Laboratory hydraulic stimulation experiments to investigate the interaction between newly formed and preexisting fractures

Hydraulic stimulation in an enhanced geothermal reservoir (EGS) often creates a complex fracture network. It is understood that a fracture network includes both preexisting natural fractures and newly formed fractures. Analyzing the interaction between preexisting fractures and newly formed fractures during hydraulic stimulation is thus necessary to better understand the creation of a fracture network. In this study, we conducted laboratory scale hydraulic fracturing experiments to investigate how a fracture network is created when a propagating hydraulic fracture and a preexisting fracture interact. We observed that a secondary fracture (wing crack) was initiated and propagated from the tip of one side of a slipping preexisting fracture when a propagating hydraulic fracture follows the preexisting fracture. The laboratory experiments were analyzed by a numerical model that fully couples fluid flow between fracture surfaces and fracture deformations. The numerical analysis shows that the stress field induced by an upstream hydraulic fracture causes asymmetric distributions of normal and shear stresses along the preexisting fracture when they intersect, which resulted in initiation of a wing crack from the fracture tip on the side with larger angles. If a preexisting fracture is sufficiently large so that those tips are far from the strong effect of stress shadowing by an upstream fracture, it is possible that wing cracks propagate from both tips of the preexisting fracture. The wing cracks observed in the experiments are an important factor in improving reservoir permeability in an EGS reservoir because it has been observed in the field that secondary fractures that form as a result of fault slip can link originally discontinuous faults and enhance hydraulic conductivity underground. This type of reservoir permeability enhancement may be expected to occur by hydraulic stimulation because slipping preexisting fractures are better connected to one another by the formation of wing cracks.