Civil Engineering Reference
In-Depth Information
27
Oil well cements
Oil well cements have been developed for use in the drilling of oil and gas wells. They
are used mainly to restrict the movements of fluids between formations at different levels.
In this way productive oil- and gas-bearing formations may be sealed from water-bearing
layers, and—in the case of multilayer deposits—the production formations may be sealed
off from each other. The hardened slurry is also intended to support and protect the
casing. The cementing work is carried out by pumping the cement slurry down the steel
casing of the well and up the annular space between it and the surrounding rock. More
specialized techniques include squeeze cementing, in which the slurry is forced through a
hole in the casing into a void or into a porous rock, and plugging, in which the casing is
blocked at a specified depth.
Oil well cements must usually perform at elevated temperatures and pressures, both of
which increase with increasing depth. The maximum temperature encountered at the
bottom of deep wells may reach 250°C, and may even exceed 300°C in geothermal wells.
Under these conditions the temperature of the slurry during pumping may reach 180°C
(bottom hole static temperature). The pressure to which the cement slurry is exposed is
equal to the hydrostatic load plus the pumping pressure, and may reach 150 MPa.
The following are the main features desired in oil well cements:
• The viscosity of the cement slurry must be low enough to allow easy pumping. The
slurry must stay sufficiently mobile for the whole time needed to complete the
pumping procedure, which usually does not exceed 3-4 hours. In the oil industry the
pumpability of the cement slurry is usually assessed by the thickening time test. In this
the consistency is measured with a high-temperature, high-pressure consistometer, in
which the temperature and pressure expected to occur during pumping are simulated.
• After setting, the slurry must attain a predetermined strength at an acceptable speed.
The rate at which the material develops its strength is relevant, as it determines the
time needed before drilling operations can continue. A retrogression of strength must
not occur under the conditions existing in the well.
• After being placed, the set cement slurry must exhibit sufficient resistance to the flow
of liquids and gas. To achieve good performance in this respect, the volume changes
that take place during setting and hardening must be negligible.
• The hardened cement slurry must be sufficiently resistant to chemical attack, especially
by sulfates. In some fields corrosion by overheated CO
2
-rich steam may also be
critical, and must be taken into consideration.
• The available cement must be highly uniform, to ensure predictable and reproducible
behavior of the slurry made from such a binder.
The requirements for oil well cements are laid down in the relevant specifications of the
