Proving your solids control works: the mass balance

Most rigs judge solids control by how the shaker screens look and how dry the cuttings feel. Both are opinions. This week: the API RP 13C mass balance that turns the whole separation train into one defensible number — the equations, the targets, a full worked example, and the caveat that stops engineers misusing it.

Why "it looks fine" is not a number

The separation train — shale shakers, desander, desilter, mud cleaner and decanter centrifuge — exists to do one thing: remove drilled solids from the active system faster than the bit makes them. When it falls behind, the consequences compound quietly. Low-gravity solids (LGS) climb, plastic viscosity rises at constant mud weight — the classic signature of a solids-control problem — dilution increases, equivalent circulating density drifts toward the fracture gradient, and both mud and disposal cost inflate together. None of that announces itself on one tour; it surfaces days later as an overspend nobody can trace.

The most common check on the rig is a glance at the shaker discharge and a squeeze of the cuttings. That tells you the equipment is running, not the quantity of solids leaving the system — and quantity is the whole game. A mass balance gives you that quantity: it weighs what the bit generated against what the system removed, and the ratio is your solids-removal efficiency (SRE).

Step 1 — How much did the bit generate?

Every foot drilled produces a known volume of rock. For a hole diameter D (inches) over an interval L (feet), the in-gauge drilled-solids volume is:

That constant is not arbitrary — it is the geometry: a cylinder's volume, converted from cubic inches to barrels (1/1029.4 = 0.0009714). Multiply by the average formation density — LGS run about 2.6 SG for common shale and sandstone — to get the mass delivered to surface. Two realities make this a floor, not a ceiling: hole washout raises the true volume above gauge, and lithology shifts density. Both push the same way, which is exactly why the removed side must be measured.

Step 2 — The API RP 13C method (the honest one)

API RP 13C gives the field procedure for the total efficiency of a water-based fluid processing system, and it is built on one insight: dilution is the price you pay for solids the equipment did not remove. Over an interval you record accurate water/mud additions and retort data, then compute a dilution factor (DF) — the ratio of the dilution actually used versus the dilution that would be needed with no removal equipment at all. From DF the method derives the system's total removal efficiency.

The companion relation every mud engineer should carry is the dilution equation itself:

It assumes the dilution fluid carries essentially zero solids. Read it the right way and it tells you the truth: the further your LGS drifts above target, the more clean fluid — and barite to re-weight it — you buy to claw it back. That is the cost of low efficiency, expressed in barrels.

Step 3 — Pick the right target

Removing all drilled solids is neither possible nor economic. Field experience puts the optimum LGS concentration at roughly 4–6% by volume; well-run weighted systems hold below ~5–6%, and advanced staged processing has reached under 4%. Two levers move the optimum: if you are losing a lot of whole mud with the cuttings, a lower SRE still holds the pit; if losses are low, you need a higher SRE to keep pit level and LGS steady. Set the target before you judge the equipment — not after.

A full worked example

What the number tells you to do

• High dilution, low removal → start at the shakers. Confirm the deck is developing 4–8 G and that screen selection is right — per API RP 13C labelling, screens finer than about API 200 begin stripping barite (median ≈ 10–75 µm), so the finest screen is not always the correct one.
• Good removal but rising ECD → the fines are winning. That is a centrifuge cut-point (D50) and LGS problem, not a shaker problem — the shakers can look perfect while the sub-40-micron fraction climbs.
• Numbers that will not close → you are losing whole mud somewhere: screen tears, poor cuttings dryness, or transfer losses. The balance itself becomes the leak detector.

The one caveat that keeps you honest

The dilution-factor method is defined for water-based muds. Oil- and synthetic-based systems are inherently inhibitive and do not suffer the same LGS dilution behaviour, so a WBM dilution factor mis-reads them. On invert-emulsion fluids, lean on retort-based solids tracking and centrifuge mass balance across the stages rather than a water-addition dilution factor. Using the right tool for the fluid is the difference between a number and a guess.

The takeaway

A mass balance takes ten minutes of arithmetic and turns three shifts of opinion into a defensible number. It is the difference between telling an operator "the shakers look good" and showing them "we removed 80% of generated solids, here is where the other 20% went, and here is what it cost." Pair it with the solids-control calculators for the dilution and G-force numbers, and the engineering standards for the acceptance criteria. That is the whole SC DrillTech method in one calculation: measured, not guessed.

Educational field guidance based on API RP 13C (drilling-fluid processing) and API RP 13B-1 (WBM testing). Constants are exact geometry; targets and benchmarks are typical field ranges — verify against your own retort, dilution and discharge records before acting on a specific well.

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