Rising low-gravity solids is the most expensive trend on a mud report, and the worst response is to reach straight for dilution. Above roughly 10% LGS the fluid becomes effectively untreatable — rheology climbs, colloidal content takes over. The skill is diagnosing why LGS is rising before you spend, because a mechanical bypass and an ultrafine overload look identical on the gauge and need opposite fixes.
Read the signal first: PV and gels
Low-gravity solids announce themselves before the retort confirms them. They show up in the mud report as rising plastic viscosity (PV) — the fine particles colliding as the viscometer turns — and in gel strengths that climb the longer the mud sits, because LGS particles build stronger bonds over time. A clean mud gives the same gel reading regardless of how long it rested; a mud whose gels keep thickening with time is telling you the fine-solids fraction is winning.
So the first branch of the tree is not ‘how much LGS’ but ‘what size.’ If PV is rising with a normal, stable gel profile, you are likely accumulating solids your equipment could catch — a mechanical problem. If PV is rising and gels are climbing with time, you are into the ultrafine, colloidal fraction that mechanical equipment cannot remove — a different branch entirely.
Branch A — is a stage bypassing? (the mechanical fault)
If the solids accumulating are within reach of the train, one of your stages has quietly dropped out, and the fix is cheap. Walk the cascade in order. The shaker: are screens torn, blinded, or one API size too coarse for the section, letting catchable solids straight through? The hydrocyclones: is the desilter underflow a rope instead of a spray — starved of feed head, or plugged — so it is dumping rather than cutting? The centrifuge: is it running at all, at the right bowl speed, for enough hours to take the fines the cyclones pass?
Any one of these bypassing feeds LGS into the active system at a rate no amount of dilution can outrun economically. This is the good branch to land on, because the corrective action — change a screen, clear an apex, run the centrifuge — is a fraction of the cost of the mud you would otherwise dilute. A mechanical bypass is a maintenance ticket, not a chemistry problem.
Branch B — ultrafine overload (the OBM/SBM trap)
If the solids are truly ultrafine — below about 8 to 10 microns — no mechanical stage can remove them, and this is the branch that catches oil- and synthetic-based systems recycled through the plant. Documented cases show OBM returning with a D50 under 5 microns and most of the solids below 5 microns; the centrifuge cannot touch that fraction, and it drives the untreatable rheology you see above 10% LGS.
Here dilution genuinely is a lever — but an expensive one, because reducing the fine fraction by dilution takes three to four times the fluid volume, consuming tank space and adding handling, transport and disposal cost. So on this branch the decision is deliberate, not reflexive: dilute what you must, recover base fluid where you can, and recognise that the real prevention was upstream — keeping the mechanical train at full efficiency so the fines never concentrated to this point in the first place.
The decision tree
1. Is PV rising? → likely LGS building. Check gels next.
2. Gels stable with time? → Branch A (mechanical): walk shaker → cyclones → centrifuge; find the stage bypassing and fix it. Cheap.
2. Gels climbing with time? → Branch B (ultrafine <8–10µm): mechanical can’t remove it; recover base fluid + deliberate dilution (3–4× volume).
Ceiling: above ~10% LGS the fluid is effectively untreatable — act before you get there.
Diagnose the size before you spend. Rising PV with stable gels is a mechanical bypass — find the stage and fix it cheaply. Rising PV with climbing gels is the ultrafine, colloidal fraction — mechanical equipment can’t remove it, and dilution costs 3–4× the volume. Reaching for dilution on a mechanical fault burns mud you didn’t need to lose.
Common questions
What causes high low-gravity solids in drilling mud?
Two families of causes: a mechanical bypass (torn or too-coarse shaker screens, starved or plugged cyclones, an idle or mis-set centrifuge) that lets catchable solids into the system, or an ultrafine overload (solids below 8–10 microns, common in recycled OBM/SBM) that no mechanical stage can remove.
How do I know if high LGS is mechanical or ultrafine?
Read PV and gels. Rising PV with gels that stay stable over time usually means catchable solids are bypassing a stage — mechanical, and cheap to fix. Rising PV with gels that climb the longer the mud sits means the fine, colloidal fraction is dominating — ultrafine, which mechanical equipment can't remove.
At what LGS level does mud become untreatable?
Above roughly 10% low-gravity solids the fluid's rheology and colloidal content make it effectively untreatable, especially in invert-emulsion systems. That is why the goal is to keep LGS well below that ceiling and to diagnose a rising trend early.
