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Use of Water-Vapor Desorption Data in the Determination of Capillary Pressures at Low Water Saturations

1988; Volume: 3; Issue: 03 Linguagem: Inglês

10.2118/16286-pa

2469-9683

J. C. Melrose,

Petroleum Processing and Analysis

Summary 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 classic Kelvin equation is obtained. With available thermodynamic data, it is also shown that the classic form of the Kelvin equation can be used to compute air/brine capillary pressures for core-plug samples. Experimentally, 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 time, 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. From these data, Kelvin capillary pressures are calculated for a range of NaCl brine compositions and temperatures. Some preliminary data obtained by this technique are reported for a pair of matched Berea core plugs of about 10 cm3 PV.