Technical Tidbits - Gravel Packs
The casing and screens should be run with centralizers, at least in the screen section, to prevent direct contact between the screen and formation. After the casing is run to t.d., it is important to circulate and condition the mud. Reduce the natural sand content and viscosity as much as possible. This will make it much better and easier to develop a gravel pack. Sometimes you may have to pump off and build new mud with fresh gel to get it right. The object is good, clean, fresh mud, with as low a viscosity as you have nerve enough to run in a given hole. Ideally, clear water would be great, but it might not "hold the hole", so a little fresh gel probably won't hurt. You can sometimes circulate the casing while you run your tremie line to the bottom, saving time.
When the tremie is on the bottom, open the top of the casing, and circulate the tremie line. Now, you are ready to pump the gravel pack. There are several ways to pump the gravel; I will discuss a couple of the most common. One way is to use a large, high pressure centrifugal pump with a hopper and water lines attached to the suction. Gravel is added to the hopper with water and pumped down the tremie. This has been used for years, but it is very hard on the centrifugal pump and may not have enough pressure to unplug the tremie if it plugs up. A better way is to use the rig pump to drive a large jet hopper. Pump chlorinated clear water through the rig pump to jet, and add gravel to the hopper. If you plug the tremie, close the valve between the hopper and jet, and use the rig pump to clear the tremie line. As soon as the discharge pressure falls, you can open the valve and add gravel. If you are packing a long screen interval, you may need to "pull" some of the tremie out as you add gravel. While graveling with this method, you will probably see returns from the casing. This is good! It allows the "fines" to return through the screen, and it also will act as a telltale when you have reached the top of the screen. After reaching the top of the screen, you should add more gravel to allow for settling. How much depends on the length of the screen section, a few feet on short screens, 20% on longer settings. After pumping the gravel, allow time for settling, and sound the pack. Don't sound the hole as soon as you shut the pump down, as the pack settles, it may trap your sounding line! (Been there-Done that!) You can usually leave the tremie line in the hole during initial development to add more gravel if needed or to grout the top of the pack.
Next comes development. Proper development of a gravel packed well is a little more involved than turning on the air compressor and going to lunch. It takes quite a bit of energy to develop out the drill mud and fines from a gravel pack. The best way is to use a "surge block". This is a piece of pipe four or so feet long, with a seal to fit the casing at the top and bottom. A small hole is drilled in the bottom to pull fines from the sump below the screen. The rest of the pipe between the seals is drilled with many holes, large enough to let largest debris in. The surge block is run in the hole to the bottom, and the air, through an inside line, is gently started. This will clear the sump and start compacting the gravel pack. The surge block should be raised and lowered through a short section of screen until no mud or fines are detected, and then raised to the next section. While jetting and surging, monitor the gravel level in the annulus with your sounding line, you may need to add gravel to prevent exposure of the top of the screen to the formation. When you have reached the top of the screen, make one more pass slowly down through the screen interval. As you jet down, watch for drill mud, or fines, if you see them, work that area until clear. Work to the bottom, and jet the sump. The gravel level should not go down much on the downward pass. If everything looks good, pull the surge block, grout the annulus, and run the test pump; you've got a well!