Safely Managing Carbon Capture and Storage Injection Risks in Legacy Production Wells

This paper discusses a successful small-scale carbon dioxide (CO2) injection test, that was performed using a temporary topside facility to inject into an onshore historic production well and depleted reservoir. This test acted as the first step of a much larger carbon capture and storage (CCS) project, and was used to simulate the early lifecycle conditions and assess the feasibility of the full-scale project. The aim of this paper is to share how this unique CO2 test was delivered successfully by utilising a fit-for-purpose injection strategy and risk mitigation methodology, that was developed through leveraging full system modelling. A comparison between the measured field injection data and modelling results is also presented. Injecting CO2 into depleted reservoirs using historic wells inherently comes with safety, operational, and integrity risks. However, it is also attractive, as the existing well and infrastructure can be repurposed to reduce capital expenditures, and the depleted reservoir allows for "refilling" up to its original pressure. During CO2 injection tests, depleted reservoir pressures will often be too low to support a static column of liquid or dense phase CO2, requiring tubing friction or downhole devices to provide the required backpressure. The small-scale CO2 injection test proposed to use a 1980s suspended production well, that was not suitable for standard CO2 pumping operations due to the small operating window. Therefore, to perform the test a custom temporary system and injection program needed to be developed that allowed for CO2 vaporisation and heating on surface, ensuring the operating limits were not exceeded and minimising the stress on the historic well. The system also needed to be fit-for-purpose, cost-effective, meet regulatory requirements and stringent timelines. To meet these requirements, an approach utilising available oilfield equipment packages and integrating them into a single injection system was chosen. As this system was not bespoke for CO2 injection, dynamic modelling was heavily used to validate the operability, safety, and integrity of the system, as well as develop custom operating procedures. Using this fit-for-purpose injection system and injection strategy proved successful during the CO2 injection operations, with the system behaving as planned, safely managing the CO2 vaporisation, providing stable injection rates and temperatures. High-quality surface and downhole injection data were captured, which validated the feasibility of the target reservoir and injection models. Safety and integrity were monitored and maintained throughout the CO2 operations, with pressure and temperature managed within the system operating window, and no dry ice, hydrates or injection issues observed. Following injection, the CO2 inside the well was successfully bullheaded into the reservoir and the well was resuspended and returned to its preexisting status.