Non-aqueous drilling fluids (NADF) — oil-based muds (OBM) and synthetic-based fluids (SBF/SBM) — are used because they outperform water-based mud in shale inhibition, lubricity, high-temperature stability and wellbore quality. But they change the solids-control job in fundamental ways. The equipment is mostly the same; the priorities, the measurements and the waste implications are not. A solids engineer who steps from a WBM job onto a NAF well without adjusting their thinking will measure the wrong thing, misread the dryer, and create a compliance problem without realising it.
The primary metric shifts from LGS to OOC
On a water-based programme, the number you manage is low-gravity solids (LGS) — the drilled formation in the active mud. On a NAF job that remains important, but a second metric takes on equal or greater weight: oil-on-cuttings (OOC) — the base oil retained on the discharged solids. OOC is simultaneously a fluid-cost metric (base oil leaving the well on the cuttings) and a compliance metric (the number regulators use to decide whether the cuttings can be discharged or must be treated and disposed of). Getting OOC down is where NAF solids control spends most of its effort.
Equipment differences — what changes and what stays the same
| Stage | WBM | NAF (OBM/SBM) |
|---|---|---|
| Shale shaker | Standard — draining water-based fluid through the screen | Oil-wet cuttings are stickier and harder to convey; finer screens flood more easily; screen life can be shorter |
| Drying stage | High-G shaker adequate for most WBM drying | Vertical cuttings dryer (VCD) needed to drive OOC below 5%; high-G shaker alone leaves too much base oil on the cuttings |
| Centrifuge | LGS removal / barite recovery | Same duties, but also processes the recovered base oil off the dryer — the dryer and centrifuge are a pair on NAF |
| Dewatering | For waste stream water clarification | Less common; NAF waste management focuses on OOC reduction rather than water clarification |
| Retort | API RP 13B-1: reads oil, water, solids volumes | API RP 13B-2: reads oil-in-water ratio, OWR, and is the basis for mud reporting on NAF — reading it correctly is more involved |
The oil/water ratio — the NAF version of LGS tracking
On a NAF job, the retort reads three phases: oil, water and solids. The oil/water ratio (OWR) — the proportion of oil to water in the liquid phase — is a primary mud property that affects emulsion stability, inhibition and rheology. Maintaining the OWR in spec is part of the mud engineer's job, and drilled-water influx from the formation can shift it toward the water side. Solids control affects OWR indirectly by controlling how much fluid is lost on the cuttings and how much dilution (base oil) is added to compensate.
Barite management — the same issue, higher stakes
NAF programmes are almost always weighted — the whole point is performance in challenging formations that also tend to be high-pressure. Barite management on NAF follows the same rules as on weighted WBM: no bare desilters (use a mud cleaner), run the centrifuge on barite-recovery duty when weight material recovery is the priority, keep pits agitated. The difference is that base oil is far more expensive than water, so every barrel of dilution you avoid — by recovering barite efficiently — is more costly to replace.
Waste — the most different part
The waste stream on a NAF job is categorically different from WBM. OBM cuttings are classified as an oily waste under virtually every regulatory regime and, offshore, typically cannot be discharged without pre-treatment. The cuttings dryer, CRI or thermal treatment are not optional enhancements — they are the waste management system. Plan the surface spread accordingly: a NAF well without a functioning vertical cuttings dryer and a receiving centrifuge is a well generating non-compliant waste from the first bit run.
Key takeaways
Non-aqueous fluids change the solids-control job in three ways: OOC joins LGS as a primary metric; the cuttings dryer becomes mandatory rather than optional; and the waste stream is a classified oily waste with compliance consequences. The equipment train is largely the same, but the priorities, measurements and operating discipline are different enough that a NAF solids programme needs to be planned as its own job — not treated as a WBM job with different mud.
