Changes

In the past, chemical, thermal, and miscible techniques have been used by the industry on a commercial scale. EOR techniques require the injection of chemical compounds dissolved in water, the injection of steam, or the injection of a gas that is miscible with the oil in place. As a result, all current EOR techniques are much more expensive to implement than normal secondary water injection projects. Therefore, the amount of oil that can ultimately be recovered by existing EOR techniques is directly related to the price of [[crude oil]].

In the past, chemical, thermal, and miscible techniques have been used by the industry on a commercial scale. EOR techniques require the injection of chemical compounds dissolved in water, the injection of steam, or the injection of a gas that is miscible with the oil in place. As a result, all current EOR techniques are much more expensive to implement than normal secondary water injection projects. Therefore, the amount of oil that can ultimately be recovered by existing EOR techniques is directly related to the price of [[crude oil]].

All EOR projects begin with an analysis of the nature, location, and causes of residual oil saturations (''S''_or) that remain after primary and/or secondary recovery operations. The main factors that control the value of ''S''_or are pore geometry, rock [[wettability]], and the properties of the displaced (oil) and displacing (injected) fluids. Fluid properties of particular interest are interfacial tension, viscosity, and density. In combination with the heterogeneity of the reservoir, these properties result in the overall recovery (''E''_R) for any recovery scheme.

All EOR projects begin with an analysis of the nature, location, and causes of residual oil saturations (''S''_or) that remain after primary and/or secondary recovery operations. The main factors that control the value of ''S''_or are pore geometry, rock [[wettability]], and the properties of the displaced (oil) and displacing (injected) fluids. Fluid properties of particular interest are interfacial tension, [[viscosity]], and density. In combination with the heterogeneity of the reservoir, these properties result in the overall recovery (''E''_R) for any recovery scheme.

The overall recovery is the product of displacement efficiency (''E''_D), and sweep efficiency (''E''_S). The displacement efficiency is inversely proportional to the residual oil saturation, while the sweep efficiency is inversely proportional to the mobility ratio (''M'') between the injected fluids and the oil in place (see “[[Waterflooding]]”). ''M'' is usually stated in terms of the [[Relative permeability]] of a fluid phase (''k''_r) divided by the phase's viscosity (''μ'') relative to the same ratio for the other phase, such as for a waterflood:

The overall recovery is the product of displacement efficiency (''E''_D), and sweep efficiency (''E''_S). The displacement efficiency is inversely proportional to the residual oil saturation, while the sweep efficiency is inversely proportional to the mobility ratio (''M'') between the injected fluids and the oil in place (see “[[Waterflooding]]”). ''M'' is usually stated in terms of the [[Relative permeability]] of a fluid phase (''k''_r) divided by the phase's viscosity (''μ'') relative to the same ratio for the other phase, such as for a waterflood:

[[file:enhanced-oil-recovery_fig1.png|300px|thumb|{{figure number|1}}Schematic diagram of chemical flooding (alkaline). © U.S. Department of Energy, Bartlesville, Oklahoma.]]

[[file:enhanced-oil-recovery_fig1.png|300px|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., and R. S. Schechter, 1971, Improved oil recovery by surfactant and polymer flooding: New York, Academic Press.</ref>

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., and R. S. Schechter, 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.

[[file:enhanced-oil-recovery_fig4.png|300px|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_fig4.png|300px|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.]]

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]]).

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]]).

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.

[[Category:Reservoir engineering methods]]

[[Category:Reservoir engineering methods]]