如题所述
Rotary drilling reverse circulation ( either using drilling mud and / or compressed air or gas) can be a useful alternative to direct circulation methods. The reverse circulation technique is particularly useful for drilling relatively shallowlarge diameter boreholes. In a typical reverse circulation operation utilizing drilling mud，the drilling mud ( or treated water) flows from the slush pump ( or mud pump) to the top of the annulus between the outside of the drill string and the inside the borehole，down the annulus space to the bottom of the borehole. At the bottom of the borehole the drilling mud entrains the rock bit cuttings and flows through the large orifice in the drill bit and then upward to the surface through the inside of the drill string. At the surface the cuttings are removed from the drilling mud by the shale shaker and the drilling mud is returned to the mud tanks.
Figure 14. 5 Dual tube ( or dual drill pipe) closed reverse circulation operation
Reverse circulation can also be carried out using air and gas drilling techniques. Figure 14. 5 shows a typical application of reverse circulation using compressed air as the drilling fluid ( or mist，unstable foam) . This example is a dual tube ( or dual drill pipe ) closed reverse circulation system. The closed system is characterized by an annulus space bounded by the inside of the outer tube and the outside of the innertube. This is a specialized type of reverse circulation and is usually limited to small single and double drilling rigs with top head rotary drives. Dual tube and dual drill pipe are available from a number of manufacturers in the United States and elsewhere in world.
Reverse circulation drilling operations require specially fabricated drill bits. Figure 14. 6 shows a schematic of the interior flowchannel of a tricone rotary drill bit designed for reverse circulation. These drill bits utilize typical roller cutter cones exactly like those used in direct circulation drill bits. These bits，however，have a large central channel opening that allows the circulation fluid flowwith entrained rock cuttings to flowfrom the bottom of the borehole to the inside of the drill string and then to the surface.
Most tri-cone drill bits with a diameter of 5 3/4 in or less are designed with the central flowchannel as shown in above figure. These direct circulation drill bits usually have three orifices that can be fitted with nozzles. Tri-cone roller cutter drill bits for reverse circulation operations are available in diameters from 4 2/3 to 31 in. The larger diameter bits for reverse circulation operations are usually custom designed and fabricated. Dual wall pipe reverse circulation operations require special skirted drill bits. These skirted drill bits are also custom designed for the particular drilling operation. Most reverse circulation tri-cone bits are manufactured by companies that specialize in geotechnical and mining drilling equipment.
Figure 14. 6 Schematic of the internal flowchannel of a tricone rotary drill bit designed for reverse circulation operations
