Joseph Osoba, Rhett Gist
Texas A&M University
Herb Carroll
Bartlesville Energy Research Center
ABSTRACT
Large volumes of gas are believed to exist in the Tight Western Gas Sands. Yet, to date there has been little exploitation of this potential resource. To aid in bringing this gas to market, research is being conducted to determine the best methods for evaluating the porosity of this resource using the density log. With accurate formation evaluation techniques, the exploitation of this resource may progress with more certainty.
The porosities of core samples from the Natural Buttes 21 Well in Uinta County, Utah, were measured in the laboratory using three techniques. Two techniques used a Helium Porosimeter, and the third technique required that the sample be saturated with kerosene. The porosities determined by the saturation method tended to be lower than the porosities measured with the
Helium Porosimeter.
Initial results indicate that for the Natural Buttes 21 Well porosities calculated from the density log might be too low if standard procedures are used. Laboratory-measured matrix densities tended to be higher than the 2.65 gm/cc normally assumed for sandstones. The cement in these sandstones appears to be a high density carbonate. Low porosity zones in the Natural Buttes 21
Well have more cementation and higher matrix densities than high porosity zones. Using standard procedures to calculate porosity from the density log could underestimate the volume of gas-in-place. Using laboratory measured porosities and matrix densities, a calibration curve has been constructed for the density log that will give better estimates of porosity and gas-in-place.