5) Plug Cementing

2

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}

3

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,

4

Side Track and Directional

Drilling

Kick Off Point

NEW HOLE CEMENT

5

Plug Back and Depleted

Zone

Depleted Zone

Cement Plug

6

Lost Circulation

7

Abandonment

CEMENT PLUG

CEMENT PLUG

8

Examples of Countries Having Drilling and Cementing

Regulations

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

Petroleum Mine Safety Regulations Petroleum Ministry

Geology and Mines Dept The Ministry of Mines Petroleum Directorate Petroleum Admin Dept of Energy

9

Test Anchor

10

Plug Placement Techniques



There are three common techniques for

placing cement plugs:



Balanced plug



Dump bailer

11

Balanced Plug

12

Balanced Plug

13

Dump Bailer Method

WIRELINE DUMP BAILER CEMENT SLURRY ELECTRICAL/ MECHANICAL DUMP RELEASE BRIDGE PLUG CASING

14

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.

15

Two Plug Method

1. Running In 4. Top Plug Landed

2. Bottom Plug Landed 5. Reverse Circulation 3. Cleaning of Aluminium Tail Pipe and Pulling Out

16

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.

17

Coiled Tubing Cement Plug

Technique

Cemen

t

Suspended

mud System

Coiled Tubing

18

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?

19

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}

20

Slurry Thickening Time and Compressive

Strength

COMPRESSIVE STRENGTH (psi) 16hrs 3500 3000 2200 24hrs 4100 3700 3800

21

Mud Contamination vs Compressive

Strength

*

*

Compressive strength is 18hr at 230F

** Contains dispersant

Neat Class H Cement

16.5 lb/gal Contamination*Effect of Mud

Mud Contamination

(% by Volume) Compressive Strength (psi at 170°F) Contamination Mud (%) Normal Slurry 15.6 lb/gal Reduced Water Slurry** 17.5 lb/gal 0 5 10 20 50 4,647 3,512 2,619 2,378 245 5,862 5,300 4,538 2,331 471 0 10 40 60 4,082 psi 2,950 psi 2,426 psi 593 psi 8,600 psi 8,237 psi 3,850 psi 2,967 psi 8 hr 16 hr

22

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

23

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

24

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.

25

Diverter Tool

8 holes phased at 45

26

Drill Pipe Centralization &

Diverter Tool

27

Bad Cement Placement

Technique

28

Reverse Circulating Excess

Cement

H

29

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.

30

Slurry Volume Calculation

7000

7500

Desired Plug

At End of Displacement

Mud Spacer Cement Drillpipe/tubing L_sp2 L_cmt L

31

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

32

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).

where,

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

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

L

= V

_C

33

Slurry Volume Calculation



Volume of Spacer Behind the Cement

V

= Volume of spacer ahead of the cement

V

=

V

34

Slurry Volume Calculation



Volume of Spacer Behind the Cement

V

= Volume of spacer ahead of the cement



Length of Spacer

Vsp2 =

Vsp

Can

x C tbg

35

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 = C_tbg x [D - (L_cmt + L_sp2)] V sp2 = V sp1

C an x C tbg