Solids Control Definition / Meaning
Solids control is the surface equipment and the practice that strips drilled solids out of the drilling mud as it returns from the well, so the mud can be pumped back down clean. Every foot drilled turns rock into cuttings that ride up the annulus in the mud; if they stay in it the mud gets heavier, thicker and more abrasive, and the whole system drifts away from the properties the mud engineer designed. Solids control is how the rig fights that drift mechanically rather than by dumping mud and mixing new.
The equipment sits in a fixed order along the flowline, each stage taking out finer material than the last, and the mud only reaches the pits and the pumps once it has passed them all.
The Equipment, in Order
| Stage | What it removes | How |
|---|---|---|
| Shale shakers | Cuttings and coarse solids | Returning mud pours onto vibrating screens; mud falls through, cuttings walk off the end. The screen mesh is chosen to match the mud and the rock. |
| Degasser | Entrained gas | Vacuum or atmospheric vessel that lets gas break out of the mud before it reaches the pumps, where it would cause cavitation and a false light reading. |
| Desander | Sand-sized solids | Large hydrocyclones: mud spins in a cone, heavier grains go to the underflow, cleaner mud to the overflow. |
| Desilter and mud cleaner | Silt-sized solids | Smaller cones; a mud cleaner adds a fine screen under them to recover weighting material from the underflow. |
| Centrifuge | The finest solids | A spinning bowl separates by density; used to strip fine drilled solids from weighted mud, or to recover barite, depending on how it is run. |
Why the Solids Have to Come Out
Low-gravity solids, the ground-up formation, are the enemy of a good mud. They raise the mud weight without adding useful hydrostatic control, thicken the mud so it needs more pump pressure, raise the equivalent circulating density in the annulus, slow the rate of penetration, wear pumps and bits, and build thick filter cake on permeable formations. In weighted muds the problem is sharper still, because the expensive barite has to be kept while the worthless drilled solids are removed, and both are fine and heavy.
Dilution Versus Removal
There are only two ways to bring the solids content down: throw mud away and replace it with clean fluid, or take the solids out mechanically. Dilution is simple and costly, in fluid, in additives and in disposal, and it is the only option once solids are ground finer than the equipment can catch. Good solids control catches them early, at the shakers, while they are still coarse, which is why the shaker screens and the way the flowline is fed get more attention than any other stage.
Usage Example
“We were building weight by a tenth of a pound every stand until we screened down two sizes on the shakers and put the centrifuge on the active; after that the mud held at 11.2 for the rest of the section.”
On the Mud Report
The mud engineer tracks the result with a retort test that splits a mud sample into water, oil and solids by volume, and separates the solids into high-gravity weighting material and low-gravity drilled solids. The drilled-solids percentage, and how fast it is climbing, is the daily verdict on whether the solids-control equipment is winning.