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I would like to know how the core is cut off at the bottom of the drill. I also would like to know how the core is brought up to the surface without slipping out and falling back down.

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Coring is an important method to get detailed information about the formations, however, it's very time-consuming and costive, so it's only applied when geologists and reservoir engineers need high-resolution data. It is more common in ore prospecting and mining, mapping for infrastructure and basic geological understanding of the 3-dimentional structures. Lake sediment cores are used for varve chronology. Marine sediments are cored to understand the oceanographic development, date events as slope processes and changes in glaciation and climate. Ice is cored to measure variations in the atmosphere to understand past climate and vulcanism.

To sum up, coring is a very important tool for geologists, not only in oil fields. Cheaper, and faster, alternatives to coring is mud logging, a very common practise when drilling in oil fields.

So how to get the cores up from the borehole?

There are some different systems used, depending on the rock/sediment and investment involved.

For deep drillings in hard formations, a core catcher is used in a two-barrel system. The corehead (the drill bit) is connected to the outer barrel, while the core is pushed into an inner barrel. The lower part of the inner barrel is called the shoe just above the drill. In the shoe, there is a core catcher that wedges between the barrel and the core. The core catcher is a split ring with a rough inside that holds the core. When you pull the core, it snaps off as the lower and upper shoe squeezes the catcher. The core catcher works somehow like an expansion anchor, but inverted, grabbing the core on the inside of the barrel rather than the wall on the outside. The inner barrel also protects the core from the rotating outer barrel.

For softer sediments, the core catcher is made out of springs that close inwards as the core is pulled. It is sometimes called basket retainer and can be in metal or plastic.

For smaller gravity corer, there is a valve on top of the tube that is closed and the underpressure in the tube holds the core in place. Sometimes...

Check the linked pictures, they probably explain better how it works, but I couldn't find any good CC.

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  • $\begingroup$ Core springs are also used in very hard rock - at least for relatively small diameter core. I have used them in Archean meta-sediments and igneous rocks. $\endgroup$ – haresfur May 11 '18 at 1:51
  • $\begingroup$ "core catcher" link doesn't work for me. $\endgroup$ – Keith McClary May 13 '18 at 3:15
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Coring is not done continuously in oil drilling, but is done selectively when it is determined that the information obtained is worth the cost. As blacksmith37 notes the crushed rock flushed from the hole by the drilling fluid is the primary way of determining information about the formations the well is going through. But if an oil company needs a solid sample for measuring formation properties for, for instance, modeling a reservoir on a computer they have special drills that can be put down the hole and that capture the core produced. The core drill assembly at the base, just inside the core has a slip collar that while drilling slides down the core but when the drill string is pulled up grabs onto the core, holding it in the sample tube and also breaking off the core at the bottom. To see a diagram check out page 8 of this brochure on coring services

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From my personal experience there is two types of coring. 1st being conventional coring. Conventional coringis preformed using third party cutting tools as (BHA) bottom hole assembly. This consists of a rotating cutting head made up of tungsten carbide/diamond. The core barrel sits on top of cutting bit than is fasten by an x/o (cross over), to the drill string and Kelly or top drive (rig specific).

This (bha) equipment is delicate as far as rig equipment goes. Special attention is required while cutting and retrieving core. Torque, weight, pump pressure (if required), rotary speed among other things must all be monitored closely. Once the core is cut. It is delicately fasten inside the barrel with a series of little metal prongs in a circular pattern.

A trip speed is required as to not damage or shake the core out. Trip speed is time it takes to lay out (dp) drill pipe.

2nd method is wire line retrieval. This uses a Logging truck with gamma tools that log data and X-ray the intended zones. This system requires a gun barrel system on the end of a wireline. The drilling rigs purpose is basically just a plat form.

This is just a very basic description from my personal experience. Feel free to ask more.

I’ve provided some examples of bits & barrels as well links with detailed descriptions of both processes.

Schlumberger‘s coring methods&tools

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I doubt this technique is used in petroleum, the wells are usually too deep for this to be practical. It is used for mineral deposits that are not so deep : As I remember "Longear" makes these coring tubulars. I expect the core rock breaks at natural fissures. Petroleum formations are evaluated by examining the drillings in the drilling mud.

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  • $\begingroup$ I'm quite sure it is commonplace to use this technique in petroleum. I just visited a huge laboratory with an innumerable amount of cylindrical samples all which were collected from the oil field. However, I believe you are right about examining the drillings(cuttings) in the mud. They do both types of evaluations(core and cuttings) and each type of evaluation has its purpose(s). $\endgroup$ – Mike K Jul 8 '17 at 22:41
  • $\begingroup$ After "googling" Longear , i am going to stick with my answer. Coring is limited to relatively shallow holes ( one thousand ft .?). In petroleum this would be tar sands, shale oil and general info on a shallow formation that extends to greater depth at another location . Although wire line retrieval permits greater depth of operation. $\endgroup$ – blacksmith37 Jul 10 '17 at 15:32
  • $\begingroup$ I must backtrack; occasionally a sample larger than chips is needed for tests like permeability , then a core sample is taken from a deep formation as described in the first answer. $\endgroup$ – blacksmith37 Jul 11 '17 at 16:11

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