5) Plug Cementing

PLUG CEMENTING

Objectives By 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:

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

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.

Side Track and Directional Drilling Kick Off Point

CEMENT PLUG

NEW HOLE

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Plug Back and Depleted Zone

Cement Plug

Depleted Zone

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Lost Circulation Drill Pipe

Open Hole

Drill Pipe CEMENT CEMENT PLUG PLUG

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ThiefZone

Abandonment

CEMENT PLUG

CEMENT PLUG

CEMENT PLUG

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Examples of Countries Having Drilling and Cementing Country Abu Dhabi Australia Austria Canada

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Regulations Agency

Colombia France Germany Ireland Italy Japan

Ministry of Petroleum Department of Mines Oberste Bergbehorde Ontario-Dept of Mines and Northern affairs Alberta-Oil & Gas Conservation Board Saskatewan-Dept of Mineral Resources Minister of Mines & Petroleum Direction Generale des Mnes Bureau of Mines Offshore Operating Committee - London National Mining Bureau for Hydrocarbons Bureau of Mines

Libya Malaysia Mozambique The Netherlands Norway Turkey United Kingdom Venezuela

Petroleum Mine Safety Regulations Petroleum Ministry Geology and Mines Dept The Ministry of Mines Petroleum Directorate Petroleum Admin Dept of Energy Dept of Hydrocarbons

Test Anchor

Test String

Zone to be Tested

CEMENT PLUG

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Weak Formation

Plug Placement Techniques There 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

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Balancing

Balanced Plug Displ. Fluid

Balancing

Reversing

Spacer

Cement Slurry Plug Length

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Dump Bailer Method WIRELINE

DUMP BAILER

CEMENT SLURRY ELECTRICAL/ MECHANICAL DUMP RELEASE BRIDGE PLUG CASING

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Dump Bailer Method Advantages:  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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Two Plug Method

1. Running In 2. Bottom Plug Landed 3. Cleaning of Aluminium Tail Pipe

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4. Top Plug Landed 5. Reverse Circulation and Pulling Out

Two-Plug Method: Tell-Tale Catcher Sub System Advantages of this method are:  Isolation ahead and behind the cement.  Pipe cleaned down to the lower end of the tail

pipe.  Breakable tail pipe that can be abandoned if

stuck. 16

Coiled Tubing Cement Plug Technique Coiled Tubing

Cemen t

Suspended mud System

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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 Properties Density - lighter for Lost Circulation Density - heavier for Sidetracking Density - homogeneous - batch mixing Rheology - higher for Lost Circulation Rheology - lower for placement with Coiled Tubing Compressive Strength - higher for Sidetracking Compressive Strength - less important for Lost

Circulation Compressvie Strength - minimum 500 psi for drill out Thickening Time - enough for placement - 1.5 to 2 hours

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Slurry Thickening Time and Compressive Strength API Class G Cement Mixed at 15.8 lb/gal

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Conc D81R gal/sk

Depth (ft)

BHCT (oF)

BHST (oF)

THICKENING TIME (hrs:min)

None

10,000

144

228

1:20

0.04

10,000

144

228

2:25

0.08

10,000

144

228

3:40

Conc D81R gal/sk

Depth (ft)

None

10,000

228

3050

3500

4100

0.04

10,000

228

2500

3000

3700

0.08

10,000

228

1200

2200

3800

BHST COMPRESSIVE STRENGTH (psi) (oF) 8hrs 16hrs 24hrs

Mud Contamination vs Compressive Strength Effect of Mud Contamination*

Neat Class H Cement 16.5 lb/gal

Mud Compressive Strength Normal Mud Contamination Contamination (psi at 170°F) Slurry (% by Volume) (%) 15.6 lb/gal 8 hr 16 hr 4,647 5,862 0 4,082 psi 0 3,512 5,300 5 2,950 psi 10 2,619 4,538 10 2,426 psi 40 2,378 2,331 20 593 psi 60 50

245

471

Reduced Water Slurry** 17.5 lb/gal 8,600 8,237 3,850 2,967

psi psi psi psi

Compressive strength is 18hr at 230F ** Contains dispersant *

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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 Conclusions Place 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 45 Bull Plug

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Drill Pipe Centralization & Diverter Tool DRILLPIPE CENTRALIZED 9.0 lb/gal MUD SPACER DIVERTER TOOL 16.0 lb/gal CEMENT

9.1 lb/gal VISCOUS BENTONITE PIL

9.0 lb/gal MUD

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Bad Cement Placement Technique 9.0 lb/gal MUD SPACER 13.8lb/gal 15.8 lb/gal 17.5 lb/gal CEMENT BENTONITE PILL

9.0 lb/gal MUD

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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 Drillpipe/tubing

Lsp2

Spacer

7000

Cement L

Lcmt 7500

Desired Plug

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Mud

At End of Displacement

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, 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).

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

Lcmt =

Vcmt Can + Ctbg

where, Can = Capacity of annulus between tubing or drill pipe and open hole(ft3ft). Ctbg = Capacity of tubing or drill pipe (ft3/ft).

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Slurry Volume Calculation  Volume of Spacer Behind the Cement

V sp 1 x C tbg V sp2 = C an Vsp1 = Volume of spacer ahead of the cement

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Slurry Volume Calculation  Volume of Spacer Behind the Cement Vsp 1 xC Vsp2 = tbg Can Vsp1 = Volume of spacer ahead of the cement

 Length of Spacer

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

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Slurry Volume Calculation  Volume of Spacer Behind the Cement V sp2 =

V sp 1 C an

x C tbg

Vsp1 = Volume of spacer ahead of the cement

 Length of Spacer L sp2 = length of spacer behind (ft) =

V sp2 / C

tbg

 Displacement Volume VVd = Ctbg x [D - (Lcmt + Lsp2)] D = depth of work string (bottom of cement plug) (ft)

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