Long Turn Radius Drilling
A long turn radius well is a well having at least one section with a build rate of between 2 ° and 6° per 100 ft, and a build radius of 1000 ft or more. Long radius applications include:
- Drilling multiple, extended-reach wells from offshore platforms or other single surface locations.
- Reaching otherwise inaccessible locations.
- Drilling exploratory wells over long intervals.
- Drilling wells that require zone isolation and selective completion/stimulation.
The advantages of long radius drilling over other horizontal drilling methods include minimal dog-leg severity, attainment of horizontal departure while drilling the build section, and the ability to employ either conventional rotary bottomhole assemblies or steerable drilling systems. In addition, long radius methods impose no restrictions on hole diameter, bit type, coring LWD MWD capabilities, and they permit various options with respect to completion, stimulation and artificial lift.
Long-radius drilling also has some limitations, including high torque-and-drag tendencies, and greater difficulty in hole cleaning.
A limiting factor in designing a long radius well is the ability to rotate the pipe at the surface as the bit approaches total depth.
There may also be constraints imposed by shallow pay zones, lease boundaries or other restrictions.
The use of conventional rotary assemblies to drill long radius wells is quite common. There are a number of software packages available for optimizing and predicting BHA operating parameters. The BHA designs that are generated can prove very successful, assuming that accurate information on formation anomalies is available. If this information is not available, then frequent trips may be required to correct the well course, resulting in high drilling costs. In such cases, it may be more economical to use a steerable drilling system.
A steerable, or navigation drilling system, is a bottomhole assembly that can be controlled from the surface while it is in the hole. Thus, it is possible to direct such a system along a pre-selected well trajectory without having to trip it out of the well. Drilling cost saving, along with reduced dog-leg severity problems, result from the steerable system’s ability to build, drop, turn or hold angle, continually monitor well trajectory and control drilling parameters from the surface.
The cost effectiveness of a steerable system depends, of course, on effective well planning and on the system’s proper design and operation.
The tools that make up a steerable system generally consist of a bit, a bent housing positive displacement motor (PDM), bent subs, measurement-while-drilling tools, drill collars and stabilizers. The system can operate in an oriented mode, where drill bit rotation is induced only by the PDM, or a rotary mode, where the main source of bit rotation is still the PDM, but the drill string is rotated as well to negate deviation tendencies.
