U.S. patent number RE34,371 [Application Number 07/824,022] was granted by the patent office on 1993-09-07 for lightweight proppant for oil and gas wells and methods for making and using same.
This patent grant is currently assigned to Norton-Alcoa. Invention is credited to Paul R. Lemieux, David S. Rumpf.
United States Patent |
RE34,371 |
Rumpf , et al. |
September 7, 1993 |
Lightweight proppant for oil and gas wells and methods for making
and using same
Abstract
A lightweight oil and gas well proppant made by simultaneously
mixing and compacting a mixture of kaolin clay which has been
calcined at a temperature low enough to prevent the formation of
mullite and crystobalites to an LOI of 12 or less when tested at
1400.degree. C., and amorphous to microcrystalline silica both of
which have been milled to an average agglomerated particle size of
7 microns or less to form green pellets, and then drying, screening
and sintering the pellets to form proppant pellets having a
specific gravity of 2.7 or less, the proppant having a conductivity
of at least 3,000 millidarci-feet as measured by the Stim-Lab
Technique after 50 hours at 8,000 psi and 275.degree. F. in the
presence of deoxygenated aqueous 2% solution of KCl using sandstone
shims.
Inventors: |
Rumpf; David S. (North
Tonawanda, NY), Lemieux; Paul R. (Ft. Smith, AR) |
Assignee: |
Norton-Alcoa (Ft. Smith,
AR)
|
Family
ID: |
26970357 |
Appl.
No.: |
07/824,022 |
Filed: |
January 22, 1992 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
297876 |
Jan 17, 1989 |
04921820 |
May 1, 1990 |
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Current U.S.
Class: |
501/128;
166/280.2; 501/127; 501/133; 501/144 |
Current CPC
Class: |
E21B
43/267 (20130101); C09K 8/80 (20130101) |
Current International
Class: |
C09K
8/60 (20060101); C09K 8/80 (20060101); E21B
43/267 (20060101); E21B 43/25 (20060101); C04B
035/10 () |
Field of
Search: |
;501/133,144,127,128
;166/280 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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101855 |
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Mar 1984 |
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EP |
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116369 |
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Aug 1984 |
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EP |
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169412 |
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Jan 1986 |
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EP |
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8503327 |
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Aug 1985 |
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WO |
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Primary Examiner: Group; Karl
Attorney, Agent or Firm: Weil, Gotshal & Manges
Claims
What is claimed is:
1. A low density proppant comprising solid sintered ceramic pellets
which prior to sintering consist essentially of a mixture of a
kaolin clay and amorphous to micro-crystalline silica, said clay
being substantially free of quartz and, prior to sintering, having
been maintained at a temperature low enough to prevent a phase
transformation of said kaolin clay to mullite and crystobalite,
said pellets having specific gravity of less than 2.70, such
proppant having a conductivity of at least 3,000 md-ft after 50
hours at 8,000 psi and 275.degree. F. in the presence of
deoxygenated aqueous 2% solution of KCl as measured by the Stim-Lab
Technique using sandstone shims.
2. The proppant of claim 1 wherein said pellets have a specific
gravity of 2.6 or less.
3. The proppant of claim 1 wherein said pellets are produced by the
method comprising the steps of:
pelletizing an initial amount of material, which consists
essentially of a mixture of milled, calcined kaolin clay and
amorphous to microcrystalline silica by simultaneously mixing and
compacting said material while adding water at a controlled
rate;
adding additional amounts of said material at a controlled rate
while continuing said simultaneous mixing and compacting to form
pellets therefrom in a desired size range; and
drying and sintering said pellets.
4. The proppant of claim 3 wherein said step of pelletizing further
includes dispersing an organic binder in said material prior to
adding water.
5. The proppant of claim 3 wherein said kaolin clay is calcined at
a temperature of less than 900.degree. C.
6. The low density proppant of claim 3 wherein said method further
includes the steps of:
milling said kaolin clay and said silica to an agglomerated
particle size of 7 microns or less.
7. The proppant of claim 6 wherein said calcined kaolin clay and
silica are milled together.
8. The low density proppant of claim 6 wherein said calcined kaolin
clay and silica are milled to an agglomerated particle size of 3
microns or less.
9. The proppant of claim 5 wherein said kaolin clay is calcined to
an LOI of 12 w/o or less when tested at 1400.degree. C.
10. The proppant of claim 9 wherein said kaolin clay is calcined to
an LOI of 2% w/o or less when treated at a temperature of
1400.degree. C.
11. The proppant of claim 1 having a conductivity of at least 4,000
md-ft after 50 hours at 8,000 psi and 275.degree. F. in the
presence of deoxygenated aqueous 2% solution of KCI as measured by
the Stim-Lab Technique using sandstone shims.
12. The proppant of claim 1 wherein said clay contains less than 1%
quartz prior to sintering.
13. The proppant of claim 1 .[.consisting essentially of.].
.Iadd.comprising .Iaddend.mullite and crystobalite.
14. A low density proppant comprising solid ceramic pellets
.[.consisting essentially of.]. .Iadd.comprising .Iaddend.mullite
and crystobalite and having a dry specific gravity of about 2.6 or
less, said proppant having a conductivity of at least 3,000 md-ft
after 50 hours at 8,000 psi and 275.degree. F. in the presence of
deoxygenated aqueous 2% solution of KCL as measured by the Stim-Lab
Technique using sandstone shims.
15. The low density proppant of claim 14 wherein said proppant has
a conductivity of at least 4000 md-ft after 50 hours at 8000 psi
and 275.degree. F. in the presence of a deoxygenated aqueous 2%
solution of KCl as measured by the Stim-Lab Technique using
sandstone shims. .[.16. The support of claim 14 wherein said
pellets contain between 35 w/o and 60 w/o mullite, between 35 w/o
and 60 w/o crystobalite, and less than 10 w/o
of a glassy phase..]. 17. A low density proppant comprising
sintered pellets .[.consisting essentially of.]. .Iadd.comprising
.Iaddend.mullite and .[.at least 35%.]. crystobalite and which
prior to sintering consist essentially of a mixture of between 0
w/o to 90 w/o calcined kaolin clay and 10 w/o to 100 w/o amorphous
to micro-crystalline silica, said kaolin clay containing less than
2 w/o quartz and less .[.that.]. .Iadd.than .Iaddend.1 w/o iron,
said mixture being substantially free of mullite and crystobalite
prior to sintering, said pellets having a density of less than
about 2.6, said calcined kaolin clay and silica having been milled
to
an average particle size of less than 7 microns. 18. The proppant
of claim 17 wherein said calcined kaolin clay and silica are milled
to an
agglomerated particle size of 3 microns or less prior to sintering.
19. The proppant of claim 18 wherein said kaolin clay is calcined
to an
LOI of less than 12 w/o when tested at 1400.degree. C. 20. The
proppant of claim 17 having a conductivity of at least 4,000 md-ft
after 50 hours at 8,000 psi and 275.degree. F. in the presence of
deoxygenated aqueous 2% solution of KCI as measured by the Stim-Lab
Technique using sandstone shims. .[.21. A low density proppant
comprising about from 35 w/o to about 60 w/o mullite and about 35
w/o to about 60 w/o crystobalite, having a specific gravity of less
than 2.70, and having a conductivity of at least 3,000 md-ft after
50 hours at 8,000 psi and 275.degree. F. in the presence of
deoxygenated aqueous 2% solution of KCl as measured by the Stim-Lab
Technique using sandstone shims, with less than 10% of said
proppant being a glassy phase..].
Description
FIELD OF INVENTION
This invention relates generally to lightweight proppants for oil
and gas wells and more particularly to lightweight proppants for
oil and gas wells which are lighter in weight than existing
lightweight proppants but which have strength and conductivity
similar to and preferably substantially higher than such existing
lightweight proppants, and to methods of making and using such
proppants.
BACKGROUND
Hydraulic fracturing is a process of injecting fluids into a
selected oil or gas bearing subsurface earth formation traversed by
a well bore at sufficiently high rates and pressures such that the
formation fails in tension and fractures to accept the fluid. In
order to hold the fracture open once the fracturing pressure is
released a propping agent (proppant) is mixed with the fluid which
is injected into the formation.
Hydraulic fracturing increases the flow of fluids from an oil or
gas reservoir to a well bore in at least three ways: (1) the
overall reservoir area in communication with the well bore is
increased, (2) the proppant in the fracture generally has
significantly higher permeability than that of the formation,
thereby allowing fluids to flow more easily and (3) the high
conductivity channel causes large pressure gradients to be created
in the reservoir past the tip of the fracture.
Proppants are generally strong, preferably substantially spherical,
particulates that should be able to withstand the high temperatures
and pressures and corrosive environments experienced in the
subsurface formations surrounding an oil or gas well. Early
proppants were formed of material such as glass beads, sand, walnut
shells and aluminum pellets. These materials did not have
sufficient strength or resistance to corrosion to be successful in
many wells, particularly where closure pressures above a few
thousand psi were experienced.
U.S. Pat. No. 4,068,718 to Cooke relates to a proppant which Cooke
states is formed of "sintered bauxite" that has a specific gravity
greater than 3.4. Cooke states that specific gravities above 3.4
are required in order that the proppant have sufficient compressive
strength to resist fragmentation under the high stress levels
experienced in use. While the proppant described in Cooke's example
proved to have sufficient strength to resist crushing, the high
specific gravity was undesirable since it required the use of
higher viscosity fracturing fluids and resulted in a lower
volumetric proppant concentration for a given weight of proppant
loading in a fracturing fluid when compared with that achieved by a
proppant of lower specific gravity. In general, the higher the
volumetric concentration of the proppant in the fracturing fluid,
the wider the propped fracture will be after the fracturing
pressure is released.
U.S. Pat. No. 4,427,068 to Fitzgibbon relates to intermediate
strength composite proppants made by mixing calcined diaspore clay,
burley clay or flint clay with alumina, "bauxite" or mixtures
thereof such that the ratio of alumina to silica in the composite
mix is between nine to one and one to one. The powered starting
materials are mixed in an Eirich mixer and while the mixing is in
progress sufficient water is added to cause formation of composite
spherical pellets from the powered mixture. Fitzgibbon states that
the rate of water addition is not critical. The pellets are dried
and then furnaced to sinter the pellets. The sintered pellets have
a specific gravity of between 2.7 and 3.4.
U.S. Pat. No. 4,522,731 to Lunghofer relates to an intermediate
strength proppant having an alumina content between 40% and 60%
which is produced using a spray agglomeration process and which has
a density of less than 3.0 gr/cc. In a preferred embodiment
Lunghofer produces his proppants from "Eufaula bauxite" which it
states is bauxitickaolin type material deposited in and around
Eufaula, Alabama. According to Lunghofer, the Eufaula bauxite
preferably contains at least some (above 5%) gibbsite.
U.S. Pat. No. 4,668,645 to Khaund relates to an intermediate
strength proppant made from a mined "bauxitic clay" having a
specified chemical composition.
The proppants described in the Fitzgibbons, Lunghofer and Khaund
patents have specific gravities lower than that of the earlier
Cooke proppant and proppants having such lower specific gravities
have been used with some success in intermediate depth wells where
the stress on the proppant is in the 5,000 to 10,000 psi range. It
will be desirable, however, to have still lighter weight proppants
which are easier to transport in the fracturing fluid and are
therefore carried farther into the fracture before settling out and
which will yield a wider propped fracture than the known lower
specific gravity proppants. The lighter weight proppant should,
however, have a conductivity rating at least as high as and
preferably substantially higher than those obtainable with the
presently available "lightweight" proppants.
The conductivity of a proppant under specific conditions of stress,
temperature, corrosive environment and time is the single most
important measure of its quality. The conductivity of a packed
proppant such as might be deposited in a fracture is defined as the
permeability of the proppant pack multiplied by the width of the
propped fracture and is usually stated in units of millidarci-feet
("md-ft").
The conductivity of currently available intermediate strength
proppants is frequently measured by the tentative API 8 hour
procedure, "Tentative Fifth Draft of Recommended Practices For
Evaluating Short Term Proppant Pack Conductivity", (March 1987)
(hereinafter the "API 8 hour Procedure"), which procedure is hereby
incorporated by reference.
Recently a consortium of some twenty-eight organizations involved
in various aspects of the fracturing and stimulation business has
sponsored research on ways of evaluating and improving stimulation
techniques. Stim-Lab, Inc. of Duncan, OK acts as the testing arm of
the consortium to develop consistent and repeatable testing
procedures for proppants including tests for determining their
permeability and conductivity. The long term conductivity testing
techniques developed by Stim-Lab have been widely accepted in the
industry and are described in a publication of the Society of
Petroleum Engineers, No. SPE 16900, entitled "An Evaluation of the
Effects of Environmental Conditions and Fracturing Fluids on the
Long-Term Conductivity of Proppants" by G. S. Penny of Stim-Lab,
Inc., which publication is hereby incorporated by reference. It
should be understood that any gap in the description in the SPE
publication should be filled in by reference to the API 8 hour
Procedure. The testing techniques used by the applicants to
determine the conductivity of the proppants of the present
invention as they are intended to be supplied to a customer
(referred to as the "Stim-Lab Technique") are essentially identical
to those described in SPE publication No. 16900 using Monel-K 500
or sandstone shims in the conductivity cells, as noted herein. A
single cell was used rather than stacking 4 cells in the manner
described in the SPE publication. This however should have no
effect on the measured results. The Stim-Lab Technique is
considered to yield conductivity measurements that are repeatable
to within about 5-10%.
SUMMARY OF THE INVENTION
In accordance with the invention there is provided a low density,
high strength proppant which is formed of solid ceramic particles
having a dry specific gravity less than 2.70 and preferably 2.60 or
less and a conductivity of at least about 3,000 md-ft and
preferably at least 4,000 md-ft after 50 hours at 8,000 psi and
275.degree. F. in the presence of a deoxygenated 2% aqueous
solution of KCl as measured by the Stim-Lab Technique using
sandstone shims. Most preferably the conductivity is at least 4400
md-ft as measured under the above conditions.
The proppant of the invention may be produced by milling calcined
kaolin clay and an amorphous to microcrystalline silica to an
average particle size of less than 7 microns, and preferably about
3.0 microns or less and pelletizing a mixture of the resulting
powers in a compacting mixer. Preferably the mixture contains
between 55 weight percent ("w/o") and 90 w/o kaolinite and between
45 w/o and 10 w/o silica, although the percentages may fall outside
these ranges. Kaolinite is normally the major component of the
mixture. However, in the limit the silica content could reach 100%
yielding a proppant of essentially all crystobalite having a
specific gravity of about 2.33. The kaolinite and silica are
preferably milled together.
The kaolin clay is calcined at a temperature of less than
900.degree. C. to reduce the loss on ignition ("LOI") to 12 w/o or
less when tested at 1400.degree. C., and preferably to about 2 w/o
LOI when tested at 1400.degree. C. It is important that the
calcining be done at a low enough temperature that the kaolin clay
does not undergo a phase transformation to mullite and
crystobalite. The silica should not contain any significant amount
of crystalline quartz, other than amorphous to microcrystalline
quartz as hereinafter defined, and is preferably dried to a LOI of
about 1 w/o when tested at 1000.degree. C.
Preferably, an organic binder is first dispersed in the milled
material in the mixer and then water is added to the power at a
controlled rate while mixing and compacting the power to form rough
pellets of a desired size. Next, with the mixer still running,
additional dry, milled material is slowly added to yield smooth,
spherical pellets. The pellets are then dried and fired to convert
the material to proppant pellets that preferably comprise
.[.between about 35 (w/o) and 60 (w/o mullite, between about 35 w/o
and 60 w/o.]. .Iadd.primarily a mixture of mullite and
.Iaddend.crystobalite and a minor amount (less than 10 w/o) of a
glassy phase. The proppant has a specific gravity of less than 2.70
and preferably of 2.60 or less. .[.Most preferably the pellets
comprises between 35 w/o and 50 w/o mullite and 50 w/o to 60 w/o
crystobalite..].
In accordance with another aspect of the invention, the proppant is
mixed with a fluid and injected into a subterranean formation under
high pressure to open a fracture, with the proppant remaining in
the fracture to prop it open after the fracturing pressure is
removed. The measured conductivity of the proppants of the present
invention are equal to and usually substantially better than those
of other known lightweight proppants which have higher specific
gravities and higher weight per unit of volume.
DETAILED DESCRIPTION
The preferred raw materials for use in making the low specific
gravity, high strength proppants of the present invention are
kaolin clay and amorphous to microcrystalline silica. Preferably
the kaolin clay consists largely of kaolinite (Al.sub.2 Si.sub.2
O.sub.5 (OH).sub.4) and is essentially free of sand (.Iadd.i.e.,
.Iaddend.quartz). One source of such material is from C. E.
Minerals, headquartered in King of Prussia, Pennsylvania. The
kaolinite deposits owned by C. E. Minerals are mined at C. E.
Minerals' Mulcoa operations in Andersonville, Georgia. The
Andersonville kaolin deposits owned by C. E. Minerals are well
described in a report by Alfred D. Zapp entitled "Bauxite Deposits
of the Andersonville District, Georgia", U.S. Geological Survey
Bulletin 1199-G. This report is incorporated herein by reference.
The report states that the deposit consists largely of the mineral
kaolinite and is essentially sand free. The chief impurities are
compounds of iron and titanium and small amounts of gibbsite. The
kaolin clay is deposited in tabular lenticular masses. Raw
materials containing significant amounts of sand (free silica or
quartz) produce a weaker proppant, while raw materials containing
gibbsite or other hydrated aluminas yield a product having an
undesirable high specific gravity.
By amorphous to microcrystalline silia is meant silica which is
truly amorphous or is "amorphous" in the sense that the ultimate
submicron particles, as seen with the aid of a scanning electron
microscope, do not have the angular shape of crystals. One source
of amorphous to microcrystalline silica is from Illinois Minerals
Company in Cario, Illinois. This silica, as described by Illinois
Minerals, is an extremely fine-grained microcrystalline silica
formed by weathering of silica-rich limestone. Materials from other
deposits of kaolinite and of amorphous to microcrystalline silica
can also be used in making the proppants of the present
invention.
In general the kaolin clay most useful as a raw material in making
the proppants in accordance with the invention may contain by
chemical analysis about 45 w/o alumina and 52 w/o silica, less than
1 w/o iron oxide and less than 2 w/o (preferably less than 1 w/o)
free quartz. Preferably the kaoline clay approaches 100% kaolinite.
Most preferably the amount of free quartz is non-detectable.
Other naturally occurring minerals which may be present in minor or
trace amounts include anatase and rutile. Minerals whose presence
in the raw material appear to be somewhat detrimental to the
properties of the final product include quartz, pyrite, marcasite,
siderite, micas and montmorillonites. The amorphous to
microcrystalline silica most useful as a raw material in making the
proppants in accordance with the invention should contain by
chemical analysis at least 94 w/o (preferably 100 w/o) silica.
The kaolin clay is preferably calcined before further processing in
order to remove water and organics. In accordance with the
invention, applicants have found that the calcining should be
performed at a temperature low enough that the kaolin clay does not
undergo a phase change to form mullite or crystobalite. Applicants
have found that the presence of any significant amount of mullite
or crystobalite in the kaolin clay before the final drying and
firing steps has a severely detrimental effect on the properties of
the proppants produced. Therefore the calcining should be performed
at a temperature below 900.degree. C. for a sufficient time that
the loss on ignition ("LOI") of the kaolinite is 12 w/o or less and
preferably 2 w/o or less when tested at 1400.degree. C. The
calcining may be performed relatively quickly at temperatures on
the order of 700.degree. to 800.degree. C. or may be performed more
slowly at lower temperatures. If the calcining temperature is above
about 450.degree. to 500.degree. C., the kaolinite is converted to
amorphous alumina and silica (sometimes referred to as "meta
kaolinite"). Such a transformation, however, has no adverse effect
on the product of the invention and such transformed material will
still be referred to herein as kaolinite. The silica should be
dried after mining to an LOI of less than 1 w/o when tested at
1,000.degree. C. This drying is typically done at temperatures of
between about 150.degree. and 200.degree. C.
The calcined material and the silica is then reduced in particle
size, preferably by dry ball milling them in a closed loop system
containing a particle classifier. The average agglomerated particle
size in this milled material is less than 7 microns and preferably
about 3.0 microns or less as measured by a Sedigraph (Micro
Meritics Instrument Corp.) or a Granulometer' (Cilas Compagnie
Industrielle des Lasars). The true ultimate particle size, however,
of the milled raw material (both the kaolin and silica) is much
finer than 3.0 microns and is believed to be made up of submicron
flakes or particles. Such submicron flakes or particles tend to
agglomerate to form the composite particles which are sensed by the
measuring instruments. The calcined kaolin clay and silica are
mixed together and .[.preferable.]. .Iadd.preferably .Iaddend.are
milled together. The mixture preferably contains between 55 w/o and
90 w/o kaolinite and between 45 w/o and 10 w/o silica, although
different percentage mixtures may be used. Kaolinite is normally
the major component of the mixture. However, in the limit the
amorphous to microcrystalline silica content can reach 100%,
yielding a proppant that is essentially entirely crystobalite
having a specific gravity of about 2.33. Such a super lightweight
proppant would be very useful in shallow wells when closure
stresses are on the order of 4000 psi or less.
The milled raw material is pelletized preferably in a compacting
mixer. The preferred commercially available machines for this
purpose are the Eirich Countercurrent Intensive Mixers which are
manufactured in several sizes of different capacities by the
Maschinenfabrik Gustav Eirich of Hardheim, West Germany and
distributed in the United States by Eirich Machines, Inc., New
York, N.Y.
The Eirich mixer has a rotation mixing pan forming the bottom of
the mixing chamber which pan can be either horizontal or inclined
at an angle and a "mixing star" which rotates in the opposite
direction from the pan. The mixing star rotates about an axis
parallel to and offset from that of the pan and has a diameter of
about 20 to 25 percent of that of the pan. The preferred form of
mixing star for the pelletizing operation is referred to by Eirich
as a suction type rotor and has a plurality of vertically extending
trapidzoidally shaped mixing elements which extend from the
periphery of the rotating element. For the Model R7 machine, the
pan has a single rotational speed of about 45 rpm, while the mixing
star has a low and a high speed. The low speed is about 700 rpm and
the high speed is about twice that, about 1400 rpm. The rotational
speeds of the elements of the larger mixers are adjusted so that
rotating elements have similar tip speeds. The mixer may also
contain a second mixing star upon which can be mounted plows for
cleaning the sides of the rotating pan. This second mixing star is
also mounted eccentrically in the mixer and rotates in the opposite
direction from the mixing pan. An example of another compacting
mixer that is believed suitable for use in the present invention is
the Littleford Lodige mixer.
The milled powder charge is preferably first mixed dry in the mixer
with up to about 3 w/o (calculated on the basis of the initial
milled powder charge) of an organic binder such as gelatinized
cornstarch for a period of about 30 seconds with the suction rotor
on the low speed setting in order to fully disperse the binder in
the raw material powder.
It is preferred that such binders be such that they are pyrolized
or driven off during later firing of the proppant pellets. The
amount of the binder is normally on the order of 1 w/o to 3 w/o of
the initial milled powder charge. It is possible to make pellets
with the method of the invention without a binder but it is
believed that the use of a binder results in improved properties
and yield.
Next water is added at a controlled rate to the mixer with the
suction rotor running at the low speed setting to initially
pelletize the material. The rate of water addition should be
controlled properly to facilitate the formation of the pellets in
the mixer and can affect the quality of the final product.
Typically the total amount of water for the Model R7 Eirich mixer
is from about 35 to 55 w/o of the initial starting material and
preferably about 43 to 50 w/o. Preferably between about 40 and 60
w/o of the expected total amount of water is added after the binder
is dispersed and mixed for about 2 to 4 minutes. Thereafter
additions of about 8 to 10% of the total water are made about every
0.75 to 1.5 minutes until irregularly shaped pellets ranging in
size from about 1/16 to 1/8 inch in diameter are formed.
Once pellets have formed in the mixer, which at this point are
normally large and irregularly shaped, the suction rotor is
switched to the high speed setting and additional milled calcined
raw material (referred to as "dust") is slowly added to the mixer
over a period of about 5 to 7 minutes in order to cause the pellets
to become smaller, well shaped spheres of a narrower size
distribution, typically 0.02 inches to 0.047 inches. Typically, the
amount of dust required to form the desired size spheres is about
42 to 70 w/o of the starting raw material. Preferably about 46 to
54 w/o dust is added.
In some cases the suction rotor is switched back to the low speed
setting for a later portion of the 5 to 7 minute period after a
major portion of the dust has been added to the mixer. After adding
the dust, the suction rotor is then preferably set back to the low
speed setting if it has not already been and the mixer run for a
further period of about 30 seconds in order to improve the
sphericity of the pellets and to increase yield of the desired size
range.
The amount and rate of water and dust additions to the mixer can
have a significant impact on the conductivity of the proppant
produced by the method of the invention. If too much water is
added, the particles are poorly shaped and are too large. If the
dust is added too quickly it results in a poor yield of particles
in the desired size range. If not enough dust is added the
particles are poorly shaped and will stick together in agglomerated
masses. If too much dust is added the particles have a rough
surface and unconsolidated dust remains in the mixer. When the
right amount of dust is added, the process yields smooth,
spherical, well compacted pellets.
The speherical pellets are next discharged from the mixer and dried
to a free moisture content of less than 10% and preferably less
than 5%. The drying temperature and time does not appear to be
critical and it has been found, for instance, that drying at
temperatures of 150.degree. C. overnight, or of 260.degree. C. for
10 to 15 minutes are suitable. Once dried, the desired size
spherical pellets are separated by screening for instance to a
18/35 mesh size. The larger than 18 mesh and finer than 35 mesh are
recycled to be repelletized. The screened particles in the desired
size range are then sintered, for instance in a rotary kiln, at a
temperature between 1300.degree. and 1500.degree. C., preferably
between 1375.degree. to 1425.degree. C. for about 15 to 30 minutes
at temperature. The total time in the kiln is normally from about 1
to 11/2 hours. The particles shrink upon firing by about 15% to
20%. The desired finished size is typically between about 0.0167
inches and 0.0331 inches but can be larger or smaller as needed.
The particular temperature to which the screened particles are
fired is selected in order to maximize their strength. This
temperature depends on several variables, some of which are the raw
material mineralogy, the milled particle size, the pellet size and
the volume of material in the kiln.
After firing, the pellets are again screened to the desired final
size. A typical product size is 20/40 mesh which contains 90 w/o of
its pellets of between 0.0167 inches and 0.0331 inches in size and
preferably 90 w/o of the pellets between about 0.0232 inches and
0.0331 inches in size.
The proppants of the invention have been found to have suprisingly
and unexpectedly high conductivities for having such low specific
gravities. More particularly, the conductivities of the proppants
of the preferred embodiment of the present invention are at least
3000 md-ft and preferably at least 4000 md-ft after 50 hours at
8000 psi and 275.degree. F. in the presence of a deoxygenated 2%
aqueous solution of KCl as measured by the Stim-Lab Technique using
sandstone shims. Most preferably the conductivity is at least 4500
md-ft when measured under the above conditions. The proppants of
the present invention have specific gravities of less than 2.70,
and preferably 2.60 or less, which are lower than those of other
commercially available lightweight proppants. In fact, the
preferred proppants of the present invention are even lighter than
sand (specific gravity 2.64), the lightest proppant which has
heretofore been in common commercial use. The conductivity of the
proppants of the present invention are between about 3 and 20 times
that of sand, however, depending upon operating conditions. Sand is
of little use at pressures on the order of 8000 psi while the
preferred proppants of the present invention still have high
conductivities, on the order of as much as 4500 md-ft, at such
pressures.
Compared to another heavier commercially available proppant which
is considered to be in the lightweight range, the preferred
proppants of the present invention have substantially higher
conductivities. The commercially available "lightweight" proppant
has been measured in accordance with the Stim-Lab technique to have
a conductivity of 6067 md-ft after 50 hours at 250.degree. F. and
6000 psi closure stress in the presence of deoxygenated aqueous 2%
KCl solution using sandstone shims. A proppant in accordance with
the preferred embodiment of the present invention had conductivity
values of 7855 md-ft when measured under the same conditions. The
same commercially available "lightweight" proppant had
conductivities measured by the Stim-Lab technique of 3616 to 3700
md-ft after 50 hours at 275.degree. F. and 8000 psi while the
proppant of the present invention had a conductivity of 4459 md-ft
under similar conditions.
The Loose Pack Bulk Density (LPBD) of the proppant of the invention
used in the above example is 1.45 gms/cm.sup.3. The LPBD of the
commercially available "lightweight" proppant used for comparison
in the above example is 1.61 gm/cm.sup.3. Their specific gravities
were 2.56 and 2.70 respectively.
From a mineralogical point of view the sintered pellets of the
present invention are preferably .[.between about 35 w/o and 60 w/o
mullite and between about 35 w/o and 60 w/o.]. .Iadd.primarily a
mixture of mullite and .Iaddend.crystobalite and a minor amount
(less than 10 w/o) of a glassy phase. It is believed that the
glassy phase is best minimized for the highest quality product.
EXAMPLES
The invention is further illustrated by reference to the following
non-limiting examples wherein all percentages are by weight unless
otherwise specified. When applicable calcining conditions and
milling techniques for each raw material are given with each
example. Unless otherwise indicated, the pellets are screened to an
18/35 mesh size after drying and to a 20/30 mesh size after
sintering wherein 90% of the pellets are between about 0.0232
inches and 0.0331 inches in size. The typical yield of properly
.[.size.]. .Iadd.sized .Iaddend. pellets in the first screening is
about 70%. The chemical compositions of the raw materials used in
the following examples are summarized in Table I. The mineralogical
composition of the calcined kaolin clays as measured by the X-ray
diffraction technique is summarized in Table II. The kaolinite in
both the Huber 40-C clay and the Mulcoa clay have been transformed
to amorphous alumina and silica by being calcined at 750.degree. C.
and thus are not detected by X-ray diffraction. The figures in
Table II for the ACCO clay are for the uncalcined material so that
kaolinite is detected. Mineralogically the silicas used in the
examples is of such a fine grain size as to be virtually
undetectable as measured by X-ray diffraction techniques. The
components in Table II are divided into major minerals which
comprise about 95 w/o of the detected minerals present and the
minor minerals which together make up less than 5 w/o of the
detected minerals present. Kaolinite is approximately 45 w/o
Al.sub.2 O.sub.3. After calcining or drying all the raw materials
are milled to an average particle size of about 3 microns as
measured by a Granulometer. About 90% of the particles were
measured to be less than 10 microns in size.
TABLE I ______________________________________ Example 1 2 3 Huber
40-C ACCO Mulcoa 4 Chemistry Clay Clay Clay (Silica)
______________________________________ Al.sub.2 O.sub.3 45.0 45.9
43.7 0.7 SiO.sub.2 53.0 51.4 51.8 99.0 TiO.sub.2 1.8 1.8 1.7 0.5
Fe.sub.2 O.sub.3 0.2 0.9 0.8 0.5
______________________________________
TABLE II ______________________________________ Example Mineralogy
1 2 3 ______________________________________ Major Minerals
Kaolinite ND Major ND Minor Minerals Anatase Major Major Trace
Rutile ND Trace ND Quartz ND ND Trace Mullite ND ND ND Amorphous
Yes No Yes ______________________________________ Where "ND" means
nondetectable
EXAMPLE I
One hundred (100) lbs. of a kaolin clay/silica mixture was produced
by combining 86 lbs. of calcined Huber 40-C kaolin clay
(manufactured by J. M. Huber Corporation) and 14 lbs. of dried
Imsil A-108 microcrystalline silica (manufactured by Illinois
Minerals Company) in an Eirich Model R-7 Compacting Mixer and
mixing the two ingredients together for 60 seconds. Both the
calcined kaolin clay and the silica had been milled separately to
an average particle size of 3 microns as measured by a
Granulometer. Hereafter in this example this mixture will be
referred to as "blended material".
A 50 lbs. charge of blended material was placed in an Eirich
Compacting Mixer having an inclined pan and a suction type rotor
along with 1/2 lbs. of gelatinized cornstarch binder and mixed for
30 seconds with the suction rotor in its low speed setting in order
to fully disperse the binder in the blended material. With the
suction rotor rotating at its low speed setting, 9 lbs. of water
was added to the mixer and mixed for a period of two minutes. As
the suction rotor continued to operate in the low speed setting,
seven 2 lb. additions, followed by 1 lb. addition and then a 1/2
lb. addition of water were added sequentially to the mixer with a
45 second mixing period after each addition. At this point
irregularly shaped pellets had formed.
The rotor was then switched to the high speed setting and an
additional 17 lbs. of the same blended material was slowly added to
the mixer over a three minute period. Then the rotor was reset to
its low speed setting and additional 11 lbs. of the blender
material was added over a period of two minutes. The mixing was
continued with the suction rotor in the low speed setting for an
additional period of 30 seconds. At this point the particles had
fairly good sphericity but had rather rough surfaces. An additional
7 lbs. of the blended material was slowly added to the mixer over a
1 minute period and the material mixed for an additional 30 seconds
with the suction rotor continuing in the low speed setting. The
pellets now had a good spherical shape and smooth surfaces. The
pellets were discharged from the mixer and dried overnight at
150.degree. C. in a box oven. After drying the pellets were
screened to an 18/35 mesh and fired at 1405.degree. C. in a rotary
kiln for a period of about 30 minutes at temperature with a total
time in the kiln of approximately 1 hour 15 minutes. After cooling
the pellets were screened to a 20/30 mesh. The dry specific gravity
of the material was 2.56 as measured by a Beckman air comparison
pycnometer Model 930.
A conductivity test using the Stim-Lab Technique referred to above
was conducted with final closure stress of 6,000 psi at 250.degree.
F. in the presence of deoxygenated aqueous 2% solution of Kcl for
50 hours. In accordance with this procedure 63.06 grams of the
screened proppant pellets were loaded into a API Hasteloy-C 10
in.sup.2 linear flow cell to give a loading of 2 lbs./ft.sup.2 of
proppant and leveled loosely with a universal bevel blade device. A
3/8 inch thick sandstone core was placed on top of the test pack
followed by an O ring fitted to a piston which was lightly coated
with vacuum grease. The loaded test cell was then placed in a 150
ton Dake press and the closure stress was increased to 500 psi at a
rate of 100 psi/min. The cell was saturated with deoxygenated
aqueous 2 w/o KCl solution and then purged of air at the ambient
laboratory temperature of 70.degree. F. to 80.degree. F. A Validyne
DP15- 30 differential pressure transducer connected across the cell
was calibrated with water columns to 0.0001 psi accuracy. Closure
stress was then raised to 1,000 psi at a rate of 100 psi/min. A
Reservoir Accumulator, made up to two 5 gal and two 1 gal nitrogen
driven fluid reservoir accumulators which were filled with a 2 w/o
KCl aqueous solution that had been deoxygenated with nitrogen to a
level of less than 15 ppb and preferably less than 5 ppb of oxygen
was connected to the test cell and set at a driving pressure of 400
psi. The connection of the Reservoir Accumulator to the cell is
made through two 150 ml sample cylinders filled with 100 mesh
Oklahoma #1 sand with ceramic band heaters in order to saturate the
test fluid with silica. It should be noted that the closure
stresses on the proppant pack in the cell are stated in terms of
the net closure stress on the pack which is equal to the gross
pressure applied by the press minus the 400 psi pressure applied by
the reservoir accumulator.
The system was allowed 30 minutes to come to equilibrium and a
series of five conductivity measurements were taken and averaged.
The conductivity was calculated from the darci relationship:
where
kwf=Conductivity (md-ft)
26.78=factor to account for a 11/2.times.5 inch flow area and
pressure in psi
.mu.=Viscosity of flowing fluid at temperature (cp)
Q=Flow rate (ml/min)
P=Pressure differential across 5 inch flow path.
After the readings were taken at ambient temperature, the
temperature was increased to 250.degree. F. and held for 8 hours
for temperature uniformity. Next readings with the system at
250.degree. F. were taken at 1,000 psi, 2,000 psi, 4,000 psi and
6,000 psi with the closure stress being raised between levels at a
rate of 100 psi/min. After reaching each of the 1,000, 2,000 4,000
and 6,000 closure stress levels, the system was held at 250.degree.
F. for 1.5 hours before the conductivity readings were taken. At
the 6,000 psi level the cell was held at 250.degree. F. for 50
hours during which conductivity measurements were taken at 10 hour
intervals. The measured conductivity at the 50 hour time was 7,351
md-ft and 7,084 md-ft on a second sample of the raw material run at
the same time.
EXAMPLE II
In this example a kaolin clay was obtained from the American
Cyanamid Co., Andersonville, Ga. The chemical and mineralogical
composition of the uncalcined clay as indicated in Table I and II.
This kaolin clay was calcined at 480.degree. C. for approximately
24 hours then crushed to less than 1/8". The crushed kaolin clay
was combined with 250 grade air-floated microcrystalline silica
provided by the Illinois Minerals Company. This mixture contains 75
w/o clay and 25 w/o silica and hereafter will be referred to as
"blended material" in this example. The mixture was tumbled in a
"V" blender for approximately 2 minutes to insure complete
mixing.
The blended material was next reduced in particle size using a jet
mill manufactured by the Fluid Energy Processing Equipment Company
of Hatfield, Pa. The average particle size of this milled material
was 3.68 microns as measured by the Leads and Northrup Microtrac II
particle size analyzer.
A 45 lb. charge of this blended material was placed in the Eirich
Model R-7 Compacting Mixer described in Example I along with 11/2
lbs. of cornstarch binder and mixed dry for 30 seconds at the low
speed setting. Water was then added to the mixer with the mixer
operating at the low speed setting with 9 lbs. being added and
mixed for 2 minutes followed by 4 additions of 2 lbs. each, one
addition of 1 lb. and three additions of 1/2 lb., with a 45 second
mixing period after each addition.
The mixer was then set to the high speed mode and 10 lbs. of the
blended material was slowly added to the mixer over a minute
period. The mixer was then switched back to the low speed setting
and an additional 7 lbs. of the blended material was slowly added
over a 2 minute period, followed by a 30 second low speed mixing.
An additional 3 lbs. of the blended material was added over the
following minute followed by an additional 30 second mixing period
at the low speed. Then a final 3 lbs. of the blended material was
added over the following minute followed by an additional 30 second
mixing period at low speed. The particles were thereafter
discharged from the mixer and screened, dried and fired in the same
manner as described in Example I with the exception that the
sintering temperature was 1395.degree. C. After cooling the pellets
were screened to a 20-30 mesh size. The dry specific gravity of the
material was 2.52. The Stim-Lab conductivity test described in
connection with Example I was then performed except that the
readings at the elevated temperature were taken at 275.degree. C.
and the maximum stress was 8000 psi. The test yielded a measured
conductivity after 50 hours at 8,000 psi and 275.degree. F. of
3,351 md/ft.
EXAMPLE III
In this example a kaolin clay was obtained from C. E. Minerals.
This clay was mined at their Mulcoa operation in Andersonville, Ga.
The chemical and mineralogical composition of the calcined clay is
indicated in Tables I and II. For this example the clay was
calcined in a rotary kiln at 750.degree. C. and held at that
temperature for 2 hours. This clay was next crushed to minus 1/8
inch then milled in a similar method as the material in Example II.
The average particle size of the milled clay in this example was
3.01 microns as measured by the Leads and Northrup Microtrac II.
This milled clay was then mixed with Imseil A-108 silica in a
similar manner as in Example I, the ratio again being 86% clay and
14% silica. Again as in the previous examples this mixture will be
referred to as "blended material" hereafter in this example.
A 45 lbs. charge of this blended material was placed in the Eirich
Model R-7 Compacting mixer described in Example I along with 11/2
lbs. or cornstarch binder and mixed with water in the same manner
as described in Example I with the exception of adding 2 lb.
additions to 17 lbs. then 1lb. additions to 21 lbs. then a 1/4 lb.
addition.
Next 13 lbs. of blended material was slowly added over a minute
period with the mixer in the high speed setting. Next the mixer was
switching back to the low speed setting and an additional 8 lbs. of
blended material was added over a period of 2 minutes after which
the mixer continued to be operated for an additional 30 seconds
mixing time. The pellets formed of this procedure were slightly wet
but had a good spherical shape. The pellets were then removed from
the mixer and dried, screened (both before and after sintering) and
sintered in the same sintering temperature used was 1,385.degree.
C.
The Stim-Lab conductivity test described in Example II was then
performed on the proppant resulting in a conductivity measurement
of 4,459 md-ft. after 50 hours at 8,000 psi at 275.degree. F. The
dry specific gravity of the proppant was 2.59.
In accordance with the method of the present invention the low
density high strength proppant particles of the present invention
may be injected into fractures in subsurface formation as a
propping agent. In fracturing treatment a viscous fluid, often
referred to as a "pad" is injected into the well at extremely high
pressure to cause the formation to fail in tension and fracture to
accept the fluid. The fracturing fluid may be an oil base, water
base, acid, emulsion, foam or other fluid. Normally the fluid
contains several additives such as viscosity builders, drag
reducers, fluid loss additives, corrosion inhibitors, cross linkers
and the like. The fluid of the pad is injected until and fracture
of sufficient geometry is obtained to permit the placement of the
proppant pellets. Normally the treatment is designed to provide a
fracture at the well bore of at least 21/2 times the diameter of
the largest proppant pellet. Once a fracture of the desired
geometry is obtained, the proppants are carried suspended in the
fluid pad and placed in the fractures. Following placement of the
proppant, the well is shut-in for a length of time sufficient to
permit the pressure to bleed off into the formation which in turn
causes the fracture to close and exert closure stress on the
proppant particles. The shut-in period may vary from a few minutes
to several days. The proppant particles of the present invention
are particularly suitable for use as propping agents in wells of
depths less than about 14,000 feet.
* * * * *