Under the auspices of the PTRC in its STEPS program, Devon NEC Corporation is planning to implement a solvent-vapor-extraction (SVX) pilot in the Manatoken East heavy oil field. SVX processes involve very large and important changes to the properties of heavy oils, and a thorough, quantitative understanding of the phase behaviour — vapour-liquid equilibrium, asphaltene precipitation, and sometimes liquid-liquid equilibrium — is necessary for reliable project simulation and design. Because relatively little of the necessary data has been gathered on the properties and phase behavior of Manatoken East oil, a comprehensive study is proposed. Propane is being considered as the main solvent on the basis of its commercial availability and its suitability at the reservoir pressures that are anticipated.
Technically, the “solvents” of potential importance include not only propane, but also carbon dioxide and methane, even though they will have much lower solubilities in the oil phase. CO2 and methane are potential carrier gases that can be mixed with the propane to control the dewpoint pressure and avoid unnecessarily large amounts of solvent being left behind in liquid form in the reservoir. Methane is probably already present in significant amounts as the main constituent of solution gas.
The approach for data gathering will be to examine the interaction of each of these gases one by one so that the parameters needed for reservoir fluid characterization and later numerical simulation can be determined most accurately. The tests will be conducted at two temperatures, the reservoir temperature, and an elevated temperature. At each temperature, four VLE points will be taken for each single-solvent system. The higher temperature will be approximately 75°C, which is far enough from the reservoir temperature to allow the determination of the parameters that describe property changes with temperature. This feature will permit the prediction of the effects of both the small temperature changes that will occur in the field, and of larger changes that may be of interest with thermal EOR, or for use in design and operation of field treaters/separators. To allow testing/validation of the fluid characterization, one small data set that includes a point with both CO2 and methane mixed in oil, and two points with all three solvents, will also be obtained at the reservoir temperature.
To define the boundaries at which an additional phase appears — probably asphaltenes, but possibly a second liquid — the concentrations at which this occurs will be determined for a reference condition (propane in flashed oil at reservoir temperature), and for three other conditions that will expose the effects of temperature, CO2 concentration, and methane concentration on the boundary conditions.
Finally, the acquired data will be used to prepare an equation-of-state characterization to test for consistency, and to provide the necessary input for numerical simulations.