Drilling 101

A basic guide to drilling technology

The following discussion is focused primarily on drilling methods and tooling best suited to lightweight ultraportable drilling as described in projects

There are 2 basic objectives that are part of any drilling system:

Objective 1 Cutting, breaking or grinding the formation into material that can be transported out of the hole. Depending on the type of formation being drilled, this can take the form of a cutting action (auger or drag bit for softer materials, diamond core for harder materials), a grinding action (Tricone bit used in mud or air rotary, soft to medium hard material), or a breaking action (percussion drilling: top hammer, dth hammer, capable of drilling the hardest materials).

Objective 2 Transporting the drilled material out of the hole. This can be done mechanically (auger), with fluid (mud rotary, core) or with air (top hammer, dth hammer)

Basic Components of a Top Rotary Drill Rig

Tracked rig built for PBO project

1 FEED: Feed motor, feed chain, guide (drill mast). The feed system supplies the linear motion, lowering or raising the drill tooling in the hole.

2 ROTATION HEAD: Supplies the rotary motion to the drill tooling.

3 Tool Table: At bottom of drill mast, this is where wrenches or clamps are applied to make or break tooling sections.

4 TOOLING: The 'wear parts' that actually go down the hole and perform the drilling, such as drill rods, DTH hammer and bit (DTH percussion drilling), or auger sections and bit (auger drilling).

5 Carrier: The machine or structure that the drill mast is mounted on. This can vary from a tracked excavator type machine to a simple frame for heliportable drills. The carrier will have some type of articulation to position the drill mast for drilling.

6 Power Source: Most commonly, a diesel engine driving a hydraulic pump delivers power to the rotation head, feed motors and other hydraulic components. For strict rotary drilling such as auger, this is all the power required. Percussion drilling (top hammer or DTH) however requires an additional power source: an air compressor. Other accessory sources may be required as well, such as a water pump for core drilling.

Types of Tooling

Tooling refers to the wear parts that go down into the hole and perform the actual drilling.

Auger: Formation is cut at the face by teeth or blades moving in a rotary motion. Cut material is transported out of the hole by the auger flighting, which acts as a screw. Auger drilling requires torque, weight and stability as all the drilling action is performed from the top of hole by rotary action and down pressure alone. This method is not ideally suitable to light, low powered equipment.

Hollow Stem Auger (HSA) which is used for drilling in overburden and unconsolidated formations, has a hollow center which allows for the installation of elements such as DDBM legs (PBO), or a soil sampling barrel . This method requires fairly soft ground, and plenty of power and weight.

Core drilling: There are many types of core drills, ranging from large diameter core barrels with carbide teeth, down to 1 1/2" diamond tooling used in deep wireline sampling. Core drill tooling cuts only the perimeter of the bore hole, creating a 'core' of intact material that must then be extracted from the hole.

In practice, the only core drilling method suitable application for the ATA project was the use of 7" core barrels similar to those used in concrete coring. Drawbacks for this method included the need for solid contiguous rock, difficulty in extracting drilled core, and also the need for large quantities of flushing water.

Percussion Drilling

Rotary methods (as described above) use downpressure, rotation and a flushing mechanism such as fluid, air or auger flighting for removal of drill cuttings. Percussion drilling, as the name implies, adds the element of a hammer, mounted either on the rig (top hammer), or down at the bottom of the hole (DTH hammer).

Top hammer: The striking force is generated by a hammer above ground on the drill rig itself, and transmitted down to the drill bit through the actual drill rods or pipes. This technology is best suited to smaller holes, good solid ground and limited depth. Flushing or removal of drill cuttings is limited by the small amount of air that can be injected through the drill rods.

DTH (Down the hole) hammer: The striking force is generated by a pneumatic (air driven) hammer that is mounted on the bottom of the drill rods, and incorporates the drill bit itself. Hammer force is transferred directly to the bit without going through the drill rods as in top hammer drilling. This has huge advantages for smaller drill rigs, as the actual drilling power is down in the hole (driven by an air compressor), and does not require the significant down pressure or torque of other methods. Reactive force from the hammer is largely absorbed by the mass of the hammer and drill rods rather than the rig itself. Drill rods can be lightweight and large diameter, allowing a large amount of air to be delivered to the hammer and providing plenty of velocity for flushing drill cuttings out of the hole.

DTH technology is the most comprehensive method for drilling relatively large holes in any formation with lightweight drill rigs.

Percussion Overburden Drilling

Drilling Sequence with percussion overburden tooling

Percussion drilling through caving ground such as gravels, sands, highly fractured formations or areas with high water table require the use of a casing which is advanced with the drill bit. After completion of the borehole, the inner drill string is removed, leaving the casing in the hole. Casing can remain permanently in the hole (as in the ATA cased borehole), or extracted after installation of an element such as a DDBM leg (PBO). This method uses a pilot bit attached to the DTH hammer, and a ring bit attached to the casing itself.

Casing, ring bit and pilot bit. Pilot (in center) is attached to hammer and is removed from hole after completion. During drilling, the pilot and ring bit are locked together and work as one unit. After completion of borehole, hammer and pilot are retracted, and the ring bit remains in hole with casing.

Round buttons on pilot and ring bit are carbide inserts that transmit the breaking force from hammer to the rock. Air holes and channels in pilot bit allow exhaust air from hammer to return up through the casing, carrying the drilled material out of the hole.

A 'shoe' welded to the bottom of casing provides an inner shoulder for pilot bit to drive casing into hole. Pilot bit and ring bit rotate together, casing and shoe do not rotate.

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