When drilling a well, the standard procedure is to set casing to whatever depth is required by law or that conditions dictate. Then, you drill below the casing to find the aquifer. This is when it gets interesting. The driller has to not only find and identify the producing formations, he also has to minimize damage to the formation.
Mud drilling requires a mud weight slightly higher than the formation pressure to keep the hole stable. But this also damages the formation by intrusion of fines and cuttings into the producing aquifer. It is not as bad with air drilling, because the pressure in the wellbore is always less than the formation pressure. Still, the force of the air and the hammer does push some cuttings into vugs, crevices and other parts of the formation. The least damaging method of drilling is air-assist reverse circulation. With that method, the formation pressure is always higher than the pressure at the face of the bit.
Once the driller has reached total depth and either set a screen or made an open hole completion, it’s time for development. The idea is to get all the cuttings, fines and wall cake out of the well, and develop the formation to make optimum, clear water for the customer. To do this, you need to vigorously agitate the well bore with both positive and negative pressure. If you just put a pump in and let it run till tomorrow, the formation never sees positive pressure. This leaves behind embedded fines that will sooner or later lead to service calls. Those calls usually start like this: “My well is muddy.” Of course it is, because it was never developed.
The cheapest and easiest way to develop most water wells is with air. Most every driller I know uses this method, but a basic understanding of what’s happening down hole helps. After all, we can’t see down there any better than Ray Charles could have, so we have to use our knowledge, senses and the clues we see at the surface to do a good job. Generally, we lower an airline — connected to a large enough compressor to lift — into the well and produce the total well capacity.
If depths permit, it is usually better to leave the airline above the production zone. Let me explain. When you first turn on the air, the well is at static pressure and the pressure difference between the wellbore and the formation is zero. When you turn on the air, it starts lifting a column of water causing a positive pressure on the producing formation. This forces water into the formation, hopefully loosening fines, debris and wall cake. As soon as the first head is out of the well, the pressure at the wellbore reverses violently. This will bring the loosened fines into the wellbore and out. Then the well can start producing “new” water. Usually by the second — or third — head, you are seeing “new” water.
One trick I use is to time how long it takes between the first head and the second head. This gives you a rough estimate of the recovery rate. If it comes back quickly, the well is a good producer. If it takes a while, the formation either doesn’t produce much or it is plugged with fines, drill mud and miscellaneous BS. After a few minutes, it may clear and look pretty good. You might think you’ve got it! Not likely. Shut down the air and wait a while (your mileage may vary).
When you turn the air back on, the same thing happens. The pressure in the wellbore forces water into the formation, and then quickly reverses — same as the first time. But, if development is going well, you will see the second “head” arrive at the surface more quickly. This is because the formation is cleaner and has a higher transmissivity rate. If you repeat this procedure a number of times, you will, hopefully, see two things. One, the time between the first head and the second head stabilizes. Second, the well shows no cloudiness or sand, and each repetition produces clear water. Congratulations! Set the pump and sell the well.
Sometimes, everything doesn’t go quite as planned. If you have an open-hole well that always makes sand, you may need to set a screen liner in it to fix this. If that is the plan, it is a good idea to sieve the sand and see what size screen you need. I have a sieve set just for this reason. A good choice would be a screen that retains 60 to 80 percent of the sand. This allows the fine sand to come through, and the coarser sand, which is retained, to form a “gravel pack” around the screen.
Another problem that can show up, especially in open-hole wells, is clay. Since clay doesn’t usually produce water, but can slowly flake off into the wellbore, it can be hard to develop. In the old days, we just kept developing until the well was no longer cloudy. I learned years later via camera inspections that, often, the clay would erode, leaving a much larger washout that was not subject to the velocity and turbulence of the rising water, thus clearing the well. This works with fairly stable clays, but some clays are very water soluble and will cause problems forever. In this case, you have two choices. First, there are chemicals that stabilize clay, essentially anchoring it in place. The other choice is to case off the clay section and be done with it. Neither one is cheap, but they will save the well.
If you have any questions or comments, call, message or email me. Advice is free and worth every penny!
For more Wayne Nash columns, visit www.thedriller.com/wayne.