Geometric Concerns for Accurate Measurement of Capillary Pressure Relationships With Centrifuge Methods (includes associated paper 27739 )

Abstract

Summary Several numerical methods for converting centrifuge experiment data tocapillary pressure-saturation relationships were compared. Differentialreduction methods included those proposed by Hassler and Brunner, van Domselaar, and Rajan. An integral reduction method suggested by Bentsen and Anli was also tested. In another integral reduction method, the radialdistribution of applied capillary pressure throughout a core plug wasaccurately modeled. Synthesized centrifuge data were the basis for allcomparisons. Introduction Capillary pressure relationships are routinely measured for rock samplesfrom many oil producing reservoirs. Techniques for capillary pressuremeasurement with reservoir fluids may be classified under two headings:porousplate methods, and centrifuge methods. Measurements of capillary pressure bymercury injection are used to probe the structure of porous media. The subjectof this paper is the set of techniques used to convert data obtained incentrifuge experiments into capillary pressure relationships. Hassler and Brunner were first to prescribe a method for convertingcentrifuge data to capillary pressure relationships. Their method requiresdifferentiation of centrifuge data in the framework of a iterative scheme. Forsmall ratios of rock sample length to centrifuge radius, they found the firstterm in the iteration to be sufficient. The truncated Hassler-Brunner method(just the first term) is likely the most common data-reduction scheme in usetoday. In more recent literature, Bentsen and Anli matched raw data with cubicpolynomial splines, which they differentiated according to the truncated Hassler-Brunner method to obtain capillary pressure curves. Amaefule and Masuo" smoothed" their raw data, then matched the "smoothed" datawith three functions (one for each of three regions of the raw centrifugedata), finally, the functions were differentiated. King et al. extended a datareduction procedure similar to the complete Hassler-Brunner method to accountfor drainage of discontinuous fluid ganglia.