Section 7Plug-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 7•1 Cementing 1 Plug-Back Cementing Use for Section Notes… 7•2 Cementing 1 . Topic Areas The units included in this section are: A. plug-back cementing serves many purposes. it can serve several purposes.1). Fluid migration can cause loss of production or an increase in lifting costs. High-pressure from a squeeze job performed above the pay zone may force cement or wellbore fluids into a pay zone. The plug eliminates the chances of sticking your pipe below the testing assembly. • It prevents fluid migration up the pipe or annulus by isolating a high-pressure zone from a lower pressure zone.Plug-Back Cementing Introduction During its life. 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 • • It prevents damaging fluids from entering a producing formation. unlike primary cementing jobs. every well will require a plugback cementing job. Plug-Back Cementing Calculations Learning Objectives Upon completion of this section. It can be used to block off a problem area. Setting high quality cement plugs may be difficult for several reasons: state regulations. 7. formations. 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. and the conditions in the hole. These reasons and the types of jobs associated with them are discussed in this unit. For example. causing damage and loss of production. However. Purposes of Plug-Back Cementing B. Plugging to Isolate Zones When you use a cement plug to isolate zones (Fig. as you will see in this section. Slurry is pumped down drillpipe or tubing and into the annulus. It isolates an upper zone by forming a new bottom for the well from which drill stem tests can be run. • 7•3 Cementing 1 . a permanent cement plug can be placed above a depleted zone without affecting the producing zones above the plug. the cement levels in the annulus and inside the tubular goods are the same. A plug may be run with spacers containing special chemicals which block water flow. a thixotropic or gilsonite cement may be used to block off the lost circulation zone. because drilling fluids or cement slurries flow into the fractures. Directional drilling or whipstocking is done by setting the plug. Circulation loss generally occurs in porous or fractured formations. the hole is deviated. At times.2).2 – Cement plug used to stop lost circulation. 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 7•4 Cementing 1 . and then rotating the bit off the plug in another direction.Plug-Back Cementing Figure 7. A cement plug can be used for this purpose (Figure 7. The cement plug is called a whipstock when used in this way. you need a seat or a bridge on which to set the tool.3). A cement plug helps combat this problem since before the plug’s cement sets. In severe cases. Plugging for Directional Drilling Plugging to Stop Lost Circulation A cement plug is sometimes set during drilling or cementing operations to stop lost circulation. Perhaps an object is blocking the path downwards (for example. Before directional drilling can be performed. a broken string of pipe). Figure 7. or you want to drill toward a target which is off to the side of the hole.1 – Cement plug used for zone isolation. you do not or cannot perform vertical drilling. it drifts into the cavities to block them off (Figure 7. 4 – Cement plugs used for well abandonment. and causes unfavorable surface conditions.Plug-Back Cementing zones or prevent fluid migration. highpressure zones may be uncovered. Plugging for Well Abandonment When a dry or depleted well is abandoned. 7•5 Cementing 1 . wells were plugged for abandonment with anything from cotton seed hulls to ground wood. 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. a portion of the casing that was left uncemented may be pulled from the hole. This allows fluid to migrate to the surface. In the past. Although these rules vary. Figure 7. the federal and state governments set forth rules for plugging wells for abandonment. However. In addition. these materials did not isolate Figure 7. Today. This leaves freshwater zones unprotected.3 – Cement plug used in directional drilling.4). plugs are set across and above _________ formations. check your answers in the Answer Key at the back of this section. 2. A cement plug stops lost circulation because its cement _________ into a formation’s pores or fractures. A plug can help protect a producing formation against __________. When plugging to abandon. 3. 1. A plug used for this purpose is called a _____________. In addition. Now. A cement plug can prevent ____________ migration by _________ a high-pressure zone from a lower pressure zone. Plug-back cementing differs from primary cementing because the level of cement in the ___________ is the same as in the __________. 6. 7.Plug-Back Cementing Unit A Quiz Fill in the blanks with one or more words to test your progress on Unit A. 5. 7•6 Cementing 1 . A plug may be used to form a new _______________ for a well from which drillstem tests may be conducted. The bit is ____________ off the plug to drill in another direction. The purpose of a cement plug in directional drilling is to provide a ______________ for the tool. 4. they are set above and below the bottom of ________ and at __________ level. and above and below ___________ zones. Plug calculations are easier to reason out when you draw two wellbores: • • The wellbore during job execution The wellbore after job completion 7•7 Cementing 1 . One of the most unique calculations in the oilfield is the balanced plug. this job is performed with open-ended drillpipe. the calculations are simple.5): A Drillpipe size 6 5 B Bottom of plug C 1 D B Mud Spacer Wellbore after job completion (drillpipe out) Wellbore during job execution (drillpipe in) C Top of plug 2 3 A 4 D Hole size Balanced Plug Cementing Cement Figure 7. More often than not. In this unit. The height of cement with the workstring out of the plug. such as drillpipe ID or hole size is incorrectly reported. The volume of the spacer needed behind the cement The volume of cement mixing water Following are the basic plug-back problem calculations: With drillpipe out: 1 Volume of cement (bbl). When performed correctly. several calculations must be performed. 2 Sacks of cement 3 Minimum water requirements.5 – Well parameters. you will learn how to calculate • • • • • The amount of cement needed for a balanced plug The amount of cement with the workstring in the plug. 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. Problems with this job occur when one of the known parameters.Plug-Back Cementing Unit B: Plug-Back Cementing Calculations Before a plug-back cementing job begins. 100 ft 107.4176 ft3/ft = 41. We know the cement occupies a known space in an open wellbore.93 ft Therefore.3869 ft3/ft 107. 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. This is due to the fact that the wall thickness of the drillpipe displaces some of the volume of cement.93 ft Mud Spacer Wellbore after job completion (drillpipe out) Wellbore during job execution (drillpipe in) Cement Example Using Equalization Point Formula For this example we are given the following information: • • 8 ¾ in. To get the wellbore to this state we must run drillpipe into wellbore and “balance” the fluid column hydrostatically. with the drillpipe in.6 lb/ft drillpipe 100 ft of cement plug in open hole. which is: 100 ft × 0.0798 ft /ft 0.307 ft /ft + 0. Unknown Values: 7•8 Cementing I . as it is with most other jobs.6). 4 ½ in. is to work from the known values to solve for the unknown values. That same cement also occupies a larger space with the drillpipe in. our height of cement increases from 100 ft to 107. Notice that the heights of all fluids are taller when the drillpipe is in the wellbore.76 ft3 3 3 0. 16.76 ft3 of cement.6 – Fluid heights during and after job..76 ft3 = 41. Section 240. spacers and mud that you have placed downhole The trick to calculating balanced plugs. page 12: 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 In our example we calculate as follows: h= = 41. hole. Known Values: • • • • • • • • Hole Size Drillpipe Size Volume of Cement Volume of spacer Height of Cement (Drillpipe In) Height of Spacer (Drillpipe In) Volume of spacer to pump behind cement Height of mud (drillpipe in) Use the equalization point formula.Plug-Back Cementing • 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.93 ft (Figure 7. Figure 7. 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. Table 213.06 ft3/sk yield 20 bbl water 8 7 Calculations (Drillpipe Out) 6 5 1 2 Cement Required Section 210.6 lb/ft EUE 6.0798 ft /ft 0.8 ft Sacks Of Cement 208. We can multiply this known volume by the fill factor of the 7•9 Cementing I .67 ft 5 Height of Spacer Ahead (HOS) Known: 20 bbl water ahead Sometimes we are given the volume of spacer to pump ahead.8 ft3 ÷ 1.800 ft 500 ft Class H 1.3 gal/sk = 847. The following table presents the well parameters for our example job: Well Parameters Hole Size Drillpipe Size Plug Depth Length of Plug Cement Type Mixed at 16. Red Book. (Average) 4 ½ in.3071 ft /ft + 0. Section 240. 7. page 12: 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. 16.8 ft3 3 3 0. 4 Height of Cement (HOC) We use the equalization point formula. Page 13 3 3 500 ft × 0.7 – Single geometry wellbore.Plug-Back Cementing Balanced Plug Job One (One Wellbore Geometry) In a single wellbore geometry.3869 ft3/ft = 539.. you have the same size hole (on average) throughout the area involved in the calculations.06 ft3/sk = 197 sks Mixing Water 197 sks × 4. as shown in Fig.8 ft3 208.7.1 gal C 1 D B Mud Spacer Wellbore after job completion (drillpipe out) Wellbore during job execution (drillpipe in) 2 3 A 4 3 Calculations (Drillpipe In) Cement Figure 7.4 lb/gal Spacer Ahead 8 ¾ in.4176 ft /ft = 208. In this case we know we have to pump 20 bbl of water ahead. 06892 bbl/ft Figure 7.Plug-Back Cementing annulus and thereby solve the spacer height in the annulus: 20 bbl × 18.61 ft 5.72 ft × 0.00 bbl (Total) –15.20 bbl 7 Height of Mud Drillpipe Depth HOC (Drillpipe in) HOS (Drillpipe in) 6800 ft .894.01422 bbl/ft = 5.13 bbl behind 7 • 10 Cementing I .539.61 ft × 0. Therefore the spacer height is 290.2804 ft/bbl = 365.82 bbl Cement 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 0.87 bbl 6 Volume of Spacer (Behind Cement) 20.61 ft 6 Volume of Spacer Behind Knowing the height of spacer in the annulus.198 ft 20 bbl 0.01422 bbl/ft = 83.87 bbl (Ahead) 4.198 ft and the volume of spacer ahead (of cement) is 290.72 ft Mud Spacer Wellbore after job completion (drillpipe out) Wellbore during job execution (drillpipe in) 6 5 8 Volume Of Mud Behind (To Balance) 5894.19 ft × 0.01422 bbl/ft = 290.8 – Working with a fixed amount of spacer. (Section 210.0547 bbl/ft = 15.67 ft .0547 bbl/ft + 0.365. page 9) 365. it is easy to calculate the volume of spacer to pump behind the cement. 000 ft 1. from 1000 ft . first calculate what you know (the volume in the 9 ½ in.81 bbl 2 3 7 Calculations (Drillpipe In) 5 6. Then use the equalization point formula h= Job Completed (Drillpipe Out) Job Execution (Drillpipe In) N C +T Figure 7. (If you have a third hole size then you continue to work from the bottom up.) Now. all of our calculations will be incorrect.44 ft 3 100 ft × 0. hole.91 ft3 3 Total = 133. because you know the volume factor and length).6140 ft 9 ½ in.15 ft3/sk = 116 sks Mixing Water 116 sks × 5.000 ft Cement Required Section 210. Page 13 3 3 200 ft × 0.140 ft 6 4B 6.6 lb/ft EUE 6340 ft 300 ft Class G 1.15 ft3/sk.9.9 – Two wellbore geometries.35 ft3 ÷ 1.Plug-Back Cementing Balanced Plug Job Two (Two Wellbore Geometries) In a wellbore with two geometries.040 ft 6. from 6140 ft – 6340 ft 4 ½ in.. 16.35 ft Sacks Of Cement 133. to solve for the actual height.4922 ft /ft = 98.0 gal/sk 15 bbl Calculations (Drill Pipe Out) Mud Spacer Cement 1 8 1.3491 ft /ft = 34.3 40 ft 4 Height of Cement (HOC) Work from the know values to solve for the unknown.0 gal/sk = 580 gal 580 gal ÷ 42 gal/bbl = 13. We are trying to determine the cement height with the drillpipe in. with a washout to 9 ½ inches below that point. solving for the volume until you get to the top hole size that contains cement and then use the equalization point formula. 5.8 lb/gal Spacer Ahead 8 in. We have two different hole sizes to work with. working from the bottom up: 7 • 11 Cementing I . Then subtract this volume from the total cubic feet of slurry. Table 213. 7. as shown in Fig. Well Parameters Hole Sizes Drillpipe size Plug Depth Length of Plug Cement Type Mixed at 15.1 40 ft 1 2 3 4A 6. you have two hole widths within the area involved in the calculations. This wellbore is 8 inches down to 6140 ft. To solve this problem. If we don’t take the increased hole size into consideration. page 169) 5658. annulus.96 ft3 92. hole with 4 ½ in.03 ft3 41.0798 ft /ft 0.32 ft3 3 5 Height of Spacer (HOS) Given: Pump 15 bbl of spacer ahead of cement.Plug-Back Cementing 4A – 9 ½ in.2 ft × 0.352.0798 ft3/ft 3 = 76.86 ft 7 • 12 Cementing I .02 bbl 7 Height of Mud HOC (Drillpipe In) 6140 ft – 128.94 ft = 5658.94 ft 6 Volume of Spacer Behind Cement Volume of spacer behind cement. page 69) 352.3184 ft3/ft = 128.2 ft 8 Volume of Mud (To Balance) (Section 210. drillpipe 200 ft annulus × 0.46 bbl Subtract this volume from the known total cement volume: 133.23 ft3 = 41.03 cubic feet of slurry is remaining to fill into the 8 in.14 ft HOS (Drillpipe In) .36 ft = 15.94 ft × 0.3918 ft /ft 200 ft drillpipe × 0.35 ft3 – 93.2386 ft /ft + 0.04122 bbl/ft = 5.03 ft3 4B – Therefore. 41.5295 ft/bbl = 352. (Section 210.01422 bbl/ft = 80.03 ft3 3 3 0.86 ft = 6011. From this point you can use the equalization point formula: N h= C +T h= = 41. page 137) 15 bbl × 23. (Section 122. 1.Plug-Back Cementing Unit B Quiz Fill in the blanks with one or more words to test your progress on Unit B. check your answers in the Answer Key at the back of this section. 7 • 13 Cementing I . Now. Balanced plug jobs are usually performed with _____________________ drillpipe. 3. The heights of all fluids are _____________ when the drillpipe is in the wellbore. What are the well parameters needed for the plug-back calculations? ____________________________ ____________________________ ____________________________ ____________________________ 2. annulus. rotated 7. Drillpipe size Bottom of plug Top of plug Hole size 2.Plug-Back Cementing Answers to Unit Quizzes Items from Unit A Quiz 1. lost circulation 4. bottom 5. fluid. freshwater. ground Refer to Page 7-3 7-3 7-3 7-3 7-4 7-4 7-4 Items from Unit B Quiz 1. casing. producing. seat or bridge. whipstock. open-ended Refer to Page 7-7 7-8 7-7 7 • 14 Cementing I . isolating 3. penetrates 6. taller 3. tubular goods 2.