Natural thermal convection in fractured porous media

Abstract:

In the crust, fractures/faults can provide preferential pathways for fluid flow or act as barriers

preventing the flow across these structures. In hydrothermal systems (usually found in fractured rock masses), these discontinuities may play a critical role at various scales, controlling fluid flows and heat transfer.

The thermal convection is numerically computed in 3D fluid satured fractured porous media. Fractures are inserted as discrete objects, randomly distributed over a damaged volume, which is a fraction of the total volume. The fluid is assumed to satisfy Darcy's law in the fractures and in the porous medium with exchanges between them. All simulations were made for Rayleigh numbers (Ra) < 150 (hence, the fluid is in thermal equilibrium with the medium), cubic boxes and closed-top conditions. Checks were performed on an unfractured porous medium and the convection cells do start for the theoretical value of Ra, namely 4p². 2D convection was verified up to Ra=800.

The influence of parameters such as fracture aperture (or fracture transmissivity), fracture density and fracture length is studied. Moreover, these models are compared to porous media with the same macroscopic permeability.

Preliminary results show that the non-uniqueness associated with initial conditions which makes possible either 2D or 3D convection in porous media (Schubert & Straus 1979) is no longer true for fractured porous media (at least for 50<Ra<150). The influence of fracture density and fracture aperture on the Nusselt number (Nu) is highly Ra dependent. The effect of the damaged zone on Nu is roughly proportional to its size.

All these models also allows us to determine for which range of fracture density the fractured porous medium is in good agreement with an unfractured porous medium of the same bulk permeability.