Types Of Casing
The purpose of drilling a well is to evaluate one or more prospective producing zones and provide a means of producing the hydrocarbons that may be found in those horizons. It also can be said that this is primary purpose of the casing
While drilling of oil & gas wells, it is necessary to line the walls of a borehole with steel pipe which is called casing.
When drilling wells, hostile environments, such as high- pressured zones, weak and fractured formations, unconsolidated formations and sloughing shales, are often encountered.
Hence, wells are drilled and cased in several steps to seal off these troublesome zones and to allow drilling to the total depth.
Different casing sizes are required for different depths.
The five general casings used to complete a well are: conductor pipe, surface casing, intermediate casing, production casing and liner.
CONDUCTOR CASING
Normal size - 18 5/8”- 30”
(always cemented upto surface)
To protect near surface unconsolidated formations.
Seal off shallow water zones.
Provide protection against shallow gas zones.
To protect the foundation of the platform in off-shore operations.
It is used to support subsequent casing strings and well head equipment or alternatively the pipe is cut off at the surface after setting
SURFACE CASING
Normal size - 13 3/8” – 18 5/8”or 20”.
Should be set in competent rocks.
To prevent caving of weak formations and maintain hole integrity.
Cover fresh water sands and prevent their contamination by fluids from deeper formation.
Minimize lost circulation into shallow permeable zones.
Cover weak zones that are incompetent to control kick imposed pressures.
Provide a means for attaching the blow-out preventers.
Support the weight of all casing strings (except liners) run below the surface pipes.
INTERMEDIATE CASING
Normal size - 95/8” – 13 3/8”
It is set at a depth between the surface and production casing.
Troublesome zones encountered include those with abnormal formation pressures, lost circulation, unstable shales and salt section.
Good cementation of this casing must be ensured to prevent communication behind the casing between the lower hydrocarbon zones and upper water formations. Longer cement columns are sometimes necessary to prevent casing buckling.
Multi-stage cementation may be opted for.
PRODUCTION CASING
Normal size - 5” - 7”
(May be set above, mid-way or below pay zone)
Isolate the producing zone from other formations.
provide a work shaft of a known diameter to the pay zone.
protect the production tubing and other equipments.
LINERS
Normal size - 7” – 9 5/8”
Liners are the pipes that do not usually reach the surface, but are suspended from the bottom of the existing largest casing string. It is about 100 – 150 m overlap between the two strings. Liners are used for the same purpose as intermediate casing.
Drilling liners are used to isolate lost circulation or abnormally pressured zones to permit deeper drilling.
Production liners are run instead of a full casing to provide isolation across the production or injection zones.
ADVANTAGES OF LINERS
Low cost completion.
Case off open hole more rapidly and easily.
Complete wells with less weight landed on well-head and surface pipe.
Prevent lost circulation.
Provide safer operations.
Large ID up the hole accommodates large dual strings. On very deep wells, the liner allows for higher strength and large OD section tubing.
Permit drilling with tapered drill-string.
Provide good well control while drilling and completing.
Where rig capacity cannot handle full string.
To provide an upper section of casing (tie back) which has seen no drilling.
Allow deepening of old wells.
Permit testing lower zone of a new well economically before plugging back to primary zones.
Accommodate large volume pumps for artificial lift.
Provide ability to reciprocate high angle holes while cementing.
DISADVANTAGES OF LINERS
Possible leak across a liner hanger.
Difficulty in obtaining good primary cementation due to the narrow annulus between the liner and the hole.
Liner seals sometimes give trouble due to occasionally disengagement from the run – in string may be difficult or impossible.
Clearance between a liner and the previous string of casing is often smaller than is usual with other casings.
OTHER PURPOSE LINERS
Slotted liners, perforated liners and gravel packed liners, which are set opposite producing zones for the purpose of preventing sand from entering the well.
PROPERTIES OF CASING
All oil well tubulars including casing have to meet the requirements of the API (American Petroleum Institute) Specification 5CT.
Two basic processes are used to manufacture casing: seamless and continuous electric weld.
Seamless Pipe: It is a wrought steel pipe manufactured by a seamless process. A large percentage of tubulars and high quality pipes are manufactured in this way.
Welded Pipe: In the continuous electric process, pipe with one longitudinal seam is produced by electric resistance welding without adding extraneous metal. In the electric flash welding process, pipes are formed from a sheet with the desired dimensions and welded by simultaneously flashing and pressing the two ends.
PROPERTIES OF CASING
The physical properties of oil-field tubular goods include grade, pressure resistance, drift diameter, and weight.
These properties relate to the pipe’s ability to meet the demands of the imposed drilling conditions. The limitations of the properties must be considered before final pipe selection.
Grade: The pipe grade is a designation that defines the pipe’s yield strength and certain special characteristics. eg – N – 80, P – 110.
PROPERTIES OF CASING
API RANGES:
Range – I: 16 to 25 ft.
Range - II: 25 to 34 ft.
Range - III: 34 to 46 ft.
TYPES OF COUPLING AND ELEMENTS OF THREADS
Coupling is a short section of casing used to connect two casing joints.
In general casing and coupling are specified by the type of threads (or connection) cut in the pipe or coupling.
Following are the most widely used
connections :
TYPES OF COUPLING AND ELEMENTS OF THREADS
Eight round threads per inch, having V-shape with an included angle of 60°. Clearance between crests after mating = 0.003 in. approx. to be filled with special sealing compound.
Two types :-
STC and LTC
STC & LTC connections are weaker than the pipe body and LTC is capable of transmitting higher axial loads.
BUTTRESS THREAD COUPLING
Capable of transmitting higher axial loads than API round threaded coupling. It has 5 threads per inch.
VAM THREAD COUPLING
Modified buttress thread, providing double metal to metal seal at pin end
It has 5 threads per inch.
EXTREME LINE THREAD COUPLING
Externally and internally threaded on internal-external upset ends. Upset ends to increase wall thickness, to compensate for the loss of metal due to threading. The thread profile is trapezoidal, providing metal to metal seal at both the pin end and the external shoulder. This makes the extreme line casing suitable for use in elevated temperature and pressures. The joint is gas tight and can transmit high axial, tensile and compressive loads.
6 threads per inch for sizes 5 in. to 7 5/8 in.
5 threads per inch for sizes 85/8 in. to 10 ¾ in.
CASING SPECIFICATIONS
Casings are specified according to the following:
Size:
Size is specified for outside diameter of the casing pipe.
Nominal weight:
The term nominal weight is primarily used for the purpose of identification of casing type during ordering.
It is the theoretical wt. per foot for a 20 ft. length of threaded & coupled casing joint
= 10.68(D-t)t+0.0722D (PPF )
where, D=outside dia.(”),
t=wall thickness(”)
CASING SPECIFICATIONS
Plain end weight:
Excludes threads & couplings = 10.68(D-t)t (PPF)
Threaded and coupled weight:
It is the average wt. of a joint including the threads at both ends & a coupling at one end when power tight.
=1/20{Wn[20-(N1+2J)/24]+coupling wt. - wt. removed in threading two pipe ends. (ppf)
where, N1 =coupling length (inch)
J =Distance from end of pipe to
centre of coupling in the
power tight position (inch)
Wn =Plain end wt. (ppf)
Normal size - 18 5/8”- 30”
(always cemented upto surface)
To protect near surface unconsolidated formations.
Seal off shallow water zones.
Provide protection against shallow gas zones.
To protect the foundation of the platform in off-shore operations.
It is used to support subsequent casing strings and well head equipment or alternatively the pipe is cut off at the surface after setting
SURFACE CASING
Normal size - 13 3/8” – 18 5/8”or 20”.
Should be set in competent rocks.
To prevent caving of weak formations and maintain hole integrity.
Cover fresh water sands and prevent their contamination by fluids from deeper formation.
Minimize lost circulation into shallow permeable zones.
Cover weak zones that are incompetent to control kick imposed pressures.
Provide a means for attaching the blow-out preventers.
Support the weight of all casing strings (except liners) run below the surface pipes.
INTERMEDIATE CASING
Normal size - 95/8” – 13 3/8”
It is set at a depth between the surface and production casing.
Troublesome zones encountered include those with abnormal formation pressures, lost circulation, unstable shales and salt section.
Good cementation of this casing must be ensured to prevent communication behind the casing between the lower hydrocarbon zones and upper water formations. Longer cement columns are sometimes necessary to prevent casing buckling.
Multi-stage cementation may be opted for.
PRODUCTION CASING
Normal size - 5” - 7”
(May be set above, mid-way or below pay zone)
Isolate the producing zone from other formations.
provide a work shaft of a known diameter to the pay zone.
protect the production tubing and other equipments.
LINERS
Normal size - 7” – 9 5/8”
Liners are the pipes that do not usually reach the surface, but are suspended from the bottom of the existing largest casing string. It is about 100 – 150 m overlap between the two strings. Liners are used for the same purpose as intermediate casing.
Drilling liners are used to isolate lost circulation or abnormally pressured zones to permit deeper drilling.
Production liners are run instead of a full casing to provide isolation across the production or injection zones.
ADVANTAGES OF LINERS
Low cost completion.
Case off open hole more rapidly and easily.
Complete wells with less weight landed on well-head and surface pipe.
Prevent lost circulation.
Provide safer operations.
Large ID up the hole accommodates large dual strings. On very deep wells, the liner allows for higher strength and large OD section tubing.
Permit drilling with tapered drill-string.
Provide good well control while drilling and completing.
Where rig capacity cannot handle full string.
To provide an upper section of casing (tie back) which has seen no drilling.
Allow deepening of old wells.
Permit testing lower zone of a new well economically before plugging back to primary zones.
Accommodate large volume pumps for artificial lift.
Provide ability to reciprocate high angle holes while cementing.
DISADVANTAGES OF LINERS
Possible leak across a liner hanger.
Difficulty in obtaining good primary cementation due to the narrow annulus between the liner and the hole.
Liner seals sometimes give trouble due to occasionally disengagement from the run – in string may be difficult or impossible.
Clearance between a liner and the previous string of casing is often smaller than is usual with other casings.
OTHER PURPOSE LINERS
Slotted liners, perforated liners and gravel packed liners, which are set opposite producing zones for the purpose of preventing sand from entering the well.
PROPERTIES OF CASING
All oil well tubulars including casing have to meet the requirements of the API (American Petroleum Institute) Specification 5CT.
Two basic processes are used to manufacture casing: seamless and continuous electric weld.
Seamless Pipe: It is a wrought steel pipe manufactured by a seamless process. A large percentage of tubulars and high quality pipes are manufactured in this way.
Welded Pipe: In the continuous electric process, pipe with one longitudinal seam is produced by electric resistance welding without adding extraneous metal. In the electric flash welding process, pipes are formed from a sheet with the desired dimensions and welded by simultaneously flashing and pressing the two ends.
PROPERTIES OF CASING
The physical properties of oil-field tubular goods include grade, pressure resistance, drift diameter, and weight.
These properties relate to the pipe’s ability to meet the demands of the imposed drilling conditions. The limitations of the properties must be considered before final pipe selection.
Grade: The pipe grade is a designation that defines the pipe’s yield strength and certain special characteristics. eg – N – 80, P – 110.
PROPERTIES OF CASING
API RANGES:
Range – I: 16 to 25 ft.
Range - II: 25 to 34 ft.
Range - III: 34 to 46 ft.
TYPES OF COUPLING AND ELEMENTS OF THREADS
Coupling is a short section of casing used to connect two casing joints.
In general casing and coupling are specified by the type of threads (or connection) cut in the pipe or coupling.
Following are the most widely used
connections :
TYPES OF COUPLING AND ELEMENTS OF THREADS
Eight round threads per inch, having V-shape with an included angle of 60°. Clearance between crests after mating = 0.003 in. approx. to be filled with special sealing compound.
Two types :-
STC and LTC
STC & LTC connections are weaker than the pipe body and LTC is capable of transmitting higher axial loads.
BUTTRESS THREAD COUPLING
Capable of transmitting higher axial loads than API round threaded coupling. It has 5 threads per inch.
VAM THREAD COUPLING
Modified buttress thread, providing double metal to metal seal at pin end
It has 5 threads per inch.
EXTREME LINE THREAD COUPLING
Externally and internally threaded on internal-external upset ends. Upset ends to increase wall thickness, to compensate for the loss of metal due to threading. The thread profile is trapezoidal, providing metal to metal seal at both the pin end and the external shoulder. This makes the extreme line casing suitable for use in elevated temperature and pressures. The joint is gas tight and can transmit high axial, tensile and compressive loads.
6 threads per inch for sizes 5 in. to 7 5/8 in.
5 threads per inch for sizes 85/8 in. to 10 ¾ in.
CASING SPECIFICATIONS
Casings are specified according to the following:
Size:
Size is specified for outside diameter of the casing pipe.
Nominal weight:
The term nominal weight is primarily used for the purpose of identification of casing type during ordering.
It is the theoretical wt. per foot for a 20 ft. length of threaded & coupled casing joint
= 10.68(D-t)t+0.0722D (PPF )
where, D=outside dia.(”),
t=wall thickness(”)
CASING SPECIFICATIONS
Plain end weight:
Excludes threads & couplings = 10.68(D-t)t (PPF)
Threaded and coupled weight:
It is the average wt. of a joint including the threads at both ends & a coupling at one end when power tight.
=1/20{Wn[20-(N1+2J)/24]+coupling wt. - wt. removed in threading two pipe ends. (ppf)
where, N1 =coupling length (inch)
J =Distance from end of pipe to
centre of coupling in the
power tight position (inch)
Wn =Plain end wt. (ppf)
CASING SPECIFICATIONS
Internal Diameter:
Average internal diameter of the pipe used for inside volume calculations.
Drift diameter:
The largest diameter equipment (bit, packer etc.), which can be safely run inside the casing.
STRENGTH PROPERTIES
Casing strength properties are normally specified as:
1. Yield strength for (a) pipe body and (b) coupling.
2. Collapse strength for (a) plain pipe & (b) coupling.
Burst (or internal yield) strength of pipe body.
LOADING CONDITIONS – BURST
GRADES OF STEEL
Seven different API grades available
The numbers followed by alphabets denotes minimum yield strength.
DESIGN CRITERIA
AXIAL TENSION:
Tension due to the casing dead weight
Wn = Nominal weight of casing (Kg/M )
W = Wn / L , L = Length of casing (M )
Bending, drag, shock loading, casing pressure testing all adds to the tension.
Bending force due to build up, side tracking, deviation
Be(Kgf) = 29 * RC * D * Wn , RC = Radius of curvature (deg/ 30 M)
D = Casing O.D. ( Inches)
Shock loading = 1.55 * 10 * V * Wn , V = Peak velocity while R/I (M/S)
The top most casing joint is subjected to the maximum tension.
DESIGN CRITERIA
COLLAPSE PRESSURE:
The increase in external pressure due to difference in fluid column / formation pressure results in a tendency to collapse the casing.
Collapse pressure is maximum at the bottom & zero at the surface.
Collapse pressure = L*G/10 , G = Mud density (Gms/cc )
It is assumed that the inside of casing is empty for surface & production casing, & partially empty for intermediate casing.
Collapse pressure is calculated taking true vertical depth (TVD) for directional wells.
DESIGN CRITERIA
BURST PRESSURE:
The increase in internal over external pressure results in a bursting tendency.
The criteria is based upon maximum formation pressure resulting from a kick during drilling of next hole section.
It is assumed that the entire inside volume is displaced by formation fluid.
The burst pressure is maximum at the top & minimum at the casing shoe due to external pressure resistance.
CASING DESIGN
Choose design factors.
Make assumptions.
Draw resultant burst load line.
Draw resultant collapse load line.
Select sec.I of casing for collapse from available casing.
Select sec.II for collapse.
Re-calculate the depth of sec.II for collapse considering bi-axial stresses.
Select sec.III for collapse.
Re-calculate the depth of sec.III for collapse considering bi-axial stresses.
Extend the depth of sec.III upwards & check it for burst & tension.
If unsafe, then select next higher grade for sec.IV & so on till burst & tension design factors are satisfied.
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