Patent Publication Number: US-2016237774-A1

Title: Plug for well drilling

Description:
TECHNICAL FIELD 
     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 ART 
     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 (referred to as “fracturing” or “hydraulic fracturing”) in the productive layer using fluid pressure have received attention. 
     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. 
     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. 
     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. 
     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. 
     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 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. 
     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). 
     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 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. 
     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. 
     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. 
     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. 
     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 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. 
     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. 
     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. 
     CITATION LIST 
     Patent Literature 
     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 A1 specification 
     Patent Document 3: US Patent Application Publication No. 2003/0183391 A1 specification 
     Patent Document 4: US Patent Application Publication No. 2005/0205266 A1 specification 
     SUMMARY OF INVENTION 
     Technical Problem 
     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. 
     Solution to Problem 
     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. 
     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. 
     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. 
     (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.
 
(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.
 
(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 Pa·s.
 
(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 structure 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.
 
     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 method comprising degrading a part or all of the plug for well drilling after blocking a borehole. 
     Advantageous Effects of Invention 
     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. 
     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. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         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 
     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 . 
     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 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. 
     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 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. 
     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 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. 
     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. 
     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 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. 
     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. 
     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 are 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. 
     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. 
     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  1  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 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. If the 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. 
     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 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. 
     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. 
     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”. 
     Slips and Wedges 
     A combination of slips  2   a  and  2   b  and wedges  3   a  and  3   b  (in  FIGS. 1A and 1B , two slip-wedge combinations are illustrated (slip  2   a  and wedge  3   a  and slip  2   b  and wedge  3   b ), 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  2   a  and  2   b  formed from a metal, inorganic substance, resin, or similar material are placed so as to be in slideable contact with the upper surface of the wedges  3   a  and  3   b  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  2   a  and  2   b  ride up on the upper slant surface of the wedges  3   a  and  3   b  and move outward orthogonal to the axial direction of the mandrel  1 . The outermost circumferential surface of the slips  2   a  and  2   b , 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  2   a  and  2   b  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  2   a  and  2   b  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  3   a  and  3   b  advance to the lower surface of the slips  2   a  and  2   b  due to pressure in the axial direction of the mandrel  1  being applied to the wedges  3   a  and  3   b . As a result, the slips  2   a  and  2   b  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. 
     Pair of Ring-Shaped Fixing Members 
     At least one combination of the slips  2   a  and  2   b  and the wedges  3   a  and  3   b  are preferably placed between the pair of ring-shaped fixing members  4   a  and  4   b  so that the wedges  3   a  and  3   b  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  4   a  and  4   b  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  3   a  and  3   b  by coming into contact directly or indirectly with the end part along the axial direction of the wedges  3   a  and  3   b . Individual shapes and sizes of the pair of ring-shaped fixing members  4   a  and  4   b  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  3   a  and  3   b , the edge surfaces of the pair of ring-shaped fixing members on each side contacting the wedges  3   a  and  3   b  are preferably flat. Each ring of the pair of ring-shaped fixing members  4   a  and  4   b  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  4   a  and  4   b , 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 ). 
     The pair of ring-shaped fixing members  4   a  and  4   b  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  4   a  and  4   b  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  4   a  and  4   b  slides on the outer circumferential surface of the mandrel, each of the ring-shaped fixing members of the pair of ring-shaped fixing members  4   a  and  4   b  respectively contact the edges along the axial direction of the wedges  3   a  and  3   b . However, there is no limitation on the configuration that one of the rings of the pair of ring-shaped fixing members  4   a  and  4   b  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  4   a  and  4   b  are integrally formed (in this case, the ring-shaped fixing member cannot slide freely with respect to the mandrel  1 ); 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 for well drilling in which the mandrel  1  and one of the rings of the pair of rings  4   a  and  4   b  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. 
     The plug for well drilling may be provided with a plurality of pairs of the pair of ring-shaped fixing members  4   a  and  4   b . In this case, at least one of each of the combinations of the slips  2   a  and  2   b  and the wedges  3   a  and  3   b  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  4   a  and  4   b.    
     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  4   a  and  4   b  on the outer peripheral surface orthogonal to the axial direction of the mandrel  1 . Preferably, the pair of ring-shaped fixing members  4   a  and  4   b  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 contacting the pair of ring-shaped fixing members  4   a  and  4   b , 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 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  5  is fixed in a compressed state by the pair of ring-shaped fixing members  4   a  and  4   b , 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. 
     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 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. 
     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  4   a ,  4   b , with respect to the plurality of diametrically expandable circular rubber members  5  may be selected as desired. 
     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 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. 
     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. 
     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 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. 
     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 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. 
     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. 
     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 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). 
     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-ε-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 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. 
     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. 
     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, ε-caprolactone, and similar lactones having from 3 to 10 carbons, and the like. 
     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 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. 
     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. 
     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-ε-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. 
     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 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, preferably from 90:10 to 10:90, and more preferably from 80:20 to 20:80, may be used. 
     Melt Viscosity 
     The aliphatic polyester, and preferably as the PGA, PLA, and/or PGLA typically has a melt viscosity from 50 to 5000 Pa·s, but preferably has a melt viscosity from 150 to 3000 Pa·s and more preferably from 300 to 1500 Pa·s. 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 φ×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 sec −1 . 
     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 Pa·s. 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 Pa·s. 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 μl of the solution to be measured is obtained by dissolving 10 mg of the PGA in hexafluoroisopropanol (HFIP) in which sodium trifluoroacetate 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
 
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
 
     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. 
     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 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. 
     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. 
     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. 
     Composite Material 
     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. 
     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. 
     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. 
     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 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. 
     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 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. 
     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. 
     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 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. 
     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. 
       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 of 45 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. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 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 
               
               
                   
               
            
           
         
       
     
     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. 
     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. 
     5. Plug for Well Drilling 
     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. 
     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), 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. 
     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 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. 
     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. 
     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 via integral molding by injection molding, extrusion molding (including solidification- and extrusion-molding), centrifugal molding, or the like, or by cutting or similar machining. 
     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 
     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. 
     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 
     
         
           1 : Mandrel 
           2   a  and  2   b : Slips 
           3   a  and  3   b : Wedges 
           4   a  and  4   b : 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