The retort gives you total solids, but total solids is the wrong number to manage. What drives dilution cost and drilling performance is the split — how much is barite you paid for, and how much is drilled solids you need gone.
Why the split is the number that matters
High-gravity solids (barite, SG 4.2) are weighting material you want to keep. Low-gravity solids (drill solids and clays, SG 2.6) are the load your shakers, cyclones and centrifuge exist to remove. A retort that reads 20% total solids could be a healthy weighted mud or a system drowning in drill solids — only the split tells you which.
The material balance
Assume the accepted specific gravities: low-gravity solids at 2.6 and barite at 4.2. The mud weight fixes the average density of everything in the barrel, and the retort fixes the total solids volume. That is two equations in two unknowns — the volume of LGS and the volume of HGS — and they solve exactly.
The equation
For a fresh-water mud, with mud weight MW (ppg) and total retort solids Vs (vol%):
2.6·VLGS + 4.2·VHGS = 100·(MW÷8.33) − Vwater
VLGS + VHGS = Vs
Solved for the drilled-solids fraction:
VLGS = 62.5·(1 − MW÷8.33) + 2·Vs, VHGS = Vs − VLGS
Track LGS as vol% or as an LGS/HGS trend, not total solids. When drilled solids climb past roughly 6–7 vol% in a weighted mud, removal has fallen behind generation and dilution cost starts to run away.
Common questions
What SG values are standard for LGS and HGS?
Low-gravity solids (drill solids and clay) are taken at 2.6 and barite at 4.2. Hematite, where used, is about 5.5 — swap that in for the HGS density if the mud is weighted with hematite.
Does drilled solids equal low-gravity solids?
Almost, but not exactly. Low-gravity solids include added bentonite as well as drill solids. To isolate true drilled solids, subtract the known commercial bentonite from the calculated LGS.
