Thursday, 9 January 2020

UNDER GAUGE HOLE IN DRILLING

mechanical stuck pipe undergauge hole

Under gauge Hole

Under gauge Holes in drilling is a phenomena which means that the drilled hole diameter is less than the used Drilling Bit diameter. It always happens after the Drilling Bit and the Stabilizers gauges got worn while drilling abrasive rock, so the following section using the same Drilling Bit will produce less diameter hole or under gauge hole. And if it was decided to run new in gauge Drilling Bit after that, it will have difficulty to run in the last under gauge section and reaming will be required to pass this under gauge section. If it was decided to run quickly without reaming, the Drilling Bit can jam in the under gauge hole and become Stuck.
 
Under gauge hole may also be caused by a plastic formation. After being penetrated by the Drilling Bit with a fluid giving a hydro static pressure less than the overburden, which is the usual pressure of the plastic formation, this formation deforms causing the hole to close. One of the formations which most often causes such a situation is moving salt.

The only preventive measure is to use a drilling fluid dense enough to give overburden pressure at the depth of the salt. This will usually be impossible due to the risk of creating fractures elsewhere in the hole. In that case the drilling fluid density should be kept as high as feasible as the flow rate of the salt is proportional to the pressure difference. If the Drill String become Stuck the solution (!) is simple - a pill of fresh water. If this is done the effect of introducing a light fluid on well control aspects has to be taken into account. But here we will talk about the under gauge hole caused mechanically

So we can summarize when the under gauge hole happens into simple points.
  • After using a new Drilling Bit.
  • Coring is one method of collecting rock and sediment samples for geological research. Some coring heads are often slightly smaller than bit sizes and will produce under gauge hole so reaming is preferred for the cored sections.
  • When changing bits from Roller Cone Bit To PDC Bit in the same section.
  • When drilling hard abrasive rocks. The wearing speed of the drilling bit will increase and will lead to drill with worn less diameter drilling bit. So It is a good practice to check your Bit Dullness after POOH.
  • By plastic formations (we will not talk about this for now)
undergauge hole in abrasive sand stone


Preventative Action To Avoid Under Gauge Hole Stuck Pipe Type.


What Is The Indications For Under Gauge Stuck Pipe ?

  • As its reason is the under gauge of Drilling Bits and Stabilizers so first thing will be the pulled Drilling Bit or Stabilizers are under gauge.
  • And as it will happen after RIH with new in gauge Drilling Bit so the second indicator will be when RIH.
  • Sudden set-down weight.
  • Circulation is unrestricted or slightly restricted.
  • As the under gauge hole may happen after coring operations so Drilling Bit may Stuck at the top of a cored section.

How we could work on Freeing such mechanical stuck pipe type?

You can consider the following tips while freeing such type of Stuck Pipe
  • Jarring up with maximum trip load. 
  • Do not Jar down. 
  • Consider the use of an acid pill.
  • Consider applying torque as a last resort. But put in your considerations that Drill Pipe, Heavy Weight Drill Pipe HWDP yield strength will be affected with such torque and will be decreased.

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