Every problem solids control exists to solve comes down to one quantity: low-gravity solids (LGS) — a fraction that includes drilled formation solids and added bentonite. LGS is not the same as drilled solids alone. Weight material (barite, ~4.2 SG) is a high-gravity solid you want; drilled solids (~2.6 SG) are the low-gravity intruders you spend the whole surface system trying to remove. Read the LGS right and the rest of the programme — dilution, rheology, even ROP — starts to make sense.
Why LGS is the number behind the number
As LGS rises, plastic viscosity climbs, the mud gets harder to pump and condition, equipment wears faster, and ROP suffers. The only two ways to control it are to remove the solids mechanically or to dilute them away with fresh fluid. Removal is cheap; dilution is expensive and recurring. So LGS is both the symptom you measure and the driver of the mud bill.
Where the numbers come from — the retort
You cannot manage LGS without measuring it, and the measurement starts with the retort. Per API RP 13B-1 (water-based) and 13B-2 (oil-based), a known mud volume is heated until the liquids boil off and condense, giving you the volume split:
The retort gives total retort solids. Apply the required salinity corrections before estimating suspended solids and separating weighting material from LGS; the LGS estimate still includes added bentonite.
Splitting total solids into barite and LGS
Total solids alone does not tell you whether the mud is heavy with useful barite or fouled with drilled fines. You separate them using mud weight and the known specific gravities — barite at ~4.2 SG and average drilled/low-gravity solids at ~2.6 SG. Combining the retort solids volume with the measured mud weight lets you back out how much of the solids load is weight material and how much is LGS (API RP 13B-1 sets out this low-gravity-solids and weighting-material calculation directly).
A worked feel for it: two muds can both read 20% total solids on the retort, yet one is a healthy 14-ppg weighted mud carrying mostly barite, and the other is an over-loaded mud whose solids are mostly drilled fines. Same retort number, completely different health — which is exactly why you must do the split, not just read total solids.
MBT: the reactive-clay early warning
Not all LGS behaves the same. The methylene blue test (MBT) measures the cation-exchange capacity of the solids — in plain terms, how much reactive clay (bentonite-equivalent) the mud is carrying. In water-based mud, a rising MBT may reflect added bentonite or formation-derived clay; it does not by itself establish dispersion or particle size. Review additions, formation and test repeatability before changing treatment. Conventional centrifugation does not guarantee removal of the colloidal fraction.
From measurement to action
- LGS rising, MBT flat — you are generating coarse-to-fine drilled solids the equipment is missing. Check screens, feed head and centrifuge hours; you can mechanically remove these.
- LGS and MBT both rising — reactive clay is dispersing. Mechanical removal alone won’t hold it; expect to lean on the centrifuge and controlled dilution, and review fluid chemistry.
- LGS in band, dilution still high — suspect a bypass or a measurement error before adding fluid.
Key takeaways
LGS is the quantity solids control is really managing; the retort and the LGS/barite split are how you see it; MBT tells you what kind of solids you are fighting. Track all three and dilution stops being a reflex and becomes a decision — the difference between a mud programme you steer and one that steers you.
