Impact of Electrical Surface Charges and Cation Exchange on Oil Recovery by Low Salinity Water

Abstract

Abstract Low salinity water is a promising technique for improving oil recovery in sandstone reservoirs. However, the mechanisms of low salinity water are still uncertain; that makes the optimum conditions are unknown. In this paper, we study the effect of cation type and concentration in the injected water to identify the optimum salinity. Coreflood experiments of waterflooding and cation exchange tests were conducted to investigate the impact of cations on oil recovery, and understand the rock/brine interactions. In addition, zeta potential at oil/brine and rock/brine interfaces were measured to examine how the cation type and concentration change the electrical surface charges, and relate these measurements to the oil recovery obtained from coreflood. Berea sandstone cores were used for waterflooding and cation exchange experiments; all coreflood experiments run at HPHT. Three concentrations of NaCl, CaCl2, and MgCl2 were tested with two crude oils of different compositions and properties. ICP-OES, and a spectrophotometer were used to analyze the ions concentrations in the effluent fluids for both waterflooding and cation exchange tests. Zeta potential was measured for solutions of oil/brine, and solid/brine; the solids used were Berea sandstone and four types of clays. This work contributes to the understandings of the impact of electrical surface charge and cation exchange on oil recovery by low salinity waterflooding. Zeta potential results showed that Na+ changes the electrical charge at both oil/brine and rock/brine interface to highly negative, which results in repulsion forces between the two interfaces, and hence wettability alteration and improvement in oil recovery. Moreover, waterflood experiments of NaCl solutions improved oil recovery significantly compared to CaCl2, and MgCl2. Analysis of core effluent fluid proved the interaction between brine and rock; injection of single cation brine leached some of the ions from the rock surface by exchanging cations, which could contribute to improvement of oil recovery. Results demonstrate that the existing cations in the injected water brine have more dominant effect on the recovery factor than the water salinity. These findings can help in defining the brines that have higher potential for oil recovery improvement.