Most horizontal wells in unconventional reservoirs are drilled in the direction of the minimum stress. The preferred far-field fracture orientation thus favors hydraulic fractures transverse to the wellbore. The near-wellbore stress concentration, however, sometimes favors the initiation of fractures in a plane defined by the well axis. Transverse and axial hydraulic fractures can thus both initiate in some situations and cause significant near-wellbore tortuosity. We investigate the competition between these two types of fractures by comparing their energy requirement during hydraulic fracture initiation and propagation. First, we investigate the limiting cases of slow and fast pressurization where fluid flow and fracture mechanics uncouple. We then use recently developed numerical models for the initiation and propagation of hydraulic fractures from an open hole accounting for fluid flow in the newly created crack, wellbore stress concentration and injection system compressibility.
Compared to a simple tensile stress analysis, the methodology described here provides a way to quantify the occurrence of only transverse or both transverse and axial hydraulic fractures as well as the maximum length of the axial fractures in the latter case. Based on dimensional analysis and numerical simulations for a range of relevant formation properties and far-field stress conditions, our results show that the critical defect length that favors transverse fracture over longitudinal is less than a borehole radius in the slow pressurization limit. For realistic injection conditions, if the initial defect length favors axial fractures, the distance over which transverse fractures become energetically favorable can become much larger than its slow pressurization value, especially for large dimensionless viscosity. Smaller pressurization rate and less viscous fluid ultimately favor the propagation of transverse fractures compared to axial ones.