Electrical resistivity of partially saturated sandstones
Creators: name: Tercier, Paulette E.
Publisher: name: University of British Columbia
Year: 2008

description: Electrical resistivity measurements are used to evaluate the in situ nature of pore fluids. These measurements can provide information on the level of water saturation, Sw, and the pore-scale fluid distribution. The objective of this thesis was to study the effect of fluid distribution on electrical resistivity measurements. In this study two methods were used to change the fluid/air distribution within sandstone and teflon cores. The first method involved changing saturation technique, imbibition or drainage; the second method involved varying wetting conditions. Electrical resistivity measurements were made on partially saturated sandstone and teflon cores. The resistivity measurements were made at various levels of water saturation on three sandstone cores during the imbibition and drainage of distilled water. The relationship between electrical resistivity and Sw was shown to be non-unique. Measurements made during imbibition differed from those measurements made during drainage; this is a reflection of varying fluid geometries within the pore space. Electrical resistivity measurements were also made at various levels of saturation on a teflon core during drainage of nonwetting (distilled water) and a wetting (methanol) conducting fluids. Results from experiments on the sandstone and teflon cores were analyzed in terms of Archie's equation. The results from the sandstones indicate Archie's equation can be extended into the low Sw region for both imbibition and drainage. The saturation exponents were found to vary between imbibition and drainage indicating different pore-scale fluid distributions for the two saturation processes. It is suggested that during imbibition the water forms thick surface layers which are separated by a continuous thin cental air phase. During drainage this continuous air phase is not recreated and therefore the fluid/air distribution will be different for drainage than imbibition. The saturation exponent for the nonwetting system, water/teflon, was found to be higher than the saturation exponent for the wetting system, methanol/teflon. The higher saturation exponent found for the nonwetting system indicates a different pore-scale fluid geometry. Wetting conducting fluids will form a connected path along the surface of the pores at very low levels of saturation. This surface layer is not formed when the conducting fluid is nonwetting and therefore higher levels of saturation are necessary for conduction. The drainage data were assessed in light of percolation theory. It is suggested that Archie's saturation exponent from the water saturated sandstones and the methanol saturated teflon can be compared to the critical exponent for conductivity in two-dimensions. Archie's exponent for the water saturated teflon can be compared to the critical conductivity exponent in three-dimensions. For the samples studied, it appears that if the conducting fluid wets the insulating matrix the system will be dominated by a two-dimensional conducting mechanism. This is seen in the drainage data for both the water-wet rocks and the methanol-wet teflon. If the conducting fluid no longer has an affinity for the insulating matrix, conduction is through a three-dimensional system. This is seen in the drainage data for the water saturated teflon sample.