show that low-salinity brine can still increase oil recovery even in the absence of wettability alteration. This is attributed to the formation of a more visco-elastic brine-crude oil interface upon exposure to low-salinity brine, leading to a more continuous oil phase. Two additional experiment pairs were conducted on Berea and TC oil at high temperature (70°C). Two cores were subject to the same experimental conditions except that one was allowed to age with crude oil for 3weeks. This aging process was found to render the rock more oil wet, resulting in more pronounced capillary hysteresis and less oil recovery. Our study shows effects of two mechanisms, i.e. fluid–fluid interfacial visco-elasticity and wettability alteration, separately. The interpretation of our unique capillary pressure datasets sheds light on low-salinity waterflooding mechanisms.

Mechanistic Study of Oil/Brine/Solid Interfacial Behaviors during Low-Salinity Waterflooding Using Visual and Quantitative Methods

Despite the many studies of low-salinity waterflooding (LSWF), its underlying mechanisms are not well understood as a result of the complexity of the oil/brine/solid interfacial behaviors. Moreover, the widely held belief that LSWF is effective only under certain conditions has not been conclusively proven. Therefore, the static and dynamic interfacial behaviors during LSWF were visually and quantitatively studied in this work to elucidate the underlying mechanism(s). The results showed that LSWF effectively promotes oil recovery by causing double-layer expansion (DLE) and hydrocarbon solubilization. The DLE resulted from multicomponent ion exchange (MIE), as revealed by contact angle, ζ potential, and conductivity measurements. Emulsions prepared with low-salinity brines (≤0.21 wt %) were noticeably heavier and quite stable, demonstrating the significant solubilization effect of LSWF. The oil displacement dynamics observed in a visual micromodel indicated that LSWF increased the oil recovery factor by 4%...

Effect of injection brine composition on wettability and oil recovery in sandstone reservoirs

Wettability alteration and enhanced oil recovery (EOR) of sandstone by low salinity waterflooding (LSF) have become the focus of many studies. However, there is no clear explanation of how the ion composition of brines influences the LSF effect. In this paper, imbibition, ζ-potential measurements, core displacement, and microscopic displacement experiments were adopted to investigate the effect of brine composition on the wettability and oil recovery of sandstone reservoirs and the Ca2+ bridge action between rock surface and the acidic components in crude oil. Besides, the surface force model in a cylindrical pore was also developed for investigating the mechanisms behind wettability alteration and EOR by LSF. The results show that Ca2+ in the brine acted like a bridge between the negatively charged sites on rock surface and dissociated acidic components in crude oil, and the acidic components in crude oil played an important role in the Ca2+ bridge action. Divalent cations (e.g. Ca2+) in the brine hindered the low salinity effect, and thus the low-salinity brine without divalent cations showed optimal low salinity effect. Lowering the salinity of 1:1 electrolyte solution (e.g., NaCl solution) caused the expansion of the electrical double layer and increasing disjoining pressure between rock/brine and oil/brine interfaces, and thus the rock surface changed to be more water-wet and a thicker wetting-water film could be stable between rock surface and crude oil. When injecting low salinity NaCl brine, more water-wet state improves the ability of water imbibition in small pore and a thicker wetting-water film promotes destabilization of oil layers adhering to sandstone surface, thus resulting in EOR.

Visualizing Low-Salinity Waterflooding

Technology Update Low-salinity waterflooding has recently attracted considerable interest from governments, researchers, and oil companies, who are evaluating the method’s potential to increase oil recovery. While laboratory work and field tests have indicated that low-salinity waterflooding can have positive results, and implementation of the technology is planned, there are still a number of challenges and uncertainties related to the technique. It is generally accepted that the effects of low-salinity waterflooding are caused by wettability alteration, but the processes involved are still not fully understood. Heriot-Watt University has been conducting research to investigate the mechanisms of interactions between low-salinity water, crude oil, and porous media at reservoir conditions. Industry support for the work was secured through Industry Technology Facilitator (ITF), and professor Mehran Sohrabi, director of the Center for Enhanced Oil Recovery (EOR) and CO2 Solutions at the university, is leading the project team. Low-Salinity Water Injection Waterflooding improves oil recovery by displacing oil, and injection water is usually taken from the nearest available source (which is seawater for most offshore fields). The first effects of low-salinity water injection were published in the 1990s, when work at the University of Wyoming indicated that injecting water with a total dissolved-solids content of less than 5,000 ppm resulted in a higher oil recovery than that attained by injecting high-salinity water such as seawater. Since then, a number of studies with varying results have been published, and the recovery benefits described are likely to be field-dependent. Low-salinity waterflooding has a number of potential benefits. It has relatively low costs compared with other techniques. It can be implemented in ongoing and new waterflood projects onshore and offshore and has shown potential for sandstone and carbonate reservoirs. In addition, it can be used alone or in conjunction with other EOR techniques, e.g., polymer injection, as a technology enabler. It also can alleviate problems associated with conventional waterflooding, such as scale formation and souring.

Critical review of low-salinity waterflooding

It was observed that higher oil recovery could be obtained when low-salinity (LS) water flooded a core of high-salinity initial water about 15 years ago. Such low-salinity waterflooding benefit or effect has drawn the oil industry attention since then. In the recent years, many researchers conducted laboratory corefloods, and several companies carried field tests. The objectives of these efforts were (1) to conform the benefits and (2) find the mechanisms of such benefit. Although most of the results confirmed the positive effect, some results showed no benefit. Many mechanisms have been proposed, but there is no consensus of the dominant mechanism(s). The oil industry is continuing the effort to discover the effect. This paper is to provide a critical review of the results and to summarize the achievements of the industry׳s effort. This paper aims to provide the status of the art. The information provided in this paper hopefully will help to speed up our further efforts to explore this effect. The following contents are reviewed: (1) history of low-salinity waterflooding; (2) laboratory observations; (3) field observations; (4) working conditions of low-salinity effect; (5) mechanisms of low-salinity waterflooding; and (6) simulation of low-salinity waterflooding. In this paper, the mechanisms proposed in the literature and their validity are discussed. • Provide a comprehensive review of lab results, field observation and all proposed mechanisms. • Discuss the validity of each mechanism. • Provide the personal view of possible effects of low-salinity waterflooding in real field-scale.