Numerical computation of stress-permeability relationships of fracture networks in a shale rock

Abstract: We present stress-sensitive permeability relationships for two-dimensional fracture networks in the Opalinus Clay from the Mont Terri underground rock laboratory. These relationships may be used as a proxy for fracture network permeability in numerical models that resolve large spatial scales and are used in a variety of GeoEnergy applications involving flow in shaly rocks. To obtain these relationships we present a numerical procedure that uses experimentally determined stress-permeability relationships to numerically compute the effective permeability of the network. The material discontinuities stemming from the fractures are treated by a simple contact-interaction algorithm that accounts for normal interaction between fracture walls, allowing us to calculate the permeability of a fracture network under different stress conditions. We apply the procedure to four fracture networks digitized from two galleries of the Mont Terri rock laboratory. These fracture networks are mapped from the damage zone of the Main Fault that intersects the Opalinus Clay. The networks show a maximum variation of four orders of magnitude when stress ranges from 1MPa to 20 MPa. Our numerical procedure not only establishes representative stress-permeability relationships for a fractured rock mass under stress, but also provides a proxy for fracture network permeability for simulation in fractured formations.