Centrifuge Measurements of Capillary Pressure: Part 1-Outflow Boundary Condition

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Abstract
Summary: The capillary pressure curve is estimated from centrifuge measurements with the assumption that Pc =0 at the outflow end of the core. With the proper end piece to support the core sample, this boundary condition is valid in practically all circumstances. If the end piece is wetted by the produced liquid, however, experiments show that the capillary pressure curve is altered significantly. Theoretical analysis demonstrates that film drainage can displace the zero-capillary-pressure boundary condition to the bottom of the end piece or farther. Accordingly, we recommend that the displaced phase not wet the end piece; in the case of water, a teflon end piece is recommended over the commonly used rubber one. When the end piece is not a problem, the condition for 100% liquid saturation at the outflow face can be expressed as a critical Bond number, which is rarely exceeded. Saturation profiles for an experiment just below the critical Bond number were in excellent agreement with predictions. Pendant drops at the outflow face can result in a nonzero capillary pressure at the outflow face, but this effect is insignificant. Good agreement was obtained between mercury injection and centrifuge-measured capillary pressure curves. Attention is drawn to the importance of core jacketing, core cleaning, and equilibration times in providing reliable centrifuge-measured capillary pressure curves.