Field air injection tests to determine the effect of a heat cycle on the permeability of welded tuff

As part of a series of prototype tests conducted in preparation for site characterization at Yucca Mountain, air-injection tests were conducted in the welded tuffs in G-Tunnel at the Nevada Test Site. The objectives were to characterize the permeability of the highly fractured tuff around a horizontal heater emplacement borehole, and to determine the effect of a heating and cooling cycle on the rock-mass permeability. Air was injected into packed-off intervals along the heater borehole. The bulk permeability of the rock adjacent to the test interval was computed from the air-flow rate, temperature, and pressure at steady state. The permeability varied from a minimum of 0.08 D to a maximum of over 144 D. Higher permeabilities seemed to correlate spatially with the mapped fractures. The rock was then heated for a period of 6.5 months with an electrical-resistive heater installed in the borehole. After heating, the rock was allowed to cool down to the ambient temperature. the highest borehole wall temperature measured was 242{degrees}C. Air injection tests were repeated following the heating and cooling cycle, and the results showed significant increases in bulk permeability ranging from 10 to 1830% along the borehole.