MECHANICAL STRATIGRAPHIC CONTROLS ON FRACTURE FLOW PATHS

Mechanical stratigraphy describes mechanical units and the interfaces (e.g. bed contacts) between these units. Mechanical units and the interfaces between units control fracturing and thus fluid flow within low-matrix permeability rocks. A field study in Door County, WI, demonstrates that the strength of stratigraphic horizons controls fracture termination. Within the Austin Chalk, TX, the thickness of unfractured marl layers controls fracture termination within the interbedded fractured carbonate layers. Modeling studies support these observations and demonstrate that localized opening along mechanical interfaces and distributed shear within mechanical layers (e.g. within clay-rich layers) can act to terminate propagating fractures. Numerical experiments with distributed shear within mechanical units suggest that marl layers within the Austin Chalk as thin as 1 cm can act as mechanical interfaces. Similarly, numerical experiments on fractures approaching mechanical interfaces indicate that low strength interfaces such as organic partings can terminate fractures. The combination of empirically determined relationships from field observations and results of numerical experiments can guide prediction of fracture network from the sedimentary stratigraphy. The key to predicting the fracture network flow paths lies in understanding and characterizing the mechanical stratigraphy