Borehole Instability in Fissile, Dipping Shales, Northeastern British Columbia
Authors: McLellan P.J. , Cormier K. 
Published: December 2007

Abstract
A detailed investigation of borehole instability in the Foothills region of Northeastern British Columbia, Canada is described. Costly problems due to hole collapse have been experienced during the drilling of deviated wells through dipping, fissile shales in the region. An integrated field and laboratory research study of the Fernie Formation was undertaken including the cutting of an 18 m core, petrophysical and rock mechanics testing, and borehole stability modelling. Key insights into the mechanisms of instability have been gained from the logging of discontinuities in the core and ultrasonic borehole imagery acquired in the well. Triaxial strength measurements are used to model the development of yielding in the rock mass and along bedding plane parallel fractures using 3D geomechanical software. Recommendations for selecting appropriate drilling fluids, planning hole trajectories and reducing problem time in similar settings are provided.

Introduction
Borehole instability related problems often arise when drilling weak, fissile shales in the Foothills region of Western Canada. Total or partial hole collapse can occur rapidly or over an extended period of time resulting in stuck pipe, poor hole cleaning, poor logging conditions and excessive cement and mud volumes. A limited number of case histories describing borehole instability in Western Canada have been published, although there is very little written about Foothills drilling problems. Recently, Last et al described an integrated effort to evaluate and manage borehole instability in the Cusiana field of Columbia, which is located in a similar thrust belt.
Shell Canada has historically been an active gas explorer and producer from the Foothills region of Western Canada. A review of its drilling costs in the late 1980's revealed that 56% of all drilling problem costs, or 12% of total drilling costs were directly or indirectly related to borehole instability. A multidisciplinary team was formed to investigate the underlying causes of the problems and to develop operational procedures to reduce or eliminate these costs. Several studies were undertaken including:–FMI and FMS studies of fractured Fernie shale sections–Area studies of caliper logs and their relationship to drilling problem time–In-situ stress evaluations using log-based techniques–Rock mechanical property testing–Geomechanical modelling of hole collapse and enlargement processes–On-site monitoring of hole enlargement including the continuous measurement of cavings
Learnings from this team were conveyed to operational personnel through presentations, workshops, computer programs and planning studies for individual wells. Unfortunately, mechanistic predictive modelling has been hampered by the lack of good quality, representative rock strength and stress data.
This paper focuses on an integrated investigation in the Boulder field of Northeastern British Columbia. Borehole collapse problems had been experienced by Shell and other operators in the vicinity, especially where long sections of dipping, fissile shales of the Fernie Formation were encountered. Severe borehole enlargement occurred in the two previous Boulder wells, with related problem time and poor logging conditions. The structural complexity of the region and the often uncertain geology complicated well planning and occasionally resulted in the need for wells to be sidetracked. In 1994 Shell Canada took advantage of the opportunity to obtain logs and a shale core from the problematic Fernie Formation in order to improve its understanding of borehole instability.
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