Page 212 - Drilling Technology in Nontechnical Language

Chapter 9 – CASING AND CEMENTING 203

The top joint of casing in this case will therefore have to resist 707,000 lb
(320 tonnes) of tensile force (again neglecting buoyancy).
Burst. Casings must be able to withstand internal pressure. Internal
pressure will come from downhole formation pressures, hydrostatic
pressures, and pressure tests.

Collapse. The opposite of internal pressure is where the pressure

outside of the casing is higher than the pressure from the fluids inside
the casing. It is possible for casing to be squashed flat like a ribbon by

external pressure.
Cemented casing is much harder to collapse (takes a much higher
pressure) than uncemented casing.
Flowing salt was discussed in chapter 1. Thick salt deposits flow under

pressure from the rocks above. Thus salt acts a lot like a hydraulic fluid in

these conditions and can impose very high collapse pressures on a string
of casing. Two factors then are important when cementing in flowing salts.

The casing has to be very strong (thick wall and/or high-strength steel),
and the cement around the casing must form a complete sheath. If there are

unfi lled areas without cement, the salt can flow into that area and place a
very high point loading (as opposed to an even loading) on the casing. No
casing can resist a high point loading.

Driving forces. Conductor pipes are sometimes driven (hammered
in place) by a pile driver into the ground to allow the well to be spudded
with a closed circulating system. Conductor is thick-walled pipe (often 1"
or greater wall thickness), so the pipe itself is strong enough to drive. The
connections must be selected to be suitable to transmit the heavy shock
loads of driving.
Temperature. When a steel casing gets hot, it expands. Where the
casing is cemented, this does not cause any problems, but the uncemented
pipe between the top of cement and the surface wellhead may buckle as
expansion takes place. This can be compensated for by stretching the
casing before setting it in the wellhead.
Steel also loses strength as temperature increases. In a deep, hot well,
this loss of strength can be significant. At a temperature of 200°C, steel will

have lost 19% of its strength. This has to be accounted for when designing
casings for high-temperature wells.

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