On unweighted mud, the retort’s solids number is almost the whole story. On weighted mud it hides the thing you actually care about: how much of those solids is the barite you paid for, and how much is drilled solids you need to remove. Average specific gravity — ASG — is the calculation that separates the two, using nothing but the mud weight and the retort you already run. It is the quiet number that turns “percent solids” into a real drilled-solids gauge.
What ASG is
Average specific gravity is exactly what it sounds like: the volume-weighted average density of all the suspended solids in the mud. Formally, ASG = (ρHGS × VHGS + ρLGS × VLGS) / (VHGS + VLGS) — the two solid populations, each weighted by how much of it there is. The densities are standard: barite is taken as 4.2, low-gravity (drilled) solids as 2.6, water as 1.0. If every solid in the mud were barite, ASG would be 4.2; if it were all drilled solids, 2.6. Anything in between tells you the mix.
That is the whole point. A weighted mud reporting, say, 20% solids by retort could be mostly barite doing its job or half drilled solids wrecking your rheology — and the solids percentage alone can’t tell you which. ASG is the single number that resolves it, and it needs no extra test: you back it out from the mud weight and the retort you already have. Running that back-calculation cleanly, correcting for salt and oil, is one of the routine computations Rig IQ handles.
Working the split
The method is a material balance. From the retort you get the volume fractions of oil, water and total solids; the mud weight gives the overall density. Subtract the liquid contributions (correcting water for dissolved salt and using the right base-oil density) from the mud’s density, divide by the solids fraction, and you have the ASG of the solids. Once you know ASG, the split falls straight out: VLGS = Vs × (4.2 − ASG) / (4.2 − 2.6), and VHGS is the remainder.
A worked case makes it concrete. Take a 12.0-ppg freshwater mud reading 20% total solids by retort. Its density implies an ASG of about 3.2 — and running the split gives roughly 12.5% low-gravity solids and 7.4% barite. So two-thirds of that “20% solids” is drilled solids and clay, not weighting material — a very different picture from the raw number, and one that tells you the removal train and dilution have work to do. Turning the retort and mud weight into that drilled-solids split every check is exactly what Rig IQ automates.
Why ASG is a drilled-solids gauge
Because ASG is fixed by the LGS/HGS ratio, its trend is a direct read on drilled-solids accumulation. As you drill and low-gravity solids build up, the average density of the solids falls — ASG drifts down toward 2.6. A falling ASG at constant mud weight means the barite fraction is being diluted by drilled solids: the removal train is losing ground even if the total solids number looks steady. That makes ASG a more honest early-warning signal than percent solids on a weighted system.
It also guards against a costly mistake. On weighted mud you cannot simply centrifuge or cyclone “the solids” away, because much of that solid is barite you want to keep — the barite-recovery problem. ASG is what quantifies the target: it tells you how much drilled solids you’re actually carrying, so you can size dilution and separation to remove those without discarding the weighting material. Reading ASG as the gauge of the drilled-solids fraction — and watching its trend rather than a single value — is the discipline Rig IQ is built around.
ASG and the split
ASG = (ρHGS×VHGS + ρLGS×VLGS) / (VHGS+VLGS) — volume-weighted average solid density. Barite = 4.2, drilled solids = 2.6.
The split: VLGS = Vs × (4.2 − ASG)/(4.2 − 2.6) · VHGS = remainder. Back ASG out from mud weight + retort — no extra test.
Example: 12.0 ppg, 20% solids → ASG ≈ 3.2 → ~12.5% LGS + 7.4% barite.
Falling ASG = drilled solids building — an earlier signal than percent solids on weighted mud.
Average specific gravity (ASG) is the volume-weighted average density of the suspended solids: ASG = (ρHGS×VHGS + ρLGS×VLGS)/(VHGS+VLGS), with barite 4.2 and drilled solids 2.6. Back it out from mud weight and the retort — no extra test — then split: VLGS = Vs×(4.2−ASG)/1.6. A 12.0-ppg mud at 20% solids gives ASG≈3.2, about 12.5% LGS and 7.4% barite. A falling ASG means drilled solids are building — an earlier warning than percent solids on weighted mud.
Common questions
What is average specific gravity (ASG) of solids?
It is the volume-weighted average density of all the suspended solids in the mud: ASG = (density of barite times its volume, plus density of low-gravity solids times its volume) divided by the total solids volume. Using standard densities of 4.2 for barite and 2.6 for drilled solids, ASG tells you the mix — 4.2 would be all barite, 2.6 all drilled solids, and values in between give the split.
How do you calculate the LGS/HGS split from ASG?
First back ASG out of the mud weight and retort: subtract the liquid contributions (water corrected for salt, plus oil) from the mud's density and divide by the solids fraction. Then split with VLGS = Vs × (4.2 − ASG) / (4.2 − 2.6), and VHGS is the remainder. For example, a 12.0-ppg mud with 20% solids gives ASG ≈ 3.2, about 12.5% low-gravity solids and 7.4% barite.
Why does ASG matter on weighted mud?
Because 'percent solids' alone can't tell you how much of the solid is valuable barite versus damaging drilled solids. ASG resolves that split using data you already have. Its trend is also a drilled-solids gauge: as low-gravity solids accumulate, ASG falls toward 2.6, so a declining ASG at constant mud weight warns that drilled solids are building even when the total solids number looks steady.
