Civil Engineering Reference
In-Depth Information
Physical and performance requirements
Mix water (% by weight of cement)
Class G
44
Class H
38
Schedule 5 thickening time (min.)
90-120
Max. consistency during first 15-30 min of schedule 5 test (Bc)
30
Compressive strength (MPa)
8 h atmospheric pressure, 38°C (min.)
2.1
8 h atmospheric pressure, 62°C (min.)
10.3
I
n terms of chemical composition, Class G and Class H oil well cements are low-C
3
A
Portland cements, and the specifications of both are typically met by high-iron, sulfate-
resistant Portland cements (see also section 2.8). The free lime content in these cements is
not limited by the API specifications, but it should be below 0.5 wt% to avoid difficulties
with cement slurry rheology and retarder response. In producing Class G and Class H oil
well cements, iron oxide (in the form of hematite or pyrites residues) must usually be
added to the raw mix to produce more ferrite phase at the expense of tricalcium
aluminate. The required amount of Fe
2
O
3
is greater when a cement of HSR rather than
MSR grade is produced.
Up to temperatures of about 100°C the hydration products formed in the hydration
process do not differ significantly from those formed at ambient temperature, but the
hydration is accelerated unless retarders are added to the slurry. Of the two calcium
silicate phases, dicalcium silicate is accelerated by elevated temperature more effectively
than tricalcium silicate. In general, the C-S-H phase formed at elevated temperatures has
a higher C/S ratio, and the silicate anion size is shifted upwards. It also contains higher
amounts of incorporated aluminum and sulfate ions. The strength of the hardened slurry
at shorter hydration times is obtained mainly from the hydration of C
3
S, whereas after
longer hydration times the contribution of C
2
S to strength development increases
significantly. Above about 70°C the AFt phase becomes unstable, and the AFm phase is
formed as the sole calcium aluminate (ferrite) sulfate hydrate.
The properties of slurries made with Class G and Class H cements may be modified by
a variety of additives in order to cover a wide range of well depths and temperatures,
pumping times, and other requirements.
To control the bulk density, low- or high-density materials may be added to the mix.
To lowering the density, bentonite (sodium montmorrilonite based clay), typically in
amounts between 2 and 12 wt%, may be used. Bentonite works by increasing the water
demand of the slurry, as it can hold several times its own weight of water. The
effectiveness of bentonite may be increased and its dosage lowered significantly if it is
mixed with water and allowed to swell before being added to the slurry. Other materials
that may be added to the slurry to lower its density include pulverized fly ash,
diatomaceous earth, or expanded perlite. To increase the density of the slurry, finely
