It is increasingly recognised as essential for decarbonisation, with many countries enacting laws and regulations to support its implementation and developing programmes to advance technologies to support it. In this blog, we will share our experience and lessons learned in assessing the suitability of offshore wells at the end of their commercial life for repurposing and integration into a CCS transport and storage network. 

Reducing Costs through Well Repurposing

Developing a CCS network from scratch demands a significant upfront capital investment, rendering such projects economically unfeasible. However, there is potential to significantly reduce costs by repurposing old wells connected to depleted oil and gas reservoirs and integrating them into a CCS network.

Depleted reservoirs can be ideal geological storage sites for carbon dioxide. Unfortunately, not every well that has reached the end of its producing life is suitable for CO2 injection. This could be due to degradation during the producing life of the well or because the structural loads during injection lie outside the original design envelope. Therefore, it is crucial to assess whether individual wells can support CO2 injection before including them in CCS networks.

Technical Challenges and Considerations

Extending a well's commercial life by repurposing it for a different type of operation poses significant technical challenges. Firstly, oil and gas production wells are typically not designed for scenarios involving CO2 injection at the end of their life. Transitioning from production to injection alters the pressures and temperatures within the well, affecting the structural loads along the conductor and other well casings. The injection operation will require high-pressure conditions to ensure efficient injection and dispersion of the CO2 within the reservoir, which can put significant stress on downhole tubulars. Therefore, the pressure exerted by the injected CO2 is critical and it should be assessed under various scenarios, including maximum injection rates, to ensure the integrity of the well tubulars and other components.

For platform wells, where the conductor is supporting the well through the water column, understanding the change in the conductor's response during injection is crucial. The CO2 injection at low temperatures may generate compressive loads on the conductor that exceed those considered during design. The risk of the conductor buckling under increased compression must be assessed. Greater compression also reduces the effective stiffness of the conductor, changing how it responds to environmental forces like waves and currents. In this scenario, the deflections on the conductor will increase, which will have a detrimental effect on its strength and fatigue performance.

For ageing wells, the technical challenges described are exacerbated by wear and tear from the previous life of the well. As wells reach the end of their producing life, significant corrosion may have occurred. Corrosion diminishes the capacity of the well tubulars by reducing the amount of steel available to resist external loads. The corrosion rates tend to be fastest in the splash zone – the section over which the sea level varies due to tide and wave action – where the presence of oxygen from the atmosphere hastens the process. The wall thickness in this section will be the most reduced, and it will be the weaker section of the conductor. Injecting CO2 into the well also carries degradation risks. CO2 acts as a corrosion agent, accelerating steel corrosion, so an accelerated loss of wall thickness in the well tubulars should be expected in the coming years of injection. CO2 can also degrade the cement, potentially leading to leaks or loss of zonal isolation.

The original well design may have incorporated lateral supports where the conductor passes through decks or guides attached to the platform structure. These supports are often used to control the buckling risk. It is common for the centralizers used to control the amount of clearance at these supports to be lost during service due to corrosion or fatigue. Any missing centralizers may need to be reinstated before injection operations can start.

Lastly, the conductor will have accumulated fatigue damage from exposure to environmental loads during its previous life and this will need to be accounted for when assessing its remaining fatigue capacity for future operations. In extreme cases, fatigue is known to cause through-wall cracking and parting of the conductor at critical welds or connections. In these cases, significant remediation work may be needed to ensure sufficient fatigue capacity for CO2 injection.

Expert Assessment for Successful Repurposing

To address these challenges effectively, a holistic approach that understands the interaction between downhole conditions and external environmental factors in the overall structural performance of the well is required. Proper assessment can provide vital information for selecting wells and guidelines for trouble-free CO2 injection through them.

2H offers comprehensive analysis services and expertise in assessing the structural integrity and fatigue life of wells. Our experience, deep domain knowledge, and advanced analytical capabilities allow us to deliver assessments that maximise value for our clients, ensuring successful repurposing for their CCS initiatives.