A hydraulic-fracturing silica sand is not simply a sand with high SiO₂. To work as a proppant, it must be transportable into the fracture, form a sufficiently permeable pack and withstand the stresses that tend to close the fracture after pumping.
The current international reference is ISO 13503-2:2024, which provides consistent testing procedures for proppants used in hydraulic fracturing and gravel packing. The standard states that it supplements API Std 19C, 2nd edition 2018, whose requirements remain applicable except where ISO specifies exceptions [1].
For the buyer, the consequence is important: qualification should be performed on a defined size class and a representative lot, using comparable test methods. A strong chemical analysis or a photograph of rounded grains is not enough.
Key takeaway: a proppant is a system of properties. Particle size, sphericity, roundness, cleanliness, density and crush resistance must be read together and tied to well conditions.
1. What is the proppant actually supposed to do?
Hydraulic fracturing creates or reactivates a network of fractures in a low-permeability formation. When pumping pressure falls, the rock tends to close those fractures.
The proppant is placed in the fracture to maintain a conductive pathway between the formation and the well. Performance therefore depends not only on the strength of each grain, but on the quality of the entire pack.
Once loaded, several mechanisms can reduce conductivity:
- grain crushing;
- generation and migration of fines;
- grain embedment into the rock;
- rearrangement of the pack;
- interaction with fluids;
- changes in stress, temperature and reservoir chemistry [5–6].
A laboratory result should therefore be treated as a comparison and qualification tool, not as a perfect prediction of field performance.
2. ISO 13503-2:2024 and API 19C: what should buyers understand?
ISO 13503-2:2024 covers procedures for evaluating proppants used in hydraulic fracturing and gravel packing. In its scope, “proppant” includes sand, ceramics, resin-coated proppants and other materials [1].
ISO explicitly states that its 2024 edition supplements API Std 19C, 2nd edition 2018. API still lists Std 19C under the title Measurement of and Specifications for Proppants Used in Hydraulic Fracturing and Gravel-packing Operations and shows the standard under ongoing revision activity [2].
A supplier contract should therefore state precisely:
- the normative reference;
- the edition;
- the test method;
- the size class tested;
- the sample-preparation condition;
- the acceptance criterion chosen by the operator.
Writing only “API compliant” or “ISO compliant” without defining the test and product class is too vague for a serious industrial dossier.
3. Particle size: the first property to lock down
Proppants are purchased in size classes, often expressed through sieve designations such as 20/40, 30/50 or 40/70. Those labels correspond to intervals measured by sieving; they do not by themselves describe overall proppant quality [3–4].
Particle size creates several trade-offs:
- larger grains can create more open flow channels if the pack remains intact;
- smaller grains can be transported more easily into some narrower or more distant fractures;
- excessive out-of-size material changes packing and pore space;
- fines already present before testing can complicate interpretation of a crush test.
The class must therefore be fixed before comparing suppliers. Comparing a 20/40 result with a 40/70 result without stating the tested size makes little sense.
For lot control, the supplier should document at least the complete sieve curve, principal fraction and out-of-spec fractions according to the agreed method.
4. Sphericity and roundness are different properties
Sphericity describes how closely the overall grain shape approaches a sphere. Roundness mainly describes how blunt or sharp the edges and corners are.
A grain can therefore be relatively spherical while retaining sharp edges, or be well rounded without being perfectly spherical.
The distinction matters because shape influences:
- how grains contact each other;
- local stress concentration;
- packing stability;
- pore space between grains;
- transport in the fracturing fluid;
- sensitivity to crushing [3–5].
USGS studies based on historical API practice use the classic Krumbein and Sloss scales and cite reference values for roundness and sphericity. Those values are useful for understanding qualification logic, but they should not be copied as a universal current specification without checking the applicable standard edition and the customer’s purchasing requirements [4].
5. Crush resistance: measuring fines generation under stress
Crush resistance is intended to characterize what happens when the proppant pack is subjected to compressive stress.
If grains break, they generate smaller particles. Those fines can:
- fill pore space between grains;
- reduce pack permeability;
- increase pressure losses;
- migrate toward critical flow pathways [5–6].
Recent testing reviews emphasize that the standard crush procedure is a static laboratory test. It allows materials to be compared consistently, but it does not reproduce the full geometry, stress history, rock, temperature, fluid chemistry and time-dependent behavior of a real fracture [5].
For procurement, two mistakes should be avoided:
- claiming that a sand “holds X psi” without stating the procedure and tested size class;
- assuming that a good crush result alone guarantees well conductivity.
6. Mechanical strength also depends on shape and mineralogy
A high quartz content is generally sought in natural frac sands, but mineralogy alone does not guarantee performance.
USGS describes high-quality frac sand as material with high quartz content, good crush resistance, high sphericity/roundness and relatively uniform size distribution [3].
The physical logic is straightforward:
- very angular edges can concentrate stress;
- a heterogeneous shape distribution can produce less regular contacts;
- weak grains produce more debris under load;
- different minerals or inclusions can change local strength.
Qualification should therefore connect chemistry, mineralogy, texture and mechanics. To distinguish bulk chemistry from mineral phases, see our guide to XRF and XRD analysis of silica sand.
7. Turbidity, cleanliness and fines: why grain surfaces matter
A proppant can have the correct nominal size while still carrying dust, very fine particles or surface contaminants.
Historical API practices and technical literature include turbidity among proppant cleanliness controls [4–5]. The objective is not cosmetic: fines introduced into the pack can reduce flow space or interact with fluids.
Supplier control can therefore include:
- visual cleanliness;
- fines fraction;
- turbidity using the agreed method;
- documented washing and drying;
- prevention of recontamination during storage and transport.
A turbidity result should still not be interpreted without the procedure, test water, sample preparation and product context.
8. Acid solubility: an indicator of reactive material, not an absolute purity test
Historical proppant-qualification procedures also include acid-solubility testing [4–5]. These tests are used to identify constituents that dissolve under the acid conditions defined by the method.
Depending on the material, higher solubility can indicate carbonates, mineral cements or other reactive phases.
Interpretation must remain careful:
- results depend on the specified reagent and conditions;
- the test does not replace mineralogical analysis;
- it does not predict behavior in every fracturing fluid;
- it is not a universal measure of “quartz purity.”
The right use is to treat acid solubility as one complementary qualification parameter, never as a single shortcut.
9. Density: proppant transport and pack logistics
Density matters at two levels.
Loose bulk density helps estimate the mass contained in a storage or transport volume. Grain density has more influence on settling and transport in the fluid.
Natural quartz proppants, resin-coated sands and ceramic proppants do not have the same densities or transport behavior [1,5].
A buyer should therefore avoid comparing a density measured with one method against a value obtained under a different packing state or method.
A contract should specify:
- density type;
- method;
- dry or conditioned state;
- particle-size class;
- temperature where relevant.
10. Why pack conductivity cannot be reduced to the crush test
Conductivity of a propped fracture depends on pack permeability × propped width. It can decline even when the material has acceptable mechanical strength.
Scientific reviews identify several mechanisms:
- crushing;
- embedment into the rock;
- fines migration;
- pack rearrangement under stress;
- interaction with fluids;
- temperature and time effects [5–6].
Supplier qualification should therefore distinguish two levels:
Material level - size, shape, density, cleanliness, strength and lot consistency.
System level - behavior of the proppant with fluid, rock, closure stress, temperature and fracture geometry.
A supplier certificate mainly addresses the first level. The second belongs to engineering tests and treatment design.
11. What should a silica-proppant supplier dossier contain?
A useful buyer dossier should go beyond a generic CoA.
| Item | Why it matters |
|---|---|
| Lot identity and origin | Connects results to the material actually delivered |
| Particle-size class | Allows equivalent products to be compared |
| Full sieve analysis | Shows the actual distribution and out-of-size material |
| Sphericity / roundness | Documents grain geometry |
| Crush resistance | Quantifies fines generation at a defined stress |
| Turbidity / cleanliness | Tracks fines and surface contamination |
| Acid solubility | Indicates reactive phases under the test method |
| Densities | Supports transport, storage and formulation calculations |
| Mineralogy / chemistry | Verifies quartz and relevant impurities |
| Methods and standard edition | Makes results reproducible and comparable |
The report should also identify the laboratory, date, sample and sample-reduction method.
12. An eight-step purchasing procedure
To qualify a new silica-sand source as a proppant:
- define the well type and target proppant class;
- fix the standard edition and test methods;
- obtain a representative lot sample;
- confirm particle-size distribution;
- measure shape, cleanliness, densities and solubility according to the test plan;
- perform the crush test at the condition defined by the specification;
- compare multiple lots to verify consistency;
- add conductivity or compatibility testing where the project requires it.
This sequence avoids turning one strong sample into a promise of long-term supply consistency.
Conclusion
Silica sand intended for hydraulic fracturing becomes a qualified proppant only when it is evaluated as a performance material, not simply as a quartz source.
ISO 13503-2:2024 provides a consistent methodology and supplements API Std 19C. For buyers, the key parameters remain particle-size class, grain shape, cleanliness, density and crush resistance, followed by compatibility with actual well conditions.
Reference values found in studies or historical editions should not be copied as universal thresholds. The purchasing specification should state the method, standard edition, product class and use context.
References
- ISO, ISO 13503-2:2024 - Measurement of properties of proppants used in hydraulic fracturing and gravel-packing operations.
- American Petroleum Institute, API Standards Plan - Std 19C.
- U.S. Geological Survey, Frac Sand in the United States - A Geological and Industry Overview, 2015.
- U.S. Geological Survey, Suitability of River Delta Sediment as Proppant, 2017.
- Journal of Petroleum Science and Engineering, Recent progress on proppant laboratory testing method, 2021.
- Journal of Natural Gas Science and Engineering, Impact of reservoir, fracturing fluid and proppant characteristics on proppant crushing and embedment, 2021.