7. Plug-Back Cementing

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 Topic Areas

During its life, every well will require a plugback 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.

The units included in this section are: A. Purposes of Plug-Back Cementing B. Plug-Back Cementing Calculations

Learning Objectives

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

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

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

•

the conditions under which cement plugs are used

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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

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Lost circulation stoppage

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Directional drilling

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Well abandonment

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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.

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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.

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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.

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.

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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:

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get around non-retrievable objects

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correct excessive vertical deviations of the hole

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drill a relief well

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reduce unwanted water production

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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

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above and below freshwater zones

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

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at ground level (Figure 7.4).

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, highpressure zones may be uncovered. This allows fluid to migrate to the surface, and causes unfavorable surface conditions.

Figure 7.4 – Cement plugs used for well abandonment.

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

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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 •

The amount of cement needed for a balanced plug

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The amount of cement with the workstring in the plug.

With drillpipe out: 1 Volume of cement (bbl).

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The height of cement with the workstring out of the plug.

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The volume of the spacer needed behind the cement

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The volume of cement mixing water

2 Sacks of cement 3 Minimum water requirements. With drillpipe in: 4 Height of cement 5 Height of spacer ahead of cement (with volume given) 6 Volume of spacer behind cement 7 Height of mud 8 Volume of mud (to balance)

8

7

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

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5

B Bottom of plug

C Top of plug

C 1 D

2

D Hole size

4

A

3

Balanced Plug Cementing

B Mud Spacer Wellbore after job completion (drillpipe out)

Wellbore during job execution (drillpipe in)

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.

Cement

Figure 7.5 – Well parameters.

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

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The wellbore during job execution

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The wellbore after job completion

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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.

107.93 ft

100 ft

Example Using Equalization Point Formula

Mud Spacer Wellbore after job completion (drillpipe out)

Wellbore during job execution (drillpipe in)

Cement

For this example we are given the following information:

Figure 7.6 – Fluid heights during and after job. To get the wellbore to this state we must run drillpipe into wellbore and “balance” the fluid column hydrostatically.

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

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100 ft of cement plug in open hole, which is: 100 ft × 0.4176 ft3/ft = 41.76 ft3 of cement.

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=

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: •

Hole Size

•

Drillpipe Size

•

Volume of Cement

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Volume of spacer

In our example we calculate as follows: h=

Height of Cement (Drillpipe In)

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Height of Spacer (Drillpipe In)

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Volume of spacer to pump behind cement

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Height of mud (drillpipe in)

= 41.76 ft3 = 41.76 ft3 3 3 0.307 ft /ft + 0.0798 ft /ft 0.3869 ft3/ft 107.93 ft

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

Unknown Values: •

N C+ T

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.

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Balanced Plug Job One (One Wellbore Geometry)

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

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.

8

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

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Calculations (Drillpipe Out) 6

5

C 1 D

2

4

A

1

Cement Required Section 210, Table 213, Page 13 3 3 500 ft × 0.4176 ft /ft = 208.8 ft

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

3 B

Calculations (Drillpipe In) Mud Spacer Wellbore after job completion (drillpipe out)

Wellbore during job execution (drillpipe in)

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

Cement

Figure 7.7 – Single geometry wellbore.

h=

N C+ T

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 3 3 0.3071 ft /ft + 0.0798 ft /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)

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5

365.61 ft × 0.01422 bbl/ft = 5.20 bbl 7 Height of Mud Drillpipe Depth HOC (Drillpipe in) HOS (Drillpipe in)

6800 ft - 539.67 ft - 365.61 ft 5,894.72 ft Mud

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

Spacer Wellbore after job completion (drillpipe out)

Working with a Fixed Amount of Spacer

Wellbore during job execution (drillpipe in)

Cement

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

=

= 290.198 ft 20 bbl 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

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.

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

Calculations (Drill Pipe Out) Mud Spacer Cement

1

Cement Required Section 210, Table 213, Page 13 3 3 200 ft × 0.4922 ft /ft = 98.44 ft 3 100 ft × 0.3491 ft /ft = 34.91 ft3 3 Total = 133.35 ft

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

8 1,000 ft

1,000 ft

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Calculations (Drillpipe In) 6

5 6,040 ft

4B

6,140 ft

1

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

6,1 40 ft

4A 6,3 40 ft

2 3

Job Completed (Drillpipe Out)

h=

Job Execution (Drillpipe In)

N C +T

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 3

200 ft annulus × 0.3918 ft /ft 200 ft drillpipe × 0.0798 ft3/ft

5 Height of Spacer (HOS) Given: Pump 15 bbl of spacer ahead of cement. (Section 122, page 137)

3

= 76.36 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 HOC (Drillpipe In)

From this point you can use the equalization point formula:

6140 ft – 128.86 ft = 6011.14 ft

N h= C +T h=

HOS (Drillpipe In) - 352.94 ft = 5658.2 ft

= 41.03 ft3 41.03 ft3 3 3 0.2386 ft /ft + 0.0798 ft /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

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2. fluid, isolating

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3. lost circulation

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4. bottom

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5. penetrates

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6. seat or bridge, whipstock, rotated

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7. producing, freshwater, casing, ground

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Items from Unit B Quiz

Refer to Page

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

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2. taller

7-8

3. open-ended

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