FLUID FLOW IN DUCTILE ROCKS: PERMEABILITY IN THE LIMIT OF ALMOST NO POROSITY

The ability the microstructure in ductile materials to evolve to minimize its energy allows the formation of a percolating pore network at very low porosities, if the pore-fluid is wetting the grain boundaries. Ductile materials therefore allow pore-fluid flow and the associated mass and energy transport below the transport threshold in more typical porous media. Pore-scale simulations can now compute texturally equilibrated pore spaces in real polycrystalline materials. This allows us to probe the basic physical properties of these materials, such as percolation and trapping thresholds as well as permeability-porosity relationships. Laboratory experiments in NaCl-H₂O system are consistent with the computed percolation thresholds. Field data from hydrocarbon exploration wells in rock salt show that fluid commonly invade the lower section of the salt domes. This is consistent with laboratory measurements that show that brine begins to wet the salt grain boundaries with increasing pressure and temperature and theoretical arguments suggesting this would lead to fluid invasion. In several salt domes, however, fluid have percolated to shallower depths, apparently overcoming a substantial percolation threshold. This is likely due to the intensive shear deformation in salt domes, which is not accounted for in theory and experiments.