Although it's been in use since the mid-1970s, some drillers are not yet familiar with the reverse circulation (RC) drilling technique. "It's growing in popularity, but it's not really a mainstream thing yet," says Alicia Holte of Holte Manufacturing Co., a company in Eugene, Ore., that makes complete reverse circulation systems and was kind enough to give a brief RC tutorial.

Typical reverse circulation system. Courtesy of Holte Manufacturing Co.

What Makes It Unique?

Holte says several different components are necessary for a complete system. In addition to the aforementioned pipe, essential are the RC bit and hammer at the bottom, and, at the top, a top head drive to turn the pipe and an side inlet that allows the air compressor to hook up to the RC pipe.

"You can have reverse circulation with just those components," she explains, "but if you want to have a more complete RC system, you can put a casing driver at the top to drive your casing down as you drill. You may or may not have a casing driver at the top, but if you're drilling deep and you're driving casing, you'll be more efficient with the casing driver. The casing driver can divert your cuttings and spit out the cuttings that are coming out of the hole. You can even put a tube on it and aim it at a container. If you want an even more complete system, you can make it so your top head drive also is RC compatible - all that means is that it has a hole in the center through which your cuttings can go. It will have a tube that blows to a container," Holte says.

A 24-inch reverse circulation bit. Courtesy of Numa.

Why Use RC?

Holte goes on to maintain that there is another major environmental advantage with the RC system: containment. "There are some areas where you cannot wash mud and polymer across soil; it could be because it is an urban environment or perhaps there is a water stream nearby," she says. "This containment is more environmentally sensitive."

In addition to environmental benefits, Holte claims there are operational advantages, as well. "One of the functional advantages of RC is that you don't lose as much air into the formations. The problem with large diameter holes is that you have a lot more area to lose air, and you need a huge amount of air - practically a parking lot full of compressors - to get a large diameter hole very deep. But if you use reverse circulation, you're not blasting your air, you're not losing your air and it's not being lost in the formations."

Holte also states that RC allows drillers to evacuate cuttings faster, "and when you evacuate cuttings quickly, it doesn't wear out your bit. When a bit turns and grinds on its cuttings, it wears out. It's really more crucial when you drill holes that deep and that large to replace the bit because the bits are so expensive."

This rig will drill reverse circulation to nearly 1,200 feet. Courtesy of Mt. Magnet Drilling.

Who Uses It?

Surprisingly enough, Holte Manufacturing's main reverse circulation market lies across the Pacific Ocean. "Most of our business is in Hong Kong because there they put in so many skyscrapers and apartment buildings - such as 30-story apartment buildings in clusters of 10 - and they have to be seated in bedrock. They drill really huge holes - about 2-meter holes. We sell a lot of 24-inch bits over there," Holte says.

Besides foundation drilling, RC also is popular with contractors who do various environmental and exploration projects, she says. "One thing that RC can be used for at any diameter, especially really small diameters, is exploration drilling. Contractors will have 100 acres where they drill 100 holes and map out what they've found. RC drilling is going to be fast and inexpensive compared to, say, core drilling where they take an undisturbed core out of the ground, which is slow and expensive. And RC will be far more accurate than flushing the cuttings out across the ground and trying to sample that way."

RC in Action

Sober says the company, which also has an office in Ventura, Calif., has been using reverse circulation in Hawaii for four or five years. Valley Well Drilling decided to use RC for several reasons. "It's a good clean drill system for a nice straight plumb borehole, and you can also do large diameter boreholes with it," he relates. "There are not a whole lot of companies who use reverse circulation, so they are surprised of how well the water samples come out. It does a great job as far as cleaning the formation out. And all of your returns come right there above your bit, so you don't require the big amount of air that you normally would."

Sober also says that RC is particularly useful in Hawaii's terrain. "We use 7-inch diameter reverse circulation pipe, so the inner pipe is almost 4 inches. Your returns can be quite large, and you can still bring them up through your inner pipe, which is good when you drill in Hawaii because you have a lot of large, loose cinders or clinkers," he relates.

Dave Traut employs a reverse circulation system that evolved out of his company's desire to be able to adapt and address situations as they arose. Traut and his partner took over Mark J. Traut Wells Inc., Waite Park, Minn., from his partner's father in 1982.

"We pretty much were a straight-forward mud rotary only driller - doing some cable tool work, Traut recalls. "Then we kept getting into larger and larger diameter work and soon discovered that every type of drilling has its limitations."

Traut explains that up in Minnesota, they have a very diverse geology. "In the northern part of the state, we have glacial drift sands, clays, rock and boulders on top of granite," he explains. "In the center part of the state, we have anywhere from 100 feet to 300 feet of drift, and then we have a lot of fractured limestone. If you drill with a rotary - whether it's direct or reverse - the problem you encounter is when you get through the drift and hit the fractures, you lose all of your circulation. With these circumstances, work was primarily done with cable tools because the pipe could be driven down to and through the fractured rock zone; two or three casings later, you'd get to where you want to go.

"I'm not bashing cable tools, but in my mind, life is too short to be spending that much time on a hole. There's definitely a place for cable tooling, but it's too slow for me. So a couple of years ago, we started playing around with some different things. There was some reverse circulation projects happening at that time that didn't turn out well at all and sort of gave reverse a black eye. I reviewed those projects and did some homework and I came to the conclusion that it wasn't a bad method or technology, it's just that it wasn't being properly applied. So we started doing some reverse work by attaching the reverse tooling to a conventional rotary rig and had some limited success. We'd have some really good days and say 'Hey, this is the best thing in the world.' Then the next day we'd be working with a different geology and it would be nothing but problems."

Traut decided that if he could successfully case off the drift, the reverse tools would work very well in the fractured limestone zones. "We use a Foremost rig to simultaneously case and drill off the drift down to bedrock," he says. "Once we get down to that zone, we've got protection; we don't have to worry about the hole caving in. Now we can do a lot of things. We typically will drill reverse rotaries through the fractured zones because we can drill at whatever static Mother Earth gives us. If the static is low, we don't care; it will still drill reverse, we can still remove our cuttings and continue on from there. It's working pretty well for us. We're still learning; we've only been using this system for 16 months. Reminds Traut: "By the time you think you have everything figured out, you get humbled again."

Bit selection is paramount, says Traut. "Different things that are more critical with reverse than direct are the types and designs of the bits (see photo on page 9). You have to use a different designed bit if you have clay vs. sand vs. limestone or whatever. It's not as forgiving as going direct mud rotary where you can choose one or two bits and that's pretty much your arsenal. With reverse, it's different. Things will be going right along and then Mother Nature throws you a curve and you've got to go back to the drawing board and redesign or modify a bit.

"Another reason we've been doing it the way we are is because I'm not too intrigued with the idea of using a downhole hammer because I don't want to let air escape off into the fractures and fissures and possibly promote the growth of iron bacteria. So we haven't worked with any downhole large diameter hammers with reverse. However, I have some ideas I'm working on in order to use a hammer, speed up my production and still prevent air from going off into formation, but right now, that's on the wish list."

Traut's firm almost exclusively does municipal work with this reverse system. "We also do monitoring wells and recovery where the client doesn't want any drill fluids," he relates. "It works out really well in conjunction with our casing driving unit because we can control the sloughing of the hole and take up less space on the site. The type of tooling lends itself nicely to the casing advancement capability of the drill. When we did do straight reverse circulation with a conventional rotary drill, it worked fine, but not having the capability to bring the casing down made for a whole other set of problems to deal with. It's got some good advantages - certainly shallow, and especially if you have unstable soil conditions in the first 100 feet. It's nice to be able to run that pipe down and continue to drill from there."

Asked what major advantage reverse circulation drilling provides, Traut answers, "You can get excellent samples. You can drill with little or even no mud, depending on the formation. This gives you much cleaner samples than you'd get with straight mud rotary because you don't have the bentonite getting mixed in there. The engineers like that because a better representation of what's really down there is provided. That's the biggest thing - the quality of the samples. There also is a speed advantage involved, but I say that reservedly. What I mean by that is when things go well when you're drilling reverse, they go really well and you can get some impressive footage in a day. But when things go bad, they go very bad. With a mud rotary going direct, you can have a bad day and still have made some footage, but on a good day, you've made a little better progress than normal; when it goes bad, it doesn't go as bad.

"Geology dictates how well a drilling system will perform," notes Traut. "What I like about reverse circulation is it puts another weapon in our arsenal for situations in which we previously had struggled. I might choose today to still use the reverse pipe, but I'll drill direct. I might drill direct to a given depth then all of a sudden run into a problem. For example, maybe I don't have the right technique but very sticky caramelized-type clays are a problem. I can drill faster going the direct method than I can in reverse. After I get through the sticky clays, I rotate my hoses around and within an hour, I'm going back in the other direction. I've hit a different formation level and now I can be efficient going reverse again. I have the flexibility to go back and forth. If we have course sand and gravel and rocks on top, and then sticky clays in the middle and then limestone and cemented sands on the bottom, we have to trip out three times to change the bit - which isn't any fun. But ultimately you want to have a successful straight hole and have it clean when you're done."

Traut tells us that to him, it seems like the deeper he goes, the better it works. "I find that interesting and it's another reason we looked at reverse circulation," he explains. "With conventional downhole hammers, when you get down to about 500 feet or 600 feet, you have to start running boosters. Another thing I like about reverse circulation is when I am drilling down deep, I don't need to have the well make a lot of water. I can do recirculations so I don't have to have a large area to work in. A lot of times, I may work on a city lot with houses on either side of me and I have a 100-square foot piece of ground on which to set up my entire rig - and the well's right in the middle. So as opposed to somebody doing reverse work in the middle of an irrigation patch who has 160 acres on which to make a mess, I have to keep it quite confined. I may have to drill and dump it into a dumpster because I can't dig a pit for one reason or another. It gives me a lot of flexibility to react as the situation downhole changes."

Looking ahead, Traut has this forecast: "I see a lot of potential for reverse coupled with the dual rotary capability in applications such as elevator shafts, bridge abutments and landfills. With a conventional reverse rotary machine, when people are drilling a hole in Nebraska - probably the birthplace of most of the reverse rotary techniques - they like to move in, set up, drill, and they're out of there in a day or two. We do a lot of work for engineers who want to drill down a specific site a specific depth, pull samples and wait to do some testing. If I didn't case the hole as I drilled reverse, I couldn't really feel safe and leave that hole standing there. But when I couple the casing method with the reverse rotary, I can drill to a given depth, and we can stop and think and wait on the engineers as long as they desire. Then we can go another 10 feet or whatever they want. It gives us a lot of flexibility that way as opposed to a conventional reverse system. We can stop at will and evaluate the situation. That's not something ordinarily done in a water well application, but it might be important on a landfill or a highly sensitive site that's going to have monitor wells."

"Typical well design in the Minneapolis area is an open hole construction with anywhere from 100 yards to 1,000 yards of sand removed. That's how we complete them," says Traut. "We've discovered that reverse circulation is a very efficient system for removing sand. It's like having a giant Shop -Vac. Being able to have a bit on the end, if any chunks fall in, you can break them up and get them out of there in short order. The customary procedure had been a cable tool system whereby you suck the sand out until you run into a chunk and then you go down with the bit to beak up the chunk. Then you go back down and either bail it out or suck it out. It's grossly slow; doing the type of work we do, it's not uncommon to see a cable tool setup on a well for six months. You might spend two or three months drilling and cementing and then another two or three months doing the development procedure.

"Again, the best drilling technique is dictated by the geology. I can brag about my system being the best thing ever invented - for my geology in my backyard. But if I go into your backyard, I might have to modify my tooling to make it work there. It still may be sand, but it may be a different type of sand. What's the blend and the mix? I've discovered that uniformity is a key. If you have uniform formations - either uniform sand or uniform gravel, for example - you can change your tooling and you can be pretty efficient at it. But when you get a borehole that doesn't know what it wants to be - clay one minute and 10 feet later it's boulders and rocks and switching back and forth - it's more difficult to drill.

As for Mark J. Traut Wells Inc., he says, "We have a pretty diverse company. In the summer, we have as many as 55 employees; in the winter we have about 35. We have three mud rotary rigs and we have one cable tool rig, but I don't like to run it; we have the dual rotary, a sonic and an auger rig - and lots of support equipment. We do lots of municipal pump repair; we have a division that does underground sprinklers, a division that does remediation for gas station cleanup and sewage pretreatment systems. We have our own on-site state-certified lab to test fresh water and wastewater. We even have some of our competitors come by and drop off samples."