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Abstract The development of a new motor designed specifically for use in holes drilled with air and mist is chronicled, from initial use of modified mud motors in air drilling applications, through development to date of an air drilling motor currently in use.
Until recently, air drilling was accomplished using either an air hammer or, for a more directionally controlled well, a Moineau type positive displacement mud motor (PDM) adapted to an air drilling environment. In such cases, bottom hole assembly performance proved heavily dependent on motor life and lithology type. Greatly reduced stator life and accelerated damage to bearings designed for mud lubrication meant that conventional motor runs on air usually were shorter than with mud systems. Ultimately, conventional PDMs exhibit many economic limitations when used on air for directional drilling.
The new air drilling motor, which features a Moineau motor with a novel geometrical design of the power section, an adjustable kick off sub and a modified bearing assembly, represents the latest step in the continuing development of a downhole motor suitable for directional and horizontal drilling using air or mist. This design has been successful in US air drilling applications, which are detailed in case histories.
Introduction Since the first air-drilled well was drilled in Utah in 1953, air has been used to drill oil and gas wells around the world. Air drilling techniques are now routine in the Appalachian Mountain region of the eastern US, the Arkansas oil and gas fields, the San Juan Basin of New Mexico, and in some West Texas oil and gas fields. In drilling areas where lost circulation problems are prevalent, such as the Rocky Mountain region, problems are prevalent, such as the Rocky Mountain region, air and gas (natural gas) drilling has been used extensively, while air drilling techniques have been used in some offshore operations to drill through potential lost circulation formations. With the advent and continued improvement of horizontal drilling technology, air/gas drilling in enhanced recovery operations where the geology is well known and the potential producing formations are of rather low pressure represents an producing formations are of rather low pressure represents an excellent joint venture of the two technologies.
SPECIAL CONSIDERATIONS Currently, successful application of air drilling techniques is determined by formation hardness, the available capacity and pressure rating of the equipment and the water influx from pressure rating of the equipment and the water influx from drilled formations. For example, soft formation cuttings are usually too large to lift with air at applicable depths, and are not usually suitable for air drilling. On the other hand, hard formation cuttings can be lifted with an adequate volume of air.
Water influx is also a significant factor. Influx from drilled formations reduces the hole-cleaning efficiency when "dusting" or drilling only with air. The addition of soaps and other chemicals through mist injection into the drill string, called "misting", allows air drilling to continue even with significant water influx.
In addition, mud motors have been identified as having too many economic limitations when used on air. Mud motors are designed to be run using drilling mud as the power source, for lubrication and for heat (friction) dissipation. The significant differences between drilling mud and air have led to problems in the application of conventional mud motors when using air.
Because air is compressible, flow rate changes with pressure. Also, because of its much lower lifting capacity, air requires annular velocities much greater than drilling mud. However, the higher air volumes exceed the recommended flow rates for mud motors, often causing premature failure. Typically, the air volume required to clean the hole is three times greater than the recommended flow rate for the conventional mud motor.
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