Borehole wall instability as a common problem in oil well drilling

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The first important thing to evaluate is that the instability present in the walls of the hole while drilling an oil well is a condition that is not normal, and far from being a problem in drilling wells that is undesirable.

This undesirable character makes engineering study the various ways to solve this problem and provide a guarantee that an oil well can be drilled and constructed without the problem of instability in the walls of the hole.

One of the ways we can know that there is instability in the hole being drilled is by analyzing the amount of formation solids present in the drilling fluid, in addition to this, if when we finish a section of the well we run a profile to evaluate the average diameter of the hole and we realize that it did not maintain its caliber in size and shape, it is because it is very likely that we have had problems of hole instability.

Causes of hole instability.

Some pit instability problems may be intrinsically associated with the geological conditions of the subsurface formation such as mechanical failures caused by stresses in place. There are other causes associated with pit instability that have to do with poor operational practices.

One of the bad operational practices associated with hole instability refers to not taking care of some properties of the drilling fluid such as filtrate, it should also be taken into account that when we drill a well with a drilling fluid that circulates through the well with a high flow rate it is very likely that erosion will occur on the walls of the hole.

It is important to have a previous knowledge about the geology of the rock strata of the subsoil, since it is based on this knowledge that we can prepare a chemical formulation for the preparation of the drilling fluid so that it can have the corresponding chemical additives that do not make unfavorable reactions that promote the instability of the hole.

Consequences associated with hole instability.

The consequences associated with letting our hole present instability is that other problems are presented in the well as the narrowing and total closure of the well, this means that due to the instability of the walls of the well they present landslides that lead to the total closure of the well and therefore stop the drilling of the well.

Not only can the hole at the bottom be plugged and cause the drilling to stop, but also due to instability and erosion, there can be segments of the well with larger diameters as a result of erosion, as shown in the following image:

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References.

Aadnoy, B.S. 1988. Modeling of the Stability of Highly Inclined Boreholes in Anisotropic Rock Formations (includes associated papers 19213 and 19886 ).