This paper presents a discussion around themes, challenges and opportunities relating to subsea system design for CO2 injection in CCS applications. The scope includes field development considerations, system operation and control, intervention, monitoring, and explores challenges with barrier testing.

A global review of existing and planned CCS developments is conducted, with key drivers and variables identified. These observations are used to present common system characteristics and core functional requirements. Applicable technologies are discussed as well as key challenges, risks and opportunities. A transient flow assurance analysis is also performed to inform potential methodologies for testing of barrier valves, as a common challenge in these applications.

Economics of CCS developments are different to hydrocarbon production, and lowest cost per tonne of stored CO2 is key. Uptime is essential, meaning that highly available systems must be developed with reliable equipment and constrained budgets. There is a need for simplicity and fit-for-purpose design, whilst ensuring CO2 is stored in a safe manner.

Some system aspects are project specific (for example the infield architecture driven by well placement), however common themes and characteristics drive operation and functionality. These include formation type (saline aquifer vs depleted reservoir), offset distance, CO2 transportation method, the upstream process (or collection network) and fluid composition. Challenges with managing flowrate variability, and constraints to ensure CO2 is transported in a dense phase also drive operational philosophies, as well as material selection and corrosion management.

Ensuring captured CO2 is stored permanently and safely is also paramount, through monitoring programs for CO2 plume imaging and leak detection for example. Life of field intervention and maintenance requirements may include well washing to maintain injectivity, intelligent pigging, and periodic testing of barrier valves. Venting fluid from the wellbore is identified as a method to achieve differential pressures for testing of the downhole safety valve. A transient flow assurance analysis demonstrated the feasibility and limitations of this approach.

This paper brings together topics around CO2 transportation, injection and storage across multiple disciplines, to provide holistic guidance and commentary specifically for subsea system design as it relates to CO2 injection for CCS applications.

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