As the cost of replacing strings increased due to string cost itself and associated downtime, service companies were eager to find solutions to improve the useful life of the tubing. These included, but were not limited to, carefully matching the string to the job at hand, real-time weld location monitoring during the job, and post-job corrosion and biocide treatments.
To achieve the most life from a string, a service provider would review fatigue graphs from its available strings and match one to the job expected to be performed. This technique ensured the strings met the customers' requirements while extending the life by placing expected fatigue in areas of the sting that were underused. While this technique was, and still is, effective, the issue arises where a field or play was, and is, developed, all the wells were drilled relatively the same with the same job type needing to be performed. For example, milling frac plugs on a 10,000-ft vertical well with a 10,000-ft horizontal section. This similarity negated matching the string to the well as all strings had fatigue in the same location.
Monitoring weld locations was the next advancement in extending life. While electronic systems had existed for some time, little emphasis was taken. Operators would often not pay attention to the systems during the job and would cycle the tubing at a weld with high pumping pressure, accelerating the fatigue on a weld. Another common issue was inaccurate information entered into the systems regarding footage removed from the whip end from mismeasurement or forgetfulness to enter anything. To combat this inaccurate information, engineers were sent to the field to set up the job in the computer system and accurately update the system's string file, along with operator training on how and when to cycle the tubing.
Through analysis of failed strings, service companies noticed a pattern arising from microbial-induced corrosion (MIC) contributing to pinholes and general corrosion pitting leading to parted strings (pitting creates a stress riser, Fig. 3). These issues were magnified by E&Ps using recycled water to reduce completion costs. Service companies began developing and performing post-job treatments on strings with specialty biocides and corrosion inhibitors to mitigate the use of recycled water. These treatments were somewhat effective in preventing issues, provided the field personnel implemented them correctly.