Hello friends and stem content lovers.
In this opportunity I would like to share with you all how we could symptomatologically treat an oil well that is being drilled and whose downhole drill cuttings are visible at the surface with a particular geometry shape.
If the well is being drilled with good operational practices and at the same time we are drilling through a lithological formation whose shales are not crumbling, then it is very likely that we will see normal shale cuttings at the surface as a result of drilling without problems. Below is an image where we can see the size and shape of the drill cuttings that belong to shales:
Image author: @carlos84, using Microsoft PowerPoint design tools.
Roughly speaking, relatively small shale cuttings can be observed, which give an indication that they are cuttings from drilling where the drill bit cuts the shale into relatively small pieces.
The other circumstance under which another shape and size of the shale can be achieved is that as we go deeper in the drilling of a well, we find ourselves drilling a shale zone whose formation pressure is that of a normal pressure and suddenly we fall into a shale formation with abnormal pressure, when falling into a zone of abnormal pressure the formation pressure will be greater than the pressure exerted by the drilling fluid on the walls of the well.
When drilling becomes an overbalanced condition with a drilling fluid pressure value that is above the formation pressure value and goes into reverse, where the formation pressure is greater than the pressure exerted by the drilling fluid on the borehole walls, this generates an underbalance in circumstances where the formation pressure pushes the borehole walls and where the shales crumble, making the shale cuttings do not look small at the surface, but are sharp and elongated cuttings product of this underbalance.
Logically, an underbalance condition does not only occur under circumstances where an abnormally pressured formation is being traversed, because if for example, due to poor operational practices, the fluid density drops, it can cause an underbalance in wellbore pressures that are not the result of an increase in formation pressure.
The way of interpreting a shale breakdown is that if the shape of the cuttings are elongated and large, and if there are no operational reasons that cause these shapes in the drill cuttings, then we are probably drilling a formation with abnormal pressure, so we must act quickly and raise the mud density to a value where we can recover the overbalance condition in the well.