The Effect of Reservoir Conditions and Wettability on Electrical Resistivity

Abstract

Abstract Archie saturation exponents have been determined on sandstone core plugs from three North Sea reservoirs from capillary pressure measurements using the porous plate method. Traditionally, restored or native state samples are compared with cleaned samples in studies of the effect of wettability on the electrical properties of porous media. In this study cleaned samples were restored during a capillary pressure drainage test. This is supposed to be more similar to the process taking place during migration of the oil in the reservoir. Archie saturation exponents, from experiments where brine was displaced with refined oil at reservoir and ambient temperature, both with ambient pressure and reservoir conditions, (reservoir temperature and pressure), are presented. Next saturation exponents, crude oil displacing brine, at reservoir and ambient temperature both with ambient pressure are presented. Finally saturation exponents, live crude oil displacing brine, at reservoir conditions are presented. The wettability preference of the samples were determined both prior to and after the electrical resistivity tests. The results from the electrical resistivity measurements at reservoir temperature, show that the relationship between the resistivity index and water saturation is as expected from Archie's equation, when water were displaced with refined oil. The Archie saturation exponents obtained at reservoir temperature are equal to the Archie saturation exponents obtained at reservoir conditions but lower than the Archie saturation exponents obtained at ambient conditions. The saturation exponents obtained with crude oil displacing brine are higher than the saturation exponents obtained with refined oil displacing brine, for the measurement at ambient temperature. The results from the electrical resistivity measurements with live crude oil at reservoir conditions, (reservoir temperature and pressure) and the measurement with crude oil show that the relationship between the resistivity index and water saturation is not as expected from Archie's equation. The saturation exponents (n*) increased with decreasing brine saturations. The increase of the saturation exponents can be related to a wettability change.