To be able to continue making a hole it is necessary to remove the chips, cuttings, or core from the hole.
Fluid clearing – reverse circulation/dual tube
As its name implies, reverse circulation (RC) has the circulation medium low down the hole between the wall of the hole and the drill rods through the bit and up the inside of the drill rods.
The reverse circulation technique uses the hydrostatic pressure of the column of luid in the hole or an outer tube to stabilise the wall primarily, although drilling muds may be introduced to provide wallbuilding
Of particular beneit are the low luid velocities against the wall of the hole with high velocities carrying the cutings inside the drill rods.
Large diameter boreholes can be drilled quickly with conventional RC for water supply or dewatering wells requiring minimal development. Dual tube RC in its various forms has many sampling and exploration uses.
Conventional reverse circulation
Circulation is obtained by the following methods:
1. A suction lift pump is introduced between the drill rig swivel and mud pit providing a suction lift to the luid contained in the drill pipe.
Circulation is then achieved by displacement.
2. Two air pipes are installed on the side of the drill pipes and are capable of being alternated to allow air to be introduced to the drilling luid inside the drill pipe at typically 10–20 metres (30–60 ft) submergence.
Introducing the air makes the drill luid in the drill rods less dense than the luid on the outside between the hole wall and rods. Circulation is achieved by displacement.
Reverse circulation dual tube
In this system the drill rods contain an inner and outer tube. Air is passed between the inner and outer tube, through the bit, and then returns with cutings up the inner tube.
Dual tube is also successfully used with a downhole hammer. Two methods can be used:
1. Using a purpose designed hammer with a hollow piston and inner tube that is positioned in the centre of the downhole hammer. Air travels down the annulus between the rod and the inner tube. Cutings and return air travel from the bit face to the surface through the centre of the inner tube. This is the preferred method.
2. Using a crossover sub employed between the dual rods and a conventional hammer diverting the air lowing down between the inner and outer rod into the hammer.
Cutings and air low up the outside of the hammer and are diverted back into the inside of the inner tube for return to the surface.
These methods are particularly suited to fractured or caved formations or when accurate uncontaminated samples are required. The circulating luid is largely isolated from the wall of the hole. The following table
summarises these diferent methods.
Hole clearing and drilling methods