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