A mechanical thruster tool is a piston device that can be included in the drill string above the drilling bit or a motor. It is a specialist tool originally developed for slim hole drilling and is now available in sizes from 1-11/16″ up to 9-1/2″ in diameter.
As the pressure inside the thruster increases, it causes the thruster to be opened with a greater force. The thruster opens or closes, keeping the WOB constant and thus allowing the WOB to be controlled by pump pressure. A thruster also acts as a shock absorber, decoupling vibrations created by the bit from the drill string above. The principle of operation is shown in Figure 1.
Mechanical Thruster Applications
The applications are:
- Where bit bounce, or drilling vibration is very high
- When high drag makes it difficult to transfer the weight of the drill collars to the bit
- Where WOB needs to be controlled to optimize ROP (rate of penetration)
The actual design of mechanical thrusters is more complex than the simple schematic shown in Figure 1. The following two schematics show a more realistic, but still schematic (more wellbore schematics), view of the internal workings of a thruster drilling tool.
Thrusters are designed to have two surfaces set WOB settings for a given flow rate. The driller controls the setting by raising or lowering the drill string to alter the position of the choke spear. The pressure drop across the tool is different for each position of the choke spear. Thus, by observing the pressure drop, the driller can tell which position the tool is in.
In Figure 2 the choke spear is above the restriction inside the tool body, giving a low pressure drop, showing the tool is fully extended.
Figure 3 shows the mechanical thrusters’ choke spears in the high WOB setting, with the highest pressure drop. The driller can detect the tool position by observing the pressure gauge. The pressure would drop if the string lowered further, indicating to the driller that the tool was about to close.