Rapid Depressurization and Corrosion Evaluation of OCTG Connections for CCS Injection Wells

Carbon capture and storage (CCS) is a key technology to decarbonize many industrial processes. Premium OCTG connections are key components in CCS wells ensuring structural and tightness performance between the casing and the tubing pipes. Vallourec research and development departments continue to develop a comprehensive connection test protocol for CCS injection wells and investigate impact of low temperature and the corrosion performance of premium OCTG connections with liquid CO₂.

CO₂ phase change can occur in the tubing during the process of injection or during the shut-in of the subsurface safety valve generating the Joule-Thomson effect due to rapid depressurization. The temperature can plummet to '60°C in regular operations and could lead to the failure of the tubing in the well posing risks to human safety and the environment due to uncontrolled CO₂ release. A rapid depressurization test has been put in place to reproduce this phenomenon.

The aim of this paper is to describe the tests performed under combined load of tension and external pressure with liquid CO₂. The specimens tested in rapid depressurization were in super duplex stainless steel. They were inspected after these tests to check if corrosion or pitting were present inside the connection.

A premium OCTG connection in 5-1/2″ 20.00# 25Cr 80ksi with dope free solution and compatibility configuration has been tested according to the CCS test rapid depressurization evaluation. The first test was performed at 80% of pipe capacity with 100 bar of liquid CO₂ with an ambient temperature at 25°C. The specimens experienced a temperature of '60°C in 7 minutes without any leak or structural failure. A second test was carried out under pure external pressure at 473 bar and a third test with combined loads: tension at 510 kN and external pressure at 473 bar. The liquid CO₂ pressure was at 100 bar and 20°C as initial temperature condition. During the rapid depressurization the temperature went down from 20°C to '60°C in 10 minutes without any leak or structural failure. A failure test was also conducted under depressurization above 100% of connection capacity.

The connections with metal-to-metal seal tested in rapid depressurization were compared to configurations tested for their corrosion resistance in autoclave. All the configurations tested show no indication of pitting nor corrosion on the seal.

The size tested with rapid depressurization evaluation demonstrated this product is compatible for CCS injection wells in the most severe conditions. The additional physical data also allowed to better understand the temperature drop in full-scale tests. The metal-to-metal seal with dope free solution presented no corrosion after these tests and confirmed the suitability of premium OCTG connections to liquid CO₂.