GEOMECHANICAL CONSIDERATIONS IN HYDRAULIC FRACTURING DESIGN

Several new fracturing designs for unconventional horizontal wells have been introduced recently. Although some of these designs and technologies have been tested in the field, none has been exposed to a thorough geomechanical study and its implications in final recommendations for a field pilot. In this thesis, analytical and numerical methods are implemented to investigate the geomechanical effects of a fracturing treatment on the rock system and developed a new fracturing design for future applications. Stress changes in the neighborhood of an elliptical, penny shaped and a semi-infinite fracture are calculated. Applying the principle of superposition, the change in stresses in the neighborhood of multiple fractures in an elastic- static medium are calculated for the both conventional and the new fracturing designs. Consecutive fracturing, alternating fracturing, simul-frac, zipper-frac, and the newly developed Modified Zipper Frac (MZF) are among the fracturing designs under the study in this work. It has been shown that the likelihood of creating far field complexity with simul-frac and zipper-frac is minimal, while the alternating fracturing and MZF designs have potential to enhance a complex network in the half-length window between the horizontal laterals. The effect of stress interference (stress shadow) on the widths of fractures is numerically investigated displacement by implementing discontinuity method. As was expected, fractures in MZF design maintained larger widths compared to other designs. Later, a fracture propagation criterion was added to the displacement discontinuity model to investigate the effect of stress changes on the fracture propagation path in a dynamic system. The stress intensity factors calculated from the in-house displacement discontinuity code were validated with previous experimental and analytical results. Fracture propagation angle and fracture path were predicted for different internal fluid pressure and different far field stresses. The result show that in a strong anisotropic medium, if the horizontal well is drilled in the direction of maximum horizontal stress, even with excessive injection pressures, the fracture would experience reorientation toward the direction of maximum stress and can develop a pinch point and may result in a pre-mature screen out. Fracture reorientation is also investigated in multi-well pad designs where two fractures propagate toward one another. Results show that fracture spacing highly influences the change in stresses near the tips and the orientation of the fractures change when the tip of two fractures overlap. This could result in hydraulic communication of closely spaced clusters in a down spaced well pad. It is shown that in an MZF design the middle fracture propagates more stably in the half-length window and further through the area between the two fractures from other wellbore. The results of this study can be directly implemented in current well spacing and fracture interval designs in unconventional horizontal well drilling and stimulation.