Abstract
Fractured reservoir permeability, water-breakthrough and hydrocarbon recovery cannot be extrapolated from experimental data obtained on cores. Instead computer simulations allow these processes to be studied in discrete- fracture models at an intermediate scale. We represent intersecting natural and stochastically generated fractures in massive or layered porous rocks accurately with novel unstructured hybrid finite-element meshes. This highresolution discretization allows us to compute two-phase flow with an original implicit-pressure implicit-saturation formulation. Computations incorporate unique relative permeability-saturation relationships and capillary pressure curves for individual material domains.
The steady-state flow velocity varies over many orders of magnitude. Velocity spectra have multiple characteristic peaks and show significant overlaps between fracture and matrix domains. In models with well interconnected fractures, residual saturations greatly exceed those initially assigned to the rock matrix. Total mobility is extremely sensitive to small saturation changes. Hence, grid-block averaged relative permeabilities offer little predictability and a new formalism is needed to upscale from core to grid-block scale.
Fracture matrix counter-current imbibition is surprisingly inefficient as it occurs only over a small fraction of the total fracture-matrix interfaces and only where fracture flow velocities are high enough to remove the drained oil. There is also a strong time dependence of the flow. Water breakthrough in transient models occurs earlier than in steady state ones.
