Joint geophysical and numerical insights of the coupled thermal-hydro-mechanical processes during heating in salt

Salt is an ideal medium for permanent isolation of heat-generating radioactive waste due to its tightness regarding fluid flow, high thermal conductivity, and ability to creep close fractures. Understanding the thermal-hydrological-mechanical (THM) processes, including the resulting brine migration, is a key part of the scientific basis for safe radioactive waste disposal in salt formations. Underground at the Waste Isolation Pilot Plant (WIPP), to study brine migration near an excavation during active heating, we conducted joint in-situ geophysical monitoring experiments using electrical resistivity tomography (ERT) and high-resolution fiber optic-based distributed temperature sensing (DTS), during a controlled heating experiment. ERT electrode arrays and fiber optic sensors were cemented into parallel horizontal boreholes. In addition, DEM (Discrete Element Model) based numerical simulations were conducted to simulate the THM processes during heating to understand better the mechanisms that led to changes in these geophysical measurements. During heating, resistivity changes near the heater can be explained well with a simple temperature effect. Yet, at more distant regions that were cooler, the resistivity decreased much more than is predicted from temperature effects alone. DEM simulations indicate that brine migration, driven by a pore pressure gradient, is likely the primary reason for the significant resistivity decrease beyond temperature effects. Comparison between the predicted ERT responses and observations is much improved when considering the DEM simulated brine migration effects. In support of our understanding of salt for radioactive waste disposal, this geophysical and simulation evidence provided mechanistic insights into field-relevant THM processes.