Special Report: Surveillance combats corrosion
Ken Feather, chief marketing officer, TGT Oilfield Services, explores well system integrity and combatting corrosion in the Middle East
Well systems vary in complexity and cost, but they all share one fundamental duty – to transport pressurised fluids between the subsurface and surface safely, productively and profitably. And they must do so unfailingly, whilst exposed to extreme working conditions for many years, often decades.
This article will look at some of the key issues surrounding integrity management, including threats to well system integrity such as corrosion and new diagnostic strategies that are addressing them in the Middle East.
Maintaining the integrity of a well system is critical for its long-term future. Integrity failures vary in severity but in most cases can prevent the well system from functioning properly. An article by R.J. Davies contains integrity statistics from 25 different studies that reveal failure proportions around the world from 2% to 75% in more than 380,000 wells. Other frequently quoted statistics refer to alarming failure proportions in major producing areas such as 45% in the Gulf of Mexico and 34% in the UK North Sea. Integrity failures at these levels would never be tolerated by other industries, such as the airline or chemical processing sectors – so we clearly have a responsibility to improve integrity performance in ours.
Technically, well integrity depends on the collective performance of multiple ‘barriers’ that are designed to ensure that the right well fluids consistently travel to and from the right places – without compromise. Such barriers include steel tubulars, cement, valves, wellheads and a plethora of other components that form and protect precise fluid flowpaths.
To achieve this, well barriers need to have two fundamental properties – they need to be in good mechanical ‘condition’, and they need to ‘seal’. Surprisingly, despite the intimate relationship between these properties, satisfying one doesn’t imply that the other is also satisfied. A barrier may appear to be in good condition, but may not be sealing, or a barrier may be in poor condition yet still be sealing. The latter condition may be temporarily fortuitous, but poor barrier condition will eventually lead to seal failure.
This strangely inconsistent relationship between condition and seal becomes more pronounced when we try to evaluate each properly using industry-accepted diagnostic methods.
Take acoustic ‘cement bond’ or imaging techniques – used for decades to infer the success of a ‘cement job’ and annulus integrity. Advanced ultrasound scanning systems can reliably assess the strength and annular extent of the cement but cannot confirm if the cement is sealing. Similarly, multifingered calipers provide an accurate means of measuring the internal diameter of ‘primary tubulars’ and is often used to infer ‘wall thickness’ and condition – but this assumes that the outside surface diameter is ‘nominal’ or unscathed, and this may not be the case. Other examples of imperfect diagnostic techniques include using stand-alone temperature measurements or ‘spinners’ to locate primary tubular leaks. These work in certain circumstances, but not all.
These factors point to the need to follow three key surveillance principles in the quest for effective integrity management: firstly, evaluate both the condition and sealing performance of critical well barriers; secondly, monitor these properties routinely and proactively; finally, use the right diagnostic system for the task.
One area where these principles particularly apply is in the surveillance of ‘tubular condition’ – sometimes referred to as ‘tube integrity’ or ‘corrosion’ monitoring.
The most prolific barriers in the well system and the backbone of its integrity are the steel tubulars, casings and cement that fuse together with the subsurface to form the well system. Often operating in extreme conditions, tubulars must be in good condition to ensure containment and provide mechanical support. But many factors conspire against the integrity of the tubes making integrity management extremely challenging.
The first is accessibility. Most well tubulars exist behind the primary tubing, making them difficult to access for remediation. The second is surveillance. The hidden nature of outer casings means that corrosion and even failure is particularly insidious and challenging to detect. The third is that whilst steel barriers are strong, they are vulnerable to one fundamental weakness – they can corrode. The fourth is the environment within which tubulars need to perform. Heat, pressure, mechanical loading, aquifers and corrosive fluids such as carbon dioxide, hydrogen sulphide (H2S) and brine play havoc with steel. Lastly, the fifth factor is time. Well systems are expected to endure extreme conditions for decades.
The Middle East has more than its fair share of corrosion challenges. Amplifying the factors listed above are region-wide aquifers, such as the Umm er Radhuma, which inevitably corrode well systems from the outside, and high-H2S production, which can corrode from the inside. Operators in the region use several methods to combat this, including chrome completions that are resistant to attack. Some Middle East operators are also leading the way in adopting at least two of the surveillance principles above – namely, implementing proactive tube integrity surveillance, and using the most appropriate diagnostic system for the task.
TGT is a specialist in ‘through-barrier diagnostics’ for the oil and gas industry and is at the forefront of tube integrity surveillance. The company has designed and engineered a diagnostic system which harnesses electromagnetic [EM] energy to assess the condition of multiple tubulars from within the production tubing. This in-house design combines features with advanced 4D modelling to deliver impressive accuracy across multiple barriers.
Notably, the system also works in chrome completions – prolific in the Middle East and a hostile environment for ordinary EM devices. The system is being used across the region to evaluate existing corrosion issues, and proactively to track and predict tube failures before they happen.
Maintaining well system integrity is a persistent challenge, but with the correct approach in evaluating barrier condition and seal proactively, and by using reliable diagnostic systems, operators can maintain well performance – without compromise.