Note: Descriptions are shown in the official language in which they were submitted.
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POROUS PROPPANTS
CLAIM OF PRIORITY
This application claims priority to provisional U.S. application no.
61/549,878,
titled "Porous Proppant," and filed on October 21, 2011, which is incorporated
by
reference in its entirety.
TECHNICAL FIELD
This invention relates to porous proppants for use in hydraulic fracturing,
and
methods of making and using these.
BACKGROUND
Hydraulic fracturing, or fracking, is a common stimulation technique used to
enhance production of fluids from subterranean formations. In a typical
hydraulic
fracturing treatment, fracturing treatment fluid containing a proppant
material is injected
into the formation at a pressure sufficiently high enough to cause the
formation or
enlargement of fractures in the reservoir. Proppant material remains in the
fracture after
the treatment is completed, where it serves to hold the fracture open, thereby
enhancing
the ability of fluids to migrate from the formation to the well bore through
the fracture.
Many different materials have been used as proppants including sand, glass
beads,
walnut hulls, and metal shot. Sand-based proppants are commonly used due to
the low
cost of sand. However, these proppants cannot often be used at depths where
pressures
are greater than about 2500 psi. The relatively recent rise of use of
hydraulic fracturing,
often referred to as fracking, has presented a need for proppants having
increased crush
strengths.
Many fracking wells are at depths greater than a few hundred feet and can
subject
proppant materials to pressures in excess of 10,000 psi. Therefore,
strengthening coatings
on sand and sintered ceramic proppants have been utilized to achieve greater
crush
strengths.
Two important properties of proppants are crush strength and density. High
crush
strength can be desirable for use in deeper fractures where pressures are
greater, e.g.,
greater than about 2500 psi. As the relative strength of the various materials
increases, so
too have the respective particle densities. Proppants having higher densities
can be more
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costly to use, for example due to transportation costs. Accordingly, there is
a need for
ultra-lightweight proppants having increased crush strength.
SUMMARY
Ceramic ultra-lightweight porous proppants can be cost-effective for use in
hydraulic fracturing operations. Silicon carbide and silicon nitride can
advantageously
provide a high degree of strength while having sufficient porosity to remain
lightweight
and facilitate fluid transport. Oxycarbides and oxynitrides of silicon are
also suitable
lightweight proppant materials.
In one aspect, a porous proppant has a generally spherical shape with a
particle
diameter between 100 and 2,000 microns, median pore sizes between 1 and 50
microns,
and a porosity between 10 and 70% of the total spherical volume.
For a plurality of porous proppants, each porous proppant individually can
form a
proppant pack that has a crush strength of at least 2,000 psi and an apparent
specific
gravity of 1.0 g/cc or less; a crush strength of at least 4,000 psi and an
apparent specific
gravity of 1.3 g/cc or less; a crush strength of at least 6,000 psi and an
apparent specific
gravity of 1.6 g/cc or less; a crush strength of at least 8,000 psi and an
apparent specific
gravity of 1.8 g/cc or less; a crush strength of at least 10,000 psi and an
apparent specific
gravity of 2.0 g/cc or less; or a crush strength of at least 12,000 psi and an
apparent
specific gravity of 2.2 g/cc or less.
For a plurality of porous proppants, each porous proppant individually can
form a
proppant pack that produces 10% or less fines in a crush test.
The porous particles can include silicon carbide, silicon nitride, or a
combination
thereof. The porous particles can include 90% or greater silicon carbide. The
porous
particles can have a sphericity of 0.91 or greater, or 0.95 or greater. The
porous particles
can have a roundness of 0.91 or greater, or 0.95 or greater.
In another aspect, a composition includes a plurality of particles including
silicon
carbide, silicon nitride, or a combination thereof, forming a porous proppant
having a
generally spherical shape with a particle diameter between 100 and 2,000
microns,
median pore sizes between 1 and 50 microns, and a porosity between 10 and 70%
of the
total spherical volume.
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For a plurality of compositions, each porous proppant individually can form a
proppant pack that has a crush strength of at least 2,000 psi and an apparent
specific
gravity of 1.0 g/cc or less; a crush strength of at least 4,000 psi and an
apparent specific
gravity of 1.3 g/cc or less; a crush strength of at least 6,000 psi and an
apparent specific
gravity of 1.6 g/cc or less; a crush strength of at least 8,000 psi and an
apparent specific
gravity of 1.8 g/cc or less; a crush strength of at least 10,000 psi and a an
apparent
specific gravity of 2.0 g/cc or less; or a crush strength of at least 12,000
psi and an
apparent specific gravity of 2.2 g/cc or less.
For a plurality of compositions, each porous proppant individually can form a
proppant pack that produces 10% or less fines in a crush test.
In the composition, particles can have a sphericity of 0.91 or greater, or
0.95 or
greater. The particles can have a roundness of 0.91 or greater, or 0.95 or
greater.
In another aspect, a method of using a composition of claim 15, comprising
injecting the composition into a hydrofracture.
In another aspect, a method of making a porous proppant, includes heating a
composition including a carbon source and a silicon source between 10 and 70%
porosity
of the total proppant volume thereby forming a porous silicon carbide
proppant.
The porous silicon carbide proppant can have a particle diameter between 100
and
2,000 microns, median pore sizes between 1 and 50 microns, and a porosity
between 10
and 70% of the total spherical volume.
Other aspects, embodiments, and features will be apparent from the following
description, the drawings, and the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1-2 are SEM images of a porous proppant.
FIGS. 3A-3B show results of short term conductivity and permeability testing
of
porous proppants.
FIGS. 4A-4B show results of long term conductivity and permeability testing of
a
porous proppant.
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DETAILED DESCRIPTION
Two important physical attributes of proppant packs - pack strength and pack
porosity - depend on many factors. Proppant density is also an important
attribute. These
three important attributes strongly influence overall performance of well
conductivity.
Although there are many factors that determine compressive strength, porosity
and
density to achieve overall conductivity, they can be categorized into four
levels of
importance.
The first and most important level (the goal) is conductivity. This determines
the
performance of the well. Permeability and other related flow terminology is
associated
with conductivity. It is well known that strength and porosity of the proppant
pack are
primary factors in determining conductivity. Accordingly, proppants providing
enhanced
well performance, e.g., proppants having increased strength and/or porosity,
are desirable.
The second level of importance is combined strength and porosity. A proppant
pack must be strong in compression and not produce fines that will plug the
pores of the
proppant pack in the well. When proppants are crushed they produce small
fractions
called fines that can reduce well performance. Therefore strong, porous
proppant packs
are most desirable for conductivity.
A third level of importance is proppant density. Although density does not
affect
conductivity once a proppant pack is in place, a less dense proppant can be
delivered
further into the well before settling. Lighter proppants flow with water,
brine or other
fluid mediums to allow deeper penetration into the well.
Fourth-level attributes that contribute to higher level important attributes
include,
but are not limited to: primary material composition; secondary material
composition;
necking size of primary material composite grains with itself or secondary
composition;
sintered grain size of primary material composition; porosity volume ¨ total
volume in the
proppant; pore size; pore shape; open vs. closed pores; sphericity/roundness;
proppant
particle size (e.g., sphere diameter); proppant particle size distribution;
nature of size
distribution (e.g., bi-modal, single mode size distribution, or other).
While many variables determine overall performance, the combined properties of
strength and porosity most heavily influence conductivity. A desirable
proppant is one
that has low density yet high compressive strength.
The failure mode of proppant packs typically involves fracturing of individual
proppants, under well formation pressure, thus producing smaller proppant
particles
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(fines). The plugging failure mode results from fines produced from proppant
crushing
yielding in poorer conductivity when more fines are produced.
With reference to FIG. 1, a porous proppant is indicated generally by numeral
100. Porous proppant 100 can be generally spherical, ovoid, elongate,
columnar, or other
shape, including an irregular shape. For example, the porous proppant can be
spherical
and have a Krumbein sphericity of at least about 0.5, at least 0.6 or at least
0.7, at least
0.8, or at least 0.9, and/or a roundness of at least 0.4, at least 0.5, at
least 0.6, at least 0.7,
at least 0.8, or at least 0.9. The term "spherical" can refer to roundness and
sphericity on
the Krumbein and Sloss Chart by visually grading 10 to 20 randomly selected
particles.
Sphericity and roundness of at least .9 is most desired to achieve higher
strength at lower
densities.
Porous proppant 100 can be formed of any suitable oxide, carbide, or nitride
of
silicon, boron, aluminum, zirconium, iron, titanium, zinc, tin, chromium,
manganese,
magnesium or calcium. For example, the porous proppant can be formed of a
silicon
carbide, a silicon nitride, a silicon oxide, an aluminum oxide, a boron
carbide, or a
combination thereof. In some cases, porous proppant 100 can be composed of at
least
90% silicon carbide, at least 95% silicon carbide, at least 98% silicon
carbide, or at least
99% silicon carbide. In some cases, porous proppant 100 can be composed of at
least
90% silicon nitride, at least 95% silicon nitride, at least 98% silicon
nitride, or at least
99% silicon nitride. Porous proppant 100 can have a diameter ranging from
about 1
micron to about 3,000 microns, e.g., between about 100 and 2,000 microns. In
some
embodiments, porous proppant 100 has a diameter of about 500 microns.
The median pore sizes of the porous proppant can be between, e.g., about 1
micron and about 50 microns, and the porosity can account for about 10% to
about 70%
of the total spherical volume. The pore sizes can be tailored in size and
volume to achieve
different crush strengths for different well formations.
The porous proppant can have a crush strength greater than 10,000 psi with a
specific gravity of less than 2.2 g/cc. The porous proppant can have a crush
strength
greater than 11,000 psi, greater than 12,000 psi, or higher. The porous
proppant can have
a specific gravity of less than 2.0 g/cc, less than 1.8 g/cc, less than 1.6
g/cc, less than 1.5
g/cc, or less than 1.4 g/cc, or lower. The porous proppant desirably combines
properties
of high crush strength and low density. For example, the porous proppant can
have a
crush strength greater than 10,000 psi with a specific gravity of less than
2.2 g/cc; a crush
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strength greater than 11,000 psi with a specific gravity of less than 2.0
g/cc; a crush
strength greater than 12,000 psi with a specific gravity of less than 1.8
g/cc; or even
higher crush strengths combined with even lower specific gravities.
FIG. 2 shows a proppant at greater magnification than FIG. 1. Porous proppant
100 has a scaffold 110 forming heterogeneous pores 120. The truss structure of
scaffold
110 imparts increased strength to proppant 100 so that the proppant can
withstand crush
strengths greater than 12,000 psi. Moreover, pores 120 provide permeability so
that, once
injected into a hydrofracture, released fluid can pass through the pores of
the proppant as
well as around the spaces formed by the packing of the particles. Non-porous
proppants,
or those proppants modified with external surface treatments, are limited in
fluid
extraction as fluid can only pass through the tortuous path created by the
packing of the
particles. Thus, porous proppants can greatly increase the amount of fluid
extracted and
also extracts the fluid more quickly than proppants used currently.
Porous proppant 100 can be formed by reducing silica- and carbon-based
materials, e.g., to provide a silicon carbide porous proppant. In one
embodiment, a carbon
source is reacted with a silicon source to form a porous silicon carbide by
controlling the
reaction to prevent densification. Alternatively, the pores can be formed
during a
sintering process. Templating approaches can also be used to form pores.
A suitable carbon source can be derived from coal. Other suitable carbon
sources
of include graphite or carbon black.
In some embodiments, a carbon source is combined with a silicon source (such
as
a silicon dioxide, e.g., silica, or sand) and reduced in the presence of
reducing agents to
produce silicon carbide. Porosity resulting from the off-gassing of the oxygen
can impart
porosity to the resulting silicon carbide. Silicon carbide powder can also be
pressureless
sintered to produce porous proppants. Reaction bonding is another process that
can be
utilized to produce porous proppants. Any suitable method to process a solid
material into
spherical particles can be used, such as e.g., milling, spray drying,
spheronization,
encapsulation, granulation or extruding. In most embodiments, spherical
particles are
desirable. For example, the porous non-sintered source can have a Krumbien
sphericity of
0.8 or higher, 0.9 or higher, 0.95 or higher, 0.98 or higher, or 0.99 or
higher.
Sintering can be carried out using any suitable method of heating a silicon
carbide
source, or a carbon source and a silicon source, including, for example,
resistance,
radiation, convection, induction, plasma, laser, microwave, or other methods.
Additional
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sintering aids may optionally be included, such as a polymeric binder or
organic binders.
The extent of sintering can be controlled by adjusting the temperature and
duration. In a
first phase of forming a porous proppant, a reduction step of a carbon source
and a silicon
dioxide produces a porous silicon carbide. Thus, a carbon source of
particulate carbon can
produce particulate porous silicon carbide. In an optional second phase of
forming a
porous proppant, sintering particles of the particulate porous silicon carbide
can produce a
controllable degree of fusion. Thus, "necking" can occur between particles of
porous
silicon carbide, i.e., the formation of bridges joining particles of porous
silicon carbide.
The bridges thus formed are desirably composed of silicon carbide, rather than
a silicon
oxide, which would result in a weaker proppant than similar material with
bridges
composed of silicon carbide. Amounts of less than 10% of oxides are preferable
in the
necking regions (e.g., oxides such as silicon oxide, alumina, zirconia, glass,
mullite, and
other clay bonding) can be acceptable, whereas 90% or more of the porous
proppant is
composed of silicon carbide or silicon nitride. Boron carbide and boron
nitride also are
acceptable in the necking region at levels of less than 10%. Preferably
silicon carbide is
bonded to silicon carbide as the necking region.
The necking process can form a structure having an additional level of
porosity,
i.e., the porosity formed between particles that are joined by bridges. Thus
the resulting
material can have a larger-scale porosity (e.g., on the order of one micron to
fifty
microns) between particles; and smaller-scale porosity (e.g., on the order of
less than one
micron to ten microns). Control over this larger-scale porosity can be
achieved by
controlling the degree of fusion between particles. Higher temperatures and
increased
time promotes a higher degree of fusion. When fused to a higher degree, the
bridges
between particles become larger and more numerous; individual particles become
less
distinct and more agglomerated.
Fines of less than 10% can be generally acceptable in crush tests. 90% or
greater
original particle sizes must be retained in the sieve during a crush test
procedure. Crush
tests are not a substitute for conductivity or actual well performance but are
a suitable
gauge of proppant performance, and for comparisons of different proppant
materials.
Flow back is another issue that can result in poor well conductivity
performance.
The strength of the proppant pack is not only determined by the compressive
strength of
the proppants but also how well they stay in the pack. Lower density proppants
can have
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negative flow back issues, so traditional coatings (resins) can be used on the
porous
proppants mentioned herein to reduce or prevent flow back issues.
Proppants randomly packed (similar to bulk density packing methods) yield in
greater than 30% volume to less than 70% volume of the proppant porous packs.
However, this does not include the porosity of the proppant itself as it only
includes the
porous volume in the pack between the proppants.
Many attributes and variables determine the porous volume of a proppant pack
such as packing method, particle size, particle shape and particle
distribution. However,
these properties combine to form a total pack porosity that determines
ultimate
conductivity in conjunction with pack strength.
Specific gravity is the density of the material and is also defined as the
skeletal
density of the porous proppant. The apparent specific gravity is the adjusted
density of the
proppant when considering the addition of the pore density with the proppant
material
density.
For example, silicon carbide may have a specific gravity of 3.2 g/cc yet the
proppant may have an apparent specific gravity of 1.6 g/cc when considering
50%
porosity volume. The term 'density' of the proppant herein refers to the
apparent specific
gravity, not bulk density or any other density term that may be used
elsewhere.
Sphericity and roundness of at least .9 is most desired to achieve higher
strength at
lower densities.
Suitable proppant particle sizes in many cases are 20/40 mesh. However other
mesh sizes can realize similar results of strength and density attributes.
A mesh size range is determined by retaining all proppant particles in the
smaller
mesh screen (such as 40 mesh) and allowing all other proppant particles to
pass through
the larger mesh screen (such as 20 mesh).
The following discussion provides an example of the relationship between dense
proppant strength and porous pack strength.
Solid silicon carbide having a proppant strength of 540,000 psi can yield
180,000
psi for a single solid sphere, then yielding 60,000 psi for a porous proppant
pack of non-
porous (dense) spheres. The result can less than 10% fines after crush
testing.
Solid spheres made from silicon carbide can be 'overkill' for most rock
formations so porous silicon carbide yields a strong, light weight solution
compared to
sand and sintered ceramics. Starting with higher levels of compressive
strength allow
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porous silicon carbide provide similar strength levels as sand and ceramics,
yet at more
desirable lower densities.
The following discussion provides an example of porous proppant strength in
relation to porous pack strength.
Given 60,000 psi for 60% porous block, yielding 20,000 psi for a single 60%
porous sphere, then yielding 6,000 psi for a 50% porous proppant pack
consisting of 60%
porous spheres.
Table 1 below shows that silicon carbide has desirable for a lightweight
proppant.
Boron carbide can also be a good choice for proppants, but may be cost
prohibitive. Only
widely available raw materials such as sand, certain clays, carbon, and forms
of
aluminosilicates are acceptable in terms of cost. Conversion of sand and
carbon into
porous silicon carbide is a preferred embodiment for low cost, high strength,
low density
propp ants.
Table 1
Proppant Compressive Density Strength to
Material Strength (psi) (gram/cc) Density Ratio
silica 165,343 2.6 63,593
mul lite 188,549 2.8 67,339
alumina 377,098 3.8 99,236
boron carbide 415,442 2.5 166,177
silicon carbide 565,647 3.2 176,765
(Compressive
Strength per g)
EXAMPLES
Example 1. Short Term Conductivity and Permeability
FIG. 3A shows the results of a short term conductivity test using a silicon
carbide
proppant (diamonds), a commercial sintered bauxite proppant (squares), and a
commercial mixed aluminum oxide/silicon oxide proppant (triangles). FIG. 3B
shows the
results of short term permeability tests for the same materials.
Example 2. Long Term Conductivity and Permeability
FIG. 4A shows the results of a long term conductivity test using a silicon
carbide
proppant; FIG. 4B shows the results of a long term permeability test using the
same
material.
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Example 3. Strength Measurements
Porous Proppant Compressive Density % Fines Mesh
Grade Strength (psi) (gram/cc) Generated Size
99% Si C w/1% oxide 5,000 1.4 9% 30
90% Si C w/10% mu I lite 8,000 1.6 7% 20/40
99% Si C w/1% oxide 10,000 1.8 6% 30
98% Si C w/2% oxide 12,000 2.2 9% 20/40
Other embodiments are within the scope of the following claims.
