As previous post.

Mud has likely been static for a long period so it is not ideal for cementing. So principle reason is to get mud into cementing shape. Secondary would be;

• Verify mud pumps are good.
• Verify pump pressures (for low rate prior to bump etc)

• Aides in mud gel removal. (in association with test pipe may be reciprocated and/or slowly rotated in some cases) 

Some companies therefore would always perform this to ascertain maximum displacement rate possible (by staging up rates as per previous post) as they would insist on displacing with the rig pump vs the cement pump. Particularly in larger top, surface, intermediate casing where large operating margins exist between pore and fracture pressure.


Rule of thumb; Based on drilling flow rates used and AV's present (and no losses resulting). One can estimate the equivalent circulating rate for the casing and open hole geometry now present. (so this is the ballpark figure one would expect to be able to displace at)  

Prior to cementation, it is recommended to circulate at least 150 % of the casing volume.

During the circulation the circulating pressures at different pumping rates to be recorded for each pump.

Recommended to start circulation at a very low rate and gradually increase in increments of 2bpm.

Prudent to wait 5 min for observing losses and for pressures to stabilize prior recording.

Take the opportunity to ensure all PVT instrumentation are functional and cross check with manual level measurements.

Compare between calculated pump discharges and actual fluid pumped by directing returns to a known volume pill tank.(assuming no losses/gain).

Reduce the pump rate if losses are observed or at max ECD allowable.

Determine the final cement slurry displacement rate and note the circulation pressures at this rate.

The circulation test basically guides one to choose the appropriate circulation rates for cement displacement, pump volumetric efficiencies and the detection of pressure peak on bumping plug(if used) and/or to avoid over displacement.