Material Selection, Compound Development Criteria and Elastomer Grade Optimization of Packer Element in CCUS Storage Well

Abstract CO2 storage is an integral part of CCUS, whereas packer element for CO2 storage well is an essential component that must perform reliably for decades. The key to superior reliability is a high performing elastomer. This paper intends to compare the elastomers in oilfield applications and recommend the best fit for packer element under field application. It identifies the major packer element failure mode, establishes the compound development criteria, and ultimately guides elastomer grade optimization. The sealing performance of packer element in CO2 well was reviewed, and the major failure modes identified. The compound development criteria were established successively. The characteristics of the elastomers in oilfield were evaluated for packer element application in CO2 storage well. The best elastomer was selected based on the criteria established. Special attention was paid to the optimal selection of elastomer grades. The effect of macromolecular structures on the compatibility of CO2 was studied from the thermodynamics aspect and later investigated by the immersion testing of different elastomer in supercritical CO2 and brine mixture under downhole temperature and pressure. The major cause of long-term failure of packer element has been found to be the compatibility between CO2 and elastomer. The major cause of short-term failure is the rapid gas decompression (RGD) damage. Hence, the compound development criteria are set to be CO2 compatibility, RGD resistance, low temperature flexibility and chemical resistance. Upon the characteristics comparison of ethylene propylene diene monomer rubber (EPDM), Nitrile Butadiene Rubber (NBR), Hydrogenated Nitrile Butadiene Rubber (HNBR), and Fluroelastomer (FKM), HNBR is the best candidate meeting all criteria under field condition. To maximize the material life of HNBR packer element, one needs to design the formulation with a HNBR grade that is thermodynamically incompatible with supercritical CO2. However, selecting an optimal HNBR grade is a debatable subject in the industry. Some prefer low ACN grades while many others prefer high ACN ones. The dispute on HNBR grade selection may be reconciled by our immersion testing results indicating that as the ACN content of HNBR goes up, the material suffers more mass loss as well as volume increases and tends to have more bubbles and blisters on surface. On the other side, the higher ACN grades retain the mechanical properties better than the lower ACN ones. High performing elastomer for packer element can significantly improve the reliability and reduce the operation cost of CO2 storage wells and overall CCUS value chain. Selecting the best elastomer is essential to the packer element performance. This paper is one of the first attempts to address the issues of elastomer selection, compound development criteria, and elastomer grade optimization. Our results may resolve a long-standing dispute in the industry.