Stress Regime and Fault Orientation Influence in the Propagation of Hydraulic Fractures

Hydraulic fracture mechanics has become a predominant technology to enhance the permeability of host rock formations and facilitate fluid circulation. This has been applied to enhance geothermal reservoirs as well as for unconventional hydrocarbon recovery. In the last three decades, consistent and important scientific advances have been achieved in the mechanics of hydraulic fractures from experiments, to analytical and numerical methods. One of the predominant open questions regards the interaction between newly created hydraulic fractures and preexisting fractures and faults at depth. Faults within the rock mass are known to act as either shield or anti-shield mechanisms, i.e., favouring or halting the propagation of hydrofractures depending on the material property contrast and/or loading conditions. Yet, the prediction of fracture growth in such circumstance to date still remains a great challenge as standard numerical methods require meshes conforming to the existing discontinuities, whereas analytical methods typically need to rely on certain heuristic criteria. Phase-field is one of the most promising and reliable numerical tools to investigate hydraulic fracture propagation under complex conditions. We tackle the problem of induced vs preexisting fractures interaction with a phase-field based method where both are regularised by a smooth function. One of the main advantage is that within the proposed method, pre-existing fractures can be treated as an additional phase-field order parameter without the necessity of conforming meshes: this is possible because changes in the discontinuity topology require merely changes in the profile of the phase-field variable. This enables a very efficient uncertainty assessment on geological discontinuities such as fault inclination.We first present a formulation of the phase-field model of brittle fracture extended to hydraulic fracture and then introduce a regularisation method for induced fracture and pre-existing geological discontinuities. The model is implemented into a parallel simulation environment using the open-source software OpenGeoSys. We investigate the role of preexisting fracture orientation (sub-vertical and sub-horizontal) in correlation with the acting faulting regime (normal and reverse faulting).