“Smart Water” for Oil Recovery from Fractured Limestone: A Preliminary Study

Abstract

Seawater is characterized as an enhanced oil recovery (EOR) fluid for hot, fractured chalk oil reservoirs because it is able to modify the wetting conditions and improve the displacement of oil. The chemical mechanism for the wettability alteration has been described previously, and it was verified that Ca2+, Mg2+, and SO42− played an important role because of their reactivity toward the chalk surface. Chalk, which is purely biogenic CaCO3, consists of fragmentary parts of calcite skeletons produced by plankton algae known as coccolithophorids, and it is believed to have a more reactive surface than ordinary limestone. To validate seawater as an EOR fluid also for limestone and dolomite, the affinities of these ions toward the rock surfaces must be evaluated. The present paper describes some preliminary experimental studies of the affinity of SO42−, Ca2+, and Mg2+ toward the surface of reservoir limestone cores at temperatures ranging from room temperature to 130 °C. The results confirmed that the ions interacted with the rock surface, and that the established chemical equilibrium was sensitive to the relative concentrations of the ions. It was also observed that the adsorption of Ca2+ and Mg2+ from a NaCl solution onto the limestone surface was quite similar at room temperature but that Mg2+ adsorbed more strongly at higher temperatures. At high temperatures, T = 130 °C, Mg2+ in seawater was able to substitute Ca2+ on the surface but the reactivity was less than for chalk. These findings indicate that seawater will act as an EOR fluid in limestone as well but its potential is probably smaller than for chalk. This was also confirmed by spontaneous imbibition tests performed at 120 °C.