All casing systems are a compromise of features. There is not one system that is a “do all”. They have the common goal of installing casing into ground conditions that will not allow the hole to remain open. They are universally bad at drilling in clay. As clay is 98% self supporting this is not an issue. In running sand and very loose formations the drilling is generally easy. So the casing is able to slip into the ground with a little help from the rig. It is for these types of ground conditions that the systems have been created.

In the past mud rotary has been used in collapsing bore hole conditions. Betonite mud was pumped into the bore, this swells to make the ground strong enough to be self supporting. This is a very messy process and it not suited to drilling through rock formation without the weight of a large rig.

The advent of efficient down the hole hammers has allows hard rock drilling to be preformed without the use of massive down force. Hence air rigs are more popular than mud rigs. However the problem of a collapsing hole has to be overcome. Enter the down the hole hammer casing systems. These are the results from the most creative minds in the drilling industry. There are two main types’ top drive and bottom drive or shoe drive.

Top Drive.

Uses right hand thread casings that are rotated by the drill rig rotation head via a duplex drill adaptor.

Advantage: Simple to use. No casing driver shoe required. Casing can be advanced on its own if needed in soft ground conditions.
Disadvantage: Needs to have a duplex drive adaptor to couple the rotation head to the drill stem and the casing. This not a wearing item so the cost is limited to the original purchase.

Types of bit:

Eccentric one piece. Simple, but replacement cost is high when the outer bits wears.

Outer teeth on the eccentric have to cut more rock than the other teeth hence the wear faster that the rest of the drill bit.

Eccentric two piece. Allows you to replace only the worn parts. System cost is higher than the one piece but has lower running costs.

All eccentric systems must finish drilling in rock. It needs the firm rock formation to push against the eccentric bit with reverse rotation and swing it closed. Can not be used if the final material is soft weather rock, sands or clays. Eccentric will not retract and has to be removed with the casing thus wasting the hole.

Super Jaw System
Drilling wings are pushed upward and outwards in the drill bit to increase the drilling diameter of the hole to allow the casing to follow behind the hammer drill bit.  When at depth the bit is pilled back against the casing. This forces the drilling wings down and inwards. The bottom of the casing acts as the retracting driver, not the ground. Can be used in soft ground conditions and still retracted.

Allows you to replace only the worn parts. System cost is higher than the one piece eccentric but has lower running costs. Is best all round for geothermal.