We need to stabilise the hole before we start drilling. Hole stability is important.
A stable hole stays open during and after drilling. A hole that has low stability can collapse in, wasting time and money.
There are three different ways to stabilise holes:
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| Use drilling fluids | Use casing | Use cement, grout or hole-packing products |
Click on a method for more information.
More information about stability
Holes in solid rock have high stability, whereas holes in loose sand have low stability. Other rock characteristics that can affect hole stability are:
- the degree of consolidation
- the degree of fracturing, jointing, shearing, faulting etc
- the degree of weathering
- sensitivity of the rock to changes in pore pressure
- reaction of the rock to water or other fluids.
Using drilling fluids
For more information about mixing drilling fluids, see Mixing drilling fluids. Otherwise, read on.
Using casing to stabilise a hole
When it's not economical to use drill fluids or cement to stabilise the hole, we can use casing to support the walls of the hole. To case a hole, the driller and driller's assistant have to:
- pull the rods out of the hole
- lower the casing
- seal the casing
- lower a smaller rod string through the casing. In some cases the rod string that was pulled out of the hole is used as casing (minus the barrel).
Let's go through the process in more detail.
Did you know?
Casing is sometimes used to enable the hole to be drilled through large cavities or old mine workings.
Calipers
1. Pull the rods out of the hole
Assist the driller to pull the rods out of the hole.
Use calipers to measure and check the size of the casing so that you can replace the drill bit with a smaller one that will fit into the casing. If a down the hole hammer (DTHH) is being used then you may need to install a smaller hammer.
2. Lower the casing
The driller uses an oversized bit to start the hole.
The driller will choose the most appropriate type of casing. It can be:
- Steel casing with no threads. These need to be welded together as they are lowered into the hole by someone who is trained and competent in welding.
- Steel with threads
- PVC.
If steel casing is used, you need to collect the required casing. Sometimes it needs to be cut to size; you must be trained and competent to use the cutting equipment to cut steel casing. Clean and grease all the threads of the casing. Threads that are not easily accessible will be cleaned and greased as the casing is lowered down the hole. If the hole is a water bore or for environmental sampling, thread grease might not be used. Make sure the correct adaptor coupling is screwed onto the hoist plug.
If PVC casing is used, use a handsaw to cut a piece to the required length. Any joins in PVC casing are normally glued together. Study the Material safety data sheets for the glue before starting. PVC casing is usually lowered using a rod sling attached to the hoist rope.
Assist the driller to lower the casing down the hole using the main hoist and hoist plug, or for a top drive machine, the head casing can be used to ream or wash the casing into position. Be careful when handling steel casing as it is very heavy.
3. Seal the casing
Sometimes the casing has to be cemented in place, or sealed using special mud products. If so, study the Material safety data sheets before starting, and make sure you use the recommended Personal protective equipment.
Assist in mixing the grout and placing the grout or mud product.
4. Lower a smaller rod string through the casing
After the casing is sealed, lower the rod string down the hole.
Continue drilling.
Using cement, grout or hole-packing products to stabilise a hole
Sometimes cement or hole-packing products such as polyurethane are used to stabilise a hole. This is one of the most expensive ways to stabilise a hole, and mistakes can be costly or impossible to fix.
Did you know?
Polyurethane foam expands rapidly and fills the gap between the outside of the casing and the wall of the hole. It's made by mixing part "A" and part "B" together quickly and pouring the mixture down between the wall of the hole and the outside of the casing.
Polyurethane foam floats on water, so don't use it if the top of the hole is filled with water.