The retort is the small still every mud engineer runs: heat a known volume of mud, boil off the liquids, and read what is left. It gives you oil, water and solids by volume — the raw scorecard for density, rheology and solids control. But the headline ‘percent solids’ is not the number that decides anything. The number that matters is buried inside it: how much of that solid is low-gravity drilled solids versus the barite you paid for.
What the retort actually measures
The method is deliberately simple. A known volume of mud — from a 10, 20 or 50 cm³ chamber — is heated until the liquid components vaporise, pass through a condenser, and collect in a graduated receiver where you read the oil and water volumes directly. The solids are then obtained purely by difference: total sample volume minus the liquid volume you collected. Nothing weighs the solids in the standard volumetric method; they are simply what did not boil off.
One accuracy note worth carrying: the 50 cm³ retort is more reliable than the 10 or 20 cm³ versions, where ‘missing mass’ and reading errors creep in — which is why API adopted a 50 cm³ gravimetric method for oil-based muds. And ‘retort solids’ technically include dissolved salts and charred organics, not just the suspended particles you care about. Getting from a raw retort reading to the numbers that actually drive decisions is exactly the arithmetic Rig IQ runs for you.
Suspended vs dissolved — the correction that matters
Here is the trap. The solids the retort reports include dissolved solids — the salts in the water phase — as well as the suspended solids you actually manage. On any salt-containing mud, if you skip the dissolved-solids correction you will overstate your suspended solids and chase a solids problem that is partly just brine. The correction uses the filtrate chloride and calcium analyses to work out the volume the dissolved salts occupy, which is then backed out.
What you are left with after that correction is the number that counts: suspended solids = weighting material (high-gravity solids) + drilled solids (low-gravity solids). That single figure is the input to everything downstream — and on a weighted mud it is meaningless until you split it, because 20% total solids could be mostly barite you want or mostly drilled solids you don't. Carrying the salinity correction and the suspended-solids figure through cleanly is the kind of book-keeping Rig IQ keeps honest.
Splitting the solids — where the value is
The retort plus the mud weight is enough to split the suspended solids into low-gravity (drilled) solids and high-gravity (barite) solids, using a simple material balance built on their densities: low-gravity solids at specific gravity 2.6, barite at 4.2, against the measured mud weight. Solve the balance and you get the one number that actually tells you whether your solids control is working: the volume percent of low-gravity solids.
A worked case makes it concrete. A 12.0 ppg freshwater mud carrying 20% solids by volume works out to about 12.5% low-gravity solids — the rest being barite. That 12.5% is the figure you trend, compare to target, and act on; the raw ‘20% solids’ on its own would have told you almost nothing. Running that low-gravity-solids split from your retort and mud weight, every tour, is precisely what Rig IQ is built to do — turning a raw retort reading into a decision.
Reading a retort
1. Read directly: oil and water volumes in the receiver. Solids = total volume − liquid volume.
2. Correct for salt: back out dissolved solids (from chloride/calcium) → true suspended solids = HGS + LGS.
3. Split with mud weight (material balance): LGS at SG 2.6, barite at SG 4.2 → % low-gravity (drilled) solids.
The LGS % is the number you act on — not the raw total solids. Use the 50 cm³ retort for accuracy.
The retort gives oil, water and solids by volume — but the total solids figure alone decides nothing. Correct for dissolved salts to get true suspended solids, then use the mud weight to split them into low-gravity drilled solids (SG 2.6) and barite (SG 4.2). The low-gravity-solids percentage is the number you trend and act on. A 12.0 ppg mud at 20% solids, for instance, carries about 12.5% LGS.
Common questions
What does a retort measure in drilling mud?
It measures the volume percent of oil, water and solids in a mud sample. A known volume is heated so the liquids vaporise and condense into a graduated receiver, where oil and water are read directly; the solids are obtained by difference (total volume minus liquid volume). It is the standard API RP 13B test for solids and liquid content.
How do you calculate low-gravity solids from a retort?
Take the suspended-solids volume from the retort (after correcting for dissolved salts), then use the mud weight in a material balance based on the densities of low-gravity solids (SG 2.6) and barite (SG 4.2). Solving the balance gives the volume percent of low-gravity solids. For example, a 12.0 ppg freshwater mud at 20% solids works out to about 12.5% LGS.
Why correct retort solids for dissolved salts?
Because the retort reports dissolved salts as part of the solids. On any salt-containing mud, skipping the correction overstates your suspended solids and makes the solids load look worse than it is. The chloride and calcium filtrate analyses are used to back out the volume the dissolved salts occupy, leaving the true suspended solids.
