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

Abstract

SummaryLow-salinity waterflooding (LSW) is ineffective when reservoir rock is strongly water-wet or when crude oil is not asphaltenic. Success of LSW relies heavily on the ability of injected brine to alter surface chemistry of reservoir crude-oil brine/rock (COBR) interfaces. Implementation of LSW in carbonate reservoirs is especially challenging because of high reservoir-brine salinity and, more importantly, because of high reactivity of the rock minerals. Both features complicate understanding of the COBR surface chemistries pertinent to successful LSW. Here, we tackle the complex physicochemical processes in chemically active carbonates flooded with diluted brine that is saturated with atmospheric carbon dioxide (CO 2 ) and possibly supplemented with additional ionic species, such as sulfates or phosphates.When waterflooding carbonate reservoirs, rock equilibrates with the injected brine over short distances. Injected-brine ion speciation is shifted substantially in the presence of reactive carbonate rock. Our new calculations demonstrate that rock-equilibrated aqueous pH is slightly alkaline quite independent of injected-brine pH. We establish, for the first time, that CO 2 content of a carbonate reservoir, originating from CO 2 -rich crude oil and gas, plays a dominant role in setting aqueous pH and rock-surface speciation.A simple ion-complexing model predicts the calcite-surface charge as a function of composition of reservoir brine. The surface charge of calcite may be positive or negative, depending on speciation of reservoir brine in contact with the calcite. There is no single point of zero charge; all dissolved aqueous species are charge determining. Rock-equilibrated aqueous composition controls the calcitesurface ion-exchange behavior, not the injected-brine composition. At high ionic strength, the electrical double layer collapses and is no longer diffuse. All surface charges are located directly in the inner and outer Helmholtz planes.Our evaluation of calcite bulk and surface equilibria draws several important inferences about the proposed LSW oil-recovery mechanisms. Diffuse double-layer expansion (DLE) is impossible for brine ionic strength greater than 0.1 molar. Because of rapid rock/brine equilibration, the dissolution mechanism for releasing adhered oil is eliminated. Also, fines mobilization and concomitant oil release cannot occur because there are few loose fines and clays in a majority of carbonates. LSW cannot be a low-interfacial-tension alkaline flood because carbonate dissolution exhausts all injected base near the wellbore and lowers pH to that set by the rock and by formation CO 2 . In spite of diffuse double-layer collapse in carbonate reservoirs, surface ion-exchange oil release remains feasible, but unproved.Introduction LSW refers to improved oil recovery achieved by waterflooding a reservoir with brine of lower salinity than that of the field brine. The process is alluring because of simplicity, favorable economics, and large potential. In some applications, seawater is inj...

Chemical Compositions in Salinity Waterflooding of Carbonate Reservoirs: Theory

“…Tables 1 and 2 describe the species that participate in the aqueous phase-carbonate reactions, along with their equilibrium constants at ambient temperature for bulk and surface chemistry, respectively (Yutkin et al 2018). Units in the equilibrium constants are listed to aid the reader.…”

“…Units in the equilibrium constants are listed to aid the reader. Table 2 is slightly changed from that of Yutkin et al (2018)) to include sodium surface species in reaction S 6 . The symbol > in Table 2 denotes species at the calcite surface in surface concentration units.…”

Mechanisms of smart waterflooding in carbonate oil reservoirs - A review

“…For a positively charged mineral, like calcite, the carboxylic acid components in the oil are more effective, than its basic counterparts, in changing wettability of the mineral through adhesion. Yutkin et al (2016) studied the calcite/brine bulk kinetics and equilibria, as well as the calcite/brine surface equilibria with an ion comlexation model. It was pointed out that the double layer expansion is not possible unless the ionic strength is lower than 0.1 M. The rock dissolution cannot be a LSW mechanism because of the rapid rock/brine equilibration (carbonate could rapidly equilibrate with a brine in a short distance).…”

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