Long-term Surveillance of Plugged and Abandoned Wells for Immediate Detection of CO2 Leakage in Geologic Carbon Storage Sites

Contact: Sue Hovorka

Manuscript by Bakhshian et al. coming soon

Background

A primary risk when storing CO₂ in geologic formations is the potential for CO₂ leakage from the storage site (Romanak et al., 2013; Pawar et al., 2015). Unintended leakage of CO₂, or formation fluids, from CO₂ storage sites may impact various stakeholders and the environment, leading to financial and environmental costs, and challenging the social and political acceptability of the technology. Very old legacy wells, associated with oil and gas exploration and production operations, are possible leakage pathways due to the quality of their construction and abandonment practices (Jenkins et al., 2015; Bai et al., 2016; Lackey et al., 2019). To mitigate risk, researchers at the Gulf Coast Carbon Center deemed monitoring CO₂ storage sites for signs of leakage through leaky wells imperative.

Through regulation, deep wells are constructed and maintained to isolate hydrocarbon-bearing and saline zones from overlying freshwater resources. During construction, isolation is engineered in; when a well is properly plugged and abandoned, the casing is blocked by cement plugs (Wisen et al., 2020). Many P&A wells in injection projects perform well in zonal isolation; nevertheless, screening based on well records is not fool-proof, resulting in environmental damage and liability if a failure occurs. Where records of past wellbore management are poor, the regulator may currently have no choice but to require that many wells be re-entered, which is difficult, time-consuming, and expensive. These failures can be avoided by implementing an efficient leak detection system.

The purpose of this study was to create and implement an efficient and cost-effective near-surface monitoring package that provides real-time and long-term surveillance of plugged and abandoned (P&A) wells. The project involved conducting pilot and field-scale controlled CO₂- and water release experiments to identify parameters that are most effective for detection of leakage into the near-surface vadose zone above P&A well stubs.  A well stub is known as the remaining length of a casing string that is “left in the ground after the upper portion of the casing has been cut and removed during wellbore completion or abandonment; it is a remnant of the pipe that was used to line the wellbore and is often left in place, extending from the surface down into the well.” These experiments were designed to artificially simulate point-source leakage of CO₂ and water through a proxy well from the depth of around 3 ft, the typical cut-off depth for surface casing. Sentinel surveillance of proxy wells was conducted through a modular monitoring package consisting of soil sensors sensitive to fluid migration signals. The sensor package consisted of four soil sensors to measure soil moisture, temperature, and electrical conductivity (EC) at different depths and distances from the well casing. The system was designed for continuous monitoring during release and post-release experiments in real-time. The early detection capability of a continuous monitoring network of sensors is influenced by the natural variability of meteorological conditions (e.g., wind speed, wind direction, precipitation, atmospheric stability, and ambient temperature), which were measured simultaneously during the experiments.

Experiments

Leakage experiments were conducted in a field laboratory owned by The University of Texas at Austin, located at the Brackenridge Field Laboratory (BFL). The experimental plot was developed to support experiments of controlled released of CO₂ and water in the vadose zone, a few feet below the surface over a leaky proxy well stub.

Initially, a trench with the area of 6𝑓𝑡 ×6𝑓𝑡 and depth of 5 ft was excavated (see Fig. 1). A metal well casing was placed in the middle of the trench, while its surrounding was filled with engineered coarse sand (8-16 filter sand). Two PVC piped screened at the end were placed in the coarse sand region behind the casing for injection of CO₂ and water in the controlled release experiments. CO₂ pipe was connected to a CO₂ cylinder at the surface and the water pipe was connected to city water. The water passed through a flowmeter and water heater before injecting into the trench.

The trench was then filled with a mixture of fine sand and natural soil (native soil) (50-50 vol%). The top layer of the trench was overlain by clayey soil. Four soil sensors were placed at different depths and distance from the well casing in the vadose zone above the injection points (Fig. 1).

Conclusion

The experimental field setup successfully responded to CO₂ and water leakage. Soil EC was found to be the most sensitive soil signature to CO₂ and water leakage. The leaked CO₂ elevated the soil EC signature due to dissolution of CO₂ in the soil water and release of carbonic acid. The level of EC increase during CO₂ leakage depends on the soil moisture content. Casing temperature and soil temperature at deeper locations promptly responded to the hot water release, with the response of casing being more noticeable due to the higher thermal conductivity of metal compared to the one for soil.

References

Romanak, K., Sherk, G.W., Hovorka, S., and Yang, C., 2013, Assessment of alleged CO₂ leakage at the Kerr farm using a simple process-based soil gas technique: Implications for carbon capture, utilization, and storage (CCUS) monitoring. Energy Procedia, v. 37, p. 4242–4248.

Pawar, R. J., Bromhal, G. S., Carey, J. W., Foxall, W., Korre, A., Ringrose, P. S., and White, J. A., 2015, Recent advances in risk assessment and risk management of geologic CO₂ storage. International Journal of Greenhouse Gas Control, v. 40, p. 292–311.

Jenkins, C., Chadwick, A., and Hovorka, S. D., 2015, The state of the art in monitoring and verification—ten years on. International Journal of Greenhouse Gas Control, v. 40, p. 312–349.

Bai, M., Zhang, Z., and Fu, X., 2016, A review on well integrity issues for CO₂ geological storage and enhanced gas recovery. Renewable and Sustainable Energy Reviews, v. 59, p. 920–926.

Lackey, G., Vasylkivska, V. S., Huerta, N. J., King, S., and Dilmore, R. M., 2019, Managing well leakage risks at a geologic carbon storage site with many wells. International Journal of Greenhouse Gas Control, v. 88, p. 182–194.

Wisen, J., Chesnaux, R., Werring, J., Wendling, G., Baudron, P., and Barbecot, F., 2020, A portrait of wellbore leakage in northeastern British Columbia, Canada. Proceedings of the National Academy of Sciences, v. 117(2), p. 913–922.

Acknowledgements

Special thanks to Dr. Sahar Bakhshian, now at Rice University, for conducting these experiments.

For this project, our team would like to acknowledge funding support by the Energy Institute at The University of Texas at Austin through the 2023-2024 Energy Seed Grant Program. Financial support was also provided by the Department of Energy under DOE Award Number 655 DE-FE 0031830 (known as the SECARB-USA), Advanced Energy Consortium (AEC-RPA), and Bureau of  Economic Geology’s External Support Leveraging Program.

Last Updated: October 3, 2025