Measurements of the Relationship Between Microstructure, pH, and the Streaming and Zeta Potentials of Sandstones

Abstract

A large number (1253) of high-quality streaming potential coefficient (C sp ) 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 (C f ) and rock microstructure. All samples were saturated with fully equilibrated aqueous solutions of NaCl (10 −5 and 4.5 mol/dm 3 ) upon which accurate measurements of their electrical conductivity and pH were taken. These C sp 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 C sp measurements follow a pH-sensitive power law behaviour with respect to C f at medium salinities (C sp = − 1.44 × 10 −9 C − 1.127 f , units: V/Pa and mol/dm 3 ) and show the effect of rock microstructure on the low salinity C sp clearly, producing a smaller decrease in C sp per decade reduction in C f 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 C sp measurements include 313 made at C f > 1 mol/dm 3 , which confirm the limiting high salinity C sp 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 C sp 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 C f at low to medium salinities (ζ = 0.01133 log 10 (C f ) + 0.003505, units: V and mol/dm 3 ) and a limiting zeta potential (zeta potential offset) at high salinities of ζ o = − 17.36 ± 5.11 mV in the pH range 6-8, which is also pH dependent. The sensitivity of both C sp and ζ to pH and of C sp to rock microstructure indicates that C sp 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.

Experimental Measurement of Frequency-Dependent Permeability and Streaming Potential of Sandstones

“…Recycling was stopped only when the fluid was in equilibrium with the sample as indicated by a stable fluid conductivity and pH. It was noted that this time was much shorter than encountered in the previous DC streaming potential measurements (Walker et al 2014;Walker and Glover 2018) and this was attributed to the much smaller sized sample used in this case. During equilibration, the cell was checked for gas bubbles by partially unscrewing all lead-throughs and ports until they leaked, and by carrying out flow with a back-pressure regulator on the downstream side set to 200 psi.…”

“…It is isotropic and homogeneous and is composed of well-sorted sub-rounded grains in the range 200-2000 μm. The grains are predominantly quartz (80% to 90%), feldspar (albite and microcline, 7% to 10%) and some mica (muscovite, 2% to 3%) and are cemented by quartz, with overgrowths of quartz microcrystals and evidence if some feldspar digestion as well as trace amounts of iron sulphides and clays (Walker and Glover 2018).…”

“…Initial measurements were made on three samples of Boise sandstone from the same batch samples that had been used in previous steady-state streaming potential measurements (Walker and Glover 2018). Each sample was subjected to a range of background (16) C spIP = C spo cos( )sin( t), and 17C spQP = C spo sin( ) sin ( t + ∕2).…”

“…This value depends on frequency as well as other factors such as porosity and permeability (Peng et al 2018a(Peng et al , b, 2019. For steady-state coupling (C spo ), there is a growing body of experimental measurements on rocks (e.g., Walker and Glover 2018;Walker et al 2014;Vinogradov et al 2010;Alkafeef and Alajmi 2007;Pozzi 1995a, b, 1997) and glass-bead packs (Glover and Déry 2010), together with an increasingly well-understood supporting theoretical framework (Glover 2018;Glover et al 2012a;Glover and Déry 2010, Revil and Glover 1997, 1998Revil et al 1999). However, the situation is very different for frequency-dependent coupling.…”

A brief review of the correlation between electrical properties and wetting behaviour in porous media

“…change at the nanoscale level. The streaming potential method has been widely employed from a single pore to intact core sample (Vinogradov et al, 2010;Walker and Glover, 2018). The macroscopic properties of streaming potential can be inferred to the zeta potential at the mineral-water interface accurately, and it is quite sensitive to the change of interface phenomena.…”