I wrote last month about the top-most part of a cable tool drill string: the socket. This column discusses jars, the next item on the drill string (which we may or may not want to use).
Jars for cable tool work look like two elongated lengths of a chain. Drilling jars have a gap of about 4 to 5 inches between the upper and lower parts. Fishing jars, which I will write about in a future column, have a much longer stroke or “free play.” For drilling, the jars come between the socket and the drill stem. They serve to bump loose, or “jar,” a stuck drill bit. How does a drill bit get stuck? When drilling in drift, a pebble or stone can wedge between the expanded or dressed-out part of the drill bit and the casing. In bedrock, the drill bit could get stuck in a crack or fissure.
It takes lot of pressure to push a nail straight into wood, yet you can easily hammer it in. Drilling jars work on the same principle. When drilling with jars in the string, we want the drill line just loose enough that the jar parts open a very small amount on each stroke — hopefully less than 1 inch. This allows the drill bit as weighted by the drill stem to apply maximum force to the formation. We do not want to let the jars open much beyond that 1 inch. Doing so wears hard on the jars themselves and may result in them breaking. A drill stem with a broken set of jars presents a difficult fishing job.
Jars have no actual benefit to a drilling program. They are a safety feature to be used only to get the driller out of trouble if the string sticks in the hole. They add extra weight, which on a small machine might present a problem. They also add length to the drill string, which may challenge a short-masted rig. The added length of the jars may make it difficult to get over the top of our casing if we are, indeed, driving casing. If we have landed the casing on bedrock, then the casing is only going to project a foot or two above ground and the extra length of the jars is not a problem.
Drillers have debated the very use of jars since, I guess, the invention of the cable tool method. I have known some drillers who never use jars while drilling in the drift (clay, sand and gravel). Some of these fellows would not even use jars in bedrock. I know other good drillers who would not drill a foot deep without jars in the drill string. I have always used a combination. I would start from ground surface without jars in the string. Drilling in the drift areas predominant in Michigan, I would continue this way until the tools stuck the second time within a half an hour.
Stuck tools can be loosened using a jar bumper, another tool I will write about in the future, but this bumper runs on the sand line, and you lose time taking off the bailer and running the bumper in the hole. I always figured if the stones or pebbles were going to stick my tools regularly, it was worth running jars on my tool string.
As I wrote earlier, new jars have a stroke or free play length of around 4 or 5 inches. This stroke increases with use and, at some point, the jars wear out and have to be discarded. I don’t know any hard and fast rules on the life of a set of drilling jars, but if the stroke increases to 12 or 14 inches, I’d retire them. A couple manufacturers of cable tools still operate in the United States. If you run cable tools and use jars, I would contact them to get recommendations on the useful life of jars.
As far as weight, a set of drilling jars can range from as little as 65 pounds for a 3-inch string to over 700 pounds for drill holes 12-inches or larger.
Jars have male joints on the upper end and female on the lower end, and are connected with tool wrenches just like all other cable tools. If you are driving casing with drive blocks, these are always attached to the upper square of the drill stem and never, ever to the lower square of the jars.
I would say that jars are a necessary nuisance in a drill string but running them beats an expensive or unsuccessful fishing job. Next time, I will write about the drill stem, an utterly simple yet absolutely necessary part of the drill string.
I write this the day after Labor Day and we have had weird weather here in Michigan. Through much of August, we saw heat and humidity with no breeze. We also had several fierce storms, and some areas like Detroit experienced bad flooding. Lightning from these storms knocked out many submersible pumps. I have spoken recently to driller friends running a week behind on service calls — they had that many calls. Lately the weather has improved some, but we have had very little rain and the lawns are looking a bit brown (or at least a weak shade of green). Even the corn in farmers’ fields is drying up — too early for that to happen in Michigan. Until next time, keep working hard, for goodness sake work safe and take pride that you are a driller.
For more John Schmitt columns, visit www.thedriller.com/schmitt.