Exploration and Production Technologies Advanced Drilling - Advanced Tools & Methods - Coiled Tubing
Up until the late 1980s, coiled tubing (CT) was used primarily onshore in well production services that included well cleaning to improve flow, isolating zones, stimulating and fracturing, sand control completions, and plug-and-abandonment activities.
With a number of technological advances that include larger and stronger tubing, better rig controls and injectors, and improved bottomhole assemblies, CT drilling is becoming an attractive alternative to drilling with rotary drilling rigs and jointed pipe.
CT drilling uses continuous pipe stored on a reel at the surface, combined with downhole mud motors to drill faster, more cost-effective wellbores than one can with conventional rotary drilling. Since CT is a continuous length of ductile steel tubing, it eliminates having 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.
CT does not apply to all drilling scenarios, but with proper planning and reservoir analysis, it can be highly effective in multi-well environments. CT is one application that makes possible slimhole drilling (wellbores and related casing of less than 6 inches in diameter) and even microhole drilling (ultrasmall-diameter boreholes with 4½-inch-diameter casing or less).
Additional advantages of CT are that it provides for safer drilling, reduces formation damage, minimizes pressure surges, allows for improved well control, and has a smaller environmental footprint. CT drilling can be an effective way to implement underbalanced drilling and can reduce wellbore skin damage. DOE has funded a project to enhance the commercialization of underbalanced drilling for domestic gas fields. Both equipment and software program development are included in this research.
CT also is an enabling technology for NETL's Microhole Technology (MHT) program. Under that program, a group of NETL-funded research projects is developing an infrastructure for CT drilling of microholes.
MHT focuses on using small, rapidly deployed, easily transportable CT drilling rigs combined with miniaturized downhole instrumentation to drill wells more cheaply, quickly, and with less environmental impact. This can be accomplished by allowing operators to enter and exit wells quickly; use smaller drilling equipment; drill with faster penetration times—up to 200 feet per hour; mobilize and demobilize rigs quickly; and operate with fewer people than with a rotary drilling rig, thereby reducing labor costs.
CT drilling is particularly well-suited to drilling microholes. CT-drilled microhole advantages include:
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.
Drilling sidetracks and laterals from existing wellbores increases wellbore contact with the reservoir and increases production.
Although CT tubing grassroots drilling—as opposed to well-servicing operations—has flourished in Canada, that has not been the case in the Lower 48 states. In the United States, there are 257 CT rigs in operation. Ten rigs are in constant use in Alaska, where CT technology is used to offset the North Slope's natural oil production decline by drilling lateral extensions of 2¾-4⅛ inches diameter out of horizontal wells at half the cost of rotary drilling. CT drilling is well-suited to arctic environments: faster mobilization and demobilization times and penetration rates accommodate the short drilling seasons and lighter equipment, and smaller footprints minimize impact on fragile arctic tundra.
There is currently one hybrid rig (rotary plus CT) working in a shallow gas play in western Kansas. It is the first hybrid CT drilling rig built and operated in the United States and is associated with a NETL-funded project.
Drilling experience with that rig was found to decrease the cost of drilling wells by up to 38%. The rig successfully drilled 124 gas wells in 7 months, totaling 300,000 feet of borehole.
Much of the DOE-sponsored CT drilling research in the 1990s and early 2000s focused on such field demonstrations and other information to encourage independent operators to apply the technology to their domestic fields. Current projects are formulating and testing new ideas and technologies in a number of areas, such as real-time CT flaw detection that can extend the life of the tubing and development of CT-deployed spallation technology to produce large cavities at the bottom of deep vertical bores for gas storage.
Research in the metallurgy and treatment of the tubing indicates that, with further development, CT can be used in deeper wells, withstand greater lateral stresses, and last longer. A current NETL project is working toward that end. Researchers are using microwave technology to produce CT that has twice the yield strength, which can extend tubing life and operating depth.
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