Page 220 - Drilling Technology in Nontechnical Language

Chapter 9 – CASING AND CEMENTING 211

Table 9–1. The API class G cement mixtures by weight

Bentonite
Slurry weight Cement required Water required required
14.2 ppg 8.2 lb 0.66 (US) gal 0.33 lb

13.2 ppg 6.7 lb 0.73 (US) gal 0.54 lb

12.6 ppg 5.6 lb 0.76 (US) gal 0.67 lb

Other materials may also be used as extenders. Hollow glass or ceramic
microspheres can be added to neat cement, as can materials with low
specific gravity, such as powdered coal or crushed volcanic glass. Cement

can also be made into a foam by mixing with nitrogen, which can give
very light slurries for weak zones that cannot handle much hydrostatic
pressure. Slurries down to 7 ppg (0.364 psi/ft) can be created using foam.
This cement would float in water when set.

It may also be necessary to mix up very heavy slurries that are denser
than neat cement. In this case, heavy materials such as barite or hematite
are added to the slurry.
The lower limit of cement slurry density is dictated by the requirement

to always maintain a hydrostatic overbalance on pore fluid pressures while
pumping cement around and into place. The upper limit is dictated by the
strengths of downhole formations.

Thickening time

The thickening time and compressive strength buildup is dependant
on well temperature. Higher temperature gives faster setting and faster
strength buildup. The slurry must have sufficient pumpable time to

complete the job, with a safety margin in case of problems. Also the
thickening time should not be so long that rig operations are unnecessarily
delayed while waiting for cement to set. The thickening time is determined
in the laboratory using samples of cement, chemicals, and mix water sent
in from the rig.

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