The Effect of Nuclear Stimulation on Formation Permeability and Gas Recovery At Project Gasbuggy

Abstract

At Project Gasbuggy, permeability was affected for some 220 feet beyond the chimney. However, irrespective of the permeability of the close-in region and of chimney size, gas eventually must flow into the chimney from the outer reaches of the drainage area of the well. Thus the preshot formation permeability is a highly critical factor in calculating anticipated recovery from nuclear stimulated reservoirs. Introduction Project Gasbuggy is a joint experiment by the Atomic Project Gasbuggy is a joint experiment by the Atomic Energy Commission, the Dept. of the Interior, and the El Paso Natural Gas Co. to investigate the feasibility of using underground nuclear explosives to stimulate production and increase ultimate recovery of natural gas from a low-permeability gas-bearing formation. The low-permeability Pictured Cliffs formation found in the San Juan basin of the Four Corners area of New Mexico, Arizona, Utah, and Colorado was selected for the experiment. The actual test site is about 55 miles east of Farmington, N. M., in the southwest quarter of Sec. 36, T29N, R4W, Rio Arriba County (Fig. 1). To supplement reservoir and production data available from the eight original field wells in the Gasbuggy test area, two preshot test wells, GB-1 and GB-2 (Fig. 2), were drilled in 1967. Extensive production tests were conducted on GB-1. The Project production tests were conducted on GB-1. The Project Gasbuggy nuclear explosive of 29-kiloton yield was detonated on Dec. 10, 1967, at a depth of 4,240 ft, approximately 40 ft below the base of the Pictured Cliffs formation (Fig. 3). Postshot re-entry of the emplacement hole, designated as GB-ER, began on Dec. 12, 1967, and terminated at a total depth of 3,916 ft on Jan. 10, 1968. Initial computations indicated a collapsed chimney about 160 ft in diameter extending from the 4,240-ft detonation depth to about 3,900 ft, the top of the 300-ft-thick Pictured Cliffs gas sand. In addition to GB-ER, preshot well GB-2 was reentered as GB-2RS, and a new well, GB-3, was drilled in 1969 at a surface distance of 250 ft from GB-ER. GB-ER has undergone extensive testing covering a period of 3 years. During this period, pressures were monitored in GB-2RS and GB-3. Presented in this paper is an analysis of the effectiveness of nuclear stimulation on increasing the formation permeability and the resulting gas recovery at Project Gasbuggy as determined from a flow model. Effect on Reservoir Rock Studies of a number of contained underground nuclear explosions have yielded a rather consistent representation of the geometric features produced. The items of primary interest related to the breakage and displacement of rock resulting from a nuclear stimulation include (1) chimney configuration, (2) chimney void volume, (3) chimney permeability, and (4) permeability of the rock outside the chimney. Boardman has discussed these as a function of explosive yield, depth of burst, and rock type. The chimney configuration and chimney void volume as reported by Boardman and by Atkinson et al. are in close agreement. As to the permeability in the chimney and the permeability outside the chimney, Boardman reported no quantitative answer. For the nuclear stimulation of hydrocarbon reservoirs, as for other stimulation processes, the quantitative value of the permeability increase outside the chimney is vital to the prediction of the ultimate recovery of hydrocarbons. JPT P. 1199