ADVANCES IN COILED TUBING

What’s new in coiled tubing?

Multistage fracs on coiled tubing

NCS Energy Services has developed Multistage Unlimited technology as a more efficient, more economical alternative to plug-and-perf and ball-sleeve frac systems, both of which bullhead fluids down the casing, with no feedback about formation response at the frac zone, no recourse in the event of a screen-out, and no way to manage water and chemical consumption. Both methods do, however, have a big appetite for pumping horsepower.

The coiled tubing (CT) solution. In developing Multistage Unlimited technology, NCS faced two challenges:  1) Overcome the limitations of one-way fluid movement; and 2) Find a more efficient way to isolate each stage during completion. NCS overcame both challenges by adopting CT to operate the frac-isolation system, and to open the company’s grip/shift sliding sleeves.

The CT work string provides a circulation path to the frac zone and also allows the use of a resettable frac plug that can be repositioned to isolate each stage for stimulation. Current CT capabilities are more than adequate to handle multi-stage completion operations. Already, more than 7,500 mi of CT runs have been executed to complete more than 32,000 wells.

Circulation vs. one-way fluids. With the ability to circulate, or reverse circulate, during completions, Multistage Unlimited systems offer more operating control:

• Acid and other leading-edge fluids can be circulated down to the isolated zone before frac pressure is applied, with returns back up the CT.
• Screen-outs are less likely and can be quickly removed, allowing more aggressive sand loading.
• The CT is open during fracs, to monitor formation response at the target zone in real time, allowing sand placement to be optimized.
• Sand-jet perforating can be used to access the formation, to add stages in blank casing when running the Multistage Unlimited sliding sleeve system.

Coiled-tubing-operated resettable frac plug. The system’s resettable frac plug eliminates the need for composite frac plugs, which have to be run and set at every stage and drilled out before the well is put into production, and ball-seat sliding sleeves that cause real problems, when they don’t shift and also restrict the wellbore. With this resettable plug, there is nothing to run in the well between fracs, nothing left in the well, and nothing to drill out. With it, most multi-stage completions can be performed in a continuous, one-trip operation.

Sliding sleeves or sand-jet perforating. In both cemented and open-hole completions, Multistage Unlimited systems use either grip/shift sliding sleeves or sand-jet perforating to gain access to the formation. The sand-jet perforating sub is run as part of all of the system’s frac-isolation assemblies. For completions without sliding sleeves, perforating is used for all stages. The sand-jet sub can be also used to add stages in blank casing, without tripping out of the hole.

The grip/shift sliding sleeves are shifted by setting the frac plug in the inner sleeve, then applying pressure above the seal. The inner sleeve shifts down to open frac ports at the top of the outer sleeve, and the frac is pumped down the coiled tubing annulus. More than 7,600 Grip/Shift sleeves have been installed to date.

For sand-jet perforating, an abrasive slurry is pumped down the coiled tubing, and through the sand-jet sub, cuts perforations through the casing and cement, and into the formation. After perforating, the frac is also pumped down the coiled tubing annulus.

Other CT-operated systems from NCS Energy Services include a straddle-type ReNew refrac system for restimulating underperforming zones.

While unconventional plays worldwide have created a boom in E&P activity, these resource developments present numerous challenges for the CT service industry.

With Global Tubing’s new SMARTaper technology, customers have an opportunity to improve tube performance while conducting CT operations in horizontal and extended reach wells. CT strings are custom-manufactured by joining flat steel strips together, end to end, and then milling the entire length into a continuous length of tube, often exceeding 20,000 ft. During its useful life, the tubing is potentially spooled hundreds of times at different well sites. Plastic bending fatigue from being spooled consumes some “life” each time the tube enters a well. The steel coil end welds are typically de-rated and tracked in the fatigue models.

Maximizing performance at strip-end welds. To minimize well site risk on traditional CT, oil companies and CT operators force early string retirement, due to fatigue spikes at strip-end welds. The residual tubing fatigue life between the welds is typically never utilized. SMARTaper is a patent-pending design that increases the performance of strip end welds by making the wall thickness heavier at each weld. The end result is maximized performance at strip-end welds, enhanced utilization, and engineered tubing design to match the most demanding applications.

Sizes and tube grades are available to handle the higher pressures and demands of extended-reach horizontal wells.