Page 221 - Drilling Technology in Nontechnical Language

212 Drilling Technology in Nontechnical Language Second Edition

Accelerators or retarders (chemical additives) can be used to lessen or
lengthen the pumpable time and will similarly affect the rate of compressive
strength buildup.

Compressive strength

To perform the various functions, cement is designed to provide a high
compressive strength. A sample of the cement slurry is made up and placed
into a 2" cubic mold. Once the cement is set, this cube is taken out of the
mold and placed into a hydraulic press, where it is squashed until it breaks

(fig. 9–8). The pressure required to break this cube is measured.

Fig. 9–8. Cement testing lab equipment

A good compressive strength to support the casing is considered to
be 500 psi; 2,000 psi is considered the minimum for cement that will
be perforated.

Temperature rating

Under static (nonpumping) conditions, the well will have a temperature
gradient as the formations get hotter with depth. Normal temperature
gradient in sedimentary basins is about 1.4°C per 100 ft of vertical depth.
Circulating will decrease the local temperature around the wellbore. Thus

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