ABSTRACT:

This paper presents results from a coupled thermal, hydrological and mechanical analysis of thermally-induced permeability changes during heating and cooling of fractured volcanic rock at the Drift Scale Test at Yucca Mountain, Nevada. The analysis extends the previous analysis of the four-year heating phase to include newly available data from the subsequent four-year cooling phase. The new analysis of the cooling phase shows that the measured changes in fracture permeability follows that of a thermo-hydro-elastic model on average, but at several locations the measured permeability indicates (inelastic) irreversible behavior. At the end of the cooling phase, the air-permeability had decreased at some locations (to as low as 0.2 of initial), whereas it had increased at other locations (to as high as 1.8 of initial). Our analysis shows that such irreversible changes in fracture permeability are consistent with either inelastic fracture shear dilation (where permeability increased) or inelastic fracture surface asperity shortening (where permeability decreased). These data are important for bounding model predictions of potential thermally-induced changes in rock-mass permeability at a future repository at Yucca Mountain.

1. INTRODUCTION

The Yucca Mountain Drift Scale Test (DST) is a multiyear, large-scale, underground heating test conducted by the U.S. Department of Energy in unsaturated fractured volcanic tuff at Yucca Mountain, Nevada (Fig. 1a) [1]. The DST, which started in 1997, included a four-year period of forced heating, followed by a four-year period of unforced (natural) cooling (Fig. 1b). Heating was provided by nine floor heaters within the heated drift, as well as 50 rod heaters, referred to as ?wing heaters,? placed into horizontal boreholes emanating from the heated drift. During the experiment, a volume of over 100,000 m3 of intensively fractured volcanic tuff was heated, including several-tens-of-thousands of cubic meters heated to above boiling temperature (Fig. 1a). This massive heating induced strongly coupled thermal-hydrological-mechanical-chemical (THMC) changes that were continuously monitored by thousands of sensors embedded in the fractured rock mass. Of particular interest to this study is the periodic active pneumatic (air-injection) testing used to track changes in air permeability within the variably saturated fracture system around the DST. Air-injection testing was conducted in several-meters-long packed-off sections in 44 hydrological boreholes, in 3 clusters forming vertical fans that bracket the heated drift and the wing heaters (Fig. 2). In total, about 700 air-injection tests were performed in the 44 packed-off sections during the course of the four-year heating and four-year cooling periods. Previous coupled thermal-hydrological-mechanical (THM) analyses of the initial four-year heating period (lasting from December 1997 through January 2002) indicated that the observed air-permeability changes were a result of both thermal-mechanical (TM) changes in fracture aperture and thermal-hydrological (TH) changes in fracture moisture content [2, 3, 4]. Moreover, those previous analyses indicated that the TM-induced changes in fracture aperture and intrinsic permeability would be mostly reversible. However, the prediction of reversible behavior was based on analysis of data from the four-year heating period and did not include the data from the subsequent four-year cooling period. In this paper, we present results from an extension of our THM analysis to include thermally induced permeability changes measured during the four-year cooling period, from January 2002 through November 2005.

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