Evaluation of Low-Permeability Gas-Bearing Formations in Rio Blanco County, Colorado
Authors: Boardman Charles R. , Hammack Gregory W. , Fertl Walter H. , Atkinson Charles H. 
Published: December 2007

This evaluation was conducted in connection with an experiment in stimulating formations with multiple nuclear explosions. The values for reservoir properties were obtained by core and computerized log analyses for the nuclear stimulated well and by analysis of pressure buildup tests on another well nearby. Gas production was predicted on the basis of the values obtained.

Introduction
The existence of major volumes of gas in thick, low-permeability sandstone reservoirs in Colorado, Wyoming, and Utah has tantalized producers for decades. Conventional efforts to obtain commercial production from these resources have been generally production from these resources have been generally unsuccessful, not only because the permeabilities are extremely low (on the order of several microdarcies to several tens of microdarcies) but also because the gas-bearing sandstone beds are generally scattered throughout sand/shale intervals greater than 1,000 ft thick. Typical tight gas reservoirs are found in the Tertiary Fort Union and the Cretaceous Mesaverde formations of the Piceance Basin in northwestern Colorado. These formations have been drilled and conventionally fractured in Rio Blanco County without successfully obtaining commercial production. New gas production operations in this area have been sparked, however, by the potential for successfully stimulating gas production with multiple nuclear explosions. The 94,000-acre Rio Blanco Unit area (Fig. 1) has been established and a nuclear stimulation treatment has been carried out in Well RB-E-01 with three 30-kt explosives. Emplaced at depths of 5,839, 6,230, and 6,690 ft, these explosives should have effectively fractured a 1,300-ft gross section of the Fort Union and Mesaverde formations from 5,530 to 6,830 ft, subsurface. The evaluation of a stimulation treatment's effectiveness is, of course, dependent upon a knowledge of reservoir characteristics, particularly permeability and net pay thickness. The development of such knowledge for conventionally commercial reservoirs is facilitated by a rather extensive base of proved empirical and theoretical relationships. The extent to which this base is applicable to low-porosity and low-permeability reservoirs, such as those in the Fort Union and Mesaverde formations, is uncertain. Because of this uncertainty, greater than normal effort was made to evaluate cores, logs, and production tests. This report presents the reservoir property values obtained and the predictions of gas production based upon these values. production based upon these values.

Area Geology
The Piceance basin is a northwest trending structural downwarp (Fig. 1). It contains a sedimentary section dating from lower Paleozoic to Tertiary with a maximum thickness of about 30,000 ft. The east-west cross-section (Fig. 2) shows the asymmetric nature of the basin, with gentle dips on the west and steep dips on the east. The surface rock throughout the unit area is the Tertiary Green River formation, which is composed of oil shales, marlstones, and sandstones. The Wasatch formation, consisting of brightly colored clays and shale with minor sandstone, underlies the Green River formation and overlies the Fort Union formation.