Air Rotary
The direct air rotary drilling method involves a similar rig setup as mud rotary drilling, except that the circulated fluid is compressed air instead of drilling mud. The carrying capacity (viscosity) of the compressed air is improved by adding water mist, surfactant (soap), and polymer.
Direct air rotary drilling rigs can be equipped with any of the drill bit types that are used for mud rotary drilling and can also be equipped with downhole hammers and hammer bits. This versatility makes air rotary drilling appropriate for both hard and soft formations.
In general, penetration rates are higher for air rotary boreholes compared to mud rotary boreholes, and because there is minimal wall cake accumulation during air rotary drilling, well development requirements will be greatly streamlined.
As shown in Figure 2, the compressed air is circulated up the standpipe to the kelly hose, which conveys the compressed air down the center of the drill pipe to the bit. The compressed air carries the cuttings upward as it flows up the annulus to the blooey line at the land surface. Mud pits are shown in the figure, but those are not always used with this type of drilling.
Note that in the figure the fluid level in the borehole does not extend all the way to the land surface. While compressed air is being circulated, the entire borehole is under positive pressure and will be stabilized. However, when the drilling crew “makes a connection” to add another joint of drill pipe, the compressor must be shut off, so the fluid level will fall to the water table and the upper portion of the borehole will be exposed to atmospheric pressure.
Figure 5. Generalized diagram of the cable tool drilling method. The movement of the walking beam (red arrow) causes the cable to reciprocate upward and downward in the borehole, which allows the drill bit to break up the formation into drilled cuttings. The cuttings are removed from the borehole by bailing, and the casing is driven into the borehole as it is drilled.
Thus, in locations with unstable or unconsolidated formations, the viability of the air rotary drilling method may be limited because the upper borehole could slough in against the drill string while a connection is being made.
Another limitation of air rotary drilling is in areas where the aquifer produces groundwater at a high rate. Situations may occur where groundwater flows into the borehole faster than the compressed air can remove it. Such conditions will cause the drill bit to be in contact with a thin layer of water instead of in contact with the formation at the base of the borehole. Thus, the penetration rate will be impeded. This situation is sometimes referred to as “waterlogging” or “being flooded out.”
Some drilling contractors convert the same drilling rig between air rotary and mud rotary by simply switching out the circulation system, since the overall drilling rig configuration is unchanged. This versatility enables the driller to adjust the drilling method in response to whatever hydrogeologic conditions are encountered.
A challenge with both direct air rotary and direct mud rotary drilling methods arises when large-diameter boreholes are being drilled. This challenge results from the need to achieve adequate uphole velocity to bring the cuttings to the land surface. The circulation velocity can be much lower for mud rotary drilling compared to air rotary drilling, but both those direct drilling methods rely on creating enough fluid (mud or air) velocity in the annulus to transport the cuttings.
When a large borehole is drilled, the cross-sectional area of the annulus becomes quite large. For example, if a 10-inch-diameter borehole is drilled with a 3½-inch-diameter drill string, then the annular cross-sectional area between the drill pipe and the borehole wall is about 1.9 square feet. If that same drill string was used for a 20-inch-diameter borehole, the annular cross-sectional area increases to almost 8.5 square feet (an area almost 4.5 times larger).
This means that applying direct circulation to remove cuttings from a large borehole would necessitate a high fluid flow rate to achieve the required uphole velocities to clear the cuttings from the hole. In addition, for the case of mud rotary drilling, the larger volume of drilling fluid required to fill the large-diameter borehole would be expensive.
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