Losing the drilling borehole integrity is a trenchless construction hazard that can lead to borehole collapse. Depending on the drilling borehole function, hole fragility causes damage to equipment or potential injury to employees.
Workers need to know the signs of a hole that is losing structural stability and the different ways the structure can be destabilized.
Types of Borehole Instability
There are several types of borehole instability, each with a unique cause. Understanding the different types can allow workers to ensure proper preventative measures are in place.
When a borehole begins to narrow or close, workers refer to it like a creep under the overburden pressure. Hole closure causes increased pipe sticking, increased difficulty of casings landings and increased torque and drag before total loss of the borehole:
Narrowing of the hole can lead to another type of instability, total borehole collapse.
Borehole fracturing occurs when pressure from drilling fluid exceeds the formation fracture pressure threshold. The borehole walls begin to develop fractures. This structural instability is also associated with the loss of circulation.
Enlargement of the Borehole
Borehole enlargement is another type of instability. Erosion in the hole is the most common reason behind this condition. However, it can also be caused by shale sloughing and drill string abrasion.
Enlargement has unique problems associated with it. Once a borehole begins to grow, deviation from the original drilling path is a concern. Cementing and logging operation difficulty also has a bearing on how to proceed. (Read on in "Working With Drilling Deviations.")
Of course, the most well-known type of borehole instability is total borehole collapse. Fracturing, enlargement, and narrowing can all lead to the borehole falling in on itself. However, destruction of the hole can also be the result of pipe sticking, or the drilling fluid pressure is too low. (Also Read "How to Avoid Tunnel Collapse When Boring Big.")
Causes of Borehole Instability
While there are several types of borehole instabilities, there are two main reasons a hole will begin to lose integrity: mechanical failure or shale instability. (Read "Common Issues That Come With Larger Diameter Boreholes.")
Mechanical failure of the borehole is caused when stresses acting on the rocks exceed the tensile or compression strength. Additionally, if the mud weight is to low, a compressive mechanical failure may occur. On the flipside, if the mud weight is in excess, the integrity may cause tension failure.
With shale, instability can also have a mechanical cause from the in-situ stress. However, it can also be caused by chemical factors due to the drilling fluid and shale interaction. With the drilling fluid involved, erosion due to circulation is a potential problem.
Borehole Collapse Prevention Methods
Constant monitoring of borehole integrity is essential to preventing integrity loss. Workers monitoring the situation can adjust trajectory, mud weight, and hole stability.
In addition to monitoring the borehole, workers must ensure they are using the proper borehole fluids. Improper fluids can cause issues with the hole structural soundness. Additionally, the wrong liquids can damage equipment. While using the suitable fluids is essential, it is also vital that workers calculate and use the appropriate mud-weight. Using mud with low or excessive weight can cause structural problems.
Finally, the most straightforward preventative measure it to limit time in the open hole. Reduced usage helps the borehole maintain its integrity.
In many cases, borehole integrity is maintainable if workers apply the prevention methods early on. Calculating the proper mud mix weight along with using the appropriate drilling fluids go a long way to ensuring the structure remains stable. However, knowing what signs show that a hole is destabilizing helps workers to compensate during the drilling process.