Performance Of Oil-Based Drilling Fluids On Offshore Canadian Lands
Authors: MacDonald C.A. , Grieve W.R. 
Published: March 2010

Abstract
In 1985, Esso Resources Canada Limited successfully introduced oil-based drilling fluids to their Beau/ort Sea and East Coast operations. During the past two years, Esso Resources has drilled a total of six offshore wells using these mud systems. The performance of oil-based drilling fluids, on these wells, will be reviewed with respect to the primary objectives of eliminating wellbore instability and increasing wellbore lubricity. The use of this technology has proven to be very successful and it is recommended for future drilling on Offshore Canada Lands.

Introduction
Esso Resources Canada Limited (ERCL) recently drilled six offshore wells using a variety of low-toxicity, invert emulsion oil-based muds. Two of these wells were drilled off the East Coast of Newfoundland and four were drilled in the Beaufort Sea. Oil-based mud was determined to be a cost-effective method of reducing the problem time associated with historically troublesome formations. Two of the Beaufort Sea locations were drilled to angles in excess of 55 degrees. At these wells, the oil-based mud proved to be very effective at preventingtorque and drag problems. Oil-based mud has been used world-wide to prevent hole problems(1). The goal of this paper is to summarize the operating experience gained by ERCL(2), and to highlight the benefits observed.

East Coast Operations

Baccalieu 1–78/ Kyle L-11
ERCL, in partnership with fourteen companies, re-entered the East Coast drilling scene in 1985. As operator, ERCL was responsible for the planning and implementation of a two-well program in the Flemish Pass. Baccalieu 1–78 and Kyle L-11 were drilled in 1985 and 1986 respectively (Fig. 1).
Oil-based mud was used at these two locations primarily because of serious wellbore instability which had occurred at the nearest control well, Gabriel C-60. At Gabriel, a massive shale interval below 4500 mKB sloughed severely in the presence of a seawater-bentonite drilling fluid. This problem resulted in extensive reaming and eventually stuck pipe.
Increased reaming time on successive trips indicated that the sloughing problem was increasing in severity, and the well was finally abandoned before it had stabilized. Geological evaluation of Gabriel was difficult because the sloughing wellbore hampered cuttings correlation and the rugose nature of the hole resulted in extremely poor wireline log information.
The troublesome shale encountered at Gabriel posed a more serious threat to the successful drilling and evaluation of Baccalieu and Kyle because a greater portion of each wellbore would be located below the shale. A study to determine the nature of the sloughing was initiated so that a repeat of the problem encountered at Gabriel could be prevented.
Shale samples taken from Gabriel were analyzed using x-ray diffraction and were found to be predominantly illitic in composition These hard brittle shales are known to develop extremely high swelling pressure when exposed to water(3). It was concluded that the sloughing at Gabriel was likely caused by shale hydration and subsequent rock failure. Abnormal pore pressure or abnormal tectonic stresses did not appear to be the likely causes of the sloughing problem.