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Several mechanisms of liquid flow in rock fractures are possible, but because we cannot see into natural fractures, these mechanisms are not readily visible. Mathematical, numerical, and physical models have been used, however, to predict and observe these mechanisms. When heat is applied to a part of a fracture containing a liquid, the fluid flow will be impacted, particularly if the heat source warms the fracture to temperatures above the boiling temperature of the liquid. Such thermally-induced effects are expected to take place when high-level nuclear wastes are emplaced in a geologic repository. In order to test our understanding of heat driven flows in fractures, we are performing laboratory experiments on transparent models of fractures and transparent replicas of actual rock fractures manufactured from rock fracture samples and visually observing the effects on liquid flow when part of the fracture is warmed to temperatures exceeding the liquid boiling point.
For example, in Figure 1, pentane (a volatile liquid) was introduced into a fracture model composed of a sheet of flat glass glued on the edges to a sheet of obscure glass which had been roughened by sandblasting. The reason for using pentane in these experiments is that it has a lower boiling point than water, enabling experiments to be conducted at moderately elevated temperatures. The model is about 21.5 x 33 cm in size and the space between the glass plates (aperture) ranges from near 0 to less than 1 mm. The bottom portion is submerged in a heated bath with the temperature exceeding the pentane boiling temperature by about 1.3 C. The brightest regions are where pentane saturates the aperture, the next brightest regions are where pentane is present as a thick film on the glass. Above the warmed region, pentane is present in a condensation halo both in films and in saturated islands. When enough pentane collects in a saturated island, it becomes unstable and flows downward, often colliding with other saturated islands collecting more pentane, and penetrates the warmed region as a finger. The fingering is intermittent, occurring only after sufficient pentane collects in saturated islands in the condensation halo. In the region labeled "Film", pentane is flowing downward from above the warmed region in a film on the glass. No saturated islands were present in this region. In the warmed region, pentane arriving by finger or film evaporates, and is transported upwards by advection and diffusion. It then condenses to begin the cycle again. Warmer temperatures result in films and fingers that do not penetrate the warmed region as far, and larger volumes of pentane result in longer fingers.
In Figure 2, pentane was introduced into a fracture model composed of two sheets of sandblasted obscure glass. Two circular shaped regions of the fracture were warmed, resembling a fracture extending through two tunnels (drifts) in which high-level radioactive waste may be disposed of. Small flows of pentane evaporate prior to reaching the "drifts", and the condensing pentane forms a condensation halo on the top, sides, and bottom of the warmed region. This represents the ideal case in which water flowing in fractures would not contact the nuclear waste packages. Large flows, however, like the one seen, may penetrate the drifts. In a nuclear waste repository, large flow events may bring water into the drifts where it may contact the nuclear waste packages.