In Situ Upgrading of Heavy Oil

Abstract

Low temperature oxidation (LTO) of hydrocarbon liquids generally results in a more viscous end product; this has clearly been shown in the literature of the past 30 years. However, under the right conditions, LTO can be used to achieve viscosity reduction in heavy oils. The In Situ Combustion Group at the University of Calgary conceived of a two-stage LTO process whereby oil is contacted with air, first at low, then at elevated, temperatures. The first, low temperature, step incorporates oxygen into some of the hydrocarbons, yielding labile bonds that should break at lower-than-usual temperatures. Once these free radicals are formed, the second step promotes bond cleavage at higher temperatures, resulting in shorter chain hydrocarbons. In a field situation, this process would be analogous to first injecting air into a formation at low temperature, then starting a steam soak or steam flood. Experimental runs carried out on Athabasca bitumen examined the effects of oxygen partial pressure, temperature, reaction time, and the presence of rock and brine. On completion of each experiment, the gas composition was determined using gas chromatography, water acidity (pH) was measured, and the hydrocarbon products were analysed for coke and asphaltenes contents, viscosity, and density. Some instances of viscosity reduction have been observed; these are linked to lower oxygen partial pressures, higher second stage temperatures and longer run times. This paper discusses the experimental work, and estimates the optimum conditions for successful viscosity reduction of a given heavy oil. Introduction Heavy oil and oil sands are important hydrocarbon resources that total over 10 trillion barrels, nearly three times the conventional oil in place in the world. The oil sands of Alberta alone contain over two trillion barrels of oil. In Canada, approximately 20﹪ of oil production is from heavy oil and oil sand resources(1). The application of thermal energy to increase heavy oil recovery has become more popular as conventional reserves decline. Steam injection accounts for the majority of the thermal recovery projects currently in operation; however in situ combustion offers many theoretical advantages if the operational characteristics of the process are incorporated in the design and operation of the field project. A major difficulty encountered in operating in situ combustion processes is low temperature oxidation (LTO), which involves oxygen addition reactions that occur at temperatures lower than 300 °CDATA [C. Typically, the byproducts of these reactions are oxidized hydrocarbons that have an increased polarity. This makes them more viscous, and thus detrimental to the in situ combustion process. Because of the major impact that LTO can have on the performance of an in situ project, a significant number of investigations have been carried out on the nature and effect of LTO reactions(2-27). In some circumstances, however, it may be beneficial to subject oil to LTO. The experimental results of Cram and Redford showed that air/steam combinations can provide better recovery rates and better thermal efficiencies than steam alone, at comparable volumes of steam injected, when the process is carried out in the low temperature oxidation region((28). It is believed that energy generation by the exothermic oxidation reactions is a significant factor in the LTO process.

Compositional Changes for Athabasca Bitumen in the Presence of Oxygen Under Low Temperature Conditions

“…The lower temperature was 80~120 o C, the higher was 200-220 o C. Duration of the tests ranged from 4 to 24 days, the series was designed to compare Xu's [5] data and test the LTO effect on the heavy oil properties.…”

“…Two stage thermal cracking experiments on Athabasca bitumen in a nitrogen atmosphere were conducted to provide a non-oxidation base case for Xu's [5] oxidation experiments. The experimental apparatus was designed and constructed in 1993 for the Wichert's study [6] .…”

“…5%, 10% and 50% were chosen for the oxygen molar percentage in the feed gas, at a pressure of 4,100 kPa. Xu's [5] study was based on oxidation work carried out in a two-stage LTO process using a 250 ml batch reactor that was designed for Wichert [6] . The twostage LTO process included an initial oxidation period at temperatures between 80 o C to 120 o C, then after several days the temperature was elevated to approximately 200-220 o C. Air was used as the injected gas and the vapor space was charged only once, hence the tests were performed under conditions of depleting oxygen.…”

The effect of oxygen partial pressure on oxidation of an oil base liquid

“…The compositions of the gases after each test are determined by gas chromatograph. A detailed explanation of this experimental setup is given elsewhere (Xu, 2001).…”

Catalytic Oxidative Cracking of Heavy Wax Distillates, Slack Wax, and Polyethylene Wastes to Yield Liquid and Gas Fuels

“…Dwindling reserves of light and medium crudes all over the world promote the scientists to the cracking of heavy crude, heavy petroleum residues, plastic wastes, shale oil, etc. to meet the great demand for light-refined products and petrochemical feedstocks (Xu et al, 2001).…”