Measurements of the relationship between microstructure, pH and the streaming and zeta potentials of sandstones

A large number (1253) of high-quality streaming potential coefficient (Csp) measurements have been carried out on Berea, Boise, Fontainebleau, and Lochaline sandstones (the latter two including both detrital and authigenic overgrowth forms), as a function of pore fluid salinity (Cf) and rock microstructure. All samples were saturated with fully equilibrated aqueous solutions of NaCl (10- 5 and 4.5 mol/dm3) upon which accurate measurements of their electrical conductivity and pH were taken. These Csp measurements represent about a fivefold increase in streaming potential data available in the literature, are consistent with the pre-existing 266 measurements, and have lower experimental uncertainties. The Csp measurements follow a pH-sensitive power law behaviour with respect to Cf at medium salinities (Csp=-1.44×10-9Cf-1.127, units: V/Pa and mol/dm3) and show the effect of rock microstructure on the low salinity Csp clearly, producing a smaller decrease in Csp per decade reduction in Cf for samples with (i) lower porosity, (ii) larger cementation exponents, (iii) smaller grain sizes (and hence pore and pore throat sizes), and (iv) larger surface conduction. The Csp measurements include 313 made at Cf> 1 mol/dm3, which confirm the limiting high salinity Csp behaviour noted by Vinogradov et al., which has been ascribed to the attainment of maximum charge density in the electrical double layer occurring when the Debye length approximates to the size of the hydrated metal ion. The zeta potential (ζ) was calculated from each Csp measurement. It was found that ζ is highly sensitive to pH but not sensitive to rock microstructure. It exhibits a pH-dependent logarithmic behaviour with respect to Cf at low to medium salinities (ζ= 0.01133 log 10(Cf) + 0.003505 , units: V and mol/dm3) and a limiting zeta potential (zeta potential offset) at high salinities of ζo=-17.36±5.11 mV in the pH range 6–8, which is also pH dependent. The sensitivity of both Csp and ζ to pH and of Csp to rock microstructure indicates that Csp and ζ measurements can only be interpreted together with accurate and equilibrated measurements of pore fluid conductivity and pH and supporting microstructural and surface conduction measurements for each sample.