Measurement of the Streaming Potential Coupling Coefficient in Sandstones Saturated with High Salinity Natural and Artificial Brines at Elevated Temperature

Streaming potentials are generated by the flow of water through porous rock, and measurements of the streaming potential component of the self-potential (SP) have been proposed to monitor subsurface flow in a number of settings. Interpretation of SP measurements is made easier by an understanding of the coupling between the fluid flow and associated SP current sources, typically characterised using either the streaming potential coupling coefficient (C, in VPa^-1) or the excess charge transported by the flow (Q, in Cm^-3). Numerous studies report laboratory measurements of the streaming potential coupling coefficient in intact rock samples of various lithology and mineralogy at laboratory temperature. However, in many subsurface settings, such as deep saline aquifers, geothermal fields, and hydrocarbon reservoirs, temperatures are considerably higher. Yet published measurements of streaming potential at elevated temperature are sparse, and the temperature dependence of the streaming potential coupling coefficient and associated zeta potential exhibits contradictory and inconsistent behaviour. Moreover, the measurements were obtained using simple NaCl or KCl electrolytes at relatively low concentration (typically up to one tenth of seawater salinity) but natural brines are often significantly more saline and contain a wide variety of ionic species. We report measurements of the streaming potential on intact sandstone core samples saturated with high salinity natural and artificial brines at elevated temperatures. We measure streaming potential using an experimental set-up that incorporates in-situ measurements of saturated rock conductivity, brine temperature, pressure difference and voltage at temperatures up to 160^oC. At temperatures below 100^oC we also measure the brine pH and electrical conductivity. Using constant rate pumping we obtain the streaming potential coupling coefficient from the gradient of a linear relationship between pressure and voltage difference measured across the core. Measurements of saturated rock conductivity, brine conductivity, formation factor, and published temperature dependencies for brine viscosity and brine electrical permittivity are then used to interpret the zeta potential (ζ) and excess charge Q from the measured coupling coefficient. Measured values of C and ζ at laboratory temperature are in good agreement with previous published data. We find that measurements of brine pH show significant variation with temperature; moreover, at the lower salinities (c. 10^-2 M) investigated in previous studies, the ratio of the brine electrical conductivity to the saturated core conductivity (σ_f/σ_rf) increases with temperature, most likely because surface conductivity contributes less as the brine conductivity increases. Previous studies assumed this ratio (equated with the formation factor) is independent of temperature, which has a significant impact on the interpreted temperature dependence of ζ and Q.