We have performed modeling studies on the simultaneous transport of heat, liquid water, vapor, and air in partially saturated, fractured porous rock. Formation parameters were chosen as representative of the potential nuclear waste repository site in the Topopah Spring unit of the Yucca Mountain tuffs. The presence of fractures makes the transport problem very complex, both in terms of flow geometry and physics. The numerical simulator used for our flow calculations takes into account most of the physical effects believed to be important in multiphase fluid and heat flow. It has provisions for handling the extreme nonlinearities that arise in phase transitions, component disappearances, and capillary discontinuities at fracture faces. We model a region around an infinite linear string of nuclear waste canisters, taking into account both the discrete fractures and the porous matrix. Thermohydrologic conditions in the vicinity of the waste packages are found to depend strongly on relative permeability and capillary pressure characteristics of the fractures, which are unknown at the present time. If liquid held on the rough walls of drained fractures is assumed to be mobile, strong heat pipe effects are predicted. Under these conditions the host rock will remain in two‐phase conditions right up to the emplacement hole, and formation temperatures will peak near 100°C. If it is assumed that liquid cannot move along drained fractures, the region surrounding the waste packages is predicted to dry up, and formation temperatures will rise beyond 200°C. A substantial fraction of waste heat can be removed if emplacement holes are left open and ventilated, as opposed to backfilled and sealed emplacement conditions. Comparing our model predictions with observations from in situ heater experiments reported by Zimmerman and coworkers, some intriguing similarities are noted. However, for a quantitative evaluation, additional carefully controlled laboratory and field experiments will be needed.

This paper presents an effective continuum approximation for modeling of fluid and heat flow in fractured porous media. The approximation is based on the thermohydrologic behavior observed in detailed simulations with explicit consideration of fracture effects (see the companion paper, part 1.) The crucial concept in the development of an effective continuum approximation is the notion of local thermodynamic equilibrium between rock matrix and fractures. Where applicable it provides a substantial simplification of the description of fluid and heat flow in fractured porous media. We derive formulas for effective continuum characteristic curves (relative permeabilities and capillary pressures) in terms of the properties of fracture and matrix continua, respectively. Numerical simulations demonstrate that under favorable conditions the effective continuum approximation closely matches predictions obtained from an explicit modeling of fracture effects. It is also demonstrated that the approximation breaks down under unfavorable conditions (very tight rock matrix). A simple criterion for the applicability of an effective continuum approximation is derived from consideration of diffusive processes. A quantitative evaluation shows the criterion to be consistent with results of our numerical simulations.