U.S. patent application number 11/192711 was filed with the patent office on 2007-02-01 for sintered spherical pellets useful for gas and oil well proppants.
This patent application is currently assigned to CARBO Ceramics Inc.. Invention is credited to Steve Canova, Claude A. Krause.
Application Number | 20070023187 11/192711 |
Document ID | / |
Family ID | 37693033 |
Filed Date | 2007-02-01 |
United States Patent
Application |
20070023187 |
Kind Code |
A1 |
Canova; Steve ; et
al. |
February 1, 2007 |
Sintered spherical pellets useful for gas and oil well
proppants
Abstract
Sintered, spherical composite pellets having high strength and
low density, are described, along with processes for their
manufacture. One method includes forming a green pellet from a
mixture of clay, bauxite or a clay-bauxite mixture with a
sacrificial phase such that upon sintering of the pellet, the
sacrificial phase is removed from the pellet. The use of such
sintered pellets in hydraulic fracturing of subterranean formations
is also described.
Inventors: |
Canova; Steve; (Gray,
GA) ; Krause; Claude A.; (St. Martinsville,
LA) |
Correspondence
Address: |
HAYNES AND BOONE, LLP
901 MAIN STREET, SUITE 3100
DALLAS
TX
75202
US
|
Assignee: |
CARBO Ceramics Inc.
Irving
TX
|
Family ID: |
37693033 |
Appl. No.: |
11/192711 |
Filed: |
July 29, 2005 |
Current U.S.
Class: |
166/280.2 ;
507/204; 507/924 |
Current CPC
Class: |
C04B 38/009 20130101;
C04B 2235/77 20130101; C04B 2235/96 20130101; C09K 8/805 20130101;
C04B 38/0058 20130101; C04B 38/0074 20130101; C04B 38/0054
20130101; C04B 38/068 20130101; C04B 33/04 20130101; C04B 38/009
20130101; C04B 33/04 20130101; C04B 2235/5472 20130101; C04B
35/62635 20130101; C04B 35/62695 20130101; C04B 35/636 20130101;
C04B 2111/00724 20130101; C09K 8/80 20130101 |
Class at
Publication: |
166/280.2 ;
507/924; 507/204 |
International
Class: |
E21B 43/267 20060101
E21B043/267 |
Claims
1. A gas and oil well proppant comprising a plurality of composite,
sintered, spherical pellets, said pellets being prepared from a
mixture of at least one of clay and bauxite, and a sacrificial
phase material.
2. The proppant of claim 1, wherein the pellets are made from a
mixture that comprises a sacrificial phase material selected from
the group consisting of coal, wheat flour, rice hulls, wood fiber
and sugar.
3. The proppant of claim 1, wherein the pellets are made from a
mixture that comprises from about 5 to about 35 percent by weight
of the sacrificial phase material.
4. The proppant of claim 1, wherein the pellets are made from a
mixture that comprises from about 20 to about 25 percent by weight
of the sacrificial phase material.
5. The proppant of claim 1, wherein the pellets comprise a
clay-bauxite mixture containing from 0 to 100 percent by weight of
clay and from 0 to 100 percent by weight of bauxite.
6. The proppant of claim 1, wherein the pellets have an apparent
specific gravity of from about 1.80 to about 2.50.
7. The proppant of claim 1, wherein the pellets have a bulk density
of from about 1.05 to about 1.35 g/cm.sup.3.
8. The proppant of claim 1, wherein the sacrificial phase material
comprises coal and the pellets have a crush of less than 4.0
percent by weight at a pressure of 4,000 psi.
9. The proppant of claim 1, wherein the pellets are coated with a
resin.
10. The proppant of claim 9, wherein the resin is selected from the
group consisting of phenol-aldehyde resins, urea-aldehyde-resins,
melamine-aldehyde resins, epoxy resins, furfuryl alcohol resins,
polyester resins, alkyd resins and copolymers of such resins.
11. A method for propping fractures in subterranean formations
comprising: mixing with a fluid a proppant comprising a plurality
of composite, sintered, spherical pellets, the pellets being
prepared from a mixture of at least one of clay and bauxite, and a
sacrificial phase material, and introducing the mixture into a
fracture in a subterranean formation.
12. The method of claim 11, wherein the mixture from which the
pellets are prepared comprises a sacrificial phase material
selected from the group consisting of coal, wheat flour, rice
hulls, wood fiber and sugar.
13. The method of claim 11, wherein the mixture from which the
pellets are prepared comprises from about 5 to about 35 percent by
weight of the sacrificial phase material.
14. The method of claim 11, wherein the mixture from which the
pellets are prepared comprises from about 20 to about 25 percent by
weight of the sacrificial phase material.
15. The method of claim 11, wherein the mixture from which the
pellets are prepared comprises a clay-bauxite mixture containing
from 0 to 100 percent by weight of clay and from 0 to 100 percent
by weight of bauxite.
16. The method of claim 11, wherein the pellets have an apparent
specific gravity of from about 1.80 to about 2.50.
17. The method of claim 11, wherein the pellets have a bulk density
of from about 1.05 to about 1.35 g/cm.sup.3.
18. The method of claim 11, wherein the sacrificial phase material
comprises coal and the pellets have a crush of less than 4.0
percent by weight at a pressure of 4,000 psi.
19. The method of claim 11, wherein the pellets are coated with a
resin.
20. The method of claim 19, wherein the resin is selected from the
group consisting of phenol-aldehyde resins, urea-aldehyde-resins,
melamine-aldehyde resins, epoxy resins, furfuryl alcohol resins,
polyester resins, alkyd resins and copolymers of such resins.
21. A method for making a gas and oil well proppant comprising a
plurality of composite, sintered, spherical pellets comprising: (a)
forming a mixture of at least one of clay and bauxite, and a
sacrificial phase material in a high intensity mixture; (b) while
stirring the mixture adding sufficient water to cause formation of
composite spherical pellets from the mixture; (c) drying the
pellets at a temperature ranging from about 100.degree. C. to about
300.degree. C.; and (d) sintering the dried pellets at a
temperature ranging from about 2,400.degree. F. to about
2,800.degree. F. for a period sufficient to enable recovery of
sintered spherical pellets.
22. The method of claim 21, wherein the mixture of at least one of
clay and bauxite, and a sacrificial phase material comprises a
sacrificial phase material selected from the group consisting of
coal, wheat flour, rice hulls, wood fiber and sugar.
23. The method of claim 21, wherein the mixture of at least one of
clay and bauxite, and a sacrificial phase material comprises from
about 5 to about 35 percent by weight of the sacrificial phase
material.
24. The method of claim 21, wherein the mixture of at least one of
clay and bauxite, and a sacrificial phase material comprises from
about 20 to about 25 percent by weight of the sacrificial phase
material.
25. The method of claim 21, wherein the mixture of at least one of
clay and bauxite, and a sacrificial phase material comprises a
clay-bauxite mixture containing from 0 to 100 percent by weight of
clay and from 0 to 100 percent by weight of bauxite.
26. The method of claim 21, wherein the pellets have an apparent
specific gravity of from about 1.80 to about 2.50.
27. The method of claim 21, wherein the pellets have a bulk density
of from about 1.05 to about 1.35 g/cm.sup.3.
28. The method of claim 21, wherein the sacrificial phase material
comprises coal and the pellets have a crush of less than 4.0
percent by weight at a pressure of 4,000 psi.
29. The method of claim 21, wherein the pellets are coated with a
resin.
30. The method of claim 29, wherein the resin is selected from the
group consisting of phenol-aldehyde resins, urea-aldehyde-resins,
melamine-aldehyde resins, epoxy resins, furfuryl alcohol resins,
polyester resins, alkyd resins and copolymers of such resins.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to oil and gas well proppants
and, more particularly, to sintered proppants in a broad range of
applications.
[0002] Oil and natural gas are produced from wells having porous
and permeable subterranean formations. The porosity of the
formation permits the formation to store oil and gas, and the
permeability of the formation permits the oil or gas fluid to move
through the formation. Permeability of the formation is essential
to permit oil and gas to flow for production of the well. Sometimes
the permeability of the formation holding the gas or oil is
insufficient for economic recovery of oil and gas. In other cases,
during operation of the well, the permeability of the formation
drops to the extent that further recovery becomes uneconomical. In
such cases, it is necessary to fracture the formation and prop the
fracture in an open condition by means of a proppant material or
propping agent. Such fracturing is usually accomplished by
hydraulic pressure, and the proppant material or propping agent is
a particulate material, which is carried into the fracture in a
slurry of fluid and propping agent. This propping agent must have
sufficient strength to resist crushing by the closure stresses of
the formation. The deeper the well, generally the stronger the
proppant needs to be to resist crushing. Thus, the proppants used
in shallower wells need not be quite as strong as the proppants
used in deeper wells.
[0003] It has long been known that sintered bauxite having an
alumina content of about 85% is strong enough to withstand crushing
at well depths of greater than 20,000 feet. However, these high
strength propping agents have high densities, i.e. apparent
specific gravities above 3.4 g/cc, and require high viscosity
pumping fluids or high pumping rates to keep them in suspension
during the pumping operation. The use of higher viscosity pumping
fluids required to transport the high density proppants can cause
more damage to the formation fractured face and the resulting
propped fracture as residues from the high viscosity fluids become
concentrated along the fracture face during pumping and if not
adequately broken remain within the propped fracture, therefore
reducing the propped fracture permeability. Because of the
disadvantages associated with the use of high viscosity fracture
fluids, the use of high density proppants are limited to use in
wells where high strength is the controlling attribute. As a result
of the negative effects of high viscosity fracture fluids, a
variety of proppants have been developed with lower densities and
less strength for use in shallower wells. These lower density
proppants will require lower viscosity fracture fluids that will
generate less damage to fracture surface and the final propped
fracture.
[0004] Intermediate density proppants, generally having an apparent
specific gravity of from about 3.1 to 3.4 g/cc, have been found to
have sufficient strength to provide adequate permeability at
intermediate depths and pressures. In these intermediate density
proppants, the density was lowered primarily by reducing the
alumina content to about 75%, as described in U.S. Pat. No.
4,427,068 to Fitzgibbon. Intermediate density proppants are
generally recommended for use in wells having a depth of from about
8,000 to about 12,000 feet.
[0005] A low density proppant is described in U.S. Pat. No.
5,120,455, which issued to Lunghofer, using kaolin clay having a
50% alumina content. This low density proppant has an apparent
specific gravity of 2.62 to 2.80 g/cc and is used in wells having a
depth of up to about 8,000 feet.
[0006] An even lower density proppant, having an apparent specific
gravity of from 2.20 to 2.60 g/cc, is described in U.S. Pat. No.
5,188,175 to Sweet, using a starting material having an alumina
content of from 25% to 40%. As noted in U.S. Pat. No. 5,188,175,
the reduced density means that the pumping fluid can be less
viscous and the pumping rate can be lowered, both of which are cost
saving features. Therefore, there is a desire for a proppant that
has an even lower density than the Sweet proppant, such as an
apparent specific gravity of 2.10 g/cc or less.
[0007] As can be seen from the prior art, reducing the alumina
content of the material generally results in a lower density
proppant. However, when the alumina content is reduced too much
there is generally a concomitant increase in silica content which
leads to a rather significant loss of strength. Therefore, efforts
to develop an even lighter proppant by using lower alumina content
materials generally have failed. Nevertheless, there is a need for
a very low density proppant having an apparent specific gravity of
2.10 g/cc or less, that is strong enough to be used in shallow
wells, for instance, wells at depths of up to about 7500 feet.
DETAILED DESCRIPTION
[0008] In accord with the present invention, composite, spherical
pellets or particles, having apparent specific gravities of about
1.80 to about 2.50, are produced. The spherical particles are
useful as oil and gas well proppants. The proppant of the present
embodiments has moderate strength and is effective at closure
stresses of up to about 5000 psi.
[0009] The proppant comprises substantially round and spherical
sintered pellets formed from naturally occurring materials and
includes about 65 to 95 weight percent of clay, bauxite or
clay-bauxite mixtures and from about 5 to about 35 weight percent
of a sacrificial phase material. The ingredients for forming the
proppant particles have an average particle size of less than about
15 microns and, preferably, less than about 10 microns and, most
preferably, less than about 5 microns. In general, the proppant can
be made from any aluminosilicate material that can be combined with
a sacrificial phase material, that will pelletize into spherical
particles, and that can be dried and sintered to remove the
sacrificial phase material from the pellet so as to form a porous
final pellet having desired properties, such as those described
herein.
[0010] Suitable clay materials for use in the compositions for
producing the proppant of the present embodiments include kaolin
clay, diaspore clay, burley clay and flint clay.
[0011] Suitable bauxite materials for use in the compositions for
producing the proppant of the present embodiments include natural
bauxite which contains mainly alumina (Al.sub.2O.sub.3) and various
impurities including iron oxide, aluminum silicate, titanium
dioxide and quartz.
[0012] In another embodiment of the present invention, the bauxite
materials may be substituted with an alumina material. A suitable
alumina material for use in the compositions for producing the
proppant of the present embodiments is the alumina fines dust
collector by-product of alumina purification using the Bayer
process. According to the Bayer process, the aluminum component of
bauxite ore is dissolved in sodium hydroxide, impurities are
removed from the solution and alumina trihydrate is precipitated
from the solution and then calcined to aluminum oxide. A Bayer
process plant is essentially a device for heating and cooling a
large recirculating stream of caustic soda solution. Bauxite is
added at the high temperature point, red mud is separated at an
intermediate temperature, and alumina is precipitated at the low
temperature point in the cycle. The alumina fines that are useful
for the preparation of the proppant pellets according to the
present embodiments are a by-product this process. A preferred
alumina fines product has an alumina content of about 99% and a
loss on ignition of about 13%-22%. The term "loss on ignition"
refers to a process, well known to those of ordinary skill in the
art, in which samples are dried at about 100.degree. C. to drive
off free moisture and are then heated to about 1000.degree. C. to
drive off chemically bound water and other compounds. For the
purpose of this patent application, the term "bauxite" will be
understood to include the alumina fines dust collector by-product
described above.
[0013] According to certain embodiments, the clay or bauxite
materials may be calcined, partially calcined or uncalcined. If the
materials are calcined, the materials may be calcined by methods
well known to those of ordinary skill in the art, at temperatures
and times to remove sufficient water of hydration to facilitate
pelletization and achieve a higher strength final product.
[0014] Suitable sacrificial phase materials for use in the
compositions for producing the proppant of the present embodiment
include coal, wheat flour, rice hulls, wood fiber, sugar and other
organic or inorganic materials that will ignite and burn or can
otherwise be removed from the pellets leaving behind pores in its
place. Such materials are referred to as constituting a
"sacrificial phase" as they can be removed from the pellets to
generate porosity and consequently reduce the density of the
pellets. In certain embodiments, wheat flour is the sacrificial
phase material. In certain embodiments, the composition for
producing proppant may include 10 weight percent of wheat flour. In
certain embodiments, coal is the sacrificial phase material as it
ignites and burns leaving behind pores and an ash residue at
typical sintering temperatures of the pellets. The coal thus lends
a desired degree of porosity to the proppant pellets. In certain
embodiments, the compositions for producing proppant may include 5,
10, 15, 20, 25, or 35 weight percent of coal.
[0015] Those of ordinary skill in the art will recognize that other
suitable sacrificial phase materials for use in the compositions
for producing the proppant of the present embodiments include any
material that partially or wholly decomposes to a gas during
heating.
[0016] The materials for use in the compositions for producing the
proppant of the present embodiments are compatible with, and may be
used as a matrix for, a wide variety of proppant materials, and, in
this manner, a wide variety of composite proppants may be produced,
which may be customized to particular conditions or formations.
Thus, the properties of the final sintered composite pellets, such
as strength, porosity, apparent specific gravity, and bulk density
may be controlled through variations in the initial component
mixture.
[0017] Unless stated otherwise, all percentages, proportions and
values with respect to composition are expressed in terms of
weight.
[0018] One advantage of the lower density proppant of the present
embodiments is that fewer pounds of this proppant are required, as
compared to higher density proppants, to fill a given void in a
formation. Since proppants are generally sold by the pound, the
user buys fewer pounds of proppant for a particular application.
Another advantage of this low density proppant is the ability to
use a lower viscosity fluid during pumping operations, resulting in
lower overall fluid costs, reduced damage to the fracture interface
and propped fracture pack versus the use of heavier or denser
proppants.
[0019] The present invention also provides a process for propping
fractures in oil and gas wells at depths of up to about 7,500 feet
utilizing the proppant of the present embodiments. According to
such processes, a viscous fluid, frequently referred to as a "pad",
is injected into the well at a rate and pressure to initiate and
propagate a fracture in the subterranean formation. The fracturing
fluid may be an oil base, water base, acid, emulsion, foam, or any
other fluid. Injection of the fracturing fluid is continued until a
fracture of sufficient geometry is obtained to permit placement of
the propping pellets. Thereafter, pellets as hereinbefore described
are placed in the fracture by injecting into the fracture a fluid
into which the pellets have previously been introduced and
suspended. The propping distribution is usually, but not
necessarily, a multi-layer pack. Following placement of the
pellets, the well is shut-in for a time sufficient to permit the
pressure in the fracture to bleed off into the formation. This
causes the fracture to close and apply pressure on the propping
pellets which resist further closure of the fracture. In wells at
depths as described above, the compressive stress upon the proppant
generally is up to about 5,000 psi.
[0020] In a method of the present embodiments, the sintered,
spherical pellets are produced according to the following
method:
[0021] 1. Uncalcined, partially calcined or calcined clay, bauxite
or clay-bauxite mixtures and the sacrificial phase material are
ground into a fine particle size dust, such as a dust in which
about 90-100% of the particles have a size of less than 325 mesh.
The clay, bauxite or clay-bauxite mixtures and sacrificial phase
material can be ground independently and blended, or they can be
co-milled. In either case, the sacrificial phase material is
homogenously mixed with and distributed in the blend of clay,
bauxite or clay-bauxite mixtures. The clay, bauxite or clay-bauxite
mixtures and sacrificial phase material along with water are added
in a predetermined ratio to a high intensity mixer.
[0022] 2. The clay, bauxite or clay-bauxite mixtures, sacrificial
phase material and water are stirred to form a wet homogeneous
particulate mixture. Suitable commercially available intensive
stirring or mixing devices have a rotatable horizontal or inclined
circular table and a rotatable impacting impeller, such as
described in U.S. Pat. No. 3,690,622, to Brunner, the entire
disclosure of which is incorporated herein by reference.
[0023] 3. While the mixture is being stirred, sufficient water is
added to cause formation of composite, essentially spherical
pellets of desired size from the mixture of clay, bauxite or
clay-bauxite mixtures and sacrificial phase material. The intense
mixing action quickly disperses the water throughout the
particles.
[0024] In general, the total quantity of water which is sufficient
to cause essentially spherical pellets to form is from about 15 to
about 30 percent by weight of the mixture of clay, bauxite or
clay-bauxite mixtures and the sacrificial phase material. The total
mixing time usually is from about 2 to about 15 minutes. Those of
ordinary skill in the art will understand how to determine a
suitable amount of water to add to the mixer so that substantially
round and spherical pellets are formed.
[0025] Optionally, a binder, for example, various resins or waxes,
starch, or polyvinyl alcohol, may be added to the initial mixture
to improve the formation of pellets and to increase the green
strength of the unsintered pellets. Suitable binders include but
are not limited to corn starch, polyvinyl alcohol or sodium
silicate solution, or a blend thereof. Liquid binders can be added
to the mixture and bentonite and/or various resins or waxes known
and available to those of ordinary skill in the art may also be
used as a binder. A suitable binder is corn starch which may be
added at levels of from about 0 percent by weight to 1.5 percent by
weight. In certain embodiments, the starch may be added at an
amount of from about 0.5 percent by weight to 0.7 percent by
weight. In other embodiments, a suitable binder may be added at an
amount of from about 0.25 percent by weight to about 1.0 percent by
weight of the raw material, or any other amount so as to assist
formation of the pellets. Whether to use more or less binder than
the values reported herein can be determined by one of ordinary
skill in the art through routine experimentation.
[0026] 4. The resulting pellets are dried and screened to an
appropriate pre-sintering size that will compensate for shrinkage
that occurs during sintering in the kiln. Rejected oversized and
undersized pellets and powdered material obtained after the drying
and screening steps may be recycled. The pellets may also be
screened either before drying or after firing or both.
[0027] 5. The dried pellets are then fired at a sintering
temperature for a period sufficient to enable recovery of sintered,
spherical pellets having an apparent specific gravity of between
1.80 and 2.50 and a bulk density of from about 1.05 to about 1.35
g/cm.sup.3. The specific time and temperature to be employed is
dependent on the relative amounts of clay, bauxite or clay-bauxite
mixtures and sacrificial phase material and is determined
empirically according to the results of physical testing of pellets
after firing. The finished pellets may be tumbled to enhance
smoothness.
[0028] According to the present embodiments, when the sacrificial
phase material is coal, upon sintering of the green pellets at a
temperature of about 2400.degree. F. to about 2800.degree. F., the
coal is ignited and burned, producing carbon dioxide (CO.sub.2),
varying amounts of sulfur dioxide (SO.sub.2), depending on where it
was mined, and ash. The burning of the coal thus leaves a small
amount of ash and pores in its place. Because the coal is
homogenously distributed in the green pellets, the pores left
behind after sintering are homogenously distributed throughout the
sintered pellets resulting in porous sintered pellets having low
density and high strength. The pore structure left behind by the
coal has been determined by apparent specific gravity and mercury
porosimetry tests to be relatively unconnected. Also, as confirmed
by helium pycnometer, the proppant pellets are fully sintered.
[0029] The utility of the proppants of the present embodiments can
be extended into high compressive stress applications by adding a
resin coating to the proppant. The resin coating may be cured or
curable. In one embodiment, the proppant pellets are coated with a
resin dissolved in a solvent. In this embodiment, the solvent is
evaporated and then the resin is cured. In another embodiment, the
proppant pellets are mixed with a melted resin, the melted resin is
cooled to coat the pellets, and, then the resin coating is cured.
Alternatively, the resin coating is curable, but not substantially
cured prior to use. In this embodiment, the resin is cured after
injection into the well formation by techniques well known to those
of ordinary skill in the art.
[0030] Resins suitable for coating the proppant pellets are
generally any resins capable of being coated on the substrate and
then being cured to a higher degree of polymerization such as epoxy
or phenolic resins. Examples of such resins include phenol-aldehyde
resins of both the resole and novolac type, urea-aldehyde resins,
melamine-aldehyde resins, epoxy resins, furfuryl alcohol resins,
polyester resins and alkyd resins as well as copolymers of such
resins. The resins should form a solid non-tacky coating at ambient
temperatures so that the coated particles remain free flowing and
do not agglomerate under normal storage conditions.
[0031] In certain embodiments, the resins are phenol-formaldehyde
resins. These resins include true thermosetting phenolic resins of
the resole type and phenolic novolac resins that may be made
reactive to heat by the addition of catalyst and formaldehyde.
Suitable phenol-formaldehyde resins have softening points of
185.degree. F. to 290.degree. F.
[0032] In certain embodiments, the resin is a phenolic novolac
resin. Suitable phenolic novolac resins are commercially available
from Jinan Shengquan Hepworth Chemical Co., Ltd under the trade
name PF-0987 and Georgia-Pacific Corporation under the trade names
GP-2202 and GP-2207. When such resins are used, it is usually
necessary to add to the mixture a cross-linking agent to effect the
subsequent curing of the resin. Hexamethylenetetramine is a
suitable cross-linking agent and serves as a catalyst and a source
of formaldehyde.
[0033] In other embodiments, the resins are resole phenolic resins.
Suitable resole phenolic resins are commercially available from a
number of suppliers. Suitable resole resins are generally provided
in a solution of water and methanol as the solvent system. Suitable
organic solids levels are from 65 to 75%, with a water content in
the 5 to 15% level. A suitable hot plate cure time at 150.degree.
C. is in the range of 25 to 40 seconds.
[0034] The resin coating may be formed by a variety of methods. For
example, a suitable solvent coating process is described in U.S.
Pat. No. 3,929,191, to Graham et al., the entire disclosure of
which is incorporated herein by reference.
[0035] Other suitable processes such as that described in U.S. Pat.
No. 3,492,147 to Young et al., the entire disclosure of which is
incorporated herein by reference, involve the coating of a
particulate substrate with a liquid, uncatalyzed resin composition
characterized by its ability to extract a catalyst or curing agent
from a non-aqueous solution.
[0036] As stated above, suitable resins for use in embodiments of
the present invention include phenol-formaldehyde novolac resins.
When using such resins a suitable coating method is a hot melt
coating procedure. A suitable hot melt coating procedure is
described in U.S. Pat. No. 4,585,064, to Graham et al, the entire
disclosure of which is incorporated herein by reference. Solvents
may also be used to apply the resin coat. The following is a
discussion of typical coating process parameters using
phenol-formaldehyde novolac resins.
[0037] The coating of resin may be formed on the particulate
substrate by first preheating the particulate substrate to a
temperature above the melting point of the particular resin used.
Typically the particulate substrate is heated to 350.degree. F. to
500.degree. F. prior to resin addition. The heated substrate is
charged to a mixer or muller and then the resin is added at a rate
of from about 1% to about 6% by weight of substrate. A particularly
suitable amount of resin is about 2% by weight of substrate. After
completion of addition of the resin to the substrate, the substrate
and melted resin are allowed to mix in the muller for a time
sufficient to insure the formation of a uniform coating of resin on
the particulate, usually about 10 to about 30 seconds.
[0038] Following the mixing step, hexamethylenetetramine is added
to the substrate resin mixture at a rate of from about 5 to about
25% by weight of the resin. A particularly suitable amount of
hexamethylenetetramine is about 13% by weight of the resin. After
addition of the hexamethylenetetramine, the entire mixture is
allowed to mull for approximately one to five minutes until the
resin coating is completely cured. It is anticipated that by resin
coating the proppant particles of the present embodiments, the
resin will penetrate at least some of the open surface porosity of
the particles and seal or encapsulate some of the open surface
porosity, thus leading to a reduction of the apparent specific
gravity (ASG) of the particles.
[0039] The sintered composite proppant pellets of the present
embodiments are spherical in shape. The term "spherical," as used
herein refers to both roundness and sphericity and is used to
designate proppant pellets having an average ratio of minimum
diameter to maximum diameter of about 0.8 on the Krumbein and Sloss
chart (Krumbein and Sloss, Stratigraphy and Sedimentation, second
edition, 1955, W. H. Freeman & Co., San Francisco, Calif.) as
determined by visually grading 10 to 20 randomly selected
particles.
[0040] According to one embodiment, porosity on the surface of the
proppant is controlled such that the apparent specific gravity of
the proppant pellets is reduced. According to this embodiment, the
proppant pellets are sintered to final stage, and the sintered
pellets have a surface porosity of between about 6.0% and about
15.0% by volume of the pellets comprising the proppant. In some
embodiments, the sintered proppant pellets have a surface porosity
between about 6.6% and 21.8% by weight of the pellets comprising
the proppant.
[0041] The term "apparent specific gravity," as used herein, is a
number without units, but is defined to be numerically equal to the
weight in grams per cubic centimeter of volume, excluding void
space or open porosity in determining the volume. The apparent
specific gravity values given herein were determined by the
Archimedes method of liquid (water) displacement according to API
RP60, a method which is known to those of ordinary skill in the
art.
[0042] The term "bulk density", as used herein, is defined to mean
the weight per unit volume, including in the volume considered, the
void spaces between the particles. The bulk density values reported
herein were determined according to the ANSI B74.4 method by
weighing that amount of a sample that would fill a cup of known
volume. The overall particle size of the pellets is between about
0.1 and about 2.5 millimeters and, more preferably, between about
0.15 and about 1.7 millimeters.
[0043] For purposes of this disclosure, methods of testing the
characteristics of the proppant pellets in terms of apparent
specific gravity, bulk density, and crush strength are the standard
API tests that are routinely performed on proppant samples.
[0044] Another important characteristic of any proppant is its
conductivity to fluids at various closure stresses. A conductivity
test is routinely run on proppants to determine the decrease of
fluid flow rate through the proppant sample as the pressure (or
closure stress) on the proppant pack is increased. In the
conductivity test, a measured amount of proppant, e.g. two pounds
per sq. ft., is placed in a cell and a fluid (usually deionized
water) is passed through the proppant pack at various flow rates.
As pressure on the pack is increased, it causes the proppant to
crush, thereby decreasing the flow capacity that is being measured.
The conductivity of a proppant generally provides a good indicator
of its crush strength, and also provides valuable information about
how the proppant will perform in a subterranean formation. The
proppant of the present embodiments has a low density which allows
for good proppant transport while the strength and sphericity
results in good retained conductivity.
[0045] The following example is illustrative of the methods and
compositions discussed above.
EXAMPLE 1
[0046] A raw material blend comprising food grade wheat flour or
Wyoming Powder River Basin low sulfur coal and calcined kaolin clay
which is commercially available as Mulcoa.RTM. 47MK from C-E
Minerals was prepared. A kaolin clay product which is commercially
available as Mulcoa.RTM. CK 46 could also be used. In each case,
the raw material blend was added to a jar mill to reduce the
particle size to a sufficiently small size to feed a fluid energy
mill. The raw material was then fed to a fluid energy mill for
final grinding and blending to create a homogeneous mixture.
[0047] The homogeneous mixture was then fed to an Eirich R02, a
high intensity mixer commercially available from Eirich Machines,
Inc. In the present example, the mixer had a horizontal or inclined
circular table that can rotate at a speed of from about 10 to about
72 revolutions per minute (rpm), and a rotatable impacting impeller
that can rotate at a tip speed of from about 5 to about 50 meters
per second. The direction of rotation of the table was opposite
that of the impeller, causing material added to the mixer to flow
over itself in a countercurrent manner. The central axis of the
impacting impeller was generally located within the mixer at a
position off-center from the central axis of the rotatable table.
The table could be in a horizontal or inclined position, wherein
the incline, if any, was between 0 and 35 degrees from horizontal.
For forming the proppant of this Example 1, the table was rotated
at from about 20 to about 72 rpm, at an incline of about 30 degrees
from horizontal. The impacting impeller was initially rotated at
about 27 meters per second tip speed, and was adjusted as described
below, during addition of water containing dissolved starch to the
mixer.
[0048] While the raw material was being stirred in the Eirich R02,
water was intermittently added to the mixer in an amount sufficient
to cause formation of spherical pellets. In this particular
example, the water was fresh water containing starch binder, and
was fed to the mixer in an amount sufficient to maintain a
percentage based on the weight of the raw material in the mixer
from about 15 to about 30 percent by weight of the raw materials,
although this amount can vary. The water included a sufficient
amount of starch, i.e. from about 4.7 to 2.3 weight percent to
generate a starch concentration of about 0.70 percent by weight.
Those of ordinary skill in the art will recognize that the starch
may also be added to the raw material blend and milled as described
above.
[0049] The rate of water addition to the mixer was not critical.
The intense mixing action disperses the water throughout the
particles. Those of ordinary skill in the art can determine whether
to adjust the speed of rotation to values greater than or less than
those described in this Example 1 such that spherical pellets of
approximately the desired size are formed.
[0050] After about 2 to about 6 minutes of mixing, spherical
pellets were formed. The amount of mixing time can vary depending
upon a number of factors, including but not limited to the amount
of material in the mixer, speed of operation of the mixer, the
amount of water fed to the mixer, and the desired pellet size.
Those of ordinary skill in the art can determine whether the mixing
time should be greater than or less than the times described in
this Example 1 such that spherical pellets of approximately the
desired size are formed. Once pellets of approximately the desired
size were formed, additional raw material was added to the mixer in
an amount of about 10 weight percent, and the mixer speed was
reduced to about 16 meters per second tip speed. Mixing was
continued at the slower speed for about 1 to about 120 seconds, and
then the pellets were discharged from the mixer.
[0051] After discharge from the mixer, the pellets were dried. In
the present example, the pellets were dried in a forced convection
oven. Other types of drying equipment that could be suitable for
use with the methods disclosed herein include but are not limited
to rotary dryers, fluid bed dryers, direct heat dryers, compressed
air dryers and infrared dryers. Commercial sources for the dryers
described herein are known to those of ordinary skill in the
art.
[0052] The dryer was operated at a temperature ranging from about
100.degree. C. (212.degree. F.) to about 300.degree. C.
(572.degree. F.).
[0053] In this particular example, the green pellets were sintered
in a rotary kiln, operated at a temperature ranging from about
2,400.degree. F. to about 2,800.degree. F., for a residence time of
about 30 minutes. According to other examples, the residence time
can be in the range of from about 30 to about 90 minutes. Other
times and temperatures may be employed. During the sintering of the
pellets the coal was burned leaving ash and pores in its place.
[0054] Optionally, prior to sintering, the pellets can be screened
to remove pellets that are under and over a desired size. If
screening is employed, only the dried pellets having the desired
size are sent to a rotary kiln for sintering. Selection of green
pellet screens required to achieve a desired size of sintered
pellets should allow for firing shrinkage of pellets, typically 1
to 2 U.S. Mesh sizes. One of ordinary skill in the art can
determine the green pellet screens necessary to achieve a desired
size of sintered pellets through routine experimentation. Desired
fired pellet size in this example was between about 16 and about 70
U.S. Mesh after sintering, or expressed as microns, between about
1180 and 212 microns after sintering.
[0055] According to other examples, the desired size is in a range
between about 6 and 270 U.S. Mesh after sintering. According to
still other examples, the desired size is in a range of from about
3.35 to about 0.05 millimeters.
[0056] In the present example as shown in Table I, the sintered
pellets that included either a wheat flour or coal sacrificial
phase were determined to have a bulk density in the range of from
about 1.06 g/cc to about 1.33 g/cc, expressed as a weight per unit
volume, including in the volume considered, the void spaces between
the particles. The bulk density was determined for the present
example by ANSI Test Method B74.4-1992 (R 2002), which is a test
known and available to those of ordinary skill in the art. As shown
in Table I, as the amount of coal is increased, the bulk density
decreases. The 25% coal sacrificial phase proppant has a bulk
density that is about 32% lower than the frac sand which is shown
in Table I as a control. In general, the present method can be used
to make pellets having a bulk density of from about 1.05 g/cc to
about 1.35 g/cc.
[0057] Also, in the present Example as shown in Table I, the
sintered pellets were determined to have an apparent specific
gravity in the range of from about 2.11 to 2.40. The 10% wheat
flour sacrificial phase proppant has an ASG that is about 10% lower
than the frac sand which is shown in Table I as a control. The 25%
coal sacrificial phase proppant has an ASG that is about 20% lower
than the frac sand which is shown in Table I as a control. In
general, the present method can be used to make pellets having an
apparent specific gravity of from about 1.80 to about 2.50.
[0058] Moreover, in the present example, the -20 mesh/+40 mesh 10%
wheat flour sacrificial phase sintered pellets were determined to
have a crush strength of about 8.2 percent by weight fines (i.e.,
material less than 40 mesh) at 4000 psi and the -20 mesh/+40 mesh
coal sacrificial phase sintered pellets were determined to have a
crush strength of from about 1.6 percent by weight to about 3.3
percent by weight fines (i.e., material less than 40 mesh) at 4000
psi. The crush values reported herein were determined according to
API Recommended Practices RP60 for testing proppants, which is a
text known to those of ordinary skill in the art. Generally,
however, according to this procedure, a bed of about 6 mm depth of
sample that has been screened to contain pellets of between 20 and
40 mesh is placed in a hollow cylindrical cell. A piston is
inserted in the cell. Thereafter, a load is applied to the sample
via the piston. One minute is taken to reach maximum load which is
then held for two minutes. The load is thereafter removed, the
sample removed from the cell, and screened to 40 mesh to separate
crushed material. The results (i.e., the amount of "fines", or
crushed material) are reported as a percentage by weight of the
original sample.
[0059] In the present example, the coal sacrificial phase sintered
pellets were determined to have a percent surface porosity in a
range of from about 6.6% to about 14.8% by volume. The surface
porosity values were determined by mercury porosimetry at a
pressure from 30 to 60,000 psia. A mercury porosimeter is a device
whose use is known to those of ordinary skill in the art. In
general, the present method can be used to make pellets having a
percent surface porosity of from about 5% to about 15% by
volume.
[0060] In the present example, the coal sacrificial phase sintered
pellets were also determined to demonstrate a typical short term
conductivity profile, in which conductivity decreased with an
increase in closure pressure. TABLE-US-00001 TABLE I Badger 10%
20/40 frac Wheat 10% 15% 20% 20% 25% sand Flour Coal Coal Coal Coal
Coal Pellet Size Distribution Sieves (% Retained on Screen) 16 0.0
0.0 0.0 0.0 0.0 0.0 0.0 20 0.1 8.7 3.7 3.7 3.7 3.7 3.7 25 2.2 22.0
31.2 31.2 31.2 31.2 31.2 30 14.9 26.9 40.7 40.7 40.7 40.7 40.7 35
34.3 31.1 21.0 21.0 21.0 21.0 21.0 40 34.2 11.1 3.2 3.2 3.2 3.2 3.2
50 13.8 0.2 0.3 0.3 0.3 0.3 0.3 pan 0.4 0.0 0.0 0.0 0.0 0.0 0.0 BD
(g/cc) 1.57 1.27 1.33 1.18 1.11 1.14 1.06 ASG 2.64 2.40 2.40 2.19
2.16 2.22 2.11 Helium Pycnometer SG Whole Pellets (g/cc) 2.65 2.54
2.52 2.51 2.53 Ground Pellets (g/cc) 2.67 2.82 2.81 2.83 2.77 API
Crush (%) at 4,000 psi 2.0 8.2 1.6 2.1 3.3 2.0 3.3 Mercury
Porosimetry Porosity (%) 0 6.6 12.8 16.3 14.8 Apparent Skeletal
Density @ 1.60 1.74 1.74 1.46 1.48 6.83 psia Short Term
Conductivity (D-ft) at 2,000 psi 4.10 9.37 10.82 11.22 9.60 at
4,000 psi 3.23 6.99 7.55 6.78 5.83 at 6,000 psi 2.16 4.79 4.16 3.23
2.79 at 8,000 psi 1.33 2.75 2.21 1.60 1.26 at 10,000 psi 0.74 1.69
1.15 0.75 0.57 at 12,000 psi 0.50 0.96 0.68 0.46 0.32
[0061] The composite, spherical, sintered pellets of the present
invention are useful as a propping agent in methods of fracturing
subterranean formations to increase the permeability thereof,
particularly those formations having a compaction pressure of up to
about 5,000 psi, which are typically located at depths of up to
about 7,500 feet.
[0062] When used as a propping agent, the pellets of the present
invention may be handled in the same manner as other propping
agents. The pellets may be delivered to the well site in bags or in
bulk form along with the other materials used in fracturing
treatment. Conventional equipment and techniques may be used to
place the spherical pellets as a propping agent.
[0063] The foregoing description and embodiments are intended to
illustrate the invention without limiting it thereby. It will be
obvious to those skilled in the art that the invention described
herein can be essentially duplicated by making minor changes in the
material content or the method of manufacture. To the extent that
such material or methods are substantially equivalent, it is
intended that they be encompassed by the following claims.
* * * * *