Abstract

Reliable experimental capillary pressure and electrical properties as functions of saturation history are essential as inputs for static and dynamic modeling of a reservoir. The only technique that simultaneously gives both capillary pressure and resistivity index as functions of saturation history, and does not rely on a model with underlying assumptions for calculation, is the porous-plate desaturation method. The main disadvantage with this method is that it is time consuming, caused by the low flux through the porous plate or membrane.

We present drainage capillary pressure curves and resistivity index measured on reservoir rock samples by the porous-plate method at pseudo reservoir conditions. In parallel with this, another plug set has been analyzed by interrupting intermediate capillary displacement pressures before reaching equilibrium, with the objective of establishing Sw-RI relationship much faster. The results show that it is possible to establish identical Sw - RI relationship with a time-saving factor of three for the carbonate rock type under study.

The saturation data were fitted to an exponential-decay model using nonlinear regression in order to derive accurate capillary pressure curves from short-wait porous-plate measurements. A similar model was suggested to describe the resistivity index change and was found to occur at a faster rate than the water saturation change.

Introduction

The most reliable method that can simultaneously measure capillary pressure (Pc) and resistivity index (RI) as a function of water saturation (Sw) is the method known as the porous plate (PP). However, this method can be quite time consuming due to the ultralow permeability of the porous plate. That is why several methods, such as the membrane technique (Longeron et al., 1994) and the continuous-injection method (Zeelenberg and Schipper, 1991), have been developed to speed up the establishment of these properties. The continuous-injection method measures resistivity index as a function of water saturation. It is identical to the porous-plate technique except that the experiment is performed using an ultralow constant injection instead of using a stepwise constant differential pressure, hence no capillary pressure curves can be directly measured. In the membrane technique experiments, thin micropore membranes can be used to shorten experimental times, minimizing the impact of the membrane permeability.

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