Wettability Alteration during Low-Salinity Waterflooding and the Relevance of Divalent Ions in This Process

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• 作者：Jie Yang ; Zhaoxia Dong ; Mingzhe Dong ; Zihao Yang ; Meiqin Lin ; Juan Zhang ; Chen Chen
• 刊名：Energy & Fuels
• 出版年：2016
• 出版时间：January 21, 2016
• 年：2016
• 卷：30
• 期：1
• 页码：72-79
• 全文大小：537K
• ISSN：1520-5029

文摘

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 low-salinity 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 ζ-potential 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.