58 January 2007

IN RECENT YEARS the silica sand 

(industrial sand) industry has experienced 

some major changes in the processing 

and production of its many products. Two 

changes that the large producers are 

contemplating are expanding processing 

and exploring new deposits for frac or 

proppant sand.

The increased demand for energy, 

namely petroleum base products and 

natural gas, has driven oil and gas 

producing companies to stimulate 

older wells to increase fl ows. There are 

several methods to increase oil and 

gas fl ow to the well and, eventually, 

to the surface. One important method 

is hydrofracing. This is a combination 

of frac sand, a viscous gel and other 

chemicals forced down the well to prop 

open fractures.

The sands which are suitable for 

fracing must be from high silica (quartz) 

sandstones or unconsolidated deposits; 

well rounded; relatively clean of other 

minerals and impurities; fi ne, medium to 

coarse grain; and mineable.

The US Geological Survey (USGS) 

reported that the domestic sales of 

industrial sand and gravel in 2004 

increased by about 6% from 2003 sales 

due to the buoyant construction sector 

in the USA. Total US production was 

estimated at 29m. tonnes and valued 

at about $646m. Only about 8% of 

production was hydraulic fracturing 

sand, or 2.32m. tonnes.

In the Rocky Mountain region it is 

estimated that about 1.1m. tonnes of sand 

is used annually, leaving about the same 

amount in the Texas-Oklahoma basin, if 

all fi gures and reports are accurate. Other 

reports indicate that worldwide frac sand 

usage is about 4.2m. tonnes.

Frac sand specifi cations

Frac sand specifi cations are set by the 

American Petroleum Institute (API). 

The primary considerations are the 

physical aspects of the sand. The API 

recommends specifi cations on size, 

sphericity, roundness, crush resistance 

and mineralogy. 

Solubility and turbidity are also 

required, but a high silica sand is 

generally insoluble. High silica content 

sand, sphericity, roundness and crush 

resistance are the key factors in seeking a 

good frac sand.

Grain size

The API recommends the sizes as shown 

in Figure 1. The 20-40 mesh or 0.84-

0.42mm size fraction is the most widely 

used size.

Sphericity and roundness

The API standards for sphericity and 

roundness of a quartz grain are simply 

an estimate of how closely the quartz 

grain conforms to a spherical shape 

and its relative roundness. The grain is 

assessed using the average radius of 

the corners divided by the radius of the 

maximum inscribed circle.

Figure 2 shows a comparison chart 

which relates to sphericity and roundness. 

The chart was devised by Krumbein and 

Sloss in 1955. API recommends sphericity 

and roundness of 0.6 or larger. 

Crush resistance

Crush resistance is the resistance of a 

quartz grain under compressive loading. 

This is a function of grain brittleness, 

which correlates with grain shape, and 

the internal structure of the grain itself, as 

well as overgrowths on the grain.

The API specifi cations 

set down for silica 

sand used in hydraulic 

fracturing, or fracing, 

are demanding. As 

Mark Zdunczyk of 

Continental Placer 

explains, suitable US 

deposits are limited and 

geology plays no small 

part in what makes the 

grade as a frac sand The facts of frac

Courtesy Norsk Hydro.

Figure 2. Chart for visual estimation 

of sphericity and roundness (from 

Krumbein and Sloss) Sphericity

Roundness

0.9

0.7

0.5

0.3

0.1 0.3 0.5 0.7 0.9

Figure 1. Some recognised size classes 

for proppant sands (adapted from 

Hoagberg & Koerner-Moore)

Note: 90% to fall within size class designation. 

Not more than 1% of total sample on the first or 

last sieve in series

Mesh # 8/12 10/20 20/40 70/140

Screen 

size (mm)

2.38-

1.68

2.00-

0.84

0.84-

0.42

210-

105μ

US 

standard 

sieves

6 8 16 40

8 10 20 70

10 16 30 100

12 20 40 140

16 30 50 200

pan pan pan pan

DRILLING MINERALS
 January 2007 59

API requires silica sand to withstand 

compressive stresses of 4,000 to 6,000 

psi before it breaks apart or ruptures. The 

tested size range is subjected to 4,000 psi 

for two minutes in a uniaxial compression 

cylinder. In addition, API specifi es that 

the fi nes generated by the test should be 

limited to a maximum of 14% by weight 

for 20-40 mesh and 16-30 mesh sizes. 

Maximum fi nes for the 30-50 mesh size 

is 10%. Other size fractions have a range 

of losses from 6% for the 70-40 mesh to 

20% for the 6-12 mesh size.

Solubility

The solubility test measures the loss in 

weight of a 5g sample that has been 

added to a 100ml solution that is 12 

parts hydrochloric acid (HCl) and three 

parts hydrofl uoric acid (HF), and heated 

at 150°F (approx. 65.5°C) in a water 

bath for 30 minutes. The test is designed 

to determine the amount of non-quartz 

minerals present. However, a high 

silica sandstone or sand deposit and its 

subsequent processing generally removes 

most soluble materials (eg. carbonates, 

iron coatings, feldspar and mineral 

cements). API requires (in weight percent) 

losses of <2% for the 6-12 mesh size 

through to the 30-50 mesh size and 3% 

for the 40-70 mesh through to 70-140 

mesh sizes.

Turbidity

Turbidity is the amount of silt/clay sized 

minerals in the sand sample. Turbidity 

is generally not a problem because 

during the processing of frac sand, the 

material is washed. In some cases, it 

is attrition scrubbed which eliminates 

not only the fi nes but weak and crusted 

grains as well.

Geology and frac sand

Frac sand must be >99% quartz 

or silica. Silica sand deposits 

are commonly mined for use in 

glassmaking, fi ltration media, blasting 

media, ceramic products, and fi llers in a 

variety of other applications.

The silica sand industry in the USA 

and its deposits are well known. For 

many years, the larger silica sand 

companies have literally explored all 

potential silica sand sites for possible 

reserves and/or information purposes. 

In other words, most deposits in the USA 

have been discovered or, at least, are 

known to exist. Occasionally, a deposit 

is rediscovered as market circumstances 

change. For example, demand may 

increase in that market area or the sand 

in that deposit may be suitable for use in 

a new product.

High purity quartz sands are common 

throughout the USA. Some deposits 

are being mined, others have been 

abandoned, and still others are so 

remote that transportation costs render 

them unsuitable commercial sources.

Finally, some deposits have 

environmental factors that limit mining 

and which cannot be overcome. 

Since frac sand has relatively narrow 

specifi cations – notably for roundness 

and sphericity of grains – many deposits 

in the USA are automatically unsuitable.

The unconsolidated east coast 

quartzose sands of the USA are 

geologically young. Those deposits 

being mined presently in the inner and 

outer coastal plain provinces have not 

been reworked suffi ciently by geologic 

processes to acquire rounded grains. 

These unconsolidated deposits are 

generally Cretaceous to Miocene in age 

(65m. to 2m. years old). Therefore, such 

deposits can be eliminated for possible 

frac sand production. The eastern 

consolidated quartzose sandstones are 

generally sub-angular to angular in 

grain shape, or suffi ciently quartzitic (ie. 

interlocking grains) that they cannot be 

used for frac sand. 

Formations such as the Oriskany, 

Clinch, Pottsville, Antietam, Chickes, 

Sharon, Sylvania and Berea, to name a 

few, are all being mined to some extent 

for silica sand. However, they cannot 

produce frac sand. These are older 

geologic deposits that have not been 

suffi ciently worked by geologic processes 

to contain well rounded grains. 

Furthermore, tectonic movements, 

depositional history, cementation and 

pitted grains have weakened the quartz 

grains. As a result, product from these 

deposits fails the crush resistance test. 

For example, the Antietam sandstone 

of Pennsylvania was found to have 

marginal roundness and sphericity 

results of almost 6.0. It was considered 

for use as a frac sand but could not 

meet the crush resistance test. It has 

been speculated that tectonic folding 

of the sandstone exposed in the quarry 

led to stress on the quartz grains which 

caused them to become weak.

In the mid-continent, the somewhat 

friable (loosely cemented) quartz sand 

of the St. Peter and Jordan formations 

has long been mined to produce 

silica sand. Both formations have well 

rounded sand grains that meet the 

crush resistance test, as well as the 

other API tests for a suitable frac sand. 

These formations are geologically old 

(Cambrian to Ordovician) and are 

called supermature. Their deposition 

involved several geologic processes. 

The right conditions and suffi cient time 

allowed the removal of weak quartz 

grains and most impurities before these 

formations were deposited.

Well rounded grains are the result 

of either many cycles of transport or 

intensive abrasion in a specifi c type of 

environment. The St. Peter formation 

has been studied at great length. Some 

geologists believed that the sand was 

transported from coastal aeolian (ie. 

wind blown) and fl uvial-deltaic sources 

(ie. water borne) into the deeper waters 

of the inner shelf by a combination of 

shore face and river mouth by-passing 

mechanisms (Mazzullo & Ehrlich). 

Whatever its history, the St. Peter deposit 

has been worked and reworked in order 

to arrive at the present day grain shape.

The Jordan formation is equally 

geologically complex in its depositional 

history. The important factor here is that 

it is a supermature quartz arenite. The 

quartz grains have been abraded through 

fl uvial and other high energy geologic 

processes, and this makes it suitable for 

frac sand.

Several other sandstones in the USA 

also produce frac sand. They are: 

Oil Creek, Oklahoma; the Hickory 

sandstone, Cambrian Riley formation, 

Texas; and, to some extent, the Bidahochi 

Typical rounded grain quartzose sand, 

which meets API specifications for 

sphericity and roundness

DRILLING MINERALS
 January 2007 61

formation, Arizona. The Bidahochi 

formation is relatively young geologically. 

Therefore, it is used sparingly and in 

shallow wells.

Grain size variation

Whether the sand deposit is geologically 

young or old, consolidated or 

unconsolidated, or has a high silica or 

low silica content, grain size changes 

occur. When mining some of the 

deposits already mentioned, producers 

occasionally fi nd that they cannot meet 

the size requirements needed by the 

customer. In multi-cyclic sandstones 

such as the St. Peter and Jordan 

formations, consistency of grain size is 

diffi cult to predict. Attempts to relate 

the size distribution of a sandstone to 

its depositional environment have had 

limited success. 

Fluvial and tidal sand bodies commonly 

have upward declining grain size curves. 

Barrier islands have upward increasing 

size curves and marine sheet sands 

may increase or decrease depending 

upon whether the sand grains migrate 

shoreward or seaward (Pettijohn, Potter, 

and Siever).

Frac sand production

Again, specifi cations seem to determine 

how frac sand must be mined. The 

geological variations in frac sand 

deposits simply determine the product 

produced. The grain size variations in 

a deposit sometimes only allow certain 

size fractions to be produced at the 

processing plant.

If the silica plant is processing many 

products, the frac sand gradations may 

be compromised. For example, the 20-

40 and 30-50 mesh grain size fractions 

can be utilised for many non-frac sand 

silica products. Therefore, the deposit 

may yield little 20-40 or 30-50 mesh, as 

production for other applications leads 

to lower frac sand output. In addition, 

poor processing resulting from not 

regulating the hydro-sizing equipment 

and screening process can reduce the 

availability of certain grain sizes.

US frac fundamentals

It is likely that in the USA there are only 

a few geological formations suitable 

for frac sand production. The strict 

specifi cations set by the API renders many 

high silica sand deposits quite literally 

useless for this application. 

It seems that older quartzose 

sandstones have a better chance of 

providing the characteristics of a good 

frac sand. However, processing can 

upgrade some deposits if carefully 

controlled.

With the demand for frac sand 

increasing, deposit exploration can begin 

by simply comparing available sandstone 

formations for their silica content, grain 

size and roundness/sphericity. Once 

these have been established, a proper 

drilling programme can take place 

with suffi cient sampling to perform the 

required crush resistance test. The deposit 

may have grain size and roundness, but 

if it does not meet crush test, then the 

deposit is of no use for frac sand. Other 

specifi cations, such as turbidity, can easily 

be controlled.

Ten facts of frac

z There is a limited amount of mineable 

sandstone and unconsolidated sands 

that meet all specifi cations set by API

z Geology plays a signifi cant role 

in whether a deposit will meet API 

specifi cations

z Careful mine planning and geological 

exploration of new and old (already 

mined) deposits may increase yields 

of necessary grain sizes

z Some deposits, or more specifi cally 

the quartz grains in the deposits, can 

be upgraded to meet specifi cations by 

surface coating the grains

z A cloudy, crusted grain may correlate 

with fractures within the grain which 

causes brittleness and, therefore, 

weak grains

z Most high content quartz (silica) 

deposits in the USA are known. 

Exploration should concentrate 

on these known deposits and their 

geological equivalents

z Processing sand sometimes causes 

size distribution problems when 

making other silica sand products

z Geologically, older sandstones of 

Cambrian-Ordovician age seem to 

be the most suitable for frac sand

z Unconsolidated quartzose sands 

generally cannot meet all API 

specifi cations

z As energy consumption of petroleum 

base fuels and natural gas increases, 

the demand for frac sand will also 

increase

References

American Petroleum Institute, 1995. 

Recommended practices for testing sand 

use in hydraulic fracturing operations. API 

recommended practice 56, 2nd edition.

Hoagberg, R. K. and Koerner-Moore, J. 

1980. Silica sand proppants used in well 

stimulation. Sixteenth Forum on Geology 

of Industrial Minerals, St. Louis, Missouri.

Krumbein, W. C. and Sloss, L. L. 1963. 

Startigraphy and sedimentation, 2nd 

Edition. W. H. Freeman and Co., San 

Francisco, p. 1 and 2.

Mazzullo, J. M. and Ehrlich, R. 1983. 

Grain shape variation in the St. Peter 

sandstone: a record of Eolian and 

fl uvial sedimentation of early Paleozoic 

cratonic sheet sand. Journal of 

Sedimentary Research, March 1983, v. 

53; no. 1; p. 105. 

Pettijohn, F. J., Potter, P. E., and Siever, 

R. 1972. Sand and sandstone. Springer 

Verlag, New York, p. 87.

Contributor: Mark Zdunczyk, Senior 

Geologist and Director of Marketing, Mining 

Division, Continental Placer Inc., USA. Typical angular quartz sands from the eastern coastal plain region, USA

DRILLING MINERALS