U.S. patent application number 14/688131 was filed with the patent office on 2015-10-22 for proppant for use in hydraulic fracturing to stimulate a well.
The applicant listed for this patent is University of Kentucky Research Foundation. Invention is credited to Anne Elizabeth Oberlink, Thomas Lee Robl.
Application Number | 20150299560 14/688131 |
Document ID | / |
Family ID | 54321465 |
Filed Date | 2015-10-22 |
United States Patent
Application |
20150299560 |
Kind Code |
A1 |
Robl; Thomas Lee ; et
al. |
October 22, 2015 |
PROPPANT FOR USE IN HYDRAULIC FRACTURING TO STIMULATE A WELL
Abstract
A proppant for use in hydraulic fracturing to stimulate a well
is provided. The proppant is fly ash particles having a mean
particle size (d50) of between 45 .mu.m and 150 .mu.m and a size
distribution defined by (d10).ltoreq.5 .mu.m and (d98).ltoreq.250
.mu.m.
Inventors: |
Robl; Thomas Lee;
(Sadieville, KY) ; Oberlink; Anne Elizabeth;
(Lexington, KY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
University of Kentucky Research Foundation |
Lexington |
KY |
US |
|
|
Family ID: |
54321465 |
Appl. No.: |
14/688131 |
Filed: |
April 16, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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61980852 |
Apr 17, 2014 |
|
|
|
Current U.S.
Class: |
166/280.2 ;
507/269 |
Current CPC
Class: |
C09K 8/62 20130101; E21B
43/267 20130101; C09K 8/80 20130101 |
International
Class: |
C09K 8/80 20060101
C09K008/80; E21B 43/267 20060101 E21B043/267 |
Goverment Interests
GOVERNMENT INTEREST
[0002] This invention was made in part with Government support from
grants NSF SBIR Phase I, IIP-0740339 and Phase II, IIP-0923822
awarded by National Science Foundation. The Government may have
certain rights in the invention.
Claims
1. A proppant for use in hydraulic fracturing to stimulate a well,
comprising: fly ash particles having a mean particle size (d50) of
between 45 .mu.m and 150 .mu.m and a size distribution defined by
(d10).ltoreq.5 .mu.m and a (d98).ltoreq.250 .mu.m.
2. The proppant of claim 1, wherein said fly ash particles have a
particle roundness and sphericity of great than 0.8 as determined
by ISO 13503-2006 (E), part 7.
3. The proppant of claim 2, wherein said fly ash particles have a
hydraulic conductivity of 1.times.10.sup.-3 cm/sec or less as
determined by falling head method.
4. The proppant of claim 3, wherein said fly ash particles have an
absolute density of less than 2.4 g/cm.sup.3 as determined by part
10.5 of ISO 13503-2006 (E).
5. The proppant of claim 4, wherein said fly ash particles have a
bulk density of less than 1.1 g/cm.sup.3 as determined by part 10.3
of ISO 13503-2006 (E).
6. The proppant of claim 5, wherein said fly ash particles have a
void volume of greater than 45% as calculated from absolute volume
(AV) and bulk specific volume (BV) as derived from the inverse of
bulk density and absolute density.
7. The proppant of claim 6, wherein said fly ash particles have a
loss on ignition of less than 6% as determined by part 12 of ISO
13503-2006 (E).
8. A proppant for use in hydraulic fracturing to stimulate a well,
comprising: fly ash particles having a mean particle size (d50) of
between 80 .mu.m and 100 .mu.m and a size distribution defined by
(d5).ltoreq.5 .mu.m and a (d99.5).ltoreq.250 .mu.m.
9. The proppant of claim 8, wherein said fly ash particles have a
particle roundness and sphericity of great than 0.9 as determined
by ISO 13503-2006 (E), part 7.
10. The proppant of claim 9, wherein said fly ash particles have a
hydraulic conductivity of 1.5.times.10.sup.-2 cm/sec or less as
determined by falling head method.
11. The proppant of claim 10, wherein said fly ash particles have
an absolute density of less than 2.3 g/cm.sup.3 as determined by
part 10.5 of ISO 13503-2006 (E).
12. The proppant of claim 11, wherein said fly ash particles have a
bulk density of less than 1.0 g/cm.sup.3 as determined by part 10.3
of ISO 13503-2006 (E).
13. The proppant of claim 12, wherein said fly ash particles have a
void volume of greater than 50% as calculated from absolute volume
(AV) and bulk specific volume (BV) as derived from the inverse of
bulk density and absolute density.
14. The proppant of claim 13, wherein said fly ash particles have a
loss on ignition of less than 2% as determined by part 12 of ISO
13503-2006 (E).
15. A method of hydraulic fracturing of a subterranean formation
having a well bore, comprising: injecting into said well bore a
slurry containing a first proppant at sufficiently high rates and
pressures to fracture said subterranean formation to accept said
slurry, said first proppant comprising fly ash particles as set
forth in claim 1.
16. The method of claim 15, including injecting a second proppant
into said well bore with said first proppant, said second proppant
having a larger mean particle size than said first proppant.
17. The method of claim 15, including injecting a second proppant
into said well bore after said first proppant, said second proppant
having a larger mean particle size than said first proppant.
18. A method of hydraulic fracturing of a subterranean formation
having a well bore, comprising: injecting into said well bore a
slurry containing a first proppant at sufficiently high rates and
pressures to fracture said subterranean formation to accept said
slurry, said first proppant comprising fly ash particles as set
forth in claim 8.
19. The method of claim 18, including injecting a second proppant
into said well bore with said first proppant, said second proppant
having a larger mean particle size than said first proppant.
20. The method of claim 18, including injecting a second proppant
into said well bore after said first proppant, said second proppant
having a larger mean particle size than said first proppant.
Description
RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent
Application Ser. No. 61/980,852 filed on 17 Apr. 2014, which is
hereby incorporated by reference in its entirety.
TECHNICAL FIELD
[0003] This document relates generally to the oil and gas well
field and more particularly to a proppant for use in hydraulic
fracturing to stimulate a well.
BACKGROUND
[0004] Oil and gas production via fracking require the drilling of
long horizontal wells, the fracturing of shale or siltstone, by
various methods, and finally the treatment of the well to stabilize
the fractures. This final step is typically accomplished by pumping
a slurry of natural or manufactured sand product into the fractured
zone to prop them open. Hence the term "proppant". The packed bed
must maintain a high degree of hydraulic conductivity as it must
readily pass oil, gas and water.
[0005] A tradeoff exists for the smaller size range of proppants
which are capable of stabilizing smaller fractures that generate a
lot of the oil and gas, an advantage, but typically result in a
lowered permeable pack, a drawback. The development of proppants
that are small enough to prevent the healing of micro fractures
(i.e. "micro-proppants"), but still demonstrate critical hydraulic
conductivity as well as strength, represents both formidable
challenge and a great opportunity.
[0006] This document describes a small diameter proppant, having
high hydraulic conductivity and strength, from the hydraulic and
mechanical classification of ponded Class F fly ash.
SUMMARY
[0007] In accordance with the purposes and benefits described
herein, a proppant is provided for use in hydraulic fracturing to
stimulate a well. The proppant may be broadly described as
comprising fly ash particles having a mean particle size (d50) of
between 45 .mu.m and 150 .mu.m and a size distribution defined by
(d10).ltoreq.5 .mu.m and a (d98).ltoreq.250 .mu.m. In one possible
embodiment, the fly ash particles have a particle roundness and
sphericity of greater than 0.8 as determined by ISO 13503-2006 (E),
part 7.
[0008] In one possible embodiment, the fly ash particles have a
hydraulic conductivity of 1.times.10.sup.-3 cm/sec or less as
determined by the falling head method. In one possible embodiment,
the fly ash particles have an absolute density of less than 2.4
g/cm.sup.3 as determined by part 10.5 of ISO 13503-2006 (E).
[0009] In one possible embodiment, the fly ash particles have a
bulk density of less than 1.1 g/cm.sup.3 as determined by part 10.3
ISO 13503-2006 (E). In one possible embodiment, the fly ash
particles have a void volume of greater than 45% as calculated from
absolute volume (AV) and bulk specific volume (BV) as derived from
the inverse of bulk density and absolute density. In one possible
embodiment, the fly ash particles have a loss on ignition of less
than 6% as determined by part 12 of ISO 13503-2006 (E).
[0010] In accordance with an additional aspect, a proppant is
provided for use in hydraulic fracturing to stimulate a well
wherein that proppant comprises fly ash particles having a mean
particle size (d50) of between 80 .mu.m and 100 .mu.m and a size
distribution defined by (d5).ltoreq.5 .mu.m and (d99.5).ltoreq.250
.mu.m. In one possible embodiment, the fly ash particles further
have a particle roundness and sphericity of greater than 0.9 as
determined by ISO 13503-2006 (E), part 7.
[0011] In one possible embodiment, the fly ash particles have a
hydraulic conductivity of 1.5.times.10.sup.-2 cm/sec or less as
determined by the falling head method. In one possible embodiment,
the fly ash particles have an absolute density of less than 2.3
g/cm.sup.3 as determined by part 10.5 of ISO 13503-2006 (E).
[0012] In one possible embodiment, the fly ash particles have a
bulk density of less than 1.0 g/cm.sup.3 as determined by part ISO
13503-2006 (E). In one possible embodiment, the fly ash particles
have a void volume of greater than 50% as calculated from absolute
volume (AV) and bulk specific volume (BV) as derived from the
inverse of bulk density and absolute density. In one possible
embodiment, the fly ash particles have a loss on ignition of less
than 2% as determined by part 12 of ISO 13503-2006 (E).
[0013] In accordance with an additional aspect, a method is
provided for hydraulic fracturing of a subterranean formation
having a well bore. That method includes injecting into the well
bore a slurry containing a first proppant at sufficiently high
rates and pressures to fracture the subterranean formation so as to
accept the slurry. That proppant is described in this document.
[0014] Further the method may include injecting a second proppant
into the well bore with the first proppant where the second
proppant has a larger mean particle size than the first
proppant.
[0015] In yet another embodiment, the method includes injecting a
second proppant into the well bore after the first proppant where
the second proppant has a larger mean particle size than the first
proppant.
[0016] In the following description, there are shown and described
several preferred embodiments of the proppant and the related
method of use. As it should be realized, the proppant and method of
use are capable of other, different embodiments and their several
details are capable of modification in various, obvious aspects all
without departing from the proppant and method as set forth and
described in the following claims. Accordingly, the drawings and
descriptions should be regarded as illustrative in nature and not
as restrictive.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
[0017] The accompanying drawing figures incorporated herein and
forming a part of the specification, illustrate several aspects of
the proppant and related method and together with the description
serve to explain certain principles thereof. In the drawing
figures:
[0018] FIG. 1 includes two scanning electron micrographs comparing
the relatively spherical surface of the fly ash proppant described
in this document to clay minerals.
[0019] FIG. 2 is a plot of cumulative percent surface area and
volume percent as a function of particle diameter for unclassified
fly ash.
[0020] FIG. 3 is a comparison of micro proppant with raw ponded fly
ash and hydraulically classified ash.
[0021] FIG. 4 is a comparison of particle fracture versus pressure
for Northern White Quartz NWQ and micro proppants.
[0022] FIG. 5 is a comparison of changes in void volume as a
function of pressure for proppants.
[0023] FIG. 6 illustrates how micro proppants are used in
combination with a coarse gravel pack.
[0024] FIG. 7 is a plot of volume percent less than 5 micron for
raw ash and hydraulically classified ash.
[0025] Reference will now be made in detail to the present
preferred embodiments of the proppant, examples of which are
illustrated in the accompanying drawing figures.
DETAILED DESCRIPTION
[0026] A proppant for use in hydraulic fracturing to stimulate a
well comprises fly ash particles having a mean particle size (d50)
of between 45 .mu.m and 150 .mu.m and a size distribution defined
by (d10).ltoreq.5 .mu.m and a (d98).ltoreq.250. The fly ash
particles have a particle roundness and sphericity of greater than
0.8 as determined by ISO 13503-2006 (E), part 7.
[0027] The fly ash particles have a hydraulic conductivity of
1.times.10.sup.-3 cm/sec or less as determined by the falling head
method. Further, the fly ash particles have an absolute density of
less than 2.4 g/cm.sup.3 as determined by part 10.5 of ISO
13503-2006 (E).
[0028] In addition, the fly ash particles have a bulk density of
less than 1.1 g/cm.sup.3 as determined by part 10.3 of ISO
13503-2006 (E). Further, the fly ash particles have a void volume
of greater than 45% as calculated from absolute volume (AV) and
bulk specific volume (BV) as derived from the inverse of bulk
density and absolute density, or VV=(BV-AV)/BV.times.100. Still
further, the fly ash particles have a loss on ignition of less than
6% as determined by part 12 of ISO 13503-2006 (E).
[0029] In accordance with an additional embodiment, the proppant
comprises fly ash particles having a mean particle size (d50) of
between 80 .mu.m and 100 .mu.m and a size distribution defined by
(d5).ltoreq.5 .mu.m and a (d99.5).ltoreq.250 .mu.m. Further, the
fly ash particles have a particle roundness and sphericity of
greater than 0.9 as determined by ISO 3503-2006 (E), part 7. Still
further, the fly ash particles have a hydraulic conductivity of
1.5.times.10.sup.-2 cm/sec or less as determined by the falling
head method.
[0030] In addition, the fly ash particles have an absolute density
of less than 2.3 g/cm.sup.3 as determined by part 10.5 ISO
13503-2006 (E). In addition, the fly ash particles have a bulk
density of less than 1.0 g/cm.sup.3 as determined by part 10.3 of
ISO 13503-2006 (E).
[0031] The fly ash particles also have a void volume of greater
than 50% as calculated from absolute volume (AV) and bulk specific
volume (BV) as derived from the inverse of bulk density and
absolute density, or VV=(BV-AV)/BV.times.100. Still further, the
fly ash particles have a loss on ignition of less than 2% as
determined by part 12 of ISO 13503-2006.
[0032] This document also relates to a method of hydraulic
fracturing a subterranean formation having a well bore. That method
may be broadly described as injecting into the well bore a slurry
containing a first proppant at sufficiently high rates and
pressures to fracture the subterranean formation to accept the
slurry where that first proppant comprises fly ash particles as set
forth and described in this document. In one possible embodiment,
the method further includes injecting a second proppant into the
well bore with the first proppant where the second proppant has a
larger mean particle size than the first proppant. That second
proppant may comprise sand or other proppants known in the art.
[0033] In yet another embodiment, the method includes injecting the
second proppant into the well bore after the first proppant.
The following further describes the proppant and the technology. It
is critical that the emplaced proppants not only keep micro
fractures from healing but also allow good flow of oil or gas
through it. The flow through a packed bed is described by the
well-known Carman-Kozeny equation which states:
Q A = .DELTA. p .epsilon. 3 .mu. L 5 ( 1 - .epsilon. ) 2 S 2
##EQU00001##
where Q is the volumetric flow rate, A is the face area of the bed,
L is the depth of the bed, .DELTA.p is the applied pressure drop, E
is the void volume of the bed and S is the volume specific surface
of the bed. Note that particle diameter is not described in the
equation. What Carman-Kozeny teaches is the importance of void
volume and surface area to flow through a packed media.
[0034] Void Volume, .epsilon.. The void volume or porosity of any
packed sand is not a function of particle size but rather particle
size distribution, shape and packing. The porosity of a bed of
large ball bearings is the same as the porosity of a bed of BB's if
each particle has the same diameter, the same surface smoothness
and identical packing. The greatest packing density achievable for
spherical particles of the same diameter is for hexagonal close
packing which results in a density D=.pi..sup.2 {square root over
(18)}=0.74 or about 26% void volume. However this is almost never
achieved due to physical constrains that the particles place on one
another (they lock or jam each other) and most systems are best
described by cubic close packing, which achieves a packing density
D=.pi./6=0.52 or about 48% void volume (in practice .about.45% void
volume is typical). Thus the more uniform the particles are the
more void volume is present, the higher the potential flow.
[0035] The roundness or spheriodicity of the particle is an
important factor in void volume. The discussion above only relates
to particle that are round or very nearly so. The American
Petroleum Institute (API) and ISO standards.sup.2,3,4,5 specifies a
spheriodicity of 0.7 or greater (shortest cord length of a particle
divided by the longest) for proppants. It is clear from the SEM of
FIG. 1 that fly ash has an almost perfect roundness with a
spheriodicity that approaches .about.1.0 for many of the
particles.
[0036] Surface Area, S. Where particle size does enter the equation
is in the surface area component of the equation, S2. This is
critical in providing resistance to flow or drag. The more internal
surface the more resistance to flow. The specific surface area of a
packed bed is a function of two parameters, particle size and
particle surface roughness. Surface roughness can be very critical.
The smoother the surface the better the flow. This is illustrated
in FIG. 1 which compares at similar resolution SEM's of a
classified Class F fly ash (the fine fraction in this case) with
that of kaolinite and illite. The latter are two common clay
minerals that are important components of most shale. The
difference in the surfaces of these materials is obvious.
[0037] The effect of particle sizes on surface area is also
important. For example the finest proppant size recognized by the
API is 70.times.140 mesh or 210.times.140 .mu.m. below this size
range the particles are considered too small to be useful as a
proppant.
[0038] The effect of particle size on surface area for unclassified
fly ash is illustrated in FIG. 2. This is a plot of the cumulative
volume and area, as percent, as a function of particle diameter (as
measured by laser). It is found that the particles that are smaller
than 10 .mu.m represent 30% of the volume of the ash but well over
90% of the total surface area.
[0039] An Effective Micro Proppant from Fly Ash.
[0040] Coal combustion fly ash has many positive characteristics
for use as a proppant. It is almost perfectly round and has a very
smooth surface. It has, however, a very broad particle size range
which results in low hydraulic conductivity. FIG. 2 illustrates the
need to produce a proppant with a paucity of particles less than 10
.mu.m in diameter. Also the discussion on void volume clearly
illustrates the need to produce a material with a very narrow size
distribution.
[0041] The critical steps in the production of a useful
micro-proppant from fly ash is the elimination of the smallest
particles, narrowing of the size distribution and increasing the
overall average particle size of the ash.
[0042] Hydraulic Conductivity of Proppants.
[0043] Class F fly ash, as defined by the ASTM C618 standard, were
used in this test work. Three different sources of ash were used,
two from Kentucky and one from Ohio. The samples included ash
collected dry and from ponds. The composition of these fly ash are
typically and generally representative of Class F ash as defined by
the ASTM Standard. The falling head method was used to measure the
hydraulic conductivity of fly ash and ash derived proppants. This
approach is considered more appropriate for fine grain specimens
than the constant head method and provides good reproducible and
comparable data. The method defines hydraulic conductivity, K, in
units of time/length and is normally standardized to 20.degree. C.
(K.sub.20). Values less that 10.sup.-6 cm/sec (0.036 cm/hr) are
considered to be impermeable. Typical values for unprocessed Class
F fly ash are in the range of 10.sup.-5 to 10.sup.-6 cm/sec. The
two raw ash samples tested produced permeabilities in this
range.
[0044] A hydraulically classified ash recovered from a pond in
Kentucky was tested along with ash from a pond in Ohio which was
double screened which we will refer to as micro-proppant 1 (MP-1)
and a double screened sample of ash from a power plant in northern
Kentucky (MP-2). A summary of the size data is presented in Table
1. Although only somewhat coarser than the raw ash, with a d50
(which is the particle diameter at which half of the particles are
smaller than on a volume basis) of 35 microns (.mu.m) versus
.about.23 .mu.m, the improvement in hydraulic conductivity is large
with a measured K20 of 7.8.times.10.sup.-4 cm/sec or almost an
order of magnitude better than the raw material
(8.5.times.10.sup.-5 cm/sec).
[0045] The two hydraulically classified and double screened ash had
the highest hydraulic conductivity with the samples screened at
100.times.200 mesh (150.times.74 .mu.m) having a value of
1.5.times.10.sup.-2 cm/sec (.about.1 cm/min). This hydraulic
conductivity is typical of that found for fine to medium size sand,
well within the range of proppants described in the API
series..sup.2,3 The effect of fines on the overall hydraulic
conductivity was examined by adding back the minus -200 mesh into
100.times.200 mesh material. This data is plotted in FIG. 3 as +10%
and +20%. The conductivity dropped by a factor of about 2 over this
range.
TABLE-US-00001 TABLE 1 comparison of size distribution and
hydraulic conductivity for raw materials and proppant Source d10
d50 d90 HC cm/sec Raw Fly Ash, Ky 2.2 24 70 8.6 .times. 10.sup.-5
Raw Fly Ash, Ky 3.5 23 65 8.5 .times. 10.sup.-5 Hydraulically
Classified, No. 1 Ky 12 35 60 7.8 .times. 10.sup.-4 Screened 63
.times. 125 .mu.m, Ohio 18 73 115 4.7 .times. 10.sup.-2 Ohio MP-1 +
20% Fines 8 80 133 7.7 .times. 10.sup.-3 Screened 74 x 150 .mu.m,
Ohio MP-1 9 86 136 1.5 .times. 10.sup.-2 Screened 74 x 150 .mu.m,
Ky MP-2 11 83 136 -- MP-2 5000 psi 8 59 110 -- MP-2 13,000 psi 6 46
100 --
[0046] The reason for the high hydraulic conductivity of the
proppant is readily apparent from the particle size distributions
(Table 1). The micro proppant is found to have a much higher d50
and d90 compared to that of the raw and hydraulically classified
materials, i.e. lower surface area/volume.
[0047] Void Volume of Ash Derived Proppants.
[0048] The void volume of the proppants can be calculated by their
particle density, and packing volume. The latter is determined by
test procedure 10.3 Bulk Density of ISO 13503-2:20065 which
measures to volume and weight of powders that fill a standard
cylinder from a standard funnel at a defined distance. The absolute
density of the particle was measured by gas pycnometer (test
procedure 10.5 Absolute Density of ISO 13503-2:2006).
TABLE-US-00002 TABLE 2 Comparison of void volumes of proppants NWQ
MP-1 MP-2 Particle Density 2.63 1.98 2.16 g/cm.sup.3 Particle
Volume 0.38 0.51 0.46 cm.sup.3/g Bulk Density 1.63 0.76 0.95
g/cm.sup.3 Bulk Volume 0.61 1.32 1.05 cm.sup.3/g % Void Volume 38%
62% 56%
[0049] The bulk, absolute (or particle) and calculated void volume
are presented for the two 74.times.150 .mu.m micro-proppants, and
20.times.40 mesh Northern White Quartz (NWQ) in Table 2. The NWQ is
a highly rounded almost pure quartz sand mined in Wisconsin and is
one of the mainstays of the fracking industry. The difference in
particle density is significant with the ash derived
micro-proppant, being less dense by as much as 25%. A more
remarkable difference is in the packing or bulk density. The NWQ
sample had a measured bulk density of 1.63 g/cm.sup.3 which is
close to the typical value of approximately 1.60 g/cm.sup.3 as
cited in the ISO standard. The ash derived micro-proppants had bulk
densities of 0.76 g/cm.sup.3 (MP-1) and 0.95 g/cm.sup.3 much less
than half that of the quartz sand.
[0050] These differences in density are significant. For example
the Stokes law settling velocity of the NWQ is about 47 cm/s, while
that of the micro-proppant is -0.3 to 0.4 cm/sec or two orders of
magnitude lower, which means that the ash derived micro-proppants
are more stable in a slurry form and will be easier to pump and
emplace without the need for coatings to enhance buoyancy or fluid
viscosity modifiers often needed for the gravel pack.
[0051] Crush Resistance and Void Volume Under Pressure.
[0052] After fracking and emplacement the proppants are subjected
to high pressures. An important parameter is the breakdown of the
particles with pressure as defined by the 11.5 Crush-resistance
procedure, of ISO 13503-2:2006). Data comparing the 20.times.40
mesh NWQ and the 100.times.200 mesh micro-proppants are presented
in Table 3 and FIG. 4. Also in the table is the change in void
volume as a function of pressure of the sample in the pressure cell
(See also FIG. 5).
TABLE-US-00003 TABLE 3 Comparison of fracture and void volume for
proppants NWQ MP-1 MP-2 NWQ MP-1 MP-2 psi Wt % Pan Void Volume %
500 0% 0% 0% 39% 60% 56% 2000 0% 34% 26% 36% 44% 42% 5000 16% 52%
36% 34% 34% 36% 7000 33% 56% 40% 32% 31% 33% 9000 46% 57% 42% 29%
28% 31% 11000 50% 59% 42% 26% 26% 30% 13000 53% 60% 44% 24% 24%
28%
[0053] The data is presented as weight percent of particles that
pass through the smallest appropriate sieve, 40 mesh for the quarts
and 200 mesh for the micro-proppant, after being subjected to the
indicated pressure. These particles are referred to as the "pan"
fraction. The micro-proppants are found to produce a higher
proportion of fractured particles at lower pressures than the
quartz. However in the high pressure ranges from 5000 psi and
above, the quartz degrades at a much higher rate increasing by 37%
from 5000 to 13,000 psi (.about.35 to .about.90 MPa) while the
micro-proppants increase by about 8% over this range. At pressures
above 9000 psi the MP-2 micro-proppant shows less degradation than
the quartz. Thus after an initial period of rapid fracturing for
the proppant the material becomes stiff and strong. The changes in
size distribution is presented in Table 1 for the MP-2 proppant. At
5000 psi (34.5 MPa) the material still has a size distribution
similar to that of the unstressed material with a d50 and d90 still
twice that of the raw ash. Even at 13000 psi (90 MPa) the
micro-proppant maintains a d50 well above that of the raw ash and
should maintain a serviceable hydraulic conductivity even at
extreme pressure.
[0054] The relative changes in void volume for the proppants is
plotted in FIG. 5. The micro-propants have essentially the same
void volume over the critical range of 4,000 to 8,000 psi as the
quartz. At higher pressures the MP-2 micro-proppant actually
maintains higher void volume than the NWQ. This is significant as
even though the initial deformation of the micro-proppant is higher
it still maintains significant void volume.
[0055] The behavior of the ash derived proppant is best explained
by the differences found in its absolute or particle density. The
particle density for the smaller sized fractions of this ash is
found to be higher than that of courser fractions. For example, for
MP-1, the determined density of the less than 200 mesh (<74
.mu.m) fraction was 2.51 g/cm.sup.3, while that of the
100.times.200 mesh micro-proppant was determined to be 1.97
g/cm.sup.3. This difference is due to the inclusion of bubbles in
the glass themselves from gases trapped during formation as there
are no significant differences in chemical composition. Assuming
the smaller fractions have near zero internal air voids, the
100.times.200 mesh proppant is calculated to have 21.5% internal
voids. Similarly, the MP-2 micro-proppant had a density of 2.16
g/cm.sup.3 and fine material was 2.6 g/cm.sup.3. A bubble content
of 17% is estimated. The lower void content may explain why this
material appears to be somewhat stronger than MP-1. These bubbles
give the proppant buoyancy and increase the flowablity of slurries.
They also represent particles that are easily fractured early.
However the glass itself appears to be very strong and is capable
of supporting and holding open fracture that are very small in
nature.
[0056] The Utilization of Fly Ash Derived Micro-Proppant.
[0057] Although in the discussion above it is compared to Northern
White Quartz and it may be feasible to use the micro-proppant on
its own, it is envisioned to be used in combination with other
coarser proppants including both natural and synthetic proppants.
The described ash derived proppant consists of a material that has
a considerably smaller size than the lower range of the API series
(i.e. 120.times.230 mesh versus 70.times.140 mesh), but possess the
hydraulic conductivity of the coarser material. This product will
be useful in the fracking industry to help improve the packing of
oil and gas wells by propping open micro fractures that are too
small for larger proppants. Preventing these micro fractures from
closing will help to extend the life and yield of the treated oil
and gas wells (FIG. 5).
[0058] Although the proposed micro-proppant will have applications
as a standalone product, it is also envisioned that it may also be
used with a sand or gravel sized proppant package. In this
application the micro-proppant would be used in a packing strategy
to prop micro fractures, followed by more conventional coarse sand
sized material (FIG. 6).
[0059] The Production of a Micro-Proppant from Coal Combustion Fly
Ash.
[0060] The creation of this product requires a Class F fly ash that
has a significant fraction of particles in the range of 45 to 150
.mu.m range and that are amenable to classification either by
hydraulic or pneumatic methods. The production of a proppant by dry
methods would include multiple operations or passes of materials
thorough air classifiers to remove the finest fractions and produce
a concentrate. If required the material may then be passed through
a double screening deck to produce a material with narrowed
particle size range.
[0061] Another very practical approach is the production of a
micro-proppant from wet ponded fly ash using a hydraulic classifier
or hydro-cyclone in conjunction with a double wet screening
process. This approach is capable of operating on highly
heterogeneous ash and produce classified materials with very narrow
size distributions.
[0062] A two-step approach was used to produce an ash based
proppant used in this discussion. First the ash is hydraulically
classified to remove most of the fine materials, thus reducing the
surface area of the proppant and increasing the yield of the
material in the appropriate size range. An example of a material
prepared with a cross current lamellae hydraulic classifier.sup.1
is shown in FIG. 7. The difference between the hydraulically
classified ash and raw ash for this case is clearly reflected in
the d10, the diameter where the cumulative volume reaches 10%,
which is 12.0 .mu.m for the classified ash, and 2.1 to 3.1 .mu.m
for unclassified ash (Table 1). This is also illustrated in FIG. 7
which is a cumulative plot of the particles below 5 .mu.m, for raw
ponded ash and hydraulically classified ash.
[0063] The yield of the micro-proppant is also increased by
hydraulic classification. In an example from the ponded ash located
in Ohio, the 100.times.200 mesh fraction was increased from an
average of 9% by weight to .about.25% in a single pass using a
hydraulic classification. It is noted here that the
pre-concentration of the ash is not essential to the production of
the micro-proppant but pre-concentration results in greater
efficiencies in subsequent processes such as double screening and
or spiral classification and separation, increasing yield and
reducing cost.
[0064] Ash has been processed from several sources including ash
ponds at the Kentucky Utilities (KU) Ghent power plant, LG&E's
Trimble County power plant, both located in Kentucky, TVA's
Kingston power plant in Tennessee and AEP's Cardinal power plant in
Ohio. All of these materials are Class F fly ash and behave
similarly, but not identically, in classification tests. The most
extensive testing as a proppant has been conducted on the ash from
the Cardinal power plant.
[0065] Advantages of ash derived micro-proppant: within the range
of this specification, this proppant has many advantages: [0066]
Almost perfect spherical shape. The morphology of the material is
derived from a suspended molten state producing perfectly round
balls. The shape of these particles facilitates stabile flow with
creating slurries that are pumpable long distances without
settlement or phase separation issues. [0067] High hydraulic
conductivity. Testing to date indicates that hydraulic
conductivities of the ash derived proppant in the vicinity of
10.sup.-2 cm/s are achievable, well above the hydraulic
conductivity of the parent material of 10.sup.-5 to 10.sup.-6 cm/s.
Thus a small silt size particle based proppant (e.g. 70-90 .mu.m
d50) demonstrated the hydraulic conductivity of a much coarser sand
sized material. [0068] Relatively low density. Ash derived proppant
particle densities are in the range of 1.97 to 2.2 g/cm.sup.3. The
small size and low density result in low settling velocities, e.g.
0.3-0.4 cm/s compared to 47 cm/s for 20.times.40 mesh quartz sand
for example. This will allow the use of lower viscosity fluids for
transport with improved efficiency of placement and cleaner post
fracking conditions. [0069] High strength. The ash derived proppant
is composed of a strong ceramic glass, many of which have a
naturally high mullite content, which is the mineral that gives
synthetic proppants much of their strength. These materials show
exceptional strength and stiffness at pressures in the 5,000 to
13,000 psi range. [0070] Very low surface area. The micro-proppant
has surface area that are approximately 25% or less than that of
the parent material. The low surface area results in greatly
improved flow in a packed bed. [0071] Simply modified surfaces. The
micro proppant has a glassy surface that readily accepts low cost,
environmentally friendly dispersants such as poly carboxylate, and
sulfonates. The proppant also accepts silanes which can be used to
enhance its hydrophobicity further increasing hydraulic
conductivity. [0072] Chemically inert. The Class F fly ash which
the micro proppant is derived from is essentially inert in a very
broad pH range (.about.2 to .about.12). [0073] Environmentally
green material. The micro proppant is derived from ponded and
landfilled Class F ash. There is somewhere between 500 million and
1 billion tons of ash located in these storage facilities in the
Eastern United States. This material represents the recycling of
what is currently a waste material. These materials have been
recovered from wet ponds and have been subjected to extensive
washings removing any readily soluble metals or salts.
REFERENCES
[0073] [0074] 1. Robl, T. L. and J. G. Groppo, 2011, Method for
Hydraulically Separating Carbon and Classifying Coal Combustion
Ash, U.S. Pat. No. 7,963,398. [0075] 2. API, RP 56 Recommended
practices for testing sand used in hydraulic fracturing operations.
[0076] 3. API RP 60 Recommended Practices for Testing High-Strength
Proppants Used in Hydraulic Fracturing Operations.
[0077] 4. ANSI/API, 2008, Recommended Practice 19c (supersedes API
56 and 60), Measurement of properties of proppants used in
hydraulic fracturing and gravel-packing operations, 1.sup.st Ed,
2008, 30 p [0078] 5. ISO 13503-2:2006, 2006, (almost Identical of
ANSI/API 19c) Petroleum and natural gas industries Completion
fluids and materials Part 2, Measurement of properties of proppants
used in hydraulic fracturing and gravel-packing operations. 28 p.
[0079] 6. Luo, et al., 2012, Cement-based particulates and methods
of use. U.S. Pat. No. 8,183,186, May 22, 2012. [0080] 7. Reyes, et
al Ceramic coated particulates, U.S. Pat. No. 8,119,576, Feb. 21,
2012. [0081] 8. Nguyen, et al. Methods and compositions for
consolidating particulate matter in a subterranean formation, U.S.
Pat. No. 8,534,356, Sep. 17, 2013. [0082] 9. Bicerano, J., Method
for the fracture stimulation of a subterranean formation having a
wellbore by using impact-modified thermoset polymer nanocomposite
particles as proppants. U.S. Pat. No. 8,492,316, Jul. 23, 2013
[0083] The foregoing has been presented for purposes of
illustration and description. It is not intended to be exhaustive
or to limit the embodiments to the precise form disclosed. Obvious
modifications and variations are possible in light of the above
teachings. All such modifications and variations are within the
scope of the appended claims when interpreted in accordance with
the breadth to which they are fairly, legally and equitably
entitled.
* * * * *