Why the real cost of drilled solids is often created inside the circulating system before the
material ever reaches the waste-handling stage
By Othman Soliman | Founder, SC DrillTech
HOUSTON, TX
Petroleumworld.com
The waste problem begins much earlier
When drilling waste is discussed, the conversation often starts at the end of the process: cuttings at the surface, skips or bins on the rig, transport, treatment and disposal. By that point, however, much of theoperational and economic burden may already have been created.
The more useful question is what happened to the drilled solids before they became waste. How much formation was generated by the hole section? How much additional material came from washout orcavings? How much of that load reached the surface intact, how much was removed on the first pass,and how much remained in circulation long enough to become finer and harder to separate?
That is why solids control is better treated as a system-level mass-management problem than as a list ofindividual machines. The chain starts with formation removal at the bit, continues through the circulating fluid and surface processing system, and ends with the material that must be recovered, reused, treated or disposed.
Start with solids generation, not the equipment list
The theoretical formation volume generated by a drilled interval is governed first by hole size and drilledlength; the instantaneous generation rate rises with rate of penetration. The actual solids load arriving at surface can be higher or lower than a simple geometric estimate because the wellbore may enlarge, cavings may enter the return stream, some particles may disperse or erode, and transport efficiency changes with hydraulics, inclination and fluid condition.
This denominator matters. The same processing package can perform very differently on two wells because the incoming flow rate, solids loading, particle-size distribution and drilling-fluid rheology are different. A system that looks adequate at a moderate drilling rate may be overloaded when the hole produces several times the solids volume per hour.
The first principle remains straightforward: remove drilled solids as early as practical, while they are still relatively large and separable. If solids escape surface removal and recirculate, they can be exposed again to pumps, high-shear zones, bit nozzles, collisions and abrasion. Attrition does not affect every particle in the same way, but the surviving population can progressively shift toward finer sizes that are more difficult to remove mechanically.
Why an acceptable mud report can hide a solids-control problem
A drilling-fluid report is essential, but it is still a property snapshot. Routine properties can remain inside the programmed operating window because the fluid is being actively diluted, treated, weighted or conditioned. In water-based systems that may mean controlling rheology, inhibition and filtration; in nonaqueous systems it may also include oil/water ratio, wetting condition and electrical stability.
Those numbers do not, by themselves, show how much new liquid was added, how much whole mud was transferred or discarded, how much chemical treatment was required, or how much waste handling was created to keep the fluid inside specification. That requires a material-balance view alongside the mud report.
Low-gravity solids are a useful example. LGS is not exactly the same thing as drilled solids. In standard drilling-fluid terminology, LGS includes drilled formation solids plus lower-density solids such as added bentonite. The calculated concentration normally depends on mud density, retort results, salinity or chloride information and assumed component densities. It is therefore an engineering estimate, not a direct particle count. The trend is usually more informative than a single number, especially when read against hole size, drilling rate, dilution history and fluid type.
Particle size can matter as much as concentration
Two fluids can contain a similar total volume of unwanted solids and behave differently if their particle- size distributions are different. As particles become finer, their specific surface area increases and their interaction with the continuous phase and chemical system can become more significant.
A 2023 laboratory study on oil-based drilling fluid found that increasing LGS toward and beyond the
tested fluid’s solids-capacity range increased plastic viscosity, gel strength and thixotropy, increased high-temperature/high-pressure filtration, produced thicker filter cakes and reduced emulsion stability.
The study also reported a stronger adverse effect from the smaller LGS size fraction. An earlier
laboratory study likewise reported deterioration in rheology, filtration behavior and emulsion stability as simulated drilled-solids concentration increased.
The study also reported a stronger adverse effect from the smaller LGS size fraction. An earlierlaboratory study likewise reported deterioration in rheology, filtration behavior and emulsion stability as simulated drilled-solids concentration increased.
The field implication is not that every fine particle creates the same response in every mud system. Fluid chemistry, temperature, particle mineralogy and particle-size distribution all matter. The practical lesson is narrower and more defensible: allowing drilled solids to remain in circulation until they have become very fine reduces the options for selective mechanical removal and can increase the treatment required to maintain the fluid.
Dilution can control concentration, but it does not selectively remove solids
Dilution is a legitimate drilling-fluid treatment and can be the correct operational decision. Addition-only dilution lowers the concentration of solids by increasing the liquid volume; it does not selectively separate the unwanted solids from the mud.
If the surface volume must remain constant, dilution is normally accompanied by an equal transfer or discard. In that case, solids do leave the active system, but only because a portion of the whole drilling fluid leaves with them. This is the economic difference between selective solids removal and dilution- and-discard: the latter can sacrifice useful liquid phase, additives and, in a weighted system, weighting material.
If the incoming solids rate remains higher than the effective removal rate, the operation can enter a repeating cycle: solids concentration increases, fluid properties move away from target, fresh liquid and treatment are added, excess volume is transferred or discarded, and the process repeats. The resulting cost can appear in base fluid or water, chemicals, mixing time, storage, trucking or marine logistics, treatment and final waste handling.
System performance must balance solids removal with fluid conservation
Current API RP 13C is explicitly a drilling-fluid processing-system evaluation practice rather than a method for ranking similar individual machines. That distinction is important. The objective is not simply to make one discharge stream look drier or to force one laboratory value as low as possible; the objective is to manage the circulating system efficiently.
In weighted drilling fluids, overly aggressive separation can discard useful high-gravity solids such as barite. In nonaqueous systems, a wet solids discharge can carry valuable base fluid out of the active system. Good performance therefore sits between two losses: allowing unwanted drilled solids to remain in the mud, and discarding too much useful fluid or weighting material while trying to remove them.
A 2016 field study comparing two solids-control configurations reported improvements of approximately 14% in plastic viscosity, 17% in yield point and 25% in LGS after the system configuration was changed.
Those figures are specific to that study and should not be used as universal targets, but they illustrate the value of evaluating the processing train as a connected system rather than judging equipment in isolation.
Waste volume is partly created inside the circulating system
The material leaving a drilling operation is not always equivalent to the geometric volume of rock drilled.
Depending on how the project defines its waste boundary, the managed stream can include formation solids and cavings, liquid retained on discharged cuttings, dilution-related excess mud, cleaning fluids and whole drilling fluid that has been transferred for treatment or disposal.
The connection is especially visible in nonaqueous systems because retained oil or synthetic fluid on cuttings represents both a recoverable resource and an environmental-control issue. The relevant test method and allowable discharge or treatment criterion are project- and jurisdiction-specific, so retained- fluid performance should be evaluated against the governing local requirement rather than a universal number.
The same mass-balance logic applies to water-based systems. Every barrel of new fluid built to manage accumulated solids adds inventory and handling load. If it cannot be reused or transferred beneficially, it eventually becomes part of the waste-management burden.
What should be watched in the field?
A strong field review should read the following indicators together and trend them against the interval drilled:
• theoretical hole volume and solids-generation rate from hole size and rate of penetration, with washout and cavings treated as additional load rather than hidden inside the calculation;
• total circulating flow and the fraction of returns actually processed, including any bypass, overflow or unprocessed flow;
• retort and calculated LGS trends, with the calculation basis and fluid type recorded rather than treating LGS as an exact drilled-solids measurement;
• density, plastic viscosity, yield point, gel strengths and filtration trends, plus oil/water ratio and electrical stability where they are relevant to the fluid system;
• dilution, new-fluid build, transfers and intentional discards normalized to the interval drilled;
• the mass or volume of rejected solids and the amount of recoverable drilling fluid leaving with that discharge;
• for weighted systems, evidence of useful weighting-material loss as well as unwanted-solids removal;
• waste volume generated per interval, compared with the expected formation volume and with documented fluid additions and losses.
No single KPI answers the question. A low LGS value maintained by heavy dilution may show acceptable fluid condition without demonstrating efficient solids removal. Conversely, a low recorded waste volume is not automatically good if drilled solids are being recirculated and retained in the active system. The interpretation has to close the material balance.
The practical question is not “the equipment running”
On many rigs the surface equipment is running, motors are on and fluid is moving through the system.
That proves operation; it does not prove processing performance.
The more useful questions are: Is the full return stream being processed? Is the system removing solids quickly enough for the solids-generation rate? Are drilled solids being rejected before the surviving population becomes progressively finer? Is dilution per drilled interval increasing after changes in hole size and drilling rate are considered? Is valuable fluid leaving with the solids? In weighted mud, are we protecting useful barite while controlling LGS?
Those questions connect drilling, drilling-fluid control, solids control and waste management. They also shift the discussion away from equipment nameplates and toward measurable system performance.
From formation to waste
The most effective drilling-waste strategy begins before the waste stream exists. It begins with the formation, the amount and character of solids being generated, and the path those particles take through the circulating system.
When drilled solids are removed early and useful fluid is conserved, the active system is easier to maintain and the downstream waste burden can be reduced. When unwanted solids are allowed to recirculate and become finer, separation becomes more difficult and the operation can become increasingly dependent on dilution, treatment and fluid replacement.
For that reason, drilling waste should not be treated only as a disposal problem. It is the downstream expression of decisions made much earlier in the solids-management chain.
Better solids control does not simply mean cleaner mud. It means controlling drilled-solids inventory with the least practical loss of useful fluid and weighting material – and making the economics visible through a material balance.
About the author
Othman Soliman is Founder and Principal Consultant at SC DrillTech, and a Solids Controls; Drilling Waste Management specialist with more than 26 years of international field experience across land and offshore operations in the GCC and MENA region. His work spans rig and mud-system evaluation, troubleshooting, installation and commissioning, system optimization and technical training. His career has included roles with M-I SWACO, Halliburton, NOV and SLB. He writes vendor-neutral field-engineering analysis for SC DrillTech and has contributed technical articles to Oil Review Middle East.
Related field reading: The daily solids-control report: the numbers worth tracking every tour
Website: SC DrillTech | Email: support@scdrilltech.com | LinkedIn
EnergiesNet.com 09 27 2026
