Many of the problems encountered during the drilling process are ultimately caused by wellbore instability.
Deep underground prior to drilling, forces acting on rocks to compress them are balanced with forces acting to push them apart. Drilling a hole through a rock can disrupt this equilibrium. After drilling, forces that once acted on solid rock are rerouted around the edges of the hole, creating zones of unusually high stress near the wellbore. In addition, the natural pore pressure within the rock is replaced by pressure exerted by the drilling fluids. Unless the proper precautions are taken, these disruptions to equilibrium can result in loss of circulation, borehole wall failure, and other serious problems.
Let’s take a closer look at wellbore instability.
Transcript
Wellbore Stress Analysis – Azra N. Tutuncu – Colorado School of Mines
Wellbore instability is one of the most expensive complications that can be experienced during drilling or completion operations. It can lead to stuck pipe, lost circulation, or the worst case scenario…the loss of the well.
Wellbore stability is often related to the changes occurring in both the stresses and the rock properties when a new borehole is drilled. Drillers need to have a strong knowledge of the in situ stresses acting at a wellbore in order to reduce the risk of any type of instability and failure.
As a result, wellbore stress and rock property and strength analysis is an essential part of planning well operations.
When we drill a well, we are removing a column of solid rock from the ground and replacing it with a column of drilling mud and the drill string. Unlike the solid rock it is replacing, drilling mud has no shear strength. Consequently, shear stresses within the surrounding rock are transferred around the borehole rather than through it.
There are two ways a borehole will fail – it will either collapse inward in a process known as breakout, or a fracture will be induced outward. Both of these outcomes are undesirable.
Let’s take a closer look at these two modes of failure.
In many regions around the world, the two horizontal stress components are not equal. This inequality causes shear stress that is normally counteracted by the strength of the rock.
If the fluid pressure in the borehole is too low, the stress ratio between greatest and least horizontal stress may become too high and result in failure of the rock around the borehole.
Rock layers are spalled or flaked off by the high shear stresses, resulting in a hole that is elongated parallel to the axis of least stress and possibly shortened in the other direction. In this case, the stress level in the wellbore exceeds the compressive strength of the rock and breakouts are created instead of tensile fractures.
When excessive fluid pressure is experienced in the wellbore, the stress level exceeds the tensile strength of the rock and fractures are introduced.
This second mode of failure results in fractures perpendicular to the direction of least compressive stress, or SHmin, as the rock is pushed apart from within. Unlike compressive failure, which results in stuck pipe, tensile failure results in open fractures and can lead to lost circulation from the well.
A strong understanding of the local stress field and rock properties while drilling a well allows drilling engineers to design a suitable path for the well, select proper casing and tubular assemblies, and develop a mud weight plan that can reduce the risk of wellbore instability issues and any associated failure.
Even if it’s difficult to get an accurate understanding of the stress field before the drilling commences, failures in a well provide valuable information about the stress field which can be used when planning future drilling activities in the region.
Images: “Mudman” by Michael Black