RC Drilling compared to other drilling methods

Reverse Circulation (or RC) drilling is one of the most frequently used methods of drilling used across the world as it is more cost effective and efficient then other methods of drilling. When compared to RAB (Rotary Air Blast) and diamond drilling, RC drilling seems to be somewhere in the middle when it comes to cost, time, quality of samples, technique and depth.

Cost

Because it is less complex and time consuming reverse circulation drilling is usually 25 – 40% cheaper than diamond drilling. RC is more expensive then RAB which is used in initial exploration in shallow softer rock formation to a depth of p 100m.

Diamond drilling consumables are also more expensive as they are custom made to suit the rig. There are also significant cost and time reductions in the overall RC sampling process. RC can be used to drill up to 500m in depth as a precollar to diamond drilling which is a cost and time effective method that is often used.

Time

RC drilling takes less time than diamond drilling, but more time then RAB drilling because the technique is more complex. Aircore is faster than RC with a Hammer as aircore is only used to drill unconsolidated ground.

Diamond drilling is the most time consuming as drilling is done slowly to preserve the life of the parts and also because of the rock hardness at depths between 1200 – 1800m. RAB drilling takes less time as it is the simplest method of drilling.

Quality of Samples

The quality of the samples also depends on the type of drilling used. RC drilling gives accurate uncontaminated results because the sample passes up the inner tube of the rod string to the cyclone which collects up to 99% of the sample.

The cyclone separates the sample from the dust and air before it passes into the splitter which can give the geologist varying sample sizes depending on their requirements. RAB drilling creates chips of uncertain results as the sample is flushed up the outside of the hole and can be contaminated.

Although Diamond drilling creates a sample of very reliable results and also gives more geological data it is more complex and time consuming in both the drilling and the analysis process. Another advantage of RC Drilling is that it can be used in both exploration and for in pit grade control.

TECHNIQUE
Reverse circulation

Drilling creates chips (like RAB drilling) using either aircore or RC hammer and brings them to the surface inside an inner tube to prevent contamination. This provides a method of drilling that is fast and effective.

With the recent technical advantages in RC drilling and the use of auxiliary compressors which boost air, depths up to 500m can be obtained with a hammer and 300m with air core. Boosted air also keeps the sample dry which makes it easier to split and improves the general handing of the samples.

RAB drilling

Uses a pneumatic hammer or rotary blade (drag bit) compressed air is forced down the centre of the rod and the sample passes up the outside of the hole. RAB is normally used in initial exploration at relatively shallow depths and is not as accurate as reverse circulation.

Diamond Drilling

Uses a hollow, cylindrical diamond encrusted drill bit which rotates at high speed, cutting through the fresh rock and extracting a solid sample that travels up through the drill pipe. Water is also used for lubrication, cooling, and removal of drill cuttings from the hole. Although diamond drilling provides a solid core that gives more geological information it is more complex and thus relatively expensive and time consuming.

Depth

The depth achieved by the Drilling rigs depends on a variety of factors including the consumables being used, skill of the driller, air pressure and the type of ground being drilled. In exploration RAB drilling can reach depths of approx 100m, RC drilling can reach depths up to 500m and diamond drilling depths up to 1,800m.

Stabiliser Subs can be used in RC drilling to significantly improve the accuracy and efficiency of drilling programs.