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

Abstract

Abstract The research described in this paper was conducted in support of a more extensive study that has been ongoing at the University of Calgary to quantify the effect of the presence of low levels of oxygen in the unheated portions of an Athabasca Reservoir undergoing in situ combustion, and to evaluate if low temperature oxidation reactions could be used to achieve in situ upgrading. The objective of the overall program was to understand the compositional changes that might occur at temperatures ranging from those of the native reservoir to those experienced in a steam injection oil recovery process. The research program was originally started to quantify what were anticipated as detrimental compositional changes when oil is oxidized at native reservoir temperatures. The program was then extended to quantify the possible enhancement of the rate of cracking which might be achieved by oxidizing the oil at low temperatures, then heating it to temperatures typical of a steam injection operation. This paper will concentrate on the compositional changes of Athabasca bitumen in contact with nitrogen and air. The experiments were performed in an oscillating batch reactor with or without core and synthetic brine. The rate of oscillation was evaluated as a parameter to examine the role of mass transfer rates. Viscosity is reported in addition to the compositional data expressed in terms of the components: maltenes, asphaltenes, and coke. The data has direct applicability to recovery processes involving the injection of air or a gas containing oxygen as an impurity. Typical applications of this nature include in situ combustion, flue gas injection, and replacement of a gas cap with air or injection of CO2 containing oxygen as an impurity. Introduction Historically, the petroleum industry has tried to improve the recovery rate of heavy oils and oil sands that have reserves three times those of conventional oil reservoirs(1), but cannot be produced by conventional means. Current methods used to improve in situ bitumen production are cyclic steam stimulation and steam assisted gravity drainage. Steam injection increases the temperature in the reservoir, thereby reducing the bitumen viscosity and increasing its mobility. Sustained steam injection faces the obstacles of water availability, high natural gas costs, and air quality, hence air injection is again being considered as a method for in situ energy generation. In order to develop realistic designs for air injection or in situ combustion projects in bitumen reservoirs, it is necessary to understand the various reactions that are involved. Three major reactions have been reported when in situ combustion (ISC) is utilized:thermal cracking;liquid phase low temperature oxidation (LTO); and,high temperature oxidation (HTO) of vapour, liquid, and solid hydrocarbon fractions. Low temperature oxidation and thermal cracking reactions are associated with immobile fuel deposition during the in situ combustion process. Low temperature oxidation reaction (LTO) is the terminology used to describe the oxygen addition reactions that occur in the liquid phase of oils. The temperature range over which these reactions occur extends from the reservoir temperature up to a nominal limit of 300 °C. Low temperature oxygen addition reactions may occur simultaneously with bond scission reactions, which occur in the vapour phase at a temperature range of 150 to 300 °C. The transition between the temperatures where low temperature oxidation and high temperature oxidation are dominant is called the negative temperature gradient region (NTGR). This is the temperature range over which the global oxy