Assorted Q&A About the Oilfield

Since I have too much material to put in my class, I'm going to start putting stuff in here.

This represents my understanding of these topics. However, if you happen to be an expert on any of these topics, and you see an error, please send me an email at so I can fix the error.

A video of the inner workings of a LACT meter (no sound)
For definitions of words or acronyms, try this link: Minerals Management Service Glossary

For pipeline lingo, try this link: Pipeline Dictionary

Definitions I didn't find in the MMS Glossary are here. If you need a definition and it's not in the glossary, email me and I'll tell you if I know...

In general, shallow wells tend to have oil, deeper wells tend to have more gas. In California, the shallower the wells are, the heavier the oil tends to be.

This site has some good links:

On diagrams, the color scheme is usually:

  • green = oil
  • red = natural gas
  • blue = water
    On well logs, porous formations are often colored yellow.

    Sometimes, the formation does not have enough permeability to allow oil or gas to flow out. We help this process by pumping fluid down the well at high pressure to fracture the rock. Usually, we do this thru holes in the casing (pipe) in the well, but occasionally, somebody will drill a well and the rocks are hard enough that they don't have to put any pipe in it (the hole will stay open for many years by itself). Here is a picture of a fracture that was put into such an "open hole competion."

    How we drill sideways is shown in this picture, which features Baker Oil Tools illustration of their Bottomhole Assembly (BHA).

    The drill bit is connected to the bottom of a series of drill pipes. These are heavy duty steel pipe, about 3 to 5 inches in diameter. Each piece of drill pipe is called a "joint." A joint will be about 30 feet long. This picture is from an advertisement of Petron company.


    Normally, when we run pipes in the well, they are connected all the way back to the surface. This way, we can hook onto the pipes with the rig and pull them out.

    Ever wonder what happens if the pipe breaks off down there? How do they get the pipe out in that case? The answer is, "fishing." And it's a lot like fishing in a lake in that you lower something in to hook onto the pipe. In wellwork, these "hooks" are called overshots, shoes, washover pipe, grapples, or maybe some other term I haven't thought of. If you were catching a fish in a lake, you'd wait until you felt a jerk on your line due to the weight of the fish. The weight indicator on the rig is analogous to your hand: we watch the weight indicator to see if there is an increase in the weight on the end of the pipe. If the weight increases, we must have caught our fish!

    Sometimes you hear that people have "shot off" the pipe on purpose and left a fish in the hole. This is done when the bottom part of the drill string has become stuck to the point that the rig cannot pull it out. We (usually) "shoot off" the pipe with a "string shot." A string shot is an explosive charge (about 15 times as powerful as the charges used for perforating). However, rather than punching a hole thru the pipe, the charge is spread out so that it imparts a blow to the pipe without making a hole in it.

    The string shot by itself does not disconnect the pipe: before they detonate the string shot, they put torque (twisting force) into the pipe. Remember that the bottom end of the pipe is stuck, and it won't twist or move up and down. So if you turn the top of the pipe to the left, it will tend to act like a spring and store up the torque. They keep it twisted like that until they detonate the string shot. The string shot causes a blow to the pipe, and hopefully it will untwist itself by coming loose at one of the connections. This actually works most of the time. Sometimes you have to try a couple or three times to get the pipe to back off.

    This is the best picture I've ever seen that illustrates a packer. It is from a Schlumberger ad. This particular one is shown in open hole, but packers are also used inside casing. They are showing two rubber elements: many packers have only one element. Sometimes people refer to packers (and other items) as "tools."

    They have some widget inside the tool, which you cannot see, and this widget acts to inflate the rubber elements, pushing them against the walls of the hole and sealing the annulus.

    Generally, they run packers on some sort of pipe, usually tubing or drill pipe, but they can run them on wireline in some cases.

    On this particular model, it looks like they have a little test port below the bottom packer element. I'd imagine that they must have a test port between the elements as well (either that, or maybe one of the elements is a backup in case the first one doesn't seal all the way). They seal off the formations above the test port so they can make sure they are measuring the pressure (or some other parameter) of just the particular zone they are interested in. I have seen some of these tools work, and they are really pretty slick! Some of them have a little port that comes out (like a robot) and sticks itself to the formation. Then, it sucks in fluid past a sensor and you can see the electrical resistivity of the fluid on the screen in the logging truck. When you pull the tool out, the fluid will be in a chamber inside the tool, and you can run more tests on the fluid.

    How do you tell a drilling rig from a workover rig? I like my friend Mike Heimer's strategy: "if the rig has a lot of trucks parked around it, it's probably a drilling rig. If it has relatively few trucks around it, it's probably a workover rig."

    In areas where "triples" rigs are common, you can tell the drilling rigs because they are bigger. But that's not true in areas, like California's San Joaquin Valley, which use "doubles" rigs.


    Perforating - a part of many completion operations

    This photo shows some employees of Schlumberger running a test of their perforating equipment.
    Basically, the perforating gun has a bullet that makes a hole in the casing. Usually, the casing has been cemented in the hole before we perforate. Since the cement is between the casing and the rocks, and the rock contain the oil, the oil cannot get into the well unless we perforate to make a hole in the casing and cement. People call this hole the "perforation tunnel."
    One question that people have about perforating is, "how far out will the perforation tunnel extend?" We want to make sure that we get the tunnel deep enough that it goes past the cement. Usually, the casing is 1/4 to 1/3 inch thick, and the cement will be another inch or so of thickness. It's no trouble to perforate thru the pipe and cement.
    At least in theory, the farther out your perforation tunnel goes, the better connection you should have to the reservoir. What the guys in the picture have done is to test out their perf guns in a concrete model. They pour the concrete inside the shiny corrugated pipes, then shoot the perf gun into the concrete to see how far the tunnel extends.

    Cross Section of a Perf Test

    Here is a cross section of a test similar to the one being run above. Halliburton company tested a couple of perf guns using sandstone cores, and then cut them in half to see where the perfs went. The marks are in inches.

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