How Do I Drill a Straight Hole for a Well?
Wayne Nash Talks About the Causes, Cures of Non-Plumb Holes
In my travels lately, I've seen some of the problems that are caused by crooked or non-plumb holes. Sometimes, a close-fitting turbine pump wouldn't go in the hole, sometimes the casing wouldn't even go in the hole, and on those engineered jobs that require logs and plumbness tests before the casing is run, I've seen wells rejected by the engineer before the casing was set.
I got to thinking about the causes and cures of crooked holes. One thing is as sure as God made little green apples: It's easier to drill a straight hole than it is to fix a crooked one. Like I said last month about fishing, prevention is the surest cure! A lot of times the guys with crooked hole problems are the same guys I meet at the trade shows who tell me they can drill a well in nine minutes or some ridiculously short time. They never brag up the go-backs to fix the problem. Our goal is to complete a well, useable by the customer, for the intended purpose, at a reasonable price.
The first thing I've noticed is that, in a hurry to rig up, the driller doesn't always take the time to get the rig level and square over the hole. If you start out with what essentially is a directional hole, it ain't gonna be straight. Because a drill string basically is a big, long plumb-bob, it eventually will try to head straight down, leaving a hole “crooked as a dog's hind leg.” Usually, these wells have the dog-leg near the top. The casing might go in the hole, but the pump won't. Spend a minute and get the rig level.
I've also seen holes that start out vertical but gradually deviate so far they're out of spec., or the casing won't go. This usually can be traced to one of two causes: Either the drill string is not properly stabilized, or the driller is using too much pulldown. I've never been a big fan of pulldown for this reason: If you imagine the drill string a pendulum, it naturally will head toward the center of the earth, straight down. If you push on the top of the string, it will lay into the wall of the hole and head off in whatever direction. As an angle develops, it will get worse, creating a hole in a big, gradual arc.
Another problem is a severe dogleg in a well, usually fairly deep. A lot of times this is caused by drilling through alternately hard and soft formations that are at a pronounced angle to horizontal. When a bit drills a soft formation for a ways and then encounters a harder stringer that is not level, it will try to deviate. If the bedding plane is at a very steep angle, the bit will try to walk in the direction of the formation. In other words, it'll “skim” along the face of the harder formation headed “down dip.” If the bedding plane of the harder formation is at a fairly shallow angle, the bit will tend to deviate into the uphill direction, essentially “kicking off” the hole in a direction “up dip” to the top of the hole.
Either way, this won't lead to a harmonious outcome.
The cure to crooked holes? First of all, keep in mind that the drill string is a pendulum. If you hung 1,000 feet of drill pipe from a balloon there's a thought, sure would be a BIG balloon! it would point straight toward the center of the earth. If you set it on the ground and pushed on the top, guess what would happen? The weight on a bit belongs on the bottom, not the top. Get a drill collar and put the weight where it belongs.
Next: Stabilize the drill string. The closer the fit between the drill string and the hole, the straighter the hole will be. Obviously, there are limits to this, you've still got to be able to circulate out cuttings and not risk differential sticking. And be sure to leave enough room between the string and the hole to fish or wash over if disaster happens! I don't know if anyone is doing it anymore, but we used to run a square drill collar above the bit. The outside edges were almost out to bit gauge, but the flats allowed plenty of room for circulation. Plus, it was stiff as a board!
On large-diameter work, the best way is to drill a fairly close-fitting pilot hole. Drill it without pulldown, well stabilized, and get good samples. Then when you ream to final diameter, the hole will follow the pilot without too much trouble. Straightening a pilot hole is a lot easier than straightening the final bore; drilling a straight hole in the first place is a lot easier than having to ream a dog-leg or cement back to re-drill. I haven't even touched on near-bit or string stabilizers; that's a topic for another time.