U.S. patent application number 15/284137 was filed with the patent office on 2017-01-26 for hydraulic fracturing system.
The applicant listed for this patent is SCHLUMBERGER TECHNOLOGY CORPORATION. Invention is credited to Christopher Fredd, Richard Hutchins, Sergey Mikhailovich Makarychev-Mikhailov.
Application Number | 20170022411 15/284137 |
Document ID | / |
Family ID | 44226684 |
Filed Date | 2017-01-26 |
United States Patent
Application |
20170022411 |
Kind Code |
A1 |
Makarychev-Mikhailov; Sergey
Mikhailovich ; et al. |
January 26, 2017 |
HYDRAULIC FRACTURING SYSTEM
Abstract
A method is given for fracturing a formation, in particular
far-field in a tight formation, in which at least a portion of the
proppant is crushable in situ at some point during pumping, during
fracture closure, or at higher stresses experienced later during
fracture closure. The closure stress or hydrostatic stress is
estimated, then a proppant is selected that is at least partially
crushable at that closure stress, and then the fracturing treatment
is performed with at least a portion of the total proppant being
the selected crushable proppant.
Inventors: |
Makarychev-Mikhailov; Sergey
Mikhailovich; (Richmond, TX) ; Hutchins; Richard;
(Sugar Land, TX) ; Fredd; Christopher; (Westfield,
NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SCHLUMBERGER TECHNOLOGY CORPORATION |
Sugar Land |
TX |
US |
|
|
Family ID: |
44226684 |
Appl. No.: |
15/284137 |
Filed: |
October 3, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13520331 |
Oct 1, 2012 |
9458710 |
|
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PCT/RU2009/000757 |
Dec 31, 2009 |
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15284137 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C09K 2208/28 20130101;
C09K 8/62 20130101; E21B 43/267 20130101; C09K 8/80 20130101; C09K
8/805 20130101 |
International
Class: |
C09K 8/80 20060101
C09K008/80; C09K 8/62 20060101 C09K008/62; E21B 43/267 20060101
E21B043/267 |
Claims
1. A method of hydraulic fracturing a subterranean formation
penetrated by a wellbore comprising (a) estimating the closure
stress in a fracture, (b) selecting a crushable proppant that
produces more than about 20 percent fines in a crush test using
that closure stress, and (c) injecting a slurry of the proppant in
a carrier fluid into the formation.
2. The method of claim 1 wherein the crushable proppant is in the
form of spheres, plates, disks, rods, cylinders, platelets, flakes,
sheets, scales, husks, chips, shells, lumps and mixtures
thereof.
3. The method of claim 1 wherein the crushable proppant comprises
particles of at least two different shapes that have at least two
different crush strengths.
4. The method of claim 1 wherein the crushable proppant comprises
particles of at least two different materials that have at least
two different crush strengths.
5. The method of claim 1 wherein the crushable proppant is selected
from the group consisting of ceramic hollow spheres, glass or
ceramic microspheres and microballoons, ceno spheres, plerospheres
and combinations thereof.
6. The method of claim 1 wherein the crushable proppant comprises
materials with closed porosity.
7. The method of claim 6 wherein the materials with closed
porosity, are selected from the group consisting of glass and
ceramics, rocks and minerals, polymers and plastics, metals and
alloys, composite materials, biomaterials and combinations
thereof.
8. The method of claim 6 wherein the materials with closed porosity
have fibrous, arch/cellular, mesh, mesh/cellular, honeycomb,
bubble, sponge-like or foam structures and combinations
thereof.
9. The method of claim 1 wherein the crushable proppant comprises
finer material that has been formed into larger particles by
agglomeration or binding.
10. The method claim 1 wherein the crushable proppant is
coated.
11. The method of claim 1 wherein the crushable proppant comprises
from 10 to 100% of the total solids in the slurry.
12. The method of claim 1 wherein the crushable proppant produces
more than 15 percent fines in a crush test using the closure stress
of the formation.
13. The method of claim 1 wherein the crushable proppant produces
more than 10 percent fines in a crush test using the closure stress
of the formation.
14. The method of claim 1 wherein step (c) is followed by injection
of a slurry in which the proppant is not crushable.
15. The method of claim 14 where a cycle of alternating proppant
types is repeated a plurality of times.
16. The method of claim 14 wherein the crushable proppant generates
less than about 6 to about 20 percent fines in a crush test using
the closure stress of the formation
17. The method of claim 1 wherein a portion of the crushable
proppant is crushed during step (c).
18. The method of claim 1 wherein a portion of the crushable
proppant is crushed when the fracture closes after step (c).
19. The method of claim 1 wherein the formation has a permeability
of less than about 001 mD and the proppant loading is less than
about 4.88 kg/m.sup.2.
20. The method of claim 1 wherein the proppant consists of at least
10 weight percent of mica or cenospheres or mixtures thereof.
21. The method of claim 1 wherein the proppant is continuously
added to a carrier fluid injected into the formation.
22. The method of claim 1 wherein the crush strength of the
material is chosen so that at least a portion of the crush occurs
after initial cleanup of the well.
23. The method of claim 1 wherein the surface treating pressures
are reduced relative to injecting conventional proppant at similar
proppant concentrations.
24. The method of claim 1 wherein the settling velocity is less
than that of 150 micron sand.
25. A method of hydraulic fracturing a subterranean formation
penetrated by a wellbore comprising (a) estimating the hydraulic
pressure to which materials are exposed during pumping, (b)
selecting a crushable proppant that produces more than about 20
percent fines in a crush test using that hydraulic pressure, and
(c) injecting a slurry of the proppant in a carrier fluid into the
formation.
26. The method of claim 25 wherein the hydrostatic pressure is
changed during the step of injecting to control crushing of the
crushable proppant material.
27. The method of claim 26 wherein the rate of injection is
increased.
Description
BACKGROUND OF THE INVENTION
[0001] Hydraulic fracturing is an effective method of increasing
hydrocarbon production. The method involves pumping of a fracturing
fluid into a subterranean formation (i.e. reservoir) through a
wellbore under a pressure exceeding the formation stress. A
propping material is placed in the resulting fractures to prevent
them from closing, which, thus, provides unimpeded flow paths and
enhanced transport of hydrocarbons from the reservoir to the
wellbore.
[0002] The art of hydraulic fracturing is based to a great extent
on materials: the fluids with their various constituents and the
proppants with optional auxiliary particulates. The proppant
materials are intended to provide enhanced hydraulic conductivity
of a fracture under the formation closure stress. The traditional
approach to proppant design is focused on several material
characteristics, which include: a) compressive strength or crush
resistance under formation closure stress, to avoid generation of
fines, which are known to damage proppant pack conductivity; b) low
specific gravity, to place the proppant deep into a fracture with a
fluid of reasonable viscosity; c) substantially spherical proppant
particulate shape with smooth particle surface and uniform size
distribution to maximize proppant pack permeability; and d) low
material cost. These parameters are contradictory, so there is
usually a trade-off between the properties. As an example, proppant
crush resistance, which is a characteristic of material mechanical
strength, often conflicts with the required proppant low density
and low cost. The choice of proppant also strongly depends on the
properties of the targeted reservoir, which can vary significantly.
Therefore, while proppant pack conductivity is often considered as
a primary proppant characteristic, in certain cases, it can be
sacrificed to achieve other benefits. In very tight reservoirs,
even very low fracture conductivity will still result in a suitable
flow path for hydrocarbons entering from the formation.
[0003] It would be desirable to have an inexpensive proppant that
is readily transported deep into fractured formations by low
viscosity fluids and need not have low specific gravity, particle
size or shape uniformity, or strength.
SUMMARY OF THE INVENTION
[0004] One embodiment of the invention is a method of hydraulic
fracturing a subterranean formation penetrated by a wellbore
including the steps of (a) estimating the closure stress in a
fracture, (b) selecting a crushable proppant that produces more
than about 20 percent fines in a crush test using that closure
stress, and (c) injecting a slurry of the proppant in a carrier
fluid into the formation. The crushable proppant may be, for
example, in the form of spheres, plates, disks, rods, cylinders,
platelets, flakes, sheets, scales, husks, chips, shells, lumps and
mixtures thereof. The crushable proppant may be a mixture of
particles of at least two different shapes that have at least two
different crush strengths. The crushable proppant may include
particles of at least two different materials that have at least
two different crush strengths. The crushable proppant may be
entirely or partially ceramic hollow spheres, glass or ceramic
microspheres and microballoons, cenospheres, plerospheres and
combinations of those materials. The crushable proppant may be
partially or entirely made of materials with closed porosity, such
as glass and ceramics, rocks and minerals, polymers and plastics,
metals and alloys, composite materials, biomaterials and
combinations of those materials. The materials with closed porosity
may have fibrous, arch/cellular, mesh, mesh/cellular, honeycomb,
bubble, sponge-like or foam structures and combinations of these
structures. The crushable proppant may be made of finer material
that has been formed into larger particles by agglomeration or
binding. The crushable proppant may be coated. The crushable
proppant may be used at a concentration of from about 10 to about
100% of the total solids in the slurry. The crushable proppant
should produce more than about 10 percent, preferably more than
about 15 percent, fines in a crush test using the closure stress of
the formation.
[0005] In other embodiments of the invention, step (c) is followed
by injection of a slurry in which the proppant is not crushable. A
cycle of alternating proppant types may be repeated a plurality of
times. The non-crushable proppant should generate less than about 6
to about 20 percent fines in a crush test using the closure stress
of the formation, for example as delineated in API RP56 for various
mesh sizes of proppant. Optionally, a portion of the crushable
proppant may be crushed during step (c) and/or a portion of the
crushable proppant may be crushed when the fracture closes after
step (c).
[0006] In other embodiments, the formation may have a permeability
of less than about 0.001 mD and the proppant loading may be less
than about 4.88 kg/m.sup.2. The proppant optionally includes at
least 10 weight percent of mica or cenospheres or mixtures of those
materials. Optionally, the proppant may be continuously added to a
carrier fluid injected into the formation. The crush strength of
the material may be chosen so that at least a portion of the crush
occurs after initial cleanup of the well. The surface treating
pressures may be reduced relative to injecting conventional
proppant at similar proppant concentrations. The settling velocity
may be less than that of 150 micron sand.
[0007] Yet another embodiment is a method of hydraulic fracturing a
subterranean formation penetrated by a wellbore including the steps
of (a) estimating the hydraulic pressure to which materials will be
exposed during pumping, (b) selecting a crushable proppant that
produces more than about 20 percent fines in a crush test using
that hydraulic pressure, and (c) injecting a slurry of the proppant
in a carrier fluid into the formation. The hydrostatic pressure may
be changed during the step of injecting to control crushing of the
crushable proppant material, for example the rate of injection may
be increased.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 shows spherical proppant particles having a diameter
just smaller than a tube.
[0009] FIG. 2 shows packs of plate-like particles in a
fracture.
[0010] FIG. 3 shows experimental proppant pack conductivities of
muscovite mica at various proppant loadings and various closure
pressures.
[0011] FIG. 4 shows the dependence of mica pack conductivity on
proppant loading at various closure stresses.
[0012] FIG. 5 shows experimental proppant pack conductivities of
muscovite mica MD250 at a proppant loading of 0.49 kg/m.sup.2 at
various closure pressures and various flow rates.
[0013] FIG. 6 shows conductivity data for cenospheres and mica at a
proppant loading of 0.49 kg/m.sup.2 under various closure
stresses.
[0014] FIG. 7 compares experimental settling velocities of
conventional fracturing sands and mica flakes.
DETAILED DESCRIPTION OF THE INVENTION
[0015] Although the following discussion emphasizes fracturing far
into tight formations, the proppants and methods of the Invention
may be used in any fracturing setting. The invention will be
described in terms of treatment of vertical wells, but is equally
applicable to wells of any orientation. The invention will be
described for hydrocarbon production wells (gas, oil, condensate),
but it is to be understood that the invention may be used for wells
producing other fluids, such as water or carbon dioxide, or, for
example, for injection or storage wells. It should also be
understood that throughout this specification, when a concentration
or amount range is described as being useful, or suitable, or the
like, it is intended that any and every concentration or amount
within the range, including the end points, is to be considered as
having been stated. Furthermore, each numerical value should be
read once as modified by the term "about" (unless already expressly
so modified) and then read again as not to be so modified unless
otherwise stated in context. For example, "a range of from 1 to 10"
is to be read as indicating each and every possible number along
the continuum between about 1 and about 10. In other words, when a
certain range is expressed, even if only a few specific data points
are explicitly identified or referred to within the range, or even
when no data points are referred to within the range, it is to be
understood that the inventors appreciate and understand that any
and all data points within the range are to be considered to have
been specified, and that the inventors have possession of the
entire range and all points within the range.
[0016] We have found methods of hydraulic fracturing utilizing
crushable particulates, which provide sufficient and cost effective
fracture conductivity insignificantly dependent on closure stress.
Furthermore, such particulates can also be used to deliver proppant
far-field (deep into the reservoir away from the wellbore) into a
complex fracture network, where no high strength proppants can be
placed via current practices. Potential applications of the
crushable particulates include unconventional reservoirs, for
example tight gas shales, because the required proppant
conductivity in such reservoirs can be relatively low and the
proppant transport properties become much more important. (The term
proppant is used here to refer to materials with sufficient
compressive strength to hold a fracture open.) In one embodiment of
the invention, crush of the spherical or non-spherical particulates
leads to relative increase of effective porosity in the pack, due
to opening of flow channels. The closure stress impact on the
proppant pack conductivity is significantly diminished, as with
pack compression and decrease of the total pack porosity, the
fraction of effective porosity increases. In another embodiment of
the invention, hollow and/or highly porous lightweight particulates
are delivered deep into a fracture, wherein they are crushed with a
formation closure stress, and still provide sufficient conductivity
to the fracture, as it is propped with fragments of the initial
particulates.
[0017] All proppants are crushable at some closure stress, so
proppants may inadvertently have been used above their crush
strengths in the past. This has always been considered deleterious
to treatment effectiveness and success, but we have now discovered
that under certain circumstances it may be advantageous to estimate
the closure stress that will be experienced in a proposed
fracturing treatment and then to select a proppant that will be
substantially crushed under those circumstances Not all proppants
are suitable as the crushable proppants of the invention; they
should have certain properties as injected, such as slow settling,
and certain properties when crushed. For example, the base material
may have sufficient strength to support the formation stresses, but
may have defects in the structure and/or in the mechanical design
or structure of the material that results in weak points that will
break under formation closure stress. Generation of fines upon
crushing when using large proppant sizes can be a problem, as fines
restrict the conductivity of large-proppant packs. In the present
invention, however, particles are selected to crush during pumping
to create small particles that transport effectively or during
fracture closure to create adequate conductivity. Relevant
properties include size and shape; mixtures of different sizes
and/or shapes can be used. Also, crush resistance for some of the
shapes should be less than the available stress. When using a
mixture of shapes, some shapes should have substantially different
crush strengths and may exceed the available stress. For example, a
hollow sphere proppant material that crushes under hydrostatic
pressure can be used to create small, strong particles that
transport effectively and support formation closure stresses.
[0018] It is widely accepted that proppant crush is highly
undesirable, as it leads to formation of fines, which fill pores of
the proppant pack, thus decreasing its hydraulic conductivity.
Significant efforts have been made in the past to develop high
strength proppants (HSP) and many such products are available
commercially. Most HSP's are based on ceramic materials and are
characterized by relatively high cost and high specific
gravity.
[0019] Patent Application PCT/RU2008/000566 discloses plate-like
materials, which, while they possess quite low conductivity,
compensate with significantly enhanced transport properties, e.g.
slower settling compared to sand of similar density and particle
size. The reason for such slowed settling is so-called glided
settling, as the plate-like particle settling path length is
significantly increased. While conductivity of mica in packs is
quite low, it can be delivered far-field with a fluid of low
viscosity without settling issues, which deleteriously affect
spherical particulates of similar size. Note that the cost of mica
can be significantly lower than that of polymeric materials. Other
benefits of the plate-like proppants include diminished proppant
embedment into formation fracture faces, due to the greater contact
surface area of proppant particles with the formation, and enhanced
flowback control. Preferred plate-like proppants are layered rocks
and minerals; most preferred is mica, for example muscovite mica.
Some of these materials may be crushable and suitable for use in
the present Invention.
[0020] We define a plate-like particle as one which possesses three
average dimensions in which the largest dimension is at least two
times the smallest dimension and the third dimension can be smaller
or equal to the largest dimension. Thus, discs with a thickness of
less than one-half the diameter qualify as plate-like particles.
Rod-like and fiber-like particles can also be used. Preferably,
such particles should have an aspect ratio of at least about 2,
most preferably at least about 3; preferably they should have a
maximum length of about 5 mm, most preferably about 3 mm. Such
particles can be made, for example, of glass or ceramics or may be
of natural origin, for example basalt fibers, asbestos fibers and
the like.
[0021] Another trend in proppant development targets lightweight
and ultra lightweight particulates. These proppants are intended
for use in unconventional reservoirs (i.e. gas shales, tight gas
sands), where slickwater is used as a fracturing fluid. Slickwater
is typically a dilute solution of a friction reducer (a polymer is
added to decrease pumping pressure), the viscosity of which
generally does not exceed about 10 mPa-s. Slickwater treatments are
pumped in large volumes in gas shales to create complex fracture
networks, which are believed to enhance gas production. As delivery
of a proppant into the fracture network with a fluid of low
viscosity is challenging, the conventional approach to proppant
development for slickwater treatments is to reduce the proppant
specific gravity. Lightweight proppants based on polymer composites
have been commercialized; they commonly have specific gravities of
from about 1.08 to about 1.25 g/cm.sup.3. The main problem with
these proppants is their cost, but they are technically suitable
for the present Invention.
[0022] Therefore, other suitable proppants are described in U.S.
Pat. No. 4,547,468 which discloses hollow fine-grained ceramic
proppants, that have a crushing strength equal to or greater than
that of Ottawa sand at closure stresses above 5000 psi (34.5 MPa).
Also suitable, U.S. Pat. No. 7,220,454 and U.S. Patent Application
Publication 20070154268 describe high strength polycrystalline
ceramic spheres and methods of making hollow spheres of alumina or
aluminate by coating of polymeric beads with alumoxane, heating the
particles to convert alumoxane to alumina, removal of the polymeric
beads from inside the coating by washing with a solvent, and
sintering the resulted hollow particles to give high strength
.alpha.-alumina spheres. Also suitable as proppants in the
Invention are those described in U.S. Patent Application
Publication Nos. 20070154268, 20070166541, 20070202318, and
20080135245; these disclose proppants having a suitable crush
resistance and/or buoyancy as shown by specific gravity. These
proppants generally are made with a template material and a shell
on the template material; the shell is a ceramic material or oxide
thereof or a metal oxide. The template material may be a hollow
sphere and may be a single particle, such as a cenosphere.
[0023] The concept of the present Invention differs from the
aforementioned patents and patent applications, and others
describing hollow lightweight proppants, by utilization of the
hollow particle as a transport vehicle for the true proppant only.
The proppant material in the present Invention is not the particles
themselves, but rather their fragments. Such an approach completely
changes the design of the proppant material, as there is no need to
reinforce the particulates, but rather they may be crushable, and
chosen to provide the best possible conductivity of the crushed
material.
[0024] The stress distribution in a pack of spherical particles is
usually quite uniform; however, above critical compressive stresses
the particles start crushing. While proppant particle crushing has
previously been believed to be bad for the pack permeability, this
is not necessarily true. One can consider a simple notional
experiment (see Example 1 below) which demonstrates that proppant
crush can actually increase permeability under certain conditions.
If the fracturing slurry contains particulates which are subject to
crushing at the closure stresses, such a crush, however, may induce
generation of channels (flow paths) in the pack, and hence enhances
permeability, which is then insignificantly affected by closure
stress magnitude. The crushable particulates may be approximately
spherical or in the form of plates, disks, platelets, flakes,
sheets, cylinders, rods, scales, husks, chips, shells and mixtures
thereof (see Examples 2 and 3 below). If not spherical, the
crushable particulates may have any aspect ratio.
[0025] Alternatively, the fracturing slurry may contain crushable
particulates of low specific gravity. Such particulate material may
be chosen from a variety of lightweight materials, including, but
not limited to, ceramic hollow spheres, glass microspheres and
microballoons, cenospheres, plerospheres (char and ash cenospheres
which have their cavities filled by finer particles of ash and
other materials), various porous materials, including rocks and
minerals, ceramics, cements, polymers; and various composite
materials and mixtures of such materials. The preferred material is
cenospheres, hollow spherical ceramic particles made of aluminum
and silica, which are a byproduct of coal combustion and are found
in fly ash. The particles are filled with air and have apparent
specific gravities of from about 0.4 to about 0.8 g/cm.sup.3. Their
primary use is as fillers for cements to make low density concrete
(see Example 4 below). However, placement of such lightweight
particulates deep into a fracture or complex fracture network may
be easily achieved by means of fluids of low viscosity, i.e.
slickwater, because the particles are generally buoyant. When the
fracture is closed, the particulates are subjected to crush, which
generates particle fragments, which still prop the remote fractures
open, thus providing permeability sufficient for hydrocarbon
production enhancement.
[0026] Alternatively, the crushable proppants of the Invention may
be particulates made of materials with closed porosity, for example
glass and ceramics, rocks and minerals, polymers and plastics,
metals and alloys, composite materials, biomaterials and
combinations of such materials. Such closed porosity materials may
have fibrous, arch/cellular, mesh, mesh/cellular, honeycomb,
bubble, sponge-like or foam structures and combinations of such
structures. Any proppant under sufficient closure pressure is a
crushable proppant.
[0027] Alternatively, the crushable proppants of the Invention may
be crushed into fragments due to hydrostatic pressures encountered
during pumping into the reservoir. In this case, fine mesh proppant
materials are created in situ during pumping. An example is fine
mesh materials that are delivered to the location in a
granulated/pelletized form. Crushing of the aggregates during
pumping or fracture closure reduces dusting and other HSE risks
encountered at the surface. Cenospheres and other fragile
particulates fall into this category; at least a portion of their
crushing may occur during pumping, not necessarily under formation
closure stress.
[0028] Alternatively, the proppant may include a mixture of
plate-like or, as other examples, rod-like or cylinder-like and
either approximately spherical particles or irregular particles so
that the plates or, as other examples, rods or cylinders will trap
the spheres or irregular particles in between layers. This
increases the permeability of the pack of plates or, as other
examples, rod-like or cylinder-like materials. Then, under stress,
the spheres may be a failure point (if they are lower strength than
the plates) or an initiation point for the plates to crack (if they
have higher crush strength than the plates). Before any cracking of
plates occurs, the permeability should be higher with the
spheres.
[0029] It is preferred that crushing of the crushable proppants of
the Invention produces particulates which are less than an order of
magnitude smaller than the parent particles. The size distribution
of the crushed material may be determined by experiments such as
the API RP 56 test described below.
[0030] All or a portion of the crushable particles may be coated to
increase their strength, alter their wettability, provide higher
closed porosity and thus better transport properties, reduce fines
formation, decrease the friction during pumping or decrease their
adhesion to each other. Suitable materials for enhancing the
properties may include quaternary hydrophobic or hydrophilic
absorbents, adsorbed surfactants, silicones, fluorocarbons, or
polymers which impart desirable surface properties to the
particles. As another example, crushable proppant can be coated
with resin coating that would provide higher crush resistance,
thereby providing higher conductivity during the initial flowback
of the well where stresses are lower to enhance fluid cleanup, and
then the resin coated particles crush at higher stresses during
production to create a fine mesh pack. Further, the particle faces
can be etched by chemical or optical methods to make the surfaces
rough rather than smooth to enhance permeability.
[0031] The proppants and methods of the Invention are particularly
suitable to very tight formations, which, as used herein, refers to
formations having a permeability less than about 1 millidarcy, and
in various embodiments, less than about 100 microdarcy, less than
about 10 microdarcy, less than about 1 microdarcy, or less than
about 500 nanodarcy. These formations have such low permeability
that the wells can be effectively stimulated in one embodiment with
an overall or primary final fracture conductivity on the order of
0.3 to 30 mD-m (1 to 100 mD-ft) and/or with secondary and/or
tertiary fractures on the order of 0.0003 to 3 mD-m (0.001 to 10
mD-ft), where secondary fractures are understood to refer to
usually relatively smaller fractures in length and/or width,
branching from the primary fractures, and tertiary fractures refer
to usually relatively smaller fractures in length and/or width,
branching from the secondary fractures. As an example, the
crushable proppants of the Invention may be used for treating
formations with permeabilities less than 0.001 mD where the
proppant loading is less than about 4.88 kg/m.sup.2 (1
lb/ft.sup.2), preferably less than about 2 kg/m.sup.2 (0.5
lb/ft.sup.2). As another example, the crushable proppants of the
Invention may be used for treating formations where the generated
fractures are not substantially lateral but may include a mixture
of induced lateral and transverse flow paths; the proppant is
transported throughout the network of induced flow paths. Crushable
proppant can be continually injected or slugs of larger particles
can be used to promote transport.
[0032] Because the effectiveness of the final proppant pack does
not rely on the porosity or permeability of the packed matrix of
the proppant as injected to impart flow conductivity to the
fracture, the availability of the option to select a wider range of
proppant materials can be an advantage in embodiments of the
present invention. For example, the proppant may have a range of
mixed, variable diameters, shapes, strengths or other properties
that yield a suitable proppant pack after closure and crushing of
at least some of the proppant. If the proppant as injected is
uniform in properties, at least some of it must be crushed; if the
proppant is a mixture of different materials or of one material but
a mixture of, for example, sizes and/or shapes, then at least one
of the different proppants must be at least partially crushable
under closure conditions. The crushable particles make up from
about 10 percent to about 100 percent of the total particles in the
fluid, preferably from about 30 to about 100 percent. The preferred
concentration of crushable particles in the fluid is from about 0.1
to about 1200 kg/m.sup.3 (10 ppa), most preferably from about 120
kg/m.sup.3 to about 240 kg/m.sup.3 (0.1 to about 2 ppa). The other
proppant material may, for example, be conventional proppant
materials, such as sands, ceramics, sintered bauxites, glass beads,
minerals, polymers, plastics, naturally occurring and composite
materials and combinations of these materials.
[0033] Optionally, conventional proppants may be used to fill that
portion of the fracture nearer the wellbore where, because of the
size and geometry of the fracture created, the advantages of the
crushable proppants of the Invention may not be needed. (The nearer
the wellbore, typically the wider and less complex the fracture.)
This conventional proppant may have a crush strength above the
closure pressure. This conventional proppant material may, for
example, be conventional proppant materials, such as sands,
ceramics, sintered bauxites, glass beads, minerals, polymers,
plastics, naturally occurring and composite materials and
combinations of these materials.
[0034] Any surface and downhole equipment, any pumping schedule,
and any fracturing or slickwater fluids, may be used with the
crushable proppants and methods of the Invention, provided only
that the crushable particles are not substantially broken by the
equipment before they reach the final pack position. Optionally,
the equipment/proppant can be engineered to break the proppant
prior to placement--for example in the wellbore (in situ crushing).
Any additives conventionally used in fracturing or slickwater
fluids may also be used. When applied with low viscosity fluids
that have difficulty in transporting conventional proppants, such
as slickwater, higher concentrations of the crushable proppant can
be employed as that reduces the settling rate. Alternatively, the
practice of alternating slugs of slickwater without proppant and
slickwater with proppant can be eliminated and proppant can be
continuously added to the slickwater. This practice reduces the use
of water which can have a significant impact on the economics and
simplifies the pumping operation. The amounts and concentrations of
total proppant can also be the same as for treatments of similar
formations with similar fluids without crushable proppants.
[0035] While the fundamental connection is believed to exist, no
correlations have yet been reported between material compressive
strength and proppant crush resistance. The latter is usually
estimated for conventional proppants according to the API RP 56
method in a special crush cell under different applied loadings.
The fines generated are measured by means of sieve analysis and
compared with the data prescribed by the API RP. The easiest way to
define the crushable particulate proppants of the Invention is to
define them as those which do not meet the API RP 56 crush
resistance specifications. Note that therefore we define "fines" as
being any particles produced by crushing of the original
proppant.
[0036] API RP 56 describes the crush test as being done in a press
having platens that can be maintained parallel and apply a load;
the cell typically has an inside diameter of 5.08 cm (2 inches) and
a piston length of 8.89 cm (3.5 inches). Proppant of a given mesh
size range is placed in the cell at a concentration of 19.5
kg/m.sup.2 (4 lb/ft.sup.2), which for the cell described is 40 g,
and leveled by inserting the piston and rotating it in one
direction. A load is applied, taking 1 minute to reach the maximum,
and held for 2 minutes. The load is released and the proppant
removed and sieved in a sieve shaker for 10 minutes. Any particles
smaller than the smallest mesh size loaded are considered fines.
Results are compared to the suggested specifications given in Table
1:
TABLE-US-00001 TABLE 1 Mesh Size Mesh Size Stress Stress Maximum
(U.S.) (mm) (psi) (MPa) Fines 6/12 1.68-3.36 2000 13.8 20 8/16
1.19-2.38 2000 13.8 18 12/20 0.84-1.68 3000 20.7 16 16/30 0.59-1.19
3000 20.2 14 20/40 0.42-0.84 4000 27.6 14 30/50 0.297-0.59 4000
27.6 10 40/70 0.21-0.42 5000 34.5 8 70/140 0.105-0.21 5000 34.5
6
The present invention can be further understood from the following
examples.
EXAMPLE 1
[0037] A tube with a fixed inner diameter is filled with fragile
spherical proppant particles having nearly the same diameter as the
tube (see FIG. 1A). In this case, before closure stress is applied,
the tube will have almost zero permeability; even though the total
porosity of the pack might be quite high, the porosity available
for fluid flow is negligible. Once an external closure stress is
applied, and the inner diameter is reduced, the proppant particles
start crushing (FIG. 1B), the total pack volume decreases and so
does the total pack porosity. However, the proppant crushing leads
to opening of pore space and the relative effective porosity (which
is a fraction of the total porosity) increases, which results in an
increase in the tube permeability.
EXAMPLE 2
[0038] For non-spherically shaped materials, for example plates,
this effect can be even more pronounced, as the conditions in this
latter case are much closer to those encountered in the field, as
compared to those in the first example. Consider non-spherical
particles, for example plates, in a fracture having parallel walls.
Without any externally applied stress, the particles are oriented
randomly (except perhaps for some orientation induced by transport
fluid flow) and a substantial fraction of the pack porosity is
confined (see FIG. 2A). Once the stress is applied, the particles
tend to align with the walls and some of them are crushed, opening
flow channels (see FIG. 2B). The effect of closure stress on the
permeability of the pack therefore may be quite complicated, and
permeability and effective porosity generally can increase with
closure stress applied on the pack. A similar result can be
obtained with rods and/or cylinders.
Laboratory Experiments
Materials
[0039] A commercial muscovite mica sample was obtained from Minelco
Specialties Limited, Derby, UK. It was designated MD250; the number
in the code represents the approximate maximum flake diameter in
microns. The thickness of these mica particles was about 20 to 25
microns. The manufacturer described the material as dry ground,
highly delaminated potassium aluminum silicate Muscovite Mica
flakes having a melting point of about 1300.degree. C., a specific
gravity of about 2.8, a pH of about 9 as a 10% slurry in water, and
as being flexible, elastic, tough, and having a high aspect ratio.
The MD250 material is 99.9% smaller than 250 microns, 10-50%
smaller than 125 microns, and 0-15% smaller than 63 microns.
[0040] Cenospheres were obtained from Sphere Services, Inc., Oak
Ridge, Tenn., USA; they are lightweight, inert, hollow spheres made
of silica and alumina and filled with air and/or gases. Cenospheres
are a naturally occurring by-product of the burning process at
coal-fired power plants, and they have most of the same properties
as manufactured hollow-sphere products. The size given by the
manufacturer is 10 to 350 microns.
Standard Conductivity Apparatus
[0041] The conductivity apparatus consisted of a 90, 700 kg (100
ton) load press with automated hydraulic intensifiers and API
conductivity cells having 64.5 cm.sup.2 flow paths. The apparatus
could attain a maximum closure stress of 138 MPa and a maximum
temperature of 177.degree. C. The temperature of the conductivity
cells was controlled by electrically heated platens contacting the
sides of the cell. Precision metering pumps were used to pump brine
through the cell during flowback and conductivity measurements. The
pumps drew 2 wt % KCl brine from a 20 l flowback reservoir. The
brine was vacuum degassed and nitrogen sparged to prevent the
introduction of metal oxides into the proppant pack. The brine was
pumped through a silica saturation system prior to entering the
conductivity cell. Rosemount pressure gauges (with upper limits of
690 Pa, 62 kPa and 2 MPa) were used to measure the pressure drop
across the conductivity cell. Digital linear variable displacement
transducers or telescope width gauges were used to measure the
distances between the cores to monitor fracture widths. The
conductivity apparatus was automated for controlling closure stress
ramps, flow schedules and temperature, as well as providing data
acquisition and real-time conductivity/permeability
calculations.
Split Core Apparatus
[0042] A Formation Response Tester (FRT) Model 6100 obtained from
Chandler Engineering (Broken Arrow, Okla., USA) was used for split
core conductivity measurements. A proppant pack was placed between
two metal semi-cylindrical cores, which were inserted into a rubber
sleeve, in place of the traditional cylindrical rock core sample.
The confining pressure was applied to the sleeve with a manual
hydraulic pump. The fully automated core flow instrument allowed
the operator to sequence various fluids (including acids) through a
core sample. The system was designed to handle acids and other
corrosive fluids at temperatures up to 177.degree. C. The core
could be up to 3.81 cm in diameter and up to 17.1 cm long.
Operating pressure and temperature were limited to 41.4 MPa and
177.degree. C. The direction of flow was flexible; flows from top
to bottom, across the core face, and bottom to top (system flush)
could all be done. The differential pressure was measured across
the core sample using two Rosemount precision differential pressure
sensors. During the execution of a test, conductivity was measured
using a 0 to 2.75 MPa differential pressure transducer or a 0 to
41.4 MPa transducer, depending on the range and level of precision
required.
EXAMPLE 3
[0043] The conductivities of muscovite mica MD250 packs at proppant
loadings of 2.44 and 9.77 kg/m.sup.2 were measured with the
standard conductivity apparatus, while special precautions were
taken to avoid parasitic flows due to the low proppant pack
permeabilities. (Compared to conventional proppant packs, the packs
of plate-like mica particles were characterized by very low
conductivities and by non-conventional stress distributions in the
packs. Ohio sandstone cores are usually used for API conductivity
tests; however, in our case flows through such cores were possible,
which would have strongly affected conductivity results. Other
parasitic flows might have existed due to unusually high pressure
drops in the cells (up to 1.72 MPa (250 psi)), e.g. flows along the
conductivity cell walls. Other challenges in conductivity
measurements of mica packs have been faced. Special precautions
used included: a) utilization of aluminum cores instead of
sandstone ones; b) sealing the core edges with room temperature
vulcanized rubber; c) sealing the cell walls with a silicon vacuum
grease.) The conductivities of the mica packs at proppant loadings
of 0.49 kg/m.sup.2 and below were measured by means of the
split-core apparatus. The steady state conductivity data for the
mica packs are presented graphically in FIG. 3. The conductivities
of the packs depended only weakly on the proppant loadings, as
shown in FIG. 4. In this range of proppant loadings the
conductivity would have been expected to be proportional to the
proppant loading. Formation of channels in the pack decreased this
dependence, as is demonstrated in FIG. 4.
EXAMPLE 4
[0044] The conductivity of a 0.49 kg/m.sup.2 mica pack was found to
be strongly dependent on flow rates, as shown in FIG. 5. The
formation of channels in the mica packs was observed when the
conductivity cell was disassembled. The higher the flow rate, the
more channels could be seen.
EXAMPLE 5
[0045] The conductivity of 0.49 kg/m.sup.2 cenospheres was measured
and compared with that of mica. The cenospheres exhibited a drastic
drop in conductivity at closure stresses above 14 MPa, due to
particle failure, as shown in FIG. 6. However, the retained
conductivity may be sufficient to enhance production from extremely
low permeable unconventional reservoirs, for example gas shales
where conductivities as low as 1.4 mD-cm (0.05 mD-ft) are
acceptable in secondary and tertiary fractures.
EXAMPLE 6
[0046] Static proppant settling measurements were performed in
slickwater, comprising tap water containing 0.05 wt. % of a
polyacrylamide-based friction reducer. The fluid was placed into a
500 ml graduated cylinder with a ruler fixed on its side. A portion
of proppant slurry (proppant in slickwater, 1:1 by volume) was
slowly introduced into the cylinder with a spatula and the settling
front was photographed at 1-2 second intervals. The falling front
path was calculated and the terminal settling velocity was
determined from the linear part of the curve path vs. time. Three
replicate measurements were made for each material in each fluid,
and the velocities were averaged. The settling velocities are shown
in FIG. 7. Mica flakes demonstrated significantly slower settling
rates than the conventional silica sands that are widely used as
proppants. Cenospheres did not exhibit any settling at all, as the
particles were floating on the slickwater surface.
* * * * *