Two different air drilling methods use air as the primary drilling fluid – direct rotary air and down-the-hole air hammer. In conventional reverse circulation methods, air is used as an assist but not as the primary drilling fluid. In the air rotary method, air alone lifts the cuttings from the borehole. A large compressor provides air that is piped to the swivel hose connected to the top of the kelly or drill pipe. The air, forced down the drill pipe, escapes through small ports at the bottom of the drill bit, thereby lifting the cuttings and cooling the bit. The cuttings are blown out the top of the hole and collect at the surface around the borehole. Injecting a small volume of water or surfactant and water (foam) into the air system controls dust and lowers the temperature of the air so that the swivel is cooled.
Air drilling can be done only in semi consolidated or consolidated materials. Therefore, to achieve the capability to operate in completely unconsolidated as well as consolidated formations, air rotary drilling machines are often equipped with a mud pump in addition to a high-capacity air compressor. Conventional water-based drilling fluids are then used when drilling through the overlying, caving formations above the bedrock (or more consolidated formations), whereas air is used once bedrock has been reached. Thus drillers are utilizing various options of drilling technology to adjust to the different physical characteristics of the formation. In many instances, casing may have to be installed through the overburden to avoid caving or excessive erosion of the borehole wall after changing to air circulation.
Cuttings are removed by grinding the material finely enough so that the uphole velocity of the air is sufficient to lift them to the surface. The lifting capacity of the air can be enhanced by adding a small amount of surfactant and water solution to the air. Larger cuttings can then be removed, thereby increasing the drilling rate. Foam also reduces loss of air to the formation.
Roller-type rock bits, similar to those designed for drilling with water-based fluids, can be used when drilling with air. Tricone rock bits up to about 12 inches diameter are commonly used. Larger sizes are available. Button bits, made with sintered tungsten-carbide inserts set into the perimeters of steel rollers, are used successfully in many areas.
Field tests with various sizes of bits have shown that penetration rate is often faster and the bit life longer when using air as compared with water-based drilling fluids. Better bottom-hole cleaning is partly responsible for this difference in performance. If too much water comes into the hole during drilling, however, the penetration rate is no better than when drilling with water-based drilling fluids. Air also keeps the bit bearings cool and clean and causes some oxidation of the bearings; the oxidized material then becomes a lubricant. On the other hand, water-based drilling fluids are often abrasive and cause wear on the bearings.
A second direct rotary method using air is called the “down-the-hole” drilling system. A pneumatic drill operated at the end of the drill pipe rapidly strikes the rock while the drill pipe is slowly rotated. The percussion effect is similar to the blows delivered by a cable tool bit. The hammer is constructed from alloy steel with heavy tungsten-carbide inserts that provide the cutting or chipping surfaces. Tungsten-carbide is extremely resistant to abrasion, but drill bits do become dull with continued use. The inserts are sharpened by grinding when operating conditions indicate that the bit is not cutting properly. Alternatively, the bits can be provided with carbide buttons that can be periodically replaced when worn.
Rotation of the bit helps to assure even penetration and, therefore, straighter holes even in extremely abrasive or resistant rock types. The rates of penetration in several rock types are higher than those obtained by other drilling methods or other types of tools. 6-in inch and 6 ½-in hammer bits are most commonly used, although sizes range up to 17 1/2-in. Cuttings are removed continuously by the air used to drive the hammer. Unlike the conventional cable tool bit that is constantly striking previously broken rock fragments, the bit (or buttons) on the air hammer always strike a clean surface. Thus, the air hammer is highly efficient.
Compressed air must be supplied to the hammer at a pressure of 100 to 110 psi. Some tools require as much as 200 psi. To remove cuttings effectively, the upward velocity in the space outside the drill pipe should be about 3,000 ft/min or more. For drilling 4-in holes, the air supply must be at least 100 cfm. For 6-in holes, at least 330 cfm is needed. Proper rotation speed is from 10 to 30 rpm, reduced speed is best in harder and more abrasive rock.
Advantages of using air drilling methods include the following:
Disadvantages include the following: