HPHT well environments present design & operational challenges that could potentially translate into well failure with high consequences. Several risk elements can combine into a complex hazard causing serious threat to well design & integrity. Risk elements could be complex downhole environment, material deration, material incompatibility with the completion/packer fluids and other treatment fluids, metallurgy imbalances etc. This case study presents early life production tubing integrity failure highlighting gaps and suggestions to adopt an integrated risk mitigation approach.
A 5,700 m TVD keeper HPHT exploratory well was drilled in north of Pakistan with reservoir pressure, temperature exceeding 10,000 psi and 320°F respectively. Matrix acidizing was required to remove near wellbore damage in the targeted carbonate reservoir. Initial kick start stimulation efforts resulted in tubing-annulus communication indicating compromised CRA super 13CR completion string integrity. Workover followed wherein another early life completion string failure occurred. Consequently, comprehensive analysis was carried out to determine failure root causes using a systematic fault tree analysis approach.
Failure investigation consisted of two broad scopes: a) Scrutinize well design against established industry standards and best practices for HPHT wells and; b) Completion string material metallurgy tests to evaluate compatibilities with exposed well & treatment fluids, bottom hole environment and assessment of all possible risk scenarios that by itself or in combination with other risks could cause material failure. Further, detailed study work included describing bottom hole environment comprehensively, various types of corrosion risk assessments including evaluation of environment assisted cracking risks, acid inhibitor efficiency evaluation, completion/packer fluid selection, fluid compatibility assessment and fluid additives degradation at high temperatures. Mill manufacturing processes, susceptibility of CRA material passive layer because of austenite percentage were also looked-into. Based on systematic approach and extensive in-depth analysis, key observations were drawn. These observations were further investigated with material testing and possible root cause failure risk factors were arrived at. Conclusions were drawn highlighting primary and secondary failure root causes. A new basis of design and qualification protocols was proposed to mitigate various risks to ALARP.
Holistic HPHT well design approach is found to be lacking pan industry as the complex failures risk mitigation information is either not available or is discrete. Operators venturing into HPHT for the first time are challenged to comprehend requisite steps required to mitigate the risks to ALARP. Therefore, the paper aims to provide an integrated approach for completion metallurgy selection and test qualification requirements for stimulation and completion fluid in conjunction with the downhole environment.