Low Salinity EOR Effects in Limestone Reservoir Cores Containing Anhydrite: A Discussion of the Chemical Mechanism

Abstract

It is well accepted that seawater injection is able to improve the water wetness of carbonate reservoirs at high temperatures, and in that way, it can act as an enhanced oil recovery (EOR) fluid. A recent laboratory investigation showed that increased oil recovery also was obtained from carbonate reservoir cores by successively flooding composite limestone cores by 2, 10, and 20 times diluted seawater. The study confirmed that it is possible to obtain low salinity EOR effects also in carbonates, and not only in sandstones. In the present study, preserved reservoir core material from a similar limestone formation was used with the objective to obtain a chemical understanding of the mechanism for the improved oil recovery. It was verified, that the core material contained significant amounts of anhydrite (CaSO 4 ), which appeared to be the key factor for observing the low salinity EOR effect. The concentration of sulfate in the injection brine increased due to increased dissolution of anhydrite as the salinity and concentration of inactive salt, NaCl, decreased. Both an increase in the sulfate concentration and a decrease in NaCl content in the injected brine will have a positive effect on the wettability alteration process. An oil displacement test was conducted on a restored reservoir core at 100°C using 100 times diluted formation water in a tertiary flooding process, after first injecting formation water. This showed a low salinity EOR effect of 22% of original oil in place (OOIP), corresponding to an 88% increase in oil recovery. Also 30 times diluted seawater increased the oil recovery by 18% of OOIP in a tertiary flood after first flooding the core with formation water. After flooding a core successively at 100°C with formation water, seawater, and 10 times diluted seawater, the oil recovery increased gradually by 25, 30, and 33% of OOIP. The chemical low salinity EOR mechanism was discussed in terms of dissolution of anhydrite and a decrease in the NaCl concentration. This wettability alteration mechanism is, in principle, the same as that reported previously for injection of seawater and modified seawater into chalk cores, involving a symbiotic interaction between Ca 2+ , Mg 2+ , and SO 4 2− at the rock surface. In this case, supply of extra Ca 2+ and SO 4 2− was obtained by dissolution of anhydrite. The low salinity EOR technique can have a great economic potential regarding oil recovery from high temperature carbonate reservoirs containing significant amounts of dissolvable anhydrite distributed in the pore space.

Bulk and Surface Aqueous Speciation of Calcite: Implications for Low-Salinity Waterflooding of Carbonate Reservoirs

“…One proposed mechanism of improving waterflood performance is to displace asphaltic carboxylates tightly bound to the rock surface by ion complexing and bridging by tailored solution ions (Lager et al 2006(Lager et al , 2007(Lager et al , 2008aStrand et al 2006;Zhang and Austad 2006;Zhang et al 2007b;Sorbie and Collins 2010;Zahid et al 2012;Austad et al 2015;Puntervold et al 2015;Qiao et al 2016). MIE is strongly coupled to rock/brine-surface chemistry.…”

“…In particular, sodium and chloride ions are dominant species at high brine concentrations and may participate in ion-exchange equilibria. It is commonly accepted, however, that sodium and chloride ions are inactive at the calcite/water interface (Somasundaran and Agar 1967;Möller and Werr 1972;Siffert and Fimbel 1984;Thompson and Pownall 1989;Van Cappellen et al 1993;Pokrovsky et al 2000;Nystrom 2001;Wolthers et al 2008;Hiorth et al 2010;Alotaibi et al 2011;Heberling et al 2011;Austad et al 2015;Puntervold et al 2015). Unfortunately, no direct evidence is available to confirm or refute this view.…”

“…Precise definition of LSW is lacking. In principle, spiking injection water with specific anions and/or cations has the potential to improve recovery (Strand et al 2006;Zhang et al 2007b;Sorbie and Collins 2010;Alshakhs and Kovscek 2015;Austad et al 2015;Puntervold et al 2015) without need for lower salinity.Starting in the middle 1990s, the pioneering work of Morrow and colleagues popularized LSW of sandstones. Fines release was highlighted as one of several proposed recovery mechanisms (Jadhunandan and…”

Impact of SO42−, Ca2+, and Mg2+ ions in Caspian Sea ion-engineered water on the rate of wettability alteration in carbonates

“…Other laboratory studies and field trials verify the positive effect of low salinity and the importance of brine chemistry on oil recovery. Different studies have shown that for each crude oil/brine/rock (CBR) combination, there are an optimum salinity concentration and certain ion composition that could result in the most optimum incremental oil recovery (Al-Shalabi and Sepehrnoori 2016;Webb et al 2005;Zhang et al 2007;Gupta et al 2011;Honarvar et al 2020;Hiorth et al 2010;Rashid et al 2015;Rezaeidoust et al 2009;Su et al 2019;Strand et al 2003;Austad et al 2015;den Ouden et al 2015;Al-Kharusi et al 2018;Fani et al 2019;Yousef et al 2012;Al-Attar et al 2013;Alameri et al 2014;Ayirala et al 2019;Awolayo et al 2014;Fathi et al 2010;Ghandi et al 2019;Jabbar et al 2013;Jadhunandan and Morrow 1995;Karimi et al 2016;Lashkarbolooki et al 2017;Ligthelm et al 2009;Robertson 2009;Tang and Morrow 1999;Tweheyo et al 2006;Winoto et al 2012;Yousef et al 2010Yousef et al , 2011Yu et al 2008;Zaeri et al 2018;Zahid et al 2012; http://porta l.kaznt u.kz/files /publi cate/2015).…”

Research on Mechanical Properties of High-Pressure Anhydrite Based on First Principles

“…Marius manufactured a new type of high-performance composite using gypsum material and tested its mechanical properties, such as tensile strength [4]. Austa studied the chemical mechanism for low-salinity enhanced oil-recovery effects in carbonates by using a preserved core material containing dissolvable anhydrite [5]. Scholars have also undertaken a large amount of research work that investigates the physical and mechanical properties of anhydrite crystals.…”