Reverse Circulation

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 shallow large 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 (where the slush 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 1-8 shows a typical application of reverse circulation using compressed air as the drilling fluid (or mist, unstable foam) [7]. 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 (see Chapter 3 for drill pipe details).

Udr Drill Rig Top Head Drive







Outer Pipe

Inner Pipe

Outer Pipe i"l

Figure 1-8: Dual tube (or dual drill pipe) closed reverse circulation operation.

Reverse circulation drilling operations require specially fabricated drill bits. Figure 1-9 shows a schematic of the interior flow channel of a tri-cone 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 1-1). 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.

Figure 1-9: Schematic of the internal flow channel of a tri-cone roller cutter bit designed for reverse circulation operations (courtesy of Smith International Incorporated).

Most tri-cone drill bits with a diameter of 5 3/4 inches or less are designed with the central flow channel as shown in Figure 1-9 above. Figure 1-1 showed the typical tri-cone drill bit for direct circulation operations. 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 1/2 inches to 31 inches. 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 (see Chapter 3 for details). 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.

Tricone Roller Bit

Figure 1-9: Schematic of the internal flow channel of a tri-cone roller cutter bit designed for reverse circulation operations (courtesy of Smith International Incorporated).

1.3 Comparison of Mud and Air Drilling

The direct circulation model is used to make some important comparisons between mud drilling and air and gas drilling operations.

1.3.1 Advantages and Disadvantages

There are some very basic advantages and disadvantages to mud drilling and air drilling operations. The earliest recognized advantage of air and gas drilling technology was the increase in drilling penetration rate relative to mud drilling operations. Figure 1-10 shows a schematic of the various drilling fluids (the top four comprise air and gas drilling technology) and how these drilling fluids affect drilling penetration rate. The drilling fluids in Figure 1-10 are arranged with the lightest at the top of the list and the heaviest at the bottom. In general, the lighter the drilling fluid the greater the drilling penetration rate (the arrow points upward for increasing penetration rate). The lighter the fluid column in the annulus (with entrained rock cuttings) the lower the confining pressure on the rock bit cutting face. This lower confining pressure allows the rock bit to be more easily advanced into the rock (see Chapter 3 for more details).

Improved Penetration Rate

Figure 1-10: Improved penetration rate.

Figure 1-11 shows a schematic of the various drilling fluids and their respective potential for avoiding formation damage. Formation damage is an important issue in fluid resource recovery (e.g., water well, environmental monitoring, well drilling operations, oil and natural gas, and geothermal fluids). The lighter the fluid column in the annulus (with entrained rock cuttings), the lower the potential for formation damage (arrow points upward to increasing avoidance of formation damage). Formation damage occurs when the fluid column pressure at the bottom of borehole is higher than the pore pressure of the resource fluid (oil, gas, or water) in the rock formations. This higher bottomhole pressure forces the drilling fluid (with entrained rock cutting fines) into the exposed fractures and pore passages in the drilled rock formations. These fines plug these features in the immediate region around the borehole. This damage is called a "skin effect". This skin effect damage restricts later formation fluid flows to the borehole, thus, reducing the productivity of the well.

Ability to Not Cause Formation Damage

Figure 1-11: Formation damage avoidance.

Figure 1-12 shows a schematic of the various drilling fluids and their respective potential for avoiding loss of circulation. Loss of circulation occurs when drilling with drilling muds or treated water through rock formations that have fractures or large interconnected pores or vugs. If these features are sufficiently large and are not already filled with formation fluids, then as drilling progresses the drilling fluid that had been flowing to the surface in the annulus can be diverted into these fractures or pore structures. This diversion can result in no drilling fluid (with entrained rock cuttings) returning to the surface. The rock cuttings are left in the borehole and consolidate around the lower portion of the drill string and the drill bit. If this situation is not identified quickly, the drill string will begin to torque-up in the borehole and mechanical damage to the drill string will occur. Such damage can sever the drill string and result in a fishing job to retrieve the portion of the drill string remaining in the borehole.

For deep oil and natural gas recovery wells, loss of circulation can result in even more catastrophic situations. If drilling fluids are lost to thief formations, the fluid column in the annulus can be reduced resulting in a lower bottomhole pressure. This low bottomhole pressure can cause a high pressure oil and/or natural gas kick, or geothermal fluid kick (a slug of formation fluid) to enter the annulus. Such kicks must be immediately and carefully circulated out of the annulus (to the surface) otherwise an uncontrolled blowout of the well could occur. Here again heavier drilling fluids are generally more prone to loss of circulation (arrow points upward to increasing loss of circulation avoidance).

Figure 1-13 shows a schematic of the various drilling fluids and their respective potential for use in geologic provinces with high pore pressures. High pore pressures are encountered in oil, natural gas, and geothermal drilling operations. New discoveries of oil, natural gas, or geothermal fluid deposits are usually highly pressured. In order to safely drill boreholes to these deposits heavily weighted drilling muds are utilized. The heavy fluid column in the annulus provides the high bottomhole pressure needed to balance (or overbalance) the high pore pressure of the deposit.

Ability to Drill in Loss df Circulation Zones

Figure 1-12: Loss of circulation avoidance.

Figure 1-13 also shows that the heavier the drilling fluid column in the annulus the more useful the drilling fluid is for controlling high pore pressure (the arrow points downward to increasing capability to control high pore pressure). There are limits to how heavy a drilling mud can be. As was discussed above, too heavy a drilling mud results in overbalanced drilling and this can result in formation damage. But there is a greater risk to overbalanced drilling. If the drilling mud is too heavy the rock formations in the openhole section can fracture. These fractures could result in a loss of the circulating mud which could result in a blowout.

Ability to Drill in High Pore Pressure Zones

Figure 1-13: Controlling high pore pressure.

In the past decade it has been observed that drilling with a circulation fluid that has a bottomhole pressure slightly below that of the pore pressure of the fluid deposit gives near optimum results. This type of drilling is denoted as underbalanced drilling. Underbalanced drilling allows the formation to produce fluid as the drilling progresses. This lowers or eliminates the risk of formation damage and eliminates the possibility of formation fracture and loss of circulation. In general, if the pore pressure of a deposit is high, an engineered adjustment to the drilling mud weight (with additives) can yield the appropriate drilling fluid to assure underbalanced drilling. However, if the pore pressure is not unusually high then air and gas drilling techniques are required to lighten the drilling fluid column in the annulus.

Figure 1-14 shows a schematic of the various drilling fluids and their respective potential for keeping formation water out of the drilled borehole. Formation water is often encountered when drilling to a subsurface target depth. This water can be in fracture and pore structures of the rock formations above the target depth. If drilling mud is used as the circulating fluid, the pressure of the mud column in the annulus is usually sufficient to keep formation water from flowing out of the exposed rock formations in the borehole. The lighter drilling fluids have lower bottomhole pressure, thus, the lower the pressure on any water in the exposed fracture or pore structures in the drilled rock formations. Figure 1-14 shows that the heavier drilling fluids have a greater ability to cope with formation water flow into to the borehole (the arrow points downward to increasing control of formation water).

Ability to Cope with Formation Water

Air and Gas Unstable Foam Stable Foam Aerated Mud Mud


Figure 1-14: Control of the inflow of formation water.

+1 0


  • Rowan
    Does tricone drill bit have nozzles?
    6 years ago
  • Nelma
    When high viscosity drilling fluid is needed for large diameter reverse circulation drilling?
    5 years ago
  • Sian
    Can air pressure from drilling geothermal wells damage adjacent structures?
    5 years ago

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