Cement Slurry Flow Properties and Mud Displacement

12 minutes read

Mud Conditioning

The first step in an optimal cement job is drilling a good hole:

Smooth and straight (few doglegs or washouts)
In gauge (consistent diameter)
Stable (free of spalling or shale swelling and encroachment)
Clean (free of drill cuttings and excessively thick mud cake)

Before the casing string is lowered into the hole, the drillstring will likely have been out of the hole for 12 to 24 hours while logging and caliper surveys are run to determine the exact dimensions of the hole for cement volume calculations. So it is important to have the hole and mud in good condition before removing the drill string. The mud should be conditioned to reduce gel strength, yield point, and plastic viscosity to minimum values before the drillstring is removed, taking care not to impair the mud’s ability to suspend weighting agents needed to maintain proper hydrostatic head.

After the casing has been run, and before pumping the slurry, the mud should be circulated to clean the hole and remove remaining cuttings and mud cake. At the same time, the casing can be worked up and down or rotated during mud circulation. In areas where mudcake removal is expected to be difficult, scratchers or wipers should be placed on the casing to facilitate this action. Properly placed centralizers also facilitate the circulation of mud around the casing in areas where off-centered casing could be a problem.

At least two annular volumes of mud should be circulated at the highest rate possible without creating friction pressures that will cause lost returns. Here is where turbulent flow is needed to help with the cuttings and mud cake removal process. Tracers such as dye, grain, and colored beads can be used to monitor the volume of mud being circulated and make certain that at least 85% of the borehole volume is circulated before the cement slurry is pumped. By timing the travel time of these markers from the mud pump back to the shale shaker, one can compare the volume of mud pumped during that time to the calculated borehole volume.