Changes

==Chemical flooding==

==Chemical flooding==

The basic purposes of chemical flooding are to add a material (chemical) to the water being injected into a reservoir to increase the oil recovery by (1) increasing the water viscosity (polymer floods), (2) decreasing the relative permeability to water (cross-linked polymer floods), or (3) increasing the relative permeability to oil and decreasing ''S''_or by decreasing the interfacial tension between the oil and water phases (micellar and alkaline floods). The process is depicted schematically in Figure 1. Chemical additives to reduce interfacial tension are detergent type compounds such as petroleum sulfinates and are so expensive that chemical floods are often technical successes and economic failures. Successful design of chemical floods always revolves around minimizing the amount of chemicals needed to achieve the desired change in interfacial tension and/or mobility ratio.<ref name=pt10r28>Shah, D. O., Schechter, R. S., 1971, Improved oil recovery by surfactant and polymer flooding: New York, Academic Press.</ref>

[[file:enhanced-oil-recovery_fig1.png|left|thumb|{{figure number|1}}Schematic diagram of chemical flooding (alkaline). © U.S. Department of Energy, Bartlesville, Oklahoma.]]

[[file:enhanced-oil-recovery_fig1.png|thumb|{{figure number|1}}Schematic diagram of chemical flooding (alkaline). © U.S. Department of Energy, Bartlesville, Oklahoma.]]

The basic purposes of chemical flooding are to add a material (chemical) to the water being injected into a reservoir to increase the oil recovery by (1) increasing the water viscosity (polymer floods), (2) decreasing the relative permeability to water (cross-linked polymer floods), or (3) increasing the relative permeability to oil and decreasing ''S''_or by decreasing the interfacial tension between the oil and water phases (micellar and alkaline floods). The process is depicted schematically in [[:file:enhanced-oil-recovery_fig1.png|Figure 1]]. Chemical additives to reduce interfacial tension are detergent type compounds such as petroleum sulfinates and are so expensive that chemical floods are often technical successes and economic failures. Successful design of chemical floods always revolves around minimizing the amount of chemicals needed to achieve the desired change in interfacial tension and/or mobility ratio.<ref name=pt10r28>Shah, D. O., Schechter, R. S., 1971, Improved oil recovery by surfactant and polymer flooding: New York, Academic Press.</ref>

This minimization is achieved by preceding the chemical injection with a preflush to buffer the chemicals from reactions with the ''in situ'' water and following the chemical injection with the injection of a polymer solution to maintain a favorable mobility ratio for the flood. One of the major problems with the injection of surfactants and other chemicals into reservoir rock is that the chemicals are surface active. Thus, they have a great affinity for the minerals found in reservoirs, causing adsorption of chemicals from solution onto the rock in great quantities.

This minimization is achieved by preceding the chemical injection with a preflush to buffer the chemicals from reactions with the ''in situ'' water and following the chemical injection with the injection of a polymer solution to maintain a favorable mobility ratio for the flood. One of the major problems with the injection of surfactants and other chemicals into reservoir rock is that the chemicals are surface active. Thus, they have a great affinity for the minerals found in reservoirs, causing adsorption of chemicals from solution onto the rock in great quantities.

==Miscible gas flooding==

==Miscible gas flooding==

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 (CO₂), 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 (Figure 2).

Current applications of miscible flooding have concentrated on carbon dioxide (CO₂), 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]]).

[[file:enhanced-oil-recovery_fig2.png|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.]]

[[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.]]

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.

==Thermal recovery==

==Thermal recovery==

All thermal recovery processes involve the use of heat to accelerate the oil recovery process. The heat can be generated at the surface and injected into the reservoir, as in the case of steam injection (Figure 3), or generated in the reservoir by injecting a fluid such as air that is combustible with the in-place oil (Figure 4). The choice of which technique to use to add thermal energy to the reservoir depends on an analysis of the oil reservoir and the economics of generating the energy. However, a major goal of all thermal methods is to reduce the viscosity of the in-place oil. For most thermal processes, this is accomplished by heating a very heavy oil (API gravity).

[[file:enhanced-oil-recovery_fig4.png|thumb|{{figure number|4}}Schematic diagram of ''in situ'' combustion. The mobility of oil is increased by reduced viscosity caused by heat and solution of combustion gases. © U.S. Department of Energy, Bartlesville, Oklahoma.]]

[[file:enhanced-oil-recovery_fig3.png|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.]]

All thermal recovery processes involve the use of heat to accelerate the oil recovery process. The heat can be generated at the surface and injected into the reservoir, as in the case of steam injection ([[:file:enhanced-oil-recovery_fig3.png|Figure 3]]), or generated in the reservoir by injecting a fluid such as air that is combustible with the in-place oil ([[:file:enhanced-oil-recovery_fig4.png|Figure 4]]). The choice of which technique to use to add thermal energy to the reservoir depends on an analysis of the oil reservoir and the economics of generating the energy. However, a major goal of all thermal methods is to reduce the viscosity of the in-place oil. For most thermal processes, this is accomplished by heating a very heavy oil (API gravity).

 

[[file:enhanced-oil-recovery_fig4.png|thumb|{{figure number|4}}Schematic diagram of ''in situ'' combustion. The mobility of oil is increased by reduced viscosity caused by heat and solution of combustion gases. © U.S. Department of Energy, Bartlesville, Oklahoma.]]

In a thermal flood, much effort is devoted to treating the boiler water and the stack gases resulting from the burning of produced oil or gas to generate heat. Because of environmental considerations, this usually limits the technique to unpopulated areas.

In a thermal flood, much effort is devoted to treating the boiler water and the stack gases resulting from the burning of produced oil or gas to generate heat. Because of environmental considerations, this usually limits the technique to unpopulated areas.