Why Pilot Holes May Be Useful in Your Drilling Protocol

The customer deserves the best well possible in its geologic setting.

By Gary Shawver, MGWC

When I owned and operated my business, we utilized both test wells and pilot holes to determine our final well design. I will address the use of a pilot hole in this column. Pilot holes were primarily used in the domestic well construction we did for both farm and home wells.

In much of our trade area, we had limestone shallow to the surface (within 50 feet of the surface). The limestone was often overlain with glacial tills, many of which were often sandy. In some areas the limestone was extremely shallow, or it would be only a few feet below grade. In those areas the upper section of limestone may run from 100 feet to 300 feet thick, depending on the area.

Given that we live in a high agricultural area, nitrates were often found in the first water-bearing aquifers. The underlying aquifers that occurred from 200 feet to 700 feet below grade were, in most cases, devoid of any nitrate contamination due to thick layers of shale that separated the lower, underlying aquifers.

In these shallow limestone areas, we would typically set 10-inch surface casing to the bedrock and then drill a 6.25- inch pilot hole through the underlying limestone until we had penetrated the full section of upper limestone.

As we drilled through these zones, we would log where the differing zones of water would occur. Since we drilled the limestone with high pressure air, it was easy to determine where the first few zones of water were produced. Some may produce 5 or 10 gallons per minute. Others could produce 30 to 100 GPM. When we encountered the first zone or two, we would collect a sample of that water and field test it for nitrates. While the test was not a lab certified test, we would often be able to tell if the nitrate level was low or high.

While the pilot hole process was a lot of extra work, realize the fact that a well is a once in a lifetime expenditure, and we owe it to the customer to do the best we can.

After two or three zones of water were drilled through, it was more difficult to determine the nitrate levels due to the mixing of the differing zones. However, if we found the upper zones showed significant nitrate levels, we would continue our pilot holes until we had penetrated the full section of limestone. When we had completed the pilot hole, we would collect one more sample of water and test it. Often that test showed no or very little nitrate, and that’s assuming the lower portion of the limestone we were drilling through produced more water.

But as any water well contractor knows, no two wells are typically the same and that was true for many of these shallow limestone formations we drilled. All would produce various amounts of water—some from a few gallons per minute to more than 1000 GPM. Most, however, produced from 3 GPM to 50 GPM, with some having four or five zones of water production.

Three Items to Consider

Upon completion of the pilot hole, we would then evaluate three things:

  1. What the nitrate level was for the first zones of water encountered.
  2. What the nitrate level showed after collecting a sample of the water produced collectively from the entire aquifer.
  3. What the composition of the limestone was as we drilled through the aquifer.

Regarding the last item, we often would see that the upper section of limestone would be soft, sometimes fractured and highly weathered, while the lower sections were often more firm and more homogenous.

After we had analyzed all three of the above items, we would then decide at what depth we would set the permanent casing. When we had made this decision, we would then ream over the top of the 6.25-inch pilot hole with a 10-inch bit down to the level we had determined we wanted to set the casing.

We would then set the casing, which was either 7-inch outside diameter (OD) steel or 6.9-inch PVC, complete with centering guides. The centering guides ensured the casing was set in the center of the 10-inch hole and that the 6.25-inch hole below would line up with the original pilot hole.

When the casing was set, we would then install three to four sacks of graded bentonite around the base of the casing and then proceed to clean the bottom of the 6.25-inch hole from the debris left from the reaming process. We would then airlift the borehole and check the estimated production as well as collect another sample for nitrate analysis.

We would then complete the grouting of the annulus through a tremie installed in the annulus and typically would use a high-solids bentonite grout. In some cases, if the upper bedrock was fairly dry, we may use neat cement grout.

We used this procedure regularly where we had these geologic conditions, as I outlined at the beginning. We would then record the data we collected and include it in our well log database for future reference for other wells we may drill in the area in the future.

And again, while each well can and does differ, it may give us some insight as to what we may expect on the next project in that general vicinity. In some cases, if the data showed some consistency, we may skip the pilot hole, especially if we had several wells in the area that surrounded a new well location.

There were other situations we found where the use of a pilot hole on domestic well construction was beneficial, and that was to ascertain whether the upper limestone formations would actually produce water.

Again, we had some geologic formations in our trade area where the upper bedrock may produce little or no water, or the formation could produce significant amounts of water. However, until the upper limestone formation was drilled, we would not know if the formation would produce water or not. So, the use of a pilot hole would help us determine that.

If we took the attitude we will just figure this formation in this area won’t produce the water, we will just drill a larger size hole and set the casing in the top of the next water-producing formation, then start drilling only to find that the upper formation may produce a significant amount of water. So, in this situation, we always would again drill a pilot hole.

Final Thoughts

When we drilled these pilot holes (6.25 inches), we always stabilized the 6.25-inch hole with a 5.5-inch OD stabilizer to ensure the borehole was aligned properly, so that when we reamed the hole, the 6.25-inch hole would be in the center of the 10-inch hole. The 10-inch hole was also stabilized with a 9.5-inch OD stabilizer to again ensure the alignment of the final hole.

Also, if we had to set 50 to 60 feet of temporary 10-inch surface casing, we would then start the pilot hole out the bottom of the 10-inch casing with a short 9.5-inch OD stabilizer, built on a sub of about 18 inches in length. We would drill the length of a hammer, say 4 feet or so with that 9.5-inch sub attached to the top of the hammer. This would ensure that the 6.25-inch hole was centered in the 10-inch temporary casing.

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While the pilot hole process was a lot of extra work, realize the fact that a well is a once in a lifetime expenditure, and we owe it to the customer to do the best we can to ensure what we are doing for them is obtaining the best well possible for that geologic setting.

The data we collect from each of these types of wells is helpful for planning the next well we might drill in that area. Retaining the data in our well database helped those contractors drilling in that area for future wells some invaluable insight.


Gary Shawver, MGWC, is vice president of Shawver Well Co. Inc. in Fredericksburg, Iowa. He has been in the water well industry for more than 40 years and is a Master Groundwater Contractor. He served on the NGWA Board of Directors. Shawver is semi-retired, having sold his business to his employees. He can be reached at grs@shawverwell.com.