Low salinity water flooding in high acidic oil reservoirs: Impact of pH on wettability of carbonate reservoirs (original) (raw)
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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.
pH effect on wettability of oil/brine/carbonate system: Implications for low salinity water flooding
Journal of Petroleum Science and Engineering, 2018
Wettability of oil/brine/carbonate system is a critical parameter to govern subsurface multi-phase flow behaviour, thus remaining oil saturation and ultimate oil recovery in carbonate reservoirs. Despite the fact that salinity level, ionic strength, oil composition and rock chemistry (e.g., limestone and dolomite) have been extensively investigated, few work has been done regarding the effect of pH on oil/brine/rock interaction, thus wettability. We thereby measured contact angles at two different pH (pH=3 and 8) in the presence of either 1mol Na 2 SO 4 or 1mol CaCl 2 using a crude oil with acid number of 1.7 and base number of 1.2 mg KOH/g. Moreover, we performed a geochemical modelling study in light of the diffuse double layer to understand how pH controls the number of surface species at interfaces of oil/brine and brine/carbonate. Our results show that pH scales with oil/brine/carbonate wettability, demonstrating that pH is one of the controlling factors to govern the system wettability. Further, our results suggest that pH (6.5-7.5) likely triggers an oil-wet system, which is favourable for low salinity water flooding, but pH<5 usually exhibits a water-wet system, which explains why low salinity effect is not always observed in carbonate reservoirs. This also confirms that CO 2 flooding, carbonated water flooding, and CO 2 huff-and-puff EOR very likely renders a strongly water-wet system due to H + adsorption on the interface of oil/brine and brine/carbonate as a result of CO 2 dissolution.
Drivers of Low Salinity Effect in Carbonate Reservoirs
Energy & Fuels, 2017
Wettability alternation appears to be the main mechanism of low salinity water flooding in carbonate reservoirs. However, what factor(s) controls the wettability alteration is not clearly defined. We hypothesized that zeta potential at interfaces of oil/brine and brine/rock, controls the wettability alternation in carbonate reservoirs. To test our hypothesis, we removed SO 4 2ions from the aqueous ionic solutions because SO 4 2ions likely adsorb at the pore surface, triggering desorption of carboxylic materials. We examined the zeta potential of interfaces of crude oil/brines and brines/rock. We also measured the contact angle, and conducted two core-flooding
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.
Low salinity water flooding has shown great promise due to its cost-effectiveness and low environmental impact for improving and sustaining oil production. It is believed that injecting water with ionic strength lower than that of the reservoir changes the reservoir from less to more water-wet and enhances oil recovery. This alteration phenomenon is not well understood, due to complex interactions between oil, water, and rock. Here we use molecular simulations to characterize the wettability of the 10.4-face of calcite in a calcite/brine/oil system, and address how wettability is altered by changing ionic strength and salt type (NaCl vs. CaCl2). Using the test area method we calculate the superficial tension of the fluids against the solid and the surface tension between the two fluid phases. As the salinity is decreased, the wetting of calcite by brine is progressively less favored, contrary to what might be expected based on low salinity flooding. However, as salinity is decreased...
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...
IOP Conference Series: Earth and Environmental Science, 2019
Low salinity water flooding is one of the emerging enhanced oil recovery technologies as it has been proven economical and environmentally friendly. However, the recovery mechanism of low salinity water (LSW) is still under debatable due to the complex effect of low salinity water and its ionic compositions. Therefore, this study aims to discover the optimum seawater dilution salinity and influence of single and binary ionic compounds low salinity water on wettability alteration of carbonate core slices at optimum salinity. To achieve that, a modified Design of Experiments (DOE) has been implemented. Contact angle measurement was carried out to characterize the wettability of core slices at 0 hour, after 24 hours and after 48 hours. The results revealed that dilution of seawater reduced the contact angle of carbonate core slices towards more water wet until the optimum salinity of 1750ppm. Further dilution to 700ppm only shown a slight impact in shifting the wettability of the carbo...
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.
Energy & Fuels, 2017
It has been proposed that increased oil recovery in carbonates by modification of ionic composition or altering salinity occurs mainly at temperature exceeding 70-80⁰C. The argument was that elevated temperatures enhance adsorption of the potential determining ions which then modifies wettability to less-oil-wetting state. According to this rationale, it becomes questionable if diluted brines or brines without these ions can be still applicable. Therefore, the aim of this paper is to investigate if the wettability alteration truly depends on temperature and if so how the trend with temperature can be explained. We followed a combined experimental and theoretical modeling approach. The effect of brine composition and temperature on carbonate wettability was probed by monitoring contact angle change of sessile oil droplets upon switch from high salinity to lower salinity brines. IFT measurements as function of salinity and temperature along with extensive ζ-potential measurements as a function of salinity, pH, temperature and rock type were conducted. Interaction potentials between oil and carbonate surfaces were estimated based on DLVO theory and its consistency with oil droplet data was checked to draw conclusions on plausible mechanisms. Three carbonate rocks (two limestones and one dolomite) were used along with two reservoir crude oils, high salinity formation water (FW), seawater (SW) and 25 time diluted seawater (25dSW) as low salinity (LS) brine. It was observed that i) wettability alteration to less-oil-wetting state can occur at ambient temperature for specific rock types and brines, ii) there is no univocal increase in response to SW and LS brine at elevated temperature. The largest improvement in wettability was observed for dolomite while among the limestones only one rock type showed noticeable wettability improvement at elevated temperature with SW. The difference in behavior between limestones and dolomite indicate that the response to brine composition change depends on rock type and mineralogy of the sample. These observations are consistent with the ζ-potential trends with salinity at a given temperature. Dolomite generally shows more positive ζ-potential than limestones. But even the two limestones react differently to lowering salinity and exhibit different magnitude of ζ-potential. Moreover, it is observed that at specific salinity an increase in temperature leads to reduction of ζ-potential magnitude on both rock/brine and oil/brine interfaces toward zero potential. This can affect positively or negatively on the degree of wettability alteration (to less oil-wetting state) at elevated temperature depending on the sign of oil/brine and rock/brine ζ-potential in SW/LS. The observed trends are reflected in the DLVO calculations which shows consistency with contact angle trends with temperature and salinity. According to the DLVO calculation lack of response to SW/LS in some of the systems above can be explained by stronger electrostatic attractive forces under SW/LS than HS. This study concludes that a combined surface-charge-change and double-layer expansion is a plausible mechanism for the wettability alteration in carbonate rocks.