Electrokinetics of Carbonate/Brine Interface in Low-Salinity Waterflooding: Effect of Brine Salinity, Composition, Rock Type, and pH on ?-Potential and a Surface-Complexation Model

Abstract

Summary 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 (Mahani et al. 2015b) suggests that surface-charge alteration 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 value, and potential-determining ions in brines. However, in the majority of the studies, single-salt brines or model-carbonate rocks have been used and it is fairly unclear how natural rock reacts to reservoir-relevant brine as well as successive brine dilution; whether different types of carbonate-reservoir rocks exhibit different electrokinetic properties; and how the surface-charge behavior obtained at different brine salinities and pH values can be explained. This paper presents a comparative study aimed at gaining more insight into the electrokinetics of different types of carbonate rock. This is achieved by ζ-potential measurements on Iceland spar calcite and three reservoir-related rocks—Middle Eastern limestone, Stevns Klint chalk, and Silurian dolomite outcrop—over a wide range of salinity, brine composition, and pH values. With a view to arriving at a more-tractable approach, a surface-complexation model (SCM) implemented in PHREEQC software (Parkhurst and Appelo 2013) is developed to relate our understanding of the surface reactions to measured ζ-potentials. It was found that regardless of the rock type, the trends of ζ-potentials with salinity and pH are quite similar. For all cases, the surface charge was found to be positive in high-salinity formation water (FW), which should favor oil-wetting. The ζ-potential successively decreased toward negative values when the brine salinity was lowered to seawater (SW) level and diluted SW. At all salinities, the ζ-potential showed a strong dependence on pH, with positive slope that remained so even with excessive dilution. The sensitivity of the ζ-potential to pH change was often higher at lower salinities. The existing SCMs cannot predict the observed increase of ζ-potential with pH; therefore, a new model is proposed to capture this feature. According to modeling results, formation of surface species, particularly >CaSO4− and to a lower extent >CO3Ca+ and >CO3Mg+, strongly influence the total surface charge. Increasing the pH turns the negatively charged moiety >CaSO4− into both negatively charged >CaCO3− and neutral > CaOH entities. (Note that throughout this paper, the symbol > indicates surface complexes.) This substitution reduces the negative charge of the surface. The surface concentration of >CO3Ca+ and >CO3Mg+ moieties changes little with change of pH. Nevertheless, besides similarities in ζ-potential trends, there exist notable differences in terms of magnitude and the isoelectric point (IEP), even between carbonates that are mainly composed of calcite. Among all the samples, chalk particles exhibited the most negative surface charges, followed by limestone. In contrast to this, dolomite particles showed the most positive ζ-potential, followed by calcite crystal. Overall, chalk particles exhibited the highest surface reactivity to pH and salinity change, whereas dolomite particles showed the lowest.

Zeta potential in oil-water-carbonate systems and its impact on oil recovery during controlled salinity water-flooding

“…2a). These results are consistent with previous studies of natural carbonates40434547. Thus a typical low salinity waterflood yields increasingly negative zeta potential at the mineral-brine interfaces.…”

“…Three of the brine compositions tested were designed to represent the natural formation brine typical of hydrocarbon reservoirs6 (denoted FMB), natural seawater (denoted SW) and seawater diluted 20 times (denoted 20dSW) (Table 1). Similar brines have been used previously in studies of CSW in carbonates404345. The other tested brines comprised two sets each containing three synthetic brines; each set had the same ionic strength but varying Ca 2+ concentration (Table 1).…”

“…At the high Ca 2+ and/or Mg 2+ concentration typical of formation brines, the zeta potential of the calcite surface is positive. Reduction of Ca 2+ and/or Mg 2+ concentration, either selectively or by bulk dilution, can invert the polarity, yielding negative zeta potential404142434445. Addition of SO 4 2− can also yield more negative zeta potential8304046.…”

“…Previous studies have typically utilised measurements of electrophoretic mobility to determine the zeta potential of artificial and natural calcite and carbonate3031323334354243444546. In this approach, the sample is crushed to a fine powder and suspended in a solution of the electrolyte of interest.…”

Enhanced Wettability Modification and CO2 Solubility Effect by Carbonated Low Salinity Water Injection in Carbonate Reservoirs

“…This enhanced oil production was attributed to wettability modification of the rock surface. Many experimental techniques (interfacial tension measurement, contact angle measurement, NMR, -potential measurement, imbibition testing, and coreflooding testing) have suggested mechanisms 2 Journal of Chemistry responsible for wettability modifications, such as calcite dissolution [7,8], anhydrite dissolution [9], and surface charge change [10][11][12]. Although the underlying mechanism is still unclear, Yousef et al [13] confirmed the reduction of residual oil saturation through the single well tracer test in field trials.…”