(590a) Low-Salinity Waterflooding of Carbonate Reservoirs: Bulk and Surface Aqueous Speciation of Calcite

Low-salinity waterflooding (LSW) refers to improved oil recovery achieved by waterflooding a reservoir with brine of lower salinity than that of the in-situ field water. The process is alluring because of simplicity, favorable economics, and large potential, especially in carbonate reservoirs which constitute over 60 % of world oil resources.  Success of LSW relies heavily injected brine to alter reservoir surface chemistry of rock-brine-crude oil interfaces. Application of LSW to carbonate reservoirs, however, 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. Later work focuses on the important role of crude oil in LSW.

We prove that calcite rapidly equilibrates with aqueous solution within very short distances of order mm, even at the high flow rates in fractures of over 100 ft/day. Thus, injected-brine concentrations rapidly equilibrate with calcite rock and are not those of the flooding brine. We present new multi-reaction chemical-equilibrium calculations to describe rock-equilibrated brine compositions. In particular, injected-brine pH is altered substantially because of buffering due to rock dissolution. It is difficult to achieve alkalinities much different than pH ~ 8.3 quite independent of injected-brine pH. We also 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 and not that injected.

A simple ion-complexing equilibrium model is presented to predict calcite surface charge as a function of reservoir aqueous-solution composition. Our new multispecies ion-exchange reaction model illustrates that the surface charge of aqueous calcite may be either positive or negative depending on aqueous speciation. There is no single point of zero charge; all dissolved aqueous species are charge determining. We find excellent agreement with measured zeta potentials of calcite mineral.

Our evaluation of calcite bulk and surface equilibria draws several important inferences about proposed LSW oil-recovery mechanisms. Diffuse double layer expansion (DLE) is not possible unless brine ionic strength is below 0.01 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. Except at prohibitively high injected alkalinity, LSW cannot be an ultralow 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.

Our work stresses the importance of rock/water equilibration under reservoir conditions when considering LSW. A simple, but effective formulation is now available for scoping calculations of LSW performance.