Beyond the Drill Bit: Building a Successful Bottom Hole Assembly
In order to make a usable hole in the earth, there has to be a connection from the rig to the ground and a way to remove the earth as you drill. From the swivel down, this means drill pipe. But when you get near the bottom, many specialized tools are needed to successfully drill.
Starting at the bottom, obviously, the bit is the first and most important part of the bottom hole assembly (BHA). Bit selection is a complicated issue that would take more space than I have here, but most drillers have a good idea what kind of bits to run, based on overall cost per foot. A bit may drill a fast hole, but give up quickly, while another bit might last forever, but penetration leaves a lot to be desired. Compromise is necessary, based on your experience and geology. New bit designs come along, and we would be wise to keep up with the changes so we don’t get passed by our competitors. Most of the time incremental improvements help in the long run.
Above the bit, many tools are available that lead to a harmonious outcome. First would be stabilizers. A stabilizer is a tool that helps keep the bit in the middle of the hole. It should be large enough to interact with the walls of the hole, and centralize the bit without interfering with circulation. Straight blade centralizers are cheap and easy, but may cause very high torque in rough running conditions. Spiral blade stabilizers run much smoother, but may restrict circulation some, and are more expensive to produce. In either case, the stabilizer near the bit should be very close to the bit size. Full gauge, to 1/16 inch under is about right. Once a stabilizer gets worn beyond that, it has done its job and should be rebuilt or moved up the string. When stabilizers are necessary, I generally run two or three stabilizers in these conditions. I start with the newest one on bottom, and move it up as it wears, always keeping a full gauge stabilizer on bottom. This makes a smoother, straighter hole.
Above the stabilizer in the BHA are the drill collars. There can be anywhere from one to many drill collars. They provide weight for the bit, and stiffness for the BHA to drill a straighter hole. Drill collars should be sized for the hole being drilled. They should be as large as practical, but still be fishable. If you run too big, and stick or lose your BHA, you will have no room to get over it with a fishing assembly. Larger collars concentrate the weight at the bit and make a smoother, straighter hole. Enough collars should be run to provide all the bit weight. I like to run enough collars so that I don’t use more than three-quarters of the collar weight on the bit. This keeps the top collar in tension, and helps save the cross-over sub between the collars and the drill pipe. It also gives me some insurance in case I need to run more weigh on the bit to make penetration. Drill pipe, unless specially made, is not made to run in compression. It is designed to provide tensile strength and torque. Running drill pipe in compression usually results in crooked holes and very rapid wear.
At the top of the BHA is the cross-over sub that connects the drill collars to the drill pipe. This changes the size and thread pattern from one to the other. It’s usually a simple, easily-forgotten-about part of the string, but there is one important thing to keep in mind. Drill collar threads are usually large, robust threads that take a lot of torque to make up and break out, and drill pipe threads are usually smaller, requiring different torque and handling. I have seen subs that go from 6⅝ regular to 2⅜ in one step. This will be the weakest point in the entire drilling assembly, so careful measurements are in order before you run it in the hole — just in case you have to fish it out.
These are the basics of a decent BHA, but there are other tools commonly run to make a more harmonious outcome. If you are making fast hole in a formation that may heave or flow back when you make a connection, a drill pipe float is a good idea. A float valve goes just above the bit, usually in a specially bored bit sub. It allows normal circulation, but prevents flow back into the bit on connections. This can save trips to unplug the bit. There are several types — flapper, ball etc. — but they all do the same thing. Some are wire-line retrievable, if you need it. Funny thing, unless it happens to you is, to run the float upside down. You can go in the hole just fine, but you can’t circulate. Everybody laughs except the driller and that one poor hand that loaded the float.
Other “jewelry” that can be in the BHA include jars. If you are running in an area that is known to be sticky, or very fractured, I’m sure you have had the experience of drilling a connection down and not being able to pick up. Oops. Stuck. With jars in the string, you can usually pick up enough to fire the jars and free the string to continue drilling. The key to jar placement is to have them as close to the stuck point, and still be above it. This placement is a best-guess deal and, sometimes, requires guidance from on high. Too low, and the jars are stuck too. Too high, and they are so far from the stuck point that they don’t do any good.
Hope this helps keep you turning to the right economically. If I can help, contact me.
For more Wayne Nash columns, visit www.nationaldriller.com/wayne.