Coupled THM Modeling of a Heated Borehole Test in Rock Salt

ABSTRACT The very low permeability of intact salt rocks makes them promising media for the geologic disposal of nuclear waste. However, permeating flow can occur through fractures in rock salt. These fractures can be induced by excavation operations that create a localized damage rock zone where brine could migrate towards the excavation, or by strong temperature changes, around a high-heat emitting nuclear waste package, causing compressive (during heating) or tensile (during cooling) thermal stresses that could cause rock salt damage and a potential brine migration towards the heat source. Coupled thermo-hydro-mechanical simulations were performed to predict quantitatively the brine flow into a borehole excavated in rock salt as a result of excavation, heating/cooling and damage. The rate of water release was shown to rise with any change in temperature, with the greatest increase occurring during the cooling phase. Heating-induced compressive thermal stresses and creep caused the initial mechanical damage of rock salt, leading to the creation of higher-permeability zones (e.g., fractures) for pore water flow. The subsequent cooling led to an enhancement of these zones and the formation of additional ones as a result of tensile rock failure. INTRODUCTION Rock salt has been identified as a suitable medium for nuclear waste disposal due to its low permeability, high thermal conductivity, and self-sealing capacity (Sweet and McCreight, 1983; Cosenza et al., 1999; Winterle et al., 2012). However, micro-fractures in the salt host rock can compromise the hydraulic integrity of the disposal facility by providing pathways for brine migration (Hansen and Leigh, 2011). Micro-fracturing is often initiated by two phenomena: shear-induced dilatancy (Stormont, 1997) and tensile stresses exceeding the tensile strength of salt which is around 1 MPa (Hoffman and Ehgartner, 1998). Several laboratory and in situ tests have been conducted in the 1990s or earlier to characterize the movement of brine in rock salt under isothermal and heated conditions (Hohlfelder and Hadley, 1979; Hohlfelder, 1980; Ewing, 1981; Krause, 1983; McTigue and Nowak, 1987; Finley et al., 1992). In the absence of heating, the brine inflow response varies from one experiment to another, mainly due to the heterogeneity of the tested rock salt formation that the excavation intersects (Finley et al., 1992). In the presence of a heating phase, the experiments show that brine inflow rates increase with an increasing borehole temperature (Hohlfelder and Hadley, 1979; Hohlfelder, 1980; Ewing, 1981; McTigue and Nowak, 1987). Additionally, some of these tests have shown that a considerable amount of brine was released during the heater shut-down (Hohlfelder and Hadley, 1979; Ewing, 1981).