BYRNES, ALAN P., GeoCore, Loveland, CO; and * WILLIAM C. KEIGHIN, U.S. Geological Survey, Denver, CO

ABSTRACT: Effects of Confining Stress on Pore Throats and Capillary Pressure Measurements, Selected Sandstone Reservoir Rocks

Although data documenting the effects of confining stress on porosity and permeability in low permeability sandstones are available, almost no data are available that document the effect of confining stress on capillary pressure or pore size. Nineteen samples from three geographic areas, from different depths, and with different diagenetic histories were examined. Matched porosity/permeability pairs were obtained from single plugs; thin sections were prepared from end pieces of the plugs.

The magnitude of porosity and permeability decrease with confining stress was typical of low permeability sandstones. Air-mercury capillary pressure analysis to 10,000 psi was performed on one sample, and a similar analysis was performed on the second sample while it was hydrostatically confined at 3,000 psi greater than the mercury injection pressure. Confined principal pore entry throat diameters (PPETD), calculated using the Washburn relation, were from 35-65% smaller than unconfined diameters and were correlated:

log PPTED<confined> = 1.02 * log PPETD<unconfined> - 0.175

Given that the general correlation between PPETD and permeability for these samples is:

log K<insitu> = 2.09 * log PPETD - 0.777

much of the decrease in permeability exhibited by these rocks is due to a decrease in PPETD. Since these are the throats that access most of the pore volume of these rocks, these data indicate that most permeability decrease in these sample is due to pore throat closure, and not closure of stress-relief microfractures.

Understanding the nature of pore throats, and their relationship to facies distribution and effects of diagenesis, determined through petrographic examination, aids in understanding mechanisms of fluid flow in clastic rocks. This information may be valuable for calculating rates of recovery from potential reservoirs.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.