Gaps in Material Qualification Requirement and Acceptance for Severe to Extreme Sour-Sweet Corrosive HPHT Environments Impacting Completion Design
Authors: Dhawan Sandeep , Farid Syed Munib Ullah 
Published: December 2021

Severe to extreme sour-corrosive environment assisted cracking (EAC) phenomenon are complex. Mandatory test qualification requirements and acceptance criteria is non-existent, in relevant API and NACE standards for fracture toughness of the CRA's. This paper, perhaps an industry first, attempts to highlight some of these gaps and how it translates into material strength uncertainties thereby impacting tubing design and risk assessment. The materials in this context are high strength group 1 to 4 corrosion resistant alloys of API 5CRA.

Fracture toughness or critical stress intensity factor is a measure of resistance to failure due to crack propagation - a key parameter for HPHT tubing material selection and design. This material aspect of fracture toughness can be influenced by several factors like Microstructure, Strength, Hardness, Heat treatment, Anisotropy etc. Low temperature is generally considered as worst case, nevertheless at higher temperatures, well environment driven embrittlement can have a serious impact on the fracture toughness value. Therefore, with several factors influencing, its characterization is important to define the burst envelope of the tubing when exposed to severe to extreme sour-sweet corrosive environment typical of HPHT wells.

A unique approach is followed to determine the brittle burst tri-axial envelope of selected tubing based on minimum fracture toughness value of the CRA material, referred to as KIMAT for SSC (or EAC) as prescribed by the mill. Proportional radial scaling is proposed to generate scaled down von-mises brittle-burst envelope. The tubing loads and the safety factors are analyzed to the shrunken envelope to visualize the risks of tubular failure under sour-sweet corrosive environment. The analysis includes varying crack depths of 5% and 3%. In addition, a minimum KIMAT for SSC (or EAC) value required to achieve full scale VME is investigated to determine specific material property requirements.

TM0177 NACE D covers methods to measure fracture toughness KIMAT for sour service at ambient temperature only and does not address the context of EAC exposure at ambient or elevated conditions i.e., KIMAT for EAC.This implies that a methodology for evaluation of EAC risk is not available as yet. Guidance on the potential for corrosion to cause cracking of CRAs is given in Table B.1 of ISO 15156-3 with primary and secondary failure mechanisms. However, a quantitative test to cover the risk of cracking of materials by specifying minimum required KIMAT for EAC for each group type in 5CRA is non-existent. Even KIMAT for sour service minimum requirements with SSC as primary failure mechanism, e.g., group 1 CRA, does not currently exist. Consequently, KIMAT for EAC minimum requirements are considered as far-fetched. Additionally, mills prescribed KIMAT for SSC lacks basis due to gaps in the minimum fracture toughness requirement stipulations for group 1 to 4 CRA materials listed at API 5CRA. Therefore, this paper provides risk insights and potential of tubing failure that can lead to serious integrity issues on a HPHT well. A joint industry program or joint API/NACE task group is proposed as a logical next step.