Project Rio Blanco was designed as the first phase of a three-phase experimental program to demonstrate the potential of commercial development of a natural-gas field by nuclear stimulation techniques in the Piceance Basin in Kio Blanco County, Colorado. Because the gas is tightly held within the surrounding r rock, this field has not been developed by conventional stimulation methods. At the time of this writing, however, massive hydraulic fracturing experiments are underway to investigate further this technology for development of gas fields. One of these is being conducted at a site within one mile of the nuclear experiment. The first phase of the nuclear experiment consisted of the simultaneous detonation of three nuclear explosives at different depths within the Fort Union and Mesaverde formations. The detonations were designed to stimulate a 1, 3S0-foot vertical section of the Fort Union and Mesaverde formations. After a five-month waiting period, a reentry well was completed into the gas filled chimney as sodated with the upper explosive, and the reservoir testing and evaluation began. The field activities related to this reentry drilling and chimney pressure drawdown are the subject of this report. Figure 1 shows the principal locations of project activities.