Driving mechanism of low salinity flooding in carbonate rocks

Abstract

Several studies conducted mainly on the laboratory scale indicate that in carbonate rocks oil displacement can be influenced by the ionic composition of the brine, providing an opportunity to improve recovery by optimizing the brine mixture used in secondary or tertiary recovery. In industry this topic has been termed "low salinity flooding (LSF) in carbonates" while the underlying mechanisms are not very well understood. The increased oil recovery has been attributed to wettability alteration to a more water-wet state. However, in some studies a positive low salinity effect (LSE) has been ascribed to dissolution of rock, which occurs on the laboratory scale but due to equilibration of brine with carbonate minerals on larger length scales this is not relevant for the reservoir scale. Therefore, the objective of this paper is to gain a better understanding of the underlying mechanism(s) and investigate whether calcite dissolution is the primary mechanism of the LSE. We used a model system where the contact angle of crude oil deposited on planar surfaces coated with crushed carbonate rock particles was monitored as a function of brine composition. The approach is similar to the one published in Mahani et al. (2014) for sandstone rock, but instead of clay minerals we used carbonate materials from natural limestone and Silurian dolomite rocks. Furthermore, the effective surface charge at the oil-water and water-rock interfaces was quantified via zeta-potential measurements at several salinity and pH levels in order to establish a link between changes in the intermolecular interactions at the solid-liquid interface and the contact angle at the brine-oil-rock contact line, which is an indicator for wettability change. The impact of mineral dissolution was addressed by comparing the response to brines that were fully equilibrated (and hence dissolution suppressed) and the response to those completely under-saturated with calcium carbonate (leading to dissolution). The investigation was accompanied by geochemical modeling using PHREEQC. It was observed that by switching from formation water (FW) to seawater (SW), diluted seawater (dSW) and diluted seawater equilibrated with calcite (dSWEQ), the limestone surface became less oil-wet reflected in contact angle decrease. The recession of the 3-phase contact line observed for both SW and dSWEQ, which are not impacted by dissolution, suggests that the LSE occurs even in the absence of mineral dissolution. The trends observed for the zeta-potential data on brine composition clearly support the surface-charge-change mechanism for limestone, where at lower salinities the charges at the limestone-brine interface become more negative, causing lower adhesion or even repulsion between oil and rock. Dolomite rock shows a different behavior. First, there is a much smaller response in terms of contact angle change. Also, the zeta-potential of dolomite shows generally more positive charges at higher salinities and less decrease at lower salinities, where in comparison to limestone the electrostatic interaction remains attractive or becomes only weakly repulsive. In summary we conclude that a positive LSE in carbonate rock exists without any dissolution and it is driven by the brine composition dependency of electrostatic interactions between crude oil and rock. However, the magnitude of the LSE is impacted by the mineralogy of carbonate material.