Impact of cation type and concentration in injected brine on oil recovery in sandstone reservoirs

Abstract

Low salinity waterflood has a high potential to improve oil recovery. However, the optimum water chemistry for waterflooding is still not well known, and the dominant mechanisms are debated. In the present work, the effect of cation type and concentration in the injected water on the oil recovery to identify the optimum water salinity and composition was studied by performing coreflood experiments. In addition, zeta potentials at oil/brine and rock/brine interfaces were measured to examine the impact of brine composition on electrical double layer expansion and investigate if it is one of the dominate mechanisms. Furthermore, ionic exchange tests were conducted to improve the understanding of the rock/brines interactions. Berea sandstone cores were used for waterflooding and ion exchange experiments; all coreflood experiments were performed at high-pressure/high-temperature (HP/HT) conditions. Different concentrations of NaCl, CaCl2, and MgCl2 solutions were tested with two crude oils of different compositions and properties. Ion concentrations in the core effluent fluids were analyzed for both waterflooding and ionic exchange tests. The zeta potential was measured for solutions of oil/brine, and solid/brine; the solids were Berea sandstone, quartz, feldspar, and four types of clays. This work contributes to the understandings of the impacts of double layer expansion on oil recovery during waterflooding. Zeta potential results showed that Na+ changes the electrical charge at both oil/brine and rock/brine interfaces to highly negative, which results in higher repulsive forces between the two interfaces, and hence wettability alteration. Moreover, waterflood experiments with NaCl solutions improved oil recovery significantly compared to CaCl2 and MgCl2. These results showed that there seems to be a correlation between zeta-potential and oil recovery which implies that double-layer expansion could be a primary mechanism of oil recovery by low salinity waterflood. This study demonstrates that cation type has a significant impact on oil recovery, and it could be more dominant than the effect of total salinity of the injected brines. These findings can help in screening the brines that have higher potentials for oil recovery improvement.