A permeability model for naturally fractured carbonate reservoirs

Based on a detailed structural study performed on a reservoir surface analogue, a fracture permeability model for carbonate reservoirs is proposed. This involves four hierarchical systems, each one assumed to convey fluids exclusively to that of immediately higher order. New basic equations are then provided, simulating hydrodynamic reservoir response. The large scale, first-order structures consist of faults, to which high-permeability structures (damage zones) are associated. At the meter scale, stratabound joint systems, together with bedding joints bounding the mechanical layers, form a connected network transporting fluids to the fault system. At a lower scale, down to crystal size, non-stratabound joints constitute a pervasive and capillary fracture network which conveys fluids within the rock mass. The non-fractured host rock constitutes the lower permeability system (at the crystal scale). As no published works include an effective integration of detailed structural and numerical models, the present study aims at covering a significant gap in the literature, also providing appropriate theoretical basis for subsequent studies on fracture network modeling and reservoir simulation. Besides its application in the field of reservoir development and management, the illustrated model may also be useful in environmental studies involving ground fluids such as e.g. in the field of special/toxic/nuclear waste management.

► A structural model for naturally fractured reservoirs is suggested.
► New basic equations for reservoir numerical simulation and modeling are provided.
► A hierarchical organization of fracture networks and fluid flow is pointed out.
► This model provides the correct approach to structural analysis/characterization.
► It also allows a better understanding of the hydraulic behavior of fractured rocks.