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Miscible CO2 flooding
This is an enhanced oil recovery (EOR) method which uses compressed CO2 as a solvent injected into depleting hydrocarbon reservoirs to improve its oil recovery. It causes an improvement in the oil recovery by attaining miscibility with the oil and by dissolving in it to reduce its viscosity. The miscibility of CO2 with oil eliminates the capillary trapping of residual oil due to the absence of interfacial tension between the displacing fluid (CO2) and the displaced fluid (oil). The CO2 is compressed above a threshold pressure, called minimum miscibility pressure (which depends on the oil composition and the reservoir temperature) point before injection. This technique is mostly applied after primary recovery mechanisms and waterflooding has been exhausted. About 5-20% of the original oil in place (OOIP) can be extracted through primary recovery methods and up to 45% of OOIP recovered at the end of primary and secondary recovery methods. CO2 EOR has the potential to recover an additional 15% to 20% of the original oil.
Benefits of CO2
CO2 is much less dense than reservoir fluids. As opposed to waterflooding, CO2 can dissolve and blend with the produced hydrocarbon due to its miscibility. Below its minimum miscibility pressure (MMP), CO2 acts as immiscible solvent and gives a good displacement efficiency by reducing interfacial tension, swelling the reservoir oil, and reducing viscosity. However, above that pressure, it acts as a miscible solvent, bonding with the hydrocarbon. The immiscible process best applies to heavy oils and the miscible process to light oils. The critical pressure and temperature of CO2 above which it has a density similar to that of a liquid and a lower density are 1,070.6 psia and 31.1 °C. For economical and operational reasons, the CO2 must be transported at this pressure and temperature and above.
Sources of the CO2 include, industrial or anthropogenic sources, hydrocarbon reservoirs containing CO2 as an impurity and natural CO2 reserves. Using CO2 from industrial chimneys, it is helping to make the planet cleaner as it is responsible for a major share of greenhouse effects on the planet. Captured CO2 from industries like petrochemical plants is “cleaned” to remove any water and other components like H2S that may be in the stream. The CO2 is then compressed and transported through pipelines to the injection fields.
Not all reservoirs are suitable for CO2 EOR, eligibility depends mainly on reservoir temperature and pressure, oil viscosity and gravity, and reservoir depth. Temperature, pressure, and depth are essential to achieve miscibility at the reservoir conditions. Also, the geological complexity of the reservoir must support the CO2 to contact the residual crude oil, and the minimum miscibility pressure (MMP) must be reachable, hence extensive laboratory studies and reservoir modelling must be carried out prior to CO2 EOR.. The table below is a simplified view of the screening criteria proposed by the National Energy lab of the US department of Energy. Various other studies propose some more detailed screening criteria.
There must also be a good amount of residual crude oil to justify the economic viability of the entire project.
The CO2 injection technique and the miscible flood design aims to optimize the oil recovery from the reservoir. Some of the recovery methods are described below. The reservoir fluid properties and geology influence the choice of the optimum recovery method
i. Continuous CO2 injection: This method is most suited for strongly water wet reservoirs with medium to light oils. Only CO2 is injected with no other fluids. sometimes, a lighter gas is used to chase the injected CO2 slug.
ii Continuous CO2 injection followed by water: For reservoirs with low permeabilities, this method is used. The injected CO2 is chased with injected water.
iii. Conventional water-alternating-gas (WAG) followed with water: Alternating cycles of nearly equal volumes of water and CO2 are used to improve sweep efficiency by reducing CO2 channeling. It is most suitable for reservoirs with layers of contrasting permeabilities
iv. Tapered WAG: This method reduces the recycled CO2 in the produced oil. It is a similar method to the conventional WAG but has a gradual reduction of the injected CO2 volume per cycle. It is widely used.
v. WAG followed with gas: An inexpensive gas, is injected after the total volume of CO2 needed has been injected in the WAG process
A major challenge in CO2 EOR is its application to reservoirs that do not meet the above discussed screening criteria. Technological advancements and research are helping overcome this. More research is needed to increase recovery factors from reservoirs by designing optimum injection techniques. The CO2 EOR process has been met with opposition from some environmentalists due to the enhancement of the oil production
- Verma, M. K. (2015). Fundamentals of Carbon Dioxide-Enhanced Oil Recovery-A Supporting Document of the Assessment.
- U.S. Chamber of Commerce, U. (n.d.). CO2 Enhanced Oil Recovery. Institute for 21st Century Energy, U.S. Chamber of Commerce.
- (NETL), N. E. (2010). Carbon Dioxide Enhanced Oil Recovery, Untapped Domestic Energy Supply and Long Term Carbon Storage Solution. US Department of Energy.
- Jarrel, P. M., Fox, C., Stein , M., & Webb, S. (2002). Practical Aspects of CO2 Flooding. SPE Monograph Series.