Borehole Deviation

Originally written for the NGWA toolkit

By Christopher S. Johnson, PG, CHg

Boreholes should ideally be both aligned and plumb.
Think of a perfectly straight (aligned), perfectly vertical (plumb) line. This is an ideal hole. However, ideal holes are mostly rare, or at the least take effort to achieve. Let’s discuss how we can achieve a straight hole.
How the drill string is configured matters. Thicker walled and heavier drill pipe called collars will be just above the drill bit, with stabilizers either in between the collars or next in line above the collars as the drill string is constructed.

These components add weight and rigidity to the drill string, creating a bottom heavy configuration. This bottom heavy assemblage will help keep the hole straight by making the drill string hang straighter in the hole and add to the weight on the bit without resorting to pulling on the drill string with the rig. Pulling down on the drill string can cause the drill pipe to bend ever so slightly—but enough to induce deviation. How drilling is conducted will determine the amount, if any, of deviation that occurs. Weight on the bit, rotation, penetration rate, and drilling fluid circulation will all contribute to increasing or reducing the risk of deviation.

Imagine you’ve drilled into a piece of metal, and the spinning drill bit runs off in a direction other than where you want it to go. This is deviation, or not a straight hole. Ideally, the weight on the bit will be enough to lead the drill string downward with little assistance from the rig at a penetration rate satisfactory to meet drilling
goals, but not so rapid as to risk deviating.

The geologic materials into which drilling occurs also plays a major role in inducing deviation. Harder formations resist being drilled through and so increase the risk of deviation unless planned for ahead of time. Structural variations in geologic formations have the potential to steer drilling, and they too must be
accounted for. It is advisable to conduct deviation surveys on those holes where there is concern about deviation, or those holes that have strict tolerances in their project specifications regarding allowable deviation. Surveys are essentially an inclinometer sent down the interior of the drill string to a specific depth. At that depth, a timer goes off and a small electric charge makes a mark on a target that looks like a bullseye. The bullseye has graduations in fractions of a degree that tell the drilling contractor how much inclination the tool is sensing.

There is a great deal of discussion and even controversy over how much deviation is allowable. The deviation of a hole should not impair the ability to collect samples, prevent well casing from passing to the target depth, restrict or constrain the ability to place gravel pack, or create conditions of thinning in the gravel pack.

Equally, and some would argue more importantly, the deviation should not cause the casing to bend, flex, or deviate such that a pump will not pass a specific depth, or a pump is pushed to one side, or the client must settle for a lesser pump. Both the National Ground Water Association and American Water Works Association have criteria for allowable deviation. Drilling contractors across the country have their own ideas about what is allowable, or perhaps acceptable. Generally speaking, 6 inches per 100 feet seems to be one highly recognized and used standard—or expressed another way, about one half a degree per 100 feet (0.0573 degrees to be exact).

Where deviation becomes problematic is when more occurs over shorter distances, in other words a “dog leg.” Gradual deviation may be a less troublesome issue, but a great deal of deviation over a short distance
is problematic. Deviation surveys can often catch dog legs before they become problematic, but the drilling contractor needs to be prepared to correct for such deviation if it does.

In conclusion, certain steps can and should be taken to reduce the risk of deviation. Have a prior understanding of the geologic and drilling conditions that may be encountered, and how those conditions might affect drilling and induce deviation. Proper drill string organization will help, specifically enough weight
for the job but not too much for the rig. Agree on or specify the allowable deviation for the hole or well. Discuss corrective actions that can be performed by the drilling contractor ahead of time. Use a deviation survey tool, and most importantly, establish flexible survey points based on the anticipated
geologic and drilling conditions. For example, have shorter survey points or “stations” when the formation change may cause more deviation, and then increase the spacing between stations if the formations allow.

Christopher S. Johnson, PG, CHg, is the president and principal hydrogeologist at Aegis Groundwater Consulting LLC in Fresno, California. Johnson works with well owners and operators on a variety groundwater related projects, including locating new water resources, well design and construction management, aquifer testing, and well rehabilitation.

He can be reached at chris@aegisgw.com.
The views expressed in this column are the author’s opinions based on his professional experience.