Interpretation Of Centrifuge Capillary Pressure Data

Abstract

The traditional (Hassler) interpretation of cen- trifuge capillary pressure data is based on several implicit assumptions. In this paper the validity of two of these as- sumptions is examined. It is shown that, in some current centrifuge designs, the nonlinearity of the centrifugal field over the length of a core plug has a significant effect. The difference between the top end-face saturations calculated from the Hassler-Brunner and van Domselaar approximate expressions is evaluated for various situations. A second im- plicit assumption is concerned with the value of the capillary pressure at the bottom (outer boundary) of the core plug. The traditional assumption that the capillary pressure is zero at this boundary is shown to be subject to certain limitations. In particular, it is shown that for very high centrifuge speeds it is possible for the displacement pressure to be exceeded in the pores leading from the interior of the plug to the outer boundary region. When this occurs the Hassler assumption fails. For example, in the case of 1,000 md rocks it is pre- dicted that the Hassler boundary condition will fail when the airloil capillary pressure exceeds about 80 psi. This means that the failure of the Hassler assumption occurs only in very unusual circumstances.