Note: Descriptions are shown in the official language in which they were submitted.
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PLUG FOR WELL DRILLING
TECHNICAL FIELD
[0001] The present invention relates to a plug for well drilling used in
well drilling for
the purpose of producing hydrocarbon resources such as petroleum, natural gas,
or the
like; and a well drilling method.
BACKGROUND
[0002] Hydrocarbon resources such as petroleum, natural gas, and the like
have been
excavated and produced through wells (oil wells and gas wells; hereinafter
referred to
collectively as "wells") having porous and permeable subterranean formations.
As energy
consumption increases, deeper wells are being drilled, reaching depths greater
than 9000
m worldwide and greater than 6000 m in Japan. In wells that are continuously
excavated,
the productive layer is stimulated in order to continuously excavate
hydrocarbon
resources efficiently from subterranean formations of which permeability has
decreased
over time and subterranean formations of which permeability is not
insufficient
inherently. Known stimulation methods include acid treatment and fracturing
(Patent
Document 1). Acid treatment is a method in which the permeability of the
productive
layer is increased by injecting a strongly acidic compound such as
hydrochloric acid or
hydrogen fluoride into the productive layer and dissolving the reaction
components of
bedrock (carbonates, clay minerals, silicates, and the like). However, various
problems
that accompany the use of strong acids have been identified, and increased
costs,
including various countermeasures, have also been pointed out. Accordingly,
methods for
forming fractures _____________________________________________________
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(referred to as "fracturing" or "hydraulic fracturing") in the productive
layer using fluid
pressure have received attention.
[0003] Hydraulic fracturing is a method in which fractures are generated in
the
productive layer by fluid pressure such as water pressure (also simply called
"hydraulic
pressure" hereinafter). Generally, a vertical hole is drilled, and then the
vertical hole is
curved and a horizontal hole is drilled in a subterranean formation several
thousand
meters underground. Fracturing fluid is then fed into these boreholes (meaning
holes
provided for forming a well, also called "downholes") at high pressure, and
fractures are
produced by the hydraulic pressure in the deep subterranean productive layer
(layer that
produces the hydrocarbon resource such as petroleum or natural gas), and the
productive
layer is thereby stimulated in order to extract the hydrocarbon resource
through the
fractures and the like. The efficacy of hydraulic fracturing has also been
examined for the
development of unconventional resources such as shale oil (oil that matures in
shale) and
shale gas.
[0004] Fractures formed by fluid pressure such as water pressure immediately
close due
to formation pressure when the hydraulic pressure is no longer applied. To
prevent a
fracture from closing, a proppant is included in the fracturing fluid (that
is, the well
treatment fluid used in fracturing), which is fed into the borehole, thereby
distributing the
proppant in the fracture. Inorganic or organic materials are used as proppants
included in
fracturing fluid, but silica and alumina and other inorganic particles have
been
conventionally used, and sand particles such as 20/40-mesh sand have been
widely used
because they are capable of preventing fracture closure in a very deep
subterranean
environment under high-temperature and high-pressure for a long time.
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[0005] Various types of water-based, oil-based, and emulsion-based fluids are
used as
well treatment fluids such as fracturing fluid and the like. Because the well
treatment
fluid must have the function of transporting the proppant to the location
where the
fracture is generated in the borehole, it generally must have a prescribed
viscosity, good
proppant dispersibility, ease of after-treatment, and low environmental load.
Furthermore,
fracturing fluid sometimes contains a channelant in order to form flow paths
through
which shale oil, shale gas, and the like can pass among the proppant.
Accordingly, in
addition to the proppant, various additives are used in well treatment fluid,
such as
channelants, gelling agents, antiscale agents, acids for dissolving rock and
the like,
friction-reducing agents, and the like.
[0006] The following method is typically used to produce fractures by
hydraulic pressure
in the productive layer of a deep subterranean production layer (layer that
produces the
hydrocarbon resource such as petroleum such as shale oil or natural gas such
as shale gas
or the like) using fracturing fluid. Specifically, a prescribed section of a
borehole
(downhole) drilled into a subterranean formation several thousand meters deep
is partially
plugged while blocking sequentially from the tip portion of the borehole, and
fracturing
fluid is fed at high pressure into the plugged section to produce fractures in
the productive
layer. Thereafter, the prescribed section (typically before the leading
section, specifically,
the surface side section) is plugged and fracturing is carried out. This
process is repeated
until the necessary blocking and fracturing is completed.
[0007] Stimulation of the productive layer by fracturing is sometimes also
performed
again not only for drilling of new wells but for desired sections of existing
boreholes. In
this case as well, the operations of borehole plugging, fracturing, and the
like may be
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similarly repeated. Additionally, there are also cases where, to perform
finishing of the
well, the borehole is plugged to block fluid from below, and after the top
portions finished,
the plug is released.
[0008] Various methods are known for subsequent plugging and fracturing of
boreholes
from the tip portion of the borehole. For example, Patent Documents 2 to 4
disclose plugs
for well drilling capable of plugging or fixing a borehole (also called a
"frac plug",
"bridge plug", "packer", or the like).
[0009] Patent Document 2 discloses a downhole plug for well drilling (also
simply called
"plug" hereinafter), and specifically discloses a plug comprising a mandrel
(main body)
having a hollow part in the axial direction, a ring or annular member along
the axial
direction on the outer circumferential surface orthogonal to the axial
direction of the
mandrel, a first conical member and slip, a malleable element formed from
elastomer,
rubber, or the like, a second conical member and slip, and an anti-rotation
feature. Sealing
of the borehole by a downhole plug for well drilling is performed as follows.
Specifically,
by moving the mandrel in the axial direction thereof, as the gap between the
ring or
circular member and the anti-rotation feature gets smaller, the slip contacts
the slanted
face of the conical member, and by proceeding along the conical member, it
expands
radially in the outward direction, contacts the inside wall of the borehole,
and is fixed in
the borehole to seal the borehole, and also, the malleable element deforms by
diametric
expansion, contacts the inside wall of the borehole, and seals the borehole.
The mandrel
has a hollow part in the axial direction, and the borehole can be sealed by
setting a ball or
the like therein. It is described that metal materials (aluminum, steel,
stainless steel, and
the like), fibers, wood, composite materials, plastics, and the like are
widely exemplified
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as materials that form plugs, and that composite materials containing a
reinforcing
material such as carbon fibers, especially polymeric substances such as epoxy
resin,
phenol resin, and the like, are preferred, and that the mandrel is formed from
aluminum or
a composite material. On the other hand, it is described that, in addition to
the previously
described materials, a material that degrades depending on temperature,
pressure, pH
(acidic, basic), and the like may be used as the ball or the like.
[0010] Patent Document 3 discloses a packer assembly for well drilling where
each
packer is separably connected to each adjacent packer. Patent Document 3
recites a packer
provided with a mandrel having a hollow part in the axial direction and, a
slip, a slip
wedge, a resilient packer element, an extrusion limiter, and the like along
the axial
direction on the outer circumferential surface orthogonal to the axial
direction of the
mandrel.
[0011] Downhole plugs for well drilling are arranged sequentially inside the
well until
the well is completed, but must be removed at the stage when production of
petroleum
such as shale oil or natural gas such as shale gas (hereinafter collectively
called
"petroleum and natural gas" or "petroleum and/or natural gas") is begun.
Because the plug
is not designed to be released and retrievable after use, it is typically
removed by
destruction or by making it into small fragments by pulverization, drilling
out, or another
method, but substantial cost and time are required for pulverization, drilling
out, and the
like. There are also plugs specially designed to be retrievable after use
(retrievable plugs),
but since plugs are placed deep underground, substantial cost and time are
required to
retrieve all of them.
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[0012] Patent Document 4 discloses a disposable downhole tool (meaning a
downhole
plug or the like) or a member thereof containing a degradable material that
degrades when
exposed to the environment inside a well, and as the biodegradable material,
discloses a
degradable polymer such as an aliphatic polyester such as polylactic acid.
Additionally,
Patent Document 4 describes a combination of a tubular body element having an
axial-
direction flow bore, a packer element assembly comprising an upper sealing
element, a
center sealing element, and a lower sealing element along the axial direction
on the outer
circumferential surface orthogonal to the axial direction of the tubular body
element, a
slip, and a mechanical slip body. Furthermore, it is disclosed that fluid flow
in only one
direction is allowed due to the fact that a ball is set in the flow bore of
the cylindrical
body part. However, Patent Document 4 does not disclose whether a material
containing a
degradable material is used for a downhole tool or any part thereof.
[0013] With the increase in demand with regards to energy resource securement,
environmental conservation and the like, and particularly as the mining of
unconventional
resources spreads, mining requirements such as those pertaining to mining at
deeper
levels have become stricter and more diversified. Plugs for well
drilling(downhole tool or
the like) are transported to deep subterranean levels where fracturing is
performed using a
wire-like element (also called a "string", "stinger", "cable" or the like),
move various
members attached to the mandrel or the outer circumferential surface of the
mandrel
relatively so as to plug the borehole, and must withstand the pressure of high-
pressure
fluid and maintain plugging of the borehole during fracturing in which a high-
pressure
fluid is used. Specifically, plugs for well drilling such as downhole tools
and downhole
tool members must display sufficient resistance against high loads applied
thereto when
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plugging a borehole and maintaining that plugging during fracturing. For
example, when
plugging a borehole and maintaining said plugging during fracturing, a load of
approximately 45 kN (equivalent to about 10,000 pound weight) or greater is
applied.
Moreover, the environment within wells is known to reach a regular temperature
of 66 C
(equivalent to about 150 F) and, at times, exceed a temperature of 100 C.
Accordingly, it
has been desired that plugs for well drilling such as downhole tools and
downhole tool
members have mechanical properties (strength, ductility, and other tensile-
related
properties and/or compression properties) whereby the plug is not damaged
during
operation associated with fracturing.
[0014] Particularly, there are cases where a biodegradable material such as,
for example,
a degradable resin material is used as the mandrel or the various members
attached to the
outer circumferential surface of the mandrel, that is as a part or all of a
downhole tool as a
plug for well drilling or the members thereof, in order to make it possible to
remove the
plug or the members thereof via degradation after fracturing is completed. In
such cases, a
plug for well drilling is required that has the mechanical properties (tensile
properties
and/or compression properties) necessary to prevent the plug from being
damaged in the
environment within the well during the period until the completion of
fracturing.
[0015] As mining requirements such as those pertaining to mining at deeper
levels have
become stricter and more diversified, there is a demand for a plug for well
drilling and a
well drilling method by which reliable transport into the well, plugging of
the borehole is
enabled as a result of lightening the large load applied to bent portions, and
by which well
drilling costs or steps can be reduced as a result of facilitating the removal
of the plug and
the securing of the flow path.
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CITATION LIST
Patent Literature
[0016] Patent Document 1: Japanese Unexamined Patent Application
Publication No.
2003-533619A (corresponding to US Patent Application Publication No.
2003/0060375
specification)
Patent Document 2: US Patent Application Publication No. 2011/0277989 Al
specification
Patent Document 3: US Patent Application Publication No. 2003/0183391 Al
specification
Patent Document 4: US Patent Application Publication No. 2005/0205266 Al
specification
SUMMARY
[0017] A problem of the present invention is to provide a plug for well
drilling by
which reliable transport into the well, plugging of the borehole is enabled by
lightening
the large load applied to bent portions, and by which well drilling costs or
steps can be
reduced by facilitating the removal of the plug and the securing of the flow
path. A
further problem of the present invention is to provide a well drilling method
in which
said plug for well drilling is used.
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[0018] As a result of diligent research to solve the problems described above,
the present
inventors discovered that, in a plug for well drilling comprising a mandrel
and various
members attached on an outer circumferential surface orthogonal to the axial
direction of
the mandrel, high stress concentration occurs in bent portions of the mandrel
or the
various members when transportating into the well, plugging a borehole and
during
fracturing. As a result of further research, the present inventors discovered
that the
technical problems of the invention could be solved by controlling the shape
of the bent
portion in the mandrel or the various members. Thus the present invention was
completed.
[0019] Specifically, a first aspect of the present invention provides: (1) a
plug for well
drilling comprising a mandrel and members attached on an outer circumferential
surface
orthogonal to an axial direction of the mandrel, wherein at least the mandrel
or one of the
members is formed from a degradable material and a curvature radius of a bent
portion
thereof is from 0.5 to 50 mm.
[0020] As specific modes of the invention according to the first aspect of the
present
invention, the following plugs for well drilling of (2) to (19) are provided.
[0021] (2) The plug for well drilling described in (1), wherein the members
attached on
the outer circumferential surface orthogonal to the axial direction of the
mandrel comprise
at least one member selected from the group consisting of a slip, a wedge, a
pair of ring-
shaped fixing members, and a diametrically expandable circular rubber member.
(3) The plug for well drilling described in (2), wherein the pair of ring-
shaped fixing
members is capable of fixing the diametrically expandable circular rubber
member
attached on the outer circumferential surface orthogonal to the axial
direction of the
mandrel in a compressed state.
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(4) The plug for well drilling described in (2) or (3), wherein a combination
of at least one
slip and wedge is disposed between the pair of ring-shaped fixing members.
(5) The plug for well drilling described in any one of (1) to (4), wherein the
member(s),
formed from the degradable material and attached on the outer circumferential
surface
orthogonal to the mandrel, are formed from a composite material including a
degradable
material and a metal or an inorganic substance.
(6) The plug for well drilling described in any one of (1) to (5), wherein the
mandrel is
formed from the degradable material and a curvature radius of a bent portion
thereof is
from 0.5 to 50 mm.
(7) The plug for well drilling described in (6), wherein the mandrel formed
from the
degradable material comprises a hollow part along the axial direction.
(8) The plug for well drilling described in (6) or (7), wherein the mandrel
formed from the
degradable material and the member(s) attached on the outer circumferential
surface
orthogonal to the axial direction of the mandrel formed from the degradable
material are
integrally formed.
(9) The plug for well drilling described in (8), being integrally formed via
integral
molding.
(10) The plug for well drilling described in (8), being integrally formed via
machining.
(11) The plug for well drilling described in any one of (1) to (10), wherein
the bent
portion is at least one selected from the group consisting of a convex part, a
stepped part,
a flange, a groove, a thread, and a screw bottom.
(12) The plug for well drilling described in (11), wherein the bent portion
further
comprises a tapered part, a height of the tapered part being 1 mm or greater.
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(13) The plug for well drilling described in any one of (1) to (12), wherein
the degradable
material is an aliphatic polyester.
(14) The plug for well drilling described in (13), wherein the aliphatic
polyester is
polyglycolic acid.
(15) The plug for well drilling described in (14), wherein the polyglycolic
acid has a
weight average molecular weight from 180000 to 300000, and a melt viscosity
measured
at a temperature of 270 C and a shear rate of 122 sec-1 from 700 to 2000 Pas.
(16) The plug for well drilling described in any one of (1) to (15), wherein
the degradable
material comprises a reinforcing material.
(17) The plug for well drilling described in any one of (1) to (16), wherein
mandrel is
formed from polyglycolic acid.
(18) The plug for well drilling described in any one of (1) to (17), wherein
the mandrel
formed from the degradable material comprises a ratchet mechanism interlocking
part on
the outer circumferential surface, a curvature radius of the interlocking part
being from
0.5 to 50 mm.
(19) The plug for well drilling described in any one of (1) to (18), wherein
the mandrel
formed from the degradable material is provided, on the outer circumferential
surface,
with a male screw structure; one of the rings of the pair of ring-shaped
fixing members is
provided, on an inner circumferential surface thereof, with a female screw
struciure facing
the male screw structure; and the one of the rings is fixed in a state
disenabling sliding in
the axial direction of the mandrel.
[0022] Furthermore, a second aspect of the present invention provides: (20) a
well
drilling method using the plug for well drilling described in any one of (1)
to (19), the
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method comprising degrading a part or all of the plug for well drilling after
blocking a
borehole.
[0023] A first aspect of the present invention is a plug for well drilling
comprising a
mandrel and members attached on an outer circumferential surface orthogonal to
an axial
direction of the mandrel. In such a plug for well drilling, at least the
mandrel or one of
the members is formed from a degradable material and a curvature radius of a
bent
portion thereof is from 0.5 to 50 mm. In light of mining requirements such as
those
pertaining to mining at deeper levels becoming stricter and more diversified,
and as a
result of the configuration described above, a plug for well drilling is
provided by which
reliable transport into the well, plugging of the borehole is enabled as a
result of
lightening the large load applied to bent portions, and by which well drilling
costs or
steps can be reduced as a result of facilitating the removal of the plug and
the securing of
the flow path.
[0024] Moreover, a second aspect of the present invention is a well
drilling method
using the plug for well drilling described above, the method comprising
degrading a part
or all of the plug for well drilling after blocking a borehole. In light of
mining regulations
such as those pertaining to mining at deeper levels becoming stricter and more
diversified, and as a result of the configuration described above, a well
drilling method is
provided by which reliable transport into the well, plugging of the borehole
is enabled as
a result of lightening the large load applied to bent portions, and by which
well drilling
costs or steps can be reduced as a result of facilitating the removal of the
plug and the
securing of the flow path.
12
[0024a] In some embodiments, the present description relates to a plug for
well
drilling comprising: a mandrel and members attached on an outer
circumferential surface
orthogonal to an axial direction of the mandrel; at least the mandrel or one
of the
members being formed from a degradable material; and at least the mandrel or
one of the
members comprising a bent portion having a curvature radius of from 0.5 to 50
mm,
wherein the bent portion is formed by varying, along the axial direction, of a
diameter of
at least the mandrel or one of the members and/or an outside diameter of a
hollow part.
[0024b] In some embodiments, the present description also relates to a well
drilling
method using the plug for well drilling as defined herein, the method
comprising
degrading a part or all of the plug for well drilling after blocking a
borehole.
[0024c] In some embodiments, the present description also relates to a plug
for well
drilling comprising: a mandrel and members attached on an outer
circumferential surface
orthogonal to an axial direction of the mandrel, at least the mandrel or one
of the
members being formed from a degradable material, and at least the mandrel or
one of the
members comprising a bent portion having a curvature radius of from 0.5 to 50
mm;
wherein the bent portion is formed by varying, along the axial direction, of a
diameter of
at least the mandrel or one of the members and/or an outside diameter of a
hollow part;
wherein the one of the members is a pair of ring-shaped fixing members formed
from the
degradable material, and one of the pair of ring-shaped fixing members is
integrally
formed with the mandrel and fixed in a state disenabling sliding in the axial
direction of
the mandrel.
12a
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Brief Description of Drawings
[0025] FIG. 1A is a conceptual front cross-sectional view illustrating a
specific example
of a plug for well drilling of the present invention.
FIG. 1B is a conceptual front cross-sectional view illustrating a state where
a
diametrically expandable circular rubber member of the plug for well drilling
is
diametrically expanded.
FIG. 2 is a schematic front view of a mandrel comprising a flange.
Description of Embodiments
[0026] The present invention relates to a plug for well drilling comprising a
mandrel and
members attached on an outer circumferential surface orthogonal to an axial
direction of
the mandrel. In such a plug for well drilling, at least the mandrel or one of
the members is
formed from a degradable material and a curvature radius of a bent portion
thereof is from
0.5 to 50 mm. The present invention is described below while referencing FIGS.
1A and
1B.
[0027]
I. Plug for well drilling
1. Mandrel
The plug for well drilling of the present invention is provided with a mandrel
and
members attached on the outer circumferential surface orthogonal to the axial
direction of
the mandrel. The mandrel 1 provided in the plug for well drilling of the
present invention
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is normally called a "core metal," of which the cross-section has a
substantially circular
shape. The length of the mandrel 1 is sufficiently long relative to the
diameter of the
cross-section, and the mandrel 1 basically assures the strength of the plug
for well drilling
of the present invention. In the mandrel 1 provided in the plug for well
drilling of the
present invention, the diameter of the cross-section is selected as
appropriate according to
the size of the borehole (by making it slightly smaller than the inner
diameter of the
borehole, the plug can move inside the borehole, while on the other hand, as
will be
described later, there is a difference in diameter to an extent that enables
borehole
plugging via the diametric expansion of the diameter of a diametrically
expandable
circular rubber member 5 or the like). The length of the mandrel 1 is, for
example,
approximately 5 to 20 times the diameter of the cross-section but is not
limited thereto.
Typically, the diameter of the cross-section of the mandrel 1 is in a range of
5 to 30 cm.
[0028]
Hollow part
The mandrel 1 provided in the plug for well drilling of the present invention
may be solid,
but from the perspectives of securing a flow path at early stages of
fracturing, reducing
the weight of the mandrel, and controlling the degradation rate of the
mandrel, the
mandrel 1 is preferably a hollow mandrel comprising in at least a portion
thereof a hollow
part along the axial direction (that is, the hollow part may be configured to
penetrate the
mandrel 1 along the axial direction, or it may be configured not penetrate the
mandrel 1
along the axial direction). Additionally, in cases where the plug for well
drilling is forced
into the boreholes and transported using a fluid, the mandrel 1 preferably
comprises the
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hollow part along the axial direction. When the mandrel 1 has a hollow part
along the
axial direction, the cross-sectional shape of the mandrel 1 is a circular
shape formed by
two concentric circles forming the diameter (outside diameter) of the mandrel
1 and the
outside diameter of the hollow part (corresponding to the inside diameter of
the mandrel
1). The ratio of the diameters of the two concentric circles - that is, the
ratio of the outside
diameter of the hollow part to the diameter of the mandrel 1 - is preferably
at most 0.7.
The magnitude of this ratio has a reciprocal relationship with the magnitude
of the ratio of
the thickness of the hollow mandrel 1 to the diameter of the mandrel 1, so
determining the
upper limit of this ratio can be considered equivalent to determining a
preferable lower
limit of the thickness of the hollow mandrel. When the thickness of the hollow
mandrel is
too thin, the strength (in particular, the tensile strength) of the hollow
mandrel may be
insufficient when the plug for well drilling is placed inside a borehole or at
the time of
borehole plugging or fracturing, which may, in extreme cases, result in damage
to the plug
for well drilling. Therefore, the ratio of the outside diameter of the hollow
part to the
diameter of the mandrel 1 is more preferably at most 0.6 and even more
preferably at
most 0.5.
[0029] The diameter of the mandrel 1 and/or the outer diameter of the hollow
part may
be uniform along the axial direction of the mandrel 1, but may also vary along
the axial
direction. Specifically, due to the fact that the outer diameter of the
mandrel 1 varies
along the axial direction, the mandrel 1 may be configured so as to comprise
bent portions
such as convex parts, stepped parts, flanges, concave parts (grooves), and
also the screw
parts (typically the male screw structure), the hereinafter described ratchet
mechanism
interlocking part, and the like. In addition, bent portions such as convex
parts, stepped
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parts, grooves, screw parts (male screw structures or female screw
structures), or the like
may be formed on the inner circumferential surface of the mandrel 1 due to the
fact that
the diameter of the hollow part (inner diameter of the mandrel 1) varies along
the axial
direction. Furthermore, the bent portions may comprise a tapered part.
[0030] The convex parts, stepped parts, flanges, and concave parts (grooves)
provided on
the outer circumferential surface and/or the inner circumferential surface of
the mandrel 1
can be used as bearing sites when transporting the plug for well drilling into
a borehole,
and can also be used as sites for attaching and/or fixing other members to the
outer
circumferential surface and/or the inner circumferential surface of the
mandrel 1. In cases
where the mandrel 1 has a hollow part, the hollow part can be used as a seat
for holding a
ball used to control the flow of a fluid. Furthermore, the outer
circumferential surface of
the mandrel 1 may have a configuration in which, working together with the
inner
circumferential surface of the members attached on the outer circumferential
surface
orthogonal to the axial direction of the mandrel 1, an circular ratchet
mechanism is
provided by forming a plurality of interlocking parts that allow movement of
the members
in one direction along the axial direction of the mandrel and restrict
movement in the
opposite direction.
[0031]
Material forming the mandrel
The material forming the mandrel 1 provided in the plug for well drilling of
the present
invention is not particularly limited. Materials used conventionally in the
forming of
mandrels provided in plugs for well drilling can be used. Examples include,
metal
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materials (aluminum, steel, stainless steel, and the like), fibers, wood,
composite
materials, and resins. Specific examples include composite materials including
carbon
fibers or similar reinforcing materials, and particularly composite materials
including an
epoxy resin, phenol resin, or similar polymeric substances. As the plug for
well drilling of
the present invention will be able to reduce the costs and steps of well
drilling as a result
of facilitating the removal of the plug and the securing of the flow path
following the
completion of fracturing, the mandrel 1 is preferably formed from a degradable
material.
[0032]
Degradable material
In the plug for well drilling of the present invention, in cases where the
mandrel 1 is
formed from the degradable material, biodegradable materials, degradable
materials
having hydrolyzability, and other degradable materials that can be chemically
degraded
through any other process can be used as the degradable material.
[0033] In addition, materials such as aluminum and similar metal materials are
commonly used to form the mandrel provided in conventional plugs for well
drilling.
Such materials are prone to mechanical degradation such as destruction,
disintegration, or
the like and arc not suitable as the degradable material forming the mandrel 1
provided in
the plug for well drilling of the present invention. However, materials in
which the
intrinsic strength of resin decreases and the resin becomes weak due to a
reduction in the
degree of polymerization or the like, resulting in it disintegrating and
losing its shape
upon application of a very small mechanical force, also qualify as degradable
materials.
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[0034] In the plug for well drilling of the present invention, in cases where
the mandrel 1
is formed from the degradable material, as described hereinafter, the
degradable material
is preferably a hydrolyzable material that degrades in water of a certain or
higher
temperature. Additionally, the degradable material is more preferably an
aliphatic
polyester, and even more preferably polyglycolic acid (hereinafter also
referred to as
"PGA"). In other words, with respect to the mandrel 1, the plug for well
drilling is
preferably formed from PGA. Furthermore, the degradable material may comprise
a
reinforcing material, and may also comprise other compounding components.
[0035]
Curvature radius of the bent portion
In the plug for well drilling of the present invention, in cases where the
mandrel 1 is
formed from the degradable material, from the perspectives of reducing the
large load
imposed on the bent portion so as to enable reliable transport of the plug
into the well,
plugging of the borehole and carrying out of fracturing, the curvature radius
of the bent
portion of the mandrel I is set from 0.5 to 50 mm. As mining requirements such
as those
pertaining to mining at deeper levels have become stricter and more
diversified, due to the
fact that the curvature radius of the bent portion of the mandrel 1 is set to
be within the
range described above, the plug for well drilling of the present invention
will enable
reliable transport into the well, plugging of the borehole as a result of
enabling lightening
of the large load applied to bent portions, and will reduce the costs and
steps of well
drilling as a result of facilitating the removal of the plug and the securing
of the flow path.
Specifically, if the curvature radius of the bent portion of the mandrel 1
formed from the
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degradable material is too small, the bent portion may become damaged under
the high
pressures imposed during transport into the well, plugging of the borehole,
and fracturing.
lithe curvature radius of the bent portion of the mandrel 1 formed from the
degradable
material is too large, the junction (part of the outer diameter or inner
diameter of the
mandrel 1 that gradually changes in order to form the bent portion) will
become too long
and, as a result, it may be impossible to form a bent portion in the desired
shape and
location.
[0036]
2. Members attached on the outer circumferential surface orthogonal to the
axial direction
of the mandrel
The plug for well drilling of the present invention is provided with a mandrel
and
members attached on the outer circumferential surface orthogonal to the axial
direction of
the mandrel. Specifically, in the plug for well drilling, in order to
efficiently and reliably
carry out the transporting of the plug, plugging of the borehole, and
fracturing, and also
for the purpose of improving handling and the like, various members are
typically
attached on the outer circumferential surface of the mandrel. Examples of
these members
include members attached on the outer circumferential surface orthogonal to
the axial
direction of the mandrel, members attached on the outer circumferential
surface along the
axial direction of the mandrel, and members attached on the outer
circumferential surface
in another direction relative to the axial direction of the mandrel. In the
present invention,
at least one of the members attached on the outer circumferential surface
orthogonal to the
axial direction of the mandrel (hereinafter also referred to as "outer
circumferential
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surface-attached members") and/or the mandrel is formed from the degradable
material
and the curvature radius of a bent portion thereof is from 0.5 to 50 mm.
[0037] As mining requirements such as those pertaining to mining at deeper
levels have
become stricter and more diversified, due to the fact that the outer
circumferential
surface-attached members have the properties described above, the plug for
well drilling
of the present invention will enable reliable transport into the well,
plugging of the
borehole as a result of enabling lightening of the large load applied to bent
portions, and
will reduce the costs and steps of well drilling as a result of facilitating
the removal of the
plug and the securing of the flow path.
[0038] Provided that the outer circumferential surface-attached members are
members
used conventionally in plugs for well drilling, they are not particularly
limited, but
examples thereof include at least one selected from the group consisting of a
slip, a wedge,
a pair of ring-shaped fixing members, and a diametrically expandable circular
rubber
member. Note that "member" as used herein includes a meaning of "attachable
members
for attaching to the mandrel".
[0039]
Slips and wedges
A combination of slips 2a and 2b and wedges 3a and 3b (in FIGS. 1A and 1B, two
slip-
wedge combinations are illustrated (slip 2a and wedge 3a and slip 2b and wedge
3b), but
one or a plurality of slip-wedge combination may be provided in the plug for
well
drilling) is known in plugs for well drilling as a means for securing the plug
for well
drilling to the borehole. Specifically, the slips 2a and 2b formed from a
metal, inorganic
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substance, resin, or similar material are placed so as to be in slideable
contact with the
upper surface of the wedges 3a and 3b formed from a compound resin material or
the like.
Due to the movement of the wedges via force in the axial direction of the
mandrel being
applied, the slips 2a and 2b ride up on the upper slant surface of the wedges
3a and 3b and
move outward orthogonal to the axial direction of the mandrel 1. The outermost
circumferential surface of the slips 2a and 2b, orthogonal to the axial
direction of the
mandrel 1, contacts an inside wall H of the borehole and, thus, the plug is
secured to the
inner wall H of the borehole. The slips 2a and 2b may be provided with one or
more
convex parts, stepped parts, grooves, rough surfaces (corrugation), or the
like at the parts
making contact with the inside wall H of the borehole in order to make the
plugging
(sealing) of the space between the plug and the borehole even more reliable.
Additionally,
the slips 2a and 2b may be pre-divided into a predetermined number of sections
in the
circumferential direction orthogonal to the axial direction of the mandrel 1,
or, as
illustrated in FIG. 1, may be provided without breaks that stop partway- not
pre-divided
into a predetermined number of sections- from one end to the other end along
the axial
direction. Additionally, in cases where breaks are provided, the wedges 3a and
3b advance
to the lower surface of the slips 2a and 2b due to pressure in the axial
direction of the
mandrel 1 being applied to the wedges 3a and 3b. As a result, the slips 2a and
2b split and
separate along the breaks and an extended line thereof and each of the pieces
subsequently move outward orthogonal to the axial direction of the mandrel 1.
Note that
this structure is known in the art.
[0040]
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Pair of ring-shaped fixing members
At least one combination of the slips 2a and 2b and the wedges 3a and 3b are
preferably
placed between the pair of ring-shaped fixing members 4a and 4b so that the
wedges 3a
and 3b can be made to move when force in the axial direction of the mandrel 1
is applied
thereto. Specifically, the pair of ring-shaped fixing members 4a and 4b are
configured
such that they can slide along the axial direction of the mandrel 1 on the
outer
circumferential surface of the mandrel 1 and such that the spacing
therebetween can be
changed. In addition, they are configured such that a force in the axial
direction of the
mandrel 1 can be applied to the wedges 3a and 3b by coming into contact
directly or
indirectly with the end part along the axial direction of the wedges 3a and
3b. Individual
shapes and sizes of the pair of ring-shaped fixing members 4a and 4b are not
limited
provided that they can perform the functions described above. However, from
the
perspective of being able to effectively apply force in the axial direction of
the mandrel 1
to the wedges 3a and 3b, the edge surfaces of the pair of ring-shaped fixing
members on
each side contacting the wedges 3a and 3b are preferably flat. Each ring of
the pair of
ring-shaped fixing members 4a and 4b is preferably a circular ring which
completely
surrounds the outer circumferential surface of the mandrel 1, but may also
have breaks or
deformed places in the circumferential direction. In addition, as for the
shape in which the
circle is separated in the circumferential direction, the circle may be formed
as desired.
As each of the rings of the pair of ring-shaped fixing members 4a and 4b, a
plurality of
rings may be placed adjacently in the axial direction so as to form a wide
ring-shaped
fixing member (having a long length in the axial direction of the mandrel 1).
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[0041] The pair of ring-shaped fixing members 4a and 4b may have the same or
similar
compositions, shapes and structures, or the compositions, shapes and
structures may be
different. For example, each of the ring-shaped fixing members may differ in
outside
diameter or length in the axial direction of the mandrel 1. In addition, for
example, one of
the rings of the pair of ring-shaped fixing members 4a and 4b may also be
configured in a
state disenabling sliding relative to the mandrel 1, as desired. In this case,
due to the fact
that the other ring-shaped fixing member of the pair of ring-shaped fixing
members 4a
and 4b slides on the outer circumferential surface of the mandrel, each of the
ring-shaped
fixing members of the pair of ring-shaped fixing members 4a and 4b
respectively contact
the edges along the axial direction of the wedges 3a and 3b. However, there is
no
limitation on the configuration that one of the rings of the pair of ring-
shaped fixing
members 4a and 4b is in a state disenabling sliding with respect to the
mandrel 1.
Examples of such configurations include configurations wherein: i) the mandrel
1 and one
ring-shaped fixing member of the pair of ring-shaped fixing members 4a and 4b
are
integrally formed (in this case, the ring-shaped fixing member cannot slide
freely with
respect to the mandrel I); ii) a jaw clutch or similar clutch mechanism and/or
mating
mechanism is used (in this case, switching between states where the rings can
and cannot
slide with respect to the mandrel 1 is enabled); iii) the mandrel formed from
the
degradable material is provided with a male screw structure on the outer
circumferential
surface, one of the rings of the pair of ring-shaped fixing members is
provided with a
female screw structure facing said male screw structure on the inner
circumferential
surface thereof, and the one member of the pair of ring-shaped fixing members
is fixed in
a state disenabling sliding in the axial direction of the mandrel; and the
like. As the plug
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for well drilling in which the mandrel 1 and one of the rings of the pair of
rings 4a and 4b
are formed integrally, a plug for well drilling in which these components are
formed by
integral molding or a plug for well drilling formed by machining is provided.
[0042] The plug for well drilling may be provided with a plurality of pairs of
the pair of
ring-shaped fixing members 4a and 4b. In this case, at least one of each of
the
combinations of the slips 2a and 2b and the wedges 3a and 3b and/or the
diametrically
expandable circular rubber member 5 may be placed, individually or in
combination, at
positions between the plurality of pairs of rings 4a and 4b.
[0043]
Diametrically expandable circular rubber member
The plug for well drilling of the present invention may be provided with at
least one
diametrically expandable circular rubber member 5 placed at a position between
the pair
of ring-shaped fixing members 4a and 4b on the outer peripheral surface
orthogonal to the
axial direction of the mandrel 1. Preferably, the pair of ring-shaped fixing
members 4a
and 4b described above can be configured such that the diametrically
expandable circular
rubber member 5 attached on the outer circumferential surface orthogonal to
the axial
direction of the mandrel 1 is fixed in a compressed state. Specifically, due
to the
diametrically expandable circular rubber member 5 directly or indirectly cont
acting the
pair of ring-shaped fixing members 4a and 4b, force in the axial direction of
the mandrel
1 is transmitted on the outer circumferential surface of the mandrel 1. As a
result, the
diametrically expandable circular rubber member 5 is compressed in the axial
direction,
length in the axial direction is reduced (diametric reduction) and the
diameter in the
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direction orthogonal to the axial direction of the mandrel 1 expands. The
circular rubber
member 5 expands in diameter, and the outward part in the direction orthogonal
to the
axial direction comes into contact with the inside wall H of the borehole, and
additionally,
the inward part in the direction orthogonal to the axial direction comes into
contact with
the outer circumferential surface of the mandrel 1, thereby plugging (sealing)
the space
between the plug and the borehole. The diametrically expandable circular
rubber member
is fixed in a compressed state by the pair of ring-shaped fixing members 4a
and 4b, and
can maintain a state of contact with the inside wall H of the borehole while
fracturing is
subsequently performed, which yields the function of maintaining the seal
between the
plug and the borehole.
[0044] The diametrically expandable circular rubber member 5 is not limited
with regard
to its material, shape, or structure as long as it has the function described
above. For
example, by using a circular rubber member 5 having a shape in which the cross-
section
in the circumferential direction orthogonal to the axial direction of the
mandrel 1 has an
inverted U-shape, it can expand in diameter toward the vertex of the inverted
U-shape as
the tip portion of the U-shape is compressed in the axial direction of the
mandrel 1. The
diametrically expandable circular rubber member 5 comes into contact with the
inside
wall H of the borehole when diametrically expanded so as to plug (seal) the
space
between the plug and the borehole, and a gap is present between the plug and
the borehole
when the diametrically expandable circular rubber member 5 is not expanded.
Therefore,
the length of the diametrically expandable circular rubber member 5 in the
axial direction
of the mandrel 1 is preferably from 10% to 70% and more preferably from 15% to
65%
with respect to the length of the mandrel 1. As a result of this
configuration, the plug for
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well drilling of the present invention has a sufficient sealing function,
which yields a
function of assisting to secure the plug to the borehole after plugging.
[0045] The plug for well drilling may comprise a plurality of diametrically
expandable
circular rubber members 5, and by so doing, it can plug (seal) the space
between the plug
and the borehole at a plurality of positions in the axial direction of the
mandrel 1, and the
function of assisting to secure the plug to the borehole can be achieved even
more reliably.
In cases where the plug for well drilling comprises a plurality of
diametrically expandable
circular rubber members 5, the composition, shape, structure, position in the
axial
direction of the mandrel 1, and the relative positional relationship with the
pair of ring-
shaped fixing members 4a, 4b, with respect to the plurality of diametrically
expandable
circular rubber members 5 may be selected as desired.
[0046] It is necessary that the sealing function of the diametrically
expandable circular
rubber member 5 is not lost even when it comes in contact with higher
pressures and/or
the fracturing fluid used in fracturing under deep subterranean high-
temperature and high-
pressure environments. Therefore, typically, the diametrically expandable
circular rubber
member 5 is preferably a rubber material having superior heat resistance, oil
resistance,
and water resistance. For example, nitrile rubber, hydrogenated nitrile
rubber, acrylic
rubber and the like are often used. The diametrically expandable circular
rubber member 5
may be a rubber member with a structure formed from a plurality of rubber
members such
as a laminated rubber, or may be a structure formed by laminating other
members. In
addition, the diametrically expandable circular rubber member 5 may be
provided with
one or more convex parts, stepped parts, grooves, rough surfaces
(corrugation), or the like
at the parts making contact with the inside wall H of the borehole in order to
further
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ensure the plugging (sealing) of the space between the plug and the borehole
and the
assistance of the fixing of the plug to the borehole at the time of diametric
expansion.
[0047]
Material forming the member(s) attached on the outer circumferential surface
orthogonal
to the axial direction of the mandrel
The material forming the member(s) attached on the outer circumferential
surface is not
particularly limited and any material conventionally used in forming said
member(s)
provided in the plug for well drilling can be used. Examples include, metal
materials
(aluminum, steel, stainless steel, and the like), fibers, wood, composite
materials, and
resins. Specific examples include composite materials including carbon fibers
or similar
reinforcing materials, and particularly composite materials including epoxy
resin, phenol
resin, or similar polymeric substances. The plug for well drilling of the
present invention
is a plug whereby the costs and steps of well drilling can be reduced as a
result of
facilitating the removal of the plug and the securing of the flow path after
the completion
of fracturing. Therefore, just as described above with regards to the mandrel,
at least one
of the outer circumferential surface-attached members is preferably formed
from a
degradable material.
[0048]
Degradable material
In the plug for well drilling of the present invention, as described above
with regards to
the mandrel, biodegradable materials, degradable materials having
hydrolyzability, and
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other degradable materials that can be chemically degraded through any other
process can
be used as the degradable material forming at least one of the outer
circumferential
surface-attached members.
[0049]
Curvature radius of the bent portion
In the plug for well drilling of the present invention, in cases where at
least one of the
outer circumferential surface-attached members is formed from the degradable
material,
from the perspectives of reducing the large load imposed on the bent portion
of the outer
circumferential surface-attached member(s) formed from the degradable material
so as to
enable reliable transport of the plug into the borehole, plugging of the
borehole and
fracturing, a curvature radius of the bent portion of the outer
circumferential surface-
attached member(s) can be set to 0.5 to 50 mm. As mining requirements such as
those
pertaining to mining at deeper levels have become stricter and more
diversified, due to the
fact that the curvature radius of the bent portion of the outer
circumferential surface-
attached member(s) is in the range described above, the plug for well drilling
of the
present invention will enable reliable transport into the well, plugging of
the borehole as a
result of enabling lightening of the large load applied to bent portions, and
will reduce the
costs and steps of well drilling as a result of facilitating the removal of
the plug and the
securing of the flow path. Specifically, if the curvature radius of the bent
portion of the
outer circumferential surface-attached member(s) formed from the degradable
material is
too small, the bent portion may fail under the high pressures imposed during
transport
into the borehole, plugging of the borehole, and fracturing. If the curvature
radius of the
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bent portion of the outer circumferential surface-attached member(s) is too
large, the
junction (part of the outer diameter of the outer circumferential surface-
attached
member(s) that gradually changes in order to form the bent portion) will
become too long
and, as a result, it may be impossible to form a bent portion in the desired
shape and
location.
[0050]
3. Degradable material
In the plug for well drilling provided with the mandrel and the outer
circumferential
surface-attached members of the present invention, at least one of the outer
circumferential surface-attached members and/or the mandrel is formed from a
degradable
material. Moreover, the curvature radius of that bent portion is from 0.5 to
50 mm. A
description of the degradable material is given below.
[0051] The degradable material forming at least one of the outer
circumferential surface-
attached members and/or the mandrel may be a degradable material that is, for
example,
biodegradable, meaning that it is degraded by microorganisms in the soil in
which the
fracturing fluid and the like are used, or hydrolyzable, meaning that it is
degraded by a
solvent such as fracturing fluid, particularly by water, and also by acids or
alkalis if
desired. Additionally, it may be a degradable material that can be degraded
chemically by
some other method. Preferably, it is a hydrolyzable material degraded by water
of a
certain or higher temperature. Note that, as described above, materials such
as aluminum
and similar metal materials are commonly used in conventional plugs for well
drilling.
Such materials are prone to mechanical degradation such as destruction,
disintegration, or
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the like and are not suitable as the degradable material of the present
invention. On the
other hand, materials in which the intrinsic strength of resin decreases and
the resin
becomes weak due to a reduction in the degree of polymerization or the like,
resulting in
it disintegrating and losing its shape upon application of a very small
mechanical force,
also qualify as degradable materials. Examples of such degradable materials
include
composite materials including a degradable resin and a metal material
(described
hereinafter).
[0052]
Other degradable resins
From the perspectives of having both superior degradability and the expected
strength in
high-temperature and high-pressure in deep subterranean environments, the
degradable
material forming at least one of the outer circumferential surface-attached
members
and/or the mandrel is preferably a degradable resin. Herein "degradable resin"
is defined
as a resin that is biodegradable, hydrolyzable, or can be degraded chemically
by some
other method, as described above. Examples of the degradable resin include
aliphatic
polyesters such as polylactic acid, polyglycolic acid, and poly-s-caprolactone
(PCL), and
polyvinyl alcohols (partially saponified polyvinyl alcohols and the like
having a degree of
saponification of 80 to 95 mol%) and the like, but it is more preferably an
aliphatic
polyester. Specifically, the degradable material is preferably an aliphatic
polyester. The
degradable resin may be one type alone or a combination obtained by blending
two or
more types. Additionally, in cases where the outer circumferential surface-
attached
member(s) formed from the degradable material is the diametrically expandable
circular
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rubber member, examples of a degradable rubber that can be used as the
degradable
material include aliphatic polyester-based rubbers, polyurethane rubbers,
natural rubbers,
polyisoprene, acrylic rubbers, aliphatic polyester rubbers, thermoplastic
polyester
elastomers, thermoplastic polyamide elastomers, and the like.
[0053]
Aliphatic polyester
The aliphatic polyester is, for example, obtained from homopolymerization or
copolymerization of an oxycarbonic acid and/or a lactone, an esterification
reaction of
aliphatic dicarboxylic acid and an aliphatic diol, or copolymerization of
aliphatic
dicarboxylic acid, an aliphatic diol, and an oxycarbonic acid and/or a
lactone; and
preferably dissolves rapidly in water having a temperature from about 20 C to
100 C.
[0054] Examples of the oxycarbonic acid include, glycolic acid, lactic acid,
malic acid,
hydroxypropionic acid, hydroxybutyric acid, hydroxypentanoic acid,
hydroxycaproic acid,
hydroxyheptanoic acid, hydroxyoctanoic acid, and similar aliphatic
hydroxycarboxylic
acids having from 2 to 8 carbons, and the like. Examples of the lactone
include
propiolactone, butyrolactone, valerolactone, c-caprolactone, and similar
lactones having
from 3 to 10 carbons, and the like.
[0055] Examples of the aliphatic dicarboxylic acid include, oxalic acid,
malonic acid,
succinic acid, glutaric acid, adipic acid, and similar aiphatic saturated
dicarboxylic acids
having from 2 to 8 carbons; maleic acid, fumaric acid, and similar aliphatic
unsaturated
dicarboxylic acids having from 4 to 8 carbons; and the like. Examples of the
aliphatic diol
include, ethylene glycol, propylene glycol, butane diol, hexane diol, and
similar alkylene
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glycols having from 2 to 6 carbons; polyethylene glycol, polypropylene glycol,
polybutylene glycol, and similar polyalkylene glycols having from 2 to 4
carbons; and the
like.
[0056] One type alone or a combination obtained by blending two or more types
of
components may be used to form these polyesters. Furthermore, components that
form an
aromatic polyester such as terephthalic acid may be used in combination
provided that the
properties as a degradable resin are not lost.
[0057] Examples of particularly preferable aliphatic polyesters as the
degradeable resin
include, polylactic acid (hereinafter referred to also as "PLA"), PGA, and
similar
hydroxycarboxylic acid-based aliphatic polyesters; poly-e-caprolactone
(hereinafter
referred to also as "PCL") and similar lactone-based aliphatic polyesters;
polyethylene
succinate, polybutylene succinate, and similar diol-dicarboxylic acid-based
aliphatic
polyesters; copolymers of these, including, for example, poly(lactic-co-
glycolic acid)
(hereinafter referred to also as "PGLA"); as well as mixtures of these; and
the like.
Another example is an aliphatic polyester used by combining polyethylene
adipate/terephthalate or similar aromatic components.
[0058] From the perspective of the strength and degradability required in the
mandrel
and the outer circumferential surface-attached member(s), the aliphatic
polyester is most
preferably at least one type selected from the group consisting of PGA, PLA,
and PGLA,
of which PGA is even more preferred. Furthermore, the PGA encompasses not only
homopolymers of glycolic acid, but also copolymers containing not less than 50
mass%,
preferably not less than 75 mass%, more preferably not less than 85 mass%,
even more
preferably not less than 90 mass%, particularly preferably not less than 95
mass%, most
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preferably not less than 99 mass%, and above all, preferably not less than
99.5 mass%, of
glycolic acid repeating units. The PLA encompasses not only homopolymers of L-
lactic
acid or D-lactic acid, but also copolymers containing not less than 50 mass%,
preferably
not less than 75 mass%, more preferably not less than 85 mass%, and even more
preferably not less than 90 mass%, of L-lactic acid or D-lactic acid repeating
units, and it
may be a stereocomplex polylactic acid obtained by mixing a poly-L-lactic acid
and a
poly-D-lactic acid. As the PGLA, a copolymer in which the ratio (mass ratio)
of glycolic
acid repeating units to lactic acid repeating units is from 99:1 to 1:99,
prefer ably from
90:10 to 10:90, and more preferably from 80:20 to 20:80, may be used.
[0059]
Melt viscosity
The aliphatic polyester, and preferably as the PGA, PLA, and/or PGLA typically
has a
melt viscosity from 50 to 5000 Pas, but preferably has a melt viscosity from
150 to 3000
Pas and more preferably from 300 to 1500 Pas. The melt viscosity is measured
under a
temperature of 240 C and a shear rate of 122 sec-1. If the melt viscosity is
too low, the
strength required in the mandrel provided in the plug for well drilling may be
insufficient.
If the melt viscosity is too high, a high melting temperature will be required
in order to
manufacture the mandrel, which may lead to thermal degradation of the
aliphatic
polyester, insufficiency of degradability, and the like. The melt viscosity
described above
is measured using a capirograph (Capirograph 1-C, manufactured by Toyo Seiki
Seisaku-
Sho, Ltd.; diameter 1 mm p x length 10 mm). A 20 g sample was held at a
predetermined
temperature (240 C) for 5 minutes and subsequently measured at a shear rate of
122 secl.
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[0060] From the perspective of, for example, obtaining formability whereby
cracking
does not occur when molding by solidification- and extrusion-molding, and the
like, the
PGA as the aliphatic polyester particularly preferably has a weight average
molecular
weight from 180,000 to 300,000, and a melt viscosity measured at 270 C and at
a shear
rate of 122 sec-1 of 700 to 2000 Pas. Of these, the PGA is preferably a PGA
having a
weight average molecular weight from 190,000 to 240,000, and a melt viscosity
measured
at 270 C and at a shear rate of 122 sec-1 of 800 to 1200 Pas. The melt
viscosity is
measured according to the method described above (the predetermined
temperature is set
to 270 C). The weight average molecular weight is measured using gel
permeation
chromatography (GPC) under the conditions described below. 10 tl of the
solution to be
measured is obtained by dissolving 10 mg of the PGA in hexafluoroisopropanol
(HF1P) in
which sodium trifluoroacet ate is dissolved at a concentration of 5 mM to
obtain a 10 mL
solution and, thereafter, filtering the solution using a membrane filter.
GPC measurement conditions
Device: Shimadzu LC-9A, manufactured by Shimadzu Corporation
Columns: two HFIP-806M columns (connected in series) + one HFIP-LG precolumn
manufactured by Showa Denko K.K.
Column Temperature: 40 C
Eluent: HFIP solution in which sodium trifluoroacetate is dissolved at a
concentration of 5
mM
Flow rate: 1 mL/min
Detector: differential refractometer
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Molecular weight calibration: data of a molecular weight calibration curve
produced by
using five types of polymethylmethacrylates having standard molecular weights
that are
different from each other (manufactured by Polymer Laboratories Ltd.) are used
[0061]
Other blended components
The degradable material, preferably the degradable resin, more preferably the
aliphatic
polyester, and even more preferably the PGA, may also contain or be blended
with
various additives as other blended components, such as resin materials (other
resins when
the degradable material is a degradable resin), stabilizers, degradation
accelerators or
degradation inhibitors, reinforcing materials, and the like within a range
that does not
hinder the object of the present invention. The degradable material preferably
contains a
reinforcing material, and in this case, the degradable material can be called
a degradable
composite material. When the degradable material is degradable resin, it is a
so-called
degradable reinforced resin. The mandrel or outer circumferential surface-
attached
member(s) formed from the degradable reinforced resin preferably is formed
from an
aliphatic polyester containing a reinforcing material.
[0062]
Reinforcing material
As reinforcing materials, materials such as resin materials conventionally
used as
reinforcing materials with the objective of improving mechanical strength or
heat
resistance may be used, and fibrous reinforcing materials or granular or
powdered
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reinforcing materials may be used. The reinforcing materials may be contained
typically
in the amount of not greater than 150 parts by mass, and preferably in the
range of 10 to
100 parts by mass, relative to 100 parts by mass of the degradable material
such as
degradable resin.
[0063] Examples of fibrous fillers include inorganic fibrous substances such
as glass
fibers, carbon fibers, asbestos fibers, silica fibers, alumina fibers,
zirconia fibers, boron
nitride fibers, silicon nitride fibers, boron fibers, and potassium titanate
fibers; metal
fibrous substances such as stainless steel, aluminum, titanium, steel, and
brass; and
organic fibrous substances with a high melting point such as aramid fibers,
kenaf fibers,
polyamides, fluorine resins, polyester resins, and acrylic resins; and the
like. Short fibers
having a length of not greater than 10 mm, more preferably 1 to 6 mm, and even
more
preferably 1.5 to 4 mm are preferable as the fibrous reinforcing materials.
Furthermore,
inorganic fibrous substances are preferably used, and glass fibers are
particularly
preferable.
[0064] As the granular or powdered reinforcing material, mica, silica, talc,
alumina,
kaolin, calcium sulfate, calcium carbonate, titanium oxide, ferrite, clay,
glass powder,
zinc oxide, nickel carbonate, iron oxide, quartz powder, magnesium carbonate,
barium
sulfate, and the like can be used. These reinforcing materials may be
respectively used
alone or in combinations of two or more types. The reinforcing material may be
treated
with a sizing agent or surface treatment agent as necessary.
[0065]
Composite material
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The mandrel or outer circumferential surface-attached member(s) formed from
the
degradable material may be formed from a composite material including a
degradable
material and a metal or inorganic substance. Specific examples include
composite
materials in which concave portions such as recesses having a predetermined
shape are
provided in a base material formed from a degradable material such as a
degradable resin
exemplified by PGA, or the like; a metal (metal fragment or the like) or an
inorganic
substance having a shape that matches the shape of the recessed portion is
fitted therein;
and the base material and the metal or inorganic substance are fixed using an
adhesive or
pinned or wrapped with fibers so that the fixed state of the base material and
the metal
fragments or inorganic substance is maintained.
[0066]
4. Curvature radius of the bent portion
In the plug for well drilling provided with the mandrel and the outer
circumferential
surface-attached members of the present invention, at least one of the outer
circumferential surface-attached members and/or the mandrel is formed from a
degradable
material. Moreover, the curvature radius of that bent portion is from 0.5 to
50 mm. In
other words, the curvature radius of the bent portion of the mandrel or the
outer
circumferential surface-attached member(s) formed from the degradable material
is from
0.5 to 50 mm. Hereinafter, a description is given of the curvature radius of
the bent
portion of at least one of the outer circumferential surface-attached members
and/or the
mandrel formed from the degradable material.
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[0067] As described above, bent portions such as convex parts, stepped parts,
flanges,
grooves, or the like may be provided in the mandrel provided in the plug for
well drilling
or in the outer circumferential surface-attached members including, for
example, the slip,
the wedge, the pair of ring-shaped fixing members, the diametrically
expandable circular
rubber member, and the like. Furthermore, these bent portions may constitute a
ring-
shaped ratchet structure orthogonal to the axial direction of the mandrel,
which forms a
plurality of interlocking parts that allow movement in one direction and
restrict movement
in the opposite direction along the axial direction of the mandrel of each
member on the
outer circumferential surface of the mandrel and the inner circumferential
surface of the
outer circumferential surface-attached members. Additionally, screw parts
(male screw
structure or female screw structure) may be provided in the outer
circumferential surface-
attached members, the attachment members for attaching said outer
circumferential
surface-attached members to the mandrel, or the mandrel; and it goes without
saying that
the screw parts have bent portions of threads and screw bottoms. Accordingly,
in the plug
for well drilling of the present invention, the curvature radius of the bent
portions such as
the convex parts, stepped parts, flanges, grooves, threads, screw bottoms, and
the like in
the mandrel or the outer circumferential surface-attached member(s) formed
from the
degradable material is from 0.5 to 50 mm. Additionally, ratchet mechanism
interlocking
parts are provided on the outer circumferential surface of the mandrel formed
from the
degradable material, and the curvature radius of the interlocking parts can
also be
configured to be from 0.5 to 50 mm.
[0068] In other words, the plug for well drilling provided with the mandrel
and the outer
circumferential surface-attached members of the present invention enables
cooperation of
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the mandrel and the outer circumferential surface-attached members in order to
plug the
borehole and carry out fracturing. Accordingly, as mining requirements such as
those
pertaining to mining at deeper levels have become stricter and more
diversified; and in
order to enable the lightening of the large load applied to bent portions,
reliable
performance of transport into the well, plugging of the borehole, and
fracturing; a plug for
well drilling provided with a mandrel and outer circumferential surface-
attached members
is required that has the mechanical properties (tensile properties and/or
compression
properties) necessary to prevent the plug from being damaged in the
environment within
the well. For example, when plugging a borehole or carrying out fracturing,
pressure in
the order of multiple tons is applied to the plugged space, and the outer
circumferential
surface-attached members are subjected to tensile pressure and/or compressive
pressure
corresponding to this high pressure. Particularly, stress concentration occurs
in the bent
portions of the convex parts, stepped parts, threads, and screw bottoms, and
also the
flanges and ratchet mechanism interlocking parts, leading to the application
of even larger
tensile pressures and/or compressive pressures.
[0069] In the plug for well drilling of the present invention, at least one
part of the outer
circumferential surface-attached member(s) and/or the mandrel is formed from
the
degradable material for the purpose of facilitating the removal of the mandrel
or the outer
circumferential surface-attached members and the securement of the flow path
following
the completion of fracturing. In many cases, when compared to aluminum and
other metal
materials used conventionally as the material forming plugs for well drilling,
degradable
materials formed from aliphatic polyester or similar degradable resins have
inferior
mechanical properties in well environments. However, in the present invention,
due to the
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fact that the curvature radius of the bent portions of the mandrel or the
outer
circumferential surface-attached member(s) formed from the degradable material
is set to
0.5 to 50 mm, the plug has the mechanical properties (tensile properties
and/or
compression properties) necessary to prevent the plug from being damaged in
the
environment within the well. Note that in cases where the bent portions are
configured
from a plurality of curved surfaces having different curvature radii,
"curvature radius of
the bent portions of the mandrel and/or the outer circumferential surface-
attached
member(s) formed from the degradable material" shall mean the smallest
curvature radius
in the bent portion.
[0070] Due to the fact that the plug has the mechanical properties (tensile
properties
and/or compression properties) necessary to prevent the plug from being
damaged in the
environment within the well, and from the perspectives of lightening the large
load
applied to bent portions, leading to reliably performing transport into the
well, plugging
the borehole, and fracturing, the curvature radius of the bent portions in the
mandrel or
the outer circumferential surface-attached member(s) formed from the
degradable material
is in a range preferably from 1 to 40 mm, more preferably from 3 to 36 mm, and
even
more preferably from 5 to 32 mm.
[0071] Note that in the plug for well drilling of the present invention, in
cases where a
plurality of the bent portions are provided in the mandrel formed from the
degradable
material and/or the outer circumferential surface-attached member(s) formed
from the
degradable material, all of the bent portions may be configured to have a
curvature radius
of 0.5 to 50 mm, or the curvature radius of the bent portions upon which a
greater load is
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applied during transport into the well, the plugging of the borehole and
fracturing may be
configured so as to be within the ranges described in the previous paragraph.
[0072] Additionally, the bent portion of the mandrel or the outer
circumferential surface-
attached member(s) formed from the degradable material is at least one
selected from the
group consisting of a convex part, a stepped part, a flange, a ratchet
mechanism
interlocking part, a groove, a thread, and a screw bottom. In cases where the
bent portion
further comprises a tapered part, due to the fact that the large loads can be
lightened, a
height of the tapered part is preferably no less than 1 mm, more preferably
from 2 to 50
mm, even more preferably from 3 to 45 mm, and yet even more preferably from 5
to 40
mm. Herein, "tapered part" means the length along the mandrel axial direction
of the bent
portion, excluding the part having the smallest curvature radius, of the
mandrel or the
outer circumferential surface-attached member(s) formed from the degradable
material.
[0073] FIG. 2 is a schematic view illustrating a specific example of a mandrel
comprising
a flange (thickness (A): 30 mm) as a bent portion, the mandrel being formed
from PGA, a
degradable resin. In this specific example, the mandrel has a configuration in
which a
round bar-shaped flange having a large diameter is connected to the bottom of
a round
bar-shaped(pipe-shaped) mandrel via a tapered part having a height of T mm and
a
curvature radius of R mm. The uppermost end of the upper round bar is fixed
and a load
of45 kN is applied to the flange (load equivalent to the tensile forces caused
by the
pressure applied when fracturing). The curvature radius R (unit: mm) and the
height of the
tapered part T (unit: mm) are varied and results of the tensile stress applied
to the flange
(unit: MPa) is recorded in Table 1.
[0074] [Table 1]
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No. 1 No. 2 No. 3 No. 4 No. 5
Curvature radius (mm) 1 10 20 30 0.4
Height of Tapered part (mm) 0 0 10 20 0
Maximum Stress (MPa) 82 36 31 29 102
[0075] Table 1 shows that the maximum stress applied to the flange in No. 1 to
No. 4,
where the curvature radius of the bent portion was from 0.5 to 50 mm, was 82
MPa or less.
Given that the stress of PGA at a temperature of 66 C (150 F) is about 90 MPa,
it is
surmised that this mandrel that is provided with this flange and formed from
PGA will not
be damaged, even if a load of 45 kN is applied to the flange of the plug for
well drilling in
an environment having a temperature of 66 C.
[0076] Particularly, given that the maximum stress applied to the flanges of
No. 2 to No.
4 where the curvature radius of the bent portion is 10, 20, or 30 mm is 36 MPa
or less,
and the stress of PGA at a temperature of 149 C is 40 MPa or less, it is
surmised that a
mandrel comprising this flange will not be damaged, even when the plug for
well drilling
is subjected to pressure in high-temperature, deep subterranean environments.
Of the
examples shown in the Table, given that the maximum stress applied to the
flanges is 31
MPa or less, even smaller, in No. 3 and No. 4 where the height of the tapered
part is 10 or
20 mm, it is surmised that mandrels comprising these flanges can reliably
withstand
pressures applied to the plug for well drilling in deeper and higher-
temperature
environments.
[0077]
5. Plug for well drilling
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The plug for well drilling of the present invention is a plug for well
drilling comprising a
mandrel and outer circumferential surface-attached members. In such a plug for
well
drilling, at least the mandrel or one of the members is formed from a
degradable material
and a curvature radius of a bent portion thereof is from 0.5 to 50 mm. The
plug for well
drilling of the present invention can further comprise other members normally
provided in
plugs for well drilling. For example, where the mandrel has a hollow part
along the axial
direction, a ball (may be formed from a metal, resin, or similar material, and
from a
degradable material) can be provided in the hollow part for the purpose of
controlling the
flow of a fluid. Additionally, the mandrel and outer circumferential surface-
attached
members of the plug for well drilling, and also the other members described
above may be
provided with a member such as, for example, an anti-rotation member or the
like for
coupling and releasing the members with/from each other or other members. The
entire
plug for well drilling provided with the mandrel and the outer circumferential
surface-
attached member(s) of the present invention may be formed from the degradable
material.
[0078]
Plugging of the borehole
As described above, with the plug for well drilling of the present invention,
for example,
due to the forces in the axial direction of the mandrel being applied to the
pair of ring-
shaped fixing members, the forces in the axial direction of the mandrel are
transmitted to
the diametrically expandable circular rubber member and, as a result, the
diametrically
expandable circular rubber member is compressed in the axial direction of the
mandrel
and, along with the reduction of distance of the axial direction (diametric
compression),
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the diametrically expandable circular rubber member expands in a direction
orthogonal to
the axial direction of the mandrel. The circular rubber member diametrically
expands and
the outward part in the direction orthogonal to the axial direction comes into
contact with
the inside wall H of the borehole, and additionally, the inward part in the
direction
orthogonal to the axial direction comes into contact with the outer
circumferential surface
of the mandrel, thereby plugging (sealing) the space between the plug and the
borehole
(borehole plugging). Additionally, the slips ride up on the upper surface of
the slated
surface of the wedges and move outward orthogonal to the axial direction of
the mandrel.
Thereby, the outermost circumferential surface of the slips, orthogonal to the
axial
direction of the mandrel, contacts the inside wall of the borehole and, thus,
the plug can
be secured to the predetermined position of the inner wall of the borehole.
Then, in the
state where the space between the plug and the borehole has been plugged
(sealed),
fracturing can be performed.
[0079]
Degradation of the plug for well drilling
When the production of oil, natural gas or the like begins after the
completion of
fracturing in the various prescribed sections, typically, the drilling of the
well would be
completed, and the well finished, with the plug for well drilling of the
present invention,
at least one of the outer circumferential surface-attached members and/or the
mandrel or,
if desired, additionally the mandrel formed from the degradable material, can
be easily
degraded and removed, via biodegrading, hydrolyzing, or degrading chemically
by some
other method, and also, mining of hydrocarbon resources can be efficiently
carried out. As
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a result, with the plug for well drilling of the present invention,
substantial cost and time
conventionally required to remove, recover, or destroy or fragmentize, by
pulverization,
perforation, or another method, many plugs for well drilling remaining inside
a well after
the completion of the well become unnecessary, which makes it possible to
reduce the
cost or steps of well drilling.
[0080]
II. Method for manufacturing plug for well drilling
Provided that the plug for well drilling of the present invention is a plug
for well drilling
comprising a mandrel and members attached on an outer circumferential surface
orthogonal to an axial direction of the mandrel, wherein at least the mandrel
or one of the
members is formed from a degradable material and a curvature radius of a bent
portion
thereof is from 0.5 to 50 mm, the manufacturing method thereof is not limited.
For
example, the plug for well drilling may be obtained by: molding each of the
members
provided in the plug via a known method such as injection molding, extrusion
molding
(including solidification-and-extrusion molding), centrifugal molding,
compression
molding, or the like; machining each of the obtained members by cutting,
perforating, or
the like as necessary; and then assembling the members by known methods.
[0081] With the plug for well drilling of the present invention, in cases
where the
mandrel formed from the degradable material and the outer circumferential
surface-
attached member(s) formed from the degradable material are integrally formed,
the
mandrel formed from the degradable material and the outer circumferential
surface-
attached member(s) formed from the degradable material are preferably
integrally formed
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via integral molding by injection molding, extrusion molding (including
solidification-
and extrusion-molding), centrifugal molding, or the like, or by cutting or
similar
machining.
[0082]
III. Well drilling method
According to a well drilling method using the plug for well drilling
comprising the
mandrel and the outer circumferential surface-attached members of the present
invention,
in which a part or all of the plug for well drilling is degraded after the
blocking of the
borehole, when the fracturing in the various prescribed sections is completed,
or the
drilling of the well is finished and the well completed, and the production of
oil, natural
gas, or the like begins, at least one of the outer circumferential surface-
attached members
or, if desired, additionally the mandrel formed from the degradable material,
can be easily
degraded and removed, via biodegrading, hydrolyzing, or degrading chemically
by some
other method, and also, mining of hydrocarbon resources can be efficiently
carried out. As
a result, with the well drilling method of the present invention, the
substantial cost and
time conventionally required to remove, recover, or destroy or fragmentize, by
pulverization, perforation, or another method, many plugs for well drilling
remaining
inside a well after the completion of the well become unnecessary, which makes
it
possible to reduce the cost or steps of well drilling.
Industrial Applicability
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[0083] The present invention can provide a plug for well drilling comprising a
mandrel
and members attached on an outer circumferential surface orthogonal to an
axial direction
of the mandrel, wherein at least the mandrel or one of the members is formed
from a
degradable material and a curvature radius of a bent portion thereof is from
0.5 to 50 mm.
Thus, as mining requirements such as those pertaining to mining at deeper
levels have
become stricter and more diversified, reliable transport into the well,
plugging of the
borehole is made possible as a result of enabling lightening of the large load
applied to
bent portions, and reduction of the costs and steps of well drilling is
enabled as a result of
facilitating the removal of the plug and the securing of the flow path.
Therefore there is
high industrial applicability.
[0084] Additionally, the present invention can provide a well drilling method
using the
plug for well drilling described above, in which a part or all of the plug for
well drilling is
degraded after the blocking of the borehole. Thus, as mining requirements such
as those
pertaining to mining at deeper levels have become stricter and more
diversified, reliable
transport into the well, plugging of the borehole is made possible as a result
of enabling
lightening of the large load applied to bent portions, and reduction of the
costs and steps
of well drilling is enabled as a result of facilitating the removal of the
plug and the
securing of the flow path. Therefore there is high industrial applicability.
Reference Signs List
[0085] 1: Mandrel
2a and 2b: Slips
3a and 3b: Wedges
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4a and 4b: Ring-shaped fixing members
5: Diametrically expandable circular rubber member
H: Inside wall of the borehole
A: Thickness of the flange
R: Curvature radius
T: Height of the tapered part
48
