Note: Descriptions are shown in the official language in which they were submitted.
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HYDRAULIC FRACTURE INITIATION AND PROPAGATION CONTROL
IN UNCONSOLIDATED AND WEAKLY
CEMENTED SEDIMENTS
TECHNICAL FIELD
[0001] The present invention generally relates to enhanced recovery of
petroleum fluids
from the subsurface by injecting a fracture fluid to fracture underground
formations, and
more particularly to a method and apparatus to control the fracture initiation
plane and
propagation of the hydraulic fracture in a single well bore in unconsolidated
and weakly
cemented sediments resulting in increased production of petroleum fluids from
the subsurface
formation.
BACKGROUND OF THE INVENTION
[0002] Hydraulic fracturing of petroleum recovery wells enhances the
extraction of fluids
from low permeable formations due to the high permeability of the induced
fracture and the
size and extent of the fracture. A single hydraulic fracture from a well bore
results in
increased yield of extracted fluids from the formation. Hydraulic fracturing
of highly
permeable unconsolidated formations has enabled higher yield of extracted
fluids from the
formation and also reduced the inflow of formation sediments into the well
bore. Typically
the well casing is cemented into the borehole, and the casing perforated with
shots of
generally 0.5 inches in diameter over the depth interval to be fractured. The
formation is
hydraulically fractured by injected the fracturing fluid into the casing,
through the
perforations, and into the formation. The hydraulic connectivity of the
hydraulic fracture or
fractures formed in the formation may be poorly connected to the well bore due
to restrictions
and damage due to the perforations. Creating a hydraulic fracture in the
formation that is
well connected hydraulically to the well bore will increase the yield from the
well, result in
less inflow of formation sediments into the well bore and result in greater
recovery of the
petroleum reserves from the formation.
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[0003] Turning now to the prior art, hydraulic fracturing of subsurface earth
formations
to stimulate production of hydrocarbon fluids from subterranean formations has
been carried
out in many parts of the world for over fifty years. The earth is
hydraulically fractured either
through perforations in a cased well bore or in an isolated section of an open
bore hole. The
horizontal and vertical orientation of the hydraulic fracture is controlled by
the compressive
stress regime in the earth and the fabric of the formation. It is well known
in the art of rock
mechanics that a fracture will occur in a plane perpendicular to the direction
of the minimum
stress, see U.S. Patent No. 4,271,696 to Wood. At significant depth, one of
the horizontal
stresses is generally at a minimum, resulting in a vertical fracture formed by
the hydraulic
fracturing process. It is also well known in the art that the azimuth of the
vertical fracture is
controlled by the orientation of the minimum horizontal stress in consolidated
sediments and
brittle rocks.
[0004] At shallow depths, the horizontal stresses could be less or greater
than the vertical
overburden stress. If the horizontal stresses are less than the vertical
overburden stress, then
vertical fractures will be produced; whereas if the horizontal stresses are
greater than the
vertical overburden stress, then a horizontal fracture will be formed by the
hydraulic
fracturing process.
[00051 Techniques to induce a preferred horizontal orientation of the fracture
from a well
bore are well known. These techniques include slotting, by either a gaseous or
liquid jet
under pressure, to form a horizontal notch in an open bore hole. Such
techniques are
commonly used in the petroleum and environmental industry. The slotting
technique
performs satisfactorily in producing a horizontal fracture, provided that the
horizontal stresses
are greater than the vertical overburden stress, or the earth formation has
sufficient horizontal
layering or fabric to ensure that the fracture continues propagating in the
horizontal plane.
Perforations in a horizontal plane to induce a horizontal fracture from a
cased well bore have
been disclosed, but such perforations do not preferentially induce horizontal
fractures in
formations of low horizontal stress. See U.S. Patent No. 5,002,431 to Heymans.
[0006] Various means for creating vertical slots in a cased or uncased well
bore have
been disclosed. The prior art recognizes that a chain saw can be used for
slotting the casing.
See U.S. Patent No. 1,789,993 to Switzer; U.S. Patent No. 2,178,554 to Bowie,
et al., U.S.
Patent No. 3,225,828 to Wisenbaker, U.S. Patent No. 4,119,151 to Smith, U.S.
Patent No.
5,335,724 to Venditto et al.; U.S. Patent No. 5,372,195 to Swanson et al.; and
U.S. Patent No.
5,472,049 to Chaffee et al. Installing pre-slotted or weakened casing has also
been disclosed
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in the prior art as an alternative to perforating the casing, because such
perforations can result
in a reduced hydraulic connection of the formation to the well bore due to
pore collapse of
the formation surrounding the perforation. See U.S. Patent No. 5,103,911 to
Heijnen. These
methods in the prior art were not concerned with the initiation and
propagation of the
hydraulic fracture from the well bore in an unconsolidated or weakly cemented
sediment.
These methods were an alternative to perforating the casing to achieve better
connection
between the well bore and the surrounding formation and/or initiate the
fracture at a
particular location and/or orientation in the subsurface.
[0007] In the art of hydraulic fracturing subsurface earth formations from
subterranean
wells at depth, it is well known that the earth's compressive stresses at the
region of fluid
injection into the formation will typically result in the creation of a
vertical two "winged"
structure. This "winged" structure generally extends laterally from the well
bore in opposite
directions and in a plane generally normal to the minimum in situ horizontal
compressive
stress. This type of fracture is well known in the petroleum industry as that
which occurs
when a pressurized fracture fluid, usually a mixture of water and a gelling
agent together with
certain proppant material, is injected into the formation from a well bore
which is either cased
or uncased. Such fractures extend radially as well as vertically until the
fracture encounters a
zone or layer of earth material which is at a higher compressive stress or is
significantly
strong to inhibit further fracture propagation without increased injection
pressure.
[0008] It is also well known in the prior art that the azimuth of the vertical
hydraulic
fracture is controlled by the stress regime with the azimuth of the vertical
hydraulic fracture
being perpendicular to the minimum horizontal stress direction. Attempts to
initiate and
propagate a vertical hydraulic fracture at a preferred azimuth orientation
have not been
successful, and it is widely believed that the azimuth of a vertical hydraulic
fracture can only
be varied by changes in the earth's stress regime. Such alteration of the
earth's local stress
regime has been observed in petroleum reservoirs subject to significant
injection pressure and
during the withdrawal of fluids resulting in local azimuth changes of vertical
hydraulic
fractures.
[0009] Hydraulic fracturing generally consists of two types, propped and
unpropped
fracturing. Unpropped fracturing consists of acid fracturing in carbonate
formations and
water or low viscosity water slick fracturing for enhanced gas production in
tight formations.
Propped fracturing of low permeable rock formations enhances the formation
permeability
for ease of extracting petroleum hydrocarbons from the formation. Propped
fracturing of
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high permeable formations is for sand control, i.e. to reduce the inflow of
sand into the well
bore, by placing a highly permeable propped fracture in the formation and
pumping from the
fracture thus reducing the pressure gradients and fluid velocities due to draw
down of fluids
from the well bore. Hydraulic fracturing involves the literally breaking or
fracturing the rock
by injecting a specialized fluid into the well bore passing through
perforations in the casing to
the geological formation at pressures sufficient to initiate and/or extend the
fracture in the
formation. The theory of hydraulic fracturing utilizes linear elasticity and
brittle failure
theories to explain and quantify the hydraulic fracturing process. Such
theories and models
are highly developed and generally sufficient for art of initiating and
propagating hydraulic
fractures in brittle materials such as rock, but are totally inadequate in the
understanding and
art of initiating and propagating hydraulic fractures in ductile materials
such as
unconsolidated sands and weakly cemented formations.
[00010] Hydraulic fracturing has evolved into a highly complex process with
specialized
fluids, equipment, and monitoring systems. The fluids used in hydraulic
fracturing varied
depending on the application and can be water, oil, or multi-phased based.
Aqueous based
fracturing fluids consist of a polymeric gelling agent such as solvatable (or
hydratable)
polysaccharide, e.g. galactomannan gums, glycomannan gums, and cellulose
derivatives.
The purpose of the hydratable polysaccharides is to thicken the aqueous
solution and thus act
as viscosifiers, i.e. increase the viscosity by 100 times or more over the
base aqueous
solution. A cross-linking agent can be added which further increases the
viscosity of the
solution. The borate ion has been used extensively as a cross-linking agent
for hydrated guar
gums and other galactomannans, see U.S. Patent No. 3,059,909 to Wise. Other
suitable
cross-linking agents are chromium, iron, aluminum, zirconium (see U.S. Patent
No.
3,301,723 to Chrisp), and titanium (see U.S. Patent No. 3,888,312 to Tiner et
al). A breaker
is added to the solution to controllably degrade the viscous fracturing fluid.
Common
breakers are enzymes and catalyzed oxidizer breaker systems, with weak organic
acids
sometimes used.
[00011] Oil based fracturing fluids are generally based on a gel formed as a
reaction
product of aluminum phosphate ester and a base, typically sodium aluminate.
The reaction of
the ester and base creates a solution that yields high viscosity in diesels or
moderate to high
API gravity hydrocarbons. Gelled hydrocarbons are advantageous in water
sensitive oil
producing formations to avoid formation damage, that would otherwise be caused
by water
based fracturing fluids.
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[00012] Leak off of the fracturing fluid into the formation during the
injection process has
been conceptually separated into two types, spurt and linear or Carter leak
off. Spurt occurs
at the tip of the fracture and is the fracturing fluid lost to the formation
in this zone. In high
permeable formations spurt leak off can be a large portion of the total leak
off. Carter leak
off occurs along the fracture length as the fracture is propagated. Laboratory
methods are
used to quantify a fracturing fluid's leak off performance; however, analyses
of actual field
data on hydraulic fracturing of a formation is required to quantify the leak
off parameters in
situ, see U.S. Patent No. 6,076,046 to Vasudevan et al.
[00013] The method of controlling the azimuth of a vertical hydraulic fracture
in
formations of unconsolidated or weakly cemented soils and sediments by
slotting the well
bore or installing a pre-slotted or weakened casing at a predetermined azimuth
has been
disclosed. The method disclosed that a vertical hydraulic fracture can be
propagated at a pre-
determined azimuth in unconsolidated or weakly cemented sediments and that
multiple
orientated vertical hydraulic fractures at differing azimuths from a single
well bore can be
initiated and propagated for the enhancement of petroleum fluid production
from the
formation. See U.S. Patent No. 6,216,783 to Hocking et al, U.S. Patent No.
6,443,227 to
Hocking et al, U.S. Patent No. 6,991,037 to Hocking and U.S. Patent
Application No.
11/363,540. The method disclosed that a vertical hydraulic fracture can be
propagated at a
pre-determined azimuth in unconsolidated or weakly cemented sediments and that
multiple
orientated vertical hydraulic fractures at differing azimuths from a single
well bore can be
initiated and propagated for the enhancement of petroleum fluid production
from the
formation.
[00014] Accordingly, there is a need for a method and apparatus for
controlling the
initiation and propagation of a hydraulic fracture in a single well bore in
formations of
unconsolidated or weakly cemented sediments, which behave substantially
different from
brittle rocks in which most of the hydraulic fracturing experience is founded.
Also, there is a
need for a method and apparatus that hydraulically connects the installed
hydraulic fractures
to the well bore without the need to perforate the casing.
SUMMARY OF THE INVENTION
[00015] The present invention is a method and apparatus for dilating the earth
by various
means from a bore hole to initiate and propagate a vertical hydraulic fracture
formed at
various orientations from a single well bore in formations of unconsolidated
or weakly
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cemented sediments. The fractures are initiated by means of preferentially
dilating the earth
orthogonal to the desired fracture azimuth direction. This dilation of the
earth can be
generated by a variety of means: a driven spade to dilate the ground
orthogonal to the
required azimuth direction, packers that inflate and preferentially dilate the
ground
orthogonal to the required azimuth direction, pressurization of a pre-weakened
casing with
lines of weaknesses aligned in the required azimuth orientation,
pressurization of a casing
with opposing slots cut along the required azimuth direction, or
pressurization of a two
"winged" artificial vertical fracture generated by cutting or slotting the
casing, grout, and/or
formation at the required azimuth orientation. The initiation and propagation
of the hydraulic
fracture requires special consideration to the rate of the fracturing process
and viscosity of the
fracturing fluid to maintain the orientation and control of the hydraulic
fracture propagation
in unconsolidated and weakly cemented sediments.
[00016] Weakly cemented sediments behave like a ductile material in yield due
to the
predominantly frictional behavior and the low cohesion between the grains of
the sediment.
Such particulate materials do not fracture in the classic brittle rock mode,
and therefore the
fracturing process is significantly different from conventional rock hydraulic
fracturing.
Linear elastic fracture mechanics is not applicable to the hydraulic
fracturing process of
weakly cemented sediments like sands. The knowledge base of hydraulic
fracturing is
primarily from recent experience over the past ten years and much is still not
known on the
process of hydraulically fracturing these sediments. However, the present
invention provides
data to enable those skilled in the art of hydraulic fracturing a methods and
apparatus to
initiate and control the propagation of the hydraulic fracturing in weakly
cemented sediments.
The hydraulic fracturing process in these sediments involves the unloading of
the particulate
material in the vicinity of the dilation, generated pore pressure gradients
that, through
liquefaction and particulate dilation, create a path of minimum resistance for
the hydraulic
fracture to propagate further. Limits on the fracturing propagation rate are
needed to ensure
the propagating hydraulic fracture does not over run this zone and lead to a
loss of control of
the propagating process. Also the viscosity of the fracturing fluid in the
leading tip of the
hydraulic fracture needs to be maintained to ensure that the pore pressure
zone in front of the
propagating fracture is not destroyed by loss of low viscosity fracturing
fluid to the formation
being fractured.
[00017] Once the first vertical hydraulic fracture is formed, second and
subsequent
multiple vertical hydraulic fractures can be initiated by a casing or packer
system that seals
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off the first and earlier fractures and then by preferentially dilating the
earth orthogonal to the
next desired fracture azimuth direction, the second and subsequent fractures
are initiated and
controlled. The sequence of initiating the multiple azimuth orientated
fractures is such that
the induced earth horizontal stress from the earlier fractures is favorable
for the initiation and
control of the next and subsequent fractures. Alternatively multiple vertical
hydraulic
fractures at various orientations in the single well bore can be initiated and
propagated
simultaneously. The growth of each individual wing of each hydraulic fracture
can be
controlled by the individual connection and control of flow of fracturing
fluid from the
pumping system to each wing of the hydraulic fracture if required.
[00018] The present invention pertains to a method for forming a vertical
hydraulic
fracture or fractures in a weakly cemented formation from a single borehole
with the
initiation and propagation of the hydraulic fracture controlled to enhance
extraction of
petroleum fluids from the formation surrounding the borehole. As such any
casing system
used for the initiation and propagation of the fractures will have a mechanism
to ensure the
casing remains open following the formation of each fracture in order to
provide hydraulic
connection of the well bore to the hydraulic fractures.
[00019] The fracture fluid used to form the hydraulic fractures has two
purposes. First the
fracture fluid must be formulated in order to initiate and propagate the
fracture within the
underground formation. In that regard, the fracture fluid has certain
attributes. The fracture
fluid should not be pumped at rates that over run the dilating and modified
pore pressure zone
in front of the fracturing tip and also that low viscosity fracturing fluid
are not lost to the
formation and destroy the liquefied or loose zone in front of the fracturing
tip. The fracturing
fluid should have leak off characteristics compatible with the formation and
the pumping
equipment, the fracture fluid should be clean breaking with minimal residue,
and the fracture
fluid should have a low friction coefficient.
[00020] Second, once injected into the fracture, the fracture fluid forms a
highly permeable
hydraulic fracture. In that regard, the fracture fluid comprises a proppant
which produces the
highly permeable fracture. Such proppants are typically clean sand for large
massive
hydraulic fracture installations or specialized manufactured particles
(generally resin coated
sand or ceramic in composition) which are designed also to limit flow back of
the proppant
from the fracture into the well bore.
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[00021] The present invention is applicable to formations of unconsolidated or
weakly
cemented sediments with low cohesive strength compared to the vertical
overburden stress
prevailing at the depth of the hydraulic fracture. Low cohesive strength is
defined herein as
the greater of 200 pounds per square inch (psi) or 25% of the total vertical
overburden stress.
Examples of such unconsolidated or weakly cemented sediments are sand and
sandstone
formations, which have inherent high permeability but low strength that
requires hydraulic
fracturing to increase the yield of the petroleum fluids from such formations
and
simultaneously reducing the flow of formation sediments towards the well bore.
Upon
conventional hydraulic fracturing such formations will not yield the full
production potential
of the formation due to the lack of good hydraulic connection of the hydraulic
fracture in the
formation and the well bore, resulting in significant drawdown in the well
bore causing
formation sediments to flow towards the hydraulic fracture and the well bore.
The flow of
formation sediments towards the hydraulic fracture and the well bore, results
in a decline
over time of the yield of the extracted fluids from the formation for the same
drawdown in
the well. The present invention is applicable to formations of unconsolidated
or weakly
cemented sediments, such as oil sands, in which heavy oil (viscosity >100
centipoise) or
bitumen (extremely high viscosity >100,000 centipoise) is contained in the
pores of the
sediment. Even though these sediments are inherently permeable (in the Darcy
range) the
fluids are immobile due to their inherently high viscosity at reservoir
temperature and
pressure. Propped hydraulic fracturing of these sediments provides access for
steam,
solvents, oils, and convective heat to increase the mobility of the petroleum
hydrocarbons
either by heat or solvent dilution and thus aid in the extraction of the
hydrocarbons from the
formation.
[00022] Although the present invention contemplates the formation of fractures
which
generally extend laterally away from a vertical or near vertical well
penetrating an earth
formation and in a generally vertical plane in opposite directions from the
well, i.e. a vertical
two winged fracture, those skilled in the art will recognize that the
invention may be carried
out in earth formations wherein the fractures and the well bores can extend in
directions other
than vertical.
[00023] Therefore, the present invention provides a method and apparatus for
initiating
and controlling the growth of a vertical hydraulic fracture or fractures in a
single well bore in
formations of unconsolidated or weakly cemented sediments.
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[00024] Other objects, features and advantages of the present invention will
become
apparent upon reviewing the following description of the preferred embodiments
of the
invention, when taken in conjunction with the drawings and the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[00025] FIG. 1 is a horizontal cross-section view of a well casing having a
single
fracture dual winged initiation sections prior to initiation of the controlled
vertical fracture.
[00026] FIG. 2 is a cross-sectional side elevation view of a well casing
single fracture
dual winged initiation sections prior to initiation of the controlled vertical
fracture.
[00027] FIG. 3 is an enlarged horizontal cross-section view of a well casing
having a
single fracture dual winged initiation sections prior to initiation of the
controlled vertical
fracture.
[00028] FIG. 4 is a cross-sectional side elevation view of a well casing
having a single
fracture dual winged initiation sections prior to initiation of the controlled
vertical fracture.
[00029] FIG. 5 is a horizontal cross-section view of a well casing having a
single fracture
dual winged initiation sections after initiation of the controlled vertical
fracture.
[00030] FIG. 6 is a horizontal cross-section view of the hydraulic fracture at
initiation.
[00031] FIG. 7 is a horizontal cross-section view of the hydraulic fracture
during
propagation.
[00032] FIG. 8 is a cross-sectional side elevation view of two injection well
casings each
having a single fracture dual winged initiation sections located at two
distinct depths prior to
initiation of the controlled vertical fractures .
[00033] FIG. 9 is a horizontal cross-section view of a well casing having dual
fracture dual
winged initiation sections prior to the initiation of the controlled vertical
fractures.
[00034] FIG. 10 is a cross-sectional side elevation view of a well casing
having dual
fracture dual winged initiation sections prior to initiation of the controlled
vertical fractures.
[00035] FIG. 11 is a horizontal cross-section view of a well casing having
dual fracture
dual winged initiation sections after initiation of the second controlled
vertical fracture.
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DETAILED DESCRIPTION OF THE DISCLOSED EMBODIMENT
[00036] Several embodiments of the present invention are described below and
illustrated
in the accompanying drawings. The present invention involves a method and
apparatus for
initiating and propagating controlled vertical hydraulic fractures in
subsurface formations of
unconsolidated and weakly cemented sediments from a single well bore such as a
petroleum
production well. In addition, the present invention involves a method and
apparatus for
providing a high degree of hydraulic connection between the formed hydraulic
fractures and
the well bore to enhance production of petroleum fluids from the formation,
also to enable the
individual fracture wings to be propagated individually from its opposing
fracture wing, and
also to be able to re-fracture individually each fracture and fracture wing to
achieve thicker
and more permeable in placed fractures within the formation.
[00037] Referring to the drawings, in which like numerals indicate like
elements, FIGS. 1,
2, and 3 illustrate the initial setup of the method and apparatus for forming
a single controlled
vertical fracture with individual propagation control of each fracture wing.
Conventional
bore hole 4 is completed by wash rotary or cable tool methods into the
formation 7 of
unconsolidated or weakly cemented sediments to a predetermined depth 6 below
the ground
surface 5. Injection casing 1 is installed to the predetermined depth 6, and
the installation is
completed by placement of a grout 3 which completely fills the annular space
between the
outside the injection casing 1 and the bore hole 4. Injection casing 1
consists of two initiation
sections 11 and 21 (FIG. 3) to produce two hydraulic partings 71 and 72 which
in turn
produce a fracture orientated along plane 2, 2' as shown on FIG. 5. Injection
casing 1 must
be constructed from a material that can withstand the pressures that the
fracture fluid exerts
upon the interior of the injection casing 1 during the pressurization of the
fracture fluid. The
grout 3 can be any conventional material that preserves the spacing between
the exterior of
the injection casing 1 and the bore hole 4 throughout the fracturing
procedure, preferably a
non-shrink or low shrink cement based grout.
[00038] The outer surface of the injection casing 1 should be roughened or
manufactured
such that the grout 3 bonds to the injection casing 1 with a minimum strength
equal to the
down hole pressure required to initiate the controlled vertical fracture. The
bond strength of
the grout 3 to the outside surface of the casing 1 prevents the pressurized
fracture fluid from
short circuiting along the casing-to-grout interface up to the ground surface
5.
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[00039] Referring to FIGS. 1, 2, and 3, the injection casing 1 comprises a
single fracture
dual winged initiation sections 11 and 21 installed at a predetermined depth 6
within the bore
hole 4. The winged initiation sections 11 and 21 can be constructed from the
same material
as the injection casing 1. The winged initiation sections 11 and 21 are
aligned parallel with
and through the fracture plane 2, 2'. The fracture plane 2, 2' coincide with
the azimuth of the
controlled vertical hydraulic fracture formed by partings 71 and 72 (FIG. 5).
The position
below ground surface of the winged initiation sections 11 and 21 will depend
on the required
in situ geometry of the induced hydraulic fracture and the reservoir formation
properties and
recoverable reserves.
[00040] The winged initiation sections 11 and 21 of the well casing 1 are
preferably
constructed from two symmetrical halves as shown on FIG. 3. The configuration
of the
winged initiation sections 11 and 21 is not limited to the shape shown, but
the chosen
configuration must permit the fracture to propagate laterally in at least one
azimuth direction
along the fracture plane 2, 2'. In FIG. 3, prior to initiating the fracture,
the two symmetrical
halves of the winged initiation sections 11 and 21 are connected together by
shear fasteners
13 and 23, and the two symmetrical halves of the winged initiation sections 11
and 21 are
sealed by gaskets 12 and 22. The gaskets 12 and 22 and the fasteners 13 and 23
are designed
to keep the grout 3 from leaking into the interior of the winged initiation
sections 11 and 21
during the grout 3 placement. The gaskets 12 and 22 align with the fracture
plane 2, 2' and
define weakening lines between the winged initiation sections 11 and 21.
Particularly, the
winged initiation sections 11 and 21 are designed to separate along the
weakening line, which
coincides with the fracture plane 2, 2'. During fracture initiation, as shown
in FIG. 5, the
winged initiation sections 11 and 21 separate along the weakening line without
physical
damage to the winged initiation sections 11 and 21. Any means of connecting
the two
symmetrical halves of the winged initiation sections 11 and 21 can be used,
including but not
limited to clips, glue, or weakened fasteners, as long as the pressure exerted
by the fastening
means keeping the two symmetrical halves of the winged initiation sections 11
and 21
together is greater than the pressure of the grout 3 on the exterior of the
winged initiation
sections 11 and 21. In other words, the fasteners 13 and 23 must be sufficient
to prevent the
grout 3 from leaking into the interior of the winged initiation sections 11
and 21. The
fasteners 13 and 23 will open at a certain applied load during fracture
initiation and
progressively open further during fracture propagation and not close following
the
completion of the fracture. The fasteners 13 and 23 can consist of a variety
of devices
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provided they have a distinct opening pressure, they progressively open during
fracture
installation, and they remain open even under ground closure stress following
fracturing. The
fasteners 13 and 23 also limit the maximum amount of opening of the two
symmetrical
halves of the winged initiation sections 11 and 21. Particularly, each of the
fasteners 13 and
23 comprises a spring loaded wedge 18 that allows the fastener to be
progressively opened
during fracturing and remain open under compressive stresses during ground
closure
following fracturing with the amount of opening permitted determined by the
length of the
bolt 19.
[00041] Referring to FIG. 3, well screen sections 14, 15, 24 and 25 are
contained in the
two winged initiation sections 11 and 21. The screen sections 14, 15, 24 and
25 are slotted
portions of the two winged initiation sections 11 and 12 which limit the
passage of soil
particles from the formation into the well bore. The screen sections 14, 15
and 24, 25 provide
sliding surfaces 20 and 30 respectively enabling the initiation sections 11
and 21 to separate
during fracture initiation and propagation as shown on FIG. 5. Referring to
FIGS. 3 and 4,
the passages 16 and 26 are connected via the injection casing 1 top section 8
to openings 51
and 52 in the inner casing well bore passage 9, which is an extension of the
well bore passage
10 in the injection casing initiation section.
[00042] Referring to FIGS. 3, 4, and 5, prior to fracture initiation the inner
casing well
bore passage 9 and 10 is filled with sand 17 to below the lowest connecting
opening 51. A
single isolation packer 60 is lowered into the inner casing well bore passage
9 of the injection
casing top section 8 and expanded within this section at a location
immediately below the
lowermost opening 51 as shown on FIG. 4. The fracture fluid 40 is pumped from
the
pumping system into the pressure pipe 50, through the single isolation packer
60, into the
openings 51 and 52 and down to the passages 16 and 26 for initiation and
propagation of the
fracture along the azimuth plane 2, 2'. The isolation packer 60 controls the
proportion of
flow of fracturing fluid by a surface controlled value 55 within the packer
that control the
proportional flow of fracturing fluid that enters either of the openings 51
and 52 which
subsequently feed the passages 16 and 26 respectively and thus the flow of
fracturing fluid
that enters each wing 75 and 76 of the fracture. Referring to FIG. 5, as the
pressure of the
fracture fluid 40 is increased to a level which exceeds the lateral earth
pressures, the two
symmetrical halves 61, 62 of the winged initiation sections 11 and 21 will
begin to separate
along the fracture plane 2, 2' of the winged initiation sections 11 and 21
during fracture
initiation without physical damage to the two symmetrical halves 61, 62 of the
winged
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initiation sections 11 and 21. As the two symmetrical halves 61, 62 separate,
the gaskets 12
and 32 fracture, the screen sections 14, 15 and 24, 25 slide allowing
separation of the two
symmetrical halves 61, 62 along the fracture plane 2, 2', as shown in FIG. 5,
without physical
damage to the two symmetrical halves 61, 62 of the winged initiation sections
11 and 21.
During separation of the two symmetrical halves 61, 62 of the winged
initiation sections 11
and 21, the grout 3, which is bonded to the injection casing 1(FIG. 5) and the
two
symmetrical halves 61, 62 of the winged initiation sections 11 and 21, will
begin to dilate the
adjacent sediments 70 forming a partings 71 and 72 of the soil 70 along the
fracture plane 2,
2' of the planned azimuth of the controlled vertical fracture. The fracture
fluid 40 rapidly
fills the partings 71 and 72 of the soil 70 to create the first fracture.
Within the two
symmetrical halves 61, 62 of the winged initiation sections 11 and 21, the
fracture fluid 40
exerts normal forces 73 on the soil 70 perpendicular to the fracture plane 2,
2' and opposite to
the soil 70 horizontal stresses 74. Thus, the fracture fluid 40 progressively
extends the
partings 71 and 72 and continues to maintain the required azimuth of the
initiated fracture
along the plane 2, 2'. The azimuth controlled vertical fracture will be
expanded by
continuous pumping of the fracture fluid 40 until the desired geometry of the
first azimuth
controlled hydraulic fracture is achieved. The rate of flow of the fracturing
fluid that enters
each wing 75 and 76 respectively of the fracture is controlled to enable the
fracture to be
grown to the desired geometry. Without control of the flow of fracturing fluid
into each
individual wing 75 and 76 of the fracture, heterogeneities in the formation 70
could give rise
to differing propagation rates and pressures and result in unequal fracture
wing lengths or
undesirable fracture geometry.
[00043] The pumping rate of the fracturing fluid and the viscosity of the
fracturing fluids
needs to be controlled to initiate and propagate the fracture in a controlled
manner in weakly
cemented sediments. The dilation of the casing and grout imposes a dilation of
the formation
that generates an unloading zone in the soil as shown in FIGS. 5, 6, and 7,
and such dilation
of the formation reduces the pore pressure in the formation in front of the
fracturing tip.
Some of the dependent variables are defined as v, the velocity 65 (FIG. 6) of
the fracturing
fluid in the throat of the initiating and propagating fracture 68 (FIG. 7),
i.e. the fracture
propagation rate, w, the width 63 (FIG. 6) of the fracture 68 at initiation
and the estimated
width 66 (FIG. 7) during propagation, , the viscosity of the fracturing fluid
at the shear rate
during the fracturing process, p, the density of the fracturing fluid, L, the
half length 64 (FIG.
6) of the fracture during the fracturing process at initiation being the
radius of the casing and
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grout annulus during fracture initiation and the half length 67 (FIG.7) during
propagation, G,
the shear modulus of the weakly cemented sediment at reservoir pressure, and
ps, the density
of the weakly cemented sediment. Two important dimensionless numbers from
these
dependent variables are the Reynolds number Re=pvw/ and a form of the Euler
number
Eu=G/psv2. These dependent variables and the two dimensionless numbers are not
the entire
set of variables or dimensionless terms for similitude analysis, but by
limiting the
dimensionless Reynolds Number Re provides sufficient control of the dependent
variables to
initiate and propagate the hydraulic fracture in a controlled manner. The
dimensionless
number Eu infers that the fracture propagation velocity is proportional to the
square root of
the formation shear modulus, i.e. the stiffer the formation the greater the
fracture propagation
rate can be, and also that the fracture propagation is inversely proportional
to the square root
of the formation density. Because a stiffer formation typically also has a
greater density, then
Eu infers that the fracture propagation velocity is basically independent of
the formation
properties G and ps.
[00044] Numerous laboratory and field experiments of hydraulic fracture
initiation and
propagation in weakly cemented sediments have quantified that without dilation
of the
formation in a direction orthogonal to the plane of the intended fracture,
chaotic and/or
multiple fractures and/or cavity expansion/formation compaction zones are
created rather
than a single orientated fracture in a preferred azimuth direction
irrespective of the pumping
rate of the hydraulic fluid during attempted initiation of the fracture.
Similar laboratory and
field experiments of hydraulic fracture initiation and propagation in weakly
cemented
sediments have quantified that with dilation of the formation in a direction
orthogonal to the
plane of the intended fracture, if the pumping rate of the hydraulic fluid
during attempted
initiation of the fracture is not limited then chaotic and/or multiple
fractures and/or cavity
expansion/formation compaction zones are created rather than a single
orientated fracture in a
preferred azimuth direction. To ensure a repeatable single orientated
hydraulic fracture is
formed, the formation needs to be dilated orthogonal to the intended fracture
plane, the
fracturing fluid pumping rate needs to be limited so that the Re is typically -
0. 1 and certainly
does not exceed 1 during fracture initiation. At high Re, i.e. >10, chaotic
behavior is
observed. Also if the fracturing fluid can flow into the dilatant zone in the
formation and
negate the induce pore pressure from formation dilation then the fracture will
not propagate
along the intended azimuth. In order to ensure that the fracturing fluid does
not negate the
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pore pressure gradients in front of the fracture tip, its viscosity at
fracturing shear rates of -1-
20 sec-1 needs to be >100 centipoise.
[00045] For example, the casing and grout annulus have a diameter of 0.5 feet
(i.e. L at
initiation of 0.25 feet), the casing dilation is 0.5 inches (i.e. w is 0.5
inches at initiation),
fracture fluid density of 70 pounds mass/ft3 and viscosity of 1,000 centipoise
at the fracturing
fluid shear rate, pumping rate is initially 0.25 barrel per minute to dilate a
10 foot vertical
section of casing and grout annulus, then the velocity of fracture propagation
is 1.7 feet per
minute and Re is 0.1. Provided the formation is dilated by the casing and
grout annulus, and
the fracturing fluid is pumped at this rate, repeated single fractures will be
initiated in a
weakly cemented sediment at the intended azimuth, i.e. orthogonal to the
dilation plane.
Following fracture initiation the pumping rate can be increased as the
fracture propagates to
accommodate for the Carter leak off 69 (FIG. 7) directed perpendicular from
the fracture
plane into the formation) of the fracturing fluid into the formation, and the
larger the fracture
length L the greater is its ability to maintain its intended azimuth, provided
the pumping rate
and fracturing fluid do not exceed the limitation of a Re of 2.5 and the
fracturing fluid
viscosity at the tip is >100 centipoise.
[00046] Following completion of the fracture and breaking of the fracture
fluid 40, the
sand in the injection casing well bore passages 9 and 10 is washed out, and
the injection
casing acts as a production well bore for extraction of fluids from the
formation at the depths
and extents of the recently formed hydraulic fractures. The well screen
sections 14, 15 and
24, 25 span the opening of the well casing created by the first fracture and
act as conventional
well screen preventing proppant flow back into the production well bore
passages 10 and 9. If
necessary and prior to washing the sand from the production well bore passages
9 and 10 for
fluid extraction from the formation, it is possible to re-fracture the already
formed fractures
by first washing out the sand in passages 16 and 26 through the openings 51
and 52 and thus
re-fracture the first initiated fracture. Re-fracturing the fractures can
enable thicker and more
permeable fractures to be created in the formation.
[00047] Referring to FIGS. 4 and 5, once the fracture is initiated, injection
of a fracture
fluid 40 through the well bore passage 9 in the injection casing 1, into the
inner passages 16
and 26 of the initiation sections 11 and 21, and into the initiated fracture
can be made by any
conventional means to pressurize the fracture fluid 40. The conventional means
can include
any pumping arrangement to place the fracture fluid 40 under the pressure
necessary to
transport the fracture fluid 40 and the proppant into the initiated fracture
to assist in fracture
CA 02647324 2008-09-23
WO 2007/112175 PCT/US2007/063066
propagation and to create a vertical permeable proppant filled fracture in the
subsurface
formation. For successful fracture initiation and propagation to the desired
size and fracture
permeability, the preferred embodiment of the fracture fluid 40 should have
the following
characteristics.
[00048] The fracture fluid 40 should not excessively leak off or lose its
liquid fraction into
the adjacent unconsolidated soils and sediments. The fracture fluid 40 should
be able to carry
the solids fraction (the proppant) of the fracture fluid 40 at low flow
velocities that are
encountered at the edges of a maturing azimuth controlled vertical fracture.
The fracture
fluid 40 should have the functional properties for its end use such as
longevity, strength,
porosity, permeability, etc.
[00049] The fracture fluid 40 should be compatible with the proppant, the
subsurface
formation, and the formation fluids. Further, the fracture fluid 40 should be
capable of
controlling its viscosity to carry the proppant throughout the extent of the
induced fracture in
the formation. The fracture fluid 40 should be an efficient fluid, i.e. low
leak off from the
fracture into the formation, to be clean breaking with minimal residue, and to
have a low
friction coefficient. The fracture fluid 40 should not excessively leak off or
lose its liquid
fraction into the adjacent unconsolidated or weakly cemented formation. For
permeable
fractures, the gel composed of starch should be capable of being degraded
leaving minimal
residue and not impart the properties of the fracture proppant. A low friction
coefficient fluid
is required to reduce pumping head losses in piping and down the well bore.
When a
hydraulic permeable fracture is desired, typically a gel is used with the
proppant and the
fracture fluid. Preferable gels can comprise, without limitation of the
following: a water-
based guar gum gel, hydroxypropylguar (HPG), a natural polymer, or a cellulose-
based gel,
such as carboxymethylhydroxyethylcellulose (CMHEC).
[00050] The gel is generally cross-linked to achieve a sufficiently high
viscosity to
transport the proppant to the extremes of the fracture. Cross-linkers are
typically metallic
ions, such as borate, antimony, zirconium, etc., disbursed between the
polymers and produce
a strong attraction between the metallic ion and the hydroxyl or carboxy
groups. The gel is
water soluble in the uncrossed-linked state and water insoluble in the cross-
linked state.
While cross-linked, the gel can be extremely viscous thereby ensuring that the
proppant
remains suspended at all times. An enzyme breaker is added to controllably
degrade the
viscous cross-linked gel into water and sugars. The enzyme typically takes a
number of
hours to biodegrade the gel, and upon breaking the cross-link and degradation
of the gel, a
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permeable fracture filled with the proppant remains in the formation with
minimal gel
residue. For certain proppants, pH buffers can be added to the gel to ensure
the gel's in situ
pH is within a suitable range for enzyme activity.
[00051] The fracture fluid-gel-proppant mixture is injected into the formation
and carries
the proppant to the extremes of the fracture. Upon propagation of the fracture
to the required
lateral and vertical extent, the predetermined fracture thickness may need to
be increased by
utilizing the process of tip screen out or by re-fracturing the already
induced fractures. The
tip screen out process involves modifying the proppant loading and/or fracture
fluid 40
properties to achieve a proppant bridge at the fracture tip. The fracture
fluid 40 is further
injected after tip screen out, but rather then extending the fracture
laterally or vertically, the
injected fluid widens, i.e. thickens, the fracture. Re-fracturing of the
already induced
fractures enables thicker and more permeable fractures to be installed, and
also provides the
ability to preferentially inject steam, carbon dioxide, chemicals, etc to
provide enhanced
recovery of the petroleum fluids from the formation.
[00052] The density of the fracture fluid 40 can be altered by increasing or
decreasing the
proppant loading or modifying the density of the proppant material. In many
cases, the
fracture fluid 40 density will be controlled to ensure the fracture propagates
downwards
initially and achieves the required height of the planned fracture. Such
downward fracture
propagation depends on the in situ horizontal formation stress gradient with
depth and
requires the gel density to be typically greater than 1.25 gm/cc.
[00053] The viscosity of the fracture fluid 40 should be sufficiently high to
ensure the
proppant remains suspended during injection into the subsurface, otherwise
dense proppant
materials will sink or settle out and light proppant materials will flow or
rise in the fracture
fluid 40. The required viscosity of the fracture fluid 40 depends on the
density contrast of the
proppant and the gel and on the proppant's maximum particulate diameter. For
medium
grain-size particles, that is of grain size similar to a medium sand, a
fracture fluid 40 viscosity
needs to be typically greater than 100 centipoise at a shear rate of sec-1.
[00054] Referring to FIG. 8, two injection casings 91 and 92 are set at
different distinct
depths 93 and 94 in the borehole 95 and grouted into the formation by grout 3
filling the
annular space between the injection casings 91 and 92 and the borehole 95. The
lower
injection casing 91 is fractured first, by filling the well bore passage 110
with sand to just
below the lower most openings 101 and 102. The isolation packer 100 is lowered
into the
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well bore passage 110 to just below the lowest opening 101 and expanded in the
well bore
passage 110 to achieve individual flow rate control of the fracturing fluid
that enters the
openings 101 and 102 respectively. The fracture fluid 120 is pumped into the
isolation
packer pipe string 105 and passes through the isolation packer 100 and into
the openings 101
and 102 to initiate the vertical hydraulic fracture as described earlier.
Following completion
of the fracture in the first injection casing 91, the process is repeated by
raising the isolation
packer 100 to just below the lower most openings 111 and initiate the first
fracture in the
second injection casing 92, and the whole process is repeated to create all of
the fractures in
the injection casings installed in the bore hole 95.
[00055] Another embodiment of the present invention is shown on FIGS. 9, 10,
and 11,
consisting of an injection casing 96 inserted in a bore hole 97 and grouted in
place by a grout
98. The injection casing 96 consists of four symmetrical fracture initiation
sections 121, 131,
141, and 151 to install a total of two hydraulic fractures on the different
azimuth planes 122,
122' and 123, 123'. The passage for the first initiated fracture inducing
passages 126 and 166
are connected to the openings 127 and 167, and the first fracture is initiated
and propagated
along the azimuth plane 122, 122' with controlled propagation of each
individual wing of the
fracture as described earlier. The second fracture inducing passages 146 and
186 are
connected to the openings 147 and 187, and the second fracture is
initiated.and propagated
along the azimuth plane 123, 123' as described earlier. The process results in
two hydraulic
fractures installed from a single well bore at different azimuths as shown on
FIG. 11.
[00056] Finally, it will be understood that the preferred embodiment has been
disclosed by
way of example, and that other modifications may occur to those skilled in the
art without
departing from the scope and spirit of the appended claims.
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