1. Casing Design Workshop
Review of Loads
Casing Design Workshop

2. Our Example Well
Our Data so far:

3. Surface Casing Example: Burst and Collapse Loads

4. Our Example Well
Our Data so far:

5. Surface Casing Data Size: 13-3/8 Depth: 3000 ft
Mud Density: 9.2 ppg at 3000 ft
Frac Pressure at shoe: 12.3 ppg equiv at 3000 ft
Temperature at Surface and Shoe: 74 °F, 128 °F
Maximum Mud Density and temperature before next casing string: 11.8 ppg, 263 °F
Cementing Data: Displace with mud, cement to surface, 300 ft tail slurry at 15.4 ppg, 2700 ft lead slurry at 11.4 ppg, use 50% excess lead slurry, bump plug with 1000 psi above final displacement pressure.

6. Check Cement Hydrostatics
Cement density is usually different from the drilling mud
Check cement program to be sure that it does not allow formation nor cause formation fracture

7. Check Cement Hydrostatics
First let us check to see if the well can support the cement design
The fracture pressure will support the planned cement column but with only a small margin. In a real well we would want to reconsider this slurry design.

8. Surface Casing Collapse
Installation – Post Plug Bump
Inside: mud
Outside: cement (lead slurry, tail slurry)

9. Surface Casing Collapse
Drilling – Severe Lost Circulation
Inside: air
Outside: mud (mud density when casing was run)

10. Surface Casing Collapse Loads

11. Surface Casing Burst Installation: Plug Bump
Inside mud + displacement differential + bump pressure (1000 psi)
Outside: Cement (tail slurry, lead slurry)

12. Surface Casing Burst - Cmtg
Float plugs or annulus packs off
Mud outside, cement inside, pump pressure before shutoff
Cement column length inside casing will differ from annular column length
Determine inside column length of cement
Column lengths do not transform as vertical depths (unless the inclination is constant over the interval)
External column length must be transformed to internal column length and vertical depths of the top and bottom must be determined from directional data
Assume our well is vertical for this example

13. Transform Column Lengths
Use cross sectional areas to get ratio of internal length to external length:
Note we had to assume a specific ID before selecting the casing. An average ID is sufficient

14. Cementing Excess cement may fill 3000 ft of casing
(the 1.5 in the first equation is the 50% excess) Lead slurry with 50% excess will more than fill the 3000 ft of surface casing. This leads to two possibilities (next slide).

15. Two Possibilities Case A Case B Inside: lead slurry + pump pressure
Outside: mud
Case B
Inside: tail and lead slurry + pump pressure
Outside: lead slurry and mud
The severest cases for burst due to float or annulus plugging occur before the 9.2 ppg displacement fluid enters the casing.

16. Cement Case A
Calculate differential burst pressures
Case A

17. Cement Case B Determine interfaces
Determine differential burst pressures
Case B

18. Surface Casing Burst Installation – pre-drill out test (1500 psi)
Inside: mud
Outside: assume mud
Some operators test casing immediately after plug bump to avoid damaging cement sheath. While this is a good practice it is not equivalent to a test before drilling out since the unset cement is usually more dense than a mud channel. In many areas, regulations require the test after cement has set and before drill out.

19. Surface Casing Burst Drilling – Gas Kick
Inside: gas, pressure governed by the lesser of formation pressure or fracture pressure
Outside: mud or water (since this is surface casing we will assume fresh water gradient as worst case)

20. Surface Casing Burst Load
Pressure Control
Gas inside
Pressure determined by injection into weak zone below shoe
Fresh water on outside (common assumption with surface casing)

21. Surface Casing Drilling Burst
Determine fracture pressure at shoe
Determine maximum gas pressure at shoe, zone at ft has max pressure
Determine if that zone pressure exceeds frac pressure
It does by a considerable margin, hence the fracture pressure at the shoe is the governing pressure.

22. Surface Casing Drilling Burst
Formation gas pressure will exceed frac pressure at shoe so the frac pressure will be the limiting pressure. Now calculate gas pressure at the surface.
Use the frac pressure at the shoe as the maximum pressure in this calculation

23. Surface Casing Drilling Burst
Calculate differential burst loads at surface and at shoe
Now we may plot our surface casing burst loading

24. Surface Casing Burst Loads

25. Surface Casing Burst Loads
Note: we might want to reconsider our test pressure since it is less than the maximum pressure we anticipate in the case of a gas kick (more on this when we make our casing selection)

26. Surface Casing Loads Example
As already mentioned we cannot calculate the axial loading until we make a preliminary casing selection for collapse and burst
Stop Here for Surface Casing Project

27. Intermediate Casing Example: Burst and Collapse Loads

28. Our Example Well
Our Data so far:

29. Example: Intermediate Casing
Depth: 10,500 ft,
Pore pressure: 11.3 ppg
Fracture pressure: 15.7 ppg (10,500 ft)
Mud density: ft, ,000 ft
Temperature:
74°F at 0 ft, 263°F at ft, 326°F at 14,000 ft
Cement:
Cement to 500 ft inside surface casing, 1000 ft tail slurry at ppg, 7000 ft lead slurry w/50% excess at 12.0 ppg, 1000 ft spacer at ppg, displace plug w/11.8 ppg mud, bump plug with 1000 psi above final displacement pressure

30. Intermediate Casing Loads
Collapse Load
Essentially same as for surface casing
Severe lost circulation with air inside entire string is not likely for most wells
Severe lost circulation generally requires keeping the casing full with freshwater (or seawater) to prevent a blowout
Burst load
Similar to surface casing
Full formation or shoe frac pressure to surface
‘Maximum load’ method ?

31. Intermediate – Collapse
Installation – Running (no possibilities)
Installation – Post Plug Bump
Inside: 11.8 ppg mud displacement
Outside: 1000 ft 15.9 ppg cmt, 7000 ft 12.0 ppg cmt w/50% excess, ft 12 ppg spacer, and 11.8 ppg mud
Preliminary Calculations
Excess cement column height
Does cement column exceed frac pressure ?

32. Intermediate Collapse
Installation – Running
No possibilities
Installation – Post Plug Bump
Inside: 11.8 ppg mud displacement
Outside: 1000 ft 15.9 ppg cmt, ft 12.0 cmt w/50% excess, 1000 ft 12 ppg spacer, and 11.8 ppg mud
The picture does not show what is described to the left. What we will see in the next few slides is that the worst case cement will go to the surface.

33. Intermediate – Collapse (2)
Preliminary Calculations
Excess cement column height
Does cement column exceed frac pressure ?
Excess cement column
With a gage hole the excess lead slurry will easily reach the surface
Worst case then is 1000 ft 15.9 ppg and 9500 ft of 12.0 ppg cement
We run excess cement to account for hole size uncertainty so that it will reach the desired height in the well. If the cement column is more dense than the displacement fluid (and it should be in good cementing practice) the worst case post plug bump (after the bump pressure is released) is if the excess cement is higher in the well than intended. The highest it can be (discounting channeling) occurs when the hole is gage size and that is what we use to determine the cement height for collapse and hydrostatic frac margin.

34. Check Cement Program
1000 ft 15.9 ppg tail, 7000 ft 12.0 ppg lead w/50% excess, 1000 ft ppg spacer

35. Cement Program
It is also a good idea to plot the cement hydrostatic pressures to be sure that fracture pressures in other parts of the hole are not exceeded

36. Intermediate – Collapse (4)
Calculate post plug bump pressure differentials – maximum case

37. Intermediate – Collapse (5)
Drilling – Lost circulation
Inside: air or air/mud
Outside: mud 11.8 ppg
Severe lost circulation with casing empty is not possible if our data is correct
Other possibilities are drilling induced fractures near the shoe
Trip surge
Annular bridge

38. Intermediate – Lost Circulation Collapse
Assume our data is accurate:
Induced fracture – 11.3 ppg fm pressure, 15.3 ppg mud (11.3 is lowest pore pressure in OH, 15.3 is heaviest mud weight in next OH interval)
Drilling induced fractures (from pipe movement surges) can occur. After fracture the formation pressure at that point would be the limiting pressure value. The highest mud density used down hole would represent the worst collapse case.

39. Intermediate – Lost Circulation Collapse
Calculate differential collapse pressures

40. Intermediate – Lost Circulation Collapse
What if our fracture data is unreliable?
We could consider two severe lost circulation possibilities
Empty string
Fill with water
Data is usually spotty at best

41. Intermediate – Lost Circulation Collapse
Calculate the two common possibilities just for comparison
Severe lost circulation – empty string
Severe lost circulation – fill with fresh water
Common practice offshore to use sea water instead of fresh water

42. Example Collapse Load

43. Intermediate Casing Burst
Installation – plugged float (uncommon)
Inside: cement (tail slurry, lead slurry, spacer), displacement mud
Outside: mud 11.8 ppg
We must resolve the cement column lengths from annular lengths to internal lengths

44. Intermediate Casing Burst (2)
The ratio of inside to annular volume/ft is
Spacer = 0.762(1000) = 762 ft
Lead slurry = 0.762(1.5)(7000) = ft
Tail slurry = 0.762(1000) = 762 ft
Mud = – 8001 – 2(762) = 975 ft
Lead slurry is calculated with gage hole and 50% excess (1.5*7000=10500 ft)

45. Intermediate Casing Burst (3)
Calculate cementing burst pressures

46. Intermediate Casing Burst (4)
Installation – plug bump
Inside: 11.8 ppg mud
Outside: cement column (worst case)

47. Intermediate Casing Burst (5)
Drilling – gas kick
Inside: gas, lesser of
fracture pressure at shoe
or pressure from lower formation
Outside: water channel

48. Intermediate Casing Burst (6)
Determine limiting pressure at shoe
Frac pressure at shoe is the lesser

49. Intermediate Casing Burst (7)
Based on the frac pressure calculate the surface gas pressure

50. Intermediate Casing Burst (8)
Calculate the differential burst pressures

51. What if no gas is present?
In a number of fields in the world the possibility of a gas kick does not exist
An example with oil instead of gas is in the textbook
Calculations are straightforward
Oil kick – similar to gas kick except oil
Use same BHP as last, oil gravity 35 API
The burst load line here is for oil

52. Intermediate Casing Burst – Oil
See manual (pp 5 – 47,48) for details
Exceeds shoe frac pressure so:
Saudi Aramco requested this example in CAC because they have regions with no gas and never get to see a design with oil

53. Oil Kick

54. Intermediate Casing Burst
Note the mud column on top of the gas is from the “maximum load” method with a 5000 psi BOP limitation at the surface. That the oil load line (green) intersects the 5000 psi value at the surface is purely coincidental in this example and has no relation to the max load method.

55. Intermediate Casing Load Example
As already mentioned we cannot calculate the axial loading until we make a preliminary casing selection for collapse and burst
Stop Here for Intermediate Casing Project

56. Production Casing Example: Burst and Collapse Loads

57. Our Example Well
Our Data so far:

58. Example Well : Production
Casing Depth: 14,000 ft
Pore pressure: 14.8 ppg
Fracture Pressure: 16.2 ppg
Mud density: 15.3
Surface Temperature: 74°F
Bottom hole temperature: 326°F

59. Production Casing Data
Cementing Data:
Cement to intermediate casing, 1000 ft tail slurry at 16.6 ppg, 3000 ft lead slurry at 15.6 ppg, 1000 ft spacer (15.3 ppg) and displace plug w/15.3 ppg mud, bump plug with 1200 psi above final displacement pressure. Use 25% excess on all cement.
Production: gas and condensate

60. Production Casing “Philosophy”
Production casing will be in service for the life of the well
Future as well as initial conditions affect design equally
Think ahead, plan ahead

61. Production Casing Collapse
Installation – cementing, post plug bump
Inside: displacement fluid, 15.3 ppg mud
Outside: cement column, 1000 ft tail ppg, 3000 ft lead 15.6 ppg, 1000 ft spacer ppg, 25% excess on all cement

62. Production Casing Collapse
Check cement pressures:

63. Production Casing Collapse
Calculate the differential pressures at the annular interfaces

64. Production Casing Collapse
Production – empty casing
Inside: air
Outside: formation pressure or mud
We do not normally visualize this happening, but later in the life of many wells it does happen and not always intentionally

65. Production Casing Collapse
Determine differential collapse pressures with empty casing using mud on the outside

66. Production Casing Collapse Loads

67. Production Casing Burst
Installation – plug bump
Inside: brine plus bump pressure
Outside: cement and mud (use worst case)

68. Production Casing Burst (2)
Calculate pressures

69. Production Casing Burst (3)
Calculate differential burst pressures
Calculated at points where outside and/or inside fluids change densities

70. Production Casing Burst (4)
Production – tubing backup
Inside: gas from formation
Outside: mud or water
Use actual formation pressure for gas pressure, not mud weight pressure. Outside: mud or water ? Generally mud, but over time will solids settle out of mud and leave fresh water as only backup near surface? We will use water in our example.

71. Production Casing Burst (5)
Calculate gas pressures
Calculate differential burst pressures
Note: formation pressure is 14.8 ppg equivalent

72. Production Casing Burst (6)
Production – surface tubing leak
Inside: SITP on top of packer fluid
Outside: mud or water
Most tubing leaks from corrosion occur near the surface, often just below the hanger. Produced CO2 mixing with fresh water condensing from cooling gas forms carbonic acid and corrosion. Failure to design for this in gas wells can result in rupture and cross flow in the lower portion of the string or collapse tubing with loss of packer fluid. Both rupture and tubing collapse have occurred.

73. Production Casing Burst (7)
Although a case could be made for mud or formation pressure outside the lower part of the string we will use water again

74. Production Casing Burst Loads