For the oil and gas industry, wellbore stability is of vital importance. The collapse of a well can cause damage to equipment, loss of oil supply, and pose a serious potential threat to workers.

To understand wellbore stability, one first has to understand the dangers that may cause a borehole to become unstable and how engineers determine stability.

What Causes Wellbore Instability?

Wellbore instability comes from an unknown or unexpected behavior of the surrounding soil and rock. Generally, weakness in the borehole can come from either an uncontrollable or controllable source.

Uncontrollable sources are factors that occur naturally and not by human error. During the planning phase of the project, workers may identify possible instability factors. These factors may be taken into account while mapping out the plans for the wellbore and corrected before they become a problem.

However, uncontrollable instability can still occur even with proper and thorough planning.

Man-made errors often cause controlled instability. Often the collapse of these boreholes is preventable if workers take proper precautions.

Uncontrolled Instability

One uncontrollable source of instability is natural fractures or fault formations. A natural fracture occurs when rocks near faults are broken into small or large pieces.

Drill string vibrations can make the cracks worse if they are not kept to a minimum. Weak bedding is a contributor to faulty formations. When the weakness intersects the wellbore at an unfavorable angle, it provides a pathway for drilling fluid to invade and can soften the hole leading to collapse. (Read The Top 4 Drilling Fluid Mistakes You Need to Avoid.)

Another common uncontrolled source of instability is high in-situ stresses. High in-situ stress found in stiff rocks such as sandstone or conglomerates can lead to instability in the wellbore. While engineers calculate in-situ stress during the planning phase, local stress concentrations may be difficult in measuring. (Read Testing In-Situ Stress: What You Need to Know and Why It Matters.)

Other uncontrolled wellbore instability sources may come from mobile formations that squeeze into the wellbore due to compression. Unconsolidated formations sometimes fall into the wellbore when it is loosely packed.

Unconsolidated formation happens when there is no filter cake is present in the borehole. Naturally over-pressured shale occurs when geological substrates are under-compacted, and the overburden and uplift are removed during the drilling.

Controlled Factors

When it comes to controlled factors that cause instability, drill string vibrations may compromise the stability of the borehole. Drill string vibrations can enlarge the holes. It is difficult to diagnose if the issue will occur. Often, it causes instability in dispersive sediment or naturally fractured formations.

In some cases, the mud density can cause the wellbore to become unstable. If the mud flows into the new borehole too fast or too slow it can cause the rock support structure to fail and collapse of the hole.

Other controlled instability factors include drilling fluid temperature, which can cause stress on the surrounding rock. Fluid-rock interactions may soften the rock and compromise the wellbore. Well inclination and azimuth orientation see a risk of collapse based on the principal in-situ stress.

What Determines Wellbore Stability?

Predicting wellbore stability requires the understanding of geomechanical parameters that quantifies the pore pressure, rock properties, and principal stress magnitudes and orientation. It is possible to predict the stability of the newly created wellbore with known factors.

The use of optimal mud weight, location of casing seats, and drilling parameters that minimize swab and surge can help to ensure the newly created borehole does not collapse in on itself. To calculate the precise parameters, engineers turn to rock failure criterion for adequate calculations.

Using the Mohr-Coulomb Criterion and the Mogi-Coulomb Criterion to Determine Rock Failure

As rock failure is one of the uncontrollable instability factors that often lead to the collapse of a wellbore. The use of the Mor-Coulomb failure criterion is frequently employed to understand the risks of collapse.

The Mohr-Coulomb criterion

The Mohr-Coulomb criterion is expressed as:

σ1 = C0 + q σ3

q= the slope of the line as it relates to σ1 and σ3.

σ3 is the minimum principal stress of the rock formation.

σ1 is the maximum principal stress of the rock formation.

The Mohr-Coulomb criterion does not take into account the polyaxial stress state, which accounts for intermediate principal stress. The intermediate stress state has a pronounced effect on rock strength.

To determine the intermediate stress, engineers employ the following:

σm,2 = (σ13)/2

As the Mohr-Coulomb criterion does not take into account the intermediate stress state of the intermediate stress, some turn to the Mogi-Coulomb failure criterion.

The Mogi-Coulomb criterion

This equation takes into account the intermediate stress as well as the intersection of the line axis, represented as Toct and the line inclination.

The strength parameters for both a and b relate to the cohesion and friction angles of triaxial compression and uniaxial compression.

T oct = a + bσm,2

When ignoring the intermediate stress and utilize solely the Mohr-Coulomb criterion greatly underestimate the overall rock strength. Conversely, some believe the Mogi-Coulomb criterion greatly overestimates overall rock strength.

What Equipment is Used to Measure Wellbore Stability?

Technology has made advancements to allow those creating horizontal wellbore, especially those in the gas and oil industry, to measure the borehole stability as they drill.

The use of a real-time azimuthal acoustic measurement attached to a bottom-hole assembly allows engineers to take measures while drilling. The acoustic equipment fed realtime readings back to the operators as drilling commences. These readings are free from chemical sources to ensure that the wellbore stability analysis is accurate.

When using azimuthal acoustic measurements, principal stress orientations are collected from three separate sources: the borehole breakouts, compressional images, and acoustic anisotropy evaluation. Engineers compare this data to the three-dimensional geomechanical model constructed by the existing offset geomechanics data.

The use of careful calculations based on a geological survey combined with new technologies can help engineers adequately predict wellbore stability. Only with an accurate accounting can engineers calculate the appropriate mud mixture and casing placement to ensure that the wellbore remains stable for an extended period.

Project managers must take into account all possible instability factors when planning the drilling of the well.