Article pubs.acs.org/EF
Wettability Alteration during Low-Salinity Waterflooding and the Relevance of Divalent Ions in This Process Jie Yang,† Zhaoxia Dong,*,† Mingzhe Dong,‡ Zihao Yang,† Meiqin Lin,† Juan Zhang,† and Chen Chen† †
Enhanced Oil Recovery Institute, China University of Petroleum (Beijing), Beijing 102249, People’s Republic of China Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta T2N 1N4, Canada
‡
ABSTRACT: From laboratory and field test results, it is now largely agreed that reducing the overall salinity, especially the concentration of divalent ions of the injected water, can markedly improve the oil recovery. However, as a result of the complexity of the crude oil−brine−rock (COBR) system, there is no clear explanation of why the divalent cations (Ca2+) are of major significance in a low-salinity water effect (LSWE). In the present paper, spontaneous imbibition, ζ-potential measurements, static adsorption/desorption of benzoic acid (BA) onto crushed Berea, and BA self-assembled layers on silica wafer have been performed to correlate macroscopic wettability alteration during low-saline water flood with the microscopic release of the hydrophobic layers (organic acid layers) on mineral surfaces and explain the relevance of active cations (Ca2+) in this process, thereby providing a better understanding of the underlying mechanisms of a LSWE. Spontaneous imbibition results show that initial wettability of the COBR system was dominantly controlled by the initial concentration of Ca2+ rather than Na+ in brine; i.e., the initial wettability changed to be more oil-wet with an increasing concentration of CaCl2 in initial water, while changing the concentration of NaCl in initial water had little effect on initial wettability. Moreover, reducing salinity of imbibing brine can outstandingly improve oil recovery of cores aged by CaCl2 brine, whereas no obvious enhanced oil recovery (EOR) by lowsalinity water was observed for cores aged by NaCl brine. Decreasing the concentration of either CaCl2 or NaCl brine was able to make the oil/brine and brine/rock interfaces become less positively charged or even more negatively charged, observed by ζpotential measurements, resulting in increased electrostatic repulsive forces between the oil/brine and brine/rock interfaces. These results suggest that the anionic groups of organic acid from crude oil, such as carboxylate, adsorb onto negatively charged mineral surfaces mainly through calcium bridges instead of van der Waals forces or sodium bridges and reducing salinity is able to increase the electrostatic repulsive forces between mineral surfaces and carboxylate groups and then break calcium bridges to change the wettability of the rock surface to be less oil-wet, and as a result, EOR occurs. These suggestions were supported by the static adsorption/desorption studies and self-assembly experiments. After sorption to crushed Berea, the varied BA concentration in the supernatant analyzed by total organic carbon (TOC) reveals that the presence of the background electrolyte Ca2+ largely enhanced sorption in comparison to Na+. Lowering salinity was able to desorb BA from crushed Berea. Field emission scanning electron microscopy (FESEM) was used to obtain the microchemical composition and morphology of the wafer surface interacted with BA under different ionic strengths and solution cations (Ca2+ and Na+). Ca2+ ions were found to enhance the adsorption of BA self-assembled layers on a silica wafer compared to Na+, which directly demonstrates that BA molecules mainly self-assemble on SiO2 by calcium bridges rather than sodium bridges or van der Waals forces. The BA self-assembled layers were released after immersing organo-wafer into deionized water.
1. INTRODUCTION Over the last 20 years, wettability alteration has been widely believed to be an important factor for the improved oil recovery by low-salinity water.1−3 Laboratory tests have shown that divalent cations in connate water are necessary for wettability modification during low-salinity waterflooding.4−7 Lager et al.6 proposed that the oil component bonded to the negatively charged clay surface mainly by divalent cations in the brine and the multicomponent ion exchange (MIE), in which some simple cations in injected low-salinity water can exchange the pre-adsorbed oil component, resulting in wettability alteration, is the main mechanism of low-salinity water effect (LSWE), while the mechanism of low-salinity waterflooding was believed to primarily rely on double-layer expansion, which describes a change in the wetting state of the rock as a result of the release of the oil component caused by enhanced electrostatic repulsion between the negative clay surface and oil and to a lesser extent on the cation-exchange process.8 According to the © XXXX American Chemical Society
results of experiments and modeling, Omekeh et al. proposed that, during low-salinity waterflooding, the dissolution/ precipitation of various carbonate minerals and MIE are responsible for the wettability change of the rock surface, and as a result, improved oil recovery occurred.9 Because variation of wettability was not observed when refined oils free from polar components were used, Tang and Morrow4 inferred that the polar components (in particular, the acidic material) also had great impact on variation of wettability in low-salinity waterflooding. As the amount of asphaltene/polar component adsorption increased with an increase in cation valency, the ion bonding caused by electrostatic forces was proposed to be the reason for the adsorption.10−12 It has been found that the retention of polar oil components onto reservoir rock Received: August 13, 2015 Revised: December 8, 2015
A
DOI: 10.1021/acs.energyfuels.5b01847 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels
Berea and silica wafer self-assembled by BA under various concentrations of CaCl2 and NaCl solutions to microscopically investigate the influence of solution chemistry (e.g., ionic strength and solution cation) on model acid matter sorption to mineral surfaces. For the organic acids, BA was chosen to be the acidic polar compound because the surface active species, often retained on the rock surface bringing about wettability alteration, were fairly aromatic.17,23,24
depended upon both the composition of brine and the composition of the crude oil; i.e., the retention of oil components was dominated by bonding of carboxylic groups to mineral surfaces through calcium bridges.13 It has been proposed that the desorption of polar crude oil components from rock surfaces could lead to the wettability alteration during low-salinity waterflooding, and as a consequence, enhanced oil recovery (EOR) took place.14,15 A possible relation may be that, over geological time scales, the originally hydrophilic reservoir rock can be altered to be hydrophobic via the adsorption of polar compounds from the crude oil by electrostatic interactions.16−18 More precisely, these compounds contain both a polar end and a hydrocarbon chain. The polar end contacted the rock surface, exposing the hydrocarbon chain to the aqueous phase, and as a consequence, the mineral surface changed to be more oil-wet. During this process, bivalent cations, such as Ca2+ and Mg2+, are supposed to be of major significance, because they can bridge between the negatively charged polar end, such as the carboxylate group in crude oil, and negatively charged sites on the mineral surface.6,19 When low-salinity water was injected, cations to screen off the negative charges were reduced and the repulsive forces between the negatively charged rock surface and negatively charged acidic components increased. Then, the bonding between the hydrophobic layers and the rock surface was weaken, and as a result, the organic layers can be damaged or released, resulting in alteration of the rock surface toward a less oil-wet state and the expulsion of oil from the narrow pores as a result of a decrease of the required Laplace pressure.20 Feng et al.21 studied the peat humic acid (PHA) sorption to two types of clay minerals (kaolinite and montmorillonite) under various solution conditions to determine the influence of solution chemistry (ionic strength and solution cation) on PHA sorption to clay mineral surfaces. In accordance with contemporary observations, PHA sorption increased with an increasing ionic strength, and the presence of the background electrolyte Ca2+ largely enhanced sorption in comparison to Na+, suggesting that Ca2+ bridges and water bridges contribute a lot to the interactions between mineral surfaces and PHA. By studying the wettability of heavy oil/brine/chemical/sand systems, Liu et al.19 also proposed that, under alkaline conditions, the combination of Mg2+ and ionized organic acids at oil/water interfaces and the adsorption of Mg2+ at the water/sand interfaces greatly reduced the negative charges at both interfaces, resulting in an improved probability of the interaction between the oil and sand surface. More recently, Graber et al.22 studied the macroscopic soil water repellency hydrophobized by fatty acids upon exposure to water of variable pH and salt contents. They demonstrated that the persistence of soil water repellency increased at elevated pH in the presence of Ca2+ but not Na+. The mechanisms invoked to explain these observations are very similar to the previously proposed mechanism of EOR by low-salinity waterflooding. The purpose of the present research is to study the effect of bivalent cations (Ca2+) in brine macro- and microscopically, thereby leading to a better understanding of the mechanism behind the EOR by low-salinity water. In this work, we performed spontaneous imbibition experiments to macroscopically study the influence of solution cations (Ca2+ and Na+) and ionic strength of initial water on the initial wettability of Berea cores and the reduction of salinity of injected water on wettability alteration. We implemented batch sorption/ desorption of model acid [benzoic acid (BA)] onto crushed
2. EXPERIMENTAL SECTION 2.1. Materials. 2.1.1. Porous Media. Four Berea sandstones, labeled B1−B4, were selected. The basic data for all core samples, the initial water saturations (Swi), initial oil saturations (Soi), and mineralogical analysis of cores are listed in Table 1. 2.1.2. Crude Oil. Prior to use, the crude oil was centrifuged at 2000 rpm for 2 h to remove sediments and water and evacuated for 1 h at room temperature to reduce the possibility of gas evolution in crude oil. Properties of crude oils are listed in Table 2. 2.1.3. Chemicals and Solutions. All reagents and solvents were delivered by the Beijing Haisheng Chemical Industry. The brines were artificially made by dissolving desired amounts of salts in deionized water (Table 3). BA, pKa = 4.4, was dissolved in deionized water, and the pH was adjusted to ∼9.5 by NaOH to ensure that carboxylic acid groups of BA are totally deprotonated in the bulk water. The BA stock solution concentrations varied from 0.06 to 0.2 g/L. 2.1.4. Particles. The mineral particles (
