What is the maximum effective depth of RC drilling, and When core drilling would make more sense?

Quite often I have to deal with a situation where geologists are confused and can not agree on the drilling method. In most cases the choice is between core and RC (reverse circulation) drilling.

Although geologists would prefer to have as much drilling as possible, they also have to be reasonable and think about the budget, since investors want to maximize the output and minimize costs.

As a result, geologists, having limited resources, look at the modern drilling methods, such as RC drilling, as a way to get more geological data for less money. And indeed, under certain conditions, RC drilling is actually cheaper than core drilling.

Firstly, you need to take well depth into account. In the case of RC drilling, depth greatly affects the costs since diesel is the main cost item in RC drilling. That is, the deeper the well, the more compressors and booster compressors must be connected to the rig to make sure there is enough compressed air in the well for drilling. The problem is that water, for example, creates back pressure in the well which reduces the rig’s capacity to drill deeper.

Here is an example. Wells 50 and 100 meters deep can vary greatly in terms of the cost, because 100-meter wells may need a compressor and a booster compressor to cope with water inflow and keep the sample dry and uncontaminated. Extra compressors increase diesel consumption massively, require additional personnel to operate and maintain the units, and, of course, more spending on supplies and spare parts.

Normally RC drilling is done using drill pipes with a diameter of 102 mm, but to drill wells over 100 meters deep you will need 115 mm pipes, because the gap between the internal and external pipes must be big enough to let all the air from two additional compressors pass through. Larger diameter pipes cost more to buy and to use in drilling, since heavier pipes pipes will need additional equipment such as pipelayers.

As we can see, reverse circulation (RC) drilling doesn’t necessarily mean reduced exploration costs. Of course, in many cases, for example, in-mine exploration, the cost is about 20 US dollars, but you have to keep in mind the conditions (depth of up to 50 meters and the work amount of 50,000–100,000 linear meters per year) which help to come to this price.

My goal is to show turning points in the pricing of RC drilling. Having these data, it may be easier for you to choose the right drilling depth or drilling method. This may increase the chances of making your project less costly.

Marat Nugmanov
Editor-in-Chief and founder of Drilling Solutions