Almost every well constructed, and virtually all drilled wells, use casing to support the hole and allow access to the producing horizon, be it oil, gas, water or most anything else under the ground. The design of the casing is very important to the success of the well. Some offshore wells have as many as six or eight different strings of casing, set at different depths, to complete the hole. Most common house wells use only one string of casing with the possible addition of a telescoping screen, or liner. This is usually done to isolate an undesirable zone or assure that production comes from a very specific horizon.
Regardless of the casing design, there are some things that must be taken into account to run casing successfully. First, the hole should be as straight as possible. Casing is usually much stiffer than drill pipe, and doesn’t like to go around corners very well. In the case of directional wells, the bends should be as long and gradual as conditions permit. Abrupt dog-legs should be avoided.
The next consideration is the intended use. For instance, it is not uncommon to set 20 to 100 feet of “pit casing” to reach a competent enough formation to be sure that the hole will stay open during subsequent drilling. This string of casing is often pulled and recovered for use on the next well, after the “long string” is set. Used pipe works well for this because it is cheaper, and if you can’t pull it, you haven’t lost much.
If a string of casing is going to be left in the ground, and is an integral part of the well, other considerations come to mind. What we call “jewelry.” If you are going to cement the casing, what method will you use? On shallow strings, it is common to set the casing on bottom and cement outside through a tremie line. This usually requires drilling a bigger hole and making sure you can get your tremie line to bottom. Once again, a good, straight, clean hole helps. If you use this method with plastic casing, keep this in mind: figure out the collapse strength of the pipe. And second, consider heat of hydration. As cement sets, it produces heat, which will weaken PVC casing sometimes to the collapse point. One way to get around this is to hold pressure on the inside of the casing until the cement is set. Usually 6 to 12 hours will do it. If the rig doesn’t have a dedicated string of pipe to use for tremie line, PVC pipe usually works just fine, depending on depth. You can just cut it off and leave in the hole; it won’t hurt anything.
If you are going to cement using the “Halliburton method” — in other words, pump the cement through the casing to the outside — a method I prefer, a little more “jewelry” is helpful. A float shoe is basically a check valve that allows the cement to flow out, but does not let it return into the casing. When the calculated amount of cement, plus some excess for washouts, is pumped, a wiper plug is pumped until a sudden pressure increase is seen, indicating the plug is seated in the float shoe. To assure that competent cement is around the casing shoe, some drillers run a “float collar.” This is basically like the float shoe, but is made up and run in the string a couple joints above the float shoe. When landing the plug, it assures that competent cement is emplaced outside the shoe joint, assuring a good seal. It will leave a column of cement in the casing called the “shoe track.” It will be drilled out on the next bit trip. This is usually pretty easy drilling if the cement is still a little green. One way to make a cheap, quick and easy shoe is to put three or four brazing rods through the casing, about a foot off bottom, forming an X. When you have pumped your cement, wad up a couple sacks, stuff them in the casing and pump. When they get to bottom you will see a pressure increase. Stop pumping and hold pressure on the casing. Cheap and easy to drill out, too.
Another consideration is buoyancy. Cement is usually a lot heavier than drill mud, and may cause the casing to try to “float” out of the hole. On big strings this can be a lot of force. I once lifted the back of a Gardner-Denver 3000 rig 2 feet due to buoyancy! Make sure everything is well anchored.
Another factor to consider is casing weight. Not a problem with PVC, but steel casing can weigh much more than the drill string. You can probably drill a lot deeper than the rig will safely set casing in. In this case, you should consider “floating” the casing in. You will need a float valve on bottom, and to calculate your collapse pressure and fill volume. You will need to fill every joint for the first few, until the string gets enough weight to overcome the buoyancy. After that, you fill when you are starting to get close to collapse pressure, and stop filling when it gets too heavy for the rig. By the way, you should never do this with PVC casing. The chances of collapse are real.
One last item of jewelry: centralizers. A casing seal will be no good if the pipe is laying on the wall of the hole. At the very least, a couple centralizers near the bottom of the string will help assure a good seal. I’ve seen drillers run so many centralizers, usually because of a junior assistant or engineer trainee, that they couldn’t get the pipe down! Common sense helps.
If I can help with your casing or cementing questions, don’t hesitate to contact me. My advice is free, and usually worth what you pay for it.
Keep ‘em turning to the right.
For more Wayne Nash columns, visit www.thedriller.com/wayne.