Video Transcript
Hello, this is Marvin Glotfelty, here with another in NGWA: Industry Connected video. I am a hydrogeologist from Arizona and also a licensed well driller. And a lot of the workshops I’ve given on different drilling techniques, and I’ve talked in this video series previously about dual rotary drilling. There’s a lot of different types. But another couple of the standard types that are pretty universal are direct rotary drilling. There’s direct mud rotary and direct air-rotary. Oh, I want to show you some of the slides here. I’m going to share my screen, show you some information that we can then consider.
First with mud rotary drilling, it of course has advantages and disadvantages, just like all drilling types. So the advantage is, while we’re drilling, we are keeping the borehole full to the brim, full to the land surface, with drilling fluid, also called drilling mud. So what does that do? That stabilizes the borehole. It keeps it from caving in on us, even if it’s loose, unconsolidated material. And we can adjust the properties of this drilling fluid to make sure that that happens. And so that also means that we’re going to collect good, reliable cuttings and other data from the borehole as we go. That’s important.
And we can address problems with, like I said, adjusted drilling fluid. If we have swelling clays, if we have lost circulation where our drilling fluid is seeping into a porous formation. If we have hard drilling and all these different things, if there’s different properties in the formation, which there will be, we can just change the drilling fluid to address them.
So what are the downsides? The downsides is, these drilling fluids are not given away for free, they cost some money. So as long as we can manage that, the overall cost will not be exorbitant, but it is an additional cost because it’s a consumable material that we require during the drilling in mud rotary. And the other thing is we can’t tell where the water table is because the borehole’s full to the brim, not until we’ve completed and isolated a portion of the aquifer from the land surface.
So that’s okay. Here’s a cartoon of the drilling fluid circulation. So you can see that we have a mud pump shown on the back of this truck, and of course the silly colors on the truck are just so we can point out different parts of the rig. I don’t think anybody would ever paint a rig like this. But we can pull the drilling mud up through the mud pump, up through the stand pipe, the Kelly hose, and down to the drill bit. And then as it circulates up the borehole outside of the drill pipe, it’s going to carry the cuttings with it which can be deposited in that mud pit.
Now the mud pit can be below ground as shown, or it can be above ground. Either way, it’s the same difference. So this means that we can control our properties and collect our cuttings and really have a lot of good information as we go. So the big part of this though, is the drilling fluid, being able to control that and change it.
So let’s consider what that drilling fluid does. If we look close there’s in, at the microscopic level, there’s a bunch of platelets that are like little tiny sheets of paper, that are the bentonite clay. They’re not shaped like a little ball, they’re shaped like a little sheet of paper. And so if they’re dispersed, they’re floating around in the fluid mixture, in the water, and there’s a little bit of soda ash and things like that mixed in there with other chemicals perhaps.
But then when they flocculate, they stick together. And that means that the thickness, the viscosity, of the drilling fluid can be higher, even though we didn’t add additional bentonite, that’s cost some money. So that means that we can carry cuttings out of the borehole better and things like that. So that’s, when you hear people talking about the benefits of flocculation of drilling fluid, this is what we’re talking about. It had the property where it can pick it up, so at the same uphole velocity we can carry more cuttings out of the borehole, which is what we want to do.
The other thing that happens with drilling fluid is some of the water seeps out of the drilling mud and leaves behind these clay partlets stuck to the borehole wall, and this is how we form a wall cake. What we like is to have a little bit of water, not too much flow out to their formation, and make a relatively thin and hard wall cake. If we have a thin, hard wall cake, it’ll be very stable and easy to remove later on when we’re going to develop the well and finalize it. If it’s a thick, fluffy wall cake, it’s the opposite. It won’t be as stable and it’ll be more difficult to remove.
So this is a property of the drilling mud, not a property of the formation, so we can control it. And so it’s one of the things that we measure, one of many things. And I’ve got photos of how we measure things. In the upper left is a mud scale, so we’re just measuring the weight of the drilling mud. Usually, of course, water weighs about 8.3 pounds per gallon. Drilling mud might weigh 8.8, maybe nine pounds per gallon. But if we get it real heavy, like 9.4, 9.5 pounds per gallon, unless we’re intending that, and sometimes we are, but unless we’re intending that, that means that what we’re doing is recirculating solids. Fine solids that are the native silts and clays from the formation, and we’re not getting them removed as we recirculate and recirculate this drilling fluid.
That’s bad because that means our wall cake, for one thing, will be getting not as thin and hard as we’d like it. To measure that amount of water that goes out, called filtrate or water loss, that’s what’s shown in the device in the center there with the green frame. That’s a filter press, so we’re just measuring how the drilling fluid responds. And then on the right, you see the young lady with a marsh funnel measuring the viscosity or thickness of the drilling fluid.
So the mud engineer can come to the drilling site, as you see on the lower left, with a pickup truck or some sort of a vehicle to check all these things and some others too. In addition to the weight and viscosity, the mud engineer can look at chemical properties, such as pH, maybe calcium content, chloride content, things like that. They have titration devices and so they can measure these things. They can measure the rheology, the flow properties called plastic viscosity, yield point, gel strength, things like that. So there’s a lot of stuff that’s kind of exotic, but the mud engineer can tell all the folks and the parties involved whether that’s a problem or not.
And then the filtrate, that’s what we’re measuring with the filter press in the middle of the screen. And the solids content can be directly measured with a small Imhoff cone, but also is reflected by how heavy the drilling mud is. So all that stuff is good, that means we have control to some extent, as we interact with mother nature as we’re drilling in the well. And that’s a good thing, so this is a good … That’s why direct mud rotary is a very commonly used approach and it’s very successful.
But there’s other alternatives with almost the same drilling rig, such as direct air-rotary. What if we’re drilling at a place where we want … We’re going to have a stable borehole, no matter whether we have drilling fluid or not, and we’d like to give the advantages of air rotary. So with air rotary, we have a very rapid penetration rate compared to other drilling types, and we have quick bottoms-up time.
So that means, to the geologist, that when we drill cuttings at say a thousand feet, they will be at the land surface almost immediately, very quickly. So we don’t have to wonder how long it’ll take or calculate how long it’ll take for the drilling fluid to bring them to the surface. This happens very fast with compressed air. And we can identify where the water table is as we drill, can’t do that with mud rotary but we can do it air rotary.
And of course the wall cake in this case, it’s only really there because of some soap and because of natural formations, not because of any introduced material. And so it’s thin and basically minimal. So the disadvantages, I’ll show you in a cartoon that’s coming up next why it’s not feasible in some unconsolidated or unstable formations. We have to switch to mud in some cases. Or if the borehole makes water faster than the air compressor can remove it, well, then it keeps the bit from adequately turning on that formation rock. And so it makes it a problem called water logging or flooded out bit where we’ve got too much water coming in. Good problem to have, but it can be a limitation to this type of drilling.
So here’s what the cartoon looks like. Very similar to the mud rotary rig you noticed, except that instead of being … Once we label things are a little bit different. This brown device on the back of our drill rig is now an air compressor instead of a mud pump. So we blow compressed air through our stand pipe and Kelly hose, directly down the bit to remove the cuttings. And they come up and now, instead of we’re calling our discharge line a flow line, we just rename it as the blewie line.
And so notice that the borehole is not full of fluid to the land surface. This is the water table somewhere down here. And so we can fill this with foam, but we can’t fill up with water because we’re drilling with compressed air. So that means that if the upper borehole is wanting to cave in on us, that’s when we might have to switch to mud. But there are a number of things we can do to generally stabilize the bore hole while we drill, and it is a good and efficient way. And of course, I’m showing a rotary tricone drill bit cartoon on the bottom, but we can also use a down-the-hole hammer and have a pneumatic hammer type drilling, which in a hard or brittle formation is really effective.
So here’s what we can do. We can add water, just a little bit of mist, and that’s going to keep the dust down. And if you think of compressed air as a fluid, which it actually is, it’s a compressible fluid, then you’re raising the viscosity of that fluid. So you’re cleaning the hole a little bit better when you add a little water.
Further yet, if you add foam, so water plus detergent, that’s what’s shown in the upper right, then you get slugs of cuttings coming out a little bit better. So you’re cleaning the hole. And remember that it doesn’t matter how much you pulverize rock, unless you get it out of the hole you haven’t advanced the borehole at all. And then if you need higher viscosity, yet you can do stiff foam, and that’s detergent plus water, and then add a little polymer to it. And that’s what’s shown on the left.
So we have these different levels of viscosity, even in air-rotary drilling, that we can do. And once we’ve added some foam, some detergent, we’re going to have a little bit of a surfactant surface on that borehole wall. It’s going to slightly stabilizes. We have some help there, and so if it’s a hard rock formation, no problem. But if it’s a unconsolidated formation, depending on the nature, we may or may not be able to drill.
I’ve had experience where I could draw pretty deep in unconsolidated formations within a rotary, but I think it wasn’t anything that I did right it was the luck of the draw that the formation was just behaving itself. So it can be good or it can be not so good. Either way, we of course have the discharge at the land surface. That can be very high velocity as we see here, or it can be slow, little flow out. Really variable, depending on a lot of things. The nature of the borehole, the nature of the air package, all kinds of different things. And so it can just be different situations, depend from a hole to hole.
Either way, I really advocate both mud rotary and air rotary for drilling. It’s just a good way to go. I’m sharing my screen there. So with that, that’s a primer, mud and air rotary drilling 101 for you. So I hope you have a great day and we’ll talk to you next time. Thanks.
To learn more about mud rotary drilling, read this blog post by contributor Gary L. Hix, CWD/PI, RG.
