Unit 7 - Casing Flashcards
How long is a casing joint?
40 ft.
How much can casing contribute to total well cost?
20-30%
Name some reasons to case off a formation?
- 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
Anticipated loads, to which casing will be exposed will depend on which parameters?
- 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
Sizes and setting depths of casing strings depend primarily on what ?
- Geological and pore pressure conditions in particular location
Why is your casing design usually more conservative in an exploration well?
- Because loads can only be estimated and unexpected problems may occur
What is the name of the device attached to the bottom of the casing?
- casing shoe or guide shoe
What is on top of the casing ?
- the casing hanger → allows casing to be suspended from wellhead
What is the conductor casing?
- first casing string to be run → largest diameter ( 30” O.D.)
- Function: Seal-off unconsolidated formations at shallow depths
What is the surface casing?
- 20” O.D.
- run after conductor casing
main functions: - seal off any fresh water sands
- support wellhead and BOP equipment
What can happen if you set a casing too high?
- 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
What is the intermediate casing?
- 13 3/8 “ O.D
- used to isolate troublesome formations between surface casing setting depth and production casing setting depth
What kind of problems can be encountered in the interval where the intermediate casing is positioned?
- unstable shales
- lost circulation zones
- abnormally pressured zones
- squeezing salts
What is the production casing?
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
What is a liner?
- 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
Function of a liner hanger?
- 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
What are the advantages of a liner compared to a full string of casing?
- 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
What is a tie-back string?
- A casing string, which is latched onto top of the liner hanger , to extend the liner back to surface
Why could you need a tie-back string?
- to protect previous casing string from the pressures that will be encountered when the well is in production
When would a liner be called a drilling liner?
- when the liner is used as an intermediate string to case of problem zones before reaching the production zone
When is a liner called a sub liner?
- when it is used as a patch over existing casing for repairing damaged casing or for extra protection against corrosion
How are casings usually classified in manufacturers catalogues?
- terms of size (O.D.)
- weight
- grade
- connection type
Casing Size refers to what?
O.D. of main body
why do you need to measure each joint of casing, when it comes to the rig?
- 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
What is the casing weight?
- weight per foot of casing → represents wall thickness of pipe
What is the drift diameter and why do you need it?
- 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
What is the casing grade?
- different materials, which result from manufacturing
- grade consists of letter and number (e.g. N80)
- Letter: chemical composition of material
Number: minimum yield strength
How are casing joints connected?
- by coupling, which is threaded internally
What is most important about the connection of two joints of casing/
- it has to be leak proof
Why do most of the leaks in casings happen?
- because of poor make-up of connections
Reasons:
- excessive torque used in making up the connections
- dirty threads
- cross threading
- using wrong thread compound
Where is the wellhead located on a landwell or offshore platform?
- below the rig floor
BUT: when drilling offshore from the floating vessel, the wellhead is installed at the seabed
What are the functions of surface wellhead spools?
- 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
What are the two types of surface wellheads that are in common use?
- Spool type wellhead
- compact spool (speedhead)
What is a christmas tree?
- Set of valves, which controls the flow from the well when it is in production
→is installed on top of the wellhead
What are disadvantages of a spool type wellhead?
- 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
What is a compact spool (speedhead0/
- developed as an alternative to the conventional spool
⇒ enables several casing strings or tubing to be suspended from a single spool
What is a disadvantage of a compact spool?
- it can not easily be altered
→ system is less flexible than a separate spool system
What are the two types of casing hangers?
- 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
How should casing be handled?
- 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
What should you do before running the casing?
- 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
What can happen if you run the casing too quickly into the hole?
- surge pressure may be generated below the casing in open hole → risk of fracturing
Where do you usually set a casing shoe?
- 10-30ft. off the bottom
What are the three ways on how to land the casing?
- 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
When could be the time to land casing under compression?
- 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
- How can the wedge slips, which are set against the inside of the previous string be set?
mechanically (rotating the drillpipe) or hydraulically (differential pressure)
What is the basic liner running procedure/
- 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
What are the three distinct operations of the casing design process?
- 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
How are casing setting depths selected?
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
When will you encounter the highest pressure in an open hole section?
- When circulating out a gas influx
Casing sizes and string configuration are dictated by ?
- 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
What can be external loads, that act on the casing?
- 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)
Internal loads for casing?
- mud to surface
- pressure due to influx
- production tubing leak
What is a problem with putting a casing through a salt formation?
- massive salt formations can flow under temperature and pressure, which will put extra collapse pressure on the casing and cause it to shear
