Hydraulic Fracturing of Deviated Wells: An Investigation of Pressure Trends in Fracture Propagation During Two Stages of Injection
Authors: Deeg W.F.J. , Brumley J.L. , Abass H.H. 
Published: December 2007

Laboratory tests demonstrate that hydraulic fracture geometry is greatly affected by a well's orientation relative to the direction of externally applied stresses. To plan a successful fracture stimulation treatment for a deviated well, we must know both the magnitude and the direction of these in-situ stresses. If the in-situ stress directions are known, the perforation design can be optimized for a given wellbore orientation.

This paper reviews hydraulic fracture propagation and falloff pressures measured in laboratory-scale hydraulic fracturing experiments of an openhole section in deviated wells. Engineers evaluated the pressure profiles observed during two consecutive injection stages for fracture stimulation treatments performed on 6x6x10-in. blocks of hydrostone. In this paper, we discuss the impact of wellbore orientation on fracture extension and reopening pressures for both stages.

Laboratory results show that the fracture propagation and reopening pressures are influenced by the orientation of the wellbore relative to the direction of vertical and horizontal stresses. In our experiments, the well's orientation relative to the vertical stress direction was varied from 0 (vertical well) to 90 (horizontal well). In the horizontal plane, the orientation of the horizontal component of the wellbore axis was varied from 0 (aligned with the maximum horizontal principal in-situ stress direction) to 90 (aligned with the minimum horizontal principal in-situ stress direction). We performed tests for 21 different orientation combinations.

Introduction

The orientation of most wells will deviate to some degree from the principal in-situ stress directions. This deviation will affect wellbore breakdown and fracturing pressures during stimulation as well as wellbore integrity during drilling. During fracture stimulation, wellbore deviation from the preferred fracture plane will result in near-wellbore tortuosity, greater initiation and propagation pressures, and narrower fractures. Knowledge of wellbore deviation and an understanding of its effects is required to properly design a fracture stimulation treatment. If engineers do not consider deviation effects when designing the fracture treatment, premature screenouts and unexpectedly high treating pressures may be encountered when the treatment is pumped in the field.

In the following sections, we discuss the effects of wellbore deviation on fracturing pressures and summarize the results of laboratory experiments performed to investigate the pressure trends that occur in the field when wellbores deviate from the preferred orientation. Simple calculations are presented to demonstrate that the experimental observations are consistent with the elastic fracture mechanics theory. Both initiation and propagation pressures were found to depend on wellbore orientation. As the fracture grew away from the wellbore, the dependence of the propagation pressure on orientation decreased. The propagation pressure appeared to gradually approach the propagation pressure for a hydraulic fracture that was initiated and is propagating in the preferred fracture propagation plane.

Fracture initiation and propagation from deviated wells has been the subject of numerous studies. Daneshy showed that hydraulic fractures initiated in planes other than the preferred fracture plane gradually align themselves with the preferred fracture plane as they grow away from the wellbore. This reorientation proceeded in a stepwise manner, creating surface steps on the fracture faces.

El Rabaa performed numerous laboratory experiments to study hydraulic fracture initiation from horizontal wells with orientations ranging from 0 (preferred direction) to 90 (wellbore aligned with minimum horizontal principal in-situ stress). P. 331^