Hydraulic Fracturing of Deviated Wells: Interpretation of Breakdown and Initial Fracture Opening Pressure
Authors: Brumley J.L. , Abass H.H. 
Published: December 2007

This paper presents experimental results related to the hydraulic fracturing of a deviated well, specifically, the impact that wellbore orientation with respect to the in-situ horizontal stresses can have on breakdown pressure and pressure decline after breakdown. Experiments were performed on 6 × 6 × 10-in. cast hydrostone samples. This paper discusses the pressure profile observed during hydraulic fracturing of openhole wellbores with various orientations with respect to the vertical and the maximum horizontal stress. The results of the experiments provide visual evidence of the complexity of induced fractures that can occur in deviated wellbores.

Previous laboratory experiments have concentrated on stimulation of vertical or horizontal wells. This study was designed to bridge the gap between the extremes of wellbore orientations.

Introduction

A deviated well most frequently occurs where the surface location is far from the target position in the reservoir. Also, the angle buildup portion of a horizontal wellbore passes through varying inclinations. In many of the highly offset, deviated wells, the producing formations are encountered where the wellbore cannot be returned to a vertical orientation (as in the case of S-shaped wellbore trajectories). Because of the industry's inability to penetrate the formations vertically, these highly deviated wellbores must be stimulated. In many parts of the world, deviated wellbores are cased, cemented, and stimulated through perforations. Some formations have sufficient strength to be completed without the installation of casing.

These deviated and horizontal wells are frequently fractured to improve productivity. Fracturing a deviated well sometimes results in premature screenouts and high treatment pressures. This work demonstrates the various pressure profiles that were observed during the fracturing of deviated, openhole intervals. This paper shows the complexity of fracture patterns that can be created from deviated wellbores.

Several researchers have investigated critical mechanisms related to fracturing deviated wells. Daneshy showed that inclined hydraulic fractures exhibit tortuous surface "steps" as they propagate away from the wellbore toward the unaltered in-situ stress field.

In his experimental work on fracture initiation from horizontal wells, El Rabaa showed that multiple fractures were created when the perforated interval was greater than four wellbore diameters and when the orientation angle was less than 750 (a deviation angle is referenced, in his work, from the minimum horizontal stress).

Hallam et al. concluded that surface roughness of fractures initiated from deviated wellbores is caused by "starter fractures," which are individual fracture planes initiated from separate perforations.

Abass et al. showed three types of fracture geometries: multiple parallel, reoriented, and T-shaped. They showed that the breakdown pressure is a function of wellbore deviation. However, the propagation pressure did not vary much with wellbore deviation.

Experiments

A series of laboratory experiments were conducted to promote research in the area of fracturing horizontal wells. The rock samples used in this study were rectangular blocks of hydrostone (gypsum cement) with dimensions of 6 × 6 × 10 in. These blocks were cast from mixing water and hydrostone with a weight ratio of 32:100, respectively. The physical and mechanical properties of this man-made rock are as follows: porosity = 26.5%; permeability (N2) = 3.9 md; grain density = 2.32 gm/cc; bulk density = 1.71 gm/cc; P. 269