Phase Behaviour and Compositional Changes Associated With Residual Oxygen in a Miscible CO/Light Oil Reservoir

Abstract

Abstract Concern about the effects of global greenhouse gases has made people aware of the need to reduce the atmospheric concentration of industrial waste gas. Alberta, as the centre of Canada's petroleum industry, is strongly impacted by the politics of greenhouse gases. One way Alberta can achieve the goals of the Kyoto Protocol is to capture CO2 from stack gases and to inject it into oil reservoirs to enhance recovery while sequestering at least a portion of the CO2. This paper will concentrate on oil from the Swan Hills Formation reservoir operated by Devon Oil Corporation. The phase behaviour of different concentrations of CO2/O2 mixtures injected into the Swan Hills reservoir was investigated. Two PVT (Pressure-Volume-Temperature) apparatus were used to visually observe phases and to determine bubble points and swelling of the oil rich phase. The PVT measurements were performed to develop a basic understanding of the phase behaviour for the CO2 / O2 /oil system. The composition of oxidized oil was predicted using a previously developed low temperature oxidation model coupled with phase equilibrium calculations. Introduction Carbon dioxide flooding is a well established enhanced oil recovery process in regions where CO2 is available. Canada has been slow to implement CO2 floods due to the lack of natural sources. However, the public's concern with greenhouse gas emissions and Canada's support for the Kyoto Protocol provide sufficient environmental incentives to allow for the application of CO2 floodingas a combined EOR/sequestration process. Capturing the CO2 from stack gas is an expensive process and the actual cost will depend on the purity of the CO2 stream that is required. Oxygen is an important factor as its impurity can modify the native oil properties through low temperature oxidation reactions, promotion of corrosion in the injection and production piping and alter the phase behaviour of the reservoir fluids. The study described in this paper will examine how oxygen is consumed in the reservoir, and it will concentrate on the impact of oxidation reactions with regard to the composition and emulsification characteristics of the oil. Lee(1) studied the phase behaviour and displacement efficiency of a carbon dioxide-hydrocarbon system which consisted of various mixtures of n-pentane/n-hexane. The results demonstrated that multiple phase behaviour involving two liquids with and without a vapour phase occurred during carbon dioxide injection in low temperature reservoirs. A number of studies on the effectiveness of carbon dioxide displacement under miscible and immiscible conditions have been reported by Wilburn et al.(2), Dyer et al.(3), Srivastava and Huang(4, 5) and Srivastava et.al.(6). The results showed that CO2 /oil miscibility could be developed and that the swelling behaviour depended on the chemical composition of the oil. Wilburn et al.(2) also found that miscibility is strongly dependent on the volatility or boiling point distribution of the oil. Dyer et al.(3) investigated the phase behaviour of typical Saskatchewan heavy oils with carbon dioxide. Their study was divided into two categories: stock tank oil and reservoir oil, which were mixed with CO2 in different ratios. Parameters studied included viscosity, density, swelling factor and gas-oil ratio (GOR).