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).

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 Climate, 1, 5.