Experimental study on thermal cracking reactions of ultra-heavy oils during air injection assisted in-situ upgrading process

Abstract

In-situ upgrading (ISU) via down-hole heating can be an effective technique for exploitation of heavy oil reservoirs. In order to increase the heat transfer rate and reservoir energy, air or gas injection assisted in-situ upgrading process (AAISU) has been proposed. In the AAISU process, and also in the traditional in-situ combustion (ISC) process, complex thermal cracking reactions of heavy oils occur along with low temperature oxidation (LTO) in the presence of air or oxygen at high pressure conditions. In comparison with the conventional oil cracking or pyrolysis in refinery (without or with less oxygen), it has been concerned that low temperature oxidation of oil components during air injection may have some impacts on their cracking reactions. In this study, thermal cracking experiments of ultra-heavy oils in small batch reactor as well as thermogravimetry analysis (TGA) experiments have been conducted under high pressure in the presence of air and nitrogen to simulate the AAISU process. The starting temperature for cracking, the products of cracking reactions, reaction rate, and the influence of temperature and pressure on reaction kinetics have been investigated in order to reveal the influence of LTO on cracking reaction and to clarify the confusions. The experimental results demonstrated that the presence of air has some kind negative effect on the cracking of oils compared with that in the presence of nitrogen. For a typical ultra-heavy oil, the minimum starting temperature for cracking is around 350 °C in the presence of air at 3.2 MPa for a reaction time of over 30 days, while the starting temperature is around 325 °C when nitrogen is present. Less light oil components produced and lower cracking reaction rate (about 20–30%) were observed when air was present in comparison with that of nitrogen at the same pressure. That means higher activation energy is required to activate the thermal cracking reaction of oils when air or LTO reactions are involved.