Impact of wettability on laboratory measurements of streaming potential in carbonates

We demonstrate that changing the wettability of intact carbonate core samples saturated with brine and crude oil leads to measurable changes in the streaming potential coupling coefficient, zeta potential and excess charge transported by the flow. Consequently, measurements of streaming potential could be used to quantify the wetting state of core samples; they may also be used to determine if and how changes in surface charge and wetting state are responsible for improved oil recovery during low salinity waterflooding. In the experiments reported here, the streaming potential, electrical conductivity and permeability were measured on two brine-saturated carbonate plugs, then both plugs were flooded with crude oil to the irreducible brine saturation. One plug was then aged at 93 °C and 6.9. MPa confining pressure for 8 weeks, before both plugs were flooded with brine to the residual oil saturation and the streaming potential, electrical conductivity and permeability measured again. Aging of the plug caused some areas of the mineral surfaces to become oil-wet. This disrupted the continuity of the double layers at the mineral-brine interface, thus significantly reducing the excess electrical charge transported by the flow of brine, yielding zero streaming potential coupling coefficient and zeta potential within experimental error. The other sample, which was not aged, yielded a positive streaming potential coupling coefficient and zeta potential, consistent with previous measurements on carbonates. We find the most clearly expressed contrast between aged and non-aged samples is exhibited by the excess charge transported by the flow of brine.