Note: Descriptions are shown in the official language in which they were submitted.
CA 02557913 1997-12-16
APPLICATION FOR CANADIAN LETTERS PATENT
DETONATOR APPARATUS AND METHOD
TO ALL WHOM IT MAY CONCERN:
BE IT KNOWN THAT We,
Mr. Liqing Liu, 315-50th Avenue, S.W., Apt. 301,
Calgary, Alberta, T2S 1H3, Canada, Citizen of China and
Mr. Michael Norman Lussier, 203 Oakside Road, S.W., Calgary,
Alberta, T2V 4H7, Canada, Citizen of Canada, and
have invented a: DETONATOR APPARATUS AND METHOD
of which the following is a specification.
20421220.1
CA 02557913 1997-12-16
DETONATOR APPARATUS AND METHOD
FIELD OF THE INVENTION
The present invention relates generally to an apparatus and method for
remotely activating
blasting devices. Such an apparatus and method may be used, for example, in
oil and gas well
production in other industries in which remote initiation of explosive devices
occurs.
BACKGROUND OF THE INVENTION
In the production of oil and gas from underground wells, it is known to convey
a
perforating gun on a wireline down a bore hole of a well to a position where
an oil or gas bearing
stratum is located, and then to detonate shaped charges in the perforating
gun. The shaped
charges penetrate the formation, facilitating the entry of oil or gas into the
well.
Safe and reliable initiation of perforating guns or other firing devices in
the well-bore, far
removed from the surface, has been a continuing source of design challenges.
The explosive train
in the perforating gun normally comprises a detonator for setting off a
detonating cord. The cord
in turn detonates a series of connected shaped charges. The detonator is the
first element in the
explosive train and is normally the most sensitive to external stimulation.
Generally speaking, the
safety level of the perforating gun is primarily determined by the safety
level of the detonator
used. Bridge wire electric detonators have been, and are widely used. When an
electric current
of sufficient strength is applied to its lead wires the bridge wire is heated
and ignites the
pyrotechnic material surrounding it. This in turns sets off the primary and
secondary explosive
charges in the detonator.
An inherent problem with bridge wire detonators is the risk of unintentional
detonation
which may arise from stray currents. A bridge wire detonator does not possess
the ability to
distinguish between firing current and hazardous electric energy that reaches
its lead wires.
Typical sources of electrical interference which may cause unintentional
detonation are
communications equipment, whether cellular telephones or radio, standard 220V,
SOHz or 1 10V,
60Hz line current, electro-static discharges and lightning. At present when
bridge wire detonators
are used for perforating jobs, typical safety measures include shutting down
electric sources in the
20416191.1
CA 02557913 1997-12-16
-2-
well rig environment and turning off communication facilities. It would be
advantageous to
provide the oil industry a method of initiating perforating guns and a
detonator which reduces or
eliminates the need to suspend the use of without suspending the electric
devices and
communication radio in the well rig environment. An additional problem
concerns unauthorized
use of the detonators. Lost, stolen or mishandled detonators that can be set
off by commonly
available power sources, whether deliberately or accidentally used, may pose a
significant danger.
It would be advantageous to have a detonator which will resist detonation
except when initiated
by an authorized person using a specially designed blasting machine.
A known approach to the problem of unintentional detonation is to add extra
resistance
in series with the bridge wire, making a "resistorized detonator". A higher
voltage than would
otherwise be required is used to fire a resistorized detonator, making it more
difficult to set off
However, the magnitude of the electric current needed to initiate the
detonator remains the same
as non-resistorized detonators.
Another approach is to increase both the voltage and electric current needed
to fire the
detonator, so that they substantially exceed the upper limit of routine well
rig electrical signals like
the exploding bridge wire detonator or exploding foil detonator. This kind of
exploding bridge
wire or exploding foil detonator is disclosed in U.S. Patent No. 4,777,878 of
Johnson et al. issued
October 18, 1988 and U. S. Patent No. 5,505,134 of Brooks et al., issued April
9, 1996. Another
approach, as shown in U.S. Patent No. 4,708,060 of to Bickes et al., issued
November 24, 1987
and U.S. Patent No. 5,503,077 of Motley issued April 2, 1996, employs a
semiconductor bridge
wire to achieve improved safety.
Still another method of is to isolate the bridge wire, by employing a small
transformer in
the detonator. The load, generally the bridge wire of the detonator, is
connected to the secondary
winding of the transformer to form a loop and is electrically isolated from
the primary winding
of the transformer. The core material of the transformer is chosen to
attenuate, or eliminate,
spurious electrical power and radiofrequency signals and to respond to firing
currents falling
within a predetermined range of magnitude and frequency. A blasting machine
provides electric
current in the predetermined range needed to fire these inductive detonators.
20416191.1
CA 02557913 1997-12-16
-3-
A number of embodiments of transformer based detonators are shown in U.S.
Patent
No. 4,273,051 of Stratton, issued June 16, 1981. All of those embodiments
employ some form
of auxiliary energy dissipation means, whether a series or other leakage
inductance, a fusible link,
or a resistor in parallel with the primary winding.
Another example of a ferrite core, broad band attenuator is shown in U.S.
Patent
No. 4,378,738 of Proctor et al., issued April 5, 1983. U.S. Patent No.
4,441,427 of Barrett,
issued April 10, 1984 discloses an oil well detonator assembly that uses
ferrite materials to protect
against radio frequency energy.
U.5. Patent No. 4,544,035 of Voss, issued October 1, 1985 discloses the use of
two coils
to initiate a detonator in a perforating gun without the coupling of magnetic
materials. U. S. Patent
No. 4,806,928 of Veneruso, issued February 21, 1989 discloses the use of coil
assemblies
arranged on ferrite cores for data transmission between well bore apparatus
and the surface and
which may also be used to fire perforating guns.
U.S. Patent No. 3,762,331 to Vlahos, issued October 2, 1973 discloses a firing
circuit for
detonators that uses a step down transformer having a voltage reduction of
roughly 100:1 and
a secondary coil having only 1 or 2 turns. It operates at a voltage between
60V and 240V and
at a signal frequency of the order of l OKHz. It is powered by a battery in
parallel with a storage
capacitor, which discharge through an inverter circuit which includes a solid
state oscillator and
a transformer for stepping up the resulting a.c. voltage to the desired level.
This patent also
discloses the use of shunt and series capacitance connected to the primary
winding of the
detonator, and a large step down at the detonator transformer. U. S. Patent
No. 4,145,968 to
Klein, issued March 27, 1979 describes primary and secondary windings and a
fixed magnetic
screen designed to be saturated in the presence of the magnetic flux generated
by the primary
winding. U.5. Patent No. 4,297,947 to Jones et al., issued November 3, 1981
discloses the use
of a toroid or a magnetic core with removable parts as transformer cores to
couple a relatively
short (100m) firing cable and a number of detonators.
U. S. Patent No. 4,304,184 to Jones issued December 8, 1981 discloses a
transformer
circuit whose primary and secondary windings are not completely isolated.
Instead, they are
20416191.1
CA 02557913 1997-12-16
-4-
coupled not only magnetically but also electrically. While this configuration
may provide
protection against hazardous electrical currents at low values and low
frequencies, the safety
features would be more satisfactory if the two windings were completely
isolated electrically.
None of the transformer-based detonators noted above appear to be suitable for
oil well use.
A detonator that can be used in the oil industry at great depth poses special
requirements
for the coupling transformer. The electric energy supplied from the surface is
transmitted along
the wireline cable down oil wells as deep as 7,SOOm. The cable used for well
logging and casing
perforation may not be designed for high frequency transmission. The
distributed shunt
capacitance along the cable is in the order of 0. lSuF/Km. The attenuation for
high frequency
electrical energy is as high as 3dblKm (at 20 KHz). Consequently, for
effective power
transmission along the wireline, a relatively low frequency is preferred.
However, electric
currents having a frequency lower than lKHz will be attenuated by the ferrite
core transformer
and may not yield a suitable output for energizing the bridge wire in the
secondary winding.
Therefore, frequency significantly higher than 1KH? is preferable and the
blasting machine must
be powerful enough to allow energy dissipation along the wire-line and still
secure reliable
initiation of the detonator. For optimum power transmission, the inductance of
the transformer
used in the detonator must be in a certain range at a certain firing current
frequency. The
inductance of a transformer of some typical known designs may fall in the
range of 1-SO,uH.
Inasmuch as the characteristic impedance of a typical monocable used in well
logging is about 30-
SOS2, this inductance is too low for impedance matching purposes, especially
at lower frequencies
usable for oil well wirelines.
By contrast, a transformer having a relatively high primary inductance in the
order of
40mH, would be unsuitable even at the lowest usable frequencies. Also, where
the step down is
too large, the relatively high voltage needed to fire the detonator makes it
impractical for oil well
use because of the rapid attenuation of the high frequency voltage signal
along the cable. In the
view of the inventors of the present invention, the preferred frequency range
for effective power
transmission is between 3 and 20KHz, and the primary inductance of the
transformer should be
in the range of 200uH and 3mH.
20416191.1
CA 02557913 1997-12-16
-5-
A number of the transformers noted above use magnetic cores which provide a
closed
magnetic circuit. Some of them may have removable parts to accommodate the
firing cable and
detonator legwires, as disclosed by U.S. Patent Nos. 4,297,947 or 4,601,243.
When the primary
inductance needed is small and a relatively big transformer core (for example,
a toroid having
outer diameter of 20mm, placed outside the detonator body) is used, a few
turns of winding may
be sufficient. However, for a higher impedance the number of winding turns is
relatively large,
normally in the range 15-80 for the primary winding, depending on the actual
size and material
properties of the transformer core. Generally the core size of the transformer
should be
comparable to that of the outside diameter of the detonator. For an oil well
detonator this
dimension is commonly about 6~7mm. In the view of the present inventors, as a
practical matter,
it is difficult efficiently to wind such a large number of turns on a small
transformer core, such as
a toroid.
In the view of the inventors, some of these difficulties may be addressed by
using a
transformer constructed with a simple core in the form of a column having the
desired number of
primary and secondary windings on it. A column represents an open magnetic
circuit. To achieve
effciency in manufacturing, especially in mass production, it would be
advantageous to form the
primary and secondary windings by winding separate coils, and then be
assembling those coils
onto the column shaped core. Alternatively, the primary and secondary windings
could be wound
on a simple machine sequentially, with the primary winding be embedded, or
nested, within the
secondary winding, or vice versa. Different shapes of the column can be used,
such as a square
column, a plate, a tube, a U-shaped core, or other suitable form.
In an open magnetic circuit, there is energy loss associated with the high
magnetic
resistance. It would be advantageous to reduce this loss by using another
piece of magnetically
permeable material to form a closed magnetic circuit transformer core.
Examples of such materials
are nickel-iron alloys or permalloys and silicon steel, which have a high
magnetic permeability,
high curie temperature and are small in volume, low in cost and flexible to
form different shapes
as required.
The oil well use of a transformer-based detonator presents technical
challenges. In addition
to the extremely long transmission distance (up to 7,SOOm long) discussed
previously, the high
20416191.1
CA 02557913 1997-12-16
-6-
temperature environment also tends to present design challenges. Firstly,
magnetic permeability
of the core changes with increases in temperature, and drops to near zero
above the Curie
temperature. Magnetic materials lose their magnetism and the ability to
transmit signals beyond
the Curie temperature. Advantageously, magnetic materials chosen for
transformer cores should
S have a Curie temperature higher than the highest anticipated temperature in
the well, typically
180°C or higher. Secondly, the ability of most magnetic materials to
transmit energy decreases
substantially with the increase in temperature due to the decrease in
saturation flux density. For
example, for a typical maganese-zinc ferrite material, the saturation flux
density at room
temperature is 4500 Gauss. This decreases to 1750 Gauss when the ambient
temperature is
200°C. It is advantageous for the transformer detonator to be able to
transmit the required
amount of initiation energy at reduced saturation flux density. Thirdly, for
ferrite materials there
is generally an optimum temperature point at which the core loss at a minimum.
Deviation in
temperature from that point would result in increased core loss. Even though
the detonation
location well temperature may vary, it is advantageous to choose a ferrite
material which has an
optimum core loss temperature close to the expected well temperature.
A blasting machine is an electronic device which sends a high frequency
electric signal
through the wireline to fire the detonator. It is advantageous to provide a
blasting machine whose
output characteristics match the preferred frequency range of the detonator.
U.S. Patent No. 4,422,378 discloses an ignition circuit for firing detonators
having a
toroid transformer. It uses a power oscillator having a transistor to provide
a firing signal at the
resonant frequency of a network of detonators, the transistor being controlled
by a current
feedback signal. This self adjusting resonance matching is possible when the
inductance and
capacitance of the detonators connected in a net are detectable. In some
applications, such as
those in which diodes are placed in series with the wireline, the inductance
of the line can not be
obtained and it is difficult automatically to generate the resonant frequency.
U.S. Patent No. 4,422,379 discloses another ignition circuit for firing
detonators with
a toroid transformer. The oscillator of the circuit is a typical push-pull
power amplifier with the
use of an output transformer. U.S. Patent No. 4,848,232 also uses a firing
circuit in the form of
a push-pull power amplifier with an output transformer.
20416191.1
CA 02557913 1997-12-16
_ 7 _
In U.S. Patent No. 4,601,243, the electrical charge stored by a capacitor is
discharged to
detonators through a high frequency converting unit which oscillates at a
frequency between
SOKH? and lMHz.
The above referenced U. S. patents commonly have an output transformer. It
would be
advantageous to eliminate the use of such an output transformer in the
blasting machine. First,
power output tends to be limited by the size of the transformer. Long
transmission distances or
initiation of many detonators in one round tends to require a relatively big
transformer. This
weight and size disadvantage tends to be more pronounced at relatively lower
frequencies such
as the 3-20KHz range noted above. When a large, heavy transformer is used the
manufacturing
cost also tends to increase.
It would be advantageous to have an electrically activated detonator operable
at great
distances, from an electrical signal source, such as may be desired for
perforation of an oil well
thousands of metres from the surface.
It would be advantageous to have a simplified, electrically activated
detonator that is
relatively insensitive to signals from common electrical sources such as
radios, telephones, 50 and
60Hz supply signals, and other stray or static signals.
It would be advantageous to have a blasting machine for activating remote
detonators that
does not require the use of a large, heavy, and expensive iron core output
transformer.
SUMMARY OF THE INVENTION
The present invention provides, in a first aspect, a detonator for igniting
explosive
material comprising a multi-turn primary coil for connection to a detonation
signal source; a multi-
turn secondary coil connected to an explosive igniting element; and a core
magnetically linking
the coils. The core has a mandrel upon which at least one of the coils is
mounted.
In a second aspect of the invention there is a detonator for use in a well
perforating gun
comprising a transformer having a pair of mufti-turn coils linked by a
magnetically permeable
20416191.1
CA 02557913 1997-12-16
_ 8 _
core. The core has a mandrel. One of the coils is a pre-formed coil mounted
upon the mandrel.
One ofthe coils is connectible to a detonation signal source and the other
coil is connected to an
explosive igniting element with which it forms a closed circuit. Explosive
material is in contact
with the explosive igniting element.
The invention may also have a magnetically permeable closure member fit to the
mandrel
to form a closed loop magnetic circuit. Each of the coils may be a pre-formed
coil. Each of the
coils may be mounted on a mandrel of the core. The detonator may have closure
member fit to
each mandrel to form a closed loop magnetic path.
In a still further aspect of the invention there is an assembly for causing an
explosive
charge to explode comprising a blasting machine for generating a detonation
signal; a detonator
for receiving a detonation signal; and a carrier for carrying a detonation
signal from the blasting
machine to the detonator; the detonator having a transformer having a pair of
multi-turn coils
linked by a magnetically permeable core, one of the coils being connectible to
the signal carrier;
an explosive igniting element connected to the other coil to form a closed
circuit; explosive
material in contact with said explosive igniting element; and the core having
at least one mandrel,
and at least one of the coils being a pre-formed coil mounted on the mandrel.
In a further aspect of that invention, the blasting machine of the explosive
assembly further
comprises an energy storage system; a discharge system for releasing energy
from the storage
system; a switching system operable to control the discharge system to release
the detonation
signal from the energy storage system for communication of the signal to the
detonator along the
carrier.
In an even further aspect of the invention there is a blasting machine for
producing a
specific signal for setting off a signal selective detonator, comprising a
charge storage system; an
output port for connection to the signal selective detonator; a switching
system connected
between the charge storage system and the output port; a pre-set discharge
control system
operable to vary flow of charge through the switching system to produce the
specific signal.
20416191.1
CA 02557913 1997-12-16
-9-
In a further aspect of that even further aspect of the invention, the blasting
machine
further comprising a charging system selectively connectable to the charge
storage system when
the discharge control system is inoperative.
In another further aspect of that even further aspect of the invention the
charging system
includes a transformer connectable to draw power from a standard line source,
and a rectifier
connected to the transformer for converting the power to a form storable in
the charge storage
system.
In yet another aspect of the invention there is a detonator for igniting
explosive material
comprising a primary winding for connection to a detonation signal source; a
secondary winding
and an explosive igniting element connected thereto; and a core magnetically
linking the primary
and secondary windings. The core has a first portion made from a first
magnetically permeable
material for attenuating signals in a first frequency range, and a second
portion made from a
second magnetically permeable material for attenuating signals in a second
frequency range.
In a still further aspect of the invention a detonator for igniting explosive
material
comprises a mufti-turn primary coil for connection to a detonation signal
source and a mufti-turn
secondary coil and an explosive igniting element connected thereto. The coils
are co-axially
mounted and magnetically coupled by a core of low magnetic permeability.
BRIEF DESCRIPT10N OF THE DRAWINGS
For a better understanding of the present invention and to show more clearly
how it may
be carried into effect, reference will now be made by way of example to the
accompanying
drawings, which show an apparatus according to the preferred embodiment of the
present
invention and in which:
Figure 1 is a general schematic drawing indicating the general relationship of
a blasting
machine, a detonator and a perforating gun in the context of the present
invention.
Figure 2 is an electrical schematic of the detonator of Figure 1.
20416191.1
CA 02557913 1997-12-16
-10-
Figure 3a shows a cross section of the detonator of Figure 1 with a
transformer core
having a closed magnetic circuit core.
Figure 3b shows a cross section of an alternative detonator to the detonator
of Figure 1
with a transformer core not having a closed magnetic circuit transformer core,
Figure 4a shows a general view of the transformer of Figure 3a.
Figure 4b shows an alternative closed loop transformer for the detonator of
Figure 1.
Figure 4c shows an alternative transformer geometry for the detonator of
Figure 1.
Figure 4d shows a further alternative geometry for an open loop transformer
for the
detonator of Figure 1.
Figure 5 is an electrical schematic of a half bridge inverter for the blasting
machine of
Figure 1.
Figure 6 is an electrical schematic for a self oscillating driver for the
blasting machine of
Figure 1.
Figure 7 is an electrical schematic for a charging system for the blasting
machine of
Figure 1.
Figure 8 is an electrical schematic an alternative half bridge inverter for
the blasting
machine of Figure 1.
Figure 9 is an alternative electrical schematic for a full bridge inverter for
the blasting
machine of Figure 1.
Figure 10 is a timer circuit schematic for the full bridge inverter of Figure
9.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The description of the invention is best understood with reference to the
figures, in which
some proportions have been exaggerated, or shown in schematic form for the
purposes of
conceptual illustration.
The blasting machine of the preferred embodiment is useful, for example, in
the oil
industry for oil well casing perforation. As such, with reference to Figure l,
a well bore, such as
may be made for an oil, gas or other well, is shown as 20. It has an inner
steel casing 22, with a
more or less annular concrete filling 24 between casing 22 and bore 20. A
formation, or stratum
of oil bearing rock is indicated as 26. A perforating gun assembly 28 has been
conveyed down
20416191.1
CA 02557913 1997-12-16
-11-
bore 20 on the end of a wireline 30 by which it is physically located in the
well. The distance down
the well may be 1000 m, or more, up to 7,500 m beneath the ground surface.
Wireline 30 also
electrically connects assembly 28 with a high frequency blasting machine 32
located on the
surface.
Perforating gun assembly 28 has at its upper end a collar locator 34 to which
wireline 30
is attached. Depending therefrom, perforating gun assembly 28 includes a tube
36 containing a
series of shaped charges 38 connected to a detonating cord 40. Cord 40
terminates at a detonator
42 by which cord 40, and then charges 38 are ignited. In use, an electrical
signal originating at
blasting machine 32 is delivered along wireline 30 to detonator 42. When
detonator 42 is set off,
it in turn sets off cord 40 which detonates charges 38. The jets formed by
charges 38 penetrate
steel casing 22, concrete filling 24 and oil bearing stratum 26 to establish
communication between
the well and the rock formation.
Referring to the electrical representation of Figure 2 and the physical
presentation of
Figure 3a in greater detail, detonator 42 has a detonator casing shell 44 with
an internal, closed
ended, roughly cylindrical chamber 46. Explosive material 48 is packed into
the end of
chamber 46, and is covered by a partition 50 and pyrotechnic igniter material
52. The igniter
material, 52, surrounds an embedded filament in the nature of a bridge wire
54, suspended
between the extended ends of two lead wire legs 56 and 58. Legs 56 and 58 are
joined in a
closed circuit loop by a multiple turn, secondary winding 60 wound about a
magnetically
permeable, U-shaped Mn-Zn ferrite core 62. A keeper, or closure element 64,
again of
magnetically permeable material extends between the open legs 66 and 68 of U-
shaped core 62
to closed the magnetic circuit of U-shaped core 62.
Referring again to Figures 2 and 3a, wireline 30 is shielded by a grounded
sheathing 70
until it reaches perforating gun assembly 28, and is grounded through collar
locator 34. Collar
locator 34 has a coil 72 for generating an electromagnetic signal when
perforating gun
assembly 28 passes junctions in casing 22, so that the exact location of
perforating gun
assembly 28 in bore 20 can be determined relative to stratum 26. Collar
locator coil 72 has an
inductance of 11H, a typical value for such devices. Wireline 30 extends
beyond collar locator 34
20416191.1
CA 02557913 1997-12-16
-12-
to a pair of reversed diodes 74 and 76 on parallel paths. One lead wire 78 of
a mufti-turn primary
winding 80 is connected to diodes 74 and 76. Winding 80 is wound about U-
shaped core 62, and
its remaining lead wire, 82 is grounded. Diodes 74 and 76 are used to permit
communication of
the firing signal from blasting machine 32 to detonator 42 and to provide high
impedance to the
small signal generated by collar locator coil 72.
Bridge wire 54 is the part of detonator 42 most sensitive to external
stimulation. It forms
a closed loop with secondary winding 60. It is physically protected by a cast-
in-place plastic plug
84 which serves also to capture and immobilize legs 56 and 58, and bridge wire
54 in igniter
material 52. Plug 84 additionally holds diodes 74 and 76; the transformer
formed by primary
winding 80, secondary winding 60, U-shaped core 62, and closure element 64 in
place. Bridge
wire 54 is also physically and electro-magnetically protected by shell 44
which is, typically, made
of a highly conductive metal such as copper or aluminum. Consequently the loop
which includes
bridge wire 54 remains electrically neutral as it is electrically shielded by
shell 44.
Primary winding 80 and secondary winding 60 are pre-formed and then assembled
on
legs 66 and 68 of U-shaped core 62, before being locked in place by nickel
alloy closure
element 64. This method encourages relatively easy and economical assembly,
and contrasts with
the method of assembly of threaded-core detonators. Primary and secondary
windings 80 and
60 are mounted parallel to each other in an arrangement which reduces the
magnetic flux coupled
by air and shared by both windings. The number of turns may vary. It is
typically in the range of
15 to 80 for primary winding 80 and for secondary winding 60. In the preferred
embodiment the
number of turns on primary winding 80 is 24, and the number of turns on
secondary winding 60
is 12. In the preferred embodiment, the height of core 62 is 8mm, its
thickness is l.Smm, and its
width is 6mm.
The number of turns of windings 60 and 80, the permeability of core 62, and
the
geometry chosen afrect the range of frequencies to which detonator 42 is most
responsive. Core
62 is chosen so that it responds efficiently to electric currents delivered by
specifically designed
blasting machine 32, reducing or eliminating electrical hazards. In use, stray
DC signals and low
frequency AC sources carried on wireline 30 will have little or no effect on
bridgewire 54. Since
20416191.1
CA 02557913 1997-12-16
-13-
electrical frequencies in a typical well rig environment have frequencies
either below 1 kHz (e.g.,
DC, 50 or 60 Hz AC) or well above 1 Mhz (radio frequency energy in GHz), the
probability of
unintentional detonation tends to be reduced. When an appropriate firing
current is delivered to
wireline 30 the current running through primary winding 80 induces a current
in the closed circuit
loop formed by secondary winding 60, legs 56 and 58, and bridge wire 54.
Bridge wire 54 is then
heated to incandescence and ignites pyrotechnic igniter material 52. The
ignited material 52
initiates detonation of detonator 42, which thereafter sets oi~the explosive
train of cord 40 and
shaped charges 38 in perforating gun assembly 28.
The preferred material for core 62 is either a Mn-Zn or an Ni-Zn ferrite
chosen to
discourage energy transmission at frequencies falling outside the chosen
frequency range of
blasting machine 32. In the preferred embodiment, the ferrite has an operating
range between 3
and 20 kHz, which is too high for general power transmission interference, and
too low for
interference by radio or communications signals. As noted above, the ferrite
chosen must have
a Curie temperature higher than the temperature in bore 20 at the level of oil
bearing rock stratum
26. Typically a Curie temperature of 200°C or higher is preferred. In
the preferred embodiment
the ferrite core chosen has an initial permeability of 2500, a Curie
temperature of 230°C, a
saturation flux density of 5000 Gauss at room temperature and a field strength
of 15 Oersted.
The preferred material for closure element 64 is a super permalloy ('r.tvl.~,
an alloy of 80%
nickel and 20% iron having an A.C. impedance permeability in the order of
100,000 a Curie
temperature of roughly 400 ° C, and a saturation flux density of 8
Gauss. Due to its high
permeability, the thickness of closure member 64 is 0.36 mm. The material cost
is low, and the
alloy can be formed to the shape desired. Closure member 64 in other
embodiments, can also be
made of a suitable ferrite for a given frequency range or from other magnetic
materials, such as
silicon steels.
The combination of the material properties of core 62 and closure member 64
provide
relatively e~lcient, and desirable, frequency discrimination. The Mn-Zn
ferrite material responds
relatively poorly to DC and low frequency AC stimulation, but can operate
satisfactorily at higher
frequencies as high as a few MHz. By contrast, the magnetic alloy of closure
member 64 responds
20416191.1
CA 02557913 1997-12-16
- 14-
satisfactorily to low frequency AC and DC signals, but tends to attenuate high
frequency signals
as its permeability decreases with increasing frequency. Also, core losses are
approximately
proportional to the square of the frequency. Consequently low frequency (< 1
KH?) signals are
retarded by core 62 and high frequency signals (> lMHz) are attenuated by
closure member 64.
When a firing current is delivered by blasting machine 32 to lead wires 78 and
82 both core 62
and closure member 64 are energized, bridge wire 54 is heated to
incandescence, and pyrotechnic
material 52 is ignited. Thus the combined effect core 62 and closure member 64
is that of a
frequency sensitive filter.
Blasting machine 32, located at the far end of wireline 30 from detonator 42,
is illustrated
in electrical schematic form in Figures 6, 7, and 8. It supplies electric
current to fire detonator 42
as described at length above. Blasting machine 32 will be described in detail
in order of a timing
driver, controlling circuit 86, which provides an oscillating signal; a firing
circuit indicated
generally as 88, in the nature of an inverting circuit which receives the
oscillating signal; and a
charging circuit indicated generally as 90, which charges energy storage
elements of firing
circuit 88 to a desired voltage level.
The time varying signal generator, or driver, controlling circuit 86, shown in
Figure 6, has
as its principle element a commercially available IR2151 self oscillating
MOSFET and IGBT
driver chip 92 having V~~, Rt" C,, Com, Vb, Ho, V9, and Lo ports. A DC source
in the nature of a
15V dry cell 94 has a negative terminal connected to the Com port, and a
positive terminal
connected, through a switch 96, to V~~. A timing resistor 98 is connected
across the Rt and C~
ports, and a timing capacitor 100 connected to between the C, port and an
output terminal 'D'.
A voltage stabilising capacitor 102 is connected from Com to Vb. A diode 104
and capacitor 106
are used to provide high side power supply, high side power supply capacitor
106 being
connected across Vb and Vg. Vg is connected directly to an output terminal
'B'. Ho and Lo are
similarly connected to output terminals 'A' and 'C' respectively. Closure of
switch 96 will cause
chip 92 to produce a high side, low power square wave output 108 between
terminals 'A' and
'B', and an opposite, half period phase shifted low side square wave output
110 between
terminals 'B' and 'C', as indicated in Figure 5. Chip 92 is capable of
generating controlling
signals over a wide frequency range. In the preferred embodiment, a
controlling signal at 12.75
20416191.1
CA 02557913 1997-12-16
-15-
KHz is produced when resistor 98 has a value of ~6 kS2, capacitor 100 has a
value of 1000 pF and
capacitor 106 has a value of 0.47 ~cF.
Firing circuit 88 is shown in Figure 5, as a half bridge converter with input
ports 'A', 'B',
'C', and 'D' corresponding to output ports 'A', 'B', 'C', and 'D' of driver
86. Back to back
energy storage capacitors 112 and 114, whose charging will be described below,
are joined in
series at a central grounded node 116 and act as power sources for high and
low side MOSFETs
118 and 120 respectively, defining a high voltage side 122, and a low voltage
side 124. In a
preferred embodiment the storage of capacitors 112 and 114 are 470~cF
capacitors. MOSFETs
118 and 120 are of the high speed switching type with voltage and current
ratings of 1000 V and
14 A. A voltage limiting Zener diode 126 and an LED 128 are connected in
series between high
and low voltage sides 122 and 124 as well. The source of MOSFET 118 and the
drain of
MOSFET 120 are connected at a common node corresponding to input 'B', with the
drain of
MOSFET 118 connected to high voltage side 122 and the source of MOSFET 120
connected to
low voltage side 124.
The gate of MOSFET 118 is connected to input 'A' across a resistor 130 which,
in a
preferred embodiment, has a value in the range of 10 to SOOS2. Resistor 130 is
used to reduce the
quality factor of the input circuit, thereby discouraging parasitic
oscillations. Similarly the gate
of MOSFET 120 is connected to input port 'C' across a resistor 132 or the same
magnitude, for
the same purpose.
A gate to source resistor 134 having a value of 1MS2 is used to reduce
resistance from
the gate to the source of MOSFET 118. A similar resistor 136 is used with
MOSFET 120 for the
same purpose. A pair of opposed Zener diodes 138, 140 having a voltage rating
of 18V and a
power rating of 1 W each are used to protect the gates and sources of MOSFETs
118 and 120.
Further Zener diodes 142 and 144 connected between the drains and sources,
respectively of
MOSFETs 118 and 120 provide protection against voltage surges. Higher voltage
protection
could be obtained by connecting more than one such Zener diode in series.
20416191.1
CA 02557913 1997-12-16
-16-
Finally, a 3052 current limiting resistor 146 extends from input port 'B' to a
first load
terminal 148, while a second load terminal 150 is connected directly to
central grounded
node 116. The current initiating resistor is 3052 in the preferred embodiment.
A third component of blasting machine 32 is charging circuit 90. As shown in
Figure 7,
it has a small step up transformer 152 has a primary coil 154. Primary coil
154 has one lead
connected to a standard, single phase, 115V, 60Hz AC plug 156, and has another
lead, connected
through a current limiting resistor 158 and through a switch 160 to connect
with the other side
of plug 166. A current limiting resister 162 and LED 164 in series are
connected in parallel with
primary coil 154 to indicate the working conditions of the transformer.
Secondary coil 166 of transformer 152 has leads 168, 170 connected to opposite
sides of
a full wave bridge rectifier 172. The positive output of rectifier 172 is
connected to high voltage
side 122 and the negative side of rectifier 172 to low voltage side 124 of
blasting machine 32.
One of leads 168 or 170 is connected by a jumper 174 to grounded node 116, for
the purpose of
doubling the voltage level of main capacitors 112 and 114.
In operation, assuming that power storage capacitors 112 and 114 are initially
uncharged,
charging circuit 90 is plugged in to a suitable source, wireline 30 is
disconnected from output load
terminals 148 and 150, and timing circuit switch 96 is open. Charging circuit
switch 160 is then
closed to charge capacitors 112 and 114. Once capacitors 112 and 114 have been
charged to
300V, switch 160 may be opened or the power source may be disconnected.
After perforating gun assembly 28 has been conveyed along bore 20 to an
appropriate
position amidst oil bearing rock stratum 26, wireline 30, and hence,
ultimately primary
winding 80, is connected to load terminal 148. Load terminal 150 is grounded
through node 116
and primary winding 80 being connected to ground 82. When timing signal switch
96 is closed,
square wave signals 108 and 110 will be sensed at the respective gates of
MOSFETs 118 and
120, turning them on and off alternatively and giving a peak output current in
the range of 1 to
12A. When a positive voltage of 10 to 15V is applied to terminals A and B (
that is, gate to
source), MOSFET 118 conducts, capacitor 112 discharges through it and a
current runs through
20416191.1
CA 02557913 1997-12-16
-17-
current limiting resistor 146 to the load, that is, wireline 30 and the
components of detonator 42,
forming a first half cycle of electric current shown as h as shown in Figure
5. In the second half
of a cycle, MOSFET 120 conducts and MOSFET 118 is switched off. Capacitor 114
discharges
to the load, RL, that is, through detonator 42 and current limiting resistor
146 such that an
electric current indicated as IZ in the lower side. In this manner the two (2)
MOSFETs 118 and
120 will conduct alternately, yielding an alternating current in load RL until
both capacitors 112
and 114 are discharged. The alternating current produced in this manner is
carried along
wireline 30 to primary winding 80 to induce a current in secondary winding 60,
and
bridgewire 54, which in turn heats to incandescence and sets off igniter
material 52. In the
preferred embodiment, blasting machine 32 constructed using the circuitry
described herein has
a maximum peak to peak current output of 16A or a maximum peak to peak voltage
output of
900V, assuming capacitors 112 and 114 have been charged to 450V each, and
resistor 146 has
a value of SSS~ . In use the embodiment described yields a signal having
relatively high voltage,
relatively large current, relatively high momentary power output, and
relatively short duration.
The apparatus described has been found to discourage unintentional firing due
to stray
currents from common AC or DC sources, radio frequency energy, lightning and
other
electrostatic discharges. The inventors have found that it discourages firing,
even when
commonly used electric sources are applied directly to leadwires 78 and 82
(with the DC firing
current of the material 52 of 0.8 A). The inventors have found that detonators
made according
to the above description have resisted firing when exposed to 115V, 60Hz AC;
220V, SOHz AC;
380V, SOHz AC; and when connected to a 705pF capacitor charged to 600V.
Having described the preferred embodiment of the invention, it should be noted
that a
number of alternatives are possible without departing from the principles or
spirit of the invention.
The detonator of the present invention can be manufactured in different forms
to facilitate its use.
For example, a block detonator is a design that provides some space between
the detonator and
detonating cord by using a block, allowing fluid desensitization. A top fire
detonator is designed
to start a top-down detonation of the explosive train in the gun. A detonator
in capsule version
is directly exposed to the high pressure in the well. The present detonator
may be manufactured
in any of these forms.
20416191.1
CA 02557913 1997-12-16
-18-
Four alternative versions of detonator geometry are shown in Figures 3b, 4b,
4c and 4d.
Figures 3b shows a transversely mounted detonator transformer 180 having a
circular cylindrical
ferrite core 182 of a diameter of a 5 mm and a length of 6 mm. A primary
winding 184 having
60 turns and a secondary winding 186 of 30 turns are wound in a nested, co-
axial fashion about
core 182, that is to say, one winding is embedded within the other. Ferrite
core 182 is a simple
fernte bar, and, as shown, is an open magnetic circuit. The magnetic
properties of the bar are the
same as those of U-shaped core 62 of the preferred embodiment of Figure 3a.
Figure 4b shows a detonator transformer 200 having a circular cylindrical
ferrite core 202,
6mm long and Smm in diameter, about which a primary winding 204 having 60
turns, and a
secondary winding 206 having 30 turns are coaxially wound in a nested fashion.
Core 202 is then
held about its ends by a U-shaped, or half rectangle shaped, magnetic alloy
closure member 208
having a back 210 and legs 212 and 214. Closure member 208 could also be in
the form of a full
closed rectangle, or a circle or other shape making a closed loop for
capturing core 202 about its
ends.
Alternatively, the transformer core could be in the form of a bobbin or
spindle having at
one end a radially extending flanged base or shoulder, with a closure member
in the shape of a
cap, or thimble, at least partially covering the spindle with continuous
magnetically permeable
structure extending from one end of the spindle to the other. The foregoing
alternatives are only
examples of cores that could be used in the present invention, other
configurations such as a plate,
a square column, or a square or round tube, and other configurations also
being possible.
Figure 4c shows a detonator transformer 220 having a circular cylindrical
ferrite core 222
of a diameter of a Smm and a length of 6mm. A primary winding 224 having 60
turns is wound
about one portion of core 222. A secondary winding 226 of 30 turns is wound
about another
portion of core 222. There is no highly magnetically permeable closure member,
rather the
magnetic circuit of ferrite core 222 is left open.
Figure 4d shows a detonator transformer 240 having a core 242 in the form of a
C-shaped
half cylinder section, much like a half toroid but with a rectangular cross
section, having toes 244
20416191.1
CA 02557913 1997-12-16
- 19-
and 246. A primary winding 248 of 60 turns is wound about toe 244 and a
secondary
winding 250 of 30 turns is wound about toe 244. As before, there is no highly
magnetically
permeable closure member sparing the gap between toes 244 and 246 to form a
closed loop path.
It will be appreciated that the geometry of the transformer core may vary, and
it may be
S in the form of an open core, or a core having a closure member and a closed
loop magnetic path.
The core may be solid, or it may be a hollow tube, whether of circular,
square, or other section.
The relative position of the primary ans secondary windings has an effect on
output
performance. When one winding is embedded within the other, or the two
windings are coaxial
and close together or abutting, it is possible for a low or non-magnetically
permeable material,
whether air, a ceramic, paper or plastic core, to couple suil'icient magnetic
flux between the two
windings to permit detonation. For example, a sudden fluctuation in a 150 A
current can be
enough to trigger detonation. If the axes of the windings are parallel and
spaced apart an axial
distance, similar to the axial distance shown in Figure 3a, the magnetic flux
coupled by air
between the two cores is reduced, or minimized.
In all cases, the detonator transformer windings present a significant level
of impedance
to the firing current supplied by blasting machine 32 and coupled by wireline
30. This is done by
using a relatively large number of turns on both the primary and secondary
windings, rather more
than merely one or two turns. The minimum number of turns has not been
determined, but is
thought to be at least f ve. Hand threading multi-turn cores is generally
impractical, more so in
oil well detonators since a typical inside diameter for a casing, like shell
40, is 6mm, implying very
small core and winding sizes. It is more economical to form these multi-turn
windings by machine,
and this is facilitated if, at the time of manufacture, the core presents an
open ended spindle, or
mandrel, upon which a winding can be wound, or upon which a pre-formed winding
can be
slipped. The winding, or windings, can then be retained in place either by the
mechanical tightness
of the winding, an adhesive, or by mechanical means such as a fastener, a bent
over flange, or, as
in Figures 4a and 4b, by a magnetically permeable closure member. The spindle,
or mandrel,
portion, or portions of the core, may be circular in section, as in the case
of Figures 4b and 4c,
20416191.1
CA 02557913 1997-12-16
-20-
or rectangular, as in the case or Figures 4a and 4d, or some other shape or
shape as may be found
convenient.
The alternative blasting machine 260 of Figure 8 shows a half bridge structure
of an
inverting circuit using pairs of two IGBTs 262, 264 or 266, 268 in parallel in
place of MOSFETs
118 or 120, for the purpose of increasing the maximum current out put of the
blasting machine.
The full bridge 270 of Figure 9 is for use when a higher voltage output is
required than
can be produced with the similar half bridge of Figure 5. Those elements that
are unchanged from
Figure 5 are indicated by the same item numbers as above. A circuit as shown
in Figure 10 can
be used to drive full bridge 270 of Figure 9. It uses a 555 timer 272 as a
square wave signal
generator. The circuit is powered by battery 274 controlled by a switch 276.
The frequency of
the signal is determined by the values of the capacitors 278 and 280. The
output signal of
timer 272 is amplified using transistor 282 whose collector is connected to
the primary winding
284 of a small transformer 286. The transformer is coupled with a ferrite core
290 and has four
identical secondary windings 292, 294, 296 and 298 with the polarity shown, at
which output
signals identified as GS1, GS2, GS3, and GS4 are sensed.
Referring again to Figure 9, full bridge 270 has four MOSFETs 300, 302, 304,
and 306
arranged to work in diagonal pairs to produce a doubled-voltage push-pull
effect. MOSFETs 300
and 306 are driven by signals GS 1 and GS2, of the same polarity, MOSFETs 302
and 304 are
driven by signals GS3 and GS4, of the opposite polarity. MOSFETs 300 and 306
conduct
simultaneously as a pair, and alternate with the other pair formed by MOSFETs
302 and 304.
The net result, as before, is to drive an alternating current through a
current limiting resistor 310
and the load, RL. In the embodiment shown, each MOSFET 300, 302, 304 and 306
has a voltage
and current rating of 1000V and 14 amperes, respectively. As before,
capacitors 112 and 114
have a capacitance of 470 micro F each, in the preferred embodiment. They are
connected in
series and charged to 800 V, The value of current limiting resistor 310 is 80
ohms.
Full bridge 270 of Figure 9 is driven by 555 timer 272 of Figure 10 at a
frequency of
20KHz. Consequently, a blasting machine constructed using this circuitry has a
maximum peak
20416191.1
CA 02557913 1997-12-16
-21 -
to peak current of 20A, or a maximum peak to peak voltage of 1600V.
Controlling circuit of
Figure 10 is one example of a controlling circuit suitable for use with a full
wave inverter. Other
electronic circuits could be used as well.
Although only two types of power transistors, i.e., MOSFETs and IGBTs, are
used in the
description of the present invention, other types of power transistors can
also be used. Bipolar
transistors, Giant Darlington power transistors, and gate turn-o1T silicon-
controlled rectifiers can
be used in place of MOSFETs and IGBTs with corresponding changes in the
driving circuits
according to their driving requirements.
In the description of the present invention, the circuits are shown in
discrete elements.
However, it is understood that the half bridge or full bridge converter can be
integrated into a
single chip along with its driving circuit, making it more compact and less
expensive.
In each embodiment described, the blasting machine does not require an output
transformer. However, it does not exclude the use of a transformer for other
purposes, such as
for isolation of electronic circuits, or for impedance matching between the
blasting machine and
the load. In such uses, the transformer is not involved in the conversion of
the DC currents to
high frequency AC currents. The transformer is not necessary part of the
converter.
In addition to charging circuit 90 shown in Figure 7, capacitors 112 and 114
can be
charged using dry batteries, an oscillating circuit, a step up blasting
machine or other suitable
circuit. The use of commercially available single phase 115 V, 60Hz AC , as
shown in Figure 7,
corresponds to a source commonly available from truck mounted generators at
well sites.
Although developed mainly for oil well casing perforation, the apparatus of
the present
invention can also be used in other oil field applications such as
exploration, pipe cutting,
severing, and so on. Furthermore, the apparatus of the present invention can
also be used to
replace conventional bridge wire detonators in mining, construction and other
engineering projects
where the initiation of explosives is involved.
20416191.1
CA 02557913 1997-12-16
-22-
This description is made with reference to the preferred embodiment of the
invention.
However, it is possible to make other embodiments that employ the principles
of the invention and
that fall within its spirit and scope as defined by the following claims.
20416191.1
