High-pressure well intervention can pose a real challenge for both job design and pressure control.

Henrik Aslak Larsen, larsen6@sugar-land.oilfield.slb.com, is a senior engineer at Schlumberger in Sugar Land, Texas.

In less than a decade, well intervention with high-pressure coiled tubing (HPCT) has emerged as the dominant intervention technology for live wells under high pressure. In the past several years, the use of HPCT has pushed into ever deeper waters and into the new frontier of containing wellhead pressures above 10,000 psi. However, successful HPCT intervention is not merely an issue of containing increased wellhead pressure.

Schlumberger performed the industry's first HPCT well intervention in 1993 on a well in South Texas. The technology moved offshore into the Gulf of Mexico (GOM) in 1995 and now is in use worldwide. For onshore operations, the needed equipment can be transported to the site on three trucks. To transport offshore equipment requires one large supply vessel. Occasionally, offshore equipment is used onshore, if other equipment is unavailable.

How high is high-pressure?

The four categories of HPCT interventions are:
• Category I - up to 3,500 psi;
• Category II - 3,500-7,000 psi;
• Category III - 7,000-10,000 psi; and
• Category IV - 10,000-12,500 psi.

These high pressures require the most stringent safety measures. For Category III and IV operations, equipment rated to 15,000 psi is used exclusively to reduce risk and maintain appropriate safety levels.
An increasing necessity is the ability to engineer total job design, accomplished via specially designed software systems that not only measure progress downhole, but also calculate the expected forces, pressures and other factors affecting tubing and equipment. Knowing this, each job can be optimized for its precise downhole environment.

Engineering job design can mitigate risks and assure that project objectives are met cost-effectively. The safety and financial impacts of an incident can be very high. Not only in deep water, where daily rig costs run as high as US $250,000, but elsewhere, it is critical to avoid on-site corrections due to factors such as downhole pressures, loads, corrosion or the presence of poisonous gas that were not accurately quantified prior to the job.

The use of high-technology design and planning tools to engineer HPCT interventions, combined with the experience gained from performing many such jobs, has made it possible to handle interventions that were inconceivable less than 10 years ago.

New HPCT intervention software

The major software technologies that have driven the advance of HPCT interventions include:

• design tools that calculate tubing forces and expected pressures;
• data acquisition systems that monitor and record low-cycle tubing fatigue and tubing ovality;
• monitoring tools that can measure tubing diameter within an accuracy of 0.001 in. and changes in ovality as tubing is run in and out of the well;
• models that translate fatigue data into accurate percentage calculations of expired life; and
• analytical modules that determine safety margins and job limits, as well as monitor jobs in real time and generate alarms if working limits are approached.

An essential follow-up to each job is the evaluation of tubing records to determine if coiled tubing strings can be reused. Drawing upon its database and nearly a decade of experience, Schlumberger has established maximum limits for tubing fatigue life, ovality, ballooning and necking.

Today, HPCT technology has all but replaced the use of snubbing units as the preferred service for high-pressure, live well interventions. With no need for costlier jointed tubulars, time-consuming pipe connections or to station personnel atop the well, the new technology offers substantial safety and economic advantages.

One-third the time, half the cost

To give just one example, a scale clean out was performed a few years ago with a snubbing unit for an operator in the GOM over a period of 30 days at a total cost of $1.1 million. A year later on the same well, Schlumberger performed a similar service with HPCT and achieved equivalent results in only 11 days at a total cost of $600,000.

As the database on HPCT interventions expands and software solutions continue to evolve, intervention capabilities should likewise improve to meet increasing challenges posed by the highest pressure levels, environmental factors and job complexities.

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