Pilot Project for CO2 Injection using Coiled Tubing and a Retrievable Bridge Plug
Authors: Starodubtseva Ksenia , Keong Azwan Hadi , Casali Andrei , Sorensen Alla , McHardy Philip , McGowan Robin , Koldig Jesper , Gregersen Søren , Lery Pierre , Romanowski Niklas 
Published: November 2024

Abstract
Project Greensand is Denmark's leading carbon dioxide (CO2) sequestration project, expected to store 4-8 million metric tons of CO2 per year by 2030. An offshore CO2 injection trial in a depleted and inactive reservoir was recently conducted to evaluate the injection of CO2 into an existing water injector well in the Danish North Sea. The study required cyclic injection and monitoring of CO2 into the reservoir at 4,300-m measured depth / 1,800 vertical depth with a detailed monitoring concept including seismic monitoring to study the CO2 plume during the the CO2 injection cycles performed during the Pilot Injection Test.
Devising a practical downhole system for the injection of CO2 in supercritical phase presented challenges in well barriers, methodology, and material selection. Standard carbon-steel alloy coiled tubing (CT) was selected to deliver the CO2 to the reservoir whilst protecting the completion and well barriers against the potential corrosive effects of the CO2. A retrievable bridge plug (RBP) with a pump-thru check valve and long-term pressure and temperature memory gauge was first conveyed on CT to provide a zonal isolation. CT was then stabbed into the RBP to create a closed system for the injection. The CT stayed latched on the RBP continuously to eliminate swapping of fluids and to prevent disturbances during the injection cycles.
The operation required around 90 days comprising repeated injection cycles of 1-day pumping of approximately 600 metric tons of CO2 and 6 days of observation. A total of 7 pumping cycles were achieved.
CO2 can react with formation water to create an acidic environment around the injection point. This demanded all bottomhole assembly (BHA) components to be designed with proper metal and elastomeric material to withstand 100% CO2 exposure for an extended period. The fluid column above the isolation packer in the annulus between CT and tubing was filled with a monoethylene glycol (MEG) mixture serving as a fluid barrier for corrosion and contingency for hydrates in case the CT and packer should leak. Positive tubing head pressure was applied throughout the entire duration of CT in the well as a mechanism to monitor the integrity of the RBP. Liquid CO2 was transported in ISO tanks on a temporarily modified platform supply vessel. After transferring the CO2 via a transfer hose to a jack-up rig, a positive displacement pump and heat exchanger were used to reach desired injection conditions for injecting the CO2 via the CT at 500 l/min into the reservoir. Post-operation analysis of the BHA and CT found no signs of corrosion or embrittlement.
Proper understanding of the downhole behavior of CO2 was key to generating a safe operational procedure. This operation demonstrated the possibility of utilizing conventional intervention equipment and services as part of an early phase enabler for a carbon storage project. The captured learnings such as material selection, temperature, and pressures regimes during injection, and acquired seismic data will be used to scale up for future carbon storage solutions in the North Sea.