This paper describes and quantifies the benefits of residual oil vaporization in a gas injection project. Vaporized oil is recovered as natural gas liquid (NGL) when the injected gas is produced. In the reservoir application studied, 20% of the liquid hydrocarbons produced were being recovered as NGL.
Oil vaporization can be a benefit of pressure maintaining an oil reservoir by gas injection. Vaporized oil is recovered as natural gas liquids (NGL) when the injected gas is produced. The magnitude of this benefit is dependent on the "dryness" of the gas, the composition of the oil, reservoir temperature and pressure, and the reservoir residence time for the injected gas. This paper focuses on methods of quantifying oil vaporization.
The 26R Reservoir of the Elk Hills field at the Naval Petroleum Reserve No. 1, Kern County, California was evaluated using the methods presented. Although the oil vaporization may not be significant in some gas flooded reservoirs, in this case over 25% of the remaining reserves may be attributed to this mechanism. The metods discussed include field and laboratory measurements as well as analytical calculations based on a thermodynamic equation-of-state.
When two multi-component hydrocarbon phases are placed in contact, interphase mass transfer may occur. The degree of mass transfer is dependent on the concentration of molecular species in the phases. If a particular molecular species has a high concentration in one phase and low in the other, it will tend to move across the interphase boundary until an equilibrium condition is reached. The process is driven by the concentration gradient of the species in the two phases and is limited by the equilibrium condition.
For example, consider a dry hydrocarbon gas (gas phase) placed in contact with a reservoir oil (liquid phase). The dry gas will have a low concentration of propane while the oil would normally have a relatively high concentration. Driven by the concentration gradients in the two phases, propane molecules will move from the oil into the gas. This process continues until an equilibrium condition is reached. Likewise other molecular species such as ethane, butane, will move from the oil into the gas. Also since a dry gas has a high methane concentration, some of it may move from the gas into the oil as equilibrium is achieved. The partitioning of oil molecules into the gas is the oil vaporization process described in this paper.
The rate of oil vaporization is sensitive to the area-of-contact between the two phases. The quantity of oil vaporized is affected by the initial compositions of the gas and oil phases, temperature, pressure, and the relative volumes of the phases.
The oil vaporization process is not very important if the oil being vaporized is mobile and will be produced as the reservoir is depleted. Conversely, it becomes important when the oil being vaporized is non-mobile residual oil such as exists in a secondary gas cap. Here it benefits the operation by improving the recovery.
On a microscopic scale, the residual oil of a secondary gas cap consists of non-gas displacable oil globules distributed within the pore structure of the reservoir rock. Gas is a continuous phase front point to point throughout the gas cap. The area-of-contact between the phases is large per unit volume when compared to that of a laboratory PVT cell. This is an ideal condition for rapid interphase mass transfer.
As injected dry gas enters a pore it mixes with and displaces the original pore gas. An equilibrium between the gas and oil globules is established. As described above, the amount of oil vaporized depends on the initial compositions of the gas and oil. In general the smaller, lighter hydrocarbon molecules achieve the highest concentrations in the gas.
As the injected gas moves to a new cell along its path to a production well, it will contact new oil. However, little additional vaporization will take place once the gas has become saturated with vaporized oil. Again the driving force for vaporization is a concentration gradient which will only be significant if the oil in the new cell has a different composition from that in the former.