Measurements of zeta potential for improved understanding of controlled salinity waterflooding

Abstract

Improved oil recovery (IOR) processes increase the efficiency of oil extraction from subsurface reservoirs. Controlled salinity waterflooding (CSW) is an IOR process where brine of a specific ionic composition is injected into a reservoir. However, CSW does not always yield IOR and the underlying mechanism(s) responsible remain unclear. It is generally accepted that successful CSW is associated with a shift to a more water-wet state. The wetting state is thought to be controlled by the zeta potential, a measure of the electrical potential, at the mineral and oil interfaces. This thesis explores how modifying the brine composition changes the zeta potential of these interfaces and impacts wettability in sandstones and carbonates. This is primarily done by streaming potential measurements (SPM) of natural, intact rock cores at conditions relevant to CSW. Results are discussed with specific focus on their application to CSW. Under fully water saturated conditions, the zeta potential of carbonates becomes more negative with reduction of the divalent cations Ca2+ and Mg2+. Sandstones exhibit more complex behaviour; however, the bulk clay content appears to be an important control. Following wettability alteration with crude oil, the zeta potential of carbonate samples became more positive or negative with increasing oil-wetness. The direction of change is interpreted to represent the polarity of the oil-brine zeta potential. During CSW, if the injection brine yields a mineral-brine zeta potential polarity that is the same as the oil-brine zeta potential, then IOR is observed. Determining the polarity of the oil-brine zeta potential appears critical in controlling CSW but directly measuring this under relevant conditions is challenging using conventional methods. A theoretical pore network model was developed which allows for prediction of the oil-brine zeta potential under such conditions. The model predicts that this was positive in most of the carbonate experiments. However, previous literature data from conventional methods suggest positive values are rare. Therefore, a new method to directly measure the oil-brine zeta potential was also developed. Strongly oil-wet substrates were prepared using natural porous media and coated with crude oil. The zeta potential of these substrates reflects the oil-brine zeta potential. These results show Ca2+ is a key control, however, most of the data were negative, conflicting with prior results. There is an apparent difference between the zeta potential of the ‘pristine’ oil-brine interface and that of the ‘in-situ’ oil-brine interface present in the subsurface; the latter of which appears critical in controlling CSW. Understanding the in-situ oil-brine zeta potential contributes to better design and optimisation of CSW processes to maximise oil recovery.Open Acces