The equilibrium conditions applicable to a single liquid phase held by capillary forces within the pore space of a reservoir rock sample are reviewed. It is shown that, if the conditions for both chemical and capillary equilibrium are satisfied, an extended form of the classical Kelvin relationship is obtained. Using available thermodynamic data, it is also shown that the classical form of the Kelvin relationship can be used to compute air/brine capillary pressures for core plug samples. Experi-mentally, it is only necessary to allow such samples to reach equilibrium in a constant vapor pressure environment. Thus, the conventional methods for determining capillary pressures can be usefully supplemented by vapor phase desorption experiments. Although such experiments require relatively long equilibration times, they are simple to perform and are free from the difficulties common to other methods. A convenient way to establish a constant vapor pressure environment is to use saturated solutions of such salts as BaCl2, KNO3, and K2SO4. The available vapor pressure data for these solutions are reviewed and tabulated. Using these data Kelvin capillary pressures are calculated for a range of NaCl brine compositions and a range of temperatures. Some preliminary data using this technique are reported for a pair of matched Berea core plugs of approximately 10 cm3 pore volume.

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