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.
Track solids removal efficiency together with MCR, dilution and waste volume; no single value establishes both solids removal and fluid conservation. 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 gauge reading. Current API RP 13C Sixth Edition defines SRE as drilled-solids volume removed divided by excavated hole volume over the same interval. The former dilution-factor system-performance method is legacy; it must not be presented as the current metric:
Here Vremoved is the volume of drilled solids removed from the circulating system, and Vhole is excavated hole volume over the same interval and in the same units. Liquid retained in wet discard and disproportionately removed barite are not drilled-solids volume. If using the current dilution-based SRE balance, state the clean/dirty-mud basis and exclude additions unrelated to drilled-solids dilution. Keep Vremoved and Vhole on a consistent volume basis; retained liquid Vmud is accounted for separately.
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 estimate drilled solids after accounting for weighting material and other low-gravity additives; retort alone does not identify solids origin.
- 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 12¼-inch nominal hole drilled for 1,000 ft excavates approximately 146 bbl of formation volume: π/4 × (12.25/12)² × 1,000 / 5.6146. Caliper/washout corrections can increase this. Compare corrected dilution and drilled-solids balances; excess mud built alone does not identify an equipment fault.
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% | Higher measured removal; check MCR and fluid loss before calling the system healthy. | Check the coupled fluid/waste balance; higher SRE does not guarantee lower fluid loss or total cost. |
The bands below are illustrative screening prompts, not API acceptance limits. Establish a comparable baseline and assess SRE with MCR, fluid properties and waste volume. A change needs investigation against measurement uncertainty and operating conditions; it does not by itself prove a fault.
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.
