Chalk Surface Area Evolution during Flow of Reactive Brines: Does Oil Play a Role?

Abstract

Prolonged injection of magnesium chloride (MgCl2) brine into water-wet chalk cores leads to dissolution of calcite and precipitation of magnesium-bearing minerals. In low permeable and highly porous chalk, the mineral surfaces dominate multiphase flow properties. The hydrophobic/hydrophilic behavior of these mineral surfaces is subject to changes when aged in oil at high temperature over time, and from mineral dissolution and precipitation processes. In this study, we evaluate to which extent chemical interactions induced by the continuous MgCl2 brine injection modify the water wetness of chalk samples saturated by oil/water mixtures and compare the evolving results to a 100% water-saturated parallel reference test. The potential of MgCl2 brine to improve the oil recovery after the freely movable hydrocarbons are produced is also assessed. In situ wettability measurements were carried out during the injection program using chromatographic separation, where the delay in the increase of effluent concentration of the adsorbing sulfate ion was compared to a nonaffine tracer. These measurements were performed every 10 days to estimate the evolution in mineral surface area in contact with water. The results show an increased delay time for the sulfate ion, linked to an increase of the mineral surface area. This is observed in both water-wet and mixed-wet cores, but is found to be more dominating in the mixed-wet samples. This implied that oil, which was adsorbed on the mineral surfaces, got mobilized in addition to an increased overall specific surface area as new magnesium-bearing minerals precipitated and grew during the MgCl2 brine flow. This is supported by continuous effluent analysis displaying a reduced magnesium and increased calcium concentration. Petrophysical analysis of cores before and after flow displayed trends along the axis of the sample. Changes in density and specific surface area were more dominant on the inlet side than the outlet side. Even though magnesium (Mg2+) ions in the injection brine increased the available water-wet area in the samples, the nonequilibrium chemical reactions did not lead to additional oil recovery.