JPH0472957B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION BACKGROUND OF THE INVENTION The present invention relates to apparatus used to detonate explosives within a wellbore.

Explosives are used in wellbore for various purposes, such as perforating the wellbore casing for completion and formation testing, and for installing troopers and other equipment in the wellbore. Because of the time and expense involved in the operation and the explosive power of these devices, it is important that the operation be reliable. Typical wellbore environments are hostile to the use of explosive devices in the wellbore.
This is a cause of reduced reliability. For example, extremely high temperatures generally make blasting operations difficult, and heavy drilling mud and debris can interfere with ignition systems. Impact-type detonating heads can also be damaged by debris or particles in drilling mud.

Impact-type detonating heads may not be available.
During a drill shaft test, various wellbore parameters such as temperature and pressure are monitored by measuring instruments drilled in the test section and mounted between the tubing and the detonation head. These are exploratory tools and usually cannot drop the detonator rod to the detonator head. Therefore, in such cases, it is desirable to use a pressure-sensitive detonator.

A problem with pressure-sensitive detonators is that the pressure within the annulus or pipe must be manually manipulated to activate the detonator. However, there are many applications where it is desired to maintain a relatively low pressure during detonation, such as when it is desired to perforate a casing that is in a non-equilibrium state. This requirement therefore precludes the use of pressure sensitive detonators operating above hydrostatic pressure.

SUMMARY OF THE INVENTION The present invention provides an apparatus for detonating explosives within a wellbore. The apparatus comprises a first apparatus activated in response to initial pressure conditions in at least a portion of the wellbore for inducing a combustion reaction, and a second apparatus for detonating the explosive. Additionally, the device is designed to prepare for the combustion reaction induced by the initiator by creating a delay time that is sufficient time for the operator to change the initial pressure condition to the desired second pressure condition at the time of detonation. Also includes a delay device, which operates until the end of the delay time after ignition for detonation. Therefore, by the time the gun perforator is detonated, the wellbore pressure can be reduced to a desired value, e.g., the value desired for non-equilibrium firing, so that the wellbore pressure can detonate the explosive. can do.

According to a preferred embodiment of the invention, the device further comprises a device for generating a signal for transmission to the surface of the wellbore indicating activation of the first device.
This informs the operator that the delay device has been activated and the operation can begin to reduce pressure within the wellbore prior to detonation, if desired.

According to another preferred embodiment, the delay device comprises:
It is placed within the container in such a way that the combustion gases can be released as the combustion reaction progresses. Additionally, the device includes a device for discharging the combustion gases exiting the delay device from within the container to the outside of the device. Thus, heat and pressure from the delay device is released to the outside of the device as combustion progresses. This avoids an increase in temperature and pressure in the room, which would otherwise result in an uncertain delay time.

The present invention further provides, at a desired position and at a desired pressure condition within the casing near the desired position,
Provided is a method for perforating a well casing. This method involves the step of fixing the drilling device near the desired position and the drilling of the casing after a delay time. It consists of a step of increasing the pressure to the drilling pressure state, and then a step of reducing the pressure inside the casing near the desired position from the starting pressure state to the desired drilling pressure state prior to drilling the casing.

Example The illustrated embodiment will be described in detail below.

In FIG. 1, the apparatus 10 includes an upper support member 12 with threads 14 for attaching the apparatus 10 to tubing pipe for lowering the apparatus 10 into the wellbore or for attaching other well drilling equipment to the apparatus 10. do.

The lower portion 16 of the upper auxiliary member 12 has a reduced diameter and a tapered tip, and is screwed into the housing 18 and sealed with a pair of O-rings 17. The lower portion 20 of the housing 18 is threaded to connect the device 10 to a gun perforator or other explosive downhole equipment.

Immediately below the threaded groove 14 of the upper auxiliary member 12, a first hole 22 of relatively large diameter is drilled up to a ring-shaped shoulder 24. a lower, relatively smaller diameter second hole 26 starting from the inner edge of the shoulder 24;
penetrates to the lower end of the upper auxiliary member 12. The piston ram 30 includes an O-ring 34 that fits snugly into the hole 26 in the upper auxiliary member 12 and provides a fluid seal between the piston 32 and the hole 26.
Has 2. Piston 32 also extends above bore 26;
It is located concentrically with hole 22. A ring-shaped piston stopper 35 is inserted between them and screwed into the hole 22, so that the upper auxiliary member 12 is secured by the shoulder 24.
It is stopped from descending inside. The inner diameter of the stopper 35 is exactly matched to the outer diameter of the piston 32. In the embodiment shown in FIGS. 1-6, the pin 36 does not lower the piston ram 30 relative to the upper support 12 until a sufficient pressure differential is created such that the piston 32 of the piston ram 30 shears the pin 36. Thus, the piston ram 30 is fixed to the piston stopper 35. The piston ram 30 also includes several radial fins 42 that extend downwardly into a narrow diameter projection 40 and are used to partially concentrically the projection 40 with the bore 26.
Equipped with. Fins 42 also limit lowering of ram 30, as described below.

Immediately below the upper auxiliary member 12 and the piston ram 30 is a typically cylindrical upper plug 44 screwed into a hole 46 in the housing 18 . Upper plug 44 includes a pair of O-rings 48 for sealing housing 18 in hole 46 . A first hole 50 having a relatively large diameter is concentrically drilled in the upper plug 44 from the top opening of the plug 44 to the lower shoulder 52 . A second hole 54 of relatively small diameter is concentrically drilled downwardly from the inner edge of shoulder 52 and terminating in shoulder 56 . Below the inner edge of the shoulder portion 56 is a third hole 58 having a smaller diameter. A relatively narrow concentric hole 60 extends from the hole 58 to the lower end of the upper plug 44 . circular hole 60
The lower end is sealed to a stainless steel disc 62 that is spot welded to the upper plug 44.

A detonator pin 66 is secured within the hole 50 and above the hole 54 by a pin 68. detonating pin 66
the projection 40 of the piston ram 30 in order to drop the detonating pin 66 into the hole 50 of the upper plug 44.
It has an upper surface 70 for receiving impact. The lower end of the detonating pin 66 is a relatively thin protrusion 72 that applies an impact to the detonator 100 when the detonating pin 66 falls from the hole 50. The detonator 100 is secured to the hole 58 by a detonator support 102 screwed into the hole 54.
held within. The detonator support 102 has a concentric hole for receiving the lower end of the detonating pin 66, and guides the protrusion 72 so that the detonator 100 fits therein. The upper outer surface of the detonating pin 66 has a relatively large diameter, and when the detonating pin 66 is lowered, the pin 66
Several grooves are cut to allow air to pass underneath.

As shown in FIGS. 2 and 3, the detonator 100 is
It includes a normally cylindrical detonator cup 102' having a flat upper surface 104 and a lower surface 106. The lower surface 106 has a concentric circular hole 108 drilled toward the upper surface 104. Furthermore, in the cup 102',
The upper end of the hole 108 extends very close to the upper surface 104 so that there is a thin wall or web 11 therebetween.
Circular holes 110 are concentrically drilled to form 2. Hole 110 forms a ring-shaped shoulder 114 at the upper end of hole 108 . Detonator cup 10
2' is made of stainless steel, for example.

In the hole 110 there is a detonator 116 as will be described in detail later.
is filled. A stainless steel disc 118 is attached to the shoulder 11 to retain the detonating charge 116 within the bore 110.
It is attached to part 4. The disk 118 has a hole 10
8 is pressed against the upper shoulder 114 by a cylindrical stainless steel anvil 120 fitted within the cylindrical shape. The lower end 122 of the anvil 120 is the lower surface 106
and become flush with each other. Cup anvil 120 102'
Another stainless steel lid plate 124 is attached to the bottom surface 106 to provide a seal to protect the initiating charge 116 from moisture as well as gases generated from other ignitable materials within the device 10. Spot welded.

Explosive charge 116 is an explosive mixture of titanium and potassium perchlorate mixed in a weight ratio of 41% titanium and 51% potassium perchlorate. Titanium has a diameter of 1~
A powder with a diameter of 3 microns is used, and potassium perchlorate is a powder with a diameter of 10 microns or less. After the powder is thoroughly mixed, it is pressed into the hole 110 under a pressure of 40000psi (≒2800Kg/cm ² ).
It is desirable to fill the After that, the disk 118,
Anvil 120 and cover plate 124 are sequentially assembled together with cup 102 and initiator 116. Details of Explosive 116 are set out in the United States Application ``Ignition Mixtures, Percussive Explosives and Ignition Methods''(Attorney's DocKet No.
50511).

The thickness of the web 112 and the depth of the holes 110, as well as the loading of the initiating charge 116, are selected to provide the desired impact sensitivity. That is, as the thickness of the web 112 increases, the impact sensitivity of the detonator 116 in the detonator 100 decreases, and even if the hole 110 becomes deeper, the impact sensitivity decreases as well. Furthermore, increasing the density of the explosive (by increasing the filling pressure) also reduces impact sensitivity. In the example embodiment described herein, web 1
The thickness of hole 110 is nominally 0.011 inch;
The depth is nominally 0.035 inch. If this container is filled with explosives at a crystal density of 68% to 80%, the 4ft.
Impact sensitivity of -lbs. or more can be obtained.

In use, protrusion 72 of detonator pin 66 impinges on web 112 and deforms it inwardly, thus compressing detonator charge 116 against anvil 120 and igniting it. The web 112 is made thin enough to deform appropriately upon impact with the projections and to ensure ignition. After ignition, the generated hot gas ruptures the thin cover plate 118. Four axial through holes 128 are formed in the anvil 120, so that the combustion gases and debris particles of the disk 118 form four jets. This gas jet is then passed through a cover plate 124 which includes means for igniting a flash-sensitive primary explosive, such as A1A.
break.

Again, as shown in FIG.
is screwed into a hole 132 in the lower part 20 of the housing 18. A through hole 134 is formed in the center of the lower plug 130, and a thread groove is provided in the lower half of the through hole 134. The narrow lower end 138 of the elongated, typically cylindrical delay device 136 is screwed. The lower end 138 of the delay device 136 is connected to the lower end 13
8 is screwed into the hole 134 so that the end surface of the plug 130 is flush with the lower surface of the plug 130. Plug 13
An upper, thicker diameter portion 142 of the delay device 136 extends upward from 0 . Upper end surface 1 of portion 142
44 is adjacent to the circular hole 60 of the upper plug 44. A hole 146 is bored in the housing 18 to form a plenum chamber in the gap between the housing 18 and the upper portion 142 of the delay device 136.

In use, the jet of gas and hot particles ejected from the detonator 100 through the bore 60 by the impact of the protrusion 72 of the detonator pin 66 serves as a signal to induce a combustion reaction within the delay device 136. This combustion reaction lasts for a sufficient period of time to allow the downhole personnel to reduce the downhole pressure to the desired value, after which the gun perfluorator is detonated by the device 10. At the end of this delay time, the detonator at the lower end of section 138 detonates section 138 to detonate the gun.
The fuse connected to the lower end of 38 (not shown in the figure) is ignited. As combustion progresses in the delay device 136, combustion gases are released from the delay device 136 and fill the plenum chamber.

The lower plug 130 has several ventilation holes 150 and its upper end is sealed with a lid 152. As the combustion gases fill the plenum chamber, they create a slight pressure difference between the outside and outside of the lid 152, which ruptures and allows the gases to flow down through the holes 150, causing the housing 1
Gas flows into the gun support attached to the lower part 20 of 8. Below the piston 32 of the piston ram 30, the interior of the device 10 is sealed to fluid pressure, and the gun support is similarly sealed to fluid pressure, so that the pressure within the plenum chamber is approximately atmospheric. It is maintained. Additionally, heat is dissipated from the delay device 136 by exhausting the combustion gases. Therefore,
The primary factor determining the delay time provided by delay device 136 is the ambient temperature of the wellbore.

As shown in FIG. 4, delay device 136 consists of a generally cylindrical housing 160 with a circular hole 162 in the center. A cylindrical pellet 164 of A1A primary explosive is placed between the top surface of the pellet 164 and the delay device 1.
It is inserted so that it is flush with the upper end surface 144 of 36 and extends slightly downward from there. Circular hole 162
is closed by adhering a heat-resistant cover plate 166 to the upper end surface 144. When the detonator 100 is ignited, the hole 6
The jet of hot gas and particles emitted from the A1A pellet 164 ruptures the cover plate 166 and the A1A pellet 164 explodes.

The interior of the bore 162 is filled with retarded tagsten mixture disks 168 downwardly from the pellet 164 to a point approximately halfway through the lower part 138 to the end of the bore 162. In the example, 55 tungsten mixture disks (mil-T-23132)
is used, and each disk has a pressure of 30000psi (≒2100Kg/cm ² )
It is made from a 500mg mixture compressed with
It forms a column about 10 inches high. In this example, the combustion time is 460 seconds at room temperature, 420 seconds at 250〓 after heating for 100 hours at 250〓, and 420 seconds after heating at 300〓 for 100 hours.
388 seconds at 400〓, 312 at 400〓 after heating for 100 hours
It turned out to be seconds.

Immediately below tungsten disk 168, hole 1
62 is filled with second A1A pellets 170. Immediately below pellet 172 is an upper booster of lead azide (RD-1333) and a PXY or
HNS- is a detonator with any explosive filler 174. When the last tungsten retarder or disc 168 burns, it ignites the pellets 170 of A1A, which in turn ignites the filler 172. This in turn provides an explosive power to booster 174, which then detonates explosive charge 176. This explosion is transmitted to the fuse that detonates the gun performer. Therefore, this is considered a detonation signal.

A possible arrangement in a wellbore using the apparatus of FIGS. 1-4 is shown in FIG. 5 as part of a wellbore connected by a casing 190 underground. The lower end of the tubing rod 192 is a perforated nipple 194. The upper auxiliary member 12 of the device 10 is the nipple 19
4, and screwed into the lower part 20 is a rod of a gun perforator 196 which extends further downward and is opposite the portion 198 of the casing 190 which is to be perforated by the gun 196. An explosion detection device 200 is connected to the lower end of the gun and together with the explosion detection device 200 generates a signal that is transmitted up the wellbore through the tubing rod 192 after a time delay provided by the delayed combustion device. It has a role to play. The explosion detection device 200 is
For example, U.S. Patent Publication No. 505911 (1983, 7
Filed on May 20th, Applicant: Edward A. Cole.
It is similar to that described in ``Method and apparatus for detecting explosion of gun perfluorator'' by Giunia et al. After adjusting the gun 196 to the desired position, the pucker 202, moved by the tubing rod 192 and placed above the perforated nipple 194, connects the annular space of the casing below the pucker with the annular space above it. Fixed to insulate the space. If it is desired to drill into the casing with the lower annulus in sub-equilibrium conditions, the water pressure in the lower annulus is adjusted accordingly, for example by withdrawing well water from the tubing rod.
When it is desired to ignite gun 196, the heavier fluid in tubing rod 192 is replaced with a lighter fluid to achieve the desired nonequilibrium condition, which increases the pressure in the tubing rod and forces pin 36 (FIG. 1). Shearing occurs, causing the piston ram 30 to fall rapidly and strike the detonator pin 66. Thus, pin 68 securing pin 66 shears, driving protrusion 72 into detonator 100 and combustion begins within delay device 136 . Referring again to FIG. 1, piston ram 30 ceases to descend when its fins 42 strike upper plug 44. Referring again to FIG. This impact produces significant vibrations that can be detected at the top of the wellbore by sonic sensors, such as those described in the above-mentioned US Pat. No. 5,059,11.

At this point, the operator at the top of the wellbore must
Patsucar 2 so that the combustion reaction progresses within 36.
Begin to drop the pressure in the lower annulus of 02. When the desired wellbore pressure is reached, explosive charge 176 is detonated, combustion within delay device 136 is terminated, and gun 196 is thus detonated. A few seconds after the gun is detonated, the explosion detection system 200 sends a second vibration signal through the tubing rod to the surface when the fuse within the gun 196 burns its entire length.

The arrangement shown in FIG. 6 differs from that of FIG. 5 in that the device 10 is mounted below the gun perforator 196 and the upside-down arrangement causes the normal top end to be the bottom end of the device 10. different from. Because the perforated plug 206 is screwed into the end 12 of the device 10, the pressure in the lower annular space of the packer 202 can be applied to the piston 32 of the device 10. Gun 196 is suspended from a hollow liquid seal pipe 208 suspended from explosion detection device 200 . The explosion detection device 200 is connected at its upper part to a perforated nipple 194. A feature of the arrangement shown in FIG. 6 is that even if the pressure fluid is
or even if the fluid enters the pipe 208
It is a point stored in 0. By using a fluid sensing detonator in device 10, gun 196
A wet explosion of the gun is avoided in the arrangement of FIG. 6 because the fluid accumulates in the device 10 below.

Applications where long guns are to be ignited by the device 10 require the use of a booster to transfer combustion from one fuse to the next, and are preferably non-directional boosters. Such boosters are equipped with a secondary explosive that acts as both a receptor and a donor. Explosives include, for example, HMX packed in guilding metal stainless steel or amyl cups at a density of 1.71 g/cc, or 1.455 g/cc in similar cups.
PYX packed with a density of g/cc is used. The open end of the cup is mounted over the squib end.

It is also an advantage that the device according to the invention can be used for drill shaft testing. In that case, the drilling rig is suspended from the chiving rod above the gun perforator. In such a device, it would be difficult to drop the detonator rod within the tubing so as to impact the mechanical detonator head, but this would have no effect on the operation of a pressure sensitive detonator such as device 10.

A further advantageous application of the device 10 is the multi-point blast method, in which two or more sections are drilled simultaneously or at different times. Such a method is
It is described in the US patent application filed November 18, 1983 (assigned by Flint R. George) entitled "Ignition of a Tandem Gun." Furthermore, the present invention
It also applies to a United States patent application filed by Edward Cole Giunia titled "Multi-Gun Detonator for Use in Wells and Method of Using the Same."

The terms and expressions used in this text are for descriptive purposes only and are not intended to be limiting. In addition, the terms and expressions used include equivalent features or parts thereof as in the figures and explanatory text. The present invention can be implemented in various modifications within the scope of the claims.

[Brief explanation of drawings]

The drawings show an embodiment of the present invention, and FIG. 1 is a partial sectional view of the device for detonating explosives in a wellbore, and FIG. 2 is a partial sectional view of the detonator used in the device.
3 is a sectional view taken along line 3-3 in FIG. 2, FIG. 4 is a partial sectional view taken along line 4-4 in FIG.・A partial cross-sectional view of an underground well using a pipe equipped with a perforator and the equipment shown in Figures 1 to 4. Figure 6 shows a pipe equipped with a gun perfluorator and a
FIG. 5 is a partial cross-sectional view showing another arrangement of an underground well for drilling a casing using the apparatus of FIGS. 10... device, 12... upper auxiliary material, 14...
Thread groove, 16...lower part, 17...O ring, 1
8... Housing, 20... Lower part, 22... First
hole, 24... ring-shaped shoulder, 26... second hole, 30... piston ram, 32... piston,
34...O ring, 35...piston stopper,
36...Pin, 40...Protrusion, 42...Fin,
44...Plug, 46...Hole, 48...O-ring, 50...Hole, 52...Shoulder, 54...Hole, 5
6... Shoulder, 58... Hole, 60... Circular hole, 62...
...Disk, 66...Detonation pin, 68...Pin, 72
... Protrusion, 100 ... Detonator, 102 ... Detonator support, 102' ... Detonator cup, 104 ... Top surface,
106...bottom surface, 108...hole, 110...hole,
112...web, 116...detonator, 118...
...disc, 120...anvil, 122...lower part,
124...Lid plate, 128...Through hole, 130...
Lower plug, 132... hole, 134... through hole,
136...Delay device, 138...Lower end, 142...
... part, 144 ... upper end surface, 146 ... hole, 15
0...Vent hole, 152...Lid, 160...Housing, 162...Circular hole, 164...Pellet, 1
66...Lid plate, 168...Disc, 170...
pellet, 172... filler, 174... booster, 176... explosive filler, 190... casing, 192... tubing rod, 194...
...perforated nipple, 196...gun (gun perforator), 198...part, 200...explosion detection device, 202...patch car, 206...perforated plug, 208...pipe.

Claims (1)

[Scope of Claims] 1. An in-wellbore explosive detonator comprising: a first assembly including an impact piston; a first detonator assembly that is energized to initiate a combustion reaction in response to an impulse from an impulse piston of the first assembly that is sensitive to pressure conditions; and a second high power explosive detonator for energizing said explosive detonator. , providing a combustion reaction initiated by the first detonator assembly and a sustained combustion reaction for a delay time sufficient for an operator to change from the first pressure state to the second pressure state upon energization of the detonator; consists of a combustion retardation device that provides
An explosive detonator in a well, wherein the combustion delay device continues to provide a combustion reaction at the end of a delay time after the start of combustion to excite a second high-output explosive detonator. 2 a plug device in said first set for providing a signal suitable for transmission to the wellbore surface and indicating energization of the first detonator assembly when impacted by the impingement piston of said first assembly; The device according to item 1 above, which is provided three-dimensionally. 3. The device according to item 1 above, comprising a device that maintains the delay device at a predetermined pressure or lower as the combustion reaction progresses. 4. The device is suitable for mounting on the tubing rod and is suitable for generating a signal as vibrations of the anvil and transmitting the vibrations to the tubing rod, thereby transmitting the signal to the wellbore surface. The device described. 5. The apparatus of clause 1, wherein the delay device initiates the combustion reaction by means of an initiator and continues for a delay time. 6 The delay device is configured such that the wellbore ambient temperature is at least
400〓, create a time delay of at least 312 seconds,
The device according to item 5 above. 7. The device of paragraph 6, wherein the delay time is at least 312 seconds after maintaining an average ambient temperature of at least 400° for at least 100 hours before activation of the first detonator assembly. . 8 The delay device is configured such that the wellbore ambient temperature is at least
300〓, create a time delay of at least 388 seconds,
The device according to item 5 above. 9. The apparatus of paragraph 3, wherein the delay time is at least 388 seconds after maintaining an average ambient temperature of at least 300° for at least 100 hours before activation of the first detonator assembly. . 10. The apparatus of paragraph 5, wherein the delay device creates a time delay of at least 430 seconds at a wellbore ambient temperature of at least 250°C. 11. The apparatus of paragraph 10, wherein the delay time is at least 430 seconds after maintaining an average ambient temperature of at least 250°C for at least 100 hours before activation of the first detonator assembly. . 12 The delay device is installed in the container so as to release combustion gas as the combustion reaction progresses, and further includes a device for exhausting the combustion gas released from the delay device from inside the container to the outside of the device. The heat emitted from the delay device is also released to the outside as the combustion reaction progresses.
The equipment described in section. 13. Apparatus according to clause 12 above, wherein the apparatus is mounted on a tubing rod and is capable of discharging combustion gases to another element of the tubing rod. 14. No. 13 above, wherein the element is a container for explosives.
The equipment described in section. 15. No. 12 above, wherein the device communicates with the explosive container for detonation and allows combustion gases to flow into the container.
The equipment described in section. 16. The device according to item 5, wherein the delay device operates to cause a metal reaction to occur as the combustion reaction. 17. The apparatus of item 5 above, wherein the delay device has the function of sustaining the combustion reaction in the wellbore at substantially the same temperature as the ambient wellbore. 18. The apparatus according to item 5, further comprising a device for maintaining the delay device at a predetermined pressure or less as combustion progresses. 19 Set the drilling device near the desired position, increase the pressure from the first state to the second starting pressure state higher than the first state and the desired drilling pressure state near the desired position of the wellbore, and perform time-delayed combustion. A wellbore wall is formed by activating a reaction and excitation of a drilling device, and then the pressure is lowered from a starting pressure state to a desired drilling pressure state near the desired location prior to drilling the wellbore wall near the desired location. A method of drilling a hole in a wellbore wall at a desired location and at a desired drilling pressure inside a wellbore. 20. The method according to item 19, wherein a signal for the start of delayed drilling is generated from the drilling device to a remote location to indicate that the pressure near the desired location has decreased. 21 The method for time-delayed start of drilling consists of impacting the igniter with a hammer in response to the second pressure condition, and the method for signaling the start of time-delayed drilling is responsive to the second pressure condition. 21. The method of claim 20, further comprising firing the anvil with a hammer to generate a vibration signal. 22 The method of generating the signal for the start of time-delayed drilling is
22. The method of claim 21, comprising transmitting the vibration signal to a tubing rod extending from the wellbore surface to the drilling equipment. 23 When the ambient temperature of the wellbore is at least 400〓,
20. The method of claim 19, wherein the time-delayed drilling is performed at least 312 seconds after activation. 24 When the ambient temperature of the wellbore is at least 300〓,
20. The method of clause 19, wherein the time-delayed drilling is performed at least 388 seconds after activation. 25 When the ambient temperature of the wellbore is at least 250〓,
20. The method of claim 19, wherein the time-delayed drilling is performed at least 430 seconds after activation.