Two types of circulation techniques can be used for either a mud drilling system or an air or gas drilling system. These are direct (conventional) circulation and reverse circulation.
Figure 1 shows a schematic of a rotary drilling, direct circulation mud system that would be used on a typical double (and triple) drilling rig. Direct circulation requires that the drilling mud (or treated water) flows from the slush pump (or mud pump), through the standpipe on the mast, through the rotary hose, through the swivel and down the inside of the kelly, down the inside of the drill pipe and drill collars, and through the drill bit (at the bottom of the borehole) into the annulus space between the outside of the drill string and the inside of the borehole.
The drilling mud entrains the rock bit cuttings at the bottom of the annulus and then flows with the cuttings up the annulus to the surface where the cuttings are removed from the drilling mud by the shale shaker and the drilling mud is returned to the mud tanks (where the slush pump suction side picks up the drilling mud and recirculates the mud back into the well). The slush pumps used on double (and triple) drilling rigs are positive displacement piston-type pumps.
For single drilling rigs, the drilling fluid is often treated fresh water in the mud tank. A heavy-duty hose is run from the suction side of the onboard mud pump (see Figure 2) to the mud tank. The drilling water is pumped from the tank, through the pump, through an onboard pipe system, through the rotary hose, through the hydraulic top-head drive, down the inside of the drill pipe, and through the drill bit to the bottom of the well. The drilling water then entrains the rock cuttings from the advance of the bit and carries the cuttings to the surface via the annulus between the outside of the drill pipe and the inside of the borehole. At the surface, the drilling fluid (water) from the annulus with entrained cuttings is returned to a mud pit where the rock cuttings are allowed to settle out to the bottom. The pumps on single drilling rigs are small positive displacement reciprocating piston types.
Figure 3 shows a detailed schematic of a direct circulation compressed air drilling system that would be used on a typical double or triple drilling rig.
Direct circulation requires that atmospheric air be compressed by the compressor and then forced through the standpipe on the mast, through the rotary hose, through the swivel and down the inside of the kelly, down the inside of the drill pipe and drill collars, and through the drill bit (at the bottom of the borehole) into the annulus space between the outside of the drill string and the inside of the borehole. The compressed air entrains the rock bit cuttings and then flows with the cuttings up the annulus to the surface where the compressed air and the entrained cuttings exit the circulation system via the blooey line. The compressed air and cuttings exit the blooey line into a large pit dug into the ground surface (burn pit). These pits are lined with an impermeable plastic liner.
If compressed natural gas is to be used as a drilling fluid, a gas pipeline is run from a main natural gas pipeline to the drilling rig. Often this line is fitted with a booster compressor. This allows the pipeline natural gas pressure to be increased (if higher pressure is needed) before the gas reaches the drilling rig standpipe.
Rotary drilling reverse circulation (using either 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 shallow large diameter boreholes (e.g., conductor and surface casing boreholes).
In a typical reverse circulation operation utilizing drilling mud, the drilling mud flows from the 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 center opening 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 (where the pump suction side picks up the drilling mud and recirculates the mud back to the well).
Reverse circulation can also be carried out using air and gas drilling techniques. Figure 4 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 inner tube. This is a specialized type of reverse circulation and is usually limited to small single and double drilling rigs with top-head rotary drives (see Chapter 4 for drill pipe details).
Reverse circulation drilling operations require specially fabricated drill bits. Figure 5 shows a schematic of the interior flow channel 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 (see Figure 6). These bits, however, have a large central channel opening that allows the circulation fluid flow with entrained rock cuttings to flow from the bottom of the borehole to the inside of the drill string and then to the surface.
Most tricone drill bits with a diameter of 5 3/4 inches (146 mm) or less are designed with the central flow channel as shown in Figure 5. Figure 6 showed the typical tricone drill bit for direct circulation operations. These direct circulation drill bits usually have three orifices that can be fitted with nozzles. Tricone roller cutter drill bits for reverse circulation operations are available in diameters from 4 1/2 inches (114 mm) to 31 inches (787 mm). 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 specifically designed for the particular drilling operation. These specialized drill bits are usually manufactured by mining equipment companies.