Gas Process Displacement Efficiency Comparisons on a Carbonate Reservoir
- Patrick Egermann (Gaz de France) | Michel Robin (Institut Francais du Petrole IFP) | Jean-Marc N. Lombard (Institut Francais du Petrole IFP) | Cyrus A. Modavi (Abu Dhabi Co. Onshore Oil Opn.) | Mohammed Z. Kalam (Abu Dhabi Co. Onshore Oil Opn.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- December 2006
- Document Type
- Journal Paper
- 621 - 629
- 2006. Society of Petroleum Engineers
- 5.3.2 Multiphase Flow, 5.4.6 Thermal Methods, 4.1.2 Separation and Treating, 5.2.1 Phase Behavior and PVT Measurements, 5.5.8 History Matching, 5.5 Reservoir Simulation, 5.4.7 Chemical Flooding Methods (e.g., Polymer, Solvent, Nitrogen, Immiscible CO2, Surfactant, Vapex), 5.2.2 Fluid Modeling, Equations of State, 4.3.3 Aspaltenes, 4.1.4 Gas Processing, 1.6.9 Coring, Fishing, 5.3.4 Reduction of Residual Oil Saturation, 5.4.2 Gas Injection Methods, 4.6 Natural Gas, 5.4.1 Waterflooding, 5.5.2 Core Analysis, 6.5.2 Water use, produced water discharge and disposal, 5.6.2 Core Analysis, 4.1.5 Processing Equipment, 5.8.7 Carbonate Reservoir, 1.8 Formation Damage, 5.7.2 Recovery Factors, 5.4 Enhanced Recovery
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Secondary- and tertiary-recovery processes based on gas injection can extend the life of waterflooded reservoirs by maximizing the oil recovery. However, the injection strategy needs to be studied carefully to optimize the overall sweep efficiency. In particular, the impact of possible water blocking on the recovery has to be addressed. For that purpose, a series of experiments was performed under reservoir conditions on a carbonate rock type to compare the displacement efficiencies of a secondary gas injection, a tertiary gas injection, and a simultaneous water-alternating-gas (SWAG) injection.
The experiments were carried out on composite cores consisting of several carefully selected reservoir core plugs of the chosen rock type. The operating pressure was lower than the minimum miscible pressure (MMP) and reflected the current reservoir pressure. Phase exchanges were monitored continually during the hydrocarbon recovery, including the chromatographic analysis of the produced gas.
The final oil recovery resulting from the three types of experiments was very good [approximately 90% original oil in place (OOIP) at surface conditions after 6 pore-volume (PV) injection] and quite similar within the expected experimental error, regardless of the sequence of gas injection. The low remaining oil saturation (ROS) values observed were consistent with competing processes of both viscous displacement of oil by gas and phase exchanges occurring between oil and gas. Because of the nature of the injected gas (rich gas from the first separation stage), a condensing/vaporizing process had to be considered. The SWAG injection speeds up the oil recovery by mobility control of the water phase. This enhances the sweep efficiency by viscous drive. A water-blocking effect was found to be negligible because it could be anticipated due to wettability consideration.
The influence of the fluid description (equation of state, or EOS) and the three-phase relative permeability model on the simulation results was studied. An excellent agreement between simulation and production data was obtained with both gas/oil relative permeability data measured at ambient conditions on a restored composite core and an appropriate EOS (with seven pseudos). The condensing/vaporizing process that strips the intermediate compounds from the oil phase to the gas phase was properly taken into account with this appropriate EOS. The influence of the three-phase permeability model (either "geometrical construction?? or Stone1) on the results was found to be small.
For enhanced oil recovery (EOR) purposes, miscible or immiscible hydrocarbon gas injections have been applied successfully in many oil reservoirs throughout the world (Thomas et al. 1994; Lee et al. 1988). Compared to water injection, gas injection is associated with higher microscopic displacement efficiency due to the low value of the interfacial tension (IFT) between the oil and gas phases. IFT tends toward zero when miscibility is reached, which means that the oil recovery can be total in the swept area. Even when miscibility is not reached, the mass-transfer mechanisms that occur between oil and gas phases lead to low IFT values when compared to waterflooding. Even under those conditions, regarding remaining oil-saturation values, gas injection appears to be an interesting recovery process.
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