The Basics of Casing While Drilling
Casing while drilling (CWD) has been around in the rotary business for only 20 to 30 years, but cable tool drillers have been using this method since about day one. In overburden and soft formations, drill and drive has been the standard method forever. When the driller reaches a competent or hard rock formation, he can seat the casing in the rock and continue without worrying about losing his hole.
In rotary drilling, there are several different reasons for CWD. Unstable formations, flowing sand and high pressure are good reasons to consider this method. The first method is to attach the bit permanently to the drill pipe. This is probably the cheapest and simplest method. You drill with the casing, and when you have reached total depth, you perforate the casing where you want production. It doesn’t require a special casing shoe. The downside is that you can’t drill below the casing without milling up the bit. No fun.
The next method is to use a special casing shoe and latch the bit into it. The driller then recovers it at the casing point, with either a wireline tool or pipe. The casing shoe will have cutters — usually PDC — on the bottom to cut a big enough annulus for the casing to pass and for cuttings to circulate out. Penetration is usually slower than in conventional drilling, but you save time by not making trips and running casing. Some models allow you to unlatch the bit and drill below the casing — a handy time saver. Some methods allow you to keep the bit slightly ahead, even with or slightly behind the shoe. This feature allows the driller to find the “sweet spot” that optimizes penetration. If a very straight hole is required, running the bit slightly behind the casing shoe will also give you a straighter hole.
Another CWD system uses a mud motor just behind the bit to rotate the bottom hole assembly (BHA), but not the casing. This works well in abrasive formations where casing wear is an issue. Due to the power available at the bit, this system is usually used on casing sizes larger than 7 inches. However, slim hole motors don’t usually have enough torque to adequately turn the bit, and penetration suffers. This system is usually used for short liners that don’t extend far below the casing.
In the water well world, the most common system uses a latch-in bit and the casing is rotated with the tophead. This is a fast, economical system, but consideration should be given to the wall thickness and the tool joint. Thin wall pipe or short couplings will either twist off, or the penetration will be too slow to be economical. Threads should be a premium quality and rated for drilling. On large pipe, 12 inches and up, consider welded casing. It is very reliable, and a welder is a lot cheaper than premium connection. I have done 20-inch pipe this way with good luck. Woe to you, however, if you have to trip out to change a bit.
Another consideration for the CWD method is annular clearance. You must provide enough room to circulate out cuttings. Most systems will make a big enough hole for the pipe, but remember that your annular velocity will be much higher than with drill pipe. The effect on the hole is expressed in equivalent circulating density (ECD). When the pump is not running, the pressure exerted on the bottom of the hole is depth multiplied by fluid density. As soon as you start the pump and start drilling, the pressure goes up on the formation due to friction in the annulus. With drill pipe, this is usually not a big concern because the annulus is large. With the CWD method, the annulus is much smaller and ECD goes up quickly. In shales, clays and other competent formations, this is not usually a problem. However, in incompetent formations like flowing sands, pea gravel or other porous formations, the high ECD may erode the wall cake and wash out the hole. I usually slow my pump when drilling these formations to cut ECD.
Years ago, when I was consulting, I got a call to go relieve a hand in Africa. He had been there for quite a while and was pretty burned out. They were having pressure problems and having great difficulty controlling the hole. Every time they would stop drilling to make a connection, the well would come at them and start flowing. If they weighted the mud to balance it out, every time they started the pumps, the ECD would rise, the well would drink mud, lose circulation and come at them twice as hard.
By the time I got to Africa, they had been circulating on the choke for 30 days with no end in sight. No wonder the other hand was burned out. I relieved the other hand, got him on a plane home and started trying to figure out how to control the well. I quickly realized there was no way. I talked to the engineers who, although they could speak English, preferred to have technical meetings in French. My high school French is pretty rusty, but I finally convinced them to abandon the well, move over and use a different method: casing drilling! We never did get the hole under control well enough to trip out, so we ended up cementing the entire BHA, bit, motor, collars and all, and shooting the pipe up higher.
We knew right where the pressure was, so we were ready when we got to that depth on the offset well. Since there had been no problems higher up in the hole, we drilled with conventional methods until we got near the top of the trouble zone. As we drilled into the pressure zone, the well started to flow slightly, so we increased the mud weight a little. Since we already knew what the formation would stand, we got pretty good at it. When we would drill a joint down, we would circulate and get the mud as perfect as we could. Before we made our connections, we would always have a doghouse meeting to go over everybody’s job on the connection. Got pretty good at it, too. It was a good crew!
It was kind of spooky, because every time we made a connection, the well would start to flow. It wasn’t serious, but it was enough to get everybody’s attention. By the time we got the pumps back on and resumed drilling, we had gained quite a bit in the pits and had to recondition the mud. It was a slow process. The front office had told us how deep they wanted the casing set, and we were getting pretty good at making record-time connections and continuing to drill. By the time we got deep enough, we had it down pat. I called Aberdeen and asked the head engineer if we could go a little deeper to assure a good casing seal. He said, “Well, it looks like you are doing it right, so go ahead.” I made 300 more feet into a good, competent formation before we figured we were deep enough to continue the well. We cemented the casing almost back to the drill pipe, and shot it off with a wireline severing tool.
While I had drilled with the CWD method before, I had never drilled anything this deep or complicated. The French engineers thanked me and sent me home with several bottles of French champagne!
Point is: This is a method to keep in your toolbox until you need it. Might save a well, and make some money along the way!
For more Wayne Nash columns, visit www.nationaldriller.com/wayne.