Abstract
Modified or low-salinity waterflooding of carbonate oil reservoirs is of considerable economic interest because of potentially inexpensive incremental oil production. The injected modified brine changes the surface chemistry of the carbonate rock and crude oil interfaces and detaches some of adhered crude oil. Composition design of brine modified to enhance oil recovery is determined by labor-intensive trial-and-error laboratory corefloods. Unfortunately, limestone, which predominantly consists of aqueous-reactive calcium carbonate, alters injected brine composition by mineral dissolution/precipitation. Accordingly, the rock reactivity hinders rational design of brines tailored to improve oil recovery. Previously, we presented a theoretical analysis of 1D, single-phase brine injection into calcium carbonate-rock that accounts for mineral dissolution, ion exchange, and dispersion (Yutkin et al. in SPE J 23(01):084–101, 2018. https://doi.org/10.2118/182829-PA). Here, we present the results of single-phase waterflood-brine experiments that verify the theoretical framework. We show that concentration histories eluted from Indiana limestone cores possess features characteristic of fast calcium carbonate dissolution, 2:1 ion exchange, and high dispersion. The injected brine reaches chemical equilibrium inside the porous rock even at injection rates higher than 3.5 times 10^{-3} m s^{-1} (1000 ft/day). Ion exchange results in salinity waves observed experimentally, while high dispersion is responsible for long concentration history tails. Using the verified theoretical framework, we briefly explore how these processes modify aqueous-phase composition during the injection of designer brines into a calcium-carbonate reservoir. Because of high salinity of the initial and injected brines, ion exchange affects injected concentrations only in high surface area carbonates/limestones, such as chalks. Calcium-carbonate dissolution only affects aqueous solution pH. The rock surface composition is affected by all processes.
Highlights
Modified brine composition floods may increment oil recovery at low cost, and they have been studied quite intensively (Morrow and Buckley 2011; Al-Shalabi and Sepehrnoori 2016; Mahani et al 2017; Hao et al 2019; Katende and Sagala 2019)
All flow experiments were performed on 3.8 × 10−2 m by 7.6 × 10−2 m (1.5′′ by 3′′) Indiana limestone core plugs obtained from Kocurek Industries, Inc (Texas)
We find that Eq (3) provides adequate representation of calcium/sodium ion exchange on calcite
Summary
Modified brine composition floods may increment oil recovery at low cost, and they have been studied quite intensively (Morrow and Buckley 2011; Al-Shalabi and Sepehrnoori 2016; Mahani et al 2017; Hao et al 2019; Katende and Sagala 2019). First discovered with low-salinity waterfloods of crude oils from sandstone cores (Jadhunandan and Morrow 1995), application to more plentiful carbonate rocks has shown similar recoveries (Zhang and Austad 2006; Strand et al 2006; Zhang et al 2007; Yousef et al 2011). In both rock types, decreasing salinity is not a necessary condition for improved recovery. Brine design is currently accomplished by labor-intensive trial-anderror coreflooding experiments
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