Orientation of Hydraulic Fracture Initiation from Perforated Horizontal Wellbores

Abstract

The orientation of hydraulic fractures controls the productivity from hydrocarbon reservoirs. Productivity from low permeability formations is greatly improved having multiple fractures oriented transversely rather than longitudinally, relative to a horizontal wellbore. Analytical approximations from the literature for the longitudinal and transverse fracturing stresses are modified to incorporate pore pressure effects and then used to develop a criterion for the orientation of fractures initiating from perforated wells. The validity of this criterion is assessed numerically and is found to overestimate transverse fracture initiation, which occurs under a narrow range of conditions; when the formation tensile strength is below a critical value and the breakdown pressure within a "window." In horizontal wells, it is easier to achieve longitudinal fracture initiation, as transverse fracture initiation only occurs over a narrow wellbore pressure-at-breakdown window, while longitudinal fracture initiation occurs at comparatively higher wellbore pressures. The numerical study shows that in contradiction with existing analytical approximations, the tangential stress which induces transverse fracture initiation, is a stronger function of wellbore pressure just as the stress inducing longitudinal fracture initiation is. This reduces the breakdown pressure window for transverse fracture initiation compared to what the derived analytical approximations predict. Furthermore, this creates an additional constraint for transverse fracture initiation; the critical tensile strength value, which determines the maximum tensile strength for which transverse fracture initiation is possible for a given stress state. The range of the in-situ stress states where transverse fracture initiation is promoted can be visualized in dimensionless plots for perforated wells. This is useful for completion engineers; when targeting low permeability formations, wells must be made to induce multiple transverse fractures. A numerical simulation scheme performed on several stress states demonstrates frequent occurrence of longitudinal fracture initiation, implying that the propagating fracture re-orients in the near-wellbore region to become aligned perpendicular to the least compressive in-situ principal stress. This is the cause of near-wellbore tortuosity, which in turn is a cause completions and production-related problems, such as early screenouts and post-stimulation well underperformance.