Modelling the DC electrical response of fully and partially saturated Permo-Triassic sandstone

Results from a laboratory investigation into the electrical properties of fully and partially saturated Wildmoor Triassic Sandstone have been modelled using the Archie, Waxman-Smits and Hanai-Bruggeman equations. The results demonstrate the limitation of using simple relationships to describe samples when the matrix resistivity rho(r) is not significantly greater than the saturating electrolyte resistivity rho(w). In these situations Archie's parameters m and n are not accurately determined. Conversely, the more sophisticated Waxman-Smits and Hanai-Bruggeman models provide parameters that better describe the electrical properties of the rock and are able to identify heterogeneity between samples that would otherwise be missed. The ranges of values for matrix resistivity (49 <rho(r) <161 Omega m) and cementation factor (1.6 <m <2.1) obtained from the Hanai-Bruggeman model indicate significant variation between samples. Comparison of laboratory-determined values for cation exchange capacity (0.06 <Q(v) <0.51 meq/mL) and those obtained from the Waxman-Smits model (0.09 <Q(v) <0.55 meq/mL) indicates a very strong correlation, suggesting this model is appropriate for describing the rock. There is good agreement between parameters modelled using fully and partially saturated versions of both the Hanai-Bruggeman and Waxman-Smits equations, indicating that the data are consistent with these models and that the assumptions made are appropriate.