On most rigs cement powder and additives are handled in bulk, which makes blending and mixing much easier. For large volume cement jobs several bulk storage bins may be required on the rig. On offshore rigs the cement is transferred pneumatically from supply boats to the storage bins.
For any cement job there must be sufficient water available to mix the slurry at the desired water/cement ratio when required. The mix water must also be free of all contaminants.
The water is added to the cement in a jet mixer (Figure 8). The mixer consists of a funnel shaped hopper, a mixing bowl, a water supply line and an outlet for the slurry. As the mixwater is pumped across the lower end of the hopper a venturi effect is created and cement powder is drawn down into the flow of mixwater and a slurry is created. The slurry flows into a slurry tub where its density is measured. The density of the slurry should be regularly checked during the cement job since this is the primary means by which the quality of the slurry is determined. If the density of the slurry is correct then the correct amount of mixwater has been mixed with the cement powder. Samples can be taken directly from the mixer and weighed in a standard mud balance or automatic devices (densometers) can also be used.
Various types of cement pumping units are available. For land based jobs they can be mounted on a truck, while skid mounted units are used offshore. The unit normally has twin pumps (triplex, positive displacement) which may be diesel powered or driven by electric motors. These units can operate at high pressures (up to 20,000 psi) but are generally limited to low pumping rates. Most units are capable of mixing and displacing 50 – 70 cubic feet of slurry per minute. In order to minimise contamination by the mud in the annulus a preflush or spacer fluid is pumped ahead of the cement slurry. The actual composition of the spacer depends on the type of mud being used. For water based muds the spacer fluid is often just water, but specially designed fluids are available. The volume of spacer is based on the need to provide sufficient separation of mud and cement in the annulus (20 – 50 bbls of spacer is common).