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Line 58: + + − − [[file:enhanced-oil-recovery_fig3.png|left|thumb|{{figure number|3}}Schematic diagram of steam flooding. In this method, heat reduces the viscosity of oil and increases its mobility. © U.S. Department of Energy, Bartlesville, Oklahoma.]]
→Miscible gas flooding
==Miscible gas flooding==
==Miscible gas flooding==
[[file:enhanced-oil-recovery_fig2.png|300px|thumb|{{figure number|2}}Schematic diagram of carbon dioxide flooding. The viscosity of oil is reduced, providing more efficient miscible displacement. © U.S. Department of Energy, Bartlesville, Oklahoma.]]
The concept behind miscible flooding, such as carbon dioxide floods, is that the best way to eliminate the interfacial tension between the in-place oil and injected fluids is to inject a solvent for that oil (a ''solvent'' being a material that is miscible in all proportions with the dissolved material). The residual oil saturation to displacement by a solvent would be 0.0. In practice, a solvent must be found that is miscible with the oil and costs no more than the oil. Finally, the volumetric sweep efficiency of the process must be high enough to make this scheme economical. Because the relative permeability effects have been removed by the miscible nature of the displacment, the sweep efficiency is primarily a function of the viscosity of the solvent relative to the viscosity of the oil.<ref name=pt10r31>Stalkup, F. I., Jr., 1983, Miscible displacement: Richardson, TX, Society of Petroleum Engineers Monograph Series.</ref>
The concept behind miscible flooding, such as carbon dioxide floods, is that the best way to eliminate the interfacial tension between the in-place oil and injected fluids is to inject a solvent for that oil (a ''solvent'' being a material that is miscible in all proportions with the dissolved material). The residual oil saturation to displacement by a solvent would be 0.0. In practice, a solvent must be found that is miscible with the oil and costs no more than the oil. Finally, the volumetric sweep efficiency of the process must be high enough to make this scheme economical. Because the relative permeability effects have been removed by the miscible nature of the displacment, the sweep efficiency is primarily a function of the viscosity of the solvent relative to the viscosity of the oil.<ref name=pt10r31>Stalkup, F. I., Jr., 1983, Miscible displacement: Richardson, TX, Society of Petroleum Engineers Monograph Series.</ref>
Current applications of miscible flooding have concentrated on carbon dioxide (CO<sub>2</sub>), hydrocarbon gas, and nitrogen injection processes. The gas solvents tend to be much less viscous than reservoir oils so that the sweep efficiencies are often very low for miscible gas floods. Design efforts center around finding methods to improve this volumetric sweep efficiency. In the case of carbon dioxide floods, the gas is injected into the reservoir in small slugs that are alternated with water slugs as a means of lowering the mobility of the injected fluid ([[:file:enhanced-oil-recovery_fig2.png|Figure 2]]).
Current applications of miscible flooding have concentrated on carbon dioxide (CO<sub>2</sub>), hydrocarbon gas, and nitrogen injection processes. The gas solvents tend to be much less viscous than reservoir oils so that the sweep efficiencies are often very low for miscible gas floods. Design efforts center around finding methods to improve this volumetric sweep efficiency. In the case of carbon dioxide floods, the gas is injected into the reservoir in small slugs that are alternated with water slugs as a means of lowering the mobility of the injected fluid ([[:file:enhanced-oil-recovery_fig2.png|Figure 2]]).
Secondary benefits of miscible gas injection include the effects of the solubility of the solvent in the oil phase. As the carbon dioxide, hydrocarbon gas, or nitrogen dissolve in the oil phase, the oil is swelled and its viscosity lowered. Both of these phenomena add to the mobility of the oil relative to the injected solvent. In practice, although miscible displacement implies no residual oil saturation in the area swept by the solvent, a small residual saturation is left due to economic considerations of producing at high GORs and the phase behavior of the system prior to the attainment of miscibility in the reservoir.
Secondary benefits of miscible gas injection include the effects of the solubility of the solvent in the oil phase. As the carbon dioxide, hydrocarbon gas, or nitrogen dissolve in the oil phase, the oil is swelled and its viscosity lowered. Both of these phenomena add to the mobility of the oil relative to the injected solvent. In practice, although miscible displacement implies no residual oil saturation in the area swept by the solvent, a small residual saturation is left due to economic considerations of producing at high GORs and the phase behavior of the system prior to the attainment of miscibility in the reservoir.