One of the interesting facets of water well drilling is the wide variety of formations we drill in. It is pretty difficult to have a rig that will drill any formation, so most drillers own rigs that suit their particular area. Sometimes, the formations vary so much that a driller may have to have a mud rotary rig, an air rig and a cable tool rig — just to cover his home area.
Since I have usually drilled in coastal areas, mud rotary has been my choice for years, but it can be a challenge in some areas. One of the most challenging things in formation for a mud driller is lost circulation. Simply put, you can’t get as much mud back as you pump down the drill string.
Lost circulation can take several forms. Sometimes it can just be a slow loss of mud, as you are drilling ahead. Usually, adding water and bentonite can keep up with the loss, and allow you to complete the well. But, occasionally, it gets “more interesting” — like total loss of returns. Time to fire up the water truck and find the nearest source of potable water, and hope you can keep up! Once, in an area in Florida, we had a mixing hopper and pump on the truck so we could mix as we filled, and have fresh mud by the time we got back to the rig!
Sometimes the loss can be total and the hole will drink all the mud you give it. Boulders, cobbles and loose pea gravel are notorious for this. The easiest and most common cure is to mix higher viscosity mud and hope to plug the “leaks.” In my area, this usually works. But some areas are so porous that even mud thick as peanut butter won’t hold it.
In the oilfield, drillers often use “lost circulation material” to stop the losses. This can include a lot of different products. I have seen drillers use sawdust, wood chips, ground asphalt, shredded paper and cellophane — any number of things. Hey, if it works, who can complain?
In our industry, cement is the most common and easiest product to use. Typically, the driller will drill into the L.C. formation as he can until he’s out of mud, and then pull up off bottom a ways and pump a good batch of cement. The amount depends on hole size, formation type and other factors.
When you have cemented an L.C. zone and drill below it, slow down!
After pumping the cement, shut down and wait for it to set. Go back to bottom and see how much hole you have filled. If the cement is above the porous formation, you can usually go back to drilling. But if the hole drinks all the cement and it doesn’t come up at all, you are going to have to repeat the process until you stabilize the hole. This might take a while in some formations, and you may need some additives to speed up setting. Your local mud rep can help.
When you have cemented an L.C. zone and drill below it, slow down! Too much hydrostatic pressure and too heavy a cuttings load in the annulus, and you will be right back where you were with one difference: Your bit is now below the problem, and the hole may cave in. Not good! If it is a multi-string well, you may be able to get to your casing point, set pipe and have the problem behind “iron-wall-cake.” Then you can proceed as normal.
If you lose circulation in your producing formation and your casing is already set, that’s a pretty good sign of making a good well. Go ahead and turn on the water, thin up and drill the rest of the joint, but don’t fool around on bottom too long. Pick up and start developing. You can usually pull your mud and loose cuttings out pretty easily.
Lost circulation formations are common, but not usually too hard to deal with. Sometimes, no matter what you do, you can’t “heal up” the formation. Then it becomes a matter of skill and experience, to know if you can continue safely. If you can T.D. the well, you can usually make a pretty good well. I’ve drilled wells that made 500 gpm with a 500-foot water level and almost no drawdown! Scary, but doable.
For more Wayne Nash columns, visit www.thedriller.com/wayne.
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