Engineering considerations in the instability of an oil well while it is being drilled

Image source

For the drilling of an oil well to be carried out it is necessary that the walls of the well are stable and do not collapse, because if they are unstable they may collapse and thus the rotation of the drill bit would stop.

The instability of a well has several forms of expression, since not only the instability can derive in the collapse of the walls of the hole being drilled, but it also manifests itself by not maintaining the size and shape of the well bore, so drilling a well whose geological characteristics are unstable can result in an attack on the structural integrity in the construction of the oil well.

What causes a well to be unstable at the time of drilling?

When analyzing the causes of instability in a well, not only the geological conditions of the oil formation must be considered, but also the way in which any external agent may influence the internal geological condition, since the formation may present instability, however, there are operations carried out during drilling that do not favor the continuity of the well's stability.

For example, when pulling the drill pipe from the bottom of the well to the surface, it must be done with the necessary precautions to avoid damaging the formation by dragging or mechanical failures caused by tensions that cause collapse of the well walls.

Another cause of the borehole walls being unstable during drilling is the erosion caused by the circulation of the drilling fluid, so in particular I would advise to maintain a controlled flow rate of the drilling fluid, for this simply should not exceed the flow rate values established by the drilling program.

The study and geological evaluation made to the different lithological formations that will be crossed during the drilling of the well, must allow a correct design of the drilling fluid, so that in this way there are no dissolved chemical agents in the drilling fluid that can interact negatively with the formations crossed during drilling and generate instability in the walls of the well.

What problems can occur due to well instability?

Logically, if the walls of the well are unstable, the first thing that can happen is that a well closure occurs, which is an undesirable condition since the rotation of the drill bit stops and makes that the drilling of the well cannot continue.

If the geological formation in the subsoil is unstable, it may happen that the diameter of the hole is larger than planned, the logical thing is that the diameter of the hole is the same diameter of the drill bit, so if you are drilling for example with a bit of 8.5 inches, the most normal is that the diameter of the drilled hole is 8.5 inches. However, if the formation is unstable, the diameter of the hole may be larger than that of the drill bit, bringing undesirable consequences, since the planning of the well is based on a nominal value of the diameter of the drill bit.

Conclusion and recommendations

From the point of view of petroleum engineering and my experience as an engineer of drilling operations of oil wells I must tell you that the solution to the problems of well instability can be studied based on the agents causing the instability of the hole, because if for example the walls have become unstable by poor operational practices, then the solution is to correct these practices.

Analyzing the problems that can occur due to well instability, can help us to expand the importance of all the corrective actions that can be done to ensure that the instability of wells does not stop in any way the drilling of an oil well, because if the drilling of a well is stopped it brings consequences in generating unproductive times that in the long run are expressed in additional expenses to those already planned.

References

Drilling problems

PEH:Drilling Fluids

PEH:Drilling Problems and Solutions