Investigating Enhanced Oil Recovery from Sandstone by Low-Salinity Water and Fluid/Rock Interaction

Extensive research on crude oil/brine/rock (COBR) systems has shown that the injected water composition and salinity can act as a tertiary recovery method by altering the properties of the reservoir. Because of the complexity of the COBR interactions, the mechanism(s) of low-salinity (LS) enhanced oil recovery (EOR) is still being discussed and none of the suggested mechanisms has, thus far, been accepted as the main process. In this work, the intention is to contribute to the ongoing discussions. An increase of pH associated with flooding with low-salinity water (LSW) has been observed in this work and the literature. To test the hypothesis of the effect of pH, other simpler brines than synthetic seawater (SSW) and LSW (25 times diluted SSW), such as Na2SO4 (SO42-) or MgCl2 (Mg2+) brines, were used. Those single-salt brines (LS) were selected on the basis of previous work performed in our laboratories, which showed that they increase the pH. Ion tracking of the effluent solution has clearly demonstrated mineral dissolution with pH, where ions, such as potassium (K+), were measured when LS brines (K+-free brines) were used. This was confirmed by simulation and shown to be in reasonable agreement (within 12%). A sensitivity study of the pH indicated that, at the mineral/brine interface, the pH may be higher, where a better agreement when the pH of the LS brines was increased. The pressure drop across the flooded cores showed that LSW almost doubled the pressure drop compared to that with SSW. LS expanding the electrical double layer in combination with dissolution may lead to fine detachment and mobilization that divert flooding brines and enhance the sweep efficiency. Therefore, the main factors that govern LSW performance in EOR are the degree of the mineral/brine interaction that would increase the sweep efficiency. This may explain the controversial statement in the literature on the necessity of the kaolinite content.