Hydraulic fracturing of horizontal wells is a relatively novel stimulation treatment. For most depths of petroleum interest fractures away from the well are vertical and normal to the minimum horizontal stress direction. If a horizontal well is drilled at a trajectory other than the expected fracture direction a complicated fracture-to-well connection is likely to occur. Past literature has suggested a longitudinal fracture initiation followed by a tortuous turning path towards the final fracture direction, or multiple fractures.
This phenomenon happens within a distance that is a few times the well diameter. The consequences are increased treatment pressures and a potentially severe fracture width reduction at the turning point. Post-treatment production performance is likely to be affected substantially. Reservoir simulation studies were carried out demonstrating the dependence of production rate on the angle between the horizontal well and the final fracture trajectory, the length of the perforated interval and the dimensionless fracture conductivity. under an assumption of a choke effect at the turning point. An angle threshold was identified beyond which the well production decreases significantly.
Inducing hydraulic fractures is an additional, and relatively novel, technique of stimulating the performance of horizontal wells.
In general, horizontal wells are attractive alternating s to vertical wells, where the formation thickness is relatively small and the ratio of horizontal to vertical permeability is small. Furthermore, Deimbacher et al. showed that in formations that are highly anisotropic in the horizontal plane, the direction of drilling the horizontal well (with respect to the maximum horizontal permeability axis) is crucial. Unfractured horizontal wells drilled normal to the maximum horizontal permeability (which usually coincides with the maximum horizontal stress direction) can outperform both fractured vertical wells and longitudinally fractured horizontal wells. It was also shown that in thin formations a small vertical-to-horizontal permeability anisotropy variable,, is not as important.
With regard to hydraulic fracturing, as shown by Muxherjee and Economides and Economides et al., two mutually exclusive scenarios for drilling a horizontal well can be distinguished: the well may be drilled either along the direction of the maximum horizontal stress,, and, thus accept a longitudinal fracture, or may be drilled along the minimum horizontal stress,, leading to the possibility of transverse fractures. However, these limiting cases may not lend themselves in offshore locations where the well trajectories may be dictated by the logistics of drilling from a platform. All angles between horizontal well and minimum horizontal stress direction will result in a more complicated fracture geometry. When the horizontal well direction does not coincide with one of the principal stress axes, the hydraulically induced fracture will experience both shear and tensile failure.