Wayne Nash offers his expertise on RC, reverse circulation drilling.

Geologic formations such as limestone can affect drilling methods and results. Photo courtesy of the U.S. NPS.
Reverse circulation drilling (or RC) has been around for over 100 years. In each area that it is used, the method seems to be a little different. Some of the oldest RC drilling can be traced back to Bangladesh and what they call the sludger method. In this method, the drill pipe is raised and lowered either with a cathead, or sometimes by hand, and spudded into the bottom of the hole. A man sits on a scaffold near the top of the drill pipe, and every time the drill pipe comes up, he holds his hand over the open end. As the drill pipe falls, the force of the water and cuttings cause it to flow over the top of the pipe into the pit where water is added and flow back down the outside of the hole. Even today, 2-foot wells 60 feet deep are commonly drilled with this method, and depths of 200 feet are not unheard of. And, they started out with bamboo drill pipe — talk about weird tool joints!

Today, most RC drilling is done either with the direct reverse method or the air-assist method. In the direct reverse method, water or mud flows down the annulus and up the drill pipe. It is lifted by a huge centrifugal pump or a jet eductor. This method works very well with large-diameter holes with water-sensitive sands that are easily contaminated by mud intrusion. As long as the hole is kept full, the hole usually will remain stable. It will not work if the static pressure of the formation is either too low or the well is prone to flow. If the well starts to flow, weight material can be added to the mud to contain the hydrostatic pressure.

In the southeastern United States, we use the air-assist method almost exclusively. Casing usually is set into the top of the limestone with conventional mud and direct circulation and cemented. One joint typically is drilled out the bottom of the casing with mud before going reverse. Since the lime usually makes water as soon as it’s drilled, one joint below the casing shoe is sufficient to start the reverse process. A small PVC air line is run inside the drill pipe and connected to an air line permanently installed in the kelly. The depth of the air line is dependent on the static water level and the amount of air the driller has available. Typically, 200 feet to 300 feet of 3⁄4- or 1-inch PVC air line is run — the deeper the air line, the better. As the compressor is started, water is produced from the formation and lifted inside the drill pipe. Drilling proceeds, and formation water and cuttings are lifted inside the drill pipe. Most drillers use 41⁄2-inch or larger drill pipe. With the large bits sometimes used (24” or larger), it is important to advance the bit slowly in soft sections so as not to plug the drill pipe. A good, accurate air gauge in the air standpipe is a great help. The loss of as little as 3 psi to 5 psi indicates plugging. If the pipe plugs, it is sometimes possible run the pipe up and down rapidly to shake the slug loose. If it can’t be shaken loose, a trip out of the hole is necessary to unplug the bit. The bit should be lifted from the bottom and cleared.

Continued rotation helps to keep the cuttings moving. When the kelly is drilled down and it’s time to make a connection, the air is allowed to circulate for a few minutes to clear the drill pipe.

Sludge drilling, an early form of reverse circulation drilling, has been around for over 100 years and is used in countries such as Bangladesh and Nepal.
This is important because, if you shut the air down with the pipe full of cuttings, they will fall to the bit and almost certainly plug it. To make the connection, the kelly is pulled, and the pipe set on the slips. When it is broken out, the kelly, still attached to the air line, is picked up a few inches. A plate is installed on the top of the drill pipe to hold up the air line. As a new joint is picked up, a short sub, attached to a cable, is dropped through it. When the joint is picked up, the sub is screwed onto the air line and the plate removed. The tool joint is made up and run in the hole. The sub and cable support the air line. When the slips are set, the plate once again supports the air line, as the sub is disconnected and the air line in the kelly is screwed on. The plate is removed, the kelly made up and drilling continues. An easier way to handle the air line is to install a check valve at some depth in the air line, deep enough to make it float. With this method, no C plate is needed: You just drop the air line, and when you run the next joint into the hole, it will float right back to the surface to be screwed onto the air line in the kelly. A big time saver.

Since most drillers use a plastic air line, and the cuttings are very abrasive, the air line is an expendable item. A fair amount of extra air line should be taken to the location. A cut-off air line is indicated by a distinct drop in air pressure and a drop in produced water/cuttings. At this point, stop drilling, pick up and run another air line. It usually isn’t necessary to fish the old one out. It probably will fall to the bit and sit there ’til your next trip, or it may be carried up by the water and cuttings and be found in the kelly on the next connection. When it’s time to trip the pipe, pull the air line in one piece and lay it out across the location ’til the next time.

Today in the U.S., reverse circulation drilling typically is done using either the direct reverse or air-assist methods.
Occasionally, we run into a problem of not having enough air to properly lift the water and cuttings. If the water level is deep, or the formation isn’t producing enough, more air line is necessary. At some point, the compressor just won’t lift the initial head. The cure for this is to run several more joints of air line with a 1⁄8-inch hole drilled in each joint. This allows the air to “turn the corner” and start lifting water. But as the water level falls, the next hole becomes available and the next, until you are lifting water from much deeper in the drill pipe than you would have been able to do with an air line that is solid to the bottom. At some point, the air pressure and the water delivery will equalize at the maximum production of water to the available air. I’ve seen drillers run 500 feet of air line with a 125-psi compressor this way and make more water than you would think possible. Remember, the amount of water and the up-hole velocity inside the drill pipe is what moves the cuttings and speeds up penetration.

Very often, the static level of the well will be aboveground, and the well will flow. This scares the pants off the reverse drillers from the Midwest who know for sure that if a reverse drilled well starts to flow, it’s gonna collapse and stick the drill pipe. In the limestone formations in the Southeast, it’s not uncommon to see a well flow 2,000 gpm, while drilling! Hey bonus — it just develops faster! The beauty of this method is the well is being developed foot-by-foot as you drill. When you reach t.d., not only do you have some idea of the capacity of the well, you are ready for the test pump without further development.

Reverse drilling (not reverse circulation) is actually drilling where the cuttings are coming up the drill pipe. Which way the water goes in the annulus — up or down — is different from area to area. In some areas, water MUST flow down the annulus; in others, it makes no difference. Down here, we love a flow while drilling, if we have someplace to get rid of the water!

Remember, the larger drill pipe you can handle, the faster you can drill. Four and a half inches or 65⁄8 inches is common, but some drillers use 85⁄8-, 103⁄4-, or even 133⁄8-inch drill pipe. Obviously, these huge connections require a big rig and a lot of skill to handle. The safety and expense of large-diameter tool joints quite often limit what a driller can handle, but there are some new tool joints coming on the market that solve the problem of small rig/big tool joints and speed up the operation. If your supplier doesn’t have them, give me a call or e-mail; I can hook you up.