Oil Recovery Mechanism(s) by Low Salinity Brines and Their Interaction with Chalk

Application of low salinity brines (LSW) for enhancing oil recovery are considered by many oil field operators. In this work, the efficiency of different ionic strengths of the brines on oil recovery and their interaction with chalk are addressed. Practically, the ionic strength is related to the dilution ratio of seawater (SSW). Four different brines with the following dilution ratio to SSW are studied, LSW1:5, LSW1:10, LSW1:15, and LSW1:25. The chalk rock was Stevns Klint outcrops from Denmark. Core flooding rates were 4 PV/day (PV = pore volume) followed by 16 PV/day. Calcium ion concentration [Ca2+] in the effluent is shown to be consistently higher than the injected concentration in all of the flooding cases. In addition the pH in all cases has reached approximately pH 8. The difference between the inlet and effluent pH increases with the brine dilution factor. The highest recorded oil recovery was obtained in the flooding case of LSW1:10 at 4 PV/day. This coincides with the highest [Ca2+] at the effluent. Dissolution and calcium/magnesium ion exchange are identified as possible processes that contributed to the observed higher calcium concentration in all of the flooded cases. These processes may affect the chalk integrity, leading to fine migration that adds to flow resistance resulting in a better sweep of the cores. It was interesting to observe deficiency in [Mg2+] monitored in the effluent compared to the injectant. In addition to what the [Mg2+] deficiency may suggest as an exchange process with calcium, magnesium inclusion in calcite increases water wetness of the chalk surface (at pH approximate to 8), which to some extent may contribute to incremental oil recovery.