The casing design is based on assessing the different loads such as burst, collapse and tension. The most realistic scenarios should be applied to get the more accurate design. These scenarios can be faced during operations like drilling, cementing and production.
Burst design Criteria
The burst can be defined as the tensile failure which can result in the rupture created along the axis of the pipe body. The pipe body tends to burst when differential pressure between the internal and external pressure exceeds the pipe mechanical strength.
There are many situations where the pressure conditions can lead to bust failure:
- The hydrostatic mud pressure inside the casing exceeds the pressure outside the casing.
- During the well shut-in, the formation fluids can be allowed to enter to the wellbore.
- Gas bubble can be allowed to migrate inside the casing
- During the testing of the well or production, leaks can occur.
Design Scenario examples
Casing filled with gas: in this criterion, the casing is considered filled entirely with gas or formation fluids. The pressure bellow the wellhead inside the casing can be the formation pressure minus the hydrostatic pressure of the gas column, and the pressure outside the casing is the hydrostatic pressure of fluids. This scenario is a conservative design criterion and should be applied for the production casings. The sketch at Fig-1 shows the differential pressure and its trends along the casing. It is clear that the weaker zone for the burst is that part of casing below the wellhead.
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| Fig 1- Casing Filled with gas scenario for burst design |
Tubing leak criterion: in this scenario, during well testing or production, leaks can occur at the top of the tubing and leads to pressure increase in the space between the production tubing and casing. The bottom of the tubing is usually fitted with a packer. When the tubing leaks, the pressure inside it will superimposed the pressure in casing/tubing annulus, and packer will be exerted to a high hydrostatic pressure.
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Collapse Design Criteria
The casing tends to collapse when the external pressure acting outside the casing body will be greater than the pressure inside it. This outside pressure can be caused by pore pressure or drilling fluids pressure. The collapse is considered as a geometric failure. When the casing collapses, it changes the circular shape to non-circular form. The collapse resistance is largely related to the diameter-wall thickness ratio, so if the casings wears, the collapse resistance can be affected. The collapse failure can be categorized in four categories: yield strength collapse, plastic collapse, transition collapse and elastic collapse.
There are many situations which can lead to collapse failure:
- The mud level inside the casing can drop due to loss of circulation event
- High pressure outside the casing can be faced during cement squeeze jobs
- Collapse can occur also when cementing, high cement slurry density outside the casing can contribute in decreasing the collapse resistance.
- High load can be exerted on casing by the plastic salt layers which can lead casing to collapse.
- When the casing is not filled properly while running in the hole
Design Scenario examples
Mud losses scenario: while drilling through thief zones, mud losses can be faced leading to mud level to drop and with the constant pressure outside the casing, the collapse resistance can be affected leading the casing body to fail.
Collapse during cementing: at the end of the cement job, the column of the cement slurry outside the casing can exert high hydrostatic pressure on the lowest part of the casing string which is filled with the displacing fluids developing high collapse force.
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Tension Design Criteria
The casing can show three types of deformation under axial tension: elastic, elasto-plastic and plastic. If the deformation is in the elastic domain, the damage is not permanent and disappears when removing the applied force. When the deformation is beyond the elastic domain, the pipe body will suffer from permanent damage and loose its mechanical properties. It has to be mentioned that in order to keep the casing in safe range, the load should not exceed the yield strength of the material during operations.
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| Fig 4 - Pipe Body Behavior under Tension |
When designing for the tension many issues can be taken in consideration
Buoyant weight: the buoyancy force affects the weight of the casing, when the casing is in vertical position, the force is exerted on the lowest part of the casing. In horizontal sections, this force is distributed on the length of the pipe body.
Bending force: when running casing in deviated wells, the pipe body can be under tension and compression simultaneously.
Shock load: this load can be faced when setting casing on slips and it is a local applied force and for short period of time that means it is not like the suspended weight or bending force which can be exerted on the entire length of pipe body. The combination on the suspension, bending and shock loads can lead to pipe parting.
Drag force: the casing can be reciprocated during operations which can result in additional axial load due to the friction between the pipe and the wellbore. It is difficult to estimate the drag force due many reasons: hole geometry, filter cake, bore hole irregularities.
Biaxial Loading
In field operations, there is not one form of loading on casing body, a combination of different loading can be exerted on the pipe body. The collapse strength delivered by the manufacturer is for zero axial load, it can be faced in the field that the pipe body is under combination of external pressure, internal pressure and axial loads and this combination can lead to reduce the mechanical strength of the casing.
References
Hussain Rabia. Well Engineering and Construction. 2001
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