Abstract

Holes were drilled in natural oil sand and reconstituted oil-free 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 bore-hole deformation. In addition, bitumen was removed from the oil sand matrix using chemical solvent extraction to study any reduction in strength. The deformation and failure of the hollow oil sand cylinders were monitored and studied using computer tomography scanning method.

Introduction

Geotechnical properties of oil sands have been studied extensively in the past two decades (e.g., Dusseault and Morgenstern 1978; Agar et al. 1987; Kosar 1989; Wong et al. 1993; Samieh and Wong 1997). However, most of the previous research were performed based on element (triaxial compression) tests in which the stress-strain properties were determined (e.g., Dusseault and Morgenstern 1978; Vaziri 1986; Wan et al. 1991; Samieh and Wong 1998). Few attempts seem to have been made to conduct special tests simulating stress paths encountered in drilling. It is not possible to evaluate the validity of these stress-strain models. In drilling horizontal well in oil sand the drilling mud temperature may rise to a value of about 30–40 °C higher than the ambient reservoir temperature (Knoll and Yeung 1999). To reduce the risk of bore-hole instability which may cause problem during the installation of liners, liquid nitrogen, dry ice and commercial chilling unit have been used to cool off the drilling mud temperature at surface. This cooling procedure increases costs significantly in a commercial development. There is no reported study on the effect of elevated temperature on bore-hole stability in oil sand during drilling. Does the reduction of the bitumen viscosity due to heating increase the pore pressure diffusion rate triggering the collapse? This paper presents results of deformation behavior of oil sand around a drilled hole subjected to hydrostatic loading, hot water circulation action and chemical solvent extraction. The behaviour were studied using computer tomography (xray) imaging method. Though the interpretation of the testing results is complex, the testing results provide critical insight into the bore-hole stability phenomenon in oil sand under drilling.

The main objective of the paper is to analyze the deformation characteristics of oil sand around a drilled hole under drilling environments. In addition, we assessed the validity of the existing oil sand stress-strain model for prediction and analysis of the results observed in the special hollow cylinder tests.

TESTING MATERIAL AND PROCEDURE

The oil sand cores for this experimental study were recovered at a depth of 424 m from an observation well (3-66-4-3W4M) 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.

The testing equipment used in this study consisted of a standard plexiglass triaxial cell with some modifications.

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