Study of the Influence of Pre-Existing Bedding Planes and Faults on Hydraulically Induced Fracture Propagation in Shale-Gas Reservoirs

The focus of the present study is how existing pre-existing bedding planes and faults influence fracture propagation during a hydraulic fracturing operation. Hydraulic fracturing is studied by water injection into a 2 m vertical fracture, which will be called the injection fracture. Three scenarios were considered: in scenario 1, a shale-gas reservoir with a thickness of 20 m located between two confining formations each with 15 m thickness, is considered; in scenario 2, in addition to previous scenario, a horizontal bedding plane located above the injection fracture tip, is considered; in scenario 3, in addition to scenario 1, a pre-existing fault with a dip angle of 60°, located near the injection fracture, is considered. Changes in pore pressure and permeability caused by the interaction between the propagating fracture and those pre-existing geological structures, were evaluated. Results show that the fracture propagation is not influenced by the bedding plane because the slip displacement is not enough to lead to a significant pore pressure decrease and interrupt the propagating fracture. Consequently, at the injection fracture, changes in pore pressure and permeability are very similar to those obtained without the bedding plane. The pre-existing fault was found to limit the fracture propagation because of fault reactivation with shear failure. This led to a smaller increase in injection fracture permeability and a slight higher injection pressure than that observed in scenario 1.