Air injection into mature SAGD chambers is a promising method to sustain steam chamber pressures while reducing steam-to-oil-ratios (SOR) and increasing oil production in SAGD late life. The feasibility of attaining and sustaining a high temperature combustion front when injecting air into a steam chamber has been recently demonstrated with a three-dimensional (3-D) large scale experiment. In order to effectively mobilize bitumen, bond scission reactions are required to occur between the injected oxygen and the bitumen. The highly effective combustion process is accompanied by thermal cracking reactions which often produce lighter oil fractions and coke. However, low temperature oxidation reactions or oxygen addition to the bitumen may occur if oxygen rates are not sufficient to sustain the combustion front which in turn could degrade the bitumen properties. This paper focuses on the compositional changes observed on Athabasca bitumen during air injection after SAGD in a 3-D large scale experiment. Changes in bitumen properties, as measured from produced and residual oil samples, such as density, viscosity, asphaltenes content and hydrocarbon cut point distribution are presented.
The well configuration in the 3-D experiment consisted of two well-pairs located near the bottom corners of the rectangular model. At the end of the SAGD period, air was injected into one of the injection wells while fluids were produced from both wells in the well-pair located on the other end of the model. Oil properties from produced liquid samples, captured from the upper and lower production wells during the test, as well as extracted oil from selected post-test core samples were analysed and correlated with time and their location in the 3-D model, respectively.
Fluid segregation and distinctive fluid properties were observed in the samples produced from the upper and lower producer wells. A significant portion of produced oil samples exhibited lower density, viscosity and asphaltenes content than the original bitumen indicating that oil upgrading took place during the test. Post-test core samples were analysed during the excavation or unpacking of the 3-D model and their weights, visual appearance and 3-D location were recorded. Properties of oil extracted from selected post-test core samples also indicated the presence of upgraded oil in the residual hydrocarbons. The asphaltenes content in selected residual oil samples remained similar to that of the original oil.
Significant amounts of upgraded oil produced during the 3-D experiments highlights the potential added benefits of air injection into mature SAGD chambers.