A Novel Methodology for Simultaneous Estimation of Gas Diffusivity and Solubility in Bitumens and Heavy Oils
- Hadi Saboorian Jooybari (Petroleum University of Technology of Ahwaz)
- Document ID
- Society of Petroleum Engineers
- SPE Heavy Oil Conference Canada, 12-14 June, Calgary, Alberta, Canada
- Publication Date
- Document Type
- Conference Paper
- 2012. Society of Petroleum Engineers
- 4.1.5 Processing Equipment, 5.7.2 Recovery Factors, 5.2 Reservoir Fluid Dynamics, 5.4.10 Microbial Methods, 4.1.4 Gas Processing, 1.10 Drilling Equipment, 4.1.2 Separation and Treating, 5.5 Reservoir Simulation, 5.4.2 Gas Injection Methods, 2.7.1 Completion Fluids
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The most important parameters in the calculation of the rate and extent of gas dissolution during solvent-based heavy oil recovery processes are diffusion coefficient and solubility. However, there is a lack of sufficient experimental data on these parameters. Further, significant differences associated with reported values of diffusivities because of various nonphysical approximations made in development of the models used for calculation of this coefficient from the pressure-decay tests.
This paper presents an inverse solution technique for determining solubility, diffusion coefficient and interface mass transfer coefficient of gases in liquids (bitumens) using pressure-decay data. The approach, which is based on modeling the rate of pressure decline in response to gas diffusion, couples gas mass balance equation with the diffusion equation. Analytical solution for the forward problem is obtained by assigning physically meaningful initial and boundary conditions. Then, the forward solution is utilized to develop an interpretation technique for simultaneous determination of the equilibrium solubility, diffusivity and interface mass transfer coefficient of gas into oil. To evaluate the validity of the proposed technique, literature pressure-decay data of CH4 and CO2 dissolution in Athabasca bitumen at two different temperatures (50 and 90 °C) and initial pressure of 8 MPa were used. The simultaneous estimation of the three mass transfer parameters is the main advantage of the new methodology over the existing ones. Additionally, the calculation method doesn't depend on empirically-defined unknowns such as Henry's constant, density of solvent-bitumen mixture and etc. Eventually, the effect of neglecting gas-bitumen interface resistance on the predicted values of gas solubility and diffusivity was investigated.
Molecular diffusion plays very important rule in several oil recovery processes from petroleum reservoirs. When a gas or solvent is injected into an oil reservoir, it gradually diffuses into the oil and enhances its viscosity. The rate and extent of gas dissolution are controlled by different parameters. The rate of dissolution is controlled by diffusion coefficient in the bulk of the oil and interface mass transfer coefficient. While, the total amount of gas that will be dissolved in the oil depends on the injected gas solubility. Therefore, having accurate information about these three parameters is essential for proper modeling, design and implementation of any gas injection process.
Importance of bitumen and heavy oil reserves as emerging source of energy has increased in recent years to meet the growing demand of energy due to declining reserve of conventional oil reservoirs. So, bitumen and heavy oil reserves (especially in Alberta) have a great potential to cover the future gap between the demand and supply. The main challenge in efficient recovery of Canadian bitumen reserves is their very high viscosity (typically more than 100 cp). Injection of light gases, which can be carbon dioxide, nitrogen, flue gas and light hydrocarbon gases, reduces its viscosity and consequently enhances its mobility and recovery factor. Such viscosity reduction is attributed to gradual dissolution of the injected gas in the bitumen or heavy oil. Since mass diffusion is the predominant mass transfer mechanism in solvent-based heavy oil recovery processes, its proper operational design and economical evaluation require knowledge of diffusion parameters (diffusivity and solubility). This awareness encouraged the author of this paper to develop a mathematical diffusion model for the determination of the diffusion parameters using pressure records of gas phase during a pressure-decay experiment.
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