Dynamic reservoir evaluation through temporary and periodic CO₂ injection is crucial for effective Carbon Capture and Storage (CCS) project planning. This study presents a pioneering extended pilot test conducted in the Leman ultra-depleted gas field in the UK Southern North Sea, focusing on CO₂ injection in gaseous, supercritical, and liquid phases. The project utilised a temporary surface injection plant, incorporating storage tanks, well test equipment, high-pressure and low-pressure injection pumps, pushing the boundaries of industry standards for offshore CO₂ injection.

Handling and injecting liquid CO₂ at the surface from transportable iso-tanks into an ultra-depleted gas reservoir presents unique technical and operational challenges. The process requires specialised low-temperature-rated piping, pressure management systems, and non-standard equipment to ensure safe and efficient injection operations.

While most temporary CO₂ injection tests are conducted over short durations and involve only the liquid phase, this extended threemonth pilot test necessitated injection across gaseous, supercritical, and liquid phases. The complexity of switching between CO₂ phases while maintaining operational integrity sets this project apart from any previous offshore CCS pilot tests.

This paper details the design, execution, and key findings from the extended CO₂ injection pilot test in the Leman ultra-depleted gas field. The test comprised of 15 injection cycles, each involving up to 8 hours of pumping of up to 400 metric tonnes of CO₂, followed by a three-day observation period to assess reservoir response. The study also provides practical insights, operational lessons, and the best practices for industry professionals planning similar offshore CCS pilot projects.

During the test, liquid CO₂ was pumped from iso-tanks at rates up to 13 bbl/min (equivalent to 1.1 Mtpa), using two booster pumps and three high-pressure pumping units. This surface plant also included liquid CO₂ tank storage, a metering system, three steam exchangers (two standard and one high-efficiency) to control temperature and phase transitions before injection into a re-completed gas producer well.

For real-time monitoring, the well was instrumented with a comprehensive fibre-optic DAS+DTS array and multiple pressure sensors, enabling continuous tracking of CO₂ behaviour from wellhead to perforations. Special attention was given to data visualisation, processing capabilities, and safety protocols, including surface equipment preservation, start-up procedures, and pressure bleed-off management using a dedicated instrumented low-temperature-rated bleed-off line.

Offshore CO₂ injection in ultra-depleted reservoirs remains underexplored, with limited industry knowledge. This test is the first of its kind to evaluate CO₂ injection under a wide range of conditions, including gaseous, supercritical, and liquid phases, providing invaluable insights into reservoir response during CO₂ injection. The knowledge and practical experience gained from this operation are invaluable and will pave the way for future successful pilot CO₂ injection tests for evaluating CCS assets in the UK and globally.