A Combined Condensing/Vaporizing Mechanism in the Displacement of Oil by Enriched Gases
- A.A. Zick (ARCO Oil and Gas Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Annual Technical Conference and Exhibition, 5-8 October, New Orleans, Louisiana
- Publication Date
- Document Type
- Conference Paper
- 1986. Society of Petroleum Engineers
- 5.2.2 Fluid Modeling, Equations of State, 5.3.2 Multiphase Flow, 5.4 Enhanced Recovery, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 4.6 Natural Gas, 5.2 Reservoir Fluid Dynamics, 5.4.2 Gas Injection Methods, 4.3.4 Scale, 1.6.9 Coring, Fishing, 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 5.2.1 Phase Behavior and PVT Measurements
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Experimental observations, combined with equation-of-state predictions, indicate that a combined condensing/vaporizing-gas drive mechanism, rather than the traditionally believed condensing-gas drive mechanism, may be responsible for displacements of reservoir oil by enriched gases. Apparent minimum miscibility pressures and minimum solvent enrichments are observed with this mechanism, even though true miscibility is probably not developed.
Pseudoternary diagrams were first used to explain Pseudoternary diagrams were first used to explain the mechanisms of oil displacement by vaporizing-gas drives and condensing-gas drives more than 30 years ago. Since then, all displacements of oil by injected gases have been categorized as either immiscible, multicontact miscible, or first contact miscible, with the multicontact miscible displacements further divided into either condensing-gas drive processes or vaporizing-gas drive processes. Soon after those displacement mechanisms were proposed, Benham et al. proposed a method of proposed, Benham et al. proposed a method of predicting minimum miscibility conditions by predicting minimum miscibility conditions by constructing pseudoternary diagrams and estimating the placements of the critical tielines. That method placements of the critical tielines. That method has been in use ever since, although it has been updated slightly in recent years by the use of equations-of-state to generate the pseudoternary diagrams.
The problem with using pseudoternary diagrams to explain or predict multicontact miscibility is that they rigorously apply only to true ternary systems. It has generally been assumed in the past that pseudoternary diagrams, while perhaps not exact, at pseudoternary diagrams, while perhaps not exact, at least capture the basic phase behavior mechanisms of reservoir fluid displacements. That assumption may not always be justified.
For a three-component hydrocarbon system, when a rich injection gas displaces an oil that is relatively lean in the intermediate component, it is by a mechanism that is called a condensing-gas drive. In this process, which has been well described elsewhere, the oil near the injection point is enriched by repeated contacts with the injection gas. The intermediate component in the gas condenses into the oil, moving its composition toward the critical point on the phase envelope. Eventually, if the gas is rich enough, i.e., if its composition lies on the single phase side of the extended critical tieline, the oil becomes so enriched with the intermediate component that it becomes miscible with the gas. Since the miscible zone moves with the velocity of the injection gas, the oil is completely displaced. This process can be easily visualized with the aid of a ternary diagram. The ternary diagram in Figure 1 was generated by simulating a methane-butane-decane multicontact process at minimum miscibility conditions with the Peng-Robinson equation-of-state. Peng-Robinson equation-of-state. Phase envelopes for multicomponent reservoir fluids Phase envelopes for multicomponent reservoir fluids can be generated by simply mixing the fluid with light and intermediate components in various proportions, flashing the resulting mixtures, and proportions, flashing the resulting mixtures, and measuring the equilibrium compositions. Since these phase envelopes, when projected onto pseudoternary diagrams, look very similar to the phase pseudoternary diagrams, look very similar to the phase envelopes for ternary hydrocarbon systems, it has been assumed that the displacement of these multicomponent oils by enriched gases will be by the same condensing-gas drive mechanism that applies to the ternary system.
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