Abstract

One of the methods to measure the displacement pressure of a core is by the centrifuge capillary pressure experiment. It is generally accepted that this is the capillary pressure at the inlet end at the initial entry of the non-wetting fluid into the core in the primary drainage portion of the experiment. The objective of this paper is to show that this is valid unless the pressure drop across the interface between the fluids at the exit end is zero. As this quantity was considered negligible right at the initial theoretical development of the centrifuge method no measurement had been reported in the literature. There is, of course, no provision in the currently available centrifuges for such measurements. We have performed stepwise displacement experiments to show that this pressure drop may not be negligible. It is, therefore. necessary to confirm this result in the centrifuge.

Introduction

The measurement of capillary pressure saturation relation of a core by the centrifuge was proposed by Hassler and Brunner1. They formulated the governing set of equations for the equilibrium distribution of two fluids under constant centrifugal acceleration and presented two approximation methods to calculate capillary pressure saturation relation from measured data. Since then there were a large number of attempts to improve on these approximate solutions. Only four of these works1–4 will be discussed in this study. Interested readers are referred to Ruth and Chen's5 review article on centrifuge capillary pressure experiment. The underlying theory as derived by Hassler and Brunner were accepted by all these authors.

In spite of the general acceptance of the original formulation, a number of authors6–10 have some concern with the boundary condition at the exit end. Based on Hassler and Brunner the capillary pressure should be zero at the exit end, and it was interpreted6–10 as equivalent to water saturation being 100 % there. When Wunderlich6 found that it could be less than 100 at high centrifuge speeds, he considered it as an indication that the boundary condition was being violated. In this set of experiments epoxy was used as one of the fluids, and it was allowed to solidify while the centrifugal arm was in motion. Water saturation was determined by image analysis of the cut up sections of the core.

Wunderlich's6 concern was taken up by a number of other authors7–10 Theoretical derivations by Wunderlich6, Melrose7, and O'Meara et. al.8 were obtained for a critical pressure above which the saturation at the exit end would be reduced to less than 100 %. In the indirect experiments by O'Meara et. al.8, and Melrose and Mallinson9 they could not see any evidence of the violation of the assumption of zero capillary pressure at the exit end. They, therefore, concluded that it was only possible at centrifugal speeds much higher than the capability of their machines. This conclusion was in direct contradiction to Wunderlich's6 experimental results. Another experimental measurement of core saturation report by Baardsen et. al.10, however, found lower than 100 % saturation at the exit end at normal centrifugal speeds.

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