Oil and gas are resources of great
economic importance. The recent advent of horizontal drilling
in the US (and the associated hydraulic fracturing), means that
cheaper natural gas is changing the shape of the market, but
much hinges on the hard round grains known as proppants, so
called because they prop open the fractures in the rock
Proppants are generally made up of
either silica (when specifically for hydraulic fracturing it is
known as frac) sand, resin-coated silica sand, or ceramic
proppants made from sintered kaolin or bauxite.
Without these, the whole process
would fall apart but all these proppants differ considerably on
cost, processing capability, fracture conductivity, logistics,
and effectiveness in the various downhole situations.
A Cooke Conductivity Cell used for API
standards that tests conductivity downhole
Carbo Cermaics, Society of Petroleum Engineers
The importance of conductivity
The most important facet to any oil
or gas well is conductivity - the flow of oil and gas back
through the pipeline to the surface.
Conductivity is determined by the
width of the fracture multiplied by its permeability driven
primarily by proppant selection. The proppant is packed into
the pipe and into the fractures so that the oil and gas can
flow back through the space between the individual proppant
Conductivity differs both between
wells and between shales. Each well has its own unique
situation requiring petroleum engineers to use a measure of
conductivity to decide which proppant is most appropriate.
The quality [of the proppant]
is driving the conductivity (...) you can always improve
conductivity but the point is that it is going to cost you more
money to do it - so what is really needed is a cost benefit
analysis, Terry Palisch, director of petroleum
engineering at Carbo Ceramics, a leading producer of ceramic
proppants, explained to IM.
Whereas perhaps ten years ago
they [engineers] might have used all ceramic proppants in a
well, now they may use ceramics in just a part, Bryan
Shinn CEO of US Silica told IM, adding that
cost efficiency is now the driving force.
All petroleum engineers now focus
on maximising conductivity at minimum cost: The real
question that a person has to ask when selecting a proppant is
how much conductivity do I need in this well, said
After the appropriate level of
conductivity is decided, the proppant and fluid make-up is
selected in order to provide the most economical well drilling
Conductivity is measured using
American Petroleum Institute (API) and ISO standards which
involve laboratory tests using a Cooke Conductivity Cell
(see below) which determines the conductivity
downhole. This will then be used to evaluate which proppant to
At the actual well site, engineers
can also carry out a pressure transient analysis which is where
measurements of well pressure are taken when the well is
flowing and this helps show the length and conductivity of a
However, the problem is those
conditions dont necessarily mimic what is going on in the
well and so the really critical piece of proppant selection is
predicting the conductivity of the proppant with the specific
well conditions, Palisch explained to
Unfortunately, while the
modified API test provides a good indicator of the performance
of proppant under laminar conditions in the laboratory, it
drastically overestimates the conductivity of the proppant when
placed in a real hydraulic fracture, Terry Palisch and
Robert Duenckel of Carbo Ceramics pointed out in a 2007
This is because reference
values have not been corrected for effects
such as non-Darcy or multiphase flow, gel damage, filter cake,
fines plugging, cyclic stress loading, long term proppant
degradation and many other phenomena which will increase the
pressure losses within the fracture. None of these effects is
accounted for in any standard conductivity test. Indeed, it is
not uncommon to see as much as a 99% reduction in conductivity
under realistic conditions, they added.
This means determining realistic
conductivity is extremely important and can affect the choice
The perfect proppant
An ideal fracture would possess
infinite conductivity without the need for proppant. In
reality, a proppant is required that ideally would be uniformly
placed over the created length and not be subject to
Proppant characteristics, as per
API specifications, are expressed in mesh size,
roundness/sphericity, crush resistance, quartz content
(SiO2), bulk density, specific gravity, solubility
in acid and turgidity.
These are all important because
they are the factors that drive conductivity and indicate how
effectively the proppant will perform in the well.
This is the size of the particle
(or bead) of proppant. The most common mesh size used is API
certified mesh sizes of 8/12, 10/20, 20/40, or 70/140 with
20/40 being the most widely used for frac sand.
The change in pressure of closure
that proppants will stand has a lot to do with the mesh size. A
very large particle will withstand very little closure.
However, a very small particle (because there are several of
these particles in the same area) improves the closure pressure
as it is more efficiently distributed so 20/40 mesh proppant
will withstand less closure than 40/70 proppant because there
are significantly more contact points on the 40/70 per square
foot of the fracture pattern.
The standards prepared by the API
in this regard simply estimate how closely the quartz grain
conforms to a spherical shape and its relative roundness.
The grain is classified as
average radius of the corners/radium of the maximum
Krumbein and Sloss devised a chart
for the visual estimation of sphericity and roundness in 1955
as shown on the right. API recommends sphericity and roundness
of 0.6 or larger.
The roundness dictates how the
proppant will fit together in the fracture and a more round
proppant will mean that there is more space for the oil and gas
to flow back along the pipeline, improving the
API requires frac sand to be
subjected to between 4000 psi and 6000 psi pressure for two
minutes in a uniaxial compression cylinder to determine its
In addition, API specifies that the
fine particulates from the crushing of proppant beads (fines)
generated by the test should be limited to a maximum of 14% by
weight for 20-40 mesh and 16-30 mesh sizes. Maximum fines for
the 30-50 mesh size should be 10% or less. Other size fractions
have a range of losses from 6% for the 70-40 mesh to 20% for
the 6-12 mesh size, Mark Zdunczyk, consulting geologist,
outlined in IM in January 2007.
Crushing, and the production of
fines, is a function of grain brittleness, which correlates to
grain shape, and the internal structure of the grain itself, as
well as overgrowths on the grain.
The crush resistance is important
for keeping the fracture open so the proppant does not cave in
under pressure and allow the fissure to close and stop the flow
With respect to hardness, normal
silica sand might have a strength allowing it to withstand
fracture closure stresses up to about 5000 psi. In normal
oilfield operations, synthetic proppants are used where closure
forces are expected to be above roughly 5000 - 8000 psi but
there is no definite measure as it depends on well
Frac sand should also have a high
quartz content (>99% SiO2). A high
SiO2 content generally means that the proppant is
likely to have a higher crush resistance.
Also know as porosity, bulk density
is the mass of proppant per unit volume (eg gram/cubic
centimetre). The volume of both the grains themselves and the
voids between the grains are included.
Bulk density is useful to gain an
estimation of the weight of a proppant needed to fill a
fracture or a storage tank.
This is also called apparent
density and includes internal porosity of a particle as part of
its volume dictating the speed at which a particle will settle
when suspended in water or gel. It is ideal to have the
specific gravity as close to the fluid as possible without
compromising the hardness.
This test measures the loss in
weight of a sample that has been added to a 100ml solution made
up of 12 parts hydrochloric acid (HCI) and 3 parts hydrofluoric
acid (HF) and subsequently heated at 150ûFahrenheit
(approximately 65.5û centigrade) in a water bath for 30
The object of this test is to
determine the amount of non-quartz minerals present.
API specifications require that
losses by weight as a result of this test are restricted to
<2% across all mesh sizes up to 40-70 mesh where the loss
permitted rises to 3%.
Turgidity refers to the amount of
silt of clay sized particles in the sand sample. This is
generally not an issue in frac sand production as it requires a
washing process to be introduced which effectively removes
The best proppant is one that is
extremely hard (crush resistant), extremely round (sphericity),
of a similar density to water to prevent settling, and
Proppant selection in a
perfect world with unlimited supply comes down to cost verses
benefit, Palisch told IM.
Silica sand, known more
colloquially as frac sand when specifically used for hydraulic
fracturing, is the cheapest and most readily available of
proppants in the US. Frac sand was first mined from the brown
Hickory (Brady) silica sand deposits in Texas, US, where large
scale production continues, later from the white to off-white
St Peter (Ottawa) sandstone formation in the mid-west states of
Minnesota, Wisconsin and Illinois - both of which are
Cambrian-Ordovician sandstone formations.
Older silica sand formations tend
to have much older and rounder grains that are much better
suited to hydraulic fracturing than younger deposits.
With sand, mesh size is critical
and the vast majority of grains range from 12 to 140 mesh and
include standard sizes such as 12/20, 16/30, 20/40, 30/50, and
Generally, coarser proppant allows
for higher flow capacity owing to the larger pore spaces
between grains. However, it may break down or crush more
readily under stress due to the relatively fewer grain-to-grain
contact points to bear the stress often incurred in deep oil-
and gas-bearing formations.
Coarser proppants, such as
16/30 and 20/40, can be more difficult to effectively place in
fractures due to their size and higher settling rates compared
to, for example, 40/70 and 100 mesh, explained Robin
Beckwith of the Society of Petroleum Engineers.
The increasing use of 40/70 and
finer 100 mesh sands beginning in 2001Ñrising
particularly since 2006Ñ is a rather new development
resulting from the rise in high-volume slickwater fracturing of
unconventional horizontal gas wells such as those in the
Barnett, Fayetteville, Haynesville, and Marcellus shales, he
The trend in proppant sizing
in unconventional gas wells had been toward smaller proppants
in the hope that more of the proppant could be placed farther
into the reservoir. With the shift back toward oily reservoirs,
the need for higher proppant conductivity to move liquids at
high rates has caused a shift back toward larger proppant
sizes, explained Kevin Fisher in a recent American
Oil and Gas article.
The significant difference between
frac sand and other proppants is cost. When you look at
the cost of our product, frac sand versus the ceramics,
its about a ten-to-one difference. So it really makes a
dramatic difference to the end-use customers in terms of their
cost of the wells, Bryan Shinn of US Silica explained to
If frac sand has a good mesh size,
it can still fail on its roundness. The rough and uneven
granules fit together like a jigsaw puzzle and reduce
conductivity and the roughness of the surface of the sand
particles also hinders flow back along the pipeline.
As a proppant, silica sand
struggles to function in deeper wells where it is subjected to
much higher pressure (psi) and temperature.
Resin-coated sand is silica sand
that has been coated in a resin to improve it as a proppant and
accounts for about 15% of market supply.
The microscopic coating adds
to the strength of the grain that is coated, the sand grain or
the ceramic grain. Depending on the type of substrate that you
are coating it can double the closure strength or the
resistance to crush that material can withstand, Mike
Smith, vice president, FTS International, Proppants and
Coatings Division told IM.
This improves that sands
quality and means that in can be used in a well with much
higher pressure at greater depth.
But resin-coated sand still relies
on a supply of good quality silica sand: If you are
coating a substandard particle, because it is the majority of
the material, then the pressure that material will be at in the
fracture is the dominant factor in how much strength that
material will have. So you have to be coating a relatively high
quality sand, said Smith.
The raw sand grain needs to be
virtually 100% silica; it needs to have high quality roundness
and sphericity as well as very low solubility and it needs to
be monocrystalline in nature because naturally fractured sand
grains do not stand much closure, Smith outlined.
The resin has another advantage as
well in the elasticity provided by the coating that helps to
bond the particles together. When the proppant is placed in the
well the resin between the particles binds creating a wider
footprint meaning the pressure that the grain sees is now
distributed over a slightly larger area so it can withstand
The resin will also help to hold
the proppant in place if there isnt sufficient closure to
Additionally, resin-coated sand
helps to prevent fines clogging up the proppant pack in the
well and lowering conductivity, when that sand
[resin-coated] breaks, and its still going to break even
if you put the resin on it, the resin will hold all the fines
together so the shards dont flow through the proppant
pack and plug things up, Palisch told
Resin coating also helps to improve
the surface roughness of the sand particles which improves the
flow of gas back up to the surface.
application range tends to be from zero up to about 14,000
psi, said Smith.
In comparison to raw frac sand,
resin-coated sand is more expensive because of the extra
processing but it provides a smoother, harder proppant that
provides better conductivity in higher pressure and temperature
situations and the proppant in the well provides better
conductivity than raw frac sand.
It does however retain some of the
faults of raw frac sand, particularly in sphericity because of
its angular shape.
Ceramic proppants are the most
expensive of the three main types of proppants and are manmade
from aluminous-related materials, either the aluminous clay
kaolin or the alumina feedstock bauxite.
Ceramic proppants have a great
advantage over frac and resin-coated sand because they have no
angularity. Ceramic proppants are also much more crush and
thermal resistant so can survive in hotter deeper wells like
those of the Haynesville shale.
Ceramic proppants are
significantly superior to sand as proppants for high
temperature, high closure applications, and resistance to
saline dissolution (É) they have better properties than
most if not all sands, said Pickard Trepess, marketing
manager for Mineracao Curimbaba Sintered Bauxite proppants
explained in April this year.
As a proppant, ceramic is by far
the most effective source, providing excellent crush
resistance, and great roundness because they are manmade.
It is much stronger because
it is an alumina-based product and (...) a good ceramic
proppant should have a tighter sieve distribution which just
means that it is more uniform in size and is also going to be
rounder, explained Palisch.
Ceramics have always had strong
presence in the proppant market: In the early days, back
a decade or more, engineers were very conservative and
didnt need the extra performance of a ceramic but would
specify it anyway, Shinn of US Silica told
IM. This was done because ceramics give the
best conductivity for any well but have faded in recent years
because the cost of a ceramic proppant can make a well
uneconomical unless the wells individual requirements
In the environment that we
have today with relatively low natural gas prices, what we see
are ceramic wells - that by definition are the high cost wells
- get turned off first. They are the first ones to get turned
off but the first ones to be turned back on again, said
A well that uses ceramic proppant
will be reactivated when natural gas prices rise because they
are typically the more efficient wells with the best
For wells with extremely high
pressure, the ceramic proppants are coated in a resin giving
them the same extra benefits as resin-coated sand.
Although frac sand and ceramic
proppants seem to be competitors, there is in fact very little
opportunity for substitution in the individual wells. As the
three main types of proppants are used in very different
situations and cost vastly different amounts, there is little
The wars get fought at the
boundaries so if there was some displacement it would, for
example, be a high-end resin-coated sand versus a low-end
ceramic proppant, Shinn explained to IM.
Despite this slight displacement, the market shares will remain
consistent: We dont see any dramatic share shifts
in the market and we are not projecting that in the
future, he added.
A recent influx of low-cost
low-quality ceramic proppant from China has caused some
displacement but without sustained supply and consistent
quality, the trend is waning.
So in the oil field drilling
market, both proppants have their own niche and both will
continue to be used by petroleum engineers for horizontal
There can be competition within the
proppants themselves. Different types of sand, for example,
compete quite strongly with some drilling companies using Brady
brown sand and some using Ottawa white sand.
However, the majority of the
proppants market uses frac sand, followed by resin-coated sand
and ceramic with potential for expansion in all.
Substitution of resin for
Recently, Mei Yang with Cadre
Proppants published a paper pointing out that resin-coated sand
could perform as well as ceramics in deeper wells.
For tight gas reservoirs, we
correct the prejudice that natural sand proppants cannot be
applied to deeper reservoirs by showing NPV study results that
are superior to those of manmade proppants. By keeping
stimulation costs down, natural sand proppants have a much
larger range of applicability than previously thought,
Matt Zinn of FTS International
agrees, telling IM: With long-term
production results they [drillers in the Haynesville Shale]
were seeing zero performance increase in the long term from the
wells using ceramic versus our resin-coated
The Haynesville would be the highest pressure and
temperature which would be the situation most conducive, in
theory, to having issues with resin-coated proppants but there
werent any production increases from the more expensive
ceramic, he added.
Proppant prospects in the
The Middle East and India
are showing signs of potential new demand for proppants as
unconventional oil and gas resources attract
The Middle East is about to
embark on a new phase of oil and gas exploration and
development as existing reserves become exhausted and domestic
demand for energy, especially in Saudi Arabia,
The move marks a potential
new era of demand for oilfield minerals used in drilling fluids
including bariteÊ(barytes), bentonite, calcium carbonate,
calcium chloride, and haematite, as well as for proppants used
in hydraulic fracturing, such as frac sand and sintered bauxite
Trends in oilfield mineral
demand in the Middle East is the focus for the next IM
Roundtable, Oilfield Minerals Outlook: Middle East, 21-23
January 2013, Dubai (see p12-13).
Fracking from the Middle
East to Asia
The evolution of hydraulic
fracturing in the Middle East and its influence on proppants is
the subject of the presentation by Pickard Trepess, regional
sales manager Europe, Africa and the Middle East for Sintex
International, part of the Mineracao Curimbaba group of
Fracturing has been evident in the
Middle East and the central Asian region for a long time.
According to Trepess most operations have been performed using
acid, where the formation is carbonate or dolomite, but there
are many reservoirs that respond better to propped
In his presentation, Trepess will
summarise the experience from Algeria to north-east India, and
the development over time, the local constraints, and the
reasons for fracturing.
Trepess observes that many
countries are now looking very seriously at shale gas
exploitation as economies move away from oil, which can instead
be exported onto the world market.
Countries that are active in
propped fracturing include Algeria, Tunisia, Egypt, Oman, Saudi
Arabia, India, and Kuwait.
Significant plans to increase
activity in India, Oman, Saudi Arabia, and Algeria are
underway. In the future it is expected that Iraq and Pakistan
will be performing large operations, and that there will be
enhanced activity in Kuwait and in other parts of the western
regions such as in Jordan, Cyprus and Turkey.
Certainly these comments echo the
sentiments widely expressed among oilfield service companies
and oil and gas explorers at the huge Abu Dhabi International
Petroleum Exhibition and Conference (ADIPEC) held during 11-14
Halliburton is focused on
positioning ourselves for the unconventional and HP/HT [high
pressure/high temperature] business of the future, said
Mike Hugentobler, Halliburton VP Middle East and Eurasia.
Hugentobler acknowledged that gas
will be a major focus over the next few years in
the Middle East. Key Halliburton goals include unconventional
gas reservoirs, deep HP/HT well types, and remediation of
Dr Bernard Duroc-Danner, CEO of
Weatherford, said: We need to look harder at
unconventionals. There is a lot of experience coming out of the
US oil patch. We will know a lot more in ten years on how to
exploit these unconventionals, the frontiers, and the difficult
Saudi energy demand, especially
from the domestic sector (with air conditioning a top user) is
growing on average 8-10% pa. By 2030, domestic consumption will
reach 6.5m bpd, thus exceeding exports, which are expected to
decline as a result.
At ADIPEC, Khalid al-Falih, CEO of
world leading oil and gas producer, Saudi Aramco, called for
increasing conventional and unconventional gas supplies by
almost 250% over the next 20 years. Aramco is to invest $35bn
in oil and gas exploration and development over the next five
The Saudi company is also taking a
leaf out of the US shale gas book. Aramcos chief
exploration manager at its new unconventional gas division,
Saleh M Saleh, revealed that up to 50 Aramco staff are being
trained in the US by the likes of Baker Hughes and Schlumberger
in order to gain experience found in the US shale gas plays to
utilise in Saudi Arabia.
Saleh recognises that the key to
successfully developing unconventional gas plays lies within
the oilfield service sector that has developed and implemented
new technologies on a large scale in the US.
Proppant evolution in
Hallmark Minerals (I) Pvt Ltd of
India has been pioneering the manufacture of ceramic proppants
in the country. AK Dasgupta, managing director, of Hallmark
Minerals, will be presenting: The future of ceramic
proppants production in India in Dubai.
Dasgupta maintains that ceramic
proppant demand in India is expected to increase many
times in the near future owing to modification of state
policy to enhance oil exploration as well as liberalisation for
private and global producers for shale gas based on
unconventional drilling policy.
On 6 December 2012, US independent
explorer ConocoPhillips announced that it was nearing a deal
with Indias state owned Oil and Natural Gas Corporation
(ONGC) to explore and develop shale-gas resources in the
ONGC plans to explore the Damodar,
Cambay, Krishna Godavari, and Cauvery basins for shale gas.
The US Energy Information
Administration (EIA) estimates that India has a total of 64
trillion cubic feet of potential recoverable resources, and has
the 15th largest technically recoverable shale-gas
A recent report in Petroleum
Economist recorded the conclusions of a study by analysts
at Bernstein Research which cast some doubt on near term
fruition of Indian shale gas development. Indias delayed
shale-gas policy is expected to be released in April next
That said, it is reported that
there remains considerable interest in tapping the
nations shale gas deposits with horizontal drilling and
hydraulic fracture stimulation techniques, thus indicating good
prospects for proppant suppliers.
Key trends and developments in
oilfield mineral supply and demand for the Middle East will be
examined and discussed at Oilfield Minerals
Outlook: Middle East, 21-23 January 2013,
Dubai - see. p12-13 and