Bulk and surface aqueous speciation of calcite: Implications for low-salinity waterflooding of carbonate reservoirs

© Copyright 2016, Society of Petroleum Engineers. Low-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 rock-brine-crude oil interfaces. LSW in carbonate reservoirs is especially challenging because of high brine salinity and, more importantly, because of high reactivity of the rock minerals. Here, we tackle the complex physicochemical processes in chemically active carbonates flooded with diluted brine that is saturated with atmospheric CO2 and possibly supplemented with additional ionic species, such as sulfates or phosphates. Later work will focus on the important role of crude oil and multicomponent ion-exchange (MIE) in LSW. 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 carbon-dioxide content of a carbonate reservoir, originating from CO2-rich crude oil and gas, plays a dominant role in setting aqueous pH and speciation. A simple ion-complexing model predicts 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 calcite surface ionexchange behavior. 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 is preliminary but draws several important inferences about proposed LSW oil-recovery mechanisms. Diffuse double layer expansion (DLE) is not possible unless brine ionic strength is below 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 CO2. In spite of diffuse double-layer collapse in carbonate reservoirs, surface ion-exchange oil release remains feasible, but unproven