Distribution in Fracture Permeability of a Granitic Rock Mass Following a Contained Nuclear Explosion

Abstract

In situ permeability of the rock outside the Hardhat chimney was determined by pressurization of long holes with air. Experimental data indicated a remarkable difference between fracture permeability of the rock near the chimney and that at a considerable distance. Although permeable fault zones were known to exist in the granite stock, the measurements of background permeability were quite low, on the order of tenths to several millidarcies. The observed difference is about two or three orders of magnitude above this background in the region 125 to 215 ft radially from the shot point, or 80 to 165 ft radially front the vertical axis of the chimney. The presence of a collapsed drift immediately beyond this region precluded measurements which could have defined the maximum extent of the zone of high permeability. Introduction Previous exploration has revealed that contained nuclear explosions create cavities which generally collapse, resulting in rubble-filled chimneys and annular fractured zones. These geometries have been shown to results in complex flow regimes which should increase the production rate from hydrocarbon reservoirs. Empirical scaling equations have been developed for estimating chimney radii and heights for a variety of rock media. The extent of the annular fractured zone has been documented and associated permeability changes have been observed qualitatively for several explosions. This study was begun to establish the nature and magnitude of the permeability of the latter region. It is anticipated that results will be useful in estimating permeability changes and consequent production stimulation which might ensue from a contained explosion in a hydrocarbon reservoir. The post-shot environment of the Hardhat event (a 5-kt contained nuclear explosion in granitic rock) was chosen for the experimental measurements. The Hardhat device was detonated at a depth of 939 ft, and subsequent exploration revealed a chimney of broken rock with a radius of 65 to 70 ft and a height above shot point of 281 ft. Before the explosion, the Hardhat medium was highly jointed and fractured (Fig. 1). It was expected that differential rock motion along these and other weaknesses induced during cavity growth and collapse would significantly alter the permeability of the rock mass. EXPERIMENTAL METHOD Initially, three 2-in. diameter holes (Nos. 10, 12 and 19) were drilled about 25 ft outward through the walls of the 800-ft level drift (elevation 4.253 ft, Fig. 2). One was drilled near the chimney and two holes, one vertical and the other horizontal, were drilled near the shaft. Order-of-magnitude differences in permeability obtained by pressurization measurements indicated further investigation was warranted. A series of 3-in. diameter holes was then drilled with water as a coolant into the walls and roof of the drift (Fig. 2). An 11-in. long packer was set from 1 to 8 ft inside each hole and air was injected at measured flow rates. Hole pressures were monitored at the collar. Permeability was low enough in the holes near the access shaft that pressure could be maintained long enough to obtain good pressure decay rates. In the vicinity of the chimney, however, permeability was so high that this was not possible and air was injected under steady-state conditions. JPT P. 619ˆ

Modeling of Groundwater Flow and Radionuclide Transport at the Climax Mine sub-CAU, Nevada Test Site

“…A significant fraction of the material appeared to be sand-sized. Boardman and Skrove (1966), which indicate the location and measured permeability or rocks in the vicinity of the Hard Hat test.…”

“…Cavity/chimney pressurization measurements suggested that wall rock permeabilities are on the order of 1,200 to 1,400 millidarcys (Boardman, 1965) (approximately 1 m/d). More direct measurements of rock permeability near the cavity/chimney were reported to be 200 to 2,000 millidarcys (0.2 to 1.7 m/d) and decaying exponentially to below 1 millidarcy with distance from the working point (Figure 2-9) (Boardman and Skrove, 1966). Interestingly, experimentally shocked granite samples could produce permeabilities as high as 0.5 millidarcy (4.2 x 10e-4 m/d) (compared to native rock permeabilities lower than 10 -6 millidarcy).…”

“…Cavities that collapse quickly typically have glass fractions of 20 to 30 percent in the glass puddle due to the incorporation of infallen rock (Boardman, 1966). Quong (1979), Boardman and Skrove (1966), Short (1964), Mehta et al (1964), and Boardman (1965).…”

“…One such example is from USGS tests conducted between 1959 and 1961, where values of "effective" permeability were found to range between 2.5×10 -3 and 1.5×10 -2 Darcies by averaging two well tests over an interpreted high permeability zone thought to have a thickness of 12 meters (Price, 1959). Additional non-disturbed (i.e., no shock fracturing) permeability values come from Boardman and Skrove (1966), Izett (1960), and Murray (1981), which report values of 10 -4 to 10 -2 , 10 -7 , and 10 -5 Darcies, respectively. Note that these values are based on only a couple of measurements (e.g., Boardman and Skrove, 1966) or a single measurement (e.g., the value of 10 -5 Darcies is based on the infilling rate of borehole UG02 after dewatering by bailers).…”

“…Additional non-disturbed (i.e., no shock fracturing) permeability values come from Boardman and Skrove (1966), Izett (1960), and Murray (1981), which report values of 10 -4 to 10 -2 , 10 -7 , and 10 -5 Darcies, respectively. Note that these values are based on only a couple of measurements (e.g., Boardman and Skrove, 1966) or a single measurement (e.g., the value of 10 -5 Darcies is based on the infilling rate of borehole UG02 after dewatering by bailers).…”