Combustion/Oxidation Behavior of Athabasca Oil Sands Bitumen
Authors: Moore R.G. , Laureshen C.J. , Ursenbach M.G. , Mehta S.A. , Belgrave J.D.M. 
Published: December 2007

Oils that are potential candidates for in situ combustion recovery processes are often screened by means of their oxidation characteristics: in particular, the kinetics of the ignition process and the transition from low-temperature to high-temperature oxidation through what is known as the "negative temperature gradient region". These characteristics are readily studied in ramped-temperature oxidation tests, which involve the controlled heating of recombined, oil-saturated cores in a one-dimensional plug flow reactor under a flowing stream of air (or oxygen-containing gas). The purpose of these tests is to study the global oxidation behavior and reaction kinetics under controlled conditions, with the end purpose of providing realistic data for incorporation into a numerical simulator which can he used to predict field performance.

A ramped-temperature oxidation apparatus was used to conduct a detailed, two-year parametric study of the oxidation characteristics of Athabasca Oil Sands bitumen. The test matrix involved various levels of pressure gas injection rate, oxygen content of the injected gas and maximum ramp temperature. This paper details the principal findings for the 45-test study: in particular, the need to maintain high reaction temperatures &gt;380 C) in order to mobilize and produce heavy oils under conditions of dry in situ combustion. Design considerations and operational guidelines for successful field projects which can be suggested from the results of this study are also discussed.

Introduction

In order to successfully exploit the vast potential of processes based on the injection of air or an oxygen-containing gas for the recovery of conventional and heavy oils, it is necessary to understand the nature of the oxidation reactions which are involved. The traditional concept of in situ combustion, which is based on the high-temperature combustion of a coke-like fuel, will not explain the combustion behavior which is observed in many field projects or even in laboratory combustion tube experiments. For this reason, a number of experiments have been developed which concentrate on the global oxidation kinetics. These studies normally involve exposing the crude oil to a programmed rate of heating while in contact with the oxidizing gas. The oxidation kinetics are then observed using effluent gas analysis techniques, and differential thermal techniques such as the differential thermal analysis (DTA) work of Vossoughi et al., the pressurized differential scanning calorimetry (PDSC) studies of Phillips et al., and Belkharchouche and Hughes and the accelerating rate calorimetry (ARC) technique of Yannimaras et al.

Previous investigations of the oxidation reactions which occur during in situ combustion processes have shown the existence of at least two temperature ranges over which the oxygen uptake rates are significant. While Kisler and Shallcross have reported that the light (40.2 API) Australian oil which they studied exhibited at least three temperature ranges over which localized maxima in the oxygen uptake rate were observed, the majority of heavy oils for which oxidation data have been reported exhibit only two distinct local maxima in the oxidation rates. For convenience, the two temperature ranges where elevated oxygen uptake or energy generation rates are observed are denoted as the low-temperature oxidation and high-temperature combustion regions. For heavy oils, the range of temperatures associated with the low-temperature oxidation region is roughly from 150 to 300 C, while the high-temperature combustion region generally corresponds to reaction temperatures in the range from 380 to 800 C. The transition temperature range which falls between the temperatures corresponding to the low-temperature oxidation and high-temperature combustion regions is characterized by reduced oxygen uptake and energy generation rates. The lower temperature portion of this transition range in which the oxygen uptake and energy generation rates decrease with increasing temperature is the "negative temperature gradient region". The significance of this region will be more fully explored in the following sections of this paper.

Description of Test Procedure

Equipment The ramped-temperature oxidation apparatus and test procedures have been described in detail by Moore et al. The apparatus essentially consisted of a plug flow reactor with an inside diameter of 22.1 mm and an inside length of 320.7 mm. The reactor was equipped with five Inconel-sheathed Type K thermocouples which were spaced 50.8 mm apart and inserted radially to the centerline of the core. The reactor was mounted in an aluminum beating block which was equipped with strap heaters. Temperature control and monitoring were achieved using an in-house developed computer-based system. A mass-flow meter controlled the oxidizing gas injection rate and a backpressure control valve maintained the desired operating pressure at the reactor outlet.