Friday, November 2, 2018

Coiled Tubing Takes Center Stage

When it comes to coiled tubing, there can be few doubters left. What was once a fringe service has moved to center stage in the oilfield theater of operations. 

For many years, coiled tubing (CT) operations occupied the twilight zone of a fringe service offering niche solutions to specialized problems. However, over the past five years, technological developments, improved service reliability, gradually increasing tubing diameter and an ever growing need to drive down industry costs have combined to dramatically expand the uses of coiled tubing.

Today for example, coiled tubing drills slimhole wells, deploys reeled completions, logs high-angle boreholes and delivers sophisticated treatment fluid downhole. This article will look at the technical challenges presented by these services and discuss how they have been overcome in the field. 

 Drilling Slimhole Wells

Slimhole wells - generally those with a final diameter of 5 inches or less - have the potential to deliver cost-effective solutions to many financial and environmental problems, cutting the amount of consumables needed to complete a well and producing less waste. Other benefits depend on what kind of rig drills the well. Compared to conventional rigs, purpose-designed smaller rotary rigs can deliver slimhole wells using fewer people on a much smaller drillsite, which cuts the cost of site preparation and significantly reduces the environmental impact of onshore drilling.

Coiled tubing drilling combines the virtues of a small rig with some unique operational advantages, including the capability to run the slim coiled tubing drillstring through existing completions to drill new sections below. There is also the opportunity to harness a coiled tubing unit's built-in well control equipment to improve safety when drilling potential high-pressure gas zones. This allows safe underbalanced drilling- when the well may flow during drilling.

Although there were attempts at CT drilling in the mid-1970s, technological advances were needed to make it viable. These include the development of larger diameter, high-strength, reliable tubing, and the introduction of smaller diameter positive displacement downhole motors, orienting tools, surveying systems and fixed cutter bits. Furthermore, currently available coiled tubing engineering software enables important parameters to be predicted, such as lock up -when tubing buckling halts drilling progress- available weight on bit, expected pump pressure, wellbore hydraulics and wellbore cleaning capability.

Through-tubing reentry in underbalanced conditions is a category of CT drilling that may grow significantly. Reentering wells without pulling the production string is a cost-effective way of sidetracking or deepening existing well.

The development of through-tubing, reentry underbalanced drilling is of great interest in the Prudhoe Bay field on the North Slope of Alaska, USA, where operator ARCO Alaska Inc. has an alliance with Dowell to develop coiled tubing technology. The alliance has already scored a number of technical and commercial successes. For example, a 600-ft horizontal section extended using underbalanced CT drilling, resulted in production three times greater than predicted rates.

As with any mature operation, there is a need to extend field life and gain incremental reserves at a cost that reflects today's oil price. While the primary aim is to devise a strategy for for low-cost well redevelopment, a secondary aim is to improve the productivity of horizontal wells by reducing formation damage associated with conventional overbalanced drilling.

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