C-WELL: CO2 injection well integrity under operational thermo-hydro-mechanical loading (Flexible Funding 2023)

Dr Lee Hosking, Brunel University of London, was awarded funding through the UKCCSRC’s Flexible Funding 2023 call to investigate “C-WELL: CO2 injection well integrity under operational thermo-hydro-mechanical loading”, in partnership with Quintessa Ltd., Net Zero Technology Centre and The Solent Cluster.

One of the most important aspects of any CCS project is to ensure the secure, long-term storage of CO₂ within its intended subsurface reservoir. CO₂ leakage can pose challenges to environmental health, require additional monitoring and corrective measures, and have legal and regulatory implications. Any leaked CO₂ that reaches the atmosphere also undermines both the intended climate action and the investments of human capacity and funds.

In this context, the C-WELL project, delivered by Brunel University of London in collaboration with industry partners Quintessa, Net Zero Technology Centre and the Solent Cluster, has focused on one of the pathways by which CO₂ may migrate upwards from its storage reservoir: the injection wells themselves. Since these wells penetrate the confining layers of rock, they must be engineered to maintain an effective barrier to upward migration of the injected CO₂. To achieve this, each injection well is a composite of three main structural components: the steel casing, the cement sheath, and the surrounding rock itself, as shown in Figure 1. Also shown are the various defects that might develop into leakage pathways.

The C-WELL project team has employed numerical modelling and laboratory testing to investigate the performance of the cement sheath. Our focus has been two-fold: firstly, to predict the conditions and extent of debonding and microannulus growth at the interfaces of the cement sheath with the casing and cap rock, and secondly, to study the engineering properties and performance of different cement mixes for injection wells. This blog post focuses on the first of these.

Figure 1 – Potential defects in CO2 injection wells (Hosking and Zhou, 2025)

A model to predict damage evolution in CO₂ injection wells

As part of C-WELL, a new numerical model was developed to predict the thermal, hydraulic and mechanical behaviour of the composite well system, including interface damage and microannulus growth based on a cohesive zone model. These advancements allowed the performance of the cement sheath interfaces to be investigated under operational conditions, including in the presence of initial defects and for a wide range of CO₂ injection scenarios.

Importance of pre-existing defects

The role of a pre-existing defect was investigated in the form of an initially debonded surface following the cement job (Hosking and Zhou, 2025), which is a commonplace defect due to the challenges of deep well engineering. Figure 2 compares the interface damage predictions for an initially intact well to a well with an initially debonded segment at the casing-cement interface. A microannulus can be seen in both cases at the casing-cement interface – it is the blank space (white gap) close to the inner surface of the well. The peak microannulus is visibly larger for the defective case, reaching over double the mechanical aperture, highlighting the importance of accounting for the imperfect condition of wells when predicting leakage risk.

Figure 2 – Contour plots of displacement magnitude (in m) at 30-days near the centreline of the injection well. Black lines indicate original locations of the casing and cement with displacements are scaled up for clarity. Arrow surfaces show the direction and relative magnitude of displacement for (a) an initially intact well and (b) a well with a pre-existing defect in the form of a 45⁰ debonded azimuth (Hosking and Zhou, 2025)

Role of injection pressure and temperature

In research presented at the 17^th Greenhouse Gas Technologies (GHGT-17) conference held in Calgary, Canada, 20–24 October 2024 (Hosking et al., 2024), the C-WELL team identified that the damage initiation “window” for the casing-cement interface corresponds with conditions that could reasonably be expected at the bottomhole of CO₂ injection wells.

As an example, the findings of 171 simulations, each for different combinations of injection pressure and temperature, are expressed in Figure 3 in terms of the predicted aperture of the microannulus after 30 days of continuous CO₂ injection. Microannuli with mechanical apertures exceeding 0.12 mm were predicted for lower injection pressures and temperatures, with further research needed to determine the associated hydraulic aperture and axial extent of the damage. This future research direction has been facilitated by the C-WELL project supported by the UKCCSRC Flexible Funding Call 2023, and will build upon the present findings as we seek to better understand the CO₂ leakage risk posed by defects and damage development in injection wells.

Figure 3 – Surface plots of the casing-cement microannulus aperture at the end of the 30-day simulation period. Surfaces are formed from the results of 171 simulation scenarios (Hosking et al., 2024)

Improved CO₂ storage risk assessment

New quantitative evidence from C-WELL will next be used to improve the decision trees of project partner Quintessa’s TESLA decision support software, which will ultimately lead to more effective leakage risk assessments and corrective measures planning for CO₂ storage sites in the UK and internationally.

Read more on Lee’s Flexible Funding 2023 project page.

References

Hosking, L., Al-Noaimat, Y., Zhou, X., Zagorscak, R., Benbow, S., Metcalfe, R., 2024. Interface Damage and Microannulus Growth in CO₂ Injection Wells: Parametric Modelling for Pressure and Thermal Loading. In Proceedings of the 17^th Greenhouse Gas Technologies Conference, Calgary, Canada, 20–24 October 2024. https://doi.org/10.2139/ssrn.5034771

Hosking, L.J., Zhou, X. 2025. Damage modelling of CO₂ injection well interfaces under coupled thermal, hydraulic and mechanical behaviour. Deep Underground Science and Engineering.