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Drilling Engineering > Unit 7 - Casing > Flashcards

Unit 7 - Casing Flashcards

1
Q

How long is a casing joint?

A

40 ft.

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2
Q

How much can casing contribute to total well cost?

A

20-30%

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3
Q

Name some reasons to case off a formation?

A
  • prevent unstable formations from caving in
  • protect weak formations from high mudweights that may be required in subsequent hole sections → these high mudweights may fracture the weaker zones
  • isolate zones with abnormally high pressure from deeper zones which may be normally pressured
  • seal off lost circulation zones
  • when set across production interval, to allow selective access for production/injection/control of the fluids from, or into the reservoirs
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4
Q

Anticipated loads, to which casing will be exposed will depend on which parameters?

A
  • types of formation to be drilled
  • formation pore pressure
  • formation fracture pressure
  • geothermal temperature profile
  • nature of fluids in the formations which will be encountered
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5
Q

Sizes and setting depths of casing strings depend primarily on what ?

A
  • Geological and pore pressure conditions in particular location
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6
Q

Why is your casing design usually more conservative in an exploration well?

A
  • Because loads can only be estimated and unexpected problems may occur
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7
Q

What is the name of the device attached to the bottom of the casing?

A
  • casing shoe or guide shoe
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8
Q

What is on top of the casing ?

A
  • the casing hanger → allows casing to be suspended from wellhead
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9
Q

What is the conductor casing?

A
  • first casing string to be run → largest diameter ( 30” O.D.)
  • Function: Seal-off unconsolidated formations at shallow depths
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10
Q

What is the surface casing?

A
  • 20” O.D.
  • run after conductor casing

main functions: - seal off any fresh water sands

  • support wellhead and BOP equipment
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11
Q

What can happen if you set a casing too high?

A
  • Formations below casing may not have enough strength to allow well to be shut in and killed if a gas influx occurs whenn drilling the next hole section → this can result in formations around the casing to crater and influx flowing to surface around ouside of the casing
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12
Q

What is the intermediate casing?

A
  • 13 3/8 “ O.D
  • used to isolate troublesome formations between surface casing setting depth and production casing setting depth
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13
Q

What kind of problems can be encountered in the interval where the intermediate casing is positioned?

A
  • unstable shales
  • lost circulation zones
  • abnormally pressured zones
  • squeezing salts
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14
Q

What is the production casing?

A

9 5/8 “ O.D.

  • either run through pay zone or set just above pay zone ( for open hole completion or prior to running a liner)
  • main purpose of this casing:
  • isolate production interval from other formations (e.g. water bearing sands)
  • or/ and act as conduit for production tubing
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15
Q

What is a liner?

A
  • 7” O.D.
  • short ( usually less than 5000 ft.) casing string which is suspended from the inside of the previous casing string by a liner hanger
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16
Q

Function of a liner hanger?

A
  • liner hanger consists of a collar which has hydraulically or mechanically set slips (teeth) which, when activated, grip the inside of the previous string of casing

→ the slips support the weight of the liner, therefore liner does not have to extend back up to the wellhead

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17
Q

What are the advantages of a liner compared to a full string of casing?

A
  • shorter length of casing string required → cost reduction
  • liner is run on drillpipe → less rig time required to run the string
  • liner can be rotated during cementing operations → will significantly improve mud displacement process and quality of the cement job
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18
Q

What is a tie-back string?

A
  • A casing string, which is latched onto top of the liner hanger , to extend the liner back to surface
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19
Q

Why could you need a tie-back string?

A
  • to protect previous casing string from the pressures that will be encountered when the well is in production
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20
Q

When would a liner be called a drilling liner?

A
  • when the liner is used as an intermediate string to case of problem zones before reaching the production zone
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21
Q

When is a liner called a sub liner?

A
  • when it is used as a patch over existing casing for repairing damaged casing or for extra protection against corrosion
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22
Q

How are casings usually classified in manufacturers catalogues?

A
  • terms of size (O.D.)
  • weight
  • grade
  • connection type
23
Q

Casing Size refers to what?

A

O.D. of main body

24
Q

why do you need to measure each joint of casing, when it comes to the rig?

A
  • because it is not possible to manufacture it to an exact length → therefore it is necessary to measure each joint to know the exact length of the drillpipe
25
Q

What is the casing weight?

A
  • weight per foot of casing → represents wall thickness of pipe
26
Q

What is the drift diameter and why do you need it?

A
  • refers to guaranteed minimum I.D. of casing

→ is necessary because the actual I.D. of casing will vary slightly during the manufacturing process, and if you want to pass tools through the casing you need to know it

27
Q

What is the casing grade?

A
  • different materials, which result from manufacturing
  • grade consists of letter and number (e.g. N80)
  • Letter: chemical composition of material

Number: minimum yield strength

28
Q

How are casing joints connected?

A
  • by coupling, which is threaded internally
29
Q

What is most important about the connection of two joints of casing/

A
  • it has to be leak proof
30
Q

Why do most of the leaks in casings happen?

A
  • because of poor make-up of connections

Reasons:

  • excessive torque used in making up the connections
  • dirty threads
  • cross threading
  • using wrong thread compound
31
Q

Where is the wellhead located on a landwell or offshore platform?

A
  • below the rig floor

BUT: when drilling offshore from the floating vessel, the wellhead is installed at the seabed

32
Q

What are the functions of surface wellhead spools?

A
  • suspend weight of casing string
  • seal off the annulus between successive casing strings at surface
  • allow access to the annulus between casing strings
  • act as an interface between casing string and BOP stack
33
Q

What are the two types of surface wellheads that are in common use?

A
  • Spool type wellhead
  • compact spool (speedhead)
34
Q

What is a christmas tree?

A
  • Set of valves, which controls the flow from the well when it is in production

→is installed on top of the wellhead

35
Q

What are disadvantages of a spool type wellhead?

A
  • time consuming to flange up the spools
  • large number of seals → increases chance of pressure leak
  • BOPs must be removed to install next casing spool
  • A lot of headroom required which may not be available in the wellhead area of an offshore platform
36
Q

What is a compact spool (speedhead0/

A
  • developed as an alternative to the conventional spool

⇒ enables several casing strings or tubing to be suspended from a single spool

37
Q

What is a disadvantage of a compact spool?

A
  • it can not easily be altered

→ system is less flexible than a separate spool system

38
Q

What are the two types of casing hangers?

A
  • Madrel type casing hanger → most common but can not be used if there is the risk, that the casing will not reach the bottom and therefore the hanger will not land in wellhead
  • Slip type casing hanger → this hanger is wrapped around casing and then lowered until it sits inside the casing spool
39
Q

How should casing be handled?

A
  • once it arrives at rig → stacked in right order
  • length, grade, weight and connection of each joint should be checked and each joint should be numbered → recorded on tally sheet
  • drilling engineer uses tally sheet to select the joints that must be run, so that the casing shoe ends up at the correct depth
  • while casing is on racks → threads and coupling should be cleared
  • casing should always be handled with thread protectors in pace
40
Q

What should you do before running the casing?

A
  • a check trip to make sure, that there are no tight spots or ledges which may obstruct casing and prevent it from reaching the bottom
41
Q

What can happen if you run the casing too quickly into the hole?

A
  • surge pressure may be generated below the casing in open hole → risk of fracturing
42
Q

Where do you usually set a casing shoe?

A
  • 10-30ft. off the bottom
43
Q

What are the three ways on how to land the casing?

A
  • landing the casing and cementing
  • suspending the casing, conducting the cement job and then applying additional tension when cement has hardened
  • landing casing under compression
44
Q

When could be the time to land casing under compression?

A
  • could be required when the suspended tension reduces the casing’s collapse resistance below an acceptable level

→ in this case, casing is suspended from the elevators during cementing and then lowered until the desired compression is achieved before setting the slip and seal assembly

45
Q
  • How can the wedge slips, which are set against the inside of the previous string be set?
A

mechanically (rotating the drillpipe) or hydraulically (differential pressure)

46
Q

What is the basic liner running procedure/

A
  • run liner on drillpipe to required depth
  • set liner hanger
  • circulate drilling fluid to clean out liner
  • disconnect the liner hanger setting tool
  • pump down and displace cement
  • set liner packer
  • pick up setting tool, reverse circulate to clean out cement and pull out of hole
47
Q

What are the three distinct operations of the casing design process?

A
  • selection of casing sizes and setting depths
  • definition of operational scenarios, which will result in burst, collapse and axial loads being applied to the casing
  • calculation of the magnitude of these loads and selection of an appropriate weight and grade of casing
48
Q

How are casing setting depths selected?

A

on basis of an assessment of the conditions to be encountered when drilling the subsequent hole section or, in case of production casing, the completion design

49
Q

When will you encounter the highest pressure in an open hole section?

A
  • When circulating out a gas influx
50
Q

Casing sizes and string configuration are dictated by ?

A
  • size of the smallest casing string to be run in hole which is selected on the basis of operational considerations such as :
  • size and configuration of completion string
  • well testing an/or size of logging tools to be run through the casing
51
Q

What can be external loads, that act on the casing?

A
  • pore pressure in formation
  • weight of mud in which casing was run
  • pressure from a column of cement mixwater
  • pressure due to a column of cement slurry → experienced by casing until cement sets
  • blockage in annulus ( in case of a stinger operation)
52
Q

Internal loads for casing?

A
  • mud to surface
  • pressure due to influx
  • production tubing leak
53
Q

What is a problem with putting a casing through a salt formation?

A
  • massive salt formations can flow under temperature and pressure, which will put extra collapse pressure on the casing and cause it to shear
54
Q
A

Drilling Engineering (13 decks)

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