Measuring Minimum Miscibility Pressure: Slim-Tube or Rising-Bubble Method?
- A.M. Elsharkawy (Suez Canal U.) | F.H. Poettmann (Colorado School of Mines) | R.L. Christiansen (Colorado School of Mines)
- Document ID
- Society of Petroleum Engineers
- SPE/DOE Enhanced Oil Recovery Symposium, 22-24 April, Tulsa, Oklahoma
- Publication Date
- Document Type
- Conference Paper
- 1992. Society of Petroleum Engineers
- 5.4.6 Thermal Methods, 5.4 Enhanced Recovery, 4.6 Natural Gas, 5.3.2 Multiphase Flow, 4.3.3 Aspaltenes, 2.4.3 Sand/Solids Control, 5.2.2 Fluid Modeling, Equations of State, 5.2.1 Phase Behavior and PVT Measurements, 5.3.4 Reduction of Residual Oil Saturation, 1.8 Formation Damage, 4.1.4 Gas Processing, 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 5.4.2 Gas Injection Methods
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Measurements of carbon dioxide minimum miscibility pressures (MMP) using a slim tube apparatus were compared with those using a rising-bubble apparatus (RBA). MMPs were measured for twelve different oils, with gravities varying from 34 API to 51 degrees API. The results were found to compare very well when using a specific criterion for the slim-tube MMF. Although the slim tube method is often referred to as the industry standard, there is no standard design, no standard operating procedure, and no standard criterion for determining MMFs with the slim tube. It is shown that the RBA is faster and more reliable than the slim tube for determining MMF. Bubble behavior is described for both the vaporizing and condensing gas processes.
Gas injection above the minimum miscibility pressure is a widely practiced means for improving oil recovery in many reservoirs. The minimum miscibility pressure (MMP) is the lowest pressure for which a gas can develop miscibility through a multicontact process with a given reservoir oil at reservoir temperature. The reservoir to which the process is applied must be operated at or above the MMP to develop multicontact miscibility. Reservoir pressures below the MMP result in immiscible displacements and consequently lower oil recoveries.
At or above the MMP, miscibility can develop through a vaporizing process, a condensing process, or sometimes a combination of the two processes. In the vaporizing process, intermediate molecular weight hydrocarbons from the crude oil are transferred to the leading edge of the gas front enabling it to become miscible with the reservoir crude.
In the condensing process, the injected gas is enriched with light hydrocarbons, usually LPGs. The reservoir oil left behind the gas front is enriched by net transfer of the light hydrocarbons from the gas phase into the oil. Enrichment of the reservoir oil proceeds until it becomes miscible with the injected rich gas.
Miscibility can also develop through a combination of the vaporizing and the condensing processes. As in the condensing process, the light intermediate components in the injected gas condense into the crude oil, while the middle intermediates, C4+, vaporize into the gas phase. With this combination of condensation and vaporization, miscibility may never completely develop. Yet the process can result in low residual oil saturations.
Primarily two laboratory methods are used to measure MMPs: the slim-tube method, and the rising-bubble method. While there are many more slim-tube apparatus in the industry, a large portion of MMPs reported in the literature in recent years were measured with just two or three rising-bubble apparatus. Other methods, including a tedious multiple-contact method in phase behavior apparatus, have also been tested for measuring MMPs.
There are many correlations for CO2 MMPs in the literature. An adaptation for CO2 MMPs of the Benham-Dowden-Kunzman correlation for enriched natural gas MMPs was proposed by Holm and Josendal. This adaptation provides for temperature and C5+ molecular weight dependence of MMF. Orr and Silva proposed a correlation for CO2 MMPs which requires a more complete compositional description of the crude. Following the method used by Benham et al., Riedel developed a CO2 MMP correlation which includes compositional factors.
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