Section 7

Plug-Back Cementing

Table of Contents Introduction................................................................................................................................................7-3

Topic Areas ............................................................................................................................................7-3 Learning Objectives ...............................................................................................................................7-3

Unit A: Purposes of Plug-Back Cementing ...............................................................................................7-3 Plugging to Isolate Zones .......................................................................................................................7-3 Plugging to Stop Lost Circulation ..........................................................................................................7-4 Plugging for Directional Drilling ...........................................................................................................7-4 Plugging for Well Abandonment ...........................................................................................................7-5 Unit A Quiz ............................................................................................................................................7-6

Unit B: Plug-Back Cementing Calculations ..............................................................................................7-7 Balanced Plug Cementing ......................................................................................................................7-7 Example Using Equalization Point Formula ..........................................................................................7-8 Balanced Plug Job One (One Wellbore Geometry) ..............................................................................7-9 Balanced Plug Job Two (Two Wellbore Geometries)..........................................................................7-11 Unit B Quiz ..........................................................................................................................................7-13

Answers to Unit Quizzes .........................................................................................................................7-14

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Use for Section Notes…

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Introduction

During its life, every well will require a plug-back cementing job. Slurry is pumped down drillpipe or tubing and into the annulus; unlike primary cementing jobs, the cement levels in the annulus and inside the tubular goods are the same. This creates a blocked area which is referred to as a cement plug. Typical length of a cement plug is 100 to 500 ft. Halliburton recommends a 500 ft minimum.

Setting high quality cement plugs may be difficult for several reasons: state regulations, formations, and the conditions in the hole.

However, plug-back cementing serves many purposes, as you will see in this section.

Topic Areas

The units included in this section are:

A. Purposes of Plug-Back Cementing

B. Plug-Back Cementing Calculations

Learning Objectives

Upon completion of this section, you should be familiar with:

• the conditions under which cement plugs are used

• how to calculate basic plug-back cementing jobs

Unit A: Purposes of Plug-Back Cementing

Cement plugs are used for the following reasons:

• Zonal isolation

• Lost circulation stoppage

• Directional drilling

• Well abandonment

These reasons and the types of jobs associated with them are discussed in this unit.

Plugging to Isolate Zones

When you use a cement plug to isolate zones (Fig. 7.1), it can serve several purposes.

• It prevents fluid migration up the pipe or annulus by isolating a high-pressure zone from a lower pressure zone. Fluid migration can cause loss of production or an increase in lifting costs.

• It prevents damaging fluids from entering a producing formation. High-pressure from a squeeze job performed above the pay zone may force cement or wellbore fluids into a pay zone, causing damage and loss of production.

• It isolates an upper zone by forming a new bottom for the well from which drill stem tests can be run. The plug eliminates the chances of sticking your pipe below the testing assembly.

• It can be used to block off a problem area. For example, a permanent cement plug can be placed above a depleted zone without affecting the producing zones above the plug.

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Figure 7.2 – Cement plug used to stop lost circulation.

Figure 7.1 – Cement plug used for zone isolation.

Plugging for Directional Drilling Plugging to Stop Lost Circulation

At times, you do not or cannot perform vertical drilling. Perhaps an object is blocking the path downwards (for example, a broken string of pipe), the hole is deviated, or you want to drill toward a target which is off to the side of the hole. Before directional drilling can be performed, you need a seat or a bridge on which to set the tool. A cement plug can be used for this purpose (Figure 7.3).

A cement plug is sometimes set during drilling or cementing operations to stop lost circulation. Circulation loss generally occurs in porous or fractured formations, because drilling fluids or cement slurries flow into the fractures. A cement plug helps combat this problem since before the plug’s cement sets, it drifts into the cavities to block them off (Figure 7.2). A plug may be run with spacers containing special chemicals which block water flow. In severe cases, a thixotropic or gilsonite cement may be used to block off the lost circulation zone.

Directional drilling or whipstocking is done by setting the plug, and then rotating the bit off the plug in another direction. The cement plug is called a whipstock when used in this way. A whipstock plug provides a way to:

• get around non-retrievable objects

• correct excessive vertical deviations of the hole

• drill a relief well

• reduce unwanted water production

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Figure 7.3 – Cement plug used in directional drilling.

Plugging for Well Abandonment

When a dry or depleted well is abandoned, a portion of the casing that was left uncemented may be pulled from the hole. This leaves freshwater zones unprotected. In addition, high-pressure zones may be uncovered. This allows fluid to migrate to the surface, and causes unfavorable surface conditions.

In the past, wells were plugged for abandonment with anything from cotton seed hulls to ground wood. However, these materials did not isolate

zones or prevent fluid migration. Today, the federal and state governments set forth rules for plugging wells for abandonment. Although these rules vary, cement plugs are usually set:

• across and above potential oil and gas producing zones

• above and below freshwater zones

• above and below the bottom of any casing left in the hole

• at ground level (Figure 7.4).

Figure 7.4 – Cement plugs used for well abandonment.

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Unit A Quiz

Fill in the blanks with one or more words to test your progress on Unit A.

1. Plug-back cementing differs from primary cementing because the level of cement in the ___________

is the same as in the __________.

2. A cement plug can prevent ____________ migration by _________ a high-pressure zone from a

lower pressure zone.

3. A plug can help protect a producing formation against __________.

4. A plug may be used to form a new _______________ for a well from which drillstem tests may be

conducted.

5. A cement plug stops lost circulation because its cement _________ into a formation’s pores or

fractures.

6. The purpose of a cement plug in directional drilling is to provide a ______________ for the tool. A

plug used for this purpose is called a _____________. The bit is ____________ off the plug to drill in

another direction.

7. When plugging to abandon, plugs are set across and above _________ formations, and above and

below ___________ zones. In addition, they are set above and below the bottom of ________ and at

__________ level.

Now, check your answers in the Answer Key at the back of this section.

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Unit B: Plug-Back Cementing Calculations

Following are the basic plug-back problem calculations:

Before a plug-back cementing job begins, several calculations must be performed. In this unit, you will learn how to calculate With drillpipe out: • The amount of cement needed for a

balanced plug 1 Volume of cement (bbl).

2 Sacks of cement • The amount of cement with the workstring in the plug. 3 Minimum water requirements.

With drillpipe in: • The height of cement with the workstring out of the plug. 4 Height of cement

• The volume of the spacer needed behind the cement 5 Height of spacer ahead of cement (with

volume given) • The volume of cement mixing water 6 Volume of spacer behind cement

7 Height of mud

Wellbore after jobcompletion

(drillpipe out)

Wellbore duringjob execution(drillpipe in)

1

2

3

6

8

4

5

7

A

B

C

D

MudSpacer

Cement

8 Volume of mud (to balance)

Following are the well parameters needed for the calculations (Figure 7.5):

A Drillpipe size

B Bottom of plug

C Top of plug

D Hole size

Balanced Plug Cementing

One of the most unique calculations in the oilfield is the balanced plug. More often than not, this job is performed with open-ended drillpipe. When performed correctly, the calculations are simple. Problems with this job occur when one of the known parameters, such as drillpipe ID or hole size is incorrectly reported.

Figure 7.5 – Well parameters.

Plug calculations are easier to reason out when you draw two wellbores:

• The wellbore during job execution

• The wellbore after job completion

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Wellbore after jobcompletion

(drillpipe out)

Wellbore duringjob execution(drillpipe in)

107.93 ft

MudSpacer

Cement

100 ft

• Volume of mud to pump behind spacer

Working from the “known” we have the basic hole geometry and the cement volume to pump. Usually we have a predetermined amount of spacer to pump ahead of the cement.

We know the cement occupies a known space in an open wellbore. That same cement also occupies a larger space with the drillpipe in. The length of this space can be determined by taking the known volume of cement and dividing by the combined volume factors of both the annulus and the drillpipe.

Example Using Equalization Point Formula

For this example we are given the following information: Figure 7.6 – Fluid heights during and after

job. • 8 ¾ in. hole, 4 ½ in., 16.6 lb/ft drillpipe

• 100 ft of cement plug in open hole, which is: 100 ft × 0.4176 ft3/ft = 41.76 ft3 of cement. To get the wellbore to this state we must run

drillpipe into wellbore and “balance” the fluid column hydrostatically. Use the equalization point formula, Section 240,

page 12: Notice that the heights of all fluids are taller when the drillpipe is in the wellbore. This is due to the fact that the wall thickness of the drillpipe displaces some of the volume of cement, spacers and mud that you have placed downhole

h = TC

N+

where h = height of cement (drillpipe in)

N = ft3 of slurry used The trick to calculating balanced plugs, as it is with most other jobs, is to work from the known values to solve for the unknown values.

C = ft3/ft factor for annulus

T = ft3/ft factor for tubing or drillpipe Known Values:

In our example we calculate as follows: • Hole Size

h = 41.76 ft3 = 41.76 ft3 = 0.307 ft3/ft + 0.0798 ft3/ft 0.3869 ft3/ft • Drillpipe Size

• Volume of Cement 107.93 ft

Therefore, with the drillpipe in, our height of cement increases from 100 ft to 107.93 ft (Figure 7.6).

• Volume of spacer

Unknown Values:

• Height of Cement (Drillpipe In) This is the basic formula and method for calculating the height of any fluid of a known volume with the pipe suspended in the fluid.

• Height of Spacer (Drillpipe In)

• Volume of spacer to pump behind cement

• Height of mud (drillpipe in)

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Balanced Plug Job One (One Wellbore Geometry)

The following table presents the well parameters for our example job:

Well Parameters Hole Size 8 ¾ in. (Average) Drillpipe Size 4 ½ in., 16.6 lb/ft EUE Plug Depth 6,800 ft Length of Plug 500 ft Cement Type Class H Mixed at 16.4 lb/gal 1.06 ft3/sk yield Spacer Ahead 20 bbl water

In a single wellbore geometry, you have the same size hole (on average) throughout the area involved in the calculations, as shown in Fig. 7.7.

Wellbore after jobcompletion

(drillpipe out)

Wellbore duringjob execution(drillpipe in)

1

2

3

6

8

4

5

7

A

B

C

D

MudSpacer

Cement

Calculations (Drillpipe Out)

1 Cement Required Section 210, Table 213, Page 13 500 ft × 0.4176 ft3/ft = 208.8 ft3

2 Sacks Of Cement 208.8 ft3 ÷ 1.06 ft3/sk = 197 sks

3 Mixing Water 197 sks × 4.3 gal/sk = 847.1 gal

Calculations (Drillpipe In)

4 Height of Cement (HOC) We use the equalization point formula, Red Book, Section 240, page 12:

Figure 7.7 – Single geometry wellbore. h =

TCN+

where h = height of cement (drillpipe in)

N = ft3 of slurry used

C = ft3/ft factor for annulus

T = ft3/ft factor for tubing or drillpipe

Therefore:

h = 208.8 ft3 = 208.8 ft3 0.3071 ft3/ft + 0.0798 ft3/ft 0.3869 ft3/ft

= 539.67 ft

5 Height of Spacer Ahead (HOS) Known: 20 bbl water ahead

Sometimes we are given the volume of spacer to pump ahead. In this case we know we have to pump 20 bbl of water ahead. We can multiply this known volume by the fill factor of the

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annulus and thereby solve the spacer height in the annulus:

20 bbl × 18.2804 ft/bbl = 365.61 ft

6 Volume of Spacer Behind Knowing the height of spacer in the annulus, it is easy to calculate the volume of spacer to pump behind the cement. (Section 210, page 9)

365.61 ft × 0.01422 bbl/ft = 5.20 bbl

7 Height of Mud Drillpipe Depth 6800 ft HOC (Drillpipe in) - 539.67 ft HOS (Drillpipe in) - 365.61 ft 5,894.72 ft

Wellbore after jobcompletion

(drillpipe out)

Wellbore duringjob execution(drillpipe in)

6 5

MudSpacer

Cement

8 Volume Of Mud Behind (To Balance) 5894.72 ft × 0.01422 bbl/ft = 83.82 bbl

Working with a Fixed Amount of Spacer Figure 7.8 – Working with a fixed amount of

spacer.

What happens when a customer tells you that there is 20 barrels of spacer available on location and you are to decide how much to pump ahead and behind? The following shows the calculations:

5 Height of Spacer Known: 20 bbl total spacer

The trick is to use the same equalization point formula we used in Calculation 4 (substitute barrels instead of cubic feet).

h = 20 bbl 0.0547 bbl/ft + 0.01422 bbl/ft

= 20 bbl = 290.198 ft 0.06892 bbl/ft

Therefore the spacer height is 290.198 ft and the volume of spacer ahead (of cement) is

290.19 ft × 0.0547 bbl/ft = 15.87 bbl

6 Volume of Spacer (Behind Cement) 20.00 bbl (Total) –15.87 bbl (Ahead) 4.13 bbl behind

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Balanced Plug Job Two (Two Wellbore Geometries)

Well Parameters

Hole Sizes 8 in. from 1000 ft - 6140 ft 9 ½ in. from 6140 ft – 6340 ft

Drillpipe size 4 ½ in., 16.6 lb/ft EUE

Plug Depth 6340 ft

Length of Plug 300 ft

Cement Type Class G

Mixed at 15.8 lb/gal 1.15 ft3/sk, 5.0 gal/sk

Spacer Ahead 15 bbl

In a wellbore with two geometries, you have two hole widths within the area involved in the calculations, as shown in Fig. 7.9. This wellbore is 8 inches down to 6140 ft. with a washout to 9 ½ inches below that point. If we don’t take the increased hole size into consideration, all of our calculations will be incorrect.

4B

MudSpacerCement

1

2

3

1,000 ft 1,000 ft

6,140 ft 6,140 ft

6,040 ft

6,340 ft

Job Completed(Drillpipe Out)

Job Execution(Drillpipe In)

4A

6

7

8

5

Calculations (Drill Pipe Out)

1

Cement Required Section 210, Table 213, Page 13 200 ft × 0.4922 ft3/ft = 98.44 ft3 100 ft × 0.3491 ft3/ft = 34.91 ft3 Total = 133.35 ft3

2 Sacks Of Cement 133.35 ft3 ÷ 1.15 ft3/sk = 116 sks

3 Mixing Water 116 sks × 5.0 gal/sk = 580 gal 580 gal ÷ 42 gal/bbl = 13.81 bbl

Calculations (Drillpipe In)

4 Height of Cement (HOC) Work from the know values to solve for the unknown. We are trying to determine the cement height with the drillpipe in. We have two different hole sizes to work with. To solve this problem, first calculate what you know (the volume in the 9 ½ in. hole, because you know the volume factor and length). Then subtract this volume from the total cubic feet of slurry. Then use the equalization point formula

h = TC

N+

to solve for the actual height. (If you have a third hole size then you continue to work from the bottom up, solving for the volume until you get to the top hole size that contains cement and then use the equalization point formula.)

Figure 7.9 – Two wellbore geometries.

Now, working from the bottom up:

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4A – 9 ½ in. hole with 4 ½ in. drillpipe 5 Height of Spacer (HOS) Given: Pump 15 bbl of spacer ahead of cement. (Section 122, page 137)

200 ft annulus × 0.3918 ft3/ft = 76.36 ft3 200 ft drillpipe × 0.0798 ft3/ft = 15.96 ft3

15 bbl × 23.5295 ft/bbl = 352.94 ft 92.32 ft3

Subtract this volume from the known total cement volume:

6 Volume of Spacer Behind Cement Volume of spacer behind cement. (Section 210, page 69) 133.35 ft3 – 93.23 ft3 = 41.03 ft3

352.94 ft × 0.04122 bbl/ft = 5.02 bbl 4B – Therefore, 41.03 cubic feet of slurry is remaining to fill into the 8 in. annulus.

7 Height of Mud From this point you can use the equalization point formula: HOC (Drillpipe In)

6140 ft – 128.86 ft = 6011.14 ft h =

TCN+

HOS (Drillpipe In) - 352.94 ft = 5658.2 ft

h = 41.03 ft3 = 41.03 ft3 0.2386 ft3/ft + 0.0798 ft3/ft 0.3184 ft3/ft 8 Volume of Mud (To Balance)

(Section 210, page 169) = 128.86 ft

5658.2 ft × 0.01422 bbl/ft = 80.46 bbl

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Unit B Quiz

Fill in the blanks with one or more words to test your progress on Unit B.

1. What are the well parameters needed for the plug-back calculations?

____________________________

____________________________

____________________________

____________________________

2. The heights of all fluids are _____________ when the drillpipe is in the wellbore.

3. Balanced plug jobs are usually performed with _____________________ drillpipe.

Now, check your answers in the Answer Key at the back of this section.

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Answers to Unit Quizzes

Items from Unit A Quiz Refer to Page

1. annulus, tubular goods 7-3

2. fluid, isolating 7-3

3. lost circulation 7-3

4. bottom 7-3

5. penetrates 7-4

6. seat or bridge, whipstock, rotated

7-4

7. producing, freshwater, casing, ground

7-4

Items from Unit B Quiz Refer to

Page

1. Drillpipe size Bottom of plug Top of plug Hole size

7-7

2. taller 7-8

3. open-ended 7-7

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