Casing design
5.2.1 General test requirements
The casing must be tested after it is cemented. There are two purposes for this, namely the casing itself must be tested to withstand the design pressure, while the quality of
146 M o d e r n w e l l d e s i g n
the cement on the outside and the formation strength is tested with a so-called leak-off test.
The casing integrity can be tested either after the cement is set, or during cementing when the cement plug lands in the float collar. The latter is called bumping of plug.
According to regulations (NPD, 1991) the test pressure should:
• Be equal to the maximum burst pressure defined in the test scenario
• Not exceed 85% of the internal yield pressure of the casing string.
In the following, we will briefly outline the test pressure design process. Since we have different fluids in the well during testing compared to when a well control situation arises, we also need to perform a test design. We will investigate two common scenarios to illustrate the process.
T e s t p r e s s u re o f s u r f a c e c a s i n g
The surface casing is typically installed before the marine riser is installed. In the following we will determine the surface test pressures for two cases, during cementing and after the cement is set.
If the cement plug is landed (bumped) during cement placement, the casing can be tested immediately. The advantages are: saving time, and obtaining a better test since the cement is not set, creating a well-defined hydrostatic pressure profile. However, the large diameter surface casing gets a high tensional load during pressure testing.
Therefore, testing is often delayed until the cement is set.
Figure 5.6a illustrates the situation when testing a surface casing from a floating rig. During plug bumping, the external hydrostatic pressure is:
Po= 0.098{dsw(Dsb− Da)+ dce(D− Dsb)}
The inside pressure is:
Pi= 0.098dmwD
If a test pressure is applied inside the casing, the burst loading is:
Pburst = Pt+ Pi− Po= Pt+ 0.098{(dmw− dce)D+ (dce− dsw)Dsb+ dswDa} Often, sea water is used as a displacing fluid, reducing the above equation to:
Pburst = Pt+ 0.098{(D − Dsb)(dsw− dce)+ dswDa}
Inspection of this equation reveals that the test pressure is first reached under the wellhead, resulting in a test pressure of:
Pt = Pburst− 0.098dswDa (5.12)
Now we will investigate the test pressure after the cement is set. The surface casing is typically cemented along its whole length. After the cement is set, its weight is no longer providing hydrostatic pressure. However, some water exists in the cement, and
C a s i n g d e s i g n 147
a)
Da
Dsb
D
Pressure inside casing
Pressure inside casing
Pressure
Pressure Pressure
outside casing
Pressure outside casing Depth
Depth
Pressure difference across casing
Pressure difference across casing b)
Da
Dsb
D
Figure 5.6 Test scenarios for surface casing and deeper casing strings.
possibly also in voids and cracks around the wellbore. We may therefore imagine that a saltwater pressure is the only mobile component that may transmit pressure after the cement is set. The pressures then become:
Outside pressure:
Po= 0.098dsw(D− Da) Inside pressure:
Pi = 0.098dmwD
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If a test pressure is applied inside the casing, the burst loading is:
Pburst = Pt+ Pi− Po= Pt+ 0.098{(dmw− dsw)D+ dswDa} (5.13) Inspection of this equation reveals that for this case, the burst pressure is first reached at the casing shoe. Again, if sea water is used as the displacing fluid, the test pressure becomes identical to that of the bumping plug case. The only difference is that the test pressure is first reached in the top of the casing when bumping the plug, but at the bottom of the casing when testing after the cement is set.
T e s t p r e s s u r e o f d e e p e r c a s i n g s
The deeper casing strings are set through a marine riser, as shown in Fig. 5.6b. They are not cemented to the wellhead, but have a drilling fluid or a completion fluid above the top of the cement. In this demonstration we assume that the mud outside and inside the casing string have the same densities.
When bumping the plug, the following pressures arise:
Outside pressure:
Po= 0.098{dmwDce+ dce(D− Dce)} in the cemented interval Po= 0.098dmwD above the cemented interval Inside pressure:
Pi= 0.098dmwD
If a test pressure is applied inside the casing, the burst loading is:
Pburst = Pt+ Pi− Po= Pt+ 0.098{(dce− dmw)(Dce− D)
in the cemented interval (5.14)
Pburst = Pt above the cemented interval (5.15)
If the cement is heavier than the mud, the critical burst pressure will first be reached at the wellhead, which gives a surface test pressure of:
Pburst = Pt (5.16)
Finally we will investigate the pressure testing of deeper casing strings after the cement is set. In this case we assume drilling mud over the cement, and a hydrostatic sea water pressure as the only mobile phase inside the cement itself. The pressures are:
Outside pressure:
Po= 0.098{dmwDce+ dsw(D− Dce)} in the cemented interval Po= 0.098dmwD above the cemented interval Inside pressure:
Pi= 0.098dmwD
C a s i n g d e s i g n 149 If a test pressure is applied inside the casing, the burst loading is:
Pburst = Pt+ Pi− Po= Pt+ 0.098{(dmw− dsw)(D− Dce)
in the cemented interval (5.17)
Pburst = Pt at the wellhead (5.18)
We observe that for deeper casing strings the design test pressure was identical for the bumping plug and cement set cases, and both tests resulted in the test pressure being reached at the wellhead.
This chapter shows the methodology when designing test pressures. We observe that if different conditions exist, the test pressure must be redesigned. Examples are fixed platforms and cement programs involving several cement densities.
Another aspect is shown with the examples above. The kick scenario in the casing design results typically in the top or the bottom of the string becoming the most loaded position. In this chapter we have calculated the test pressure without comparison to the design scenario. We will find in some well designs, that the casing is not fully tested at its most severe position according to the design. This seems to be accepted practice in the industry. In the next chapter we will address this further.