There are several drilling methods:
Each of these methods have different characteristics and are used according to the type of formation to be drilled, as well as according to the intended diameter and depth. They all have advantages and disadvantages, depending on their efficiency potential, drilling speed and necessary equipment.
The direct circulation method involves the injection of a circulation fluid (mud or air) through the drillpipe. It then flows up the annulus space, between the borehole wall and the drillpipe, carrying the drill cuttings to the surface.
This method is frequently used for drilling in non-consolidated formations, using mud (usually bentonitic mud) as circulation fluid. It may also be used in the case of consolidated formations, using mud or air as circulation fluid, although air drilling is the preferred method for the most common diameters.
The direct circulation method allows for the obtaining of good efficiency levels, high speeds and a more accurate control over drilling fluid losses, even though it requires more complex equipment.
This method involves the use of a pneumatic hammer that crushes rock using percussive force. Compressed air flows down inside the drillpipe, passing through the hammer’s mechanism and then carries the drill cuttings upwards, along the outside of the drillpipe.
This method is used for consolidated formations and smaller drilling diameters, and allows for the achievement of rather high speeds.
In this case, the direction of the drilling fluid is opposite to that of direct circulation drilling. The fluid flows down the annulus space between the borehole and the drillpipe, and then flows up inside the drillpipe, together with the drill cuttings.
This is a very efficient method for large diameters, and allows for the achievement of high speeds and the collecting of good lithological samples, despite the need for a tighter control of possible subsidence problems. This is the preferred method for highly productive water wells, with high flow rates, such as public or industrial water supply systems.
The rotary core drilling method involves the use of a tubular diamond drill that, through rotary motion, extracts cylindrical cores. This method offers significantly lower speeds and is used when there is the need to obtain a representative sample of the formation being drilled.
The auger drilling method involves the use of a helical drill, which, through rotary motion, carries the drill cuttings up to the surface. This method allows for the obtaining of large drill diameters but it is rather limited in terms of depth. It is mainly used in geotechnical works, such as pile driving, or in the construction of infrastructures.
