There are several articles discussing about casing design and we would like to share you this excellent VDO training in this topic “Oil & Gas Well Casing”. This VDO contains the basic over view of casing starting from surface to production section. You will see nice illustrations which help you get clearer picture on this topic and we also add full VDO transcripts to provide assistance to some learners who cannot catch the content in this VDO clearly.
Oil & Gas Well Casing – VDO Transcript
By the time the crew drills the well to depth, it usually has several strings of casing in it. These strings are called conductor casing, surface casing, intermediate casing and production casing.
Notice that cased well looks something like a telescope pulled out of full-length but it is as the crew drills the well deeper, the size of the whole and the size of the casing gets smaller in diameter. Almost always, the drilling contractor cannot begin drilling at the surface and go all the way to total depth in one step.
For one thing, formations near the surface tend to crumble and cave-in easily so conductor casing prevents cave-ins. For another thing, formations near the surface may also hold freshwater that the well cannot contaminate. So surface casing prevents freshwater zones. For still another thing, deep formations are sometimes so-called troublesome formations. That is, they can be drilled by adjusting the properties of the drilling mud but once drilled, need to be sealed off to prevent problems in drilling the deeper portions of the well. So, intermediate casing seals of troublesome zones. Sometimes, deep wells required more than one intermediate casing string. Finally, once the producing zone is drilled, it needs to be protected and sealed; so production casing isolates the producing zone. The first string of casing is the conductor casing.
The hole drill first is pretty big; often as much as 36 inches or more as, almost a meter in diameter. The conductor hold has to start out with a big because as drilling goes on, the hole’s diameter decreases. In some cases, the rig will hammer the conductor casing in place if the ground near the surface is really soft. If the conductor hole is drilled, the casing is cemented in it. Using a bit whose diameter is small enough to easily go inside the conductor casing, the rig drills the cold below the conductor to a prescribed depth.
The diameter of the surface hole can still be relatively large; say 17 inches, over 400 mm or even more. The surface hole’s depth is usually set by regulatory agencies. They require that the surface hold a drilled through all freshwater zones and that surface casing be set and cemented to protect the zones from damage by additional drilling operations. This depth could be from hundreds to thousands of feet or meters. Normally, crew-members nipple up or connect the BOP’s to the surface casing at the well head. So this casing must be strong enough to support the BOP stack. In addition, it has to withstand the gas or fluid pressures the well may encounter. Surface casing, also has to be strong enough to support the addition of casing strings hung inside of it.
To drill the intermediate hole, the operator chooses are still smaller in diameter bit which easily fits inside the surface casing. A bit of about 12 inches or 300 mm in diameter, is one example of the size. Intermediate casing is also cemented into place to seal off troublesome formations like moss circulation zones or abnormally pressured zones. It is often the longest section of casing in the well; Also the crew connects or nipples of the BOP’s to the top of the intermediate casing by using an adapter and casing head or a drilling spool which is stacked on or connected to the top of the surface casing wellhead. It therefore anchors the BOP’s for the drilling that comes later. Remember, that the crew has to nipple up a stack of BOP’s to each string of casing that is run into the well; first they nipple up on the surface casing then on the intermediate casing and finally on the production casing.
To drill to final depth below the intermediate casing, the rig owner selects a bit whose diameter is small enough to fit inside the intermediate casing; say from 8 to 10 inches or 200 to 215 mm. This part of the hole penetrates the producing zone. When cemented in place, production casing seals off the producing zone and readies it for production. Production casing also houses and protects the tubing and other equipment used to produce the well. The operator usually perforates, puts holes in this casing when the well is completed or ready for work to begin.
Well completion is the term describing the activities and methods of preparing the well for production of oil and gas. Oil and gas flow into the well through the perforations. Sometimes well owners run liners instead of casing into the well. A liner, is a shortened the string of casing used the case the smaller open hole section below an existing casing string in the hole. It’s just like casing, except that a liner does not run all the way to the surface. Instead, the casing crew hangs it from the bottom of the previously run casing or liner string using a special piece of equipment called liner hanger. In this case there is an intermediate liner and a production liner. Using liners saves money since they do not extend to the surface.