Controls on Failure Mode in Fractured Chalk, and Its Influence on Fracture Geometry

The mode of failure can exert a major control on the geometry of fractures in chalk: Mode 1 dilatant fractures are typically vertical, short, bed-bound but very closely spaced (20-50cm apart), whereas Mode 2 shear fractures are much longer, cut across bed boundaries, and form inclined conjugate sets, but are more widely spaced (1-3m apart). At Flamborough Head, northeast England, we see Mode 2 shear fractures formed in the lower Welton Chalk Formation but Mode 1 dilatant fractures in the overlying Burnham Chalk Formation. This will have a big impact on fluid flow through the two units: the Burnham chalk will have a high horizontal permeability but a low vertical permeability, whereas in the Welton Chalk horizontal permeability will be lower but vertical permeability higher. We use simple mechanical models to show that the failure mode in chalk is dependent on the depth of burial, mechanical properties, fluid pressure and mechanism of fracturing.