Pore Throats and Pore Pressure: Pushing Gas into Small Spaces

Nelson, Philip H.
U.S. Geological Survey, Denver, CO

     Data from tight-gas reservoirs indicate that gas has invaded pore space with apertures less than one micrometer. How did sub-micron pore space become charged with gas? I review published investigations of low-permeability, tight-gas rocks and consider the pressures required to displace water with gas. In evaluating the initial gas charge, pore-throat size is the key parameter, not permeability.
     Siliciclastic rocks can be characterized in terms of their porethroat- size distributions, which are commonly measured with mercury injection. In very fine and fine-grained sandstones, grain size ranges from 62 to 250 micrometers (μm); corresponding pore-throat sizes measured in one sample set range from 3 to 30 μm. However, in lowpermeability tight-gas sandstones, pore-throat sizes are considerably smaller, ranging from 0.03 to 1 μm (for comparison, 0.5 μm is the smallest size of clay particles defined on the sedimentological phi scale). The distribution of pore-throat sizes in shales extends to less than 0.01 ìm.
     Gas first entering the pore space must overcome capillary pressure, which is inversely proportional to pore-throat size. Gaswater interfacial tension is a function of pressure and temperature. At the temperature and pressure conditions of maximum burial depth of 13,000 feet for the base of the Mesaverde Group in the Piceance Basin, a pore-throat size of 1.0 (0.1) ìm requires gas pressure of roughly 20 (200) psi to displace water. Capillary pressure data from the fluvial section of the Mesaverde Group show that water saturations would be reduced to less than 60 percent by gas pressure of 300 psi, a value considerably less than the present-day overpressure of approximately 3,000 psi at the base of the Mesaverde. Consequently, gas charging is attributed to conditions at maximum burial.

AAPG Search and Discovery Article #90071 © 2007 AAPG Rocky Mountain Meeting, Snowbird, Utah