Resistivity Index From Non-Equilibrium Measurements Using Detailed In-Situ Saturation Monitoring

Abstract

Abstract This paper presents a new method for determining resistivity index from non-equilibrium desaturation tests, where the detailed saturation distributions are measured accurately from X-ray scanning. The effects of uneven saturation distribution along core length are taken into account using a finite-element mathematical model. This new method applies to both Archie rocks and a wide range of non-Archie rocks (i.e., rocks showing non-linear log(I) vs. log(Sw) relationship). It is far less time consuming compared with other reliable method and it is applicable to both water wet and non-water wet rocks. Two examples of resistivity index measurements by rapid constant-rate oil injection are presented to demonstrate this now method. The measured data included the apparent resistivity index, which was not representative of the rocks due to the uneven saturation distribution, and the detailed saturation profile versus space and time. The measured resistivity and saturation distribution data were then input into a finite-element model to calculate iteratively the intrinsic resistivity index curves which would be measured directly if a uniform saturation distribution existed. A simple generalised (I vs. Sw) functional relationship has been proposed and used in this paper in addition to the conductive rock matrix (CRMM) model. Both models reduce to Archie model when additional conductance term is equal to zero. Resistivity index curves of the same samples were also measured using the equilibrium method of sleeved oil/brine porous plate desaturation. The resistivity index data from the new, fast non-equilibrium method agree with that obtained from the sleeved oil/brine porous plate method. Introduction In order to calculate the water saturation quantitatively from the wireline resistivity log, the resistivity of the rock as a function of water and hydrocarbon saturation must be known. Currently, the best way to obtain this data is by laboratory measurement of resistivity index on core samples from the formation of interest. Resistivity index (I) was defined by Archie as; I = Rt/Ro (1) where Ro and Rt are the resistivities of rocks fully and partially saturated with brine, respectively. He further proposed that; I = Sw (2) where n was called saturation exponent. This relationship assumes that the sample saturation does not vary over the space measured by the resistivity device. It also assumes a straight-line relationship of log(I) vs. log(Sw) or, in otherwords, a constant saturation exponent over the entire saturation range. An increasing number of cases, however, are being encountered where the above two assumptions are not valid. 1.1 Nonuniform Saturation Distribution Along Core Length Sprunt et al., using X-ray CT scan, showed problems in achieving uniform saturation distribution during resistivity index tests of two carbonate samples desaturated at stepwise-increasing, constant oil/brine capillary pressure. de Waal et al. also used X -ray scanning to monitor the saturation distribution during electrical resistivity measurements of core plugs desaturated by continuous injection at a very slow, constant injection rate. P. 456^