Modelling the Dissolution of a Static And Rising CO2 Bubble Into Athabasca Bitumen

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Abstract The dissolution of a single CO2 bubble into Athabasca bitumen was modelled for the cases of static and rising bubbles. The models for the dissolution of the static bubble included a quasistationary model and a simpler molecular diffusion model. For the rising bubble, the models were based on Brian Hales Levich's Nusselt number correlations and Higbie's penetration theory. The predictions for the static bubble case, over a temperature range of 300 –400 °K and a pressure range of 1 – 6 MPa, suggest that the diffusion is enhanced at lower temperatures and higher pressures due to the high CO2 solubility in bitumen. The dissolution of the rising bubble is predicted to be influenced by both molecular and convective diffusion, leading to a much faster bubble dissolution as compared to the static bubble. Due to a combination of effects, the models predict a minimum to exist in the dissolution time for the rising bubble with respect to temperature. Introduction Bitumen is a viscous, dense crude oil composed of a multitude of paraffinic, naphthenic and aromatic hydrocarbons. It is contained in extensive oil sand reserves in Alberta, principally in the Lloydminster, Peace River, Cold Lake and Athabasca regions. The diffusion of gases into bitumen is of practical interest, as it has applications in the modelling and implementation of both the in situ enhanced oil recovery techniques, such as fire-flooding and CO2 miscible flooding, and the surface viscosity reduction methods. including CO2 injection. Limited discussion of diffusion of gases into bitumen is available in the literature; therefore, there is a need for developing a predictive method for the diffusional process dealing with the dissolution of gases into the bitumen phase. It is pointed out that no experimental data exist with which the predictions can be compared. The dissolution of a gas into the bitumen could be affected by the presence of other phases in the reservoir. including the aqueous and the (inorganic) sand/clay phases. In addition, inclusion of the geometrical details of the gasliquid/ solid phases would add further complexities in the dissolution calculations. In view of the above, this study examines as a first step the dissolution of a single bubble of CO2 gas into a surrounding mass of bitumen. The models considered for the dissolution of a static bubble include a quasi-stationary approach and a semi-empirical approach in which the Nusselt number for mass transfer from the bubble was set as 2.0 (which is a theoretical value for purely molecular diffusion). For the rising bubble, essentially at the terminal velocity conditions under Stokes' law, mass transfer from the bubble is described by Higbie's penetration theory and by the Brian Hales Levich's correlations. In addition to assessing the effect of translatory motion on bubble dissolution, the parameters varied in order to consider their impact upon CO2 dissolution into bitumen include temperature, pressure initial bubble radius and mass and the initial gas concentration in bitumen. The properties of the CO2 bitumen system needed in the models Include surface tension, mass diffusivity, bitumen density, gas solubility and viscosity.