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(l) 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 new 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.