PLUG CEMENTING

Module CF110 NL08 Sep 99

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ObjectivesBy the end of this module, a trainee will be able to do the followings: List the purpose of setting a cement plugs List the three different techniques List the advantages and disadvantages of each technique Recognise the job consideration Select slurry properties to fit with the plug type Explain step by step the cement plug job procedure All necessary calculations to perform a balanced plug

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Cement Plugs Introduction Setting a cement plug in a well is a

common oil-field operation.

A cement plug involves a relatively small volume of cement slurry.

And is placed in the wellbore for various purposes:

To side track above a fish or to initiate directional drilling.

To plug back a zone or plug back a well.

To solve a lost-circulation problem during the drilling phase,

To provide an anchor for OH tests.

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Side Track and Directional Drilling

Kick Off Point

NEW

HOLE

CEMENT PLUG

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Plug Back and Depleted Zone

Depleted Zone

Cement Plug

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Lost Circulation

Drill Pipe

Open Hole

Drill Pipe

Thief ZoneCEMENT

PLUGCEMENT

PLUG

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Abandonment

CEMENT PLUG

CEMENT PLUG

CEMENT PLUG

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Plug Placement TechniquesThere are three common techniques for placing cement plugs:

Balanced plug

Dump bailer

Two-plug method

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Balanced Plug

Displ. Fluid

Spacer

Cement Slurry

Balancing

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Balanced Plug

Displ. Fluid

Spacer

Cement Slurry

Plug Length

Balancing Reversing

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Dump Bailer Method

WIRELINE

DUMP BAILER

CEMENT SLURRY

ELECTRICAL/ MECHANICAL DUMP RELEASE

BRIDGE PLUGCASING

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Dump Bailer MethodAdvantages:

Depth of cement plug is easily controlled.

Relatively cheap.

Disadvantages:

Not easily adaptable to setting deep plugs.

Quantity of cement limited to volume of dump bailer.

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Coiled Tubing Cement Plug Technique

Cement

Suspended mud System

Coiled Tubing

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Job Design Considerations Why is the cement plug being set? At what depth will the plug be set? Across which formations is the plug going to be

set? At what density should the slurry be mixed? What is the BHT? What volume should be pumped? What is the required thickening time? How to insure the cement will not be

contaminated by mud? Are pipe centralization and rotation necessary? Waiting on cement time?

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Slurry PropertiesDensity - lighter for Lost CirculationDensity - heavier for SidetrackingDensity - homogeneous - batch mixingRheology - higher for Lost CirculationRheology - lower for placement with Coiled TubingCompressive Strength - higher for SidetrackingCompressive Strength - less important for Lost Circulation

Compressvie Strength - minimum 500 psi for drill outThickening Time - enough for placement - 1.5 to 2 hours

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Reasons for Cement Plug Failures

Lack of hardness (sidetracking).

Poor isolation (plugback, 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).

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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 POH.

Slurry not designed for the specific problem (lost circulation).

Not enough cement volume.

Difference between cement and hole fluids' densities too high, causing the plug to sink.

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Plug Cementing - ConclusionsPlace the plug in a competent formation (i.e., a hard

formation).Use ample cement.Use a tailpipe through plugback intervals.Use centralizers on the tailpipe where the hole is not excessively washed out.

Use a drill pipe plug and a plug catcher.Condition the well before running the job, using low YP and PV mud, but of sufficient weight to control the well.

Ahead of the cement, run a high-vis pill that is compatible with the mud and will prevent the cement from sliding down the hole.

Use spacers and washes to combat the effects of mud contamination. Densified cements with a dispersant are also useful.

Allow ample time for the cement to set.

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Diverter Tool

8 holes phased at 450

Bull Plug

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Drill Pipe Centralization & Diverter Tool

DRILLPIPECENTRALIZED

9.0 lb/gal MUD

9.0 lb/gal MUD

SPACER

16.0 lb/gal CEMENT

DIVERTER TOOL

9.1 lb/gal VISCOUS BENTONITE PILL

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Bad Cement Placement Technique

9.0 lb/gal MUD

9.0 lb/gal MUD

SPACER

13.8lb/gal 15.8 lb/gal17.5 lb/galCEMENTBENTONITE PILL

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Reverse Circulating Excess Cement

H

L

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Job Procedure - Balanced Plug Test treating lines.

Pump spacer or wash ahead of cement slurry. Mix and pump cement slurry. Pump spacer or wash behind cement slurry. Displace calculated amount of displacing fluid. Under displace 1/2-1 bbl for safety.

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 plug. Reverse circulate if conditions allow. POOH and WOC.

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Slurry Volume Calculation

7000

7500

Desired Plug At End of Displacement

Mud

Spacer Cement

Drillpipe/tubing

Lsp2

Lcmt

L

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Slurry Volume Calculation

Volume of Cement, Vcmt

Vcmt = L x Ch x excess factor

where,

L = length of column of cement in open hole

(ft).

Ch = capacity of open hole from standard tables

(ft3/ft).

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Slurry Volume Calculation

Volume of Cement, VcmtVcmt = L x Ch x excess factor, where,L = length of column of cement in open hole (ft).Ch = capacity of open hole from standard tables (ft3/ft).

Length of Balanced Plug (with working string in place).

where,

Can = Capacity of annulus between tubing or drill pipe and open hole(ft3ft).

Ctbg = Capacity of tubing or drill pipe (ft3/ft).

Lcmt = Vcmt

Can + Ctbg

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Slurry Volume Calculation

Volume of Spacer Behind the Cement

Vsp1 = Volume of spacer ahead of the cement

Vsp2 = Vsp

1Can

x C tbg

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Slurry Volume Calculation

Volume of Spacer Behind the Cement

Vsp1 = Volume of spacer ahead of the cement

Length of Spacer

Vsp2 = Vsp

1

Can x C tbg

Lsp2 = length of spacer behind (ft) = Vsp2 / Ctbg

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Slurry Volume Calculation Volume of Spacer Behind the Cement

Vsp1 = Volume of spacer ahead of the cement

Length of Spacer

Displacement Volume

D = depth of work string (bottom of cement plug) (ft)

VVd = Ctbg x [D - (Lcmt + Lsp2)]

V sp2 = V sp

1

C an x C tbg

Lsp2 = length of spacer behind (ft) = V sp2 / Ctbg