Petroleum Engineering 405 Drilling Engineering

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3.2
Plugback Cementing
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Plugback Cementing

• Case I No Spacer
• Case II Equal Height Spacers
• Case III Spacer Ahead of Cmt. (only)
• Case IV Two Unequal Spacers
• Mixtures and Solutions

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Balanced Cement Plug

• Fig. 3.11- Placement technique used for setting
  cement plug.

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Cementing (Open-Hole Plugging)

• 1. Plug-back for abandonment
• 2. Plug-back for fishing or hole deviation
• Open-hole plugging is usually performed with
  slick drillpipe or tubing.
• In some cases, reciprocating scratchers may
  be run to enchance cement bonding.

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Types of Balanced Plugs

• Case I No water or other fluid of different
  density from that in hole is run ahead or behind
  the cement slurry.
• Case II Water or other fluid of different
  density from that hole is run ahead and behind
  cement slurry. The volume of fluid ahead and
  behind slurry is calculated so that height in
  casing is same as height inside the string.

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Displacement

• Case III Water or other fluid of different
  density from that in hole is run ahead of cement
  slurry and hole fluid only is used as displacing
  fluid.
• Case IV Water or other fluid of different
  density from that in hole is run ahead and behind
  cement slurry. In this case, the heights of
  fluid in annulus and drill string are not equal.

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Case I
C
T
Height of plug with pipe in place
Height of plug after pulling pipe
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Case II
hWD hWA
mud water cement water mud
hW
Height of plug with pipe in place
Height of plug after pulling pipe
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Pumping Sequence

• 1. Water spacer for annulus
• 10 bbls
• 2. Cement Slurry for Plug
• 3. Water spacer behind cement
• 2.0 bbls

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Pumping Sequence

• 4. Mud displacement behind second water spacer
• 148.5 bbls
• Total fluid pumped 10 56.2 2 148.5
• 216.7 bbls
• (at 10 bbl/min this would require 22 min)

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Case III
Hole fluid density gt density of
water Hydrostatic heads in DS and annulus must
balance at top of cement slurry with DS in hole.
hW
Height of plug with pipe in place
Height of plug after pulling pipe
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Case IV - General Case
Hole fluid density is greater than water
density. Hydrostatic heads in DS and annulus must
balance at top of cement slurry with DS in hole.
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Procedure in setting balanced plug

• 1. Run drillpipe in to depth where plug is to
  be set in this case 9,000 ft. (open ended).
• 2. Circulate and condition mud one complete
  circulation to make sure system is balanced.
• 3. Pump spacers and cement per calculations and
  displace w/proper amount of fluid

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Procedure in setting balanced plug

• 4. Stop pumps break connection at surface.
• A. If standing full, plug is balanced.
• B. If flowing back, a mistake in calculations
  has been made. Stab inside BOP,
• or have a heavy slug (small volume slug)
  ready to pump.

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Procedure in setting balanced plug

• 5. Once the end of the drillpipe clears the
  plug, there is a good chance the pipe will pull
  wet. This is because pressures have gone back
  into a completely balanced mud system.
• 6. If pulling wet, slug pipe and pull out of
  hole.

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Procedure in setting balanced plug

• 7. Even if plug is severely out-of-balance,
  never try to reverse cement out of hole.
• 8. Tag plug with DP at end of 8 hours. If too
  high, plug may have to be drilled out and
  another plug spotted. If too low, spot another
  plug to required height with DP just above top
  of first plug.

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Calculations to Design a Balanced Open Hole
Cement Plug

• 1. Calculate cu. ft. of slurry required for
  plug in open hole.
• 2. Multiply this volume by excess factor (50
  excess factor 1.50)

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Calculations for balanced plug - HINT

• When dealing with a washed-out hole, where an
  excess factor is required, it is usually easier
  to calculate a new, effective hole size, and use
  that instead of the excess factor.

If 50 excess is required
Use d2 for calculations This is the effective dia.
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Calculations for balanced plug

• 3. Find height (h, ft) cement will occupy when
• drillpipe is at bottom of plug during
  pumping

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Calculations for balanced plug - contd

• 4. Find height (ft) water spacer ahead of
  cement will occupy in annulus. Use d2 to
  calculate this (to account for the excess
  factor).
• Find height (ft) water spacer behind cement
  will occupy in DP. Do not use excess factor.
• 6. Pressures must balance at bottom of plug

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Calculations for balanced plug - contd

• 7.
• 8. Convert this to feet inside
  DP.

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Calculations for balanced plug - contd

• 9. Convert this footage to bbls inside DP for
  proper displacement.
• 10. To find sx cmt required, divide volume,
  V2, by yield/sk. This yield, Ysk, may be in
  the Halliburton tables.
• Number of sx reqd,

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Calculations for balanced plug - contd

• 11. If yield not shown, calculate from
  formula for mixtures. Solve for in this
  formula. Add the Vs for yield.
• 12. Total mix water will be times
  number of sacks.
• VW total (VW / sk) N