New Methods for Measuring Imbibition Capillary Pressure and Electrical Resistivity Curves by Centrifuge
Authors: Szabo Miklos T. 
Published: December 2007

Three new techniques have been developed for measuring the imbibition capillary pressure curve of small porous samples by centrifuge. The paper shows the capillary pressure and saturation distributions in the cores subjected to different speeds of rotation by each of the techniques.

Combination of these methods with measurements of electrical resistivity also makes it possible to obtain numerous resistivity-index/saturation curves or capillary-pressure/resistivity-index curves relatively quickly in either the drainage direction or the imbibition direction of saturation change.

Introduction To The Capillary Pressure Measurements

It has been known for more than 2 decades how to obtain the drainage capillary pressure curve by means of a centrifuge. Recently others have attempted to explain the mechanism of the gravity drainage of porous samples in the gravity field of a centrifuge by demonstrating the saturation distributions along the samples at different speeds of rotation. These works have led to both new methods and new evaluation techniques. However, there is still no method known by which the centrifuge can be used to obtain the capillary pressure curve in the imbibition direction. pressure curve in the imbibition direction. This paper reports the technical and theoretical considerations for thus obtaining such a curve.

Short, Single-Core Method

Both in this and in the following methods a system had to be chosen that would permit the quantity of fluid entering the sample to be controlled and regulated.

A system in which the sample is simply surrounded by water could be neglected unless the sample is intermediately wet or oil wet; however, only in the negative capillary pressure interval could it be used. The applicability of this system to the case of water-wet samples may be explained very simply. From a partially oil-saturated sample the oil will be displaced by water, and subjecting this system to a multiplied gravitational field will only accelerate this displacement process. Therefore, there is no chance to regulate the degree of imbibition. A theoretical solution cannot be considered when the side of a sample farthest from the rotary axis is in contact with water or with a water-saturated porous disc because the imbibition occurs against the centrifugal force. Although it is true that imbibition will take place, the rate of imbibition will be slower than would be expected in the disc method in the earth gravitational field. Consequently, a method had to be chosen in which the direction of phase exchange occurs as a result of the natural fluid differences. That is, the water must enter the sample moving off the rotary axis and the quantity of imbibed water must be controllable.

Fig. 1 illustrates a test cell that meets the requirements noted above. The cell can be used to obtain both imbibition and drainage data. For imbibition tests the sample is placed in contact with the filter nearest the rotary axis as shown. A fine porous filter paper is placed between the sample and the filter disc to provide good capillary contact. The water reservoir above the filter disc is partially filled.