Offshore groups investigate safety measures for drilling into CO2 reservoirs

Editor's note: This feature first appeared in the May-June 2024 issue of Offshore magazine. Click here to view the full issue.

By Jeremy Beckman, Editor-Europe

Full-scale carbon capture and storage (CCS) developments are set to go forward over the next few years across the North Sea region, offshore Australia and Southeast Asia and along the US Gulf Coast. Conventional rigs will likely drill the initial sets of wells into the saline aquifers or depleted reservoirs that will receive the CO₂ transported from industrial emitters onshore.  But the next phase of drilling, into reservoirs already storing large CO₂ volumes, represents something of a leap into the unknown.

According to Noble Corp., well control issues could arise during drilling or workover of wells for use as CO₂ injectors. “CO₂ will oftentimes be pumped and stored in a super-critical condition where it takes on properties in between a gas and a liquid,” said the company’s Senior Business Developer Zach Bruton. “Detecting and managing this could prove difficult should a CO2 influx enter the well.  And if a well control incident did occur, the gas circulated up the annulus would likely expand more significantly than a hydrocarbon kick.”

Operators and service companies are also working to determine which existing offshore oil and gas wells could safely be prepared for re-use, and which would need remediation to ensure suitability for long-term CO₂ injection. The REX-CO2 project in Europe has developed a screening process for existing wells that includes re-use without modification; workover with modification; side-tracking from a portion of the well; deepening the wellbore to access a deeper target; partial plugging of sections of the well; and re-entry of an abandoned well. The project’s accompanying software addresses key issues such as structural, primary and secondary well barrier integrity; material compatibility; and the risk of out-of-zone injection.

Among drilling contractors, Noble has been one of the early movers in assessing the likely scenarios for safe well construction in CO₂-rich reservoirs offshore. The company deployed the jackup Noble Resolve for the current CO₂ injection pilot for Project Greensand in the Danish North Sea, led by Ineos and Wintershall Dea. Here, liquidized CO₂ captured and transported from the Ineos Oxide factory in Belgium is injected via a well into the Nini West sandstone reservoir, 1,800 m (5,905 ft) below the seabed, for permanent storage.

Noble has also been collaborating with DNV and bp, as leader of the Northern Endurance Partnership in the UK, on the new CCS-Enabled Rig Design for drilling/workover operations in a CO₂-rich environment. NEP, formed in 2020, will transport CO₂ captured from industries within the East Coast Cluster in north-east England for injection 145 km (90 mi) offshore the Teesside coast into the Endurance saline aquifer. This is thought to hold storage capacity for 450 MM metric tons of CO₂, with other subsurface stores nearby potentially raising capacity to 1 billion metric tons.

The International Energy Association estimates that more than 50 MMt/yr of CO₂ is being injected globally at present with increasing numbers of wells to be drilled post-2030 as the planned rate soars to 1,200 MMt/yr. But as bp noted in its planning for Endurance CCS, until now no MODUs have been certified for drilling into CO₂-rich reservoirs, and conventional mobile rigs are not configured for operations in a pure CO2 environment.  The properties of CO₂, bp and others realized, might require modifications or changes to existing rig equipment, processes and procedures. Those concerns led to the development of a qualification program to prepare Noble’s rigs for safe drilling into CO₂ reservoirs.

For the CO₂ rig project, the main goal was to define generic requirements for qualifying an offshore rig to be ready for CO₂ drilling operations, using Noble Resolve as a base case. The program drew on DNV’s experience in CO₂ risk management, bp’s track record in CO₂ injection for enhanced oil recovery, and Noble’s feedback from participation in Project Greensand. According to Zach Bruton, “this allowed subject matter specialists to assess the equipment upgrades, rig and management system modifications needed for drilling operations with a risk of encountering a high- CO₂ concentration event, along with procedural updates, training and PPE requirements.”

The partnership prioritized technical solutions to address material exposure to very low temperatures, and mitigation measures for CO₂ influx detection. Noble highlights four main challenges when drilling into a CO₂-rich reservoir:

• Cold – when CO₂ is forced through a valve such as in the choke line it can expand rapidly. This causes its temperature to drop (Joule-Thomson effect), potentially damaging piping or leading to formation of hydrates
• Heavier than air: circulated CO₂ settles downwards instead of rising upwards like natural gas. This means new vent lines will be needed and other systems to ensure the crew’s safety
• Toxicity – CO₂ is hazardous to individuals at concentrations above 5%, so portable CO₂ detectors, personal gas masks and extra oxygen for rescue teams should be considered
• CO₂ and its compounds are corrosive, damaging steel, plastic and elastomers when in contact too long. Its solubility in oil brings a (very low) risk of uncontrolled blowout. Finally, methane CO₂ is flammable.

To devise practical recommendations, Bruton explained, “the project team studied hypothetical cases based on planned CCS operations. Focus areas included dispersion analyses, modelling venting of a CO₂ kick on board, and tests of well control equipment for embrittlement, corrosion, and temperature impacts in the presence of CO₂. The work followed the DNV Recommended Practice-RP-A203 for Technology Qualification. In 2023, initial phases of technology qualification were completed, along with technology and threat assessment. Then followed equipment testing, providing the basis for completing full equipment and management systems recommendations in 2Q 2024.

“During the first half of this year, Noble has conducted material testing including cryogenic tests, of key well control items. We have identified critical wells control equipment and materials and have performed laboratory type testing with exposure to CO₂. A selection of rubber and critical steel materials has been undergoing testing. We are also in dialogue with OEMs regarding the well control equipment and materials to find a safe, operational and cost-effective solution for the CCS Enabled Rig Design.”