Spacers are any fluids other than cement slurry or mud, pumped ahead (preflush) or behind (postflush) the cement slurry.

Preflushes are used to:

• prevent contamination of cement slurry with mud
• improve mud displacement and mud cake removal.

Postflushes are used to prevent:

• the plug from spinning when it will be drilled out (cement on top)
• contamination with the fluid used for displacement.

General Guidelines

The following guidelines shall be adhered to during the use of spacers:

• Scavenger slurries shall not be used if the top of cement (TOC) is critical eg., to allow for a possible side-track.
• Reduction in hydrostatic head due to spacers pumped must be carefully calculated. If the overbalance is calculated to be less than the minimum required overbalance, at any stage of the cementation, a weighted preflush shall be used instead of the water spacer.
• When water is used as a spacer over productive intervals, it shall be treated with 2 % KCl w/w for formation inhibition.
• Lines shall be flushed thoroughly to ensure the contamination of mud, spacers and cement slurry is minimised.
• Muds requiring extra long spacers or spacers of a special composition will be advised by the Mud and Cement Contractor.
• If oil has been added to a water base mud for any reason, then +/- 2 % v/v surfactant should be included in the water spacer.
• In general, plug flow and plug flow spacers are only used for stinger cementations where turbulent or laminar flow regimes cannot be achieved.
• It is preferable to pump spacers between cementing plugs.

Preflush Formulations

Cement scavenger slurries are dilute mixes of cement and water, pumped ahead of the cement slurry in turbulent flow conditions to remove gelled mud and filter cakes. The following guidelines shall be used when pumping scavenger slurries:

• The gradient should be between mud gradient and the main cement slurry gradient with a maximum of 14 ppg. The Mud and Cement Contractor should be consulted for the optimum gradient.
• To prevent fast setting of the scavenger slurry, a cement retarder shall be added to the mixwater depending on lab test and mixing logistics.

Recommended Preflushes for Water Based Muds

High Annular Velocity Displacements

The recommended preflushes for water based muds for high annular velocity displacements are given in the following table:

Cement Plugs and Squeezes

The recommended preflushes for fresh water muds when setting cement plugs and squeezes are given in the following table:

Postflushes

Standard Cementations

In standard cementations, after the top plug has been dropped cement slurry should be pumped behind to prevent the plug from spinning when it is being drilled out.

The postflushes listed in the table below should be used in all cementations including plugs and squeezes.

Cementations Using Stinger

The postflushes listed in the table below should be used in all cementations in including plugs and squeezes.