A weighted drilling fluid is only uniform while it is moving. Agitation is what keeps barite suspended in storage, and getting it right — adequate power, full coverage, never stopped, and backed by the fluid's own suspension properties — is the difference between supplying on-spec mud and supplying a density gradient. This page covers how stored mud is kept uniform.
Why suspension needs help
Barite has a specific gravity of roughly 4.2 against a continuous phase near 1.0–1.4, so gravity constantly pulls it down. Only two things resist that settling: the fluid's gel structure and low-shear-rate viscosity, and continuous mechanical agitation. Between transfers, agitation does the heavy lifting; the fluid's own rheology holds solids during the brief periods it is still.
Sizing and coverage for the whole tank
An agitator has to keep the entire tank volume in motion, not just stir the surface. Undersized power or a poorly placed impeller leaves quiescent zones near the floor and walls where barite settles first, so impeller type, diameter, speed and elevation are matched to tank geometry to give floor-to-surface turnover. A tank that looks agitated at the top can still be sagging at the bottom if coverage is wrong.
The rheology that backs it up
Agitation is far more effective when the fluid is designed to stay suspended: adequate low-shear-rate viscosity (the 6-rpm Fann reading / LSRV) and gel strengths let the fluid hold barite during quiet periods and re-suspend it easily. A fluid with collapsed low-end rheology will sag even under reasonable agitation, which is why suspension is a fluid-design problem as much as a mechanical one.
Idle tanks: recirculation and roll-over
Tanks that sit between uses are the highest sag risk, so where continuous agitation is impractical the plant runs a routine of recirculation and roll-over — periodically pumping the tank over on itself to re-homogenise it. A scheduled roll-over of reserve tanks keeps inventory ready to supply rather than discovering a settled bed at the moment it is needed.
The direct link to barite sag
Weak, mis-placed or stopped agitation is the leading cause of barite sag in storage: a measurable top-to-bottom density gradient and a settled bed that may be soft or hard-packed. The consequences are off-weight supply, starved or blocked suctions, and barite effectively lost into the floor — all of which trace back to a loss of suspension.
Never let a dense tank go still
The practical rule is simple: a full, dense storage tank must not lose agitation. That means adequate power, correct coverage, supportive rheology, a roll-over routine for idle tanks, and backup power to the most critical agitators so a single failure does not start sag across the farm.
Key takeaways
Stored drilling mud must be continuously agitated because barite (SG ~4.2) settles the instant the fluid goes still. Agitators are sized and placed for full floor-to-surface turnover, backed by adequate low-shear-rate viscosity and gels, with recirculation and roll-over for idle tanks and backup power on the critical agitators. Weak or stopped agitation is the leading cause of barite sag in storage; keeping dense tanks moving is non-negotiable.