The most common centrifuges used in the petroleum industry for capillary pressure measurements are made by Beckman. The Beckman centrifuge has, long been found problematic due to its rotor design. The gravity degradation phenomenon at low speeds has been identified as one of the problems for high permeability and porosity sandstone samples, in which the gravitational acceleration distorts the horizontal centrifugal force distribution inside core plugs and thus leads to inaccurate interpretation of capillary pressure information in the high saturation region. The possible remedial countermeasures to this problem may include developing a rotor head with a new configuration that minimizes the effect while maximizing the quality of row experimental data. This paper presents a theoretical analysis of the centrifugal field of rotor systems with pivoted heads. The analysis from the proposed theory shows that a pivoted rotor head makes the gravitational nd centrifugal fields more closely aligned, thereby greatly educing this effect. The new rotor configuration provides n alternative for the centrifuge experiments. A simple approximation is provided to extend the Hassler-Brunner method for use with a pivoted rotor head.
Two of the most important parameters required by petroleum reservoir engineers in order to calculate the performance of oil and gas reservoirs are capillary pressureand relative permeability. Unfortunately, these are he two most difficult parameters to measure. Since the mid-1940'S,1,2 centrifuges have been used to collect data from which capillary pressure data can be interpreted. The basis of this method is that if a sample of porous edium contains two components, one of which wets the solid, internal surface of the sample, then capillary pressure tends to hold this wetting component inside the sample. If the sample is spun in a centrifuge, the centrifugal force acts to expel the wetting component from the sample, while the capillary pressure forces act to hold the wetting component in the sample. By measuring the amount of wetting component produced as a function of the speed (RPM) at which the sample is spun, a data set may be obtained from which capillary pressure versus saturation may be interpreted. Such interpretations may be very complicated.3. In recent years, there has been increasing interest in using a centrifuge to obtain relative permeability.4 This is done by measuring the rate at which the wetting component is expelled, and interpreting this rate data, again using a complex procedure.5
Even though centrifuge techniques have been in use for almost 50 years, they have not yet been perfected. Three basic problems remain: obtaining a complete understanding of the mechanisms that are involved in fluid displacement by centrifugal forces, performing true imbibition experiments, and interpreting the data to obtain capillary pressure and relative permeability curves. Considering the first basic problem in particular, two typical phenomena have held people's attention. One is the radial effect due to core width. 6. Traditionally people assume either a constant or a linear centrifugal acceleration distribution inside the core plug. Such assumptions will cause errors when a short, large diameter core plug is used (unfortunately this is the case for most commercial Beckman centrifuges).