Plug Cementing Flashcards by Daniel Mendez

What is a Cement Plug

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

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What is the purposes of a cement plug?

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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Reason for a Side Tracking plug & Direction Drilling

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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Reasons for Plug & Abandonment

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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Lost Circulation plug reasons

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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Hydraulic Isolation Plug reasons

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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Plug Placement Techniques

Balanced Plug
Pump & Pull Method using Drill Pipe
Dump Bailer
Pump & Pull Method using Coiled Tubing

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Balanced Plug characteristics

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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Job Procedure - Balanced Plug

Pressure test treating lines
Pump spacer or wash ahead of cement slurry
Mix and pump cement slurry
Pump spacer or wash behind cement slurry to balance plug
Displace calculated amount of displacing fluid
Under displace by 1 - 2 bbl to avoid wet pull
PlugAdvisor required to optimise fluid tops while POOH
Open return lines to the displacement tank on the unit and allow plug to balance itself either by return flow or vacuum
Pull drill pipe or tubing above the top of spacer/wash (30ft/min)
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)
POOH and WOC (500 psi for most plugs unless for a KOP)

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Pump and Pull Method characteristics

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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Dump Bailer Method advantages and disadvantages

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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Pump & Pull using Coiled Tubing advantages an disadvantages

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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Pump & Pull using Coiled Tubing procedure

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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Placement Tools for plugs

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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Plug Placement Tool benefits

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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Foam balls and Wiper darts characteristics

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

Foam Balls characteristics

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

Competent Base – Viscous Pill characteristics

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

Volume Estimation for plugs

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

Slurry Properties for a plug

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

WOC for a plug?

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

Horizontal Plugs challenges

Slumping
Plug can’t balance
Plug disturbed while POOH

Horizontal Plugs mitigation of risks

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

Reasons for Cement Plug Failures

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.

Cement Plug Failures - Soft Cement or TOC too Deep

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

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

* 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

Reasons for Failure Can be Traced to the Following

* 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

Techniques to Improve Plug Success

* 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

Cement Plug Design with PlugAdvisor advantages

* 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