Drilled-solids removal efficiency — written η (eta) — is the single most useful number in solids control. It answers the one question that quietly decides the cost of an entire fluid programme: of all the solids the bit generated this section, how many did the surface equipment actually remove before they recirculated back down the hole? Screen selection, cone feed head, centrifuge speed and dilution rate are all just levers that move this one number.
If you track only one solids-control metric on a well, track η. It converts a vague impression — “the system looks like it’s running fine” — into a figure you can budget against, defend in the morning meeting, and watch drift section to section.
What η actually measures
η is not a reading off a gauge. It is calculated from data the rig already records on the daily drilling and mud reports. The dilution-based form compares the fluid you actually had to build against the fluid pure dilution alone would have required to hold the same low-gravity-solids concentration:
Here Vc is the volume of fluid actually built, and Ve is the dilution volume that dilution-only control would have demanded to keep the same LGS target. When the equipment does most of the work, Vc is small against Ve, k approaches zero, and η approaches 100%. When you are washing solids away with fresh fluid instead of removing them, Vc climbs, k rises, and η falls.
Working it from the daily report
The inputs are deliberately ones every rig already has, so η can be run each tour without new instrumentation:
- Hole volume drilled — from bit size and footage on the interval. This sets the drilled-solids generated.
- Mud built / added — from the pit and mixing records.
- Retort solids and mud weight — per API RP 13B-1/13B-2, to separate drilled (low-gravity) solids from weight material.
- LGS target — the low-gravity-solids concentration you are holding, from the mud programme.
Run those through a mass balance and you get drilled solids generated, solids discarded at surface, and — from the ratio of the two — the removal efficiency for the interval.
A quick worked feel for the numbers: a 12¼″ bit drilling 1,000 ft generates on the order of 80–100 bbl of formation solids. If your active and dilution records show you built far more new mud than that volume of solids should have forced, your equipment is leaving solids in the system — and η will show it.
The bands — and what each one means
| η band | What it tells you | Where to look first |
|---|---|---|
| Below 50% | Dilution-dominated — you are washing the problem away, not removing it. | Shakers and hydrocyclones; something coarse is being missed or bypassed. |
| 50–70% | The system works, but real money is still on the table. | A screen one step too coarse, low cone feed head, or a centrifuge on the wrong duty / too few hours. |
| Above 70% | A genuinely healthy, measurable system. | Fine-tune; every extra point now converts almost directly into fewer dilution barrels and less disposal. |
Field-typical η on a working rig lands anywhere from 30% to 70%. Weighted muds and aggressive sections sit lower; clean water-based tophole can run higher. The absolute number matters less than the trend: a four-point drop overnight is a fault to diagnose, not noise to absorb.
What moves the number
Once you know your η, the diagnosis is almost always one of a short list, in the order we see them:
- Shaker screens too coarse for the section, or blinded / holed panels passing solids that should have been caught at the first stage.
- Hydrocyclone feed head below target — most commonly a worn feed-pump impeller bleeding off head over days.
- Centrifuge on the wrong duty, or simply not running enough hours per day to keep up with fines generation.
- Bypassed or short-circuited equipment — the classic open gate valve quietly returning unprocessed fluid to the active system.
Key takeaways
The point is not to chase a perfect score — it is to know your number, watch it tour to tour, and act the moment it drifts. η turns solids control from an opinion into a measurement you can defend and improve. A crew that reports η every shift catches in hours what an un-measured rig only discovers at the reserve-pit audit.