“How much solids is too much?” is the question behind every dilution decision — and it has a surprisingly firm answer. A couple of percentage points of drilled solids is the difference between a mud that drills and one that fights you, and the field targets are well documented.
Total solids, barite and the LGS that matters
The retort splits your mud into oil, water and total solids by volume (API RP 13B). Total solids on a weighted mud can be 15–25% and still be perfectly healthy — because most of it is barite, the weight material you paid for. The number that governs mud behaviour is the low-gravity solids (LGS): the drilled clay and rock, specific gravity around 2.6, that solids control exists to remove.
So ‘normal drilled solids’ is really a statement about LGS, not total solids. You get it by taking the total solids from the retort and subtracting the barite fraction (calculated from mud weight), which leaves the low-gravity, mostly-drilled fraction. That is the number to watch.
The target: below about 5%
The widely used field target is to keep low-gravity drilled solids below roughly 5% by volume, and the best-run operations hold it lower. One documented water-based job kept drilled solids around 5.5% at a removal efficiency over 85% and considered that a realistic, low result. On unweighted muds the equivalent measure is total solids — a classic reference shows solids near 6% is where trouble starts and every point below that pays back.
Why the line sits around 5%: below it, plastic viscosity, filter cake and abrasion stay manageable and the mud drills. Above it, the fines that mechanical equipment can no longer catch begin to dominate rheology, and you are forced onto dilution and chemistry to hold properties — the expensive levers.
Why two points of solids move real money
The effect is not linear or small. A frequently cited data set shows that cutting average solids content from 4.8% to 2.6% — just 2.2 percentage points — produced roughly a 15% reduction in total rig days on the well. That is faster penetration, less bit and pump wear, and less dilution, all from a couple of points of drilled solids removed.
That is the whole economic case for solids control in one number. The percentage looks small on a mud report, but it sits upstream of ROP, equipment life, dilution volume and disposal — which is why an extra point of LGS is never just an extra point. It is the reason the removal train is the cheapest performance lever on the rig.
The numbers
What the retort and mud weight give you:
LGS (% vol) = total solids − barite fraction
Targets: low-gravity drilled solids < ~5% by volume (lower is better) · barite is desirable and does not count against you.
Read the drilled-solids number as LGS, not total solids — barite is weight you want and should never be counted against you. Keep low-gravity solids under about 5% by volume, and treat a rising trend as an early bill: a couple of points is enough to move ROP, dilution and rig days.
Common questions
What is a normal drilled-solids percentage in mud?
Low-gravity drilled solids should generally stay below about 5% by volume, and well-run operations hold it lower. Total solids can be higher (15–25% on weighted mud) because most of that is barite — the weight material you want, which doesn't count as drilled solids.
How do you find drilled solids from the retort?
The retort gives total solids by volume. Subtract the barite fraction (calculated from the mud weight) to leave the low-gravity solids — the drilled clay and rock that solids control targets. That LGS figure is the one to keep below ~5%.
How much does drilled-solids content affect drilling cost?
A lot for a small number. A well-known data set showed that cutting average solids from 4.8% to 2.6% — just 2.2 points — gave about a 15% reduction in total rig days, through faster penetration, less wear and less dilution.
