Ramped Temperature Oxidation Analysis of Athabasca Oil Sands Bitumen

Abstract

Abstract 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." A ramped temperature oxidation apparatus, consisting of two identical tubular reactors mounted in a common heating block, was developed to observe these characteristics. The active reactor contained core which was saturated with oil and water, while the reference reactor contained only clean core. Inert gas was flowed through the reference reactor and an oxygen-containing gas was flowed through the active reactor while both were simultaneously heated at a fixed rate. Measured temperatures from the reactors, and produced gas composition and post test core analysis of the active reactor, allowed determination of the oxidation mode and transition behaviour. The apparatus was used to conduct a detailed parametric study of the oxidation characteristics of Athabasca Oil Sands bitumen. The test operating procedure matrix involved various levels of pressure, gas injection rate, oxygen content of the injected gas and maximum ramp temperature. The principal finding from the 45 test study was the need to maintain high reaction temperatures (>380 ° C) in order to mobilize and produce heavy oils and bitumens under conditions of dry in situ combustion. Introduction In order to successfully recover heavy oils and bitumens using the in situ combustion process, an accurate knowledge of the process kinetics is required; in particular, the reactions that can occur between the oxygen and the oil. Two different oxygen-oil reaction regimes are important when discussing in situ combustion. At one end of the spectrum is high temperature oxidation or combustion, which is characterized by fast, bond scission type reactions that produce carbon oxides, water, energy, and, in the case of laboratory in situ combustion experiments, temperatures in excess of 400 ° C. On the other extreme, low temperature oxidation reactions are not as easy to characterize, but generally involve relatively slower oxygen addition type reactions that produce variable amounts of carbon dioxide, with temperatures generally below 350 ° C. Researchers such as Martin et al.(1), Alexander et al.(2), Bousaid and Ramey(3), Burger and Sahuquet(4), Dabbous and Fulton(5), and Fassihi et al.(6) developed equipment which was used to evaluate both low and high temperature oxidation kinetics involving native oils and petroleum cokes. Differential thermal techniques such as DTA, TGA and PDSC were successfully applied for examining oxidation kinetics; however, as was pointed out by Nickle et al.,(7) care is required when translating these experiments to correlate with either combustion tube or field experiments. Despite the high quality of the research performed to date, two important deficiencies remain in the area of oxidation kinetics. The first is that nearly all of the existing numerical models of in situ combustion have focussed solely on the high temperature reactions and have not allowed for the possibility of operation in a low temperature mode.