Optimising Material Testing for CO2 Injection Wells in Depleted Reservoirs: Tackling Low Temperatures and O2 Challenges

Injecting captured CO2 into geological formations is a cornerstone of climate change mitigation, but ensuring the long-term integrity of Carbon Capture and Storage (CCS) wells presents unique material challenges. Standard oil and gas material selection guidelines[1] offer a starting point, yet they often fall short in addressing the specific threats posed by dense-phase CO2 streams containing corrosive impurities. Corrosion fundamentally requires water; while the water content of the injected CO2 can be controlled at the surface, the uncontrollable flowback of saline formation water during inevitable shut-in periods poses a significant threat, directly wetting critical components like tubing and liners. This paper chronicles the journey undertaken to select appropriate tubing materials facing this complex challenge, compounded by corrosive H2S, impacts of transient oxygen (O2) ingress and potential low temperatures arising from Joule-Thomson effects or operational upsets. The presence of even ppm levels of O2 can drastically alter corrosion mechanisms, while low temperatures demand proven material toughness, especially when coupled with corrosive formation brine. Faced with a lack of standardized limits for these combined conditions, a rigorous qualification testing program became paramount. This study details the systematic approach taken, starting with identifying candidate Corrosion Resistant Alloys (CRAs) – S13Cr, 17Cr, and S25Cr – balancing cost and perceived performance. A comprehensive test matrix was designed to simulate normal operating (base-case) and design (worst-case) scenarios (up to 50 ppm H2S, 10 ppm O2, 135 degC, 12800 ppm Cl- from formation water, pH ~3), including dedicated evaluations for Sulphide Stress Cracking (SSC), Stress Corrosion Cracking (SCC), general/localized corrosion, and galvanic compatibility. The phased testing sequence, including crucial assessments of O2‘s impact and confirmed the robustness of 17Cr, especially when galvanic risks were mitigated by simulating realistic area ratios and copper plating. This investigation provides a practical roadmap for qualifying materials in complex CCS environments, highlighting the potential of 17Cr as a cost-effective and technically sound solution, contingent upon the successful implementation of galvanic corrosion mitigation strategies.