An EoS Model for a Middle East Reservoir Fluid with an Extensive EOR PVT Data Material
- Shahin Negahban (ADCO) | Karen Schou Pedersen (Calsep A/S) | Pashupati Sah (Calsep A/S) | Mahmoud Ali Basioni (Abu Dhabi Co. Onshore Oil Opn.) | Jawad Azeem (Calsep Dubai)
- Document ID
- Society of Petroleum Engineers
- Abu Dhabi International Petroleum Exhibition and Conference, 1-4 November, Abu Dhabi, UAE
- Publication Date
- Document Type
- Conference Paper
- 2010. Society of Petroleum Engineers
- 5.4 Enhanced Recovery, 5.5 Reservoir Simulation, 4.1.2 Separation and Treating, 5.2 Reservoir Fluid Dynamics, 5.2.1 Phase Behavior and PVT Measurements, 4.6 Natural Gas, 5.2.2 Fluid Modeling, Equations of State, 5.4.2 Gas Injection Methods, 4.1.5 Processing Equipment
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The paper presents compositional data and PVT data for a Middle East reservoir fluid with a reservoir temperature of 394 K and reservoir pressure of 287 bar. The PVT data was selected and designed to provide the best possible starting point for developing an EOS model that would accurately reproduce the phase behavior of a reservoir fluid subject to injection of either CO2 or a hydrocarbon gas.
To eliminate the uncertainty from use of default molecular weights and densities for the C7+ hydrocarbon fractions the reservoir fluid composition was analyzed using a True Boiling Point (TBP) analysis. PVT experiments, both routine and gas injection (EOR) experiments, were carried out including solubility swelling, equilibrium and multi contact experiments and slim tube tests. With both injection gases the reservoir fluid shows a combined vaporizing/condensing drive mechanism.
A 9-component EOS model was developed for the volume corrected Peng-Robinson equation of state, which shows a good match of all available data. Two methods were used to predict the vaporizing/condensing MMP; (a) a multi-component tie-line MMP algorithm and (b) a compositional 1D simulator. The CO2 MMP is considerably lower than the reservoir pressure while the MMP seen with the hydrocarbon gas is close to the reservoir pressure.
The recovery from an oil reservoir can be enhanced if the reservoir pressure is high enough to have a miscible drive form with either CO2 or a hydrocarbon gas injected. A miscible drive is attractive because the injected gas and the reservoir oil, possibly after a number of contacts, form a single phase (miscible) zone, which will spread in the reservoir carrying all the heavy constituents of the reservoir fluid. The lowest pressure at which a miscible drive can form is called the minimum miscibility pressure (MMP). Had the reservoir pressure been lower than the MMP, gas would spread in the reservoir as a separate zone and because the mobility of gas exceeds that of oil, a gas breakthrough might eventually take place at the production wells leaving the more valuable liquid components behind in the reservoir.
A compositional reservoir simulation will only give a correct picture of the influence of gas injection if the applied equation of state (EOS) model has been configured to match the changes in phase compositions after multiple contacts between gas and oil. Development and verification of the EOS model require extensive experimental gas injection (EOR) PVT data.
PVT data needed for EOR EOS modeling
The chemical composition and physical properties of reservoir fluids may change substantially when gas injection is applied. If a black oil type of reservoir fluid takes up gas components, it will gradually change to become a volatile oil. With more gas dissolved the fluid will become near-critical and it will eventually turn into a gas condensate type of fluid. That happens when the saturation point at reservoir temperature shifts from a bubble point to a dew point, which shift takes place at the critical point. Unless the reservoir fluid is highly undersaturated, not all injected gas will dissolve in the oil, but a free gas phase will form. Components from the injection gas will condense into the oil while the oil will release components to the gas phase through what is called a combined vaporizing/condensing mechanism.
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