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Week 2 > Plug Cementing > Flashcards

Plug Cementing Flashcards

1
Q

What is a Cement Plug

A

a certain volume of cement slurry placed in the wellbore for various purposes

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2
Q

What is the purposes of a cement plug?

A
  • Side track
  • Plug back a zone or well (abandonment)
  • Solve a lost-circulation problem
  • Provide an anchor for OH tests
  • Protect / isolate a low pressure zone during a work over treatment
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3
Q

Reason for a Side Tracking plug & Direction Drilling

A
  • Sidetrack above a fish
  • Drilling program may call for a pilot hole (for exploration purpose) that will be plugged back afterwards
  • During directional-drilling operations through soft formations it may be difficult to achieve the correct angle
  • Missed target Section milled window to sidetrack from a cased hole (work-over)
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4
Q

Reasons for Plug & Abandonment

A

Reasons

  • To prevent flow of formation fluids inside a wellbore between formation zones and/or to surface/seabed

Types of Abandonment

  • Undesired perforations
  • Depleted zone
  • Permanent or temporary well abandonment
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5
Q

Lost Circulation plug reasons

A
  • To place cement slurry in a thief zone where it will harden and plug the pore spaces making the zone stronger and more consolidated to withstand hydrostatic pressure until fresh formation is drilled
  • Cement needs to bond and consolidate the formation; otherwise, losses may reoccur once the cement plug is drilled out
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6
Q

Hydraulic Isolation Plug reasons

A
  • Prevent communication between an upper productive formation and a deeper depleted zone (permanent isolation)
  • Reduce excessive gas or water production
  • Safely perform a workover treatment above a productive interval (temporary isolation)
  • Provide a competent seat for an openhole packer when a mechanical bridge plug would not provide it (e.g. because the formation is too weak)
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7
Q

Plug Placement Techniques

A
  • Balanced Plug
  • Pump & Pull Method using Drill Pipe
  • Dump Bailer
  • Pump & Pull Method using Coiled Tubing
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8
Q

Balanced Plug characteristics

A
  • Most common method
  • Work string run to desired bottom of plug
  • Fluids heights balanced in annulus and string
  • Under-displacement by a barrel or two to avoid wet pull
  • Preferred method for KOP and P&A except for cases with horizontal sections and low fluid density hierarchy
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9
Q

Job Procedure - Balanced Plug

A
  1. Pressure test treating lines
  2. Pump spacer or wash ahead of cement slurry
  3. Mix and pump cement slurry
  4. Pump spacer or wash behind cement slurry to balance plug
  5. Displace calculated amount of displacing fluid
  6. Under displace by 1 - 2 bbl to avoid wet pull
  7. PlugAdvisor required to optimise fluid tops while POOH
  8. Open return lines to the displacement tank on the unit and allow plug to balance itself either by return flow or vacuum
  9. Pull drill pipe or tubing above the top of spacer/wash (30ft/min)
  10. Reverse circulate at least 1.5 DP volume if conditions allow –(500 ft above TOC or 100 ft to 150ft above expected TOC for cleaning excess cement)
  11. POOH and WOC (500 psi for most plugs unless for a KOP)
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10
Q

Pump and Pull Method characteristics

A
  • Applicable for cases where fluids are not balancing themselves when POOH, therefore, mud needs to be pumped in the drill pipe for every stand pulled out, to “force” the cement slurry out of the drillpipe
  • Horizontal sections
  • Non Horizontal sections
    • Very close density between mud, spacer and slurry
    • Use of dart or a mechanical separator behind the slurry
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11
Q

Dump Bailer Method advantages and disadvantages

A

Advantages:

  • The depth of cement plug is easily controlled
  • Reduced rig time for smaller volumes

Disadvantages:

  • Not easily adaptable to use in deep and/or highly deviated wells
  • Quantity of cement limited to volume of dump bailer. Volume per run varies with size and length of Dump Bailer. Max. volume that can be dumped in a single run is around 5.4 gal
  • Multiple runs may be required to dump desired quantity
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12
Q

Pump & Pull using Coiled Tubing advantages an disadvantages

A

Advantages

  • Accurate depth control
  • Less contamination
  • No workover rig required to pull out completion
  • Can perform operations on a live well
  • Continuous pipe, faster operations

Disadvantages

  • Small volumes of cement
  • Lower pump rates
  • Special Slurry Properties
  • Low PV & TY
  • Zero Free Fluid
  • Low Gel Strength
  • Low Fluid Loss <50 ml/30min
  • Long TT, tested at BHST
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13
Q

Pump & Pull using Coiled Tubing procedure

A
  • CT on bottom, pump washer and cement
  • Once 50 ft Cement Out, start POOH with Coiled Tubing
  • Maintain 50ft overlap during placement
  • Stop pumping when all cement is out of CT, POOH
  • Once CT is 100ft above TOC, start pumping bottoms up until returns are clean
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14
Q

Placement Tools for plugs

A

Mechanical Fluid Separators

  • Two-plug method using Plug Placement Tool (PPT)
  • Foam Balls

Competent Bases

  • Mechanical
  • Bridge Plug
  • Inflatable Packer
  • Cement Support Tool
  • Cement Plug

Fluid

  • Viscous Pills
  • Reactive Pill (not recommended in horizontal sections)
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15
Q

Plug Placement Tool benefits

A
  • Positive indication of displacement
  • Fluid separation
  • Minimize cement contamination, due to the pipe cleaning action of the wiper plug
  • Ensure correct displacement volume whatever the uncertainties on pipe ID are
  • Leave a clean drill pipe inner wall - free of cement - once the job is completed.
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16
Q

Foam balls and Wiper darts characteristics

A
  • Necessity should be assessed by PlugAdvisor depending on contamination ahead or behind
  • Can be used during direct circulation after POOH to clean DP
  • Wiper darts may be more efficient in cleaning the pipe but more complicated to use than foam balls
  • May prevent freefall of slurry while POOH so pump and pull method may be required
17
Q

Foam Balls characteristics

A
  • Displacement & post circulation at higher rates
  • Use DP wiper darts / sponge balls
  • Reverse the cement until the drill pipe is clean or at least 2 drill pipe volumes
  • Abrasive Pills like Nut shelves, LCM, sand, acid, etc.
  • Even if using a wiper plug, cement rings are expected to be left at the pipe connections
18
Q

Competent Base – Viscous Pill characteristics

A

Properties-

  • Optimum Density- MW + 1 ppg
  • Thixotropic properties
  • Highest possible Ty(~ 100lbf/100ft2)

Placement

  • Minimum pill length of 300ft, preferable = cement plug length
  • Place with balance plug method
  • Use PlugAdvisor for optimum placement volume and contamination
  • Extended slurry Cement plug with 50 psi CS can also be used
19
Q

Volume Estimation for plugs

A
  • Recommended plug length 500-800 ft
  • Volume depends on
    • Objective of the job
    • Caliper availability
      • 10 % excess over LWD or WL caliper
      • Fluid caliper
    • No caliper available
      • 10-20 % in OBM
      • 50-100 % in WBM
      • 100-200% for deviated wellbores or destabilized shale sections
      • If TOC is found too high it can be dressed off before the cement sets
    • Local Best Practices
  • Always optimize volumes using PlugAdvisor
20
Q

Slurry Properties for a plug

A

Density

  • Lighter for Lost Circulation
  • Heavier for Sidetracking
  • Homogeneous - batch mixing

Rheology

  • Higher for Lost Circulation
  • Optimum (mud removal) for Sidetracking
  • Lower for placement with CT
  • Low gel strength

Fluid Loss

  • Low for open hole plugs and squeezes

Compressive Strength

  • Minimum 500 psi for drill out
  • 50-100 psi for base plug
  • SLB recommend >5000 psi for KOP, but ultimately depends on the hardness of the formation

Free Fluid

  • Zero for horizontal plugs

Thickening Time

  • Enough for placement
  • Adhere to Thickening Time policy including time to POOH and circulate out the cement
21
Q

WOC for a plug?

A
  • Should be based on compressive strength results from laboratory testing
    • UCA or Crush Test
  • 50% safety factor should be applied over time recorded in lab
  • Minimum 500 psi CS to tag or pressure test the cement plug
  • Use CemCADE to predict the correct BHCT for UCA testing
  • Most cases 12-24 hrs to build CS is a reasonable target
  • Compressive strength should be tested with contamination between spacer and cement besides mud
22
Q

Horizontal Plugs challenges

A
  • Slumping
  • Plug can’t balance
  • Plug disturbed while POOH
23
Q

Horizontal Plugs mitigation of risks

A
  • Pump-and-pull method with DP or coil tubing
  • Sacrificial Tubing
  • Enough cement volume compensate for slumping
  • Mechanical retainer or viscous pill at bottom
  • If possible place TOC in inclined section (not horizontal)
  • POOH extremely slow
  • High yield point of slurry( ~30 lbf/100ft2)
  • Zero free water and stable slurry
  • String rotation for 360 degrees coverage
  • Design of spacer with density close to slurry
24
Q

Reasons for Cement Plug Failures

A
  • Lack of hardness (sidetracking).
  • Poor isolation (plug back, abandonment).
  • Wrong Depth (all plugs).
  • Not in place due to sinking to the bottom (all plugs).
  • Not in place due to loss to thief zone (lost circulation).
  • Not in place due to fluid swapping between cement and fluid underneath.
25
Q

Cement Plug Failures - Soft Cement or TOC too Deep

A

Well Condition

  • Deep hole
  • Inaccurate open hole size estimation
  • Inaccurate drill pipe ID
  • Incorrect temp. and pressure estimation
  • Losses, wellbore instability, potential flow zone, etc

Placement Technique and Design:

  • Contamination with mud and/or spacer: while in the pipe, annulus or during POOH
  • Contamination with base fluid (HVP..etc.)
  • Plug sinking through a lighter wellbore fluid or lack of a competent base
  • Losses due to high ECD while reversing

Slurry Design

  • Incorrect ramp time to temperature and pressure for lab testing
  • Inaccurate slurry design and lack of hardness

Poor Client Communication

  • Drilling out too soon
  • WS placement recommendations not followed

Execution:

  • Job is not placed as designed by the WS team on the rig: small deviations of fluid volumes, fluid densities, additives concentrations
26
Q

Cement Plug Failures - Cement Left in Pipe (CLIP) Reasons

A
  • Very high cement gel strength development
  • Longer pull-out time than expected
  • Absence of circulation during the pull-out resulting in leaving the cement in static condition for a long time
  • Fluid contamination that accelerates gel development and/or decrease cement thickening time
  • Inaccurate estimation of BHCT from a BHST higher than expected or not enough circulation prior to the job
27
Q

Reasons for Failure Can be Traced to the Following

A
  • Slurry not designed for enough compressive strength
  • Not enough WOC time
  • Inaccurate BHST
  • Cement contamination during displacement and POOH
  • Slurry not designed for the specific problem (lost circulation)
  • Not enough volume of cement or spacer ahead for mud removal
  • Difference between cement and hole fluids’ densities too high, causing the plug to sink
28
Q

Techniques to Improve Plug Success

A
  • Use of Cement Stinger
  • Centralization of the Work String
  • Use of Diverter Tool
  • Competent Base for plug
    • Viscous Pill
    • Mechanical Support
  • Condition the mud
  • Module
  • Use of Plug Placement Advisor* (Cement plug placement optimization software)
  • Sufficient Spacer or Wash
  • Workstring movement (rotation preferred)
  • Allow ample time for the cement to set.
  • Use a drill pipe plug and a plug catcher (PPT)
  • Use CemCADE Plug Placement
  • Use PlugAdvisor
29
Q

Cement Plug Design with PlugAdvisor advantages

A
  • Well geometry fully described and taken into account
  • Simulates intermixing of fluids on the way down the drillpipe, during placement in the annulus before pipe pullout, and after placement during pipe pullout
  • Plug interface stability is evaluated while POOH
  • Well security (losses)
  • Demonstrate the difficulty of each job
  • Increases the job success rate by estimating the uncontaminated TOC after placement
  • Display the efficiency of each placement technique
  • Enables mitigation of wet pipe pullout
  • Optimizes under displacement volume
  • Provides best practices with each type of plug

Week 2 (1 decks)

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