Most solids-control losses are invisible because the system never stops running. Nothing trips, nothing leaks onto the deck — the dilution rate just sits quietly higher than it should, and the money leaves the budget one barrel at a time. This is the anatomy of one such case (illustrative of the method), and what it took to catch and reverse it.
The situation
On an offshore jack-up, dilution on a 12¼″ section had crept to 165 bbl/day. The mud properties were in spec, the shakers were running, the cones were humming — by every casual measure the system was “fine.” No one had flagged a problem because there was no event to flag. The only thing wrong was a number nobody was calculating.
Two quiet failures, stacked
When removal efficiency was finally run from the daily reports, it came back at 58% — firmly in the “money on the table” band. Two independent failures had compounded:
- A worn desilter feed-pump impeller had dropped cone feed head below the ~75 ft target (the manifold had drifted to 41 psi, about 66 ft at the section mud weight), coarsening the cut and returning fines to the active system.
- An under-set shaker — a screen one step too coarse for the formation — was passing solids it should have captured at the first stage.
Each failure alone looked minor; either would have shaved a few points off η. Together they forced extra dilution to hold the low-gravity-solids concentration in spec — and that dilution is what showed up as cost.
The recovery
The interventions were ordinary maintenance, not capital projects: re-impeller the feed pump to restore head (back to ~49 psi / ~78 ft), and re-screen the shaker to the correct cut for the section. Within a day, dilution fell from 165 to 110 bbl/day and η climbed from 58% to 72%.
The approximately $48,000 figure is an illustrative gross fluid-cost calculation. Section duration and fluid cost are not specified separately, so it cannot be reproduced as a measured saving from this example. Verify them, subtract intervention cost and assess disposal separately before claiming net savings or payback.
Why the leak stayed open so long
Both faults were the slow kind. An impeller does not fail on a Tuesday; it erodes a little each day until, weeks later, the head has quietly fallen below the line. A screen that is one cut too coarse never announces itself — it simply lets a fraction of the solids through, every circulation, forever. Neither shows up in the parameters a driller watches. Only η, tracked against footage, makes them visible.
The real lesson
The dollar figure is not the lesson — it changes with rig, fluid and section. The lesson is that the leak was completely invisible until someone measured it. There was no alarm to wait for, because solids-control losses do not announce themselves. The only defence is a number, tracked daily, that turns “the system looks fine” into “η dropped four points yesterday — go look at the cones.”
Key takeaways
- Measure η every tour and plot it against footage.
- Walk the cones and shakers by eye — spray pattern, beach, screen condition.
- Treat a slow rise in dilution as a fault to diagnose, not a cost to absorb.
The rigs that do this routinely recover sums like this one without ever calling it a “save” — because they never let the leak open in the first place.
