Low Salinity Waterflooding Effect Research Articles

Assessing the impact of geochemical mechanism and interpolation factor selection on the precision of low-salinity waterflooding modelling: a comparative study

ABSTRACT The efficacy of low salinity waterflooding (LSWF) as an enhanced oil recovery (EOR) method in carbonate formations has been well established; however, its geochemical modelling in such reservoirs remains insufficiently explored. This study aims to bridge this gap by conducting a comparative analysis of three well-recognised geochemical mechanisms including fines migration, rock dissolution/precipitation, and multi-ion exchange (MIE). Our research encompasses two parts: experimental and modelling. First, we conducted four coreflooding experiments to study the effect of LSWF on oil recovery in core scale as well as on rock wettability by measuring the contact angle of the crude oil-brine-rock system. In the next step, we created a one-dimensional compositional model in CMG-GEMTM for LSWF, validated it using the obtained experimental results, and compared the accuracy of the three geochemical mechanisms one-by-one. The results of the experimental section, confirm the positive effect of LSWF on oil recovery: 34.3% with seawater (40,000 ppm), 42.5% with LoSal-01 (10,000 ppm), 49.7% with LoSal-02 (5,000 ppm), and 60.9% with LoSal-03 (2,000 ppm). The experiments also show the wettability alteration through the reduction in the contact angle. Furthermore, the modelling results reveal that choosing MIE as the governing mechanism and sulphate concentration ([ S O 4 2 − ]) as the relevant interpolation factor (IF) leads to the most accurate LSWF model. After identifying the most accurate mechanism, we detected the wettability alteration in the model by showing the movement of oil and water relative permeability curves intersection point on the Kr-Sw diagram toward the right side.

Pore-scale investigation of low-salinity water flooding in a heterogeneous-wet porous medium

Low-Salinity Water Flooding (LSWF) is a technique aimed at modifying the interactions between rock and fluids particularly altering wettability and reducing interfacial tension (IFT). However, there remains limited understanding of how heterogeneous wettability and the presence of Initial Water Saturation (Swi) can impact the effectiveness of LSWF. This study contributes to a deeper understanding of LSWF mechanisms in the context of heterogeneous wettability, while also considering Swi. The simulations were conducted using OpenFOAM, employing a non-reactive quasi-three-phase flow solver that accounts for wettability alteration and IFT reduction during the mixing of Low-Salinity (LSW) and High-Salinity Water (HSW). A heterogeneous pore geometry is designed, and four distinct scenarios are simulated, encompassing both heterogeneous and homogeneous wettability conditions while considering the presence of Swi. These scenarios included secondary High-Salinity Water Flooding (HSWF), tertiary and secondary LSWF. Notably, the simulations reveal that secondary LSWF consistently yields the highest oil recovery across all scenarios, achieving recovery rates of up to 96.98 %. Furthermore, the presence of Swi significantly influences the performance of LSWF in terms of oil recovery, particularly in heterogeneous wettability conditions where it boosts recovery by up to 3.5 %, but in homogeneous wettability, it decreases recovery by nearly 26 %. These simulations also underscore the pivotal role played by the distribution of oil and HSW phases in profoundly affecting the outcomes of LSWF.

Experimental investigation on fluid/fluid and rock/fluid interactions in enhanced oil recovery by low salinity water flooding for carbonate reservoirs

The low salinity water flooding (LSWF) technique has established itself as cost-effective and environment-friendly in nature but its mechanisms are still unclear. Recovery due to LSWF mainly depends on two types of interaction, viz. fluid/fluid and rock/fluid. Despite the significant investigation, there is still no consensus on which interaction mainly governs oil recovery. This study intends to provide essential answers to the aforementioned issues, as well as additional significant insights into the dilution behaviour of Sea Water (SW) to be injected into a carbonate formation with equivalent salinity. A series of experiments were designed to assess both fluid/fluid and rock/fluid interaction thoroughly viz. compatibility test, IFT, interfacial rheology, zeta potential, and contact angle. A comprehensive number of individual and sequential coreflooding were conducted on outcrop cores to quantify the oil recovery potential of optimum salinities and gain more insight into wettability shifts due to LSWF. Rock/fluid and fluid/fluid interfaces showed different optimal conditions for improved oil recovery. Investigation of fluid/fluid interaction showed an optimal salinity of 2dSW as optimal salinity due to IFT reduction and increased viscoelastic properties. Rock/fluid interaction showed an optimal salinity of 10dSW due to enhanced zeta potential and calcite dissolution. Rock/fluid interaction showed an incremental recovery of 14% and fluid/fluid interaction showed an incremental recovery of 7% over simple SW injection. Sequential flooding based on rock/fluid interaction showed an incremental recovery of 16% over simple SW. Oil recovery based on rock/fluid interaction showed the highest recovery as compared to fluid/fluid interaction. Increased interfacial elasticity and calcite dissolution lead to wettability alteration and increased oil recovery due to LSWF. The effectiveness of LSWF depends upon optimum salinity and wettability alteration capability of low salinity slug. Results of this study also indicate that considering only one interaction either rock/fluid or fluid/fluid can lead to erroneous interpretation of LSWF data. Rock/fluid interaction and rock geochemical reactions must be considered while designing the LSWF recipe and low salinity slug should be injected right from the start to reap the benefits of LSWF.

Investigation of impact of low salinity water flooding on oil recovery for sandstone reservoirs – a simulation approach

Low salinity Waterflooding (LSWF) is a proven EOR technique which has emerged in the last few decades. In this, the salinity of the injected water is reduced. Studies have unveiled that the LSWF can improve about 5–38% of oil recovery. This work aims to study the effectiveness of LSWF by modeling Sea waterflooding (SWF) and LSWF using a commercial reservoir simulator and justifying the proposed mechanisms responsible for the incremental oil recovery due to LSWF. A critical parametric study has been undertaken, which led us to conclude that LSWF is a cost-effective, and environment-friendly method that results in additional oil recovery for sandstone reservoirs.

Synergistic effect of low saline ion tuned Sea Water with ionic liquids for enhanced oil recovery from carbonate reservoirs

In recent decades, low salinity water flooding (LSWF) has gained much attention from both the industry and academia due to its effectiveness in oil recovery. Many efforts are still going on to harness the effect of LSWF combined with chemical enhanced oil recovery (CEOR). However, studies identifying Ionic Liquids (ILs) as potential candidates for CEOR with LSWF are not available in the literature. This study aims to evaluate the synergistic effect of ILs with varying chain lengths ([C8mim] [BF4], [C10mim] [BF4], [C12mim][BF4]) with the low saline ion tuned seawater (ITSW) for enhanced oil recovery (EOR) from carbonate reservoirs. A comprehensive analysis viz. stability and compatibility of ILs, IFT, zeta potential, contact angle, and adsorption analysis were performed to establish the mechanisms. Finally, oil recovery potential was quantified by core flooding. ILs showed a better synergistic effect with ITSW than SW. The IFT value was found to be decreasing with increasing chain length and reduction in salinity. ILs with ITSW demonstrated a strong ability to alter the wettability of rock surfaces to less oil-wet conditions. Zeta potential values turned towards the positive side with adding ILs at the oil/brine and rock/brine interfaces. In addition, ILs have shown significantly less adsorption density in ITSW than SW at the simulated reservoir temperature of 90 °C. IL having the longest chain length ([C12mim] [BF4]) showed superior properties compared to other ILs and reported an incremental recovery of 12% of original oil in place (OOIP) with low saline ITSW. The primary mechanisms involved for better performance of [C12mim] [BF4] with ITSW are calcite dissolution, the interplay of potential determining ions, and enhanced ion pair formation.

Visual investigation of the effect of clay-induced fluid flow diversion on oil recovery, as a low-salinity water flooding mechanism

Low-salinity water flooding (LSWF) is an EOR method applicable to both sandstone and carbonate reservoirs. In recent years, LSWF has become an attractive technique but still some uncertainties concerning the governing mechanisms exist in studies. Mechanisms such as wettability alteration, osmosis, and fluid flow diversion by fines migration are proposed to explain the incremental oil recovery by LSWF in sandstones. Flow diversion is still being debated as to whether this mechanism works and, if it does, to what extent. This mechanism occurs due to clay swelling and fines migration, which results in pores blockage, diversion of the fluid flow to unswept areas, and enhanced oil displacement. Hence, the presence of clays is claimed to be a necessity for the success of LSWF in sandstone reservoirs. Here we proposed a new approach to study effects of different mechanisms of LSWF alone and in combination of each other using micromodels to track fluid flow diversion and the reason behind it and its subsequent benefits in case of oil recovery, visually. In this paper micromodel tests in different wetting and clay presence conditions were designed and conducted to investigate the standalone fluid flow diversion mechanism and the combination of it with wettability alteration and osmosis effects during LSWF. Tests conducted in water wet and oil wet states showed that the presence of clays is not the main criterion for activating the LSWF effect and the initial wettability is more critical. Our studies showed that the fluid flow diversion share in oil recovery is only 30% of the total incremental oil displaced by LSWF, while about 70% of the oil is produced by wettability alteration and osmosis mechanisms. It is reasonable to say that, although flow diversion is an active mechanism, it is not the dominant one and on the other hand wettability alteration is.

Microfluidics experimental investigation of the mechanisms of enhanced oil recovery by low salinity water flooding in fractured porous media

Spontaneous imbibition of water from fracture into the matrix is considered as one of the most important recovery mechanisms in the fractured porous media. However, water cannot spontaneously imbibe into the oil-wet rocks and as a result oil won’t be produced, unless the capillary pressure barrier between fracture conduits and matrix is overcome. Wettability alteration is known as the main affecting mechanism for low salinity water flooding (LSWF), however, its effectiveness in fractured porous media has been less investigated, especially in the case of possible pore scale displacement mechanisms. In this study, the effectiveness of LSWF (diluted seawater) on oil recovery is compared to the formation water (FW) and seawater (SW) in four different fractured models. Each of these fractured models is representing a different sub scale of the NFR. In addition to the effect of fracture geometry, the effect of wetting conditions was studied. Contact angles, Interfacial Tension (IFT) and zeta potentials are measured to have more insight on the Crude-oil/Brine/Rock system under investigation.The results of micromodel experiments show a positive effect of LSWF in both water-wet and oil-wet states. Sessile drop and pendant drop tests show that contact angle alteration and IFT reduction cannot be the dominant mechanisms. Visual investigations on dynamic displacement tests, suggest that the formation of micro-emulsions and elastic oil/water interface are the main mechanisms of recovery in the case of LS and FW injections, respectively. Elastic FW/oil interface causes discontinuous water phase in the fracture, which in turn reduces the mobility of the FW in the fracture and facilitates the diversion of the flow from fracture towards the matrix. This mechanism results in better sweep efficiency for the case of FW compared to the case of SW. Nevertheless, the dominant formation of microemulsion in the case of LSW is more effective in the diversion of the flow from fracture into the matrix (compared to the effect of interface elasticity in the case of FW). This mechanism along with slightly better imbibition (less oil-wet state) in the case of LSWF enhances its recovery performance compared to the FW in both water-wet and oil-wet systems.

Simulating the effect of salinity and kaolinite concentration on low salinity water flooding in sandstone reservoir of Sabah Basin

Low salinity waterflooding (LSWF) is a technique that involves injection of water at low salinity to improve oil recovery of matured oilfields. The primary objective of this study is to determine the effects of LSWF at different salinity in sandstone reservoirs on oil recovery and to simulate the effect of rock hysteresis on oil recovery. Reservoir simulation softwares ECLIPSE 100 and Petrel were utilized to simulate low salinity waterflooding in sandstone reservoir of the Sabah Basin. This study considers salinities from 100 up to 500 ppm (parts per million) at 1100 STB/day and 500 psi bottom-hole pressure. The results revealed that the highest oil recovery was 17.78 MMSTB at 100 ppm, while the lowest was 17.00 MMSTB at 500 ppm. This demonstrates that lower salinity water result in higher oil recovery. The difference in eventual recovery is not substantial when compared to the rate of recovery. Additionally, there is a strong link between incremental oil recovery and kaolinite content as well as the compaction and sand production. Oil is indirectly mobilised because of the clay particle composition. In addition, compaction causes an increase in residual oil saturation and irreversible water saturation. Furthermore, the larger the amount of kaolinite clay, the more oil may be extracted progressively.

Influence of carbonate impurities on smartwater effect: Evaluation of wettability alteration process by geochemical simulation

Low salinity water flooding (LSWF) has been considered as a promising technique for enhanced oil recovery (EOR). The wettability alteration towards a more water-wet state is recognised as the main mechanism for the positive LSWF effect. Electrokinetic interactions occurring at crude oil/brine and rock/brine interfaces affect the wettability alteration. Most of the studies reported in the existing knowledge considered synthetic calcite to understand the electrokinetics of natural carbonate with brines. The mineral impurities present in natural carbonate could influence the electrokinetics and subsequently wettability alteration. In this study, the surface complexation model, thermodynamic equilibrium model, and extended Derjaguin-Landau-Verwey-Overbeek (DLVO) theory were combined to evaluate the effect of impurities (dolomite and anhydrite) in natural carbonate on wettability alteration. The surface complexation modelling parameters were first estimated by fitting the calculated zeta potential to that of measured value in various brines and then validated in smartwater. The thermodynamic equilibrium of impurities is largely insensitive to crude oil/brine interface properties. However, the calculated carbonate/brine surface or zeta potential changed from positive to negative with anhydrite equilibrium in the brines containing Ca2+ or Mg2+ ions. The attractive or repulsive forces between crude oil and carbonate were estimated from disjoining pressure using extended DLVO theory and compared with measured adhesion forces. The computed disjoining pressure was used as an indicator to evaluate wettability alteration. It is found that brine with SO42- ions can result in highest water-wet condition, followed by smartwater (brine with Ca2+, Mg2+, and SO42-) and brine with either Ca2+ ions or Mg2+ ions. The de-ionic (DI) water is unfavourable for wettability alteration. The effect of impurities equilibrium and the crude oil surface site densities on the disjoining pressure were also discussed.

The impact of the electrical behavior of oil-brine-rock interfaces on the ionic transport rate in a thin film, hydrodynamic pressure, and low salinity waterflooding effect

Wettability alteration is the principal low-salinity-effect (LSE) in many oil-brine-rock (OBR) systems. Our recent experimental results have demonstrated that wettability alteration by low salinity is slow. It is expected that the electrical behavior of oil/brine and rock/brine interfaces and the water film geometry control both the transient hydrodynamic pressure, and the time-scale of ionic transport in the film, thus the kinetics and degree of wettability alteration. In this paper, the electro-diffusion process induced by the imposed ionic strength gradient is simulated by solving Poisson-Nernst-Planck equations in a water film bound between two charged surfaces, using a finite element-based computational fluid dynamics method. Both the non-equilibrium electric-double-layer (EDL) pressure and the time-scale of diffusion under different plausible electrical boundary conditions (BCs) are determined. The numerical results show that electro-diffusion in the thin film is non-Fickian, strongly dependent on the electrical BCs, and significantly (10–20 times) slower than Fickian diffusion. Among various BCs, those which lead to the strengthening of the electrostatic force, or electric field (such as constant charge BC), are the most favorable in terms of observing LSE. Moreover, it is found that the contribution of the osmotic pressure in the vicinity of the pore (bulk) fluid is negligible and that Maxwell stress is the dominant source of EDL force build-up. This force can then trigger wettability alteration. Furthermore, while both film length and thickness influence the electrical interaction of interfaces, the film thickness affects mainly the EDL force rather than the rate of ionic transport. On the contrary, the film length has a significant effect on the time-scale of diffusion. The effect of the ionic strength gradient on the time-scale of diffusion and LSE is relatively minor. This study provides novel insights into the role of the electrical behavior of OBR interfaces and film phenomena in the rate of ionic transport and establishment of low salinity in the film. Thin film modeling is a means to develop predictive capability for LSWF, screen OBR candidates, and to determine favorable conditions to observe LSE. Moreover, the slow kinetics of LSE necessitates accounting for the time-effect in the experimental evaluation of LSWF.

Experimental investigation on the dominating fluid-fluid and rock-fluid interactions during low salinity water flooding in water-wet and oil-wet calcites

Low salinity water flooding (LSWF) has the potential to enhance the oil recovery by affecting the fluid-fluid and rock-fluid interactions. Therefore, a systematic investigation on the effect of initial wetting state (water-wet or oil-wet) of pure calcite is conducted to study the importance of these interactions on the effectiveness of LSWF. In the case of initially water-wet cores, more oil recovery efficiency is observed for more saline water cases. To shed light on the possible involved mechanisms, dynamic IFT, dynamic contact angle (CA), oil/brine and rock/brine surfaces zeta potentials, and effluent pH are measured. It is shown that the short-term effect of IFT reduction and long-term effect of wettability alteration toward neutral-wetness (due to change in the electric charge of the crude-oil/brine and calcite/brine) are the dominating recovery mechanisms. However, the oil recovery behavior of oil-wet cores is in the opposite direction of that of water-wet cores, which demonstrates the wettability dependency of the involved mechanisms. CA measurement shows that diluted sea water (dSW) has more ability to change wettability toward water-wetness. The zeta potential results show that as brine salinity reduces, both crude-oil and rock surfaces become more negatively charged. Therefore, the observed wettability alteration behavior with salinity reduction is attributed to larger repulsion between these surfaces at lower salinity. pH results suggest CO 3 2− ions are responsible to make calcite surface more negative with diluting SW brine. The oil recovery behavior is also a strong function of the initial wetting state. SW brine, recovered less oil and experienced an earlier breakthrough in an initially oil-wet core compared to an initially water-wet core. Unlike the previous case, dSW experienced higher ultimate oil recovery and almost the same breakthrough time in an initially oil-wet core compared to an initially water-wet core. The superior performance of the low salinity water in an oil-wet core is a result of the wettability alteration toward more water-wetness which triggers more piston-like advancement. • A systematic investigation on the contribution of different fluid-fluid and rock-fluid interactions in the oil recovery of low salinity water flooding in pure water-wet and oil-wet calcite rocks. • Dynamic IFT, dynamic contact angle, the zeta potential of both crude oil/brine and calcite/brine surfaces, and the effluent pH are measured to asses these interactions. • In the case of initially water-wet calcite, the oil recovery increases with the rise in the salinity. • In the case of oil-wet calcite, higher oil recovery achieved in the case of brine with lower salinity. • The effectiveness of low salinity water flooding, optimum salinity, ultimate oil recovery factor and breakthrough times are functions of rock wettability.

Pore scale visualization of fluid-fluid and rock-fluid interactions during low-salinity waterflooding in carbonate and sandstone representing micromodels

Low Salinity Waterflooding (LSWF) has become a popular tertiary injection EOR method recently. Both fluid-fluid and fluid-rock interactions are suggested as the contributing mechanisms on the effectiveness of LSWF. Considering the contradictory remarks in the literature, the dominating mechanisms and necessary conditions for Low Salinity Effect (LSE) varies for different crude oil-brine-rock (CBR) systems. The aim of the present study is to investigate LSE for an oil field in the Middle East that is composed of separate sandstone and limestone layers. Contact angles and Interfacial Tension (IFT) are measured to have more insight on the CBR under investigation. Visual experiments were conducted in micromodels to determine the dominating mechanisms of LSE. Experiments were conducted under water-wet and oil-wet conditions for both sandstone and limestone representing micromodels. Several secondary and tertiary injection scenarios were performed with different levels of salinities. Consecutive injection of high-salinity and low-salinity water illustrated that LSE occurs under certain conditions. For both rock types, during the secondary high salinity injection, the oil trapping mechanism was mostly snap-off under water-wet condition, while fingering/bypassing was dominant under oil-wet conditions. LSWF causes diminutive changes of saturation redistribution in water-wet micromodels. Even in rare occasions that oil ganglia started to reshape or redistribute, it became trapped again in the downstream pores of the model, and not produced effectively. Contrary to this, LSWF was very effective under oil-wetting conditions. Continuous oil films, which exist in oil-wet condition, are beneficial for the effectiveness of LSE. Measured IFT and contact angles cannot justify the observed LSE. Additionally in the designed micromodels, any contribution of clay or fine migration to the LSE is ruled out. Spontaneous formation of the micro-emulsion was a very weak mechanism in our experiments and did not contribute to pushing oil out of the dead end pores or divert the flow path to the un-swept areas. Visual investigations suggested that formation of the visco-elastic interface can be the dominating mechanism for LSE in the investigated CBRs which is more effective in the case of oil-wet system due to the lack of snap-off trapping mechanism.

Direct numerical simulation of the effects of fluid/fluid and fluid/rock interactions on the oil displacement by low salinity and high salinity water: Pore-scale occupancy and displacement mechanisms

Laboratory experiments have shown that performance of waterflooding in oil reservoirs could be significantly increased by lowering the ionic strength and/or manipulation of its composition, which is generally known as low salinity waterflooding (LSWF). The involved mechanisms in additional oil production can be generally categorized in two categories, fluid/fluid and fluid/rock interactions. The distribution of the phases and the involved displacement mechanisms would be strongly affected by the inter-relations between capillary and viscous forces. Although there have been recent advances in the simulation of the LSWF at core scale and beyond and some models are included in commercial core-scale/field-scale simulators, there are very few works that modelled and simulated this process at the pore scale. As a result the effects of wettability alteration and/or interfacial tension (IFT) change on the distribution of the phases at pore scale as well as dynamics of the displacement are not thoroughly investigated and not well understood, even in spite of many micromodel experiments. The aim of the present study is to investigate the fluids distribution, redistribution and dynamic of the displacement mechanisms at the true pore scale (inside a pore), by modelling both rock/fluid (dynamic contact angle) and fluid/fluid (dynamic oil/water IFT as well as diffusion between high salinity and low salinity water) interactions. Simulation results show that in addition to the fluid/rock interactions, fluid/fluid interactions are also important in the effectiveness of LSWF and shall be considered in simulation studies. This is especially true in the case of non-water-wet systems. Our simulations provide new physical insights into LSWF and the dynamics of two-phase flow under wettability alteration as well as IFT changes induced by the LSWF, which cannot be easily captured by laboratory experiments. The results are also used to better explain some of the observed discrepancies in larger scales such as micromodel or coreflood experiments.

Aging brine-dependent deposition of crude oil components onto mica substrates, and its consequences for wettability

While wettability alteration is widely accepted as the dominant mechanism underlying enhanced oil recovery (EOR) in sandstone reservoirs under low salinity water flooding (LSWF) conditions, the effectiveness of LSWF still varies substantially between laboratory experiments and the field indicating our incomplete understanding and control of the process. We use simple mica substrates to investigate the impact of aging conditions on the macroscopic contact angle and the microscopic properties (morphology, composition, stiffness, surface charge) of the surface. Mica samples pre-aged in brines of variable salt content prior to aging in crude oil at elevated temperature displayed macroscopic wettabilities ranging from complete water wetting to intermediate contact angles around 90° in ambient brines of variable salinity. Compared to the macroscopic wettability, variations of the microscopic properties as probed by Atomic Force Microscopy and Confocal Raman microscopy were found to be rather subtle. Clear signatures of asphaltenic deposits were observed ubiquitously, usually consisting of a combination of a thin film and larger aggregates. The deposits typically displayed a salinity-dependent elasticity and (predominantly negative) surface charge. Selective removal of ions from the pre-aging brine lead to strong variations of both macroscopic wettability and microscopic properties, including the response to low ambient salinity: For the same crude oil, reducing the salinity of the ambient brine can either increase or decrease the macroscopic oil contact angle, depending on the composition of the formation brine used to pre-age the mineral surface. These results clearly highlight the complex balance of interactions during brine-oil-rock aging and the subtle role of the connate water composition in extracting and binding different types of crude oil components and thereby setting the initial and boundary conditions for the oil recovery process.

Case study: the impact of Low Salinity Waterflooding on Sandstone Reservoirs at Egypt’s Western Desert

Surface chemistry has a great effect in enhancing oil recovery (EOR). For oil-wet sandstone reservoirs, low salinity waterflooding (LSWF) is effective as it can alter rock wettability and reduce the oil/water interfacial tension. LSWF application is related to the rock’s clay content and type. Clay hydrocarbon bonding can be formed through many mechanisms such as van deer forces and ionic bridge. LSWF effect is to weaken these bonds through two main mechanisms, Double Layer Expansion (DLE) and Multicomponent Ionic Exchange (MIE). This research figure out the impact of LSWF application through a comparsion between two fields (S & D), in Egypt’s Western Desert, which have depleted strongly oil-wet reservoirs with similar rock & fluid Properties. Field (S) is flooded by low salinity water (LSW), while field (D) is flooded by high salinity water (HSW). Fortunately, the LSWF application was with no extra desalination cost as the water source for field (S) flooding is a LSW aquifer zone, which has a salinity +/- 5000 ppm as total dissolved solids (TDS). Water Susceptibility for Field (S) rock showed good compatibility between the injected LSW, formation water and rock minerals. XRD and SEM for field (S) indicate calcareous cementation with detrital clays content around 5% which is mainly kaolinite. This composition helps to activate the LSWF effect. For field (S), the estimated ultimate recovery (EUR) is 46%, while EUR for field (D) is 39%. One of the main causes of this increase in field (S) is the successful LSWF application.

The effectiveness of low salinity waterflooding in naturally fractured reservoirs

Low Salinity (LoSal) waterflooding is a technique which promises to improve and enhance oil recovery. The method has been studied widely in the laboratories using various core plug samples of sandstones and carbonates. Most of the experimental outcomes have demonstrated the possibility of recovery increment in conventional reservoirs. Meanwhile, it is rare to find a discussion in these experiments about the effectiveness of LoSal waterflooding in naturally fractured reservoirs. In this paper, we investigate the potential benefits in fractured reservoirs using simulations.It is often reported from coreflood experiments that LoSal injection has the ability to alter the rock wettability and reduce residual oil saturations through chemical and physical interactions between the rock-oil-water system. Based on the published work, we have calculated relative permeability curves to represent the flow in regions flooded with LoSal water. This includes a qualitative representation of uncertainty. In this sense, the relative permeabilities are a proxy for any physical and chemical mechanisms which are not explicitly represented. The newly generated flow curves were applied to simulate LoSal waterflooding using a fine-scale model and a field scale model based on a fractured reservoir from the Middle East.The simulation results underline the benefits of LoSal whereby the oil recovery from fractured reservoirs can be increased. An additional benefit is the control of water production. The fine-scale modelling has provided an in-depth analysis and accurate estimation of the oil recovery without modelling artefacts or limitations, where an increment of (12–14)% of the Stock Tank Oil Initially In Place (STOIIP) were obtained under secondary and tertiary recovery scheme, respectively. The investigated scenarios demonstrated a sensitivity to the newly generated flow curves and how they affect the ultimate oil recovery. However, for the field scale model, a higher oil production rate has maintained during LoSal waterflooding up to 32 K BPD by the end of year fourteen compared with 25 K BPD in case of high salinity scenario, besides a reduction in the produced water by 35.7%. Therefore, the effectiveness of LoSal brine required a longer time frame to observe the improvements in the oil displacement or sweep efficiency.

Crude oil/brine/rock interface in low salinity waterflooding: Experiments, triple-layer surface complexation model, and DLVO theory

The determination of the electrokinetic properties of crude oil/brine and rock/brine interfaces is necessary to understand and evaluate the low salinity waterflooding (LSWF) effect on wettability alteration and enhanced oil recovery (EOR). The interface properties and characteristics of oil and rock minerals significantly affect LSWF and must be elucidated in detail. Herein, the interaction of crude oil, brine, and rock was studied to elucidate the LSWF effect in carbonate and sandstone reservoirs. A triple-layer surface complexation model was proposed to describe the calcite/brine and kaolinite/brine interfaces and was verified with zeta potential experiments at various pH and calcium, magnesium, and sulphate concentrations. Furthermore, the surface complexation model prediction of zeta potential agreed well with the measured data for the suspension in seawater, formation water, and associated dilutions. The predicted surface potential of crude oil, calcite, and kaolinite was used in Derjaguin-Landau-Verwey-Overbeek (DLVO) theory to estimate the total disjoining pressure as attractive/repulsive forces. The interaction between crude oil and calcite/kaolinite on seawater dilution and in the presence of sulphate in the diluted solution for calcite was evaluated via disjoining pressure. Moreover, the importance of electrokinetic properties on attractive/repulsive forces and the mechanisms for wettability alteration in crude oil-brine-kaolinite due to LSWF as well as in oil-brine-calcite because of sulphate addition are discussed.

Capillary pressure and relative permeability estimation for low salinity waterflooding processes using pore network models

In this work, the effect of low salinity waterflooding (LSWF) on capillary pressure and relative permeability is studied at pore scale. For this purpose, a pore-scale model implemented on OpenPNM is developed to describe fluid flow, salinity transport and wettability change in a pore network. The effect is studied by considering high salinity waterflooding followed by low salinity waterflooding. The system wettability change is introduced as modifications of the local contact angle that are induced by the water salinity reduction. These modifications are evaluated by calculating the contact angle change in each pore and throat of the network, and then integrating it. From this analysis, a methodology has been developed to study the LSWF impact on capillary pressure and relative permeability. This methodology has been successfully applied to two pore network cases available in an open rock data base; one network correspond to a sandstone and the other to a carbonate sample. The methodology developed here can be seen as a tool to complement laboratory tests needed to determine the efficiency of LSWF processes.

Novel insights into pore-scale dynamics of wettability alteration during low salinity waterflooding

Low salinity waterflooding has proven to accelerate oil production at core and field scales. Wettability alteration from a more oil-wetting to a more water-wetting condition has been established as one of the most notable effects of low salinity waterflooding. To induce the wettability alteration, low salinity water should be transported to come in contact with the oil-water interfaces. Transport under two-phase flow conditions can be highly influenced by fluids topology that creates connected pathways as well as dead-end regions. It is known that under two-phase flow conditions, the pore space filled by a fluid can be split into flowing (connected pathways) and stagnant (deadend) regions due to fluids topology. Transport in flowing regions is advection controlled and transport in stagnant regions is predominantly diffusion controlled. To understand the full picture of wettability alteration of a rock by injection of low salinity water, it is important to know i) how the injected low salinity water displaces and mixes with the high salinity water, ii) how continuous wettability alteration impacts the redistribution of two immiscible fluids and (ii) role of hydrodynamic transport and mixing between the low salinity water and the formation brine (high salinity water) in wettability alteration. To address these two issues, computational fluid dynamic simulations of coupled dynamic two-phase flow, hydrodynamic transport and wettability alteration in a 2D domain were carried out using the volume of fluid method. The numerical simulations show that when low salinity water was injected, the formation brine (high salinity water) was swept out from the flowing regions by advection. However, the formation brine residing in stagnant regions was diffused very slowly to the low salinity water. The presence of formation brine in stagnant regions created heterogeneous wettability conditions at the pore scale, which led to remarkable two-phase flow dynamics and internal redistribution of oil, which is referred to as the "pull-push" behaviour and has not been addressed before in the literature. Our simulation results imply that the presence of stagnant regions in the tertiary oil recovery impedes the potential of wettability alteration for additional oil recovery. Hence, it would be favorable to inject low salinity water from the beginning of waterflooding to avoid stagnant saturation. We also observed that oil ganglia size was reduced under tertiary mode of low salinity waterflooding compared to the high salinity waterflooding.

Impact of Brine Composition and Concentration on Capillary Pressure and Residual Oil Saturation in Limestone Core Samples

Low salinity water flooding (LSF) is a relatively simple and cheap EOR technique in which the salinit y of the injected water is optimized (by desalination and/or modification) to improve oil recovery over conventional waterflooding. Extensive laboratory experiments investigating the effect of LSF are available in the literature. Sulfate-rich as well as diluted brines have shown promising potential to increase oil production in limestone core samples. To quantify the low salinity effect, spontaneous imbibition and/or tertiary waterflooding experiments have been reported. For the first time in literature, this paper presents a comprehensive study of the centrifuge technique to investigate low salinity effect in carbonate samples. The study is divided into three parts. At first, a comprehensive screening was performed on the impact of different connate water and imbibition brine compositions/combinations on the spontaneous imbibition behavior. Second, the subsequent forced imbibition of the samples using the centrifuge method to investigate the impact of brine compositions on residual saturations and capillary pressure. Finally, three unsteady-state (USS) core floodings were conducted in order to examine the potential of the different brines to increase oil recovery in secondary mode (brine injection at connate water saturation) and tertiary mode (exchange of injection brine at mature recovery stage). The experiments were performed using Indiana limestone outcrops. The main conclusions of the study are spontaneous imbibition experiments only showed oil recovery in case the salinity of the imbibing water (IW) is lower than the salinity of the connate water (CW). No oil production was observed when the imbibing water had a higher salinity than the connate water or the salinity of the connate water and imbibing brine were identical. Moreover, the spontaneous imbibition experiments indicated that diluting the salinity of the imbibing water has a larger potential to spontaneously recover oil than the introduction of sulfate-rich sea water. The centrifuge experiments confirmed a connection between the overall salinity and oil recovery. As the salinity of the imbibing brines decreases, the capillary imbibition pressure curves showed an increasing water-wetting tendency and simultaneous reduction of the remaining oil saturation. The lowest remaining oil saturation was obtained for diluted sea water as CW and IW. The core flooding experiments reflected the results of the spontaneous imbibition and centrifuge experiments. Injecting brine at a rate of 0.05 cc/min, sea water and especially diluted sea water resulted in a significant higher oil recovery compared to formation brine. Moreover, when comparing secondary mode experiments, the remaining oil saturation after flooding by diluted sea water, sea water and formation water was 30.6 %, 35.5 % and 37.4 %, respectively. In tertiary injection mode, sea water did not lead to extra oil recovery while diluted sea water led to an additional oil recovery of 5.6 % in one out of two tertiary injection applications.