The surface charge of carbonate minerals (expressed as the zeta potential) plays a key control on reservoir wettability, and changes in zeta potential have been invoked to explain wettability alteration and the release of previously trapped oil during controlled salinity waterflooding. We report a method to characterize the zeta potential of carbonates, based on measurements of streaming potential, which can be used to probe the surface charge of mineral-brine and oil-brine interfaces within the porous medium. The advantage of our approach is that it can be applied to intact core samples, saturated with brine and crude oil, at reservoir conditions of temperature and salinity. Most reported measurements of zeta potential in carbonates were obtained using electrophoretic mobility measurements. These instruments cannot be used at reservoir conditions because (1) they require powdered samples, so the pore-space topology of the rocks is not preserved, (2) only the brine phase is present, so the impact of crude oil in the reservoir pore-space is not included, and (3) the distribution of crude oil and brine in the reservoir pore-space, including variable wettability, is not captured. We use the streaming potential method to obtain measurements of zeta potential on intact core samples at typical reservoir brine salinity and composition. We determine the impact on zeta potential of varying the total salinity, and the concentration of the potential-determining ions (PDIs) calcium, magnesium and sulfate. We investigate PDIs at concentrations naturally found in sea water, formation brines, and typical compositions used in controlled salinity waterflooding. At high calcium/magnesium concentrations, the zeta potential of the water-wet calcite surface is positively, but the magnesium concentration required for this to occur is higher than that of calcium. The effect of sulfate is more complex: increasing concentration yields a more negative zeta potential, but this is partly as a result of decreased calcium desorption which is approximately anti-correlated with sulfate. The zeta potential responds linearly to PDI concentration expressed as pPDI (analogous to pH). The zeta potential of the oil-wet calcite surface, obtained using measurements on aged core samples, is different to that of the water-wet surface, showing a higher negative charge. Our results allow us to establish a relationship between streaming potential measurements and wettability. They may also help explain the change in surface charge which may be linked to wettability alteration during controlled salinity waterflooding in carbonates.
|Publication status||Published - 14 Apr 2015|
|Event||18th European Symposium on Improved Oil Recovery - Dresden , Germany|
Duration: 14 Apr 2015 → 16 Apr 2015
|Conference||18th European Symposium on Improved Oil Recovery|
|Abbreviated title||IOR 2015|
|Period||14/04/15 → 16/04/15|