Last month, I talked about gravel-packed wells. They make a very efficient well when done right. A crucial part of this is development. When a gravel pack is first installed, it lacks some steps that make a completed project.
When gravel is poured or pumped into a well, it is not settled. There are big spaces, or even voids, between grains. And the pack probably contains fines that will pass through the screen. Also, wall cake is usually not completely removed by the gravel-pack process. This is where development comes in. Gravel pack development requires three things. One, the fine components of the gravel pack, and any fines from the formation, need to be removed and developed out. Second, the wall cake must be completely removed to allow maximum production. And, finally, the gravel pack must be settled and consolidated to a stable condition so that future pumping will not disturb it.
The most efficient tool that I have found to do this with is a dual swab. I make mine out of 4-inch pipe, about 3 or 4 feet long with multiple holes drilled in the body. The total area of the holes need to be larger than the area of the pipe. A cap on the bottom and a crossover to the run-in string completes the tool. At the top and bottom, a large ring is welded, slightly smaller than the screen interior diameter, and rubber washers are made to screen size or slightly larger. Mud flaps make good material and are readily available. This isolates the intake to the area between the “washers,” and works a small, selected area of the screen. Don’t forget to drill a hole in the bottom plug. This picks up any sand settled in the sump below the screen.
To operate the tool, run it in to near the bottom of the screen. I run mine on 4-inch or larger pipe, depending on well size. Run the air line with a pack-off at the top and a hose so you can work the pipe up and down. When you start the air, several things happen at once. The first air raises the water level in the well, and forces water out through the swab into the gravel pack. This turbulence tends to surge the gravel and help wash the wall cake in. It also loosens the fine sand in the gravel pack and any loose formation sand in the pack, and brings them in. At first the water will be very cloud, but should clear fairly quickly. This is usually the wall cake or possibly loose clay in the formation. After a few minutes of jetting, pick up the string about the length of the tool. This will uncover the next part of the gravel pack for development. After a few minutes, drop the pipe quickly and look for turbidity. This is the gravel pack settling. Come up to a new spot and repeat, working the pipe up and down as the water clears. This will settle your gravel pack and pull in most of the wall cake.
Sound the level of the gravel from time to time. It will go down. If necessary, add more gravel to completely cover the screens, plus an amount to allow for any future settling. I generally go about 30 percent of the screen length excess.
When you have vigorously swabbed the screen from bottom to top, you are ready for the final step. Jet again, this time from top to bottom. What you are looking for is turbidity. There will always be some cloudiness when you first get to a new depth, but it should clear rapidly. If it does not clear, you have a wall cake problem, or clay in the producing formation. You can deal with this with clay-block chemicals, or flocculants to clear. Just add them to the pipe and chase with clear water. They’ll go where you need.
When you have developed the screen top to bottom, slowly lower the string into the sump. This will pick up any sand that fell to bottom during development. Be careful. Don’t drop the pipe or you might knock the bottom out!
When you are done, you are ready to run your test pump, confident that the entire screen has been developed to its full potential. Sound your gravel pack one last time, and make a note of it so that if you ever have to come back you can tell if the gravel has subsided. Gravel packs will usually subside some over time. That is why you run excess gravel over the screens. One other thing to watch for if you have a falling gravel pack is voids. They can be large and eat a lot of gravel. Washouts during drilling are the same, but they usually fill up and stabilize.
Hope this helps make an efficient well. If I can help, contact me. Keep ‘em turning to the right!
For more Wayne Nash columns, visit www.nationaldriller.com/wayne.