Coiled up in tubing

Tubing has been a vital component of down-hole activity for several decades, as one of the main means of carrying out intervention operations from reservoir assessment, exploration well drilling and identifying problems during the production processes. Coiled tubing, at present is one of the fastest growing completion technologies in the well service industry, with new tubing tools and improved metallurgy helping to increase the reliability and the availability of a wide range of sizes of coiled tubing.

Up until the late 1980s, coiled tubing was primarily used onshore in well production services with applications including: well cleaning to improve flow; isolating zones; stimulating and fracturing; sand control completions; and plug-and-abandonment activities. But thanks to a number of technological advances that include larger and stronger tubing, better rig controls and injectors, and improved bottom-hole assemblies, coiled tubing drilling has become an attractive alternative to drilling with rotary drilling rigs and jointed pipe. Coiled tube drilling uses continuous pipe stored on a reel at the surface, combined with downhole mud motors to drill faster, more cost-effective well bores than is the case with conventional rotary drilling. Since it is a continuous length of ductile steel tubing, it eliminates the need to connect and disconnect threaded sections of pipe when going into and coming out of the well. The tubing can be uncoiled into the well and returned back to the reel 50 times or more before metal fatigue forces retirement.

This was previously viewed as a high risk practice only suitable for niche operations, but advances in tubing technology mean that many of the new systems for drilling, completing or remediating a well are today conveyed by coiled tubing. For example, Baker Hughes Oil Tools has reported a 200% growth in coiled tubing activity over the past few years where much of the new equipment is introducing in the United State's Permian Basin are now conveyed through coil tubing.

Coiled tubing has several applications from well cleaning to fishing and milling, isolating or stimulating and fracturing production zones, completing wells with sand control equipment, gas and water flow management, plugging and abandoning a well or side-tracking and re-entering a well. As Baker Hughes notes, using coiled tubing means that the well in question does not have to be killed, there is no skin damage to the existing casing because it avoids the need for heavy well-killing fluids and the operator does not lose production from the well. Its use can also eliminate the need for a rig and can be used for several functions down hole from cutting pipe to performing zone isolation operations.

The US Department of Energy's National Energy Technology Laboratory (NETL) believes coiled tubing will play a major role in future new drilling activity. In December 2006, it successfully demonstrated a specially designed hybrid "micro-hole" coiled tubing rig that drilled 25 test wells to penetrate a particularly intractable natural gas formation, called Niobrara, in western Kansas and eastern Colorado. The test delivered cost savings of 25% to 35% per well drilled, compared with conventional drilling equipment. As a result, about 1 trillion cubic feet of shallow gas that had been bypassed by conventional drilling was made economic - equal to around 5% of US annual natural gas consumption.

While coiled tubing rigs are used frequently to service or stimulate production in problematic oil and natural gas wells, operators have only recently begun drilling more ‘grassroots' exploratory and development wells with them. And that effort has largely been limited to higher-cost operating areas such as Alaska and Canada. According to the NETL, although coiled tubing does not apply to all drilling scenarios, with proper planning and reservoir analysis, it can be highly effective in multi-well environments. It also makes possible slim-hole drilling (well bores and related casing of less than 6 inches in diameter) and even micro-hole drilling (ultra small-diameter boreholes with 4½-inch-diameter casing or less).

Coiled tube drilling is particularly well-suited to drilling micro-holes and has several advantages including: drilling with one third the space and one third the number of equipment loads when compared with rotary drilling; drilling smaller hole sizes reduces cuttings and drilling fluids and waste-disposal costs; drilling low-cost wells enables dedicated monitoring wells to be placed at optimum locations without disrupting production; and drilling sidetracks and laterals from existing well bores increases well bore contact with the reservoir and increases production. The NETL also notes that research in the metallurgy and treatment of the tubing indicates that, with further development, coiled tube can be used in deeper wells, withstand greater lateral stresses, and last longer.