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Borehole Deviation – Part I

Once the blast is designed and plans are in place, it is time to execute. With many industries, it is easy to track progress. Quality control checks are in place to ensure standards are maintained throughout the process of a given project. With drilling and blasting this can be very difficult to track.

Quality Over Quantity

Although production numbers are easy to quantify (number of holes, total borehole footage, etc.) the quality of these holes can be difficult to determine. This is due to the fact that we can only see the top, or collar, of the borehole. A collar is the opening of the borehole on the surface. If the hole is 40’ deep, we can really only see the top couple of feet with the naked eye. It is impossible to determine the straightness of a borehole without the proper tools.

Naturally, many in the industry pay very close attention to production numbers. We certainly do. However, there is much more to drilling than just the number of holes you produced at the end of the day. For blast hole drilling, a hole is not just a hole. All blast holes are not equal, even if they are the same diameter, the same depth, and right next to each other. This is in large part due to borehole deviation, otherwise known as “bit wander”.

Bit Wander and the Collar

Bit wander can be caused by several factors, some that can be attributed to the drill rig operator and some that are out of the operator’s control. One of the main factors of bit wander is the quality of collar the driller is able to produce. The collar is an integral part of drilling a straight borehole and minimizing borehole deviation. A tight collar at the top of the hole is a great indicator of both the quality of the operator and the quality of the material being drilled. If the collar is tight and the opening of the hole on the surface is not much larger than the borehole diameter, this is a great collar in typically good material. If the collar is “cratered” and much wider than the diameter of the drill bit, this could be an indication of an inexperienced driller or it could represent less than ideal drilling conditions.

If you can picture a piston working and running through a loose bracket, it will have slop in the mechanism and ultimately start working out of line, eventually failing. Conversely, if a piston has a tight bracket it will travel in a straight line and be extremely efficient. This is very similar to blast hole drilling. The collar acts as a type of bracket for which the drill bit and steel are ran through. If the collar is not tight and clean, slop can develop in the steel and cause the bit to travel off line further down the hole. This is one way a driller can try to avoid borehole deviation, by spending the time required to establish a quality collar.

Other Factors Impacting the Borehole

Another factor that can lead to borehole deviation is impacting fractured material, dirt or clay seams, voids, or virtually any other nonconformity in what we hope is a solid column of hard rock. The inconsistencies in the rock can cause the drill bit to wander off course and again follow the path of least resistance. If the bit has a choice to drill in solid rock or dirt, it will choose dirt every time. Due to this law of nature where the bit will wander off to a lesser material if given the opportunity, the drill operator must be aware of what type of material is being drilled. Using pressure gauges, feed speed (the rate at which the bit is traveling through the rock), and sometimes even sound, the driller can identify clues that they have drilled in to an area that is no longer hard, solid rock. When this happens, the driller will need to slow down and ease their way through this area in order to do everything they can to minimize bit wander. However, even the best drillers cannot completely eliminate bit wander.

Part II is coming up next week.

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