Tue, 20 October, 2020
Right parameter selection of the drilling fluid is a critical part for successful drilling jobs and this becomes especially important when the drilling mud is also being used to drive the hammer.
You just bought a lovely painting of a cat and you want to hang it up on your wall. Now you need to drill a hole. Simple enough, but what are you actually trying to achieve when drilling a hole? The first goal is to break up the material which you are drilling into. The second is to remove that material from the space which you want your hole to occupy. After all, if the material isn’t removed, there is no hole. This approach is exactly the same when drilling into the ground to be able to harness the geothermal energy which is a few kilometers below you.
The breaking of the material is very comparable whether you are drilling into concrete or into the ground. The tip of the drill (when drilling into a wall) breaks up the material by applying a grinding, crushing motion achieved through rotational motion and the force applied onto the wall. This is comparable to the same action applied by a tri-cone drill bit in geothermal drilling. A hammer drill bit is also employed in geothermal drilling and is analogous to a power drill with a hammer setting.
When it comes to the removal of the broken material, things become a bit more complicated in geothermal drilling. One of the main purposes of the helical groves on a conventional drill bit is to carry the broken material away from the tip of the drill and out of the newly created hole in your wall. However, it is not feasible to employ the same method when drilling a hole thousands of meters deep into the ground. One practical reason is that the drill bit is larger than the diameter of the drill string, resulting in empty space between the drill string and the ground which is being drilled into. Thus, the material would just end up falling out of a helical groove. So how is crushed rock (referred to as “cuttings” in the drilling industry) brought to the surface when drilling a geothermal well?
This is where drilling mud (or drilling fluid) comes in. Drilling mud is pumped down the drill pipe, exits by the drill bit and then returns up the borehole (in the space outside the drill string) carrying the cuttings to the surface. The rheological properties of the drilling mud are critical for it to be able to carry out this task. Rheology is the science of deformation and flow of matter and is very useful when trying to describe the difference between the flow properties of tomato ketchup and orange squash. The drilling mud has to be viscous enough to carry the cuttings and it also has to have thixotropic properties. Tomato ketchup is probably the most well-known thixotropic material, as it is very thick when it is in the bottle but becomes much more fluid once it finally starts flowing out of the bottle. This “thick when still” property is exactly what we need so that the cuttings don’t drop down to the bottom of the hole when we stop pumping the drilling mud. There are a number of other purposes for the drilling mud during drilling operations and, for that reason, you will find a drilling mud specialist on most drill sites making sure that the drilling mud is just right so that it can achieve all these tasks.
Drilling mud is also very important to the innovations that are ongoing in the Geo-Drill project. To find out more see Geo-Drill Rheology testing report.
Article Courtesy: Dr. Kristján Friðrik Alexandersson, Gerosion Ltd.