Technology from Europe: Casing inspection tool monitors corrosion without interrupting production

Paul Crouzen and Diederick Bax, Shell Global Solutions;
Trevor Rea and Ian Taylor, Shell UK E&P

The North Sea is a mature basin with a well-established infrastructure where significant investment is necessary to maximize the economic recovery of oil and gas. A crucial part of this investment is to extend the lives of the offshore assets and onshore plants, in many cases to ensure they can operate safely and effectively for longer than originally envisaged in the 1970s.

Among the extensive assets and facilities that must be maintained for safety, protection of the environment and continuing production are the hundreds of wells across the North Sea. Like any other structure or equipment that is exposed to a harsh environment, they gradually deteriorate; for example, the wellhead support casings can corrode, which, in a worst-case scenario, can lead to structural failure. In this context, Shell Global Solutions has worked closely with Shell UK’s upstream business over the last few years on an inspection campaign to assess the integrity of most of its wells.

THE TECHNICAL CHALLENGE

In-situ well casing inspection was a major technical challenge, as corrosion in offshore wells occurs predominantly at sea level between two concentric, vertical tubular conduits: the surface casing and the outer conductor. These are vital well components: The conductor protects the riser against the lateral forces caused by marine currents and waves, and the surface casing supports the load of the wellhead, the subsea tree and the inner casing strings. The space between the surface casing and the conductor is called the D annulus and is usually filled with brine or seawater. In this annular space, the interface between the water and the air is the area most prone to corrosion. Severe corrosion may weaken the surface casing, so that it can no longer carry its design load and collapses. The additional load transfer to the conductor may cause it to subside, since it is not designed to take this weight. Moreover, the well’s flowlines may rupture, resulting in a gas release when the wellhead subsides.

Access to this region for inspection is extremely difficult, especially using probes, while active production operations are ongoing. Until the early 2000s, there was no system to evaluate corrosion and wall thickness reductions in narrow well annuli. Shell UK E&P considered several methods of inspection: ultrasonic, radiographic, electromagnetic volumetric and pulsed eddy current (PEC). PEC inspection technology was the method chosen, as all the others had limitations for well applications.

D-PEC TECHNOLOGY

It was clear that the work scope for casing inspection required a tool that could do the job with minimal disruption to production and platform operations, and at a reasonable cost, given the number of wells to be surveyed.

PEC technology is an established option for corrosion detection on exposed pipelines and other structures. It uses a pulsed magnetic field to induce eddy currents in the steel being examined. PEC technology determines the wall thickness of the pipe by sensing the decay of the eddy currents, Fig. 1. Inspection engineers can apply the technique even when pipes are covered with dirt, scale, deposits or corrosion product.

Fig. 1. Examples of PEC data (a) from a surface casing with no wall loss and (b) from a severely corroded surface casing, plotted as a function of distance from the top of the conductor. The highest and lowest astronomical tides (HAT and LAT) are indicated by dashed lines.

Working with Shell Global Solutions, Shell UK E&P developed the D-PEC well inspection method, which is based on PEC technology and enables operators to assess the D annulus for corrosion without interrupting production. Adapting PEC technology for well inspection involved miniaturizing the tool probes, developing a technique to control the position of the tool and determining how to control the tool during blind deployment in a fluid-filled annulus.

Conventional methods of well corrosion inspection encounter problems when the steel surface is rough because of corrosion products and/or salt scale deposits. This is not the case for PEC technology, which can measure wall thickness through deposits without contact between the sensor and the steel surface. PEC technology uses a strong electrical current to induce a signal in the steel from up to 50 mm away. The decay time of this signal is used to determine the thickness of the remaining steel.

The PEC tool designed for deployment in the D annulus more than meets the requirements for the casing inspection. A two-man crew performs a PEC survey using a portable unit, and the conductor annulus is relatively easily accessed, Fig. 2. Surveys can be done within six hours, and the results can be screened for validity on site. Repeat surveys can be performed if there is any uncertainty about the results, but so far this has been unnecessary, as the data quality has always been high. The well remains in production at all times.

Fig. 2. Inspection of the D annulus using the specialized PEC tool.

PEC technology provides accurate information on reduced wall thickness, even when the equipment is not perfectly aligned with the side of the well. As with any non-destructive method, it was important to validate the PEC inspection results. In some fields, surface casings were retrieved to surface during abandonment to enable wall thickness measurements using ultrasound for comparison with the PEC results. The PEC measurements recorded offshore matched these “post-mortem” ultrasound measurements to within ±10% for the surface casings and ±15% for the conductors (two standard deviations).

RESULTS

The inspection campaign identified clearly where remedial action was necessary and where it was not. The wall losses detected on corroded surface casings were generally concentrated near the fluid interfaces within the D annulus. In most cases, this corresponded to the region between the highest and lowest astronomical tide. The corrosion was evident over a band 0.6-to 3-m deep and was mostly uniform around the circumference.

The survey indicated that it was impossible to predict which wells were at risk from corrosion and which were not. Corrosion did not seem to correlate directly with the age of the well, or with other factors such as operating temperature or communication with seawater.

MITIGATION MEASURES

There are various ways to manage the threat of oxygen corrosion to surface casing, such as by inhibiting the effects of the brine in the D annulus or increasing the level of grouting in the D annulus from just below the mudline to the top of the conductor.

Topping up the D annulus with a biocide-inhibited rapeseed oil is another countermeasure. The rapeseed oil floats on the seawater and coats the external surface of the surface casing and the internal surface of the conductor to limit or prevent further corrosion.

In addition, controls have been put in place to monitor the variation in the wellhead height and to ensure that a positive pressure is maintained in all the well annuli, so as to have an early warning of any potential structural failure.

C ANNULUS ALTERNATIVE

In addition to the casing and the conductor, the internal guides or centralizers within the D annulus are also subject to corrosion. On occasion, these can become severely corroded to the extent of being broken as a result of varying lateral loads. The broken components can pile on top of each other to block the D annulus.

The inspection team found that, in some cases, the D annulus was not easily accessible. This lack of D annulus access prompted the team to develop a modified PEC inspection system that could access the C annulus as an alternative and still be capable of accessing the surface casing. In this case, a probe is inserted, via a lubricator, through the opened C-annulus valve and, thereby, into the generally narrower C annulus. To complicate matters, this C-annulus inspection equipment not only needed to gravity-feed past obstructions but also to work safely in the hydrocarbon-containing environment within the C annulus.

The original PEC sensor was therefore drastically miniaturized to make it orientation sensitive and intrinsically safe. Furthermore, a method was devised to deal with obstructions and position the sensor on the steel surface. To this end, an umbrella-like construction was designed. In the closed state, the sensor is lowered into the C-annular space and past any obstruction down to the desired elevation. It is then opened out at the desired position. Measurements are taken, and the sensor is subsequently hauled up to other positions of interest.

The ultra-slim device comprises a 500-mm-long chain connected to a 50-m-long umbilical, which is inserted as a unit into the well via a lubricator system. The chain contains two miniature PEC probes and a mechanical positioning system (“pushers” and ”whiskers”). The chain design enables the PEC probe links to follow the 90° turn into the C annulus. The positioning system opening mechanism is triggered by a miniature chemical timer.

The C-PEC probe offers a good alternative to the D-annulus probe when the D annulus is blocked or when a field operator does not want to drill lateral access holes in the wall of a conductor whose top is in contact with the wellhead. The C-annulus probe also provides bonus information on the integrity of the intermediate casing as a result of using parallel positioning probes.

The C-annulus PEC method uses two probes (in parallel) and can therefore simultaneously inspect the surface casing and the intermediate casing. Using this probe is a more complex operation because of the need to enter a pressurized annulus. Although, in practice, the C annulus is bled to zero pressure when carrying out the inspection, there is still a requirement to have full pressure control equipment to maintain the pressure envelope. Lubricator crews are required to connect a specially designed (bespoke, manually handled) lubricator to the C-annulus side-arm valve, Fig. 3.

Fig. 3. PEC inspection via the C annulus.

The C-annulus PEC probe made it possible to inspect wells that could otherwise not be inspected through the D annulus. Shell E&P UK completed a successful D-annulus inspection campaign on more than 340 wells in the northern North Sea in February 2008. The C-annulus probe tool was developed further during 2008, and additional C-annulus PEC measurements were taken in 2009.

CONCLUSIONS

The ability for production field operators to proactively and easily check for casing corrosion without interrupting production offers significant commercial benefits. Advanced knowledge of well integrity enables better planning of production deferment and more accurate production forecasting.

Benefits may include saved production and avoidance of unnecessary, costly remediation on non-corroded wells. Regular health checks of the corrosion status of wells will also help operators to plan more targeted and cost-effective workover programs and to focus on those wells that require urgent attention and remedial action. Besides the substantial potential cost savings, there is considerable reduction of health, safety and environmental risks.