Page 217 - Drilling Technology in Nontechnical Language

208 Drilling Technology in Nontechnical Language Second Edition

Cement

Cementitious material consists of a powder that undergoes chemical
reactions when mixed with water. The end result of these reactions is a
hard, stonelike material. Cements have been used for centuries (the ancient
Egyptians, Greeks, and Romans all used cementitious materials). Modern
cement development started with a British patent granted to Joseph Aspdin

in 1824, which defined the process used to manufacture cement for building
a lighthouse. This cement was called Portland cement because when hard,
it looked like stone from the Isle of Portland, used for building.
Portland cement has four principal components: tricalcium silicate
(C S; about 70%), dicalcium silicate (C S; not more than 20%), tricalcium
2
3
aluminate (C A), and tetracalcium aluminoferrite (C AF). These chemical
3
4
compounds are created by mixing raw materials together and firing in a

kiln at high temperatures (up to 1,500°C).
The raw materials are lime, silica, alumina, and iron oxide. Before

firing, they are finely ground up and mixed in the correct proportions.

After firing, the raw materials have been converted to a material called
clinker. After cooling, some gypsum (CaSO ·2H O) is added (3%–5%),
2
4
and the mixture is crushed to a powder. This powder is Portland cement.
The chemistry of setting cement is quite complex, and several stages

can be identified. However, in general terms, when water is added, the
components form hydrated compounds. Crystals are formed that grow
and become interlinked. Also in the early stages, materials are dissolved
in the water and later on are precipitated as solids. Some water is left
trapped in the spaces between the crystals. Cement is porous but should
be impermeable because the passages connecting the pores are sufficiently

small that movement of water is stopped.
The reaction of hydrating Portland cement is exothermic—it generates
heat. This can be a problem in arctic areas when drilling through the
permafrost. Special cements are then used that do not freeze and that have

a low heat of reaction, while developing sufficient compressive strength to
meet the requirements of the set cement.
The American Petroleum Institute established standard specifications

for oil well cements. This defined eight different cements, which were

classified according to the depths and temperatures at which they could

be used. These classes were designated A through H. The specifications

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