Proper Gravel Pack Can Keep a Well from Pumping Sand
Got a call from a driller I had met on Facebook, and we had discussed jobs for quite a while. He said he had an irrigation well that was pumping sand, and had been doing it for a while. They didn’t drill the well, but inherited the service work. He had redeveloped and surged the well several times. It would improve for a little while, and then go right back to pumping sand. A lot of sand. Enough to plug irrigation equipment, erode valves and severely shorten the life of the pump. The owner decided it was time for a new well. The area was known for plenty of water, but the producing aquifers are very fine sand. I got to thinking about my younger days on the Texas gulf coast. We had some sands that required a 10-slot screen, and some wells that would make sand unless you ran a 6 slot. That’s a pretty fine slot, and not a lot of open area so, while it might work for a house well, it isn’t enough for an irrigation well. Time to consider a gravel pack.
I have seen a lot of discussion about gravel packs, and most guys say the same thing: Run a 20 or 30 slot screen and pack with 20-40 gravel. I’ve never heard anyone mention anything about running a sieve test on the aquifer, and designing the well to fit the conditions. Worked out usually, but when it didn’t, uh-oh. My friend asked me what I would do. I said, send one of your small rigs out, drill a small test hole, set the screen of your choice, see if it produces, and send me the formation samples all the way down. After a while, I received a heavy FedEx box full of carefully labeled samples. He had drilled a 3⅞ hole, run 20 feet of 10-slot screen into the producing formation and developed 35 gallons per minute. Bingo! Good aquifer, just a matter of designing the well right.
I have a small sieve analysis system that allows me to screen the formation samples and make a plan. Here’s the key: You don’t need to hold back all the sand. There is a balance. When a well is new, it should make quite a bit of sand. After it is properly developed, sand should go down to an acceptable level. It is possible to get sand to zero, which is great for house wells, but a very small amount of sand production is not necessarily a bad thing. Even the rocket-surgeons in the U.S. government realize that a small percentage of sand production is allowable. If I recall, it is about 2 to 4 percent, by volume, which is a lot if your wife is sitting on it in the bathtub … but I digress. I like the figure of less than 1 percent for irrigation wells. No problems for the pump, or for the owner.
I sieved the formation samples and made a plan. The producing sand would have required an 8-slot screen, which was not enough open area to produce what we needed. We could have set the pump deeper and increased the drawdown, but that would have increased the entrance velocity, shortening the life of the well. The key was the gravel pack. There is a relationship between the formation, the pack and the screen. With a fine, well-sorted formation, the pack must hold out some, but not all, of the formation. The screen must hold out most of the pack. This way, during development, which includes not only air-lift, but heavy surging, some of the formation comes into the pack, some of it comes through the screen and the rest stabilizes into a stable intake area. The ratio is variable, but most good hydrogeologists like to develop some of the formation and some of the pack, and stabilize everything before they certify a well.
After design and installation, development is the key. I have seen a lot of drillers make really good wells, case, screen and develop them in record time. This is not always conducive to a harmonious outcome. Sometimes, development takes as long as, or longer, than drilling. All is not lost, though. The driller can have the crew clean up location and wash the rig, and it uses less fuel than drilling.
The idea is to settle, and consolidate, the gravel pack. Usually this is done bottom to top, then top down, with a surge assembly, working slowly, until the gravel pack consolidates on the way up. You may have to add more gravel to keep your screens covered while you do this. You did leave the tremie in the hole while you developed, didn’t you? Then, on the way down, for the final pass, just surge it until the cloudiness goes away, and the fines are gone.
Bottom line is: We made a well that test pumped 400 gpm with very small drawdown, and the sand was negligible. I watched it pump for 6 hours and caught a half cup.
If you get in this situation, your screen company can sieve your producing formations and suggest the screen, but they might not always suggest the gravel size. After all, they are not in the gravel business. There are some good books on the subject, none of which completely agree, but there is nothing like getting your Johnson in a wringer a time or two, to learn ya what ya need to know.
Remember the ratio: formation to pack to slot size. They work together to make a good well.