Air, Gas, and Mist Drilling

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Lesson 14
Air, Gas, and Mist Drilling Read UDM Chapter
2.1 - 2.4 pages 2.1-2.74
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Air, Gas, and Mist Drilling

• Circulating Pressures

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Circulating Pressures

• Calculating standpipe pressure starts with
  predicting the pressure just below the bit, and
  working your way back to the surface

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Bit Pressure Drop

• As air flows through the jets, it expands in
  response to the decrease in pressure and its
  velocity increases
• Once the pressure drop exceeds a certain level,
  the air velocity reaches the prevailing speed of
  sound.

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Bit Pressure Drop

• At this point, the air cannot expand any faster
  and the upstream pressure becomes independent of
  the downstream pressure.
• This implies that under sonic discharge
  conditions the standpipe pressure is independent
  of the annular pressure

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Bit Pressure Drop
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Bit Pressure Drop

• If the upstream pressure is more than 1.89 times
  the annulus pressure beneath the bit, flow
  through the bit will be sonic.

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Upstream bit pressure - sonic flow
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Upstream bit pressure - sonic flow
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Upstream bit pressure sub-sonic flow
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Upstream bit pressure sub-sonic flow
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Upstream bit pressure sub-sonic flow
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Standpipe pressure
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Steps to predicting standpipe pressure

• Assess whether flow through bit is sonic or
  sub-sonic
• If sonic, the pressure above the bit is
  determined with equation 2.21
• This value is used in equation 2.25 to predict
  standpipe pressure

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Steps to predicting standpipe pressure

• If flow is sub-sonic, the annulus pressure below
  the bit must be first predicted (Angels
  analysis, etc) using equation 2.12.
• The pressure above the bit is determined by
  equation 2.23
• This value is used in Equation 2.25 to determine
  standpipe pressure

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Important point

• When air drilling, large changes in annulus
  pressure may result in smaller changes in
  standpipe pressure, or in the case of sonic flow
  through the bit, no change in standpipe pressure
  at all.
• Hole problems that lead to an increase in annulus
  pressure may be indicated by small changes in
  standpipe pressure.

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Important point

• It is very important to monitor the standpipe
  pressure closely and react promptly to
  unanticipated changes.
• It is important to know if flow through the bit
  is sonic or not.
• If flow is sonic, standpipe pressure will not
  change with changes in annulus pressure

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Example

• 8.1/2 hole at 6000 drilled with 4 1/2
  drillpipe air rate is 1400 scfm. Penetration rate
  ranges up to 300 ft/hr. Bit has no nozzles in
  one example and 3- 14s in the other.

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