Chemical Compositions in Salinity Waterflooding of Carbonate Reservoirs: Theory

Abstract

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. In addition, calcite mineral reacts with aqueous solutions and can alter substantially the composition of injected water by mineral dissolution. Carefully 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 during 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. In SI, we validate the reactive transport model against analytic solutions for rock dissolution, ion exchange, and longitudinal dispersion, each considered separately. Accordingly, using an open-source algorithm (Charlton and Parkhurst in Comput Geosci 37(10):1653–1663, 2011. 10.1016/j.cageo.2011.02.005), we outline a design tool to specify chemical/brine flooding formulations that correct for composition alteration by the carbonate rock. Subsequent works compare proposed theory against experiments on core plugs of Indiana limestone and give examples of how injected salinity compositions deviate from those designed in the laboratory for water-wettability improvement.

Chemical Compositions in Modified Salinity Waterflooding of Carbonate Reservoirs -- Experiment

“…Accordingly, the rock reactivity hinders rational design of the tailored brine 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, 2021). Here we present the results of single-phase waterflood-brine experiments that verify the theoretical framework.…”

“…Namely, in Figure 4a, a high calcium-concentration washout zone is apparent with a desorption-spreading tail (Pope et al, 1978;Yutkin et al, 2021, Appendix C). Conversely in Figure 4b, a low calcium-concentration loading zone with a sharpening tail (Pope et al, 1978;Yutkin et al, 2021, Appendix C) is faintly visible. In both displacement scenarios, the ion-exchange waves are significantly smeared by large longitudinal dispersion.…”

“…Calcium histories are calculated using the aqueous equilibrium speciation in Table 5 and the calcite dissolution reaction kinetics of (Yutkin et al, 2021, SI). The spreads in the concentration history in the core, except at very near the core inlet (Yutkin et al, 2018(Yutkin et al, , 2021.…”

“…To address the calcium maximum in Figure 2a, we hypothesize 2:1 equilibrium exchange of sodium and calcium ions on the carbonate-rock surface (Yutkin et al, 2021) or…”

“…We consider finally the combined effects of dissolution reaction kinetics, ion exchange, and longitudinal dispersion (Yutkin et al, 2021). Experimental washout and loading histories in Figure 5 Only the combined dissolution/ion-exchange/dispersion theory captures the experimental behavior in Figure 5.…”