SC DrillTechShaker G-force and screen selection: the two numbers that decide
The shale shaker is the only piece of solids-control equipment that touches 100% of the returns, and the only one that removes solids before they are ever ground finer. Get the shaker wrong and every stage downstream inherits the problem. This week: the two numbers that decide shaker performance — G-force and screen cut point — and why the finest screen on the rack is often the wrong choice.
Why the shaker is the decision that matters most
Every solids-control train is a hierarchy: whatever the shaker fails to remove is handed to the cyclones and centrifuge, which work harder, cost more per barrel and grind fines even smaller. A solid removed at the shaker leaves the system dry and cheap; the same solid missed at the shaker gets recirculated, degrades, and eventually leaves as expensive dilution. So the shaker is not just the first line of defence — it is the cheapest barrel of removal on the rig. Two settings govern how well it does that job.
Number one — G-force
G-force is the acceleration the deck imparts to the cuttings, and it controls both how fast solids convey off the screen and how effectively fluid is driven through it. It is set by stroke and speed:
G = stroke (in) × RPM² ÷ 70,400
Most linear-motion shakers are designed to run in the 4–8 G band. Below about 4 G the deck under-conveys — cuttings build a bed, the pool climbs, and whole mud rolls off the end. Push too high and you gain conveyance but shorten screen life and can shear cuttings into finer particles that then defeat the cyclones. The lever is real: a deck running 0.16-in stroke at 1,600 RPM develops 0.16 × 1,600² ÷ 70,400 ≈ 5.8 G — comfortably in band. Drop the stroke or the speed and you can quietly fall under 4 G without anyone noticing, because the shaker still looks like it is running.
Number two — screen cut point (the API RP 13C way)
Since API RP 13C (internationally ISO 13501) replaced the old RP 13E, screens are no longer described by "mesh." That term became meaningless once oblong and multi-layer screens changed the relationship between weave and aperture. Instead, every compliant screen carries two tested numbers on a permanent label:
- API Screen Number — derived from the D100 cut point: the particle size above which 100% of solids are retained. It is matched to the nearest ASTM sieve. For example, a measured D100 of about 115 µm classifies as an API 140 screen.
- Conductance (kD/mm) — the screen's ability to pass fluid. All else equal, higher conductance moves more flow before the pool floods.
The single most useful consequence: same API number = same cut point, whatever the manufacturer or weave. That is what lets you compare screens honestly. A screen once sold as "200 mesh" may now be only API 100–140, because D100 is a stricter measure than the old D50.
Why the finest screen is often the wrong screen
The instinct is to fit the finest screen available and remove everything. Three realities push back. First, a finer screen has lower conductance — beyond a point the shaker floods, mud sheets off the end, and you lose whole mud (the opposite of what you wanted). Second, screens finer than roughly API 200 begin removing barite (median ≈ 10–75 µm), so on a weighted mud you throw away expensive weight material. Third, the API number describes the screen in a lab — it does not predict field performance, which also depends on fluid rheology, flow rate, ROP, shaker design and stroke. The right screen is the finest one that still passes the flow without flooding at your circulating rate.
A field example
A weighted 12.5-lb/gal WBM is running an API 170 screen and the pool is running high, with mud rolling over the end at peak flow. The instinct is a coarser screen for capacity — but the retort shows LGS holding and the discharge is wet. The real fix: confirm G-force first. The deck is measured at 0.14-in stroke, 1,500 RPM → 0.14 × 1,500² ÷ 70,400 ≈ 4.5 G — in band but at the low end. Increasing stroke to 0.18 in lifts it to ≈ 5.8 G, the pool drops, conveyance recovers, and the API 170 screen keeps its fine cut. Capacity problem solved without coarsening the cut or losing barite.
The takeaway
Shaker performance is two numbers, not a hunch: keep G-force in the 4–8 band, then select the finest API-rated screen that passes flow without flooding — and never finer than the point where you start binning barite. Confirm both with measurement, not appearance. The shaker that "looks fine" at 3.5 G with a flooded pool is quietly feeding every problem downstream.
Educational field guidance based on API RP 13C / ISO 13501 (screen designation) and standard shaker practice. The API number defines lab cut point, not field performance — verify stroke, speed, flow and screen selection against your own rig and fluid before acting.
This brief is the field summary. For the full reference, see:
Every SC DrillTech tool, in one bundle
- Field Secrets — the manual
- SC DrillTech Doctor — diagnostics
- FluidPro Suite — mud & fluids
- DrillTech ROC — cost & cuttings
- Training Series — decks & exams
- Every calculator in this brief
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