Fine: API RP 13C Particle Sizing, Shaker Screen Cutoff, and WCSB Solids Control

In drilling fluid engineering, fine is a particle-size classification that carries two related meanings. In the strict sense set out in API Bulletin 13C (now incorporated into API RP 13C, Recommended Practice on Drilling Fluid Processing), a fine particle is any solid in the 44 to 74 micron size range. That places it directly above the silt-sized particles and below the medium and coarse fractions, and it brackets a band that matters enormously for solids control because it overlaps the practical cut point of modern shale shaker screens. In the looser everyday sense used on a Western Canadian Sedimentary Basin rig, "fines" simply means any particle small enough to pass through the shaker screens and remain suspended in the active drilling fluid system. Those two definitions converge in practice: a shaker fitted with an API 200 screen has a separation potential near 74 microns, so material classified as fine under the bulletin is precisely the material that escapes the first stage of solids removal and reports downstream to the desander, desilter, and centrifuge. Particle size in a drilling fluid is never a single number; it is a distribution. Commercial barite, the primary weighting agent, is itself milled to a fine and medium range so that it stays suspended and does not get stripped out at the shaker, which is why screen selection always involves a compromise between removing drilled fines and retaining the weighting material the mud engineer paid for. Low gravity solids in the fine range, by contrast, are the enemy: ground-up Montney siltstone, Cardium sandstone, or Duvernay shale that has been recirculated and abraded into the fine band increases plastic viscosity, raises equivalent circulating density, and accelerates bit and pump wear. The accumulation of fine low gravity solids is the single most common reason a water-based mud must be diluted or partially dumped, and on a deep Montney horizontal that dilution cost is measured in tens of thousands of Canadian dollars per well. Understanding where the fine fraction sits in the size spectrum, and which piece of solids control equipment is responsible for removing it, is fundamental to controlling mud cost, hole quality, and drilling waste volume under AER Directive 050.

Where Fines Sit in the Solids Control Train

A WCSB solids control system removes particles in descending size order. The shale shaker takes the coarse and medium fractions plus the upper end of the fine band, typically down to 74 microns with an API 200 screen. Particles that pass become the hydrocyclone feed: a 10-inch desander cone removes solids to about 40 to 50 microns, and a bank of 4-inch desilter cones reaches roughly 15 to 25 microns. Below that, only a decanting centrifuge spinning at 1,800 to 3,200 rpm can recover barite-sized and ultrafine material. The fine 44 to 74 micron band is therefore split across the shaker and desander, and any fine solid that bypasses both recirculates and degrades, which is why screen integrity and cone pressure are checked every tour.

Fine Solids and Equivalent Circulating Density

On a Montney or Duvernay horizontal, fine low gravity solids accumulate fastest because long lateral sections generate large cuttings volumes that get reground by the bit and bottomhole assembly. As the fine fraction builds, plastic viscosity climbs and equivalent circulating density rises, narrowing the margin between pore pressure and fracture gradient. In a depleted Cardium or Viking interval that margin may be only 100 to 150 kPa (15 to 22 psi), so a fine-solids-driven ECD spike can trigger lost circulation. Mud engineers monitor the fraction with a retort and methylene blue test, then dilute or run the centrifuge harder. Maintaining low gravity solids below about 6 percent by volume keeps ECD predictable and protects the openhole.

Related Terms

The fine classification connects to several core mud concepts. Drilling fluid performance depends on controlling the fine solids loading, since fines directly raise plastic viscosity and shift the rheological profile. Barite is intentionally milled into the fine and medium range, so fine screening must balance solids removal against weighting-agent retention. The downstream recovery of fine material depends on the centrifuge, the only device that separates particles below the desilter cut, making it the last line of defence against fine low gravity solids buildup.

Real-World WCSB Scenario: Fine Solids on a Kakwa Montney Pad

On a four-well Montney pad in the Kakwa field west of Grande Prairie, an ARC Resources style operator drilled 3,200 metre laterals with a 1,250 kg/m3 water-based mud. By the third well, retort analysis showed low gravity solids climbing toward 9 percent as fine drilled siltstone bypassed the API 200 shaker screens and the desilter cones plugged with sticky Montney fines. Plastic viscosity rose from 22 to 38 cP and ECD increased by roughly 90 kPa, eroding the margin against a depleted zone. The mud crew added a second high-speed decanting centrifuge at a day rate near CAD 1,800 and increased dilution with fresh fluid.

The added centrifuge pulled the fine fraction back below 6 percent within two circulations, ECD stabilized, and the operator avoided a lost-circulation event that on an offset well had cost over CAD 120,000 in lost mud and rig time. The incremental solids control spend of roughly CAD 9,000 across the pad paid for itself many times over by protecting the openhole and cutting whole-mud losses to the Directive 050 waste stream.