Esimation of field-scale thermal conductivities of unsaturatedrocks from in-situ temperature data Page: 3 of 72
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66 derived from laboratory measurements. Using the field-scale thermal conductivities,
67 instead of those from core measurements, would also help to reduce uncertainty in the
68 prediction of long-term THMC conditions in the vicinity of the repository (a critical
69 factor affecting repository performance).
70 Estimating the thermal conductivities of unsaturated rock from measured temperature
71 data becomes non-trivial because of water saturation changes during the heating process.
72 The fractured welded tuff surrounding the DST has a matrix water saturation of
73 approximately 85-90% [Tsang et al., 1999; Bechtel SAIC Company, BSC, 2004] prior to
74 commencement of heating. Hence the rock can be considered "wet" under ambient
75 conditions. During the early phases of heating in the DST, heat transfer occurred entirely
76 through this wet rock. With continued heating, as the temperature approached boiling
77 near the heat source, the water in the matrix pores was converted to vapor, which then
78 moved away from the source of heating and condensed in the cooler parts of the rock.
79 The condensate thereafter flowed through the network of fractures either under gravity
80 drainage or was absorbed in the rock matrix because of stronger capillary forces in the
81 latter. Such simultaneous flow of vapor and condensate gave rise to what could be called
82 "heat-pipe" signatures, a flat zone (at the boiling temperature) in a temperature vs. time
83 or distance plot. The temperature data collected from the DST showed pervasive
84 evidence of these heat-pipe signatures [Birkholzer and Tsang, 2000; Mukhopadhyay and
85 Tsang, 2003; Birkholzer, 2006a]. With continued heating and boiling of pore water, a
86 "dryout" zone formed in the host rock in the vicinity of the heat source, and rock
87 temperature exceeded boiling point of water in the dryout zone. Outside of the dryout
88 zone, the rock continued to be "wet," with temperatures below the boiling point of water.
89 Thus, the temperature data of the DST are indicative of heat transfer occurring in three
90 distinct regimes. In the vicinity of the heat source (particularly, in the dryout zone), heat
91 transfer occurs through the superheated dry rock. At the same time, far away from the
92 heat sources, heat transfer takes place through the wet rock (see discussion below on
93 measured saturation data from the DST). In both of these regimes, the primary
94 mechanism of heat transfer is conduction. In between these two regimes is the two-phase
95 'transition' regime, where most of the boiling occurs, and where heat transfer is by means
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Mukhopadhyay, Sumit; Tsang, Yvonne W. & Birkholzer, Jens T. Esimation of field-scale thermal conductivities of unsaturatedrocks from in-situ temperature data, article, June 26, 2006; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc893068/m1/3/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.