U.S. patent application number 16/133142 was filed with the patent office on 2019-01-17 for proppant particles formed from slurry droplets and method of use.
The applicant listed for this patent is CARBO CERAMICS INC.. Invention is credited to Robert DUENCKEL, Benjamin T. ELDRED, Clayton F. GARDINIER, Brett A. WILSON.
Application Number | 20190016944 16/133142 |
Document ID | / |
Family ID | 55074038 |
Filed Date | 2019-01-17 |
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United States Patent
Application |
20190016944 |
Kind Code |
A1 |
ELDRED; Benjamin T. ; et
al. |
January 17, 2019 |
PROPPANT PARTICLES FORMED FROM SLURRY DROPLETS AND METHOD OF
USE
Abstract
Proppant particles formed from slurry droplets and methods of
use are disclosed herein. The proppant particles can include a
sintered ceramic material and can have a size of about 80 mesh to
about 10 mesh and an average largest pore size of less than about
20 microns. The methods of use can include injecting a hydraulic
fluid into a subterranean formation at a rate and pressure
sufficient to open a fracture therein and injecting a fluid
containing a proppant particle into the fracture, the proppant
particle including a sintered ceramic material, a size of about 80
mesh to about 10 mesh, and an average largest pore size of less
than about 20 microns.
Inventors: |
ELDRED; Benjamin T.;
(Houston, TX) ; WILSON; Brett A.; (Cypress,
TX) ; GARDINIER; Clayton F.; (Houston, TX) ;
DUENCKEL; Robert; (Colorado Springs, CO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CARBO CERAMICS INC. |
Houston |
TX |
US |
|
|
Family ID: |
55074038 |
Appl. No.: |
16/133142 |
Filed: |
September 17, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14802761 |
Jul 17, 2015 |
10077395 |
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16133142 |
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14502483 |
Sep 30, 2014 |
9670400 |
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14802761 |
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13608530 |
Sep 10, 2012 |
8883693 |
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14502483 |
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13357141 |
Jan 24, 2012 |
8865631 |
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13608530 |
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13045980 |
Mar 11, 2011 |
9175210 |
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13357141 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C09K 8/80 20130101; C04B
2235/444 20130101; C09K 8/62 20130101; C04B 35/111 20130101; C04B
2235/77 20130101; C04B 2235/94 20130101; C04B 2235/3208 20130101;
C04B 35/636 20130101; C04B 35/624 20130101; C04B 33/04 20130101;
C04B 2235/528 20130101; C04B 2235/60 20130101; C04B 2235/96
20130101; C04B 35/622 20130101; C04B 38/009 20130101; Y10T 428/2982
20150115; C04B 38/009 20130101; C04B 35/111 20130101 |
International
Class: |
C09K 8/62 20060101
C09K008/62; C09K 8/80 20060101 C09K008/80; C04B 35/636 20060101
C04B035/636; C04B 35/624 20060101 C04B035/624; C04B 35/622 20060101
C04B035/622; C04B 35/111 20060101 C04B035/111; C04B 33/04 20060101
C04B033/04; C04B 38/00 20060101 C04B038/00 |
Claims
1. A proppant particle, comprising: a sintered ceramic material and
having: a size of about 80 mesh to about 10 mesh; a porosity; and
an average surface roughness of from about 0.1 micron to about 4
microns.
2. The proppant particle of claim 1, wherein the sintered ceramic
material has an alumina concentration of at least about 40 wt
%.
3. The proppant particle of claim 2, wherein the sintered ceramic
material has an alumina concentration of at least about 95 wt
%.
4. The proppant particle of claim 1, further comprising a plurality
of proppant particles comprising a sintered ceramic material and
having a size of about 80 mesh to about 10 mesh, a porosity, and an
average surface roughness of from about 0.1 micron to about 4
microns, wherein the plurality of the proppant particles has a bulk
density of about 1.35 g/cc to about 2.1 g/cc.
5. The proppant particle of claim 4, wherein the proppant particles
have a specific gravity of about 2.5 g/cc to about 4.0 g/cc.
6. The proppant particle of claim 1, wherein the proppant particle
has a surface roughness of less than about 2 microns.
7. A ceramic particle for use in a subterranean formation, the
ceramic particle comprising: a sintered ceramic material; a size of
about 80 mesh to about 10 mesh; a porosity; and a surface roughness
of less than about 2 microns.
8. The ceramic particle of claim 7, wherein the sintered ceramic
material has an alumina concentration of at least about 40 wt
%.
9. The ceramic particle of claim 8, wherein the sintered ceramic
material has an alumina concentration of at least about 95 wt
%.
10. The ceramic particle of claim 7, wherein the porosity is an
interconnected porosity of at least about 25%.
11. The ceramic particle of claim 7, further comprising a plurality
of proppant particles comprising a sintered ceramic material and
having a size of about 80 mesh to about 10 mesh, a porosity, and an
average surface roughness of less than about 2 microns, wherein a
plurality of the proppant particle has a bulk density of about 1.35
g/cc to about 2.1 g/cc.
12. The ceramic particle of claim 11, wherein the proppant particle
has a specific gravity of about 2.5 g/cc to about 4.0 g/cc.
13. The ceramic particle of claim 7, wherein the proppant particle
has a surface roughness of from about 0.1 microns to about 2
microns.
14. A proppant particle, comprising: a sintered ceramic material; a
porosity; and a surface roughness of 4 microns or less.
15. The proppant particle of claim 14, wherein the sintered ceramic
material has an alumina concentration of at least about 40 wt
%.
16. The proppant particle of claim 15, wherein the sintered ceramic
material has an alumina concentration of at least about 95 wt
%.
17. The proppant particle of claim 14, wherein the porosity is an
interconnected porosity of at least about 25%.
18. The proppant particle of claim 14, wherein a plurality of the
proppant particle has a bulk density of about 1.35 g/cc to about
2.1 g/cc.
19. The proppant particle of claim 18, wherein the proppant
particle has a specific gravity of about 2.5 g/cc to about 4.0
g/cc.
20. The proppant particle of claim 14, wherein the proppant
particle has a surface roughness of from about 0.8 microns to about
1.6 microns.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation application with priority
to U.S. patent application Ser. No. 14/802,761, filed Jul. 17,
2015, which is a Continuation-in-Part of U.S. patent application
Ser. No. 14/502,483, filed Sep. 30, 2014, which is a
Continuation-in-Part of U.S. patent application Ser. No.
13/608,530, filed Sep. 10, 2012, which is a Continuation-in-Part of
U.S. patent application Ser. No. 13/357,141, filed Jan. 24, 2012,
which is a Continuation-in-Part of U.S. patent application Ser. No.
13/045,980, filed Mar. 11, 2011. The above referenced applications
are each incorporated herein by reference.
TECHNICAL FIELD
[0002] This invention relates to hydraulic fracturing of
subterranean formations in the earth. More particularly, sintered
ceramic proppant particles formed from vibration-induced dripping
from a nozzle of a slurry of finely-divided ceramic material are
provided, along with methods of use of the particles.
BACKGROUND
[0003] Hydraulic fracturing is a process of pumping liquids down a
well and into a subterranean formation at high rate and pressure,
such that a fracture is formed in the rock around the well. After
pumping a liquid volume sufficient to widen the fracture
adequately, solid particles, called "proppant," are added to the
liquid. After pumping is completed, the well is opened for
production of hydrocarbons. The production rate of fluid from the
well is usually significantly increased after the fracturing
treatment. Vast improvements in the hydraulic fracturing process
have been developed since the process was originally patented in
1949 (U.S. Pat. Nos. 2,596,843 and 2,596,844).
[0004] The material first used for proppant in hydraulic fracturing
of wells was silica sand. As wells became deeper, sand was found to
have inadequate strength. In deep wells, stress of the earth causes
the sand to crush and become much less effective in increasing the
production rate of a well.
[0005] Synthetic proppant materials were developed to provide
higher strength proppants. The original synthetic sintered proppant
was sintered bauxite. In later years, a variety of ceramic raw
materials have been used to make sintered ceramic proppants,
including bauxite containing lesser amounts of alumina and clay
minerals, such as kaolin. Generally, it has been found that the
strength of ceramic particles increases with the amount of aluminum
oxide (alumina) in the particle, all other factors remaining
constant.
[0006] A general procedure for making synthetic proppant particles
is to obtain the ceramic raw material, grind it to a fine powder,
form it into pellets (called "green" pellets), and sinter the green
pellets in a kiln. The final product is ceramic pellets in the size
range suitable for proppants, from about 70 mesh to 12 mesh (0.008
inch to 0.067 inch in diameter). Different sizes of pellets are
used depending on well conditions.
[0007] A variety of processes for forming the pellets of a proppant
have been proposed. In early work, U.S. Pat. No. 4,427,068
describes a process for forming sintered ceramic pellets by adding
dry powders of clay and alumina, bauxite, or mixtures to a high
intensity mixer (hereinafter referred to as "dry mixing method").
Powdered fine grain ceramic starting ingredients (ceramic raw
materials) are stirred to form a dry homogenous mixture. Then,
sufficient water is added to cause agglomeration of the fine
starting dust particles to form small composite spherical pellets
from the powder. Continued mixing time is allowed in order to grow
small pellets to the desired size. A broad range of sizes is
produced during the pellet-forming stage. A preferred mixing device
is obtained from Eirich Machines, Inc., and is known as the Eirich
mixer. The resulting pellets are dried and sintered into the final
proppant particles. Much of the ceramic proppant made in industry
in past years has been made with this process of forming
pellets.
[0008] U.S. Pat. No. 4,440,866 discloses an alternative process for
producing pellets that are sintered to produce high strength
pellets. A continuous spray/granulation of an aqueous aluminous ore
suspension with binder is used to form granules that are
subsequently sintered (hereinafter referred to as "spray fluidized
bed method"). All steps of this process may be carried out in a
continuous manner. An aqueous suspension containing the ceramic raw
material is continuously atomized and fed into a layer of already
partially dried small starting dust particles (often called seeds)
that are fluidized in a stream of hot drying air. The aqueous
ceramic raw material suspension is continuously sprayed and dried
onto the seed particles until the desired finished green particle
diameter is achieved. Particles produced in this process have a
size range that is less broad than those typically produced by the
dry mixing method of U.S. Pat. No. 4,427,068 but are still of
sufficient variation as to require further processing. Particles
are continuously recovered from the fluidized layer and particles
of the desired size are separated from oversized and undersized
product fractions. Material is continuously recycled in the stream
of drying air. This spray fluidized bed process has also been used
to produce large amounts of ceramic proppants in industry.
[0009] The pellet-forming methods described above have intrinsic
limitations. The dry mixing process produces an extremely wide
range of green pellet sizes due to the random nature of the
agitation of the rotor and pan. The spray fluidized bed process
produces a somewhat tighter green pellet size distribution but
still a much wider distribution than desired. These processes
require extensive screening and recycling during the manufacturing
process. Under the best manufacturing conditions about 30% of green
particles must be recycled through the pellet-forming process. Both
the dry mixing and spray fluidized bed processes also produce a
random distribution of pore sizes in pellets, including a small
percentage of very large pores that significantly degrade pellet
strength. Strength of the sintered pellets is a primary
consideration, because if the pellets break under high stress in a
fracture, the flow capacity of the fracture is decreased and the
hydraulic fracturing treatment is less effective. The sphericity
and surface smoothness of particles produced by these processes are
also important, with high sphericity and a very smooth surface
traditionally being most desirable. All of these characteristics
are strongly affected by the pellet-forming method.
[0010] U. S. Pub. No. 2006/0016598 discloses a list of
pellet-forming techniques that may be used for ceramic proppant
formation, including agglomeration, spray granulation, wet
granulation, extruding and pelletizing, vibration induced dripping
according to U.S. Pat. No. 5,500,162, spray nozzle-formed droplets
and selective agglomeration. U.S. Pat. No. 5,500,162 discloses
producing microspheres by vibration-provoked dripping of a chemical
solution through a nozzle plate, wherein the falling drops form an
envelope surrounded from all sides by flowing reaction gas. The
liquid chemical solution has no or low (i.e., 20% or less) solid
particles at the time it enters the nozzle plate, exits the nozzle
plate, and passes through the first free fall section. The reaction
gas is required to cause the precipitation (gelling) of small solid
particles (typically sub-micron) in the liquid drops as they fall
through the second free fall zone, and thereafter fall into a
reaction liquid to further gel. The reaction gas is necessary to
cause the liquid to partially gel prior to entering the reaction
liquid, and the droplets are decelerated into the liquid through a
foam or the reaction liquid is directed onto the falling drops
tangentially in the same direction in which the droplets are
falling. These two features of falling through reaction gas and
decelerating the droplets into foam are required to insure the
droplets are partially gelled during a sol-gel reaction and
therefore not deformed, for example flattened, when they strike the
reaction liquid. The reaction gas is sucked away inside or outside
the envelope. The method according to the invention can be used to
produce, for example, aluminum oxide spheres up to the diameter of
5 mm.
[0011] Vibration-induced dripping, herein called "drip casting,"
was originally developed to produce nuclear fuel pellets. Since
then it has been adapted to produce a very wide variety of metal
and ceramic "microspheres," such as grinding media and catalyst
supports. Primarily, it has been used in the food and
pharmaceuticals industries. The drip casting process is described
on the website and in sales literature of Brace GmbH. Examples of
microspheres formed by drip casting of different materials are also
provided. U.S. Pat. No. 6,197,073 discloses a process for producing
aluminum oxide beads from an acid aluminum oxide sol or acid
aluminum oxide suspension by flowing the suspension through a
vibrating nozzle plate to form droplets and pre-solidifying the
droplets with gaseous ammonia and then coagulating the droplets in
an ammonia solution. The mechanical strength of ceramic particles
formed by sintering the drip cast particles was not a factor in any
of the materials used in these references.
[0012] It is known that to produce ceramic proppant particles
having maximum strength for a given ceramic material, the particles
must contain minimum porosity, and the pores present must be kept
as small as possible, since the strength of a given proppant
particle is limited by its largest pore. What is needed is a method
of forming green ceramic particles that can be fired to have
reduced pore size and therefore maximum strength for use as a
proppant. Preferably, the particles should be spherical, have a
smooth surface and have uniform size. A method for forming the
green particles without recycling of the undesired size fraction of
green ceramic pellets is also needed.
BRIEF SUMMARY OF THE INVENTION
[0013] A method for making proppant particles is disclosed herein.
The method can include providing a slurry of ceramic raw material,
the slurry containing a reactant including a polysaccharide,
wherein the slurry has a solids content from about 25 wt % to about
75 wt %, and flowing the slurry through a nozzle in a gas while
vibrating the slurry to form droplets in the gas. The method can
also include utilizing a surface tension of the slurry with the gas
to cause the droplets to acquire and maintain a spherical shape
until contact with an upper surface of a liquid, wherein gelling
commences in the droplets upon contact with the liquid to provide
gelled droplets, and wherein the liquid contains a coagulation
agent that reacts with the reactant in the slurry to cause gelling
of the reactant in the droplets, transferring the gelled droplets
from the liquid, and drying the gelled droplets to form green
pellets. The method can also include sintering the green pellets in
a selected temperature range to form the proppant particles, the
proppant particles having a porosity of at least about 5%.
[0014] Another method for making proppant particles is disclosed
herein. The method can include providing a slurry of alumina, the
slurry containing a reactant including a polysaccharide, wherein
the slurry has a solids content from about 25 wt % to about 75 wt
%, and flowing the slurry through a nozzle in a gas while vibrating
the slurry to form droplets in the gas. The method can also include
utilizing a surface tension of the slurry with the gas to cause the
droplets to acquire and maintain a spherical shape until contact
with an upper surface of a liquid, wherein gelling commences in the
droplets upon contact with the liquid to provide gelled droplets,
and wherein the liquid contains a coagulation agent that reacts
with the reactant in the slurry to cause gelling of the reactant in
the droplets, transferring the gelled droplets from the liquid, and
drying the gelled droplets to form green pellets. The method can
also include sintering the green pellets in a selected temperature
range to form the proppant particles, the proppant particles having
a porosity of at least about 5%.
[0015] A method of hydraulic fracturing is also disclosed herein.
The method can include injecting a hydraulic fluid into a
subterranean formation at a rate and pressure sufficient to open a
fracture therein and injecting a fluid containing a proppant
particle into the fracture. The proppant particle can include a
sintered ceramic material, a size of about 80 mesh to about 10
mesh, a porosity of at least 5%, and an average largest pore size
of less than about 20 microns.
[0016] A proppant particle is also disclosed herein. The proppant
particle can include a sintered ceramic material, a size of about
80 mesh to about 10 mesh, and an average largest pore size of less
than about 20 microns. Impinging a plurality of the proppant
particle under a gas-entrained velocity of about 260 m/s onto a
flat mild steel target can result in an erosivity of the target of
about 1 to about 100 mg of target material lost due to the
impinging per kg of the plurality of the proppant particle
impinging the target. Also, a plurality of the proppant particle
can lose less than 15% of its conductivity at 20,000 psi after
being subjected to 5 cycles of cyclic loading under stresses from
about 12,000 psi to about 20,000 psi, when the proppant particle
has a specific gravity of about 3.5.
[0017] A pack of proppant particles is also disclosed herein. The
pack of proppant particles can include a plurality of proppant
particles, each proppant particle of the pack can include a
sintered ceramic material, a size of about 80 mesh to about 10
mesh, and an average largest pore size of less than about 20
microns. The pack of proppant particles having a particle size of
20-40 mesh can have a long term permeability greater than 130
darcies at a stress of 10,000 psi and a temperature of 250.degree.
F., as measured in accord with ISO 13503-5, when the proppant
particles have a specific gravity of about 2.7. Impinging the
proppant particles under a gas-entrained velocity of about 260 m/s
onto a flat mild steel target can result in an erosivity of the
target of about 1 to about 100 mg of target material lost due to
the impinging per kg of the plurality of the proppant particle
impinging the target. Also, the pack can lose less than 15% of its
conductivity at 20,000 psi after being subjected to 5 cycles of
cyclic loading under stresses from about 12,000 psi to about 20,000
psi, when the proppant particles have a specific gravity of about
3.5.
[0018] Another method of hydraulic fracturing is also disclosed
herein. The method can include injecting a hydraulic fluid into a
subterranean formation at a rate and pressure sufficient to open a
fracture therein and injecting a fluid containing a proppant
particle into the fracture. The proppant particle can include a
sintered ceramic material, a size of about 80 mesh to about 10
mesh, and an average largest pore size of less than about 20
microns. Impinging a plurality of the proppant particle under a
gas-entrained velocity of about 260 m/s onto a flat mild steel
target can result in an erosivity of the target of about 1 to
about100 mg of target material lost due to the impinging per kg of
the plurality of the proppant particle impinging the target. Also,
a plurality of the proppant particle can lose less than 15% of its
conductivity at 20,000 psi after being subjected to 5 cycles of
cyclic loading under stresses from about 12,000 psi to about 20,000
psi, when the proppant particle has a specific gravity of about
3.5.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] The present invention may best be understood by referring to
the following description and accompanying drawings that are used
to illustrate embodiments of the invention. In the drawings:
[0020] FIG. 1 is a sketch showing the principles of the
pellet-forming apparatus for proppant particles disclosed
herein.
[0021] FIG. 2 is a sketch showing a single nozzle forming droplets
from a slurry stream.
[0022] FIG. 3 is a sketch showing a multi-nozzle plate forming
droplets from a slurry stream.
[0023] FIG. 4A shows a Scanning Electron Microscope photograph at
100.times. of sintered pellets of alumina formed by the apparatus
of FIG. 1.
[0024] FIG. 4B shows a Scanning Electron Microscope photograph at
100.times. of sintered pellets of alumina formed by prior art
methods.
[0025] FIG. 4C shows a Scanning Electron Microscope photograph at
100.times. of sintered pellets of bauxite formed by the apparatus
of FIG. 1.
[0026] FIG. 4D shows a Scanning Electron Microscope photograph at
100.times. of sintered pellets of bauxite formed by prior art
methods.
[0027] FIG. 4E shows a Scanning Electron Microscope photograph at
100.times. of sintered pellets of alumina formed by the apparatus
of FIG. 1.
[0028] FIG. 4F shows a Scanning Electron Microscope photograph at
100.times. of sintered pellets of kaolin formed by prior art
methods.
[0029] FIG. 5 is a graph of long term permeability as a function of
stress of alumina pellets formed by the pellet-forming apparatus
disclosed herein and by the prior art dry mixing process using an
Eirich mixer.
[0030] FIG. 6 is a frequency plot of pore size for proppant
particles of kaolin made by the method disclosed herein and by the
prior art spray fluidized bed method.
[0031] FIG. 7 is a graph of long term permeability as a function of
stress of proppant formed from kaolin and other materials and
having different alumina contents formed by the pellet-forming
apparatus disclosed herein and by the prior art dry mixing process
using an Eirich mixer.
[0032] FIG. 8 is a graph of long term permeability as a function of
stress of proppant formed from bauxite and other materials and
having different alumina contents formed by the pellet-forming
apparatus disclosed herein and by the prior art dry mixing process
using an Eirich mixer.
[0033] FIG. 9 is a graph of erosivity as a function of proppant
velocity for bauxite proppant formed by conventional methods and
alumina proppant formed by the drip cast method of FIGS. 1-3.
[0034] FIG. 10 is a graph showing the long term conductivity of
conventional bauxite proppant and drip cast alumina, each of 20/40
mesh sizing, after subjecting each to 50 hours of 20,000 psi
closure stress, followed by 5 cycles of cyclic loading under
stresses from about 12,000 psi to about 20,000 psi, and finally
re-measuring each under 20,000 psi closure stress to determine a
decrease in conductivity due to cycling.
[0035] FIG. 11 is a graph showing the long term conductivity of
conventional bauxite proppant and drip cast alumina, each of 20/40
mesh sizing, after subjecting each to 50 hours of 14,000 psi
closure stress, followed by 5 cycles of cyclic loading under
stresses from about 6,000 psi to about 14,000 psi, and finally
re-measuring each under 14,000 psi closure stress to determine a
decrease in conductivity due to cycling.
[0036] FIG. 12 is a graph showing the long term conductivity of
conventional bauxite proppant and drip cast alumina, each of 30/50
mesh sizing, after subjecting each to 50 hours of 20,000 psi
closure stress, followed by 5 cycles of cyclic loading under
stresses from about 12,000 psi to about 20,000 psi, and finally
re-measuring each under 20,000 psi closure stress to determine a
decrease in conductivity due to cycling.
[0037] FIG. 13 is a graph showing the beta factors of conventional
bauxite proppant and drip cast alumina, each of 20/40 mesh sizing,
after subjecting each to 50 hours of 20,000 psi closure stress,
followed by 5 cycles of cyclic loading under stresses from about
12,000 psi to about 20,000 psi, and finally re-measuring each under
20,000 psi closure stress to determine an increase in beta factors
due to cycling.
[0038] FIG. 14 is a graph showing the beta factors of conventional
bauxite proppant and drip cast alumina, each of 30/50 mesh sizing,
after subjecting each to 50 hours of 20,000 psi closure stress,
followed by 5 cycles of cyclic loading under stresses from about
12,000 psi to about 20,000 psi, and finally re-measuring each under
20,000 psi closure stress to determine an increase in beta factors
due to cycling.
DETAILED DESCRIPTION
[0039] Referring to FIG. 1, pellet-forming apparatus 10 having a
single nozzle is shown to illustrate the principles of the method
disclosed herein, which is commonly called "drip casting." Nozzle
12 receives slurry 15 from feed tank 14, which contains the ceramic
raw materials suspended in water. Pressure applied to feed tank 14
by pressure supply system 16 causes slurry to flow through nozzle
12 at a selected rate, preferably in laminar flow. Below nozzle 12
is coagulation vessel 17, which receives the droplets. Vibrator
unit 18 is connected to nozzle 12 and is used to supply pressure
pulses to the nozzle or directly in the slurry flowing to the
nozzle. The resulting vibration of the slurry flow through the
nozzle causes the stream exiting the nozzle 12 to break into
droplets of uniform size. As droplets fall toward coagulation
vessel 17, surface tension effects tend to form the droplets into
spheres. Spherical particles are formed without the necessity of a
sol-gel reaction, reaction gas free fall zone, foamed layer of
reaction liquid or reaction liquid directed onto the droplets prior
to entering the reaction liquid bath.
[0040] FIG. 2 shows details of slurry 15 exiting nozzle 12 and
breaking into drops. Surface tension of the slurry drives the drops
toward minimum surface area, which is acquired in a spherical
shape, as they fall toward coagulation vessel 17. The distance of
fall is preferably selected to be great enough to allow the
droplets to become spherical before entering a liquid in vessel
17.
[0041] Slurry 15 from feed tank 14 contains a finely ground
(0.01-50 microns in size) mineral or processed powder capable of
producing a strong ceramic material after sintering, a proper
amount of dispersant necessary for keeping the solid particles in
the slurry well separated, water, and a reactant that will react
with a component in liquid 19 in coagulation vessel 17 to form a
semi-solid or insoluble compound. The solids content of the
slurries may range from about 25% to about 75%. The viscosity of
the slurries will normally be from 1 to 1,000 centiPoise, but may
be higher. Lower viscosity of the slurry aids in improving droplet
formation and formation of spherical particles and is an essential
part of the invention claimed. Optimization of the dispersant type
and concentration will reduce viscosity. Dispersants may be
selected based on cost, availability and effectiveness in reducing
viscosity of a selected slurry. Dispersants that may be used to
reduce the viscosity of slurries include sodium silicate, ammonium
polyacrylate, sodium polymethacrylate, sodium citrate, sodium
polysulfonate and hexametaphosphate.
[0042] The commonly used reactant chemical in the slurry in feed
tank 14 is sodium alginate. This is a naturally occurring
polysaccharide that is soluble in water as the sodium salt but is
cross-linked to form a gel as the calcium salt. Alginate is
typically added to the slurry at levels of 0.1% to 1.0% (weight
percent alginate solid to total slurry). Coagulation tank 17
normally contains a coagulation liquid 19 which gels the reactant
chemical in the slurry 15. The commonly used coagulation liquid for
sodium alginate is a calcium chloride solution at concentration
levels of 0.5% to 10% by weight. A variety of reactants in the
slurry flowing through nozzle 12 and in the coagulation vessel 17
may be used. This may include other polysaccharides and other
cross-linking compounds such as polyvinyl alcohol or borate
fluids.
[0043] The diameter of nozzle 12, the viscosity of slurry 15, the
ceramic particle content of slurry 15, pressure to feed the slurry
to the nozzle, along with the frequency and amplitude of vibration
applied by vibrator source 17 are adjusted to produce droplets
having a desired size. These variables are preferably set at a
constant value as spheres are produced to be formed into a batch of
pellets of propping material. Different batches may be produced
having different size pellets. Preferably, each batch will be
monosized (i.e., contained on a single sieve such as passing
through a 20 mesh sieve but staying on a 25 mesh sieve). The
pressure used to feed slurry to the nozzle is adjusted to create
laminar flow through the nozzle. The feed pressure can range from 1
to 50 psi. The frequency is adjusted for each set of slurry
conditions such that a resonance is established in the slurry
stream exiting the nozzle that then produces spherical droplets.
The frequency can range from 10 to 20,000 Hz. The pressure and
frequency are optimized iteratively to create uniform spherical
shapes. The amplitude is adjusted to improve the uniform shape of
the spherical droplets formed. The flow rate of the slurry through
a nozzle is a function of the nozzle diameter, slurry feed
pressure, and the slurry properties such as viscosity and density.
For example, for kaolin and alumina slurries through nozzles up to
500 microns in diameter the flow rate per nozzle can range from 0.2
to 3 kg/hr.
[0044] The distance between nozzle 12 and the top of the liquid 19
in coagulation vessel 17 is selected to allow droplets to become
spherical before reaching the top of the liquid. The distance can
be from 1 to 20 cm, but is more typically in the range of 1 to 5 cm
so as to reduce distortion of the droplet shape upon impact with
the liquid surface, thereby eliminating the need for a reaction
gas, foam layer, or tangentially directed reaction liquid prior to
the droplets entering the coagulation vessel 17. The reactant
chemical in the droplets of slurry reacts with the coagulation
liquid 19 in the coagulation vessel 17 and a semi-solid surface is
formed on the droplets, which helps retain the spherical shape and
prevents agglomeration of the pellets. Preferably, the residence
time of pellets in coagulation vessel 17 is sufficient to allow
pellets to become rigid enough to prevent deformation of the
spherical shape when they are removed and dried, i.e., semi-rigid.
In some embodiments, pellets may fall into a coagulation liquid
solution flowing vertically upward so that settling of the particle
through the liquid will be retarded to produce a longer residence
time in the coagulation vessel.
[0045] Pellets formed using the apparatus of FIG. 1 are washed to
remove excess coagulation agent and conveyed to other devices where
they are dried and later sintered, using well known processes in
the industry.
[0046] FIG. 3 illustrates a multi-nozzle apparatus, which is
required to apply the process on a commercial scale. Multiple
nozzles 32 are placed in vessel 30, which operates under a
controlled pressure to flow slurry through the nozzles. Large
numbers of nozzles are required for commercial production of
proppant particles. Vessel 30 is vibrated to cause vibration of
nozzles, as described above. Alternatively, variable pressure may
be induced in the slurry to cause formation of uniform sized
droplets. The droplets are collected as described before.
[0047] Pellets produced by the process described in FIGS. 1-3 are
near uniform in size. For example, Table 1 compares the pellet size
distributions for sintered alumina proppant produced by the dry
mixing process and by the drip casting process described herein,
without screening of the green pellets. Without screening of the
green pellets, dry mixing produces fired proppant with a
distribution across six screens, whereas drip casting produces
fired proppant substantially on one screen. Therefore, in a
manufacturing process for proppant, drip casting does not require
sieving the green pellets to select the size range desired and then
recycling the material in green pellets outside the selected size
range. The size pellets to be sintered into proppant are selected
by controlling the diameter of nozzle 12 or 32, the viscosity of
slurry 15, the ceramic particle content of slurry 15, pressure to
feed the slurry to the nozzle, along with the frequency and
amplitude of vibration applied by vibrator source 17.
[0048] The green pellets produced by the process described in FIGS.
1-3 can be sintered in any suitable manner and under any suitable
conditions to provide proppant particles. In one or more exemplary
embodiments, the green pellets withdrawn from the coagulation
liquid are dried and subsequently sintered at temperatures of less
than 2,000.degree. C., less than 1,900.degree. C., less than
1,800.degree. C., less than 1,600.degree. C., less than
1,500.degree. C., less than 1,450.degree. C., less than
1,400.degree. C., or less than 1,350.degree. C. to provide the
proppant particulates. In one or more exemplary embodiments, the
green pellets are sintered at a temperature of about 1,000.degree.
C., about 1,200.degree. C., about 1,250.degree. C., about
1,300.degree. C., or about 1,350.degree. C. to about 1,400.degree.
C., about 1,450.degree. C., about 1,500.degree. C., or about
1,600.degree. C.
[0049] The sintered pellets or proppant particles produced by the
process described in FIGS. 1-3 can have any suitable size. The
proppant particles produced by the process described in FIGS. 1-3
can have a size of at least about 100 mesh, at least about 80 mesh,
at least about 60 mesh, at least about 50 mesh, or at least about
40 mesh. For example, the proppant particles can have a size from
about 115 mesh to about 2 mesh, about 100 mesh to about 3 mesh,
about 80 mesh to about 5 mesh, about 80 mesh to about 10 mesh,
about 60 mesh to about 12 mesh, about 50 mesh to about 14 mesh,
about 40 mesh to about 16 mesh, or about 35 mesh to about 18
mesh.
TABLE-US-00001 TABLE 1 Sieve Distribution of Sintered Pellets
(Proppant Particles) Formed by Dry Mixing and Drip Casting 16 Mesh
20 Mesh 25 Mesh 30 Mesh 35 Mesh 40 Mesh 50 Mesh Pan Dry Mixing 0%
17.8% 23.9% 24.3% 18.4% 10.6% 4.9% 0% Drip Casting 0% 0% 0.2% 99.8%
0% 0% .sup. 0% 0%
[0050] The proppant particles produced by the process described in
FIGS. 1-3 can have any suitable composition. The proppant particles
can be or include silica and/or alumina in any suitable amounts.
According to one or more embodiments, the proppant particles
include less than 80 wt %, less than 60 wt %, less than 40 wt %,
less than 30 wt %, less than 20 wt %, less than 10 wt %, or less
than 5 wt % silica based on the total weight of the proppant
particles. According to one or more embodiments, the proppant
particles include from about 0.1 wt % to about 70 wt % silica, from
about 1 wt % to about 60 wt % silica, from about 2.5 wt % to about
50 wt % silica, from about 5 wt % to about 40 wt % silica, or from
about 10 wt % to about 30 wt % silica. According to one or more
embodiments, the proppant particles include at least about 30 wt %,
at least about 50 wt %, at least about 60 wt %, at least about 70
wt %, at least about 80 wt %, at least about 90 wt %, or at least
about 95 wt % alumina based on the total weight of the proppant
particles. According to one or more embodiments, the proppant
particles include from about 30 wt % to about 99.9 wt % alumina,
from about 40 wt % to about 99 wt % alumina, from about 50 wt % to
about 97 wt % alumina, from about 60 wt % to about 95 wt % alumina,
or from about 70 wt % to about 90 wt % alumina. In one or more
embodiments, the proppant particles produced by the process
described in FIGS. 1-3 can include alumina, bauxite, or kaolin, or
any mixture thereof. For example, the proppant particles can be
composed entirely of or composed essentially of alumina, bauxite,
or kaolin, or any mixture thereof. The term "kaolin" is well known
in the art and can include a raw material having an alumina content
of at least about 40 wt % on a calcined basis and a silica content
of at least about 40 wt % on a calcined basis. The term "bauxite"
is well known in the art and can be or include a raw material
having an alumina content of at least about 55 wt % on a calcined
basis.
[0051] The proppant particles produced by the process described in
FIGS. 1-3 can have any suitable specific gravity. The proppant
particles can have a specific gravity of at least about 2.5, at
least about 2.7, at least about 3, at least about 3.3, or at least
about 3.5. For example, the proppant particles can have a specific
gravity of about 2.5 to about 4.0, about 2.7 to about 3.8, about
3.5 to about 4.2, about 3.8 to about 4.4, or about 3.0 to about
3.5. In one or more exemplary embodiments, the proppant particles
can have a specific gravity of less than 4 g/cc, less than 3.5
g/cc, less than 3 g/cc, less than 2.75 g/cc, less than 2.5 g/cc,
less than 2.25 g/cc, less than 2 g/cc, less than 1.75 g/cc, or less
than 1.5 g/cc. For example, the proppant particles can have a
specific gravity of about 1.3 g/cc to about 3.5 g/cc, about 1.5
g/cc to about 3.2 g/cc, about 1.7 g/cc to about 2.7 g/cc, about 1.8
g/cc to about 2.4 g/cc, or about 2.0 g/cc to about 2.3 g/cc.
[0052] The proppant particles produced by the process described in
FIGS. 1-3 can have any suitable bulk density. In one or more
exemplary embodiments, the proppant particles have a bulk density
of less than 3 g/cc, less than 2.5 g/cc, less than 2.2 g/cc, less
than 2 g/cc, less than 1.8 g/cc, less than 1.6 g/cc, or less than
1.5 g/cc. The proppant particles can have a bulk density of about 1
g/cc, about 1.15 g/cc, about 1.25 g/cc, about 1.35 g/cc, or about
1.45 g/cc to about 1.5 g/cc, about 1.6 g/cc, about 1.75 g/cc, about
1.9 g/cc, or about 2.1 g/cc or more. For example, the proppant
particles can have a bulk density of about 1.3 g/cc to about 1.8
g/cc, about 1.35 g/cc to about 1.65 g/cc, or about 1.5 g/cc to
about 1.9 g/cc.
[0053] The proppant particles produced by the process described in
FIGS. 1-3 can have any suitable porosity. In one or more exemplary
embodiments, the porosity of the proppant particles can be at least
about 1%, at least about 5%, at least about 10%, at least about
15%, at least about 20%, at least about 25%, at least about 30%, at
least about 35%, or at least about 40%. The proppant particles can
have a porosity of about 2%, about 5%, about 10%, about 15%, about
20%, or about 25% to about 30%, about 35%, about 40%, about 45%, or
about 50% or more. The proppant particles can have a porosity of
about 5% to about 15%, of about 10% to about 20%, of about 15% to
about 25%, of about 20% to about 30%, or of about 25% to about 35%.
In one or more exemplary embodiments, the porosity of the proppant
particles can be an interconnected porosity.
[0054] FIGS. 4(a-e) show photographs of alumina, bauxite, and
kaolin proppant particles produced by the apparatus of FIG. 1 and
by prior art methods. FIG. 4(a) shows an alumina proppant particle
made by drip casting, as illustrated in FIG. 1, which has high
sphericity and a very smooth surface. FIG. 4(b) shows an alumina
proppant particle made by an Eirich mixer. The surfaces of the
particles are rough and the shapes are generally oblate. FIG. 4(c)
shows a bauxite proppant particle made by drip casting and FIG.
4(d) shows a bauxite proppant particle made by a commercial prior
art process using an Eirich mixer (CARBO HSP.RTM., sold by CARBO
Ceramics Inc., Houston, Tex.). FIG. 4(e) shows a kaolin proppant
particle made by drip casting and FIG. 4(f) shows a kaolin proppant
particle made by a pilot scale fluidized bed process.
[0055] The proppant particles produced by the process described in
FIGS. 1-3 can have any suitable surface roughness. The proppant
particles can have a surface roughness of less than 5 .mu.m, less
than 4 .mu.m, less than 3 .mu.m, less than 2.5 .mu.m, less than 2
.mu.m, less than 1.5 .mu.m, or less than 1 .mu.m. For example, the
proppant particles can have a surface roughness of about 0.1 .mu.m
to about 4.5 .mu.m, about 0.4 .mu.m to about 3.5 .mu.m, or about
0.8 .mu.m to about 2.8 .mu.m.
[0056] The surface roughness of each whole proppant particle shown
in FIGS. 4(a-f) was measured. A smooth, convex perimeter was drawn
around each proppant particle, establishing an average surface
level that mimicked the actual proppant particle surface as closely
as possible while still remaining convex. Then the separation
between the actual surface and the smooth, average surface was
measured around the entire perimeter at intervals of 100 .mu.m at
100.times. magnification used in FIG. 4, the separation could be
measured with a precision of about 0.5 .mu.m. The average of the
measurements from the entire perimeter is representative of the
surface roughness of the proppant particle. Table 2 shows that
proppant particles formed by dry mixing and spray fluidized bed
have surface roughness from three to seven times as large as their
drip cast counterparts.
TABLE-US-00002 TABLE 2 Surface Roughness of Drip Cast and
Conventionally- Formed Proppant Particles Average Surface Roughness
(.mu.m) Drip Cast Alumina (FIG. 4a) 1.4 Dry Mixing-Formed Alumina
(FIG. 4b) 5.8 Drip Cast Bauxite (FIG. 4c) 1.6 Dry Mixing-Formed
Bauxite (FIG. 4d) 4.9 Drip Cast Kaolin (FIG. 4e) 0.8 Spray Fluid
Bed-Formed Kaolin (FIG. 4f) 5.7
[0057] FIG. 5 compares the permeability of proppant particles
formed in the apparatus of FIG. 1 compared with proppant particles
formed by the dry mixing process. The proppant particles from the
two processes are identical in size and composition, both being a
high purity (99+%) alumina. The only variable is the pellet
formation process. The permeabilities were measured in accordance
with ISO 13503-5: "Procedures for Measuring the Long-term
Conductivity of Proppants," except that steel wafers were used
rather than sandstone wafers. The long term conductivity apparatus
described in ISO 13503-5 utilizes a steel conductivity cell that
contains an internal slot of dimensions 7 inches in length by 1.5
inches in width. An open port is placed in the cell extending from
the each end of the slot to the exterior of the cell to allow for
fluid flow through the slot. Other ports are placed along the
length of the slot also extending to the exterior of the cell for
the measurement of the internal pressure of the slot. Into this
slot are fitted a lower and upper piston, the lengths which extend
out beyond the dimensions of the cell such that a load may be
applied directly to the pistons by a hydraulic load frame. To load
the conductivity cell for the measurement of conductivity the lower
piston is first secured into the cell so as not to obstruct the
fluid or pressure ports. A seal ring is installed to prevent
pressure or fluid leakage between slot and the piston wall. A slot
sized metal shim and a sandstone wafer are then placed on the lower
piston. Alternatively a steel wafer may replace the sandstone wafer
(as was the case here). A set amount of proppant is then placed on
the wafer. In this case equal volumes of the two proppants were
loaded representing initial pack widths of about 0.19 inches. The
proppant is leveled. Then on top of the proppant is placed a second
steel wafer, metal shim, seal ring, and the upper piston. An
initial load is applied to the pistons and fluid is flowed through
the proppant pack while pressure is measured. The temperature of
the fluid and cell was maintained at 250.degree. F. Measurement of
the rate of fluid flow and pressure loss provides a measure of the
proppant pack conductivity in millidarcy-feet. The permeability of
the proppant pack is calculated by dividing the conductivity by the
measured width of the pack, which was about 0.16-0.19 inch for the
data shown in FIG. 5. The flowing fluid was a silica saturated
deoxygenated aqueous solution of 2% KCl. Conductivity was measured
at stresses of 2,000 psi to 20,000 psi in increments of 2,000 psi.
In each case the stress was held for 50 hours before measuring the
conductivity. Permeability of a proppant pack decreases as closure
stress increases due to failure of the proppant grains. Stronger
pellets will result in a higher permeability. As can be seen in
FIG. 5, proppant particles made by dry mixing (line 2) lose 78% of
their permeability as the closure stress increases from 2,000 psi
to 20,000 psi. By contrast the proppant particles made from the
apparatus in FIG. 1 (line 1) lose only 31% of their
permeability--less than one half of the permeability loss of the
proppant particles made by dry mixing. This higher permeability of
the proppant particles made from the apparatus of FIG. 1 is due to
the improved strength of the proppant particle.
[0058] The proppant particles formed by the drip cast methods
disclosed herein can have any appropriate permeability. Proppant
particles formed by the drip cast methods and having a specific
gravity of less than 2.7 can have a long term permeability greater
than about 130 darcies, about 150 darcies, about 170 darcies, about
190 darcies, about 195 darcies, about 200 darcies, about 225
darcies, or about 250 darcies at a stress of 10,000 psi and a
temperature of 250.degree. F., as measured in accord with ISO
13503-5. Proppant particles formed by the drip cast methods and
having a specific gravity of about 2.7 can have a long term
permeability greater than about 130 darcies, about 150 darcies,
about 170 darcies, about 190 darcies, about 195 darcies, about 200
darcies, about 225 darcies, or about 250 darcies at a stress of
10,000 psi and a temperature of 250.degree. F., as measured in
accord with ISO 13503-5. Proppant particles formed by the drip cast
methods and having a specific gravity of about 3.3 can have a long
term permeability greater than about 110 darcies, about 120
darcies, about 130 darcies, about 140 darcies, about 150 darcies,
about 155 darcies, about 165 darcies, or about 170 darcies at a
stress of 14,000 psi and a temperature of 250.degree. F., as
measured in accord with ISO 13503-5. Proppant particles formed by
the drip cast methods and having a specific gravity of about 3.5
can have a long term permeability greater than about 80 darcies,
about 90 darcies, about 100 darcies, about 110 darcies, about 115
darcies, about 120 darcies, about 130 darcies, about 140 darcies,
about 150 darcies, about 160 darcies, about 170 darcies, or about
185 darcies at a stress of 20,000 psi and a temperature of
250.degree. F., as measured in accord with ISO 13503-5.
[0059] The proppant particles formed by the drip cast methods
disclosed herein can have any appropriate strength. An appropriate
strength can include a decrease of less than 85%, less than 80%, or
less than 75% of long term liquid permeability, as measured in
accord with ISO 13503-5 at 250.degree. F., of a pack of test
particles, the test particles having the same composition and
method of making as the proppant particles, when a stress applied
to the pack of test particles increases from 2,000 psi to 12,000
psi and the test particles are in the size range of 20-40 mesh and
have a specific gravity of about 2.7. An appropriate strength can
also include a decrease of less than 75%, less than 65%, or less
than 55% of long term liquid permeability, as measured in accord
with ISO 13503-5 at 250.degree. F., of a pack of test particles,
the test particles having the same composition and method of making
as the proppant particles, when a stress applied to the pack of
test particles increases from 2,000 psi to 14,000 psi and the test
particles are in the size range of 20-40 mesh and have a specific
gravity of about 3.3. An appropriate strength can also include a
decrease of less than 90%, less than 80%, less than 75%, less than
70%, less than 65%, or less than 60% of long term liquid
permeability, as measured in accord with ISO 13503-5 at 250.degree.
F., of a pack of test particles, the test particles having the same
composition and method of making as the proppant particles, when a
stress applied to the pack of test particles increases from 12,000
psi to 20,000 psi and the test particles are in the size range of
20-40 mesh and have a specific gravity of above about 3.5.
[0060] The strength of a proppant particle can be indicated from
the proppant crush resistance test described in ISO 13503-2:
"Measurement of Properties of Proppants Used in Hydraulic
Fracturing and Gravel-packing Operations." In this test a sample of
proppant is first sieved to remove any fines (undersized pellets or
fragments that may be present), then placed in a crush cell where a
piston is then used to apply a confined closure stress of some
magnitude above the failure point of some fraction of the proppant
particles. The sample is then re-sieved and weight percent of fines
generated as a result of proppant particle failure is reported as
percent crush. A comparison the percent crush of two equally sized
samples is a method of gauging the relative strength. For the two
samples of proppant particles used in the conductivity test
described above the weight percent crush at 15,000 psi of the
proppant particles produced by dry mixing was 2.7% as compared to
0.8% for the drip cast proppant particles. This again indicates
that drip casting produces a stronger proppant particles.
[0061] Relative proppant strength can also be determined from
single proppant particle strength measurements. Strength
distributions of forty proppant particles from each of the two
samples of proppant used in the conductivity test described above
were measured, tabulated, and analyzed using Weibull statistics for
the determination of a characteristic strength. The characteristic
strength of the drip cast proppant particles so determined was 184
MPa as compared to 151 MPa for the proppant particles made by dry
mixing.
[0062] The proppant particles formed by the drip cast methods
disclosed herein can have any suitable pore size distribution. For
example, the proppant particles can have a standard deviation in
pore size of less than 6 .mu.m, less than 4 .mu.m, less than 3
.mu.m, less than 2.5 .mu.m, less than 2 .mu.m, less than 1.5 .mu.m,
or less than 1 .mu.m. The proppant particles formed by the drip
cast methods disclosed herein can have any suitable average maximum
or largest pore size. For example, the proppant particles can have
an average largest pore size of less than about 25 .mu.m, less than
about 20 .mu.m, less than about 18 .mu.m, less than about 16 .mu.m,
less than about 14 .mu.m, or less than about 12 .mu.m. The proppant
particles formed by the drip cast methods disclosed herein can have
any suitable concentration of pores. For example, the proppant
particles can have less than 5,000, less than 4,500, less than
4,000, less than 3,500, less than 3,000, less than 2,500, or less
than 2,200 visible pores at a magnification of 500.times. per
square millimeter of proppant particulate.
[0063] Fracture mechanics teaches that particles fail under stress
from the largest flaw in the particle. In proppant particles, the
largest flaw is believed to be the largest pore. Therefore, the
stress at failure is inversely proportional to the square root of
the size of the largest flaw. So, the ratio (R) of the stress at
failure of a drip cast proppant (DC) formed by the apparatus
disclosed herein to a conventionally (CONV) made proppant (dry
mixing or spray fluid bed processes) would be:
R=(Max pore size.sub.DC/Max pore size.sub.CONV).sup.1/2
[0064] Proppant particles made by the drip casting process and
prior art processes were examined by a scanning electron microscope
(SEM) at a magnification of 500.times.. To measure pore size
distribution in particles, cross-sections of alumina, bauxite and
kaolin proppant particles made by each process were examined in the
SEM. For each sample, a random area of approximately 252
.mu.m.times.171 .mu.m from each of ten different pellets was
photographed. The ten largest pores in each area were measured and
the equation above was used to calculate the theoretical ratio of
stress at failure of drip cast proppant particles versus
conventionally made proppant particles. The results are presented
in Table 3. For example, the average maximum pore size in the drip
cast alumina proppant particles was 16.3 .mu.m and for the dry
mixing process alumina proppant particles average maximum pore size
was 40.8 .mu.m. Using the equation above, the ratio of the stress
to failure of the drip cast proppant particles to the dry mixing
process proppant particles is 1.6. Thus fracture mechanics predicts
that drip cast high alumina proppant particles should withstand
approximately 1.6 times more stress without fracturing than dry
mixing process made proppant particles.
TABLE-US-00003 TABLE 3 Pore Sizes of Proppant Particles Formed by
Drip Casting, Dry Mixing, and Spray Fluid Bed Alumina Bauxite
Kaolin Drip Dry Drip Dry Drip Dry Cast Mixed Cast Mixed Cast Mixed
Average Largest Pore 16.3 40.8 14.3 37.5 11.1 56.0 (.mu.m) Average
of 10 Largest 10.4 19.1 9.1 20.5 6.0 18.4 Pores (.mu.m) Theoretical
Ratio of 1.6x 1.6x 2.2x Drip Cast Strength to Conventional Strength
(.mu.m)
[0065] Additional measurements were carried out on the kaolin
samples. In these, every visible pore was measured and the
composite data from all ten areas was used to calculate average
pore size, standard deviation in pore size, and number of pores per
square millimeter, as well as the largest pore data, which are
presented in Table 3. A summary of the data is presented in Table
4, and FIG. 6 shows plots of the pore size distributions for drip
cast kaolin (Curve 1) and spray fluid bed kaolin (Curve 2). The
small percentage of very large pores generated by the spray fluid
bed process shown in FIG. 6 (Curve 2) is readily visible in the
microstructures in FIG. 4f. The lack of large pores in the drip
cast material provides the strength advantage discussed above.
TABLE-US-00004 TABLE 4 Additional Pore Size Measurements for Drip
Cast and Spray Fluid Bed Kaolin Drip Cast Spray Fluid Kaolin Bed
Kaolin Average Pore Size (.mu.m) 2.0 2.8 Standard Deviation in Pore
Size (.mu.m) 1.8 6.4 Average Number of Pores Per Square 2121 5133
Millimeter
[0066] Proppant made from kaolin has a cost advantage over proppant
containing higher alumina contents, which are made from higher-cost
ores containing higher percentages of alumina. Four proppant
products having three ranges of alumina content are sold by Carbo
Ceramics, for example (data from www.carboceramics.com, searched
Dec. 19, 2011). Higher alumina content proppants generally sell for
higher prices and cost more to manufacture. The lowest alumina
contents are in the products ECONOPROP and CARBOLITE, in which the
alumina content is about 48 and 51 percent, respectively. A higher
alumina content is in CARBOPROP, in which the alumina content is
about 72 percent. The CARBOPROP is a more expensive product to make
primarily because of higher raw material costs.
[0067] The property of a proppant that is most directly related to
its performance in hydraulic fractures is permeability under
stress. Long-term permeability data for pure alumina proppant made
by a prior art method and by the drip-casting process disclosed
herein are shown in FIG. 5. FIG. 7 shows long-term permeability
data, measured using the same procedures as used to obtain the data
in FIG. 5, for proppant having different alumina contents and made
by different processes. Curve 1 represents published permeability
of 20/40 mesh ECONOPROP proppant (made from kaolin, having an
alumina content of about 48 percent) made by the Eirich-mixer
process described above. Curve 2 represents permeability of 20/40
mesh CARBOPROP proppant (made from a mixture of ores having an
alumina content of about 72 percent). Curve 3 represents the
average permeability vs stress of 15 samples of proppant (made from
kaolin, having an alumina content of about 48 percent) made by the
drip cast method disclosed herein. The drip cast process produces a
proppant made from kaolin that has about the same permeability
under stress as the higher-cost product containing 72 percent
alumina. The average long-term permeability measured at 10,000 psi
stress of 15 samples was 173 darcies. This is far above the
published long-term permeability at 10,000 psi stress (85 darcies)
of the commercial proppant (ECONOPROP) having about the same
alumina content, as can be seen by comparing Curve 3 and Curve
1.
[0068] FIG. 8 shows long-term permeability data, measured by the
same procedures as used to obtain the data in FIGS. 5 and 7, for
proppant having different alumina contents and made by different
processes. Curve 1 represents published permeability data for 20/40
mesh CARBOPROP proppant formed by the Eirich mixer process
described above (made from a mixture of ores having an alumina
content of about 72 percent). Curve 2 represents permeability data
for proppant (primarily sieved on a 25-mesh screen) made by the
drip cast method disclosed herein using bauxite with an alumina
content of 70 percent. Curve 3 represents permeability data for
20/40 mesh proppant made by the Eirich mixer process and having an
alumina content of about 83 percent alumina. The permeability of
the proppant made by the drip cast method and having an alumina
content of only 70 percent exhibits practically the same
permeability behavior as the prior art proppant made with an Eirich
mixer and having about 83 percent alumina. Since alumina is a more
expensive component of proppants, there is considerable saving by
using lower cost raw materials and the drip cast process disclosed
herein. Comparison of Curves 1 and 2 shows the benefits of the drip
cast process with about the same alumina content in the
proppant.
[0069] Methods of hydraulic fracturing using the proppant
particulates disclosed herein are also provided. The methods can
include injecting a hydraulic fluid into a subterranean formation
at a rate and pressure sufficient to open a fracture therein and
injecting the proppant particulates disclosed herein into the
fracture of the subterranean formation. Downhole tools and
equipment in place during fracturing operations oftentimes erode
due at least in part to proppant particles impinging onto the
metallic surfaces of the downhole tools and equipment when injected
during the hydraulic fracturing operation. These proppant particles
oftentimes travel at high velocities, sufficient to damage or
destroy the downhole tools and equipment. These downhole tools and
equipment include, but are not limited to, the well casing,
measurement tools, bridge plugs, frac plugs, setting tools,
packers, and gravel pack and frac-pack assemblies and the like.
Applicants have discovered that hydraulic fracturing with the
proppant produced by the drip cast methods disclosed herein instead
of conventionally made proppant particles demonstrates a surprising
and unexpected reduction in erosion to the downhole tools and
equipment. For example, replacing conventionally made proppant
particles with proppant particles made by the drip cast methods
disclosed herein can result in at least a 10%, at least a 20%, at
least a 30%, at least a 40%, or at least a 50% reduction in
erosivity to the downhole tools and equipment under same or similar
hydraulic fracturing conditions.
[0070] FIG. 9 is a graph of erosivity as a function of proppant
velocity for bauxite proppant formed by conventional methods and
alumina proppant formed by the drip cast method of FIGS. 1-3. In
this testing the wear of flat targets made of mild steel was
measured individually for each proppant at three separate proppant
velocities. The proppant was fed into a 20' long tube which had a
nitrogen gas stream of set velocity. The proppant was accelerated
by the gas stream and would exit the tube 1'' from the target at an
incident angle of 45 degrees. The proppant was fed in ten separate,
25 gram increments for a total of 250 grams for each test. Three
different nitrogen gas velocities were used to evaluate the wear
caused by each of the proppant samples. The wear was measured by
measuring the weight of the steel targets before and after impact
by the proppant samples. Erosivity was expressed as the ratio of
the weight loss of the target in milligrams to the weight of
proppant impacting the target in kilograms. The results are shown
in Table 5. The results show that the use of the proppant particles
produced by the drip cast method of FIGS. 1-3 result in a reduction
of erosivity of up to about 86%.
TABLE-US-00005 TABLE 5 Pre-test Post-test Total mass of Gas coupon
coupon Mass proppant propelled Erosivity Sample velocity (m/s) mass
(g) mass (g) loss (g) onto coupon (g) (mg/kg) Conventional 150
54.5619 54.5592 0.0027 250 10.8 Proppant 200 57.757 57.7455 0.0115
250 46 260 56.8724 56.8306 0.0418 250 167.2 Drip Cast 150 57.7018
57.7011 0.0007 250 2.8 Proppant 200 53.0541 53.0525 0.0016 250 6.4
260 52.3513 52.3327 0.0186 250 74.4
[0071] Impinging the gas-entrained proppant particles formed by the
drip cast methods at a velocity of about 160 meters per second
(m/s) onto a flat mild steel target can result in an erosivity of
about 0.01 milligrams lost from the flat mild steel target per
kilogram of proppant contacting the target (mg/kg), about 0.05
mg/kg, about 0.1 mg/kg, about 0.5 mg/kg, about 1 mg/kg, or about 2
mg/kg to about 5 mg/kg, about 7 mg/kg, about 10 mg/kg, about 12
mg/kg, or about 15 mg/kg. Impinging the gas-entrained proppant
particles formed by the drip cast methods at a velocity of about
200 m/s onto the flat mild steel target can result in an erosivity
of about 0.01 mg/kg, about 0.05 mg/kg, about 0.1 mg/kg, about 0.5
mg/kg, about 1 mg/kg, or about 2 mg/kg to about 5 mg/kg, about 7
mg/kg, about 10 mg/kg, about 12 mg/kg, or about 15 mg/kg. Impinging
the gas-entrained proppant particles formed by the drip cast
methods at a velocity of about 260 m/s onto the flat mild steel
target can result in an erosivity of about 1 mg/kg, about 5 mg/kg,
about 10 mg/kg, about 20 mg/kg, about 40 mg/kg, or about 60 mg/kg
to about 65 mg/kg, about 70 mg/kg, about 80 mg/kg, about 90 mg/kg,
or about 100 mg/kg.
[0072] In the normal operation of hydraulically fractured oil and
gas wells the operating pressures occurring in the well can vary
significantly. For example, oil and gas wells can cycle from a
shut-in condition, in which the pressure within the well is
maintained at a maximum, to a producing condition, in which the
pressure within the well is much lower. Further, the flowing
conditions can change resulting in cycles of a higher or lower
pressure within the well. This "pressure cycling" of a
hydraulically fractured well is known to cause damage to proppant
in the fracture due to rearrangement and re-stressing of the
proppant grains. This results in a less conductive proppant pack in
the fracture and adversely impacts production performance of the
well. Consequently a proppant that is resistant to pressure cycling
conductivity loss is desirable.
[0073] A pack of the proppant particles formed by the drip cast
methods disclosed herein can also have increased conductivity after
cyclic loading conditions when compared to a pack of conventionally
made proppant particles. For example, a pack of the proppant
particles with a specific gravity above 3.5 formed by conventional
methods can lose at least 16% of its conductivity at 20,000 psi
after being subjected to 5 cycles of cyclic loading under stresses
from about 12,000 psi to about 20,000 psi. Also, a pack of the
proppant particles with a specific gravity above 3.5 formed by
conventional methods can lose at least 10% of its conductivity at
14,000 psi after being subjected to 5 cycles of cyclic loading
under stresses from about 6,000 psi to about 14,000 psi. A pack of
the proppant particles with a specific gravity above 3.5 formed by
the drip cast methods disclosed herein can lose less than 15%, less
than 12%, less than 10%, or less than 8% of its conductivity at
20,000 psi after being subjected to 5 cycles of cyclic loading
under stresses from about 12,000 psi to about 20,000 psi. Also, a
pack of the proppant particles with a specific gravity above 3.5
formed by the drip cast methods disclosed herein can lose less than
10%, less than 8%, less than 6%, less than 4%, less than 2%, less
than 1%, or less than 0.1% of its conductivity at 14,000 psi after
being subjected to 5 cycles of cyclic loading under stresses from
about 6,000 psi to about 14,000 psi.
[0074] FIG. 10 is a graph showing the long term conductivity of
conventional bauxite proppant and drip cast alumina, each of 20/40
mesh sizing, after subjecting each to 50 hours of 20,000 psi
closure stress, followed by 5 cycles of cyclic loading under
stresses from about 12,000 psi to about 20,000 psi, and finally
re-measuring each under 20,000 psi closure stress to determine a
decrease in conductivity due to cycling. First, it can be observed
that the conductivity of the drip cast proppant is substantially
greater at 20,000 psi than the two conventional proppants. Second,
it can be seen that the drip cast proppant lost only 7% of its
conductivity due to the stress cycling whereas the two conventional
bauxite proppants lost 17% of their conductivity. Similarly, FIG.
11 is a graph showing the long term conductivity of conventional
bauxite proppant and drip cast alumina, each of 20/40 mesh sizing,
after subjecting each to 50 hours of 14,000 psi closure stress,
followed by 5 cycles of cyclic loading under stresses from about
6,000 psi to about 14,000 psi, and finally re-measuring each under
14,000 psi closure stress to determine a decrease in conductivity
due to cycling. First, it can be observed that the conductivity of
the drip cast proppant is substantially greater at 14,000 psi than
the two conventional proppants. Second, it can be seen that the
drip cast proppant exhibited essentially no loss of conductivity
due to the stress cycling whereas the two conventional bauxite
proppant lost 10% of their conductivity. Also, FIG. 12 is a graph
showing the long term conductivity of conventional bauxite proppant
and drip cast alumina, each of 30/50 mesh sizing, after subjecting
each to 50 hours of 20,000 psi closure stress, followed by 5 cycles
of cyclic loading under stresses from about 12,000 psi to about
20,000 psi, and finally re-measuring each under 20,000 psi closure
stress to determine a decrease in conductivity due to cycling.
First, it can be observed that the conductivity of the drip cast
proppant is substantially greater at 20,000 psi than the
conventional proppant. Second, it can be seen that the drip cast
proppant exhibited 5% loss of conductivity due to the stress
cycling whereas the conventional bauxite proppant lost 20%.
[0075] The flow of reservoir fluids through the proppant pack in a
hydraulic fracture generally occurs at velocities that are much
greater than those occurring in the reservoirs. At these very low
fluid velocities occurring in the reservoir pressure drops are
dominated by viscous flow behavior. This permits the pressure
behavior to be adequately described by Darcy's law as shown:
.DELTA.p/L=.mu.v/k, where:
[0076] 4p/L is the change in pressure per unit length, .mu. is the
fluid viscosity, v is the fluid velocity and k is the permeability
of the pack. However, inertial flow effects dominate the velocities
oftentimes found in the fracture and the Forchheimer equation is
therefore employed:
.DELTA.p/L=.mu.v/k+.beta..rho.v.sup.2
[0077] The first term in the Forchheimer equation is identical to
Darcy's law. The Forchheimer equation adds an inertial pressure
drop term that includes a velocity squared function, v.sup.2, and
the density of the fluid, p. At high velocities this inertial term
will dominate the pressure drop and thus dictate fluid flow. Also
included in the inertial term is the Forchheimer beta factor,
.beta.. Similar to permeability, the beta factor is an intrinsic
property of the porous media that will vary as a function of
confining stress. As shown by the Forchheimer equation, pressure
change (.DELTA.p) decreases as permeability increases and beta
factor decreases. Thus in high fluid velocity conditions, such as
those in a propped hydraulic fracture where inertial forces will
dominate, a low beta factor will reduce pressure losses in the
fracture resulting in higher flow rates.
[0078] A pack of the proppant particles formed by the drip cast
methods disclosed herein can also have a reduced beta factor after
cyclic loading conditions when compared to conventionally made
proppant. For example, a pack of the proppant particles formed by
conventional methods in the size range of 20/40 mesh can have an
increase in beta factor at least 0.0004 at 20,000 psi after being
subjected to 5 cycles of cyclic loading under stresses from about
12,000 psi to about 20,000 psi. Also, a pack of the proppant
particles formed by conventional methods in the size range of 30/50
mesh can have an increase in beta factor of at least 0.0004 at
20,000 psi after being subjected to 5 cycles of cyclic loading
under stresses from about 12,000 psi to about 20,000 psi. A pack of
the proppant particles formed by the drip cast methods disclosed
herein in the size range of 20/40 mesh can have an increase in beta
factor of less than 0.0005, less than 0.0002, less than 0.0001,
less than 0.00005, or less than 0.00001 at 20,000 psi after being
subjected to 5 cycles of cyclic loading under stresses from about
12,000 psi to about 20,000 psi. Also, a pack of the proppant
particles formed by the drip cast methods disclosed herein in the
size range of 30/50 mesh can have an increase in beta factor of
less than 0.0006, less than 0.0004, or less than 0.0002 at 20,000
psi after being subjected to 5 cycles of cyclic loading under
stresses from about 12,000 psi to about 20,000 psi.
[0079] FIG. 13 is a graph showing the beta factors of conventional
bauxite proppant and drip cast alumina, each of 20/40 mesh sizing,
after subjecting each to 50 hours of 20,000 psi closure stress,
followed by 5 cycles of cyclic loading under stresses from about
12,000 psi to about 20,000 psi, and finally re-measuring each under
20,000 psi closure stress to determine an increase in beta factors
due to cycling. First, it can be observed that the beta factor of
the drip cast proppant is substantially lower at 20,000 psi than
the two conventional proppants. Second, it can be seen that the
beta factor for the drip cast proppant increased only slightly when
compared to the increase in post cycling beta factor for the two
conventional bauxites. Similarly, FIG. 14 is a graph showing the
beta factors of conventional bauxite proppant and drip cast
alumina, each of 30/50 mesh sizing, after subjecting each to 50
hours of 20,000 psi closure stress, followed by 5 cycles of cyclic
loading under stresses from about 12,000 psi to about 20,000 psi,
and finally re-measuring each under 20,000 psi closure stress to
determine an increase in beta factors due to cycling. First, it can
be observed that the beta factor of the drip cast proppant is
substantially lower at 20,000 psi than the two conventional
proppants. Second, it can be seen that the beta factor for the drip
cast proppant increased only slightly when compared to the increase
in beta post cycling for the two conventional bauxites.
[0080] It is understood that modifications to the invention may be
made as might occur to one skilled in the field of the invention
within the scope of the appended claims. All embodiments
contemplated hereunder which achieve the objects of the invention
have not been shown in complete detail. Other embodiments may be
developed without departing from the spirit of the invention or
from the scope of the appended claims. Although the present
invention has been described with respect to specific details, it
is not intended that such details should be regarded as limitations
on the scope of the invention, except to the extent that they are
included in the accompanying claims.
* * * * *
References