Unconsolidated formations are loose materials and have a high tendency to cave in or wash out when drilling fluid or air pressure is applied.
It is important to understand which drilling methods are used in different geological contexts. The following table summarise the advantages of different drilling methods in unconsolidated formations.
|
Method |
Advantages |
|
Cable tool |
Good samples. Reasonable penetration rate. Cheap to operate. |
|
Auger |
Cheap and fast in dry formations. Not much good in running sands or loose gravel. |
|
Rotary mud |
Fast penetration with wall stability. Ideal for water boring. |
|
Rotary mud reverse circulation |
Good sample. Fast penetration with wall stability. Good for large diameter holes. |
|
Rotary coring |
Requires use of mud and possibly triple tube equipment. Some core loss possible depending on the skill of the driller. Slower and more expensive than other methods. |
|
Rotary air blast |
Very fast in dry cohesive formations. Good for exploration drilling, mineral production drilling and seismic drilling. Not suitable for grade control or resource definition drilling due to contamination. Water slows down drilling and increases risk of sample contamination and hole collapse. |
|
Rotary air core |
Fast and good samples in sands and clays. Hole size must be just slightly larger than the rod line. May need water injection to stabilize hole and keep sample flowing. |
|
Reverse circulation (Air) |
Fast with good accurate sampling. Hollow blade, roller, or down-the-hole hammer are used. |
|
Push probing and vibrating methods of coring, including sonic drilling |
Used only for relatively shallow holes. May require devices to catch the core. Good for undisturbed samples for geological, environmental, archaeological, and seismic studies. Little disturbance to the environment. |
