Pressure and Volume Evolution During Gas Phase Formation in Solution Gas Drive Process (includes associated papers 38340 and 38565 )
- Abbas Firoozabadi (Reservoir Engineering Research Institute) | Dimo Kashchiev (Reservoir Engineering Research Institute)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- September 1996
- Document Type
- Journal Paper
- 219 - 228
- 1996. Society of Petroleum Engineers
- 5.3.2 Multiphase Flow, 1.6.9 Coring, Fishing, 4.1.5 Processing Equipment, 4.3.4 Scale, 4.1.2 Separation and Treating, 5.2.2 Fluid Modeling, Equations of State, 5.2.1 Phase Behavior and PVT Measurements, 4.6 Natural Gas
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The pressure and volume evolution during gas phase formation in porous media is modelled by an instantaneous nucleation process. This model reveals that supersaturation (i.e., non-equilibrium state) for gas bubble formation is affected by the pressure decline rate. As a result, laboratory recovery measurements are influenced by rate effects, and material balance expressions may not be used to calculate the amount of oil and gas phases.
The evolution of gas phase in porous media for solution gas-drive process is perhaps the least understood among various recovery processes. The formation of a new gas phase when the pressure of an undersaturated oil in porous media is lowered below the bubblepoint pressure is a non-equilibrium phenomenon. In such a process, the oil normally goes from an undersaturated to a supersaturated state prior to the formation of gas bubbles. Due to supersaturation, the pressure volume relationship is often influenced by rate effects. As an example, consider an undersaturated oil with a bubblepoint pressure of say 2500 psi at an initial pressure of 3000 psi. The amount of oil recovery from a core at isothermal conditions will be a function of the rate of pressure decline. The oil recovery at a final depletion pressure of say 50 psi could be 15 percent for a low pressure decline rate and 30 percent at a higher pressure decline rate. Rate dependent recovery by a factor of two to a final pressure has been observed in the laboratory.1 The rate effect due to non-equilibrium precludes the use of material balance expression to calculate the amount of oil and gas phases.
Ref. 2 provides a review of the two main issues of the solution gas-drive process: supersaturation and critical gas saturation. Recent works not reviewed in Ref. 2 will be discussed briefly next.
In 1991, Kortekaas and van Poelgest reported critical gas saturation measurements in the presence of connate water for 6" long cores.3 Based on several experiments, these authors concluded that critical gas saturation is decreased by lowering the pressure decline rate. Yortsos and Parlar4 modelled the appearance of the gas phase in the solution-gas drive process. Nucleation, supersaturation and bubble growth in porous media were included in their model. They analyzed the onset of nucleation under both homogeneous and heterogeneous nucleation conditions. These authors concluded that the supersaturation needed for the appearance of gas bubbles should be largely independent of the rate of pressure decline. This conclusion is not in accord with the experimental results of Refs. 2 and 3. In a more recent study, Li and Yortsos5based on a network model concluded that the critical gas saturation decreases with an increase in the pressure decline rate. This conclusion is reached on the assumption that the nucleation occurs from specific active sites, each of which is activated at a specified local supersaturation. This conclusion is also different from the experimental observations.2,3,6
Recently, we have investigated the kinetics of the initial stage of isothermal gas phase formation assuming that the equilibrium pressure, pe, at isothermal condition is constant and is independent of composition.7The work of Ref. 7 also assumes a predetermined supersaturation. As a result of these assumptions, the theoretical model of Ref. 7 can not be used to describe pressure volume evolution data presented in the literature on solution gas-drive process. In this work, the variation of equilibrium pressure and supersaturation are taken into account to provide a theoretical description of pressure-volume evolution in porous media for binary and multicomponent systems under non-equilibrium conditions.
|File Size||1 MB||Number of Pages||18|