SC DrillTechThe LGS cost curve: why cleaner is not always cheaper
There are only two ways to hold low-gravity solids at target: remove them mechanically, or dilute them away. The first costs equipment; the second costs mud — and dilution is almost always the more expensive of the two. This week: the LGS cost curve, why there is an economic optimum rather than a "cleaner-is-better" answer, and how to prove where yours sits.
Two ways to control LGS, one of them cheap
Low-gravity solids — drilled clay and shale, specific gravity ≈ 2.6 — enter the mud with every foot drilled. Left unchecked they raise plastic viscosity, degrade mud properties and drive up ECD. You have exactly two levers to hold them at target: mechanical removal (shakers, cyclones, centrifuge) and dilution (adding clean fluid to reduce the concentration). Every barrel the equipment fails to remove must be diluted out instead — and dilution is the expensive path, because it does not just cost base fluid.
The dilution equation, priced out
To pull LGS from its current level down to target, the clean volume required is:
Vdilution = Vsystem × (LGScurrent − LGStarget) ÷ LGStarget
On a 1,500-bbl system sitting at 8% LGS, pulling back to a 5% target needs 1,500 × (8 − 5) ÷ 5 = 900 bbl of dilution. But that 900 bbl is only the first invoice. Each diluting barrel also needs its share of chemicals to stay in spec, barite to restore weight on a weighted mud, and — because the pits are already full — an equal volume of whole mud dumped and hauled away. One barrel of "dilution" is really four costs: base fluid + chemicals + barite + disposal. That is why dilution is the number you want to minimise.
Why the target is not zero
If dilution is expensive, why not remove everything and never dilute? Because mechanical removal has its own rising cost. Look at the same 1,500-bbl system chasing ever-lower targets:
To reach 6% LGS: 500 bbl dilution · 5%: 900 bbl · 4%: 1,500 bbl · 3%: 2,500 bbl
The dilution requirement climbs steeply as the target drops — and to avoid that dilution you would need progressively finer, faster, more numerous equipment (finer screens that flood, more centrifuge capacity, more power and maintenance). One curve rises as you clean less; the other rises as you clean more. The total cost is a U-shape, and the bottom of that U — typically an LGS target around 4–6% by volume — is the economic optimum. Cleaner than that, and the equipment cost outruns the dilution you saved.
Finding your point on the curve
The optimum shifts with conditions, so measure rather than assume. Two questions locate you:
- How much dilution did the interval actually take? Compare it to the drilled-solids you generated (last covered in the mass-balance issue). High dilution for the solids generated = your removal efficiency is low and you are paying the expensive way.
- What does a barrel of dilution cost vs a barrel of removal capacity? On an expensive oil-based or high-spec mud, dilution is punishing, so the optimum pushes toward more equipment and a lower LGS target. On a cheap water-based mud, a slightly higher target and a bit more dilution can be the cheaper answer.
The economics, not habit, set the target — and the economics change with mud cost, disposal cost and hole size.
A field example
An oil-based mud (dilution is very expensive) is holding LGS at 7% because the centrifuge is undersized. Pulling to a 5% target on the 1,500-bbl system needs 1,500 × (7 − 5) ÷ 5 = 600 bbl of dilution per cycle — at OBM prices, a large recurring bill, plus the disposal of 600 bbl of displaced mud. Adding centrifuge capacity to hold 5% mechanically costs a fraction of that dilution over the section. On this mud the optimum sits lower and leans on equipment; on a cheap WBM the same maths might favour tolerating 6% and diluting. Same equation, opposite decision — because the price of a barrel changed.
The takeaway
LGS control is an economics problem wearing an engineering costume. Dilution is the expensive lever because every diluting barrel drags chemicals, barite and disposal behind it; mechanical removal is the cheap lever until you chase the last few percent. The right LGS target is the bottom of that U-shaped cost curve — usually 4–6%, but pushed lower on expensive muds and higher on cheap ones. Price both levers, measure your dilution against your generated solids, and set the target on evidence. Measured economics, not a habit number.
Educational field guidance based on API RP 13C mass-balance principles. The dilution equation is exact; cost outcomes depend on your mud, disposal and equipment prices — verify against your own well economics before acting.
This brief is the field summary. For the full reference, see:
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- Every calculator in this brief
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