Wettability of reservoir rock and fluid systems from complex resistivity measurements

Abstract

Electrical resistivity measurements at a single low AC frequency have long been recognized as providing an indication of the wettability of reservoir rock and fluid systems. However, the resistivity response over a range of frequencies for samples of varying wettability is not so well characterized. Data is presented from reservoir core plugs of differing lithologies, permeabilities, and wettabilities. The complex resistivity response at differing saturations and wettability was measured. This research group has been investigating relationships between complex resistivity, permeability, and clay content, described in previous research papers. This study extends this work to include wettability. Electrical resistivity measurements in the low-frequency range (10 Hz–10 kHz) include an electrode polarization effect. At frequencies between 10 and 200 kHz, the electrode polarization effect is reduced and the bulk sample response measured. An Argand diagram analysis is employed to find the critical frequency ( f c) separating the electrode polarization from the bulk sample response. Samples are tested in a multi-sample rig at hydrostatic reservoir overburden stresses. The test equipment allows the measurement of resistivity in the two or four electrode configurations over a frequency range from 10 Hz to 1 MHz during drainage and imbibition cycles. Multi-electrodes down the sample length allow saturation monitoring and thus the detection of any saturation inhomogeneity throughout the samples. Sample wettability is evaluated using the Amott–Harvey wettability index (AHWI) on adjacent samples and change in Archie Saturation exponent before and after aging in crude oil. The effect of frequency dispersion was analysed in relation to pore-scale fluid distribution and, hence, wettability. The results suggest complex resistivity measurement have the potential as a non-invasive technique to evaluate reservoir wettability.