Chemical Compositions in Salinity Waterflooding of Carbonate Reservoirs: Theory

Higher oil recovery after waterflood in carbonate reservoirs is attributed to increasing water wettability of the rock that in turn relies on complicated surface chemistry. However, calcite mineral reacts with
aqueous solutions, and can alter substantially the composition of injected water by mineral dissolution. Care-
fully designed chemical and/or brine flood compositions in the laboratory may not remain intact while the
injected solutions pass through the reactive reservoir rock. This is especially true for a low-salinity waterflood
process, where some finely-tuned brine compositions can improve flood performances, whereas others cannot.
We present a 1D reactive transport numerical model that captures the changes in injected compositions dur-
ing water flow through porous carbonate rock. We include highly coupled bulk aqueous and surface carbonate-
reaction chemistry, detailed reaction and mass transfer kinetics, 2:1 calcium ion exchange, and axial dispersion.
At typical calcite reaction rates, local equilibrium is established immediately upon injection. Using an open-
source algorithm (Charlton and Parkhurst 2011), we present a design tool to specify chemical/brine flooding
packages that correct for composition alteration by carbonate rock.
Here, we present a comprehensive 1D reactive transport model and validate it against analytic solutions
for rock dissolution, ion exchange, and longitudinal dispersion, each considered separately. A companion paper
compares the proposed theory against experiments on core plugs of Indiana limestone that serve as high velocity
probes for reaction-controlled and mass-transfer-controlled dissolution. Finally, in another companion paper,
we give examples of how injected salinity compositions deviate from those designed in the laboratory for water-
wettability improvement based on contact angles, zeta potentials, surface charge densities, and ion exchange.
How to correct the design chemical packages for exposure to reactive rock is also discussed in there.