Note: Descriptions are shown in the official language in which they were submitted.
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CERAMIC PROPPANTS
CROSS-REFERENCE TO RELATED APPLICATIONS
This patent application is a continuation-in-part of and claims priority to
then co-pending U.S. Patent Application serial no. 12/986,479, filed on
January 7, 2011, and to co-pending U.S. Patent Application serial no.
14/275,226, filed on May 12, 2014.
TECHNICAL FIELD
The claimed technology relates generally to small, generally spherical
ceramic bodies and, more particularly, to alumina-silicate proppant bodies for
use
in fracturing subterranean geological formations.
BACKGROUND
Hydraulic fracturing is a technique for increasing the output and
productivity of oil and gas wells by cracking the geological formation
surrounding
and defining an oil and/or gas reserve to create pathways through which the
entrapped oil and/or gas may more easily flow for extraction. First, a highly
pressurized fluid is injected into an existing well bore at a sufficiently
high rate of
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flow to put sufficient stress on the geological formation to induce fracturing
thereof, thus creating a network of cracks in the rock defining the oil and
gas
reservoir. Next, a fluid containing a vast amount of small particulate
propping
agents, or proppants, is introduced into the crack network such that the
proppants will become positioned in the newly-opened fissures to prevent their
closure due to geological forces. In other words, the proppants literally
"prop"
the cracks open.
To do their jobs, the proppants are typically formed to have sufficient
mechanical strength to hold the cracks open against the dynamic geological
forces
that would otherwise operate to close or distort them. Typically, these
geological
forces increase with the depth of the well. Also, proppants are typically made
to
be somewhat fluid permeable and/or conductive, such that even when present in
aggregate, they do not substantially obstruct the flow of oil and/or gas
desired to
be extracted from the well. Typically, propping agents have been made stronger
through densification or by increasing the alumina content thereof. However,
denser, heavier proppants are harder to pump, more expensive to transport, and
are less permeable than lighter, more porous agents.
Another desired characteristic of proppants is that they be inexpensive to
produce, since it takes a great volume of proppants to hold open cracks in
even a
relatively small well. Sand is cheap and plentiful and is often selected as an
advantageous propping agent for maintaining the cracks formed in wells and
geological formations experiencing relatively low closure forces (i.e., 4,000
psi or
less). Moreover, the strength of the sand may be extended to withstand closure
forces of 8000 psi or more through such sorting processes as screening, sizing
and
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shaping the sand. However, the sand proppant performance drops off
dramatically as the closure forces increase, such that even highly processed
and
selected sand is inadequate under closure forces much exceeding 10,000 psi.
Further, sand tends to be nonporous, and as such is less than ideal from a
permeability standpoint. Moreover, the sorting and processing steps add
expense, thus detracting from one of the main characteristics, low expense,
making sand attractive in the first place.
Some high-alumina aluminosilicate compositions, such as bauxite with an
alumina content in the 75-90% range, offer sufficient strength to function as
proppants under relatively high closure forces and at relatively great well
depths.
However, these high-alumina proppants likewise have high densities/ apparent
specific gravities approaching or exceeding 3.5 g/cc, and thus add the
requirement
of high viscosity pumping fluids and/or high pumping rates to prevent them
from
settling out during the injection process. Increased fluid viscosity and the
requisite
high pumping rates cost precision and control of the injection operation, thus
making fracture control and high conductivity fractures more difficult to
achieve
and maintain. Moreover, the high-alumina proppants tend to be more abrasive,
and thus speed the wear of the pumping and fluid transport equipment.
Additionally, sintered high-alumina compositions are relatively expensive,
often
priced ten to fifteen times that of sand.
Intermediate density proppants, defined as those having an apparent
specific gravity in the 3.1 to 3.4 g/cc range, have been developed to provide
sufficient strength to keep cracks open at well depths of from about 8,000 to
about 12,000 feet. In these materials, lower density is achieved primarily by
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reduction of the alumina content to about 75%. Proppants having even lower
densities, such as around 3.0 g/cc, have been formed from kaolin clay
precursors
and are characterized by an alumina content of about 50%. These low density
proppants are typically intended for use at well depths up to about 8,000
feet.
An even lower density proppant has been developed having an alumina
content of from 25% to 40% and an apparent specific gravity of from 2.20 to
2.60 g/cc. While the reduced density allows for the use of less viscous
pumping
fluid and lower pumping rates (which are both desirable for prolonging
equipment life and thus reducing repair and replacement costs), the tradeoff
is in
proppant strength. Lowering the alumina content of the material generally
results
in a lower density proppant with corresponding lower strength, since the
higher
silica content results in significant loss of strength.
Accordingly, there has been a definite need for a proppant composition
enjoying both lower density and less expensive precursor materials that also
has
yield proppants having sufficient mechanical strength to withstand closure
pressures of 8000-10,000 psi or greater. The claimed novel technology
addresses
these needs.
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SUMMARY
The claimed novel technology relates to an improved formulation for
ceramic proppant bodies characterized by a relatively low alumina content. One
object is to provide an improved ceramic proppant formulation. Related objects
and advantages of the claimed technology will be apparent from the following
description.
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DESCRIPTION OF THE PREFERRED EMBODIMENTS
For the purposes of promoting an understanding of the principles of the
claimed technology and presenting its currently understood best mode of
operation, reference will now be made to the embodiments and specific language
will be used to describe the same. It will nevertheless be understood that no
limitation of the scope of the claimed technology is thereby intended, with
such
alterations and further modifications in the illustrated device and such
further
applications of the principles of the claimed technology as illustrated
therein being
contemplated as would normally occur to one skilled in the art to which the
claimed technology relates.
The claimed technology relates to a light weight sintered ceramic material
of intermediate strength, such as a propping agent or proppant useful in the
hydraulic fracturing of subterranean geological formations surrounding and
defining oil wells, gas wells and similar boreholes. The sintered ceramic
material
is typically formed as solid, substantially spherical particles or pellets and
is
typically characterized as having a silica content between about 52 and about
58
weight percent, an alumina content of between about 32 and about 39 weight
percent, an apparent specific gravity of between about 2.61 and 2.69 (more
typically between about 2.63 and 2.65), a bulk density of between about 1.41
and about 1.65 grams per cubic centimeter (more typically a loose fill density
of
between about 1.45 and 1.55, or between about 1.54 and about 1.61 g/cc),
Krumbein sphericity of at least about 0.7 (more typically at least about 0.8
and
still more typically at least about 0.9), a mechanical crush strength such
that no
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more than 10% of a test population are crushed at 7500 PSI (more typically no
more than 5% at 7500 PSI and/or no more than 10% at 8000 PSI), and an
ambient temperature permeability of at least about 100,000 millidarcies at
8,000
psi.
More particularly, the aluminosilicate proppant particles typically have a
silica (5i02) content of between about 54 and about 55 weight percent, an
alumina (A1203) content of between 30 and about 36 weight percent. Typical
proppant particle composition ranges (in weight percents) are as follows:
Typical More Typical
5i02 52-60 54-55
A1203 30-38 32-36
TiO2 1.7-3.0 1.7-2.5
Fe203 1.5-5.0 2.2-3.3
CaO 0.2-0.4 0.2-0.4
MgO 0.2-1.0 0.3-0.6
K20 0.4-2.5 1.5-2.0
5r02 0.01-0.1 0.01-0.1
Total 100.00 100.00
These proppants typically have a specific gravity of between about 2.61 and
about 2.69 and more typically between about 2.63 and about 2.65. The
proppants typically have a bulk density of between about 1.41 and about 1.65,
and more typically between about 1.45 and about 1.55 gm/cc.
Typically, the propping agent particles are made from high-iron
aluminosilicate materials or fireclay typically found deposited in and around
St.
Louis, High Hill and Mexico Missouri (i.e., Missouri fireclay). Similar
mineral
formations may also be found in such places as Ohio, Kentucky, Pennsylvania,
certain parts of Europe and the like. In general, Missouri fireclay is a
relatively
high iron-content bauxite or bauxitic material. Typically, the Missouri
fireclay will
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be blended with at least some (typically, about 5 weight percent or more) high
iron aluminosilicate, a high iron aluminosilicate source, or similar mineral
compositions.
A small amount (typically around 5 weight percent) crush strength
enhancer material may be added to the Missouri fireclay to increase the
strength
of the proppants produced therefrom. The crush strength enhancer is typically
a
material such as nepheline syenite, fused bauxite dust, wollastonite, talc,
feldspar,
rutile, bentonite, ball clay, fireclay or the like, which act to increase the
strength
of the aluminosilicate proppants particles without substantially altering
their
density, specific gravity, conductivity and the like. Typically, small
additions of
crush strength enhancers such as these operate to impede cristobalite
formation
during the sintering of the proppant particles, thus increasing their crush
strength.
The propping agent particles are typically prepared from a dry mixed
precursor including the Missouri fireclay material, any additional clays, any
strength enhancing additives, and a suitable binder to yield a composition
within
the ranges outlined above. In other embodiments, the precursors may be mixed
as an aqueous suspension or the like. The dry mixed precursor is typically a
substantially homogeneous mixture, and yields green and, later, fired
proppants
having substantially homogeneous compositions with the component oxides
homogeneously distributed therein. The mixture is then granulated through
mixture-intensive shaping into generally spherical particles. The generally
spherical particles are typically larger than the desired size of the final
proppant
particles, to allow for shrinkage to occur during the firing process (i.e.,
the final
proppant size is typically targeted to a convenient size range, such as 16/30
mesh,
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20/40 mesh, 40/80 mesh, or the like). Those fractions of undesired sizes, such
as
undersized and oversized fractions, are typically recycled into the layer of
fluidized particles. The non-recycled fractions are then typically dried,
calcined
and sintered. Sintering is typically accomplished at a temperature of between
about 2345 and about 2450 degrees Fahrenheit (or between about 1285 and
about 1327 degrees Celsius) to yield sintered proppants.
The as-sintered proppants typically have a specific gravity of between
about 2.61 and about 2.69, more typically about 2.63 to about 2.65; this
relatively low specific gravity allows for easier and less destructive pumping
of a
fluid containing the proppants for injection into crack formations. More
typically, the proppants are characterized by a Krumbein sphericity of about
0.7
or greater. Still more typically, the proppants have a Krumbein sphericity of
at
least about 0.8; yet more typically, the proppants have a Krumbein sphericity
of
at least about 0.9.
Typically, the propping agents have sufficient mechanical strength such that
80% of a sample may withstand crushing forces of 7500 psi; more typically, at
least about 90% survive. Still more typically, the propping agents have
sufficient
mechanical strength such that 90% of a sample may withstand crushing forces of
8000 psi and/or 95% of the sample may withstand a crushing load of 7500 psi.
In other words, the proppants are typically characterized by a 20% crush at
7500
psi; more typically a 10% crush at 7500 psi; and still more typically by a 10
percent crush at 8000 psi and/or a 5% crush at 7500 psi..
Aluminosilicate ore blend from St. Louis, Missouri having the following
analysis by weight after ignition at 800 degrees Celsius: 5i02 - 57%; A1203 -
35%;
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TiO2 - 1.8%; Fe203 - 3.5%; with the remainder being alkali and alkali-earth
metal
oxides. Green strength may be increased through the addition of starches,
swelling clay, KCI, NaOH, NH4CI, PVA, or other binders.
This material is generally formed through mixer granulation techniques to
yield a small, generally spherical pellet. The pellets are typically screened
and
extracted and sized in target, oversized and undersized fractions or ranges.
The
oversized fractions may be ground down to size, such as in a grinding unit,
while
the undersized fractions may be recycled or extracted for other uses. The
remaining material at target size may be calcined to remove excess moisture
and
volatilize unwanted organics. The remaining material may then be formed into
any desired shapes and sizes, and/or merely sintered in a rotary kiln at a
temperature of between about 1285 and 1325 degrees Celsius for 30 minutes or
less. The sintered substantially spherical particles are characterized by a
substantially homogeneouos distribution of silica and alumina therethrough.
The
sintered particles are then subjected to a further sieving operation to
further
control the desired particle size.
In operation, sintered proppant particles of the above compositional range
are fluidically injected into precracked geological formations surrounding and
defining oil and/or gas wells. The proppants become positioned in the cracks
in
sufficiently high numbers and concentrations to wedge or prop the cracks open,
resisting geological forces arising that would otherwise urge the cracks shut.
The
proppants provide a propping layer that is sufficiently fluid
permeable/conductive
that fluids such as oil and/or gas may readily escape through the cracks and
be
extracted from the well.
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Likewise, the proppants may be prepared and formed via any convenient
alternate aqueous process or any convenient dry or powder process.
Examples
Example 1:
A 20/40 mesh proppant sample was prepared and crush tested. The
composition of the proppant sample was analyzed via inductively coupled
plasma-atomic emission spectroscopy and was found to have the following
composition, expressed in oxide form:
A1203 35.61 wt.%
Si02 57.81 wt.%
Fe203 2.39 wt.%
1<20 1.71 wt.%
TiO2 2.00 wt.%
MgO 0.46 wt.%
CaO 0.30 wt.%
Sr02 0.06 wt.%
The specific gravity of the sample was measured to be 2.63 and the bulk
density
was measured to be 1.49. Portions of the sample were extracted for crush
testing
at various pressures. The crush test results are as follows:
Crush PSI Percentage Crushed
7500 3.8 0.3
10000 11.6 0.3
12500 26.6 0.4
15000 32.5 0.6
The sample had a crush at 7500 PSI of less than 5% and a crush at 10000 PSI of
about 10%.
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Example 2:
The sample of Example 1 was subjected to roundness and sphericity
analysis. Roundness is essentially a measure of the degree of abrasion of
particles
and may be expressed as the ratio of the average radius of curvature of the
edges
or corners of a particle to the radius of curvature of the maximum inscribed
sphere. Sphericity, while similar in concept to roundness, is a measure of how
close in overall shape a particle is to a sphere, and may be taken as the
ratio of
the surface area of an idealized sphere to the surface area of a particle of
the same
volume. Sphericity is often visually measured, such as on the Krumbein scale.
Thus, since both roundness and sphericity are ratios against ideal cases, the
closer
the value to 1.0, the more round and/or spherical the particle. Twenty random
sample particles were measured both for roundness and sphericity. A measured
mean value of 0.8 0.1 was found for both sphericity and for roundness, while
the mode was 0.9 for both.
Example 3:
A 20/40 mesh proppant sample was prepared and crush tested. The
composition of the proppant sample was analyzed via inductively coupled
plasma-atomic emission spectroscopy and was found to have the following
composition, expressed in oxide form:
A1203 35.92 wt.%
Si02 57.04 wt.%
Fe203 2.72 wt.%
1<20 1.80 wt.%
TiO2 1.96 wt.%
MgO 0.47 wt.%
CaO 0.17 wt.%
Sr02 0.05 wt.%
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The sample had a specific gravity of 2.63 and a bulk density of 1.49. The acid
solubility of the sample was measured to be 4.0% 0.0 in accordance with ISO
13503-2, section 8 procedures using 60% HF as a source.
While the claimed technology has been illustrated and described in detail
in the drawings and foregoing description, the same is to be considered as
illustrative and not restrictive in character. It is understood that the
embodiments
have been shown and described in the foregoing specification in satisfaction
of
the best mode and enablement requirements. It is understood that one of
ordinary skill in the art could readily make a nigh-infinite number of
insubstantial
changes and modifications to the above-described embodiments and that it
would be impractical to attempt to describe all such embodiment variations in
the
present specification. Accordingly, it is understood that all changes and
modifications that come within the spirit of the claimed technology are
desired to
be protected.
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