In my last two columns, I wrote about selecting a wire line socket and drilling jars for our drill or tool string. Together with the stem and bit, these components make up the three or four parts of the drill string. I say “three or four” because without jars we have a three-part string and with jars we have a four-part string. Drillers may choose to use jars (or not) in some formations.
The next part of the drill string I’ll discuss is the drill stem. Basically, we have a round steel bar with a male joint at the top and a female joint at the bottom. Running a three-part string, the male end connects to the drill socket. In a four-part string, it connects below the jars.
The stem has one purpose: to provide weight to drive the drill bit. Without the stem, our drilling program goes nowhere fast. We want the stem as heavy as possible for our operation. A stem’s diameter and length govern its weight. A cable tool catalog I have lists the smallest diameter drill stem as 2½ inches and the largest as 7 inches. The shortest stem listed is 6-feet-long and the longest is 28 feet. As you might guess, there is a tremendous difference in weight between a 2½-inch-by-6-foot stem and a 7-inch-by-22-foot stem — by a factor of about 30.
I have read in tool catalogs — and I agree — that we want to have roughly 200 pounds of weight per inch of hole diameter for good, effective drilling. That means our tool string should weigh 600 pounds for a 3-inch hole, 800 pounds for a 4-inch hole and 1,200 pounds for a 6-inch hole. If we drill a 16-inch hole, we would want 3,200 pounds and need a heavy duty rig to handle that weight. In actual practice on smaller holes (3-, 4- and 5-inch) it is nearly impossible to get the string weight up to 200 pounds per inch of diameter level. This is due to two factors: the diameter of the stem compared to the casing and the maximum length of the stem as determined by our mast height.
In actual practice, one cannot put a 3-inch diameter stem in a 3-inch hole — the hydraulics of the fluid in the hole will not allow the stem to drop. The same goes for 4-inch and 5-inch holes. For 6-inch and larger holes, this is not nearly as much of a factor. For 8-inch through 16-inch holes, it’s not a factor at all. In my experience the area of the drill stem, and tool joints for that matter, should fall between 50% and 60% of the area of the drill hole. Doing the math this comes out to a stem diameter about 75% of the casing diameter. So, for a 3-inch hole we would want to run a 2¼-inch or perhaps 2½-inch diameter stem. For a 4-inch hole, I’d suggest a 3-inch drill stem or, in rock drilling, perhaps a 3¼-inch drill stem. For a 5-inch hole we would want to run a 3½-inch stem, or perhaps a 4-inch for strictly rock drilling. For 8-inch and larger holes we have a wide latitude of choices, as interference between the stem and the casing is not a factor. In these large holes, our stem diameter is governed by the tool joint diameter (which is, in turn, dictated by the joint on the drill bit and the weight capacity of the drill rig).
As for stem length, at least one rig manufacturer recommends a stem longer than half the height of the mast. Small spudders like Bucyrus-Erie 20Ws or Speedstar 55s have a 36-foot mast. Given that, we would not want to use a stem longer than 18 feet unless drilled in rock. This recommendation comes from the fact that if we drive casing we need to get over the top of that casing with our tools when adding a section. I have run a B-E 20W drilling 4-inch holes and I used a 3-inch-by-16-foot long drill stem. I could easily get over the stem and into a 10½-foot foot casing section to drive it. On the rare occasion that I drilled rock, I would add a 3-inch-by-6-foot short stem above the regular stem. I then had nearly 34 feet of tools (including the jars) and only about 2 feet of clearance before the socket hit the crown sheeve.
With that 16-foot stem, a socket and drill bit on a 4-inch hole, I ran around 525 pounds of tools —which readers will note is nowhere near the 800-pound optimum.
I have seen some rigs specially designed for rock drilling that had a much higher than normal mast. I can remember seeing at a convention one driller running 40-feet of stem for a 4-inch hole. He had drilling jars on, as he was drilling pretty much all rock. This machine with that string would really eat up the sand stone he drilled.
Another factor to remember when selecting a stem: The tool joints and collars need to be larger than the stem. On those 4-inch holes I keep referring to, when using a 3-inch stem I would use a 1½-inch-by-2¼-inch tool joint, which has a box collar diameter of 3¼ inches. This works well in a 4-inch hole. You can extrapolate up for larger-diameter tools.
Always remember to use care handling a drill stem. Yes, it’s just a steel bar and utterly simple. However, if it is dropped in the horizontal position or bent, it causes the driller all kinds of problems until made straight again — and I do mean straight. If you run a cable tool rig, just use good common sense when selecting the drill stem part of the drill string. Use care in handling your drill tools, even if they look tough and rugged. I could go on for pages about drill stem selection but I think you get the basics. Remember, the more weight the better.
As I write this in early October, we have had nice weather here in Michigan, though a bit warm. My infamous lawn is still green and my wife mentioned recently that the lawn never did turn brown this summer, which it has on several occasions in our lifetime. Cool weather followed by cold will soon be here, but fall is our favorite season.
Until next time, be careful with those tools and keep on drilling.
For more John Schmitt columns, visit www.thedriller.com/schmitt.