Energy Excursions

How CO2 EOR Works

Why is CO2 injection such an effective method for producing remaining original oil in place when primary and secondary recovery techniques have peaked? Several physical mechanisms enhance oil production when CO2 is introduced into the reservoir for enhanced oil recovery (EOR).

CO2 EOR

Cross-section illustrating how carbon dioxide can be used to improve production of oil after primary and secondary recovery

If the technology is applied after water flooding, the goal is to produce the mobile oil that was bypassed by water and the immobile residual oil trapped by capillary force. To facilitate maximum efficiency of this ‘sweep’ of the reservoir, the CO2 and the oil must become miscible.

Two liquids are miscible if they combine in any ratio to form a homogeneous solution. For example, water and vinegar are miscible, as are water and ethanol. Water and oil are immiscible, which we observe when they separate when mixed.1UCLA College of Chemistry and Biochemistry. (n.d.). Illustrated glossary of organic chemistry. Retrieved 10/28/2020 from http://www.chem.ucla.edu/~harding/IGOC/M/miscible.html

In the desirable case where the reservoir pressure is above the minimum miscibility pressure* and the injected CO2 and residual oil are miscible, the physical forces holding the two phases apart (interfacial tension) effectively disappear. This promotes a mass transfer of light and intermediate hydrocarbons, which reduces the residual immobile oil saturation (unsticks the oil!). Additionally, the CO2-rich oil phase expands, regaining mobility. Mass transfer is improved at higher pressure, lower reservoir temperature and lighter oil.

Definition:
*minimum miscibility pressure:  At minimum miscibility pressure, the interfacial tension is zero and no interface exists between the fluids. It is the minimum pressure by which crude oil remains miscible with CO2 at reservoir temperature. Loss of full miscibility between CO2 and oil can occur when reservoir pressure drops below the minimum miscibility pressure.

The CO2 minimum miscibility pressure is an important parameter for screening and selecting reservoirs for CO2-EOR projects. For the highest recovery, a candidate reservoir must be capable of withstanding an average reservoir pressure greater than the CO2 minimum miscibility pressure.

Because CO2 EOR is a displacement process, CO2 is injected into the deep subsurface rock reservoir through an injection well to displace oil toward a production well. CO2 is produced along with reservoir fluids, separated at the surface and, commonly, re-injected into the reservoir. The cycle repeats throughout the operation.

Net Carbon Footprint of CO2-EOR

Even though large amounts of CO2 are geologically stored through EOR, the extent to which the technology can reduce greenhouse gas emissions remains an important question. We are able to address this through lifecycle analysis (LCA). LCA is a process that assesses the environmental impact that occurs throughout a product’s lifecycle, from raw materials acquisition through production, use, final treatment, recycling, and disposal.

LCA applied to CO2-EOR answers the question of whether CO2 emissions resulting from the EOR energy consumption and, more significantly, from the combustion of the incremental oil produced, are offset by the mass of anthropogenic CO2 stored. Recent studies at The University of Texas at Austin Bureau of Economic Geology on LCA for EOR conclude that the technology has potential for decarbonization during the first several years of operation.2Nuñez-López, V., & Moskal, E. (2019). Potential of CO2-EOR for Near-Term Decarbonization. Frontiers in Climate1, 5.

As part of preparing for an enhanced oil recovery program, an engineer determines that the candidate reservoir could fracture at the minimum miscibility pressure. What should the engineer recommend?

Design the project to inject at a pressure below the minimum miscibility pressure to prevent fracturing

Incorrect. 

Pressures below the minimum miscibility pressure will result in loss of full miscibility between CO2 and oil, and thus, poor recovery of oil. This reservoir is not suitable for CO2 EOR.

Recommend that the reservoir is likely not suitable for a successful enhanced oil recovery program

Correct. 

CO2 minimum miscibility pressure is an important parameter for screening and selecting reservoirs for CO2-EOR projects, and this reservoir is not highly suitable.

 

Career Spotlight: Vanessa Nuñez-López

Ms. Nuñez-López’s Academic Background

B.S. Petroleum Engineering, Universidad Central de Venezuela, July 1999
M.S. Petroleum Engineering, The University of Texas at Austin, December 2001
M.A. Energy and Mineral Resources, The University of Texas at Austin, December 2005

Vanessa Nuñez-López worked as a reservoir engineer at Chevron until 2010 when she came to work at the Bureau of Economic Geology. As a member of the Gulf Coast Carbon Center at the Bureau, she worked on enhanced oil recovery topics, especially lifecycle analysis. Her engineering expertise and communication skills led to her appoint in 2021 as Senior Advisor for the Office of Oil and Natural Gas at the U.S. Department of Energy. She is shown here taping a podcast on Carbon Neutral Oil for National Public Radio.

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