138
The Chemistry and Technology of Petroleum
interfaces exist between the hydrocarbon-enriched carbon dioxide mixture and the carbon dioxideenriched oil. Thus, by a dynamic (multiple-contact) process involving interphase mass transfer,
miscible displacement overcomes the capillary forces that otherwise trap oil in pores of the rock.
The reservoir operating pressure must be kept at a level high enough to develop and maintain
a mixture of carbon dioxide and extracted hydrocarbons that, at reservoir temperature, will be
miscible with the crude oil. Impurities in the carbon dioxide stream, such as nitrogen or methane,
increase the pressure required for miscibility. Mixing due to reservoir heterogeneity and diffusion
tends to locally alter and destroy the miscible composition, which must then be regenerated by additional extraction of hydrocarbons. In field applications both miscible and near-miscible displacements may actually proceed simultaneously in different parts of the reservoir.
The volume of carbon dioxide injected is specifically chosen for each application and usually
ranges from 20% to 40% of the reservoir pore volume. In the later stages of the injection program,
carbon dioxide may be driven through the reservoir by water or a lower cost inert gas. To achieve
higher sweep efficiency, water and carbon dioxide are often injected in alternate cycles.
In some applications, particularly in carbonate (limestone, dolomite, and chert) reservoirs where
it is likely to be used most frequently, carbon dioxide may prematurely break through to producing
wells. When this occurs, remedial action using mechanical controls in injection and production
wells may be taken to reduce carbon dioxide production. However, substantial carbon dioxide production is considered normal. Generally this produced carbon dioxide is reinjected, often after
processing to recover valuable light hydrocarbons.
For some reservoirs, miscibility between the carbon dioxide and the oil cannot be achieved and is
dependent upon the oil properties. However, carbon dioxide can still be used to recover additional oil.
The carbon dioxide swells crude oils, thus increasing the volume of pore space occupied by the oil and
reducing the quantity of oil trapped in the pores. It also reduces the oil viscosity. Both effects improve
the mobility of the oil. Carbon dioxide-immiscible flooding has been demonstrated in both pilot and
commercial projects, but overall it is expected to make a relatively small contribution to EOR.
The solution gas–oil ratio for carbonated crude oil should be measured in the normal way and
plotted as gas–oil ratio in volume per volume versus pressure. The greater the solubility of carbon
dioxide in the oil, the larger is the increase in the solution gas–oil ratio. In fact, the increase in
the gas–oil ratio usually parallels the increase in the oil formation volume factor due to swelling.
It should be noted that the gas in any gas–oil ratio experiment is not carbon dioxide but contains
hydrocarbons that have vaporized from the liquid phase. Consequently, whether the gas–oil ratio
is measured in a pressure–volume–temperature cell or from a slim tube experiment, compositional
analysis must be carried out to obtain the composition of the gas as well as that of the equilibrium
liquid phase. If actual measured values are not available, the correlation developed for crude oil
containing dissolved gases can be used but give only approximate values at best. Since the density
of pure gases is a function of pressure and temperature, for crude oil saturated with gases, the density in the mixing zone must be specified as a function of pressure and mixing zone composition.
Hydrocarbon gases and condensates have been used for over 100 commercial and pilot miscible
floods. Depending upon the composition of the injected stream and the reservoir crude oil, the
mechanism for achieving miscibility with reservoir oil can be similar to that obtained with carbon
dioxide (dynamic or multiple-contact miscibility), or the miscible solvent and in situ oil may be miscible initially (first-contact miscibility). Except in special circumstances, these light hydrocarbons
are generally too valuable to be used commercially.
Nitrogen and flue gases have also been used for commercial miscible floods. Minimum miscibility pressures for these gases are usually higher than for carbon dioxide, but in high-pressure,
high-temperature reservoirs where miscibility can be achieved these gases may be a cost-effective
alternative to carbon dioxide.
Thermal methods for oil recovery have found most use when the oil in the reservoir has a
high viscosity. For example, heavy oil is usually highly viscous (hence the use of the adjective
heavy), with a viscosity ranging from approximately 100 cP to several million centipoises at
The Chemistry and Technology of Petroleum
interfaces exist between the hydrocarbon-enriched carbon dioxide mixture and the carbon dioxideenriched oil. Thus, by a dynamic (multiple-contact) process involving interphase mass transfer,
miscible displacement overcomes the capillary forces that otherwise trap oil in pores of the rock.
The reservoir operating pressure must be kept at a level high enough to develop and maintain
a mixture of carbon dioxide and extracted hydrocarbons that, at reservoir temperature, will be
miscible with the crude oil. Impurities in the carbon dioxide stream, such as nitrogen or methane,
increase the pressure required for miscibility. Mixing due to reservoir heterogeneity and diffusion
tends to locally alter and destroy the miscible composition, which must then be regenerated by additional extraction of hydrocarbons. In field applications both miscible and near-miscible displacements may actually proceed simultaneously in different parts of the reservoir.
The volume of carbon dioxide injected is specifically chosen for each application and usually
ranges from 20% to 40% of the reservoir pore volume. In the later stages of the injection program,
carbon dioxide may be driven through the reservoir by water or a lower cost inert gas. To achieve
higher sweep efficiency, water and carbon dioxide are often injected in alternate cycles.
In some applications, particularly in carbonate (limestone, dolomite, and chert) reservoirs where
it is likely to be used most frequently, carbon dioxide may prematurely break through to producing
wells. When this occurs, remedial action using mechanical controls in injection and production
wells may be taken to reduce carbon dioxide production. However, substantial carbon dioxide production is considered normal. Generally this produced carbon dioxide is reinjected, often after
processing to recover valuable light hydrocarbons.
For some reservoirs, miscibility between the carbon dioxide and the oil cannot be achieved and is
dependent upon the oil properties. However, carbon dioxide can still be used to recover additional oil.
The carbon dioxide swells crude oils, thus increasing the volume of pore space occupied by the oil and
reducing the quantity of oil trapped in the pores. It also reduces the oil viscosity. Both effects improve
the mobility of the oil. Carbon dioxide-immiscible flooding has been demonstrated in both pilot and
commercial projects, but overall it is expected to make a relatively small contribution to EOR.
The solution gas–oil ratio for carbonated crude oil should be measured in the normal way and
plotted as gas–oil ratio in volume per volume versus pressure. The greater the solubility of carbon
dioxide in the oil, the larger is the increase in the solution gas–oil ratio. In fact, the increase in
the gas–oil ratio usually parallels the increase in the oil formation volume factor due to swelling.
It should be noted that the gas in any gas–oil ratio experiment is not carbon dioxide but contains
hydrocarbons that have vaporized from the liquid phase. Consequently, whether the gas–oil ratio
is measured in a pressure–volume–temperature cell or from a slim tube experiment, compositional
analysis must be carried out to obtain the composition of the gas as well as that of the equilibrium
liquid phase. If actual measured values are not available, the correlation developed for crude oil
containing dissolved gases can be used but give only approximate values at best. Since the density
of pure gases is a function of pressure and temperature, for crude oil saturated with gases, the density in the mixing zone must be specified as a function of pressure and mixing zone composition.
Hydrocarbon gases and condensates have been used for over 100 commercial and pilot miscible
floods. Depending upon the composition of the injected stream and the reservoir crude oil, the
mechanism for achieving miscibility with reservoir oil can be similar to that obtained with carbon
dioxide (dynamic or multiple-contact miscibility), or the miscible solvent and in situ oil may be miscible initially (first-contact miscibility). Except in special circumstances, these light hydrocarbons
are generally too valuable to be used commercially.
Nitrogen and flue gases have also been used for commercial miscible floods. Minimum miscibility pressures for these gases are usually higher than for carbon dioxide, but in high-pressure,
high-temperature reservoirs where miscibility can be achieved these gases may be a cost-effective
alternative to carbon dioxide.
Thermal methods for oil recovery have found most use when the oil in the reservoir has a
high viscosity. For example, heavy oil is usually highly viscous (hence the use of the adjective
heavy), with a viscosity ranging from approximately 100 cP to several million centipoises at