Borehole Stability in Oil Sands Under Drilling
Authors: Wong R.C.K. , Yeung K.C. 
Published: March 2010

Abstract
Holes were drilled in natural oil sands and reconstituted oilfree sand cores mounted inside a specially designed triaxial cell to investigate the borehole stability under hydrostatic loading. Tests involving hot water circulation inside the drilled hole were conducted to explore the effect of thermal heating and erosion on the borehole deformation. In addition, bitumen was removed from the oil sands matrix using chemical solvent extraction to study any reduction in strength. The deformation and failure of the hollow oil sands cylinders were monitored and studied using a computer tomography scanning method.
Geotechnical properties of oil sands have been studied extensively in the past two decades [e.g., Dusseault and Morgenstern(1); Agar et al.(2).; Kosar(3); Wong et al(4); Samieh and Wong(5)]. However, most of the previous research was performed based on element (triaxial compression) tests in which the stress-strain properties were determined [e.g., Dusseault and Morgenstern(1); Vaziri(6); Wan et al(7); Samieh and Wong(8)]. Few attempts seem to have been made to conduct special tests simulating the stress paths encountered in drilling. Thus, it is not possible to evaluate the validity of these stress-strain models. In drilling wells in oil sands, the drilling mud temperature may rise to a value of about 30 – 40 °C higher than the ambient reservoir temperature(9). To reduce the risk of borehole instability, which may cause problems during the installation of liners, liquid nitrogen, dry ice, and commercial chilling units have been used to cool off the drilling mud temperature at surface. This cooling procedure could increase costs significantly in a commercial development.
To date, there is no reported study on the effect of elevated temperature on borehole stability in oil sands during drilling. Does the reduction of the bitumen viscosity due to heating increase the pore pressure diffusion rate, triggering the collapse? Is thermal heating around the uncompleted borehole detrimental to the borehole structural integrity? This paper presents results of deformation behaviour of oil sands around a drilled hole subjected to hydrostatic loading, hot water circulation action, and chemical solvent extraction. The behaviours were studied using a computer tomography (X-Ray) imaging method. Though the interpretation of the testing results is complex, the testing results provide critical insight into the borehole stability phenomenon in oil sands under drilling.
The main objective of the paper is to analyse the deformation characteristics of oil sands around a drilled hole under drilling environments. In addition, we assessed the validity of the existing oil sands stress-strain model for prediction and analysis of the results observed in the special hollow cylinder tests.

Testing Material and Procedure

Oil Sands Specimens
The oil sands cores for this experimental study were recovered at a depth of 424 m from an observation well (3–66–4-W4M) at a site near Cold Lake, Alberta. Core sampling was carried out using a conventional rotary core barrel of 89 mm inside diameter. Cores recovered were frozen at site, and kept inside PVC tubes in a freezer. Prior to any testing, the frozen cores were X-Rayed for sample selection.