Insights into the impact of temperature on the wettability alteration by low-salinity in carbonate rocks (original) (raw)
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Insights into the Mechanism of Wettability Alteration by Low-Salinity-Flooding (LSF) in Carbonates
The low salinity effect (LSE) in carbonate rock has been less explored compared to sandstone rock. Laboratory experiments have shown that brine composition and (somewhat reduced) salinity can have a positive impact on oil recovery in carbonates. However, the mechanism leading to improved oil recovery in carbonate rock is not well understood. Several studies showed that a positive LSF effect might be associated with dissolution of rock, however, due to equilibration, dissolution may not contribute at reservoir scale which would make LSF for carbonate rock less attractive for field applications. This raises now the question whether calcite dissolution is the primary mechanism of the LSF effect. In this paper we aim to first demonstrate the positive response of carbonate rock to low salinity and then to gain insight into the underlying mechanism(s) specific to carbonate rock. We followed a similar methodology as in sandstone rock (see Mahani et al. 2015) by using a model system comprised of carbonate surfaces obtained from crushed carbonate rocks. Wettability alteration upon exposure to low salinity brine was examined by continuous monitoring of the contact angle. Furthermore, the effective surface charge at oil-water and water-rock interfaces was quantified via zeta-potential measurements. Mineral dissolution was addressed both experimentally and with geochemical modeling using PHREEQC. Two carbonate rocks with different mineralogy were investigated: Limestone and Silurian dolomite. Four types of brines were used: High salinity formation water (FW), Seawater (SW), 25×diluted SW (25dSW) and 25×diluted SW equilibrated with calcite (25dSWEQ). It was observed that by switching from FW to SW, 25dSW and 25dSWEQ, the limestone surface became less oil-wet. The results with SW and 25dSWEQ suggest that the low salinity effect occurs even in the absence of mineral dissolution, because no dissolution is expected in SW and none in 25dSWEQ. The wettability alteration to less oil-wetting state by low salinity is consistent with the zeta-potential data of limestone indicating that at lower salinities the charges at the limestone-brine interface become more negative indicative of a weaker electrostatic adhesion between the oil-brine and rock-brine interfaces, thus recession of three-phase contact line. In comparison to limestone, a smaller contact-angle-reduction was observed with dolomite. This is again consistent with the zeta-potential of dolomite showing generally more positive charges at higher salinities and less decrease at lower salinities. This implies that oil detachment from dolomite surface requires a larger reduction of adhesion forces at the contact line than limestone. Our study concludes that surface-charge-change is likely to be the primary mechanism which means that there is a positive low salinity effect in carbonates without mineral dissolution.
Scientific Reports, 2020
The injection of low-salinity brine enhances oil recovery by altering the mineral wettability in carbonate reservoirs. However, the reported effectiveness of low-salinity water varies significantly in the literature, and the underlying mechanism of wettability alteration is controversial. In this work, we investigate the relationships between characteristics of crude oils and the oils’ response to low-salinity water in a spontaneous imbibition test, aiming (1) to identify suitable indicators of the effectiveness of low-salinity water and (2) to evaluate possible mechanisms of low-salinity–induced wettability alteration, including rock/oil charge repulsion and microdispersion formation. Seven oils are tested by spontaneous imbibition and fully characterized in terms of their acidity, zeta potential, interfacial tension, microdispersion propensity, water-soluble organics content and saturate-aromatic-resin-asphaltene fractionation. For the first time, the effectiveness of low-salinity...
Wettability and oil recovery from carbonates: Effects of temperature and potential determining ions
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2006
The success of oil recovery from fractured low permeable carbonates by water injection is dictated by the wetting properties of the rock surface. Potential determining ions like Ca 2+ and SO 4 2− have influence on the surface charge of the carbonate rock and are thereby linked to its wetting properties. In the present study, zeta potentials of chalk surfaces were measured by changing the sulfate and calcium concentrations in aqueous chalk suspensions. Series of long-term spontaneous imbibition tests were conducted at 70, 100 and 130 • C using oil containing chalk cores which were close to neutral wetting conditions. The concentration of SO 4 2− in the imbibing fluids varied above and below the seawater concentration, while the Ca 2+ concentration was kept constant. Sulfate acted as a wettability modifying agent by improving the water wetting nature of the chalk. The major experimental observations were: (1) the zeta potential on chalk was determined by the relative concentration of Ca 2+ and SO 4 2− present.
Driving Mechanism of Low Salinity Flooding in Carbonate Rocks
2015
Several studies conducted mainly on the laboratory scale indicate that in carbonate rocks oil displacement can be influenced by the ionic composition of the brine, providing an opportunity to improve recovery by optimizing the brine mixture used in secondary or tertiary recovery. In industry this topic has been termed "low salinity flooding (LSF) in carbonates" while the underlying mechanisms are not very well understood. The increased oil recovery has been attributed to wettability alteration to a more water-wet state. However, in some studies a positive low salinity effect (LSE) has been ascribed to dissolution of rock, which occurs on the laboratory scale but due to equilibration of brine with carbonate minerals on larger length scales this is not relevant for the reservoir scale. Therefore, the objective of this paper is to gain a better understanding of the underlying mechanism(s) and investigate whether calcite dissolution is the primary mechanism of the LSE.
The Impact of Pore Water Chemistry on Carbonate Surface Charge and Oil Wettability
Transport in Porous Media, 2010
Water chemistry has been shown experimentally to affect the stability of water films and the sorption of organic oil components on mineral surfaces. When oil is displaced by water, water chemistry has been shown to impact oil recovery. At least two mechanisms could account for these effects, the water chemistry could change the charge on the rock surface and affect the rock wettability, and/or changes in the water chemistry could dissolve rock minerals and affect the rock wettability. The explanations need not be the same for oil displacement of water as for water imbibition and displacement of oil. This article investigates how water chemistry affects surface charge and rock dissolution in a pure calcium carbonate rock similar to the Stevns Klint chalk by constructing and applying a chemical model that couples bulk aqueous and surface chemistry and also addresses mineral precipitation and dissolution. We perform calculations for seawater and formation water for temperatures between 70 and 130 • C. The model we construct accurately predicts the surface potential of calcite and the adsorption of sulfate ions from the pore water. The surface potential changes are not able to explain the observed changes in oil recovery caused by changes in pore water chemistry or temperature. On the other hand, chemical dissolution of calcite has the experimentally observed chemical and temperature dependence and could account for the experimental recovery systematics. Based on this preliminary analysis, we conclude that although surface potential may explain some aspects of the existing spontaneous imbibitions data set, mineral dissolution appears to be the controlling factor.
Frontiers in Materials, 2022
The reservoir rock is made up of different minerals which contribute to the overall formation wettability. These minerals in their natural state differ in chemistry and structure and thus behave differently in an environment of varying composition and salinity. These have direct implications for enhanced oil recovery due to water flooding, or wettability alteration due to long-term exposure to brine. With the reservoir rock being a complex system of multiple minerals, the control of wettability alterations becomes difficult to manage. One of the dominant mechanisms responsible for wettability alteration is the mineral surface charge, which is dependent on pH, and fluid composition (salt type and salinity). For the first time, the surface charge development of barite, dolomite, and feldspar minerals in their native reservoir environments (accounting for the formation brine complexity) is presented. Also, the effect of oilfield operations (induced pH change) on minerals’ surface charge development is studied. This was achieved by using the zeta potential measurements. The zeta potential results show that barite and dolomite minerals possess positively charge surfaces in formation water and seawater, with feldspar having a near-zero surface charge. Furthermore, the surface charge development is controlled by the H+/OH− (pH), electrical double-layer effect, as well as ion adsorption on the mineral’s surfaces. These findings provide key insights into the role of fluid environment (pH, composition) and oilfield operations on mineral surface charge development. In addition, the results show that careful tuning of pH with seawater injection could serve as an operational strategy to control the mineral surface charge. This is important as negatively charged surfaces negate wettability alteration due to polar crude oil components. Also, the design of an ion-engineered fluid to control the surface charge of minerals was implemented, and the results show that reduction in the Ca2+ concentration holds the key to the surface charge modifications. Surface charge modifications as evidenced in this study play a critical role in the control of wettability alteration to enhance production.
Fuel, 2018
Wettability of the oil/brine/rock system is an essential petro-physical parameter which governs subsurface multiphase flow behaviour and the distribution of fluids, thus directly affecting oil recovery. Recent studies [1-3] show that manipulation of injected brine composition can enhance oil recovery by shifting wettability from oil-wet to water-wet. However, what factor(s) control system wettability has not been completely elucidated due to incomplete understanding of the geochemical system. To isolate and identify the key factors at play we used SO 4 2-free solutions to examine the effect of salinity (formation brine/FB, 10 times diluted formation brine/10 dFB, and 100 times diluted formation brine/100 dFB) on the contact angle of oil droplets at the surface of calcite. We then compared contact angle results with predictions of surface complexation by low salinity water using PHREEQC software. We demonstrate that the conventional dilution approach likely triggers an oil-wet system at low pH, which may explain why the low salinity water EOR-effect is not always observed by injecting low salinity water in carbonated reservoirs. pH plays a fundamental role in the surface chemistry of oil/brine interfaces, and wettability. Our contact angle results show that formation brine triggered a strong water-wet system (35°) at pH 2.55, yet 100 times diluted formation brine led to a strongly oil-wet system (contact angle = 175°) at pH 5.68. Surface complexation modelling correctly predicted the wettability trend with salinity; the bond product sum ([ > CaOH 2 + ][-COO − ] + [ > CO 3 − ][-NH + ] + [ > CO 3 − ][-COOCa + ]) increased with decreasing salinity. At pH < 6 dilution likely makes the calcite surface oil-wet, particularly for crude oils with high base number. Yet, dilution probably causes water wetness at pH > 7 for crude oils with high acid number.
Journal of Molecular Liquids, 2019
Wettability alteration has been identified as an important mechanism during low salinity water flooding in carbonate reservoirs. Oil composition, in particular, acidic and basic functional groups, plays an important role in regulating wettability. In this paper, we explored the potential of low salinity effect in reservoirs with high acidic components (acid number = 4.0 mg KOH/g and base number = 1.3 mg KOH/g) with a combination of approaches (e.g., contact angle and zeta potential measurements, and surface complexation modeling). We measured the contact angles of oil on calcite surfaces in presence of aqueous ionic solutions at different pH (3 and 8), salinity (0.01 and 1 mol/L), ion type (CaCl 2 and Na 2 SO 4) and temperatures (25-100 o C). Our results show that both salinity and ion type significantly affect contact angle at pH=8. However, at low pH (pH 3), the oil-brine-calcite system becomes strongly water-wet with minor effect from salinity, ion type, and temperature. Lowing salinity drives the zeta potential of both
Laboratory studies have shown that wettability of carbonate rock can be altered to a less oil-wetting state by manipulation of brine composition and reduction of salinity. Our recent study (see suggests that a surface-charge change is likely to be the driving mechanism of the low salinity effect in carbonates. Various studies have already established the sensitivity of carbonate surface charge to brine salinity, pH and potential-determining ions in brines. However, it has been less investigated i) whether different types of carbonate reservoir rocks exhibit different electrokinetic properties, ii) how the rocks react to reservoir-relevant brine as well as successive brine dilution and iii) how the surface charge behavior at different salinities and pH can be explained.