Barite sag is usually discussed as a downhole problem, but it starts in the tank. A weighted fluid that sits still in storage will let its barite settle, and the plant then supplies a rig with mud that is light at the top and heavy at the bottom. This page covers why sag happens in plant storage and how to keep a stored fluid uniform.
Sag starts when the fluid stops moving
Barite is roughly four times the density of water, so the only things holding it in suspension are the fluid's gel structure and continuous agitation. When a tank sits static, or is agitated too weakly, the gels relax and gravity wins: barite migrates downward and a density gradient forms from top to bottom. The denser and more weakly gelled the fluid, the faster it happens.
What causes it
Sag in storage comes from a short list of causes: idle or dead tanks with no movement, undersized or failed agitation that does not reach the tank floor, weak suspension properties (low LSRV / poor low-shear-rate gels), and simply high density, which makes every other factor worse. Often it is a combination — a heavy fluid in a tank whose agitator cannot keep the whole volume moving.
How it shows up
The plant sees sag as a density gradient in tank samples — light at the top, heavy near the bottom — and as a settled bed that can range from soft and re-suspendable to hard-packed. Downstream it shows up as off-spec supply (a rig reports the delivered mud weight is wrong), blocked or starved suctions, and barite that is effectively lost into a bed at the tank floor.
Fixing it
The cure is to keep the fluid moving and able to hold solids. That means agitators sized and positioned for the full tank volume, a routine of roll-over / recirculation for tanks that sit between uses, attention to low-shear-rate viscosity and gels so the fluid resists settling between agitation cycles, and avoiding genuinely dead tanks in the layout. Where a bed has already formed, it is re-suspended by recirculation and agitation before the tank is trusted to supply.
Key takeaways
Barite sag in storage is gravity beating a fluid's gels and agitation: in static or weakly agitated tanks barite settles into a density gradient and a bottom bed, worst in dense, weakly gelled fluids. It shows as top-to-bottom weight differences, off-spec supply and blocked suctions. The fix is adequate agitation for the full tank, regular roll-over of idle tanks, sound low-shear-rate viscosity and gels, and re-suspending any bed before supply. Tank and agitator details vary; the keep-it-moving-and-gelled principle is constant.