Downhole tool securing device and frac plug

Abstract

A downhole tool securing device and a frac plug which are excellent in setting properties and water pressure resistance and have excellent fracturing properties. A downhole tool securing device (slips) of a frac plug according to one embodiment of the present invention includes a button containing a powder metallurgy material and having a compressive elastic modulus of at least 13.5 GPa and a toughness of 0.23 GJ/m.sup.3 or greater and 1.0 GJ/m.sup.3 or less.

Claims

1. A downhole tool securing device for securing a downhole tool to a casing in a well, the device comprising: a main body; and a button attached to the main body and protruding from a surface of the main body, wherein the button includes a molded article of a powder metallurgy material, and the button has a compressive elastic modulus of at least 13.5 GPa and a toughness of 0.23 GJ/m.sup.3 or greater and 1.0 GJ/m.sup.3 or less.

2. The downhole tool securing device according to claim 1, wherein the button has an apparent density of 6.7 g/cm.sup.3 or greater and 7.2 g/cm.sup.3 or less.

3. The downhole tool securing device according to claim 1, wherein the button is formed of a surface and a core, and each of the surface and the core has a Rockwell hardness (HRC) of 20 or greater and 45 or less.

4. The downhole tool securing device according to claim 1, wherein the main body is formed of a reactive metal that is soluble in a predetermined solvent, and the button includes a molded article of an iron powder metallurgy material.

5. A frac plug comprising the downhole tool securing device described in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a side view of a frac plug including a downhole tool securing device according to an embodiment of the present invention.

(2) FIG. 2 is a partial cross-sectional view of the frac plug of FIG. 1.

(3) FIG. 3 is an enlarged cross-sectional view of a part of FIG. 1, and is a cross-sectional view illustrating a configuration of the downhole tool securing device according to the embodiment of the present invention.

(4) FIG. 4 is a partial perspective view of a slip base of a slip which is the downhole tool securing device according the embodiment of the present invention.

(5) FIG. 5 is a partial perspective view of a button which is the downhole tool securing device according to the embodiment of the present invention.

DESCRIPTION OF EMBODIMENTS

(6) Hereinafter, an embodiment of a downhole tool securing device and a frac plug according to the present invention will be described with reference to FIGS. 1 to 3.

(7) FIG. 1 is a side view of a frac plug (downhole tool) of the present embodiment. FIG. 2 is a cross-sectional view for explaining a mechanism of the frac plug in FIG. 1. FIG. 3 is an enlarged cross-sectional view of a framed portion B illustrated in FIG. 2. FIG. 4 is a partial perspective view of a slip base of a slip which is a downhole tool securing device provided in the frac plug of the present embodiment. FIG. 5 is a partial perspective view of a button of the slip which is the downhole tool securing device provided in the frac plug of the present embodiment.

(8) Frac Plug

(9) As illustrated in FIG. 1, a frac plug 100 (downhole tool) of the present embodiment includes a mandrel 101, an elastic member 102, a holding member 103 disposed adjacently to the elastic member 102 on one side of the elastic member 102, cones 104, 105 disposed to sandwich the elastic member 102 and the holding member 103, a pair of slips 106a, 106b (downhole tool securing device), and a pair of ring members 107a, 107b.

(10) In a wellbore (not illustrated), the frac plug 100 is installed in a casing 200 disposed within the wellbore, as illustrated in FIG. 2(a). When the frac plug 100 is installed in the casing 200, the mandrel 101 is moved in the axial direction indicated by P in FIG. 2(a) to reduce the distance between the pair of ring members 107a, 107b in the axial direction of the mandrel. This allows the slips 106a, 106b to ride on upper surfaces of the slopes of the cones 104, 105 and move outwardly orthogonally to the axial direction of the mandrel 101 to be in contact with the inner wall of the wellbore (the inner wall of the casing 200). As a result, the frac plug 100 is installed at a predetermined position of the wellbore. Here, in the slips 106a, 106b, a button 602 protrudes outwardly orthogonally to the axial direction of the mandrel 101 as described below. As a result, in a state where the slips 106a, 106b are in contact with the inner wall of the wellbore (the inner wall of the casing 200), a part of the button 602 is recessed into the inner wall of the casing 200. Thus, the frac plug 100 can be firmly secured to the inner wall of the wellbore.

(11) As the mandrel 101 moves in the axial direction to reduce the gap between the cone 105 and the holding member 103, the elastic member 102 is deformed to expand outward in the outer circumferential direction of the axis of the mandrel 101. Then, the elastic member 102 is in contact with the casing 200, so that the space between the frac plug 100 and the casing 200 is closed.

(12) After the frac plug 100 is installed at a predetermined position of the wellbore as described above, the wellbore is then closed by placing a ball or the like (not illustrated) in the axial hollow portion of the mandrel 101. Then, when a fluid is pumped into the closed section from the side of the cone 104 at high pressure in a state where the wellbore is closed, hydraulic fracturing is performed to create cracks in the production reservoir.

(13) The frac plug 100 is removed from the well once hydraulic fracturing is completed. The frac plug 100 of the present embodiment is a degradable frac plug formed of a degradable material that is degradable by the fluid in the well. By being exposed to the fluid in the well (the fluid flowing in the axial direction of the mandrel, that is, in the direction of arrow F1 or F2 in FIG. 2(b)) for a predetermined time, the frac plug 100 is degraded, disintegrated, dissolved, and thus removed from its contact portion with the fluid, and the closed flow path is reopened. In order to realize this, it is preferable that each of the constituent members included in the frac plug 100 is formed of a degradable resin or degradable metal. This facilitates removal of the frac plug 100 after the well treatment using the frac plug 100.

(14) In the present specification, the term degradable resin or degradable metal means a resin or metal which can be degraded or embrittled to be easily disintegrated, by biodegradation or hydrolysis, dissolution in water or hydrocarbons in a wellbore, or any chemical method. Examples of the degradable resin include aliphatic polyesters based on hydroxycarboxylic acid such as polylactic acid (PLA) and polyglycolic acid (PGA), lactone-based aliphatic polyesters such as poly-caprolactone (PCL), diol-dicarboxylic acid-based aliphatic polyesters such as polyethylene succinate and polybutylene succinate, copolymers thereof such as glycolic acid-lactic acid copolymers, mixtures thereof, and aliphatic polyesters using in combination aromatic components such as polyethylene adipate/terephthalate, or the like. Furthermore, a water-soluble resin may be used as the degradable resin. Examples of the water-soluble resin include polyvinyl alcohol, polyvinyl butyral, polyvinyl formal, polyacrylamide (which may be N, N-substituted), polyacrylic acid, and polymethacrylic acid, and furthermore copolymers of monomers forming these resins, such as ethylene-vinyl alcohol copolymer (EVOH) and acrylamide-acrylic acid-methacrylic acid interpolymer. Examples of the degradable metal include, for example, metal alloys containing magnesium, aluminum, and calcium as main components.

(15) Slip 106a, 106b (Downhole Tool Securing Device)

(16) FIG. 3 is an enlarged view of a portion B surrounded by a frame illustrated in FIG. 2(b), and illustrates a configuration of the slip 106b. Since the slip 106a has the same configuration as that of the slip 106b illustrated in FIG. 3, only the slip 106b will be described here.

(17) As illustrated in FIG. 3, the slip 106b includes a slip base 601 (main body) and a button 602 attached to the slip base 601 and protruding from the surface of the slip base 601.

(18) Slip Base 601 (Main Body)

(19) The slip base 601 is a main body portion of the slip 106b, and slides on an inclined surface of the cone 105.

(20) An outer circumferential surface 601a of the slip base 601 is provided with a recess 601b into which the button 602 is inserted. A plurality of buttons 602 are provided, and a number of recesses 601b are also formed in the outer circumferential surface 601a depending on the number of buttons 602.

(21) One embodiment of the slip base 601 will be described with reference to FIG. 4. The slip base 601 includes a plurality of slip divided pieces 612 divided by a cut 611 which ends halfway from one end to the other end along the axial direction. Each of the slip divided pieces 612 is provided with a plurality of buttons 602 on the surface 601a that is in contact with the casing 200. The embodiment of the slip base 601 is not limited to the embodiment illustrated in FIG. 4.

(22) The slip base 601 may be a degradable resin or degradable metal as described above, but is preferably formed of a reactive metal that is soluble in a predetermined solvent in the well.

(23) The reactive metal is a metallic element that degrades by readily bonding with oxygen to form a very stable oxide, reacting with water to produce diatomic hydrogen, and/or readily absorbing oxygen, hydrogen, nitrogen, or another non-metallic element to become brittle. More specifically, the reactive metal means a an elemental metal or an alloy containing the metal element as a main component, which can be degraded by a degradation reaction based on a chemical change and thus easily deprive the original shape of the downhole tool or the downhole tool member under a predetermined condition (for example, conditions such as temperature and pressure, contact with a fluid such as an aqueous fluid (preferably an acidic fluid or the like), and the like) in a well environment (hereinafter, also called downhole environment) in which the downhole tool is used.

(24) The predetermined solvent refers to a fluid such as a fracturing fluid (that is, a well treatment fluid used for fracturing), and examples thereof include various additives such as a channelant, a gelling agent, a scale inhibitor, an acid for dissolving a rock or the like, and a friction reducing material, in addition to water.

(25) A person skilled in the art can appropriately select the range of the reactive metal according to a predetermined condition such as an assumed well environment. In many cases, the reactive metal is an alkali metal or alkaline earth metal belonging to group I or group II of the periodic table, or aluminum or the like, but an alloy containing magnesium as a main component is preferable.

(26) Button 602

(27) As illustrated in FIG. 3, the button 602 is attached to the surface 601a of the slip base 601 that is in contact with the casing 200. FIG. 4 illustrates an embodiment in which four buttons 602 are provided in one slip divided piece 612, and the number of buttons 602 is not limited thereto. In addition, the number of buttons 602 provided with respect to the entire slip base 601 is not particularly limited.

(28) As illustrated in FIG. 5, the button 602 is cylindrical. The button 602 is attached to the slip base 601 such that the central axis of the button 602 is inclined with respect to the axial direction of the mandrel 101 (FIGS. 1 and 2). The attachment method is not particularly limited, and a known method of attaching the button to the slip base can be employed.

(29) The inclination angle of the central axis of the button 602 with respect to the axial direction of the mandrel 101 is, for example, 85? or less and preferably 80? or less, from the viewpoint of setting properties to a steel pipe. From the viewpoint of durability of the securing device, the inclination angle is 45? or greater and preferably 60? or greater. The button 602 may have a cylindrical shape with chamfered corners (edges).

(30) The size of the button 602 can be set as appropriate, but as an example, an outer diameter of 9 mm and a thickness (height) of 5.9 mm can be used as in an example described below.

(31) The button 602 includes a molded article of a powder metallurgy material and has a compressive elastic modulus of at least 13.5 GPa and a toughness of 0.23 GJ/m.sup.3 or greater and 1.0 GJ/m.sup.3 or less. The button 602 includes the molded article of the powder metallurgy material and has a compressive elastic modulus of at least 13.5 GPa and thus does not deform when being embedded in the casing 200 (FIGS. 1 and 2), and exhibits excellent setting properties (securing properties).

(32) In addition, the button 602 includes a molded article of a powder metallurgy material and its toughness is 0.23 GJ/m.sup.3 or greater and 1.0 GJ/m.sup.3 or less, as described above. When the toughness is 0.23 GJ/m.sup.3 or greater, the button has a strength sufficient to withstand high water pressure (for example, water pressure up to 70 MPa) during hydraulic fracturing of the production reservoir. On the other hand, since the toughness is 1.0 GJ/m.sup.3 or less, excellent crushing properties are provided. As such, the button 602 can be easily removed when the frac plug 100 (FIG. 1) is removed from the well. As described above, since the removal is easy, the frac plug 100 does not remain in the well to become a failure (production failure) of the next treatment when removed from the well.

(33) Preferably, the button 602 is formed of a molded article of a powder metallurgy material, and has an apparent density of 6.7 g/cm.sup.3 or greater and 7.2 g/cm.sup.3 or less. The button 602 can be formed of, for example, a molded article of an iron powder metallurgy material. As a result, its apparent density is smaller than the specific gravity of iron of 7.8, and the button 602 is easily broken. Therefore, it is unlikely to cause a production failure.

(34) The button 602 includes a surface and a core, and the surface and the core each have a Rockwell hardness (HRC) of 20 or greater and 45 or less. Here, the surface of the button 602 is a portion corresponding to a surface of a cylindrical shape. A portion closer to the central portion than the surface is a portion corresponding to the core. Therefore, the surface and the core referred to herein are not separate parts from each other but represent a relative positional relationship in one molded article.

(35) The HRC of the surface of the button 602 may be the same as or different from the HRC of the core. Further, the HRC of the core may be different between the surface side and the central portion of the button 602 (the central portion of a cylindrical body). For example, the HRC may be continuously varied from the surface side toward the central portion of the core. When the HRC is continuously different in the core, the HRC of the core as used herein refers to a value obtained by measuring the central portion of the core.

(36) In addition, from the viewpoint of the setting properties and the water pressure resistance, the surface and the core of the button 602 each preferably have an HRC of 30 or greater.

(37) As described above, the buttons 602 attached to the slips 106a, 106b of the present embodiment are excellent in the setting properties (securing properties) and water pressure resistance, and has excellent fracturing properties. Therefore, when the frac plug 100 (FIG. 1) is removed from the well, it can be easily removed and does not cause production failure.

Modified Example

(38) As another embodiment of the frac plug according to the present invention, the frac plug may include a mandrel and an elastic member, and further include one slip and one corresponding cone and one corresponding ring member.

(39) The present invention is not limited to the embodiments described above, and various modifications are possible within the scope indicated in the claims, which are also included in the technical scope of the present invention.

(40) Further, the downhole tool securing device according to an embodiment of the present invention (slips 106a, 106b of the present embodiment) can also be applied to a downhole tool other than the frac plug 100, and the downhole tool is also included in the scope of the present invention.

SUMMARY

(41) The downhole tool securing device (slips 106a, 106b) according to a first embodiment of the present invention is a downhole tool securing device for securing a downhole tool to a casing in a well, the device including a main body (slip base 601); and a button 602 attached to the main body and protruding from a surface 601a of the main body, wherein the button 602 includes a molded article of a powder metallurgy material, and the button 602 has a compressive elastic modulus of at least 13.5 GPa and a toughness of 0.23 GJ/m.sup.3 or greater and 1.0 GJ/m.sup.3 or less.

(42) According to the configuration of the first embodiment, it is possible to realize a downhole tool securing device having excellent securing properties, pressure resistance, and crushing properties.

(43) In a downhole tool securing device (slips 106a, 106b) according to a second embodiment of the present invention, in the first embodiment, the button 602 preferably has an apparent density of 6.7 g/cm.sup.3 or greater and 7.2 g/cm.sup.3 or less.

(44) According to the configuration of the second embodiment, it is possible to realize a downhole tool securing device having excellent crushing properties.

(45) In a downhole tool securing device (slips 106a, 106b) according to a third embodiment of the present invention, in the first or second embodiment, it is preferable that the button 602 includes a surface and a core, and the face and the core each have a Rockwell hardness (HRC) of 20 or greater and 45 or less.

(46) According to the configuration of the third embodiment, when the button is embedded into an inner wall of the well (the inner wall of the casing), deformation can be suppressed and excellent securing properties can be exhibited.

(47) In a downhole tool securing device (slips 106a, 106b) according to a fourth embodiment of the present invention, in the first to third embodiments, it is preferable that the main body is formed of a reactive metal that is soluble in a predetermined solvent, and the button includes a molded article of an iron powder metallurgy material.

(48) The frac plug 100 according to a fifth embodiment of the present invention includes the above-described downhole tool securing device (slips 106a, 106b).

(49) According to the configuration of the fifth embodiment, it is possible to realize a frac plug including a downhole tool securing device having excellent securing properties, pressure resistance, and crushing properties.

EXAMPLES

(50) Hereinafter, the button attached to the slip of the present embodiment will be described using examples.

(51) Production Method of Button

Example 1

(52) The button 602 illustrated in FIG. 5 was produced by using a powder metallurgy material (1) having a composition shown in Table 1, adjusting a material input amount and a compression amount to set its density to a predetermined value, and surface-hardening the button by heat treatment.

Example 2

(53) The button 602 illustrated in FIG. 5 was produced by using a powder metallurgy material (2) having a composition shown in Table 1, adjusting a material input amount and a compression amount to set its density to a predetermined value, and surface-hardening the button by heat treatment.

(54) TABLE-US-00001 TABLE 1 Composition [%] Material Fe C Cu Mn Mo Ni Cr Others Example Powder Remaining 0.1 1 0.05 0.2 Less 1 metallurgy constituent to to to to than 1 material 0.5 3 0.45 0.6 (1) Example Powder Remaining 0.2 1 0.2 3 Less 2 metallurgy constituent to to to to than 1 material 0.6 3 0.7 5 (2)

Comparative Examples 1 and 2

(55) As a comparative example, an extruded material of tool steel (SKD11) was cut into a button shape and heat-treated to produce a button.

Comparative Example 3

(56) A button was produced in the same manner as in Comparative Examples 1 and 2 except that an extruded material of structural alloy steel (SCM415) was used instead of the extruded material of the tool steel (SKD11), and the heat treatment condition was set to a condition for surface hardening. The compositions of the buttons of Comparative Examples 1 to 3 are summarized in Table 2.

(57) TABLE-US-00002 TABLE 2 Composition [%] Material Fe C Si Mn P S Cr Mo V Cu Ni Comparative SKD11 Remaining 1.4 0.15 0.3 0.025 0.010 11 0.8 0.2 Examples 1 constituent to to to or or to to to and 2 1.6 0.35 0.6 less less 13 1.2 0.5 Comparative SCM415 Remaining 0.12 0.15 0.55 0.030 0.030 0.85 0.15 0.3 0.25 Example 3 constituent to to to or or to to or or 0.18 0.35 0.95 less less 1.25 0.3 less less

Comparative Examples 4 and 5

(58) As a comparative example, by using a powder metallurgy material (3) having a composition shown in Table 3, a material input amount and a compression amount were adjusted to set its density to a predetermined value, and a button that was surface-hardened by heat treatment was produced.

(59) TABLE-US-00003 TABLE 3 Composition [%] Material Fe C Cu Mn Mo Ni Cr Others Comparative Powder Remaining 0.2 1 0.2 3 Less Examples 4 metallurgy constituent to to to to than 1 and 5 material 0.8 2 0.7 5 (3)

Comparative Example 6

(60) As a comparative example, a molded article of yttria-based zirconia (1) was used as a button.

Comparative Example 7

(61) As a comparative example, a molded article of magnesia-based zirconia was used as a button.

Comparative Example 8

(62) As a comparative example, a molded article of yttria-based zirconia (2) was used as a button.

(63) The compositions of the buttons of Comparative Examples 6 to 8 are summarized in Table 4.

(64) TABLE-US-00004 TABLE 4 Composition [%] Material Fe Cu Mn Mo Ni Al Ca Cr Mg Na P Si Cl Hf O Y Zr Comparative Yttria- 0.04 0.20 0.20 0.01 0.01 0.20 1.2 36 3.7 59 Example 6 based zirconia (1) Comparative Magnesia- 0.20 1.7 0.1 1.4 35 0.1 62 Example 7 based zirconia Comparative Yttria- 0.1 0.1 0.1 0.30 0.20 1.5 32 4.0 62 Example 8 based zirconia (2)
(2) Measurement Method of Characteristics

(65) Various characteristics of each button produced by the above-described production method were measured as follows.

(66) Hardness Measurement

(67) The hardness of the button surface was measured as follows. After an upper surface of the cylindrical button was polished and smoothed, an indenter of a micro Vickers hardness tester (Vickers hardness tester HV-114 available from Mitutoyo Corporation) was pressed against the button with a load 50 kgf at room temperature, and the hardness was calculated from a diagonal length of the indentation and a test load. As for the hardness of a core layer, the button was cut in a direction perpendicular to the axial direction of the cylindrical button, the cut surface was polished and smoothed, and then the hardness of the central portion of the cross section was measured in the same manner as for the surface hardness. With respect to the Rockwell hardness (HRC), the Vickers hardness obtained by the above method was converted in accordance with ASTM E140 Table 2.

(68) Density Measurement (Apparent Density)

(69) The weight of the button in air at 23? C. and the weight of the button in ion-exchanged water were measured, and the apparent density was calculated from the obtained weights and the density of the ion-exchanged water according to the Archimedes principle.

(70) Compression Test

(71) The cylindrical button was sandwiched between two tungsten carbide plates so that a bottom surface and an upper surface of the button were in contact with the tungsten carbide plates, and uniaxially compressed at a compression rate of 2 mm/min in the axial direction of the button at room temperature to obtain a strain-stress curve. The compressive elastic modulus was calculated in a section where the stress linearly changes over the strain. A crack was generated in the button with compression, and a point at which a maximum value was exhibited was defined as the compressive strength. The toughness was calculated by integrating the strain-stress curve in the section up to the strain at which the crack occurred.

(72) Composition Analysis (Ceramic)

(73) X-ray fluorescence (XRF) measurements were performed to determine the elemental composition of the ceramic buttons. Using a fundamental parameter (FP) method, the XRF peak intensity of each element was converted to a concentration ratio from the measurement results.

(74) Set Test (Test of Setting Properties)

(75) The frac plug 100 (FIG. 1) provided with the slips 106a, 106b was prepared. Polyglycolic acid (PGA) was used for the mandrel 101. Polyurethane was used for the elastic member 102. PGA was used for the holding member 103. PGA was used for the cone 104, a magnesium alloy was used for the cone 105, and in the pair of slips 106a, 106b, a magnesium alloy was used for the slip base 601 (FIG. 3) and the button described in (1) Production method of button was used for the button 602. PGA was used for the pair of ring members 107a, 107b.

(76) After the above-described frac plug was disposed in the casing (steel pipe), a compressive load of 150 kN was applied to the members including the slips 106a, 106b disposed on the side surface of the mandrel 101 to bring the members including the slips 106a, 106b into contact with the casing (steel pipe). A case where the frac plug was secured to the steel pipe was evaluated as Good, and a case where the frac plug was detached was evaluated as Poor.

(77) Water Pressure Resistance Test (Test of Water Pressure Resistance)

(78) After the frac plug was secured to the steel pipe by the method described in section Set test, water was fed and sealed in the steel pipe while the steel pipe was heated to a temperature of 200 deg F. After sealing, water pressure of 10000 psi (about 70 MPa) was applied to the frac plug by a pump, and it was checked whether the frac plug was able to hold the water pressure for 30 minutes or longer. A sample was evaluated as Good when the frac plug held a water pressure for 30 minutes or longer, a sample was evaluated as Margin when the frac plug held a water pressure for 30 minutes or longer, and the frac plug was moved by 10 mm or greater after the application of the water pressure with respect to the position of the member including the slips 106a, 106b at the time of setting the frac plug in the steel pipe, and a sample was evaluated as Poor when the securing member was damaged and the water pressure can not be held for 30 minutes or longer.

(79) Characteristics of Button

(80) The characteristics of the buttons and the frac plugs of Examples 1 to 3 and Comparative Examples 1 to 8 described above are summarized in Table 5.

(81) TABLE-US-00005 TABLE 5 Sample Water Compression test Outer Thick- Vickers Rockwell HRC pressure Elastic Compression diameter ness hardness HV (converted value) Density Set resistance modulus strength Toughness Material [mm] [mm] Surface Core Surface Core [g/cm.sup.3] test test GPal [GPa] [GJ/m.sup.3] Comparative SKD11 9 5 7.7 Good Good 19.6 5.4 or 2.05 or Example 1 greater greater Comparative 5.9 656 701 58 60 Good Good 22.0 5.4 or 2.07 or Example 2 greater greater Comparative SCM415 603 380 56 39 7.8 Good Good 22.8 5.6 or 2.03 or Example 3 greater greater Comparative Powder 269 305 25 30 6.6 Poor Good 12.5 1.8 0.28 Example 4 metallurgy material (3) (6.5) Comparative Powder 321 374 32 38 7.0 Poor Good 11.9 2.4 0.48 Example 5 metallurgy material (3) (6.9) Example 1 Powder 383 373 39 38 6.9 Good Good 15.0 2.4 0.44 metallurgy material (1) Example 2 Powder 409 397 42 40 7.0 Good Good 14.1 2.6 0.58 metallurgy material (2) Comparative Yttria- 9.525 6.35 1311 91 6.0 Good Margin 25.4 3.0 0.18 Example 6 based zirconia (1) Comparative Magnesia- 9 5.9 904 67 5.7 Good Poor 24.2 1.9 0.08 Example 7 based zirconia Comparative Yttria- 5.7 1231 84 6.0 Good Poor 26.3 2.9 0.16 Example 8 based zirconia (2)

(82) As shown in Table 5, it was shown that the buttons of Example 1 and Example 2 had good setting properties and water pressure resistance. On the other hand, all of Comparative Examples 4 to 8 were shown to have insufficient setting properties or water pressure resistance.

(83) The buttons of Example 1 and Example 2 are powder metallurgy materials having a toughness in the range of 0.23 GJ/m.sup.3 to 1.0 GJ/m.sup.3 and an apparent density in the range of 6.7 g/cm.sup.3 to 7.2 g/cm.sup.3. The buttons of Examples 1 and 2 had excellent fracturing properties after hydraulic fracturing.

(84) On the other hand, Comparative Examples 1 to 3 had a toughness exceeding 1.0 GJ/m.sup.3, and thus it was shown that the crushing properties were not sufficient.

REFERENCE SIGNS LIST

(85) 101 Mandrel 100 Frac plug (downhole tool) 102 Elastic member 103 Holding member 104, 105 Cone 106a, 106b slip (downhole tool securing device) 200 Casing 601 Slip base (main body) 601a Outer circumferential surface (surface of main body) 601b Recess 602 Button