[1] We report experimental measurements of the streaming potential coupling coefficient in sandstones saturated with NaCl-dominated artificial and natural brines up to 5.5 M (321.4 g L −1 of NaCl; electrical conductivity of 23 S m −1 ). We find that the magnitude of the coupling coefficient decreases with increasing brine salinity, as observed in previous experimental studies and predicted by models of the electrical double layer. However, the magnitude of the coupling coefficient remains greater than zero up to the saturated brine salinity. The magnitude of the zeta potential we interpret from our measurements also decreases with increasing brine salinity in the low-salinity domain (<0.4 M; 23.4 g L −1 of NaCl and 3.4 S m −1 ) but reaches a constant value at higher salinity (>0.4 M). We hypothesize that the constant value of zeta potential observed at high salinity reflects the maximum packing of counterions in the diffuse part of the electrical double layer. Our hypothesis predicts that the zeta potential becomes independent of brine salinity when the diffuse layer thickness is similar to the diameter of the hydrated counterion. This prediction is confirmed by our experimental data and also by published measurements on alumina in KCl brine. At high salinity (>0.4 M), values of the streaming potential coupling coefficient and the corresponding zeta potential are the same within experimental error regardless of sample mineralogy and texture and the composition of the brine.

[1] We present measurements of the streaming potential coupling coefficient in intact sandstone samples saturated with NaCl brines at concentrations up to 5.5 molL À1. The values we record at low salinity are consistent with those reported previously. As brine salinity increases, the coupling coefficient decreases in magnitude, but is still measureable up to the saturated concentration limit. The magnitude of the zeta potential also decreases with increasing salinity, but approaches a constant value at high salinity. This behaviour is not captured by current models of the electrical double layer. Our results suggest that streaming potential measurements may be used to monitor flow in saline subsurface environments such as deep saline aquifers and hydrocarbon reservoirs. However, they were obtained at laboratory temperature. Future work will focus on the effect of elevated temperatures at high salinity. Citation: Jaafar, M. Z., J. Vinogradov, and M. D.Jackson (2009), Measurement of streaming potential coupling coefficient in sandstones saturated with high salinity NaCl brine,

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Summary Downhole monitoring of streaming potential, using electrodes mounted on the outside of insulated casing, is a promising new technology for monitoring water encroachment toward an intelligent well. However, there are still significant uncertainties associated with the interpretation of the measurements, particularly concerning the streaming potential coupling coefficient. This is a key petrophysical property that dictates the magnitude of the streaming potential for a given fluid potential. We present the first measured values of streaming potential coupling coefficient in sandstones saturated with natural and artificial brines relevant to oilfield conditions at higher-than-seawater salinity. We find that the coupling coefficient in quartz-rich sandstones is independent of sample type and brine composition as long as surface electrical conductivity is small. The coupling coefficient is small in magnitude, but still measurable, even when the brine salinity approaches the saturated concentration limit. Consistent results are obtained from two independent experimental setups, using specially designed electrodes and paired pumping experiments to eliminate spurious electrical potentials. We apply the new experimental data in a numerical model to predict the streaming potential signal that would be measured at a well during production. The results suggest that measured signals should be resolvable above background noise in most hydrocarbon reservoirs. Furthermore, water encroaching on a well could be monitored while it is several tens to hundreds of meters away. This contrasts with most other downhole monitoring techniques, which sample only the region immediately adjacent to the wellbore. Our results raise the novel prospect of an oil field in which the wells can detect the approach of water and can respond appropriately.

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