Note: Descriptions are shown in the official language in which they were submitted.
CA 02877931 2015-01-13
SYSTEM AND METHOD FOR DETERMINING THE START TIME OF A PRESSURE
PULSE FROM A DOWNHOLE EXPLOSIVE DEVICE
Field of the Invention
[0001] The present invention relates to a system and method for determining
the start time of a
downhole pressure pulse, such as created by a downholc explosive device.
Background of the Invention
[0002] The conventional method used to determine the location of micro-seismic
events (i.e.,
those events resulting from induced seismicity related to hydraulic
fracturing) relies upon
velocity models derived from dipole sonic logs. An inversion process is used
to minimize the
misfit error between actual and theoretical event arrival times using a
multiplicity of receivers.
However, sonic tools use acoustic energy in the 10,000 to 30,000 Hz range to
measure formation
velocities, a range which greatly exceeds the typical frequency content of
micro-seismic events.
Since seismic energy in the earth is dispersive (i.e., velocity varies with
frequency), error is
introduced into the velocity model.
[0003] The seismic energy from multiple perforation shots from a perforating
gun can be used to
calibrate and improve the velocity model derived from sonic logs. Receivers
are positioned in a
nearby borehole, on the surface, or both, to detect pressure pulses created by
the perforation
shots. Every ray path between a perforation shot location and a receiver
contributes to a more
complete mapping of the three-dimensional velocity structure. If both the ray
path distance and
ray path travel time are known, then the pressure pulse velocity can be
determined for each ray
path. Information regarding velocity anisotropy enhances the understanding of
the petro-physical
properties of the rock, leading to a better correlation with surface seismic
data, and a further
reduction in event location error.
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[00041 In order to accurately determine the ray path travel time, it is
necessary to accurately
determine the detonation time of the explosive device used in the perforating
gun.
Unfortunately, the firing signal used to detonate the explosive device is not
a reliable indicator of
the actual detonation time because of variable and unpredictable delays
between the generation
of the firing signal and the detonation of the explosive device. For example,
blasting caps
typically have moisture sensors. If downhole moisture invades the blasting
cap, this can delay
detonation by wetting electrical conductors between the firing system and the
explosive device
itself. The firing system may have a safety lockout which introduces a time
delay between
generation of the firing signal and its transmission to the explosive device.
Different firing
systems are associated with different delays between sending the firing signal
and actual
detonation. Further, the electrical firing signal typically interferes with
any up-hole transmitted
signaling pulses. It has thus not been possible to reliably and accurately
determine when the
explosive was actually detonated downhole.
Summary of the Invention
[0005] The present invention relates to a system and method for determining
the start time of a
downhole pressure pulse created by the firing of a downhole explosive device.
[0006] In one aspect, the present invention comprises a downhole tool for
creating a signaling
pulse in response to a pressure pulse created by a downhole explosive device
detonated by a
firing signal. The downhole tool comprises a pressure pulse detector for
detecting the pressure
pulse and generating an input signal in response to detecting the pressure
pulse, and a circuit path
operatively connected to the pressure pulse detector, and configured to output
a signaling pulse
distinguishable from the firing signal, in response to the input signal.
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[0007] In one embodiment, the downhole tool is configured as a sub that can be
lowered
downhole by a wireline.
[0008] In one embodiment, the downhole tool is powered solely by a power
source located at the
surface and used to create the firing signal.
[0009] In one embodiment, the downhole tool circuit path is further configured
to amplify the
input signal.
[0010] In one embodiment, the downhole tool circuit path is further configured
to discriminate
the input signal from the firing signal by comparing a parameter of the input
signal to a
predetermined parameter associated with the firing signal, and to output the
signaling pulse only
if the input signal is discriminated from the predetermined parameter.
[0011] In one embodiment, the downhole tool circuit path is configured to
output the signaling
pulse with a greater voltage, a greater current, or both a greater voltage and
a greater current than
the firing signal.
[0012] In another aspect, the present invention comprises a system for
creating a start time signal
in response to a pressure pulse created by a downhole explosive device
detonated by a firing
signal. The system comprises a downhole tool, as described above, and a
surface unit. The
surface unit comprises a circuit path operatively connected to the downhole
tool circuit path, and
configured to discriminate the signaling pulse from the firing signal, and to
output the start time
signal in response to receiving the signaling pulse.
[0013] In one embodiment, the surface unit circuit path is configured to
discriminate the
signaling pulse from the firing signal by comparing a parameter of the
signaling pulse to a
predetermined parameter associated with the firing signal.
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[0014] In one embodiment, the surface unit circuit path is configured to
analogically attenuate
electrical noise associated with the firing signal that is transmitted with
the signaling pulse.
[0015] In one embodiment, the surface unit circuit path is configured to
digitally extract the
firing signal from the signaling pulse to output the start time signal.
[0016] In one embodiment, the downhole tool circuit path is operatively
connected to the surface
unit circuit path by a transmission path that also transmits the firing signal
to the downhole
explosive device.
[0017] In another aspect, the present invention comprises a method for
creating a start time
signal in response to a pressure pulse created by a downhole explosive device
detonated by a
firing signal, the method comprising the steps of:
(a) using a downhole tool for:
(i) detecting the pressure pulse and generating an input signal in response
to
detecting the pressure pulse;
(ii) in response to detecting the pressure pulse, generating a signaling
pulse
distinguishable from the firing signal;
(b) using a surface unit for:
(i) receiving the signaling pulse;
(ii) discriminating between the signaling pulse and the firing signal; and
(iii) in response to receiving the signaling pulse, outputting the start
time
signal.
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[0018] In one embodiment, the downhole explosive device comprises a
perforating gun or an
electric blasting cap.
[0019] In one embodiment, the downhole tool is powered solely by a power
source located at the
surface and used to create the firing signal.
[0020] In one embodiment, the downhole tool is further used for amplifying the
input signal.
[0021] In one embodiment, the downhole tool is further used for discriminating
the input signal
from the firing signal by comparing a parameter of the input signal to a
predetermined parameter
associated with the firing signal, and wherein generating the signaling pulse
is conditional on the
input signal being discriminated from the firing signal.
[0022] In one embodiment, the signaling pulse is distinguishable from the
firing pulse by a
greater voltage, a greater current, or both a greater voltage and a greater
current than does the
firing signal.
[0023] In one embodiment, the surface unit discriminates the signaling pulse
from the firing
signal by comparing a parameter of the signaling pulse to a predetermined
parameter associated
with the firing signal.
[0024] In one embodiment, the method further comprises, before receiving the
signaling pulse at
the surface unit, the step of analogically attenuating electrical noise
associated with the firing
signal that is transmitted with the signaling pulse to the surface unit.
[0025] In one embodiment, the method further comprises, before receiving the
signaling pulse at
the surface unit, the step of digitally extracting the firing signal from the
signaling pulse to
output the start time signal.
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[0026] In one embodiment, the surface unit receives the signaling pulse in a
transmission path
that also transmits the firing signal to the downhole explosive device.
[0027] In another aspect, the present invention provides a downhole tool for
determining the
time of a pressure pulse created by a downhole explosive device detonated by a
firing signal. The
downhole tool comprises a pressure pulse detector for detecting the pressure
pulse and
generating an input signal in response to detecting the pressure pulse, and a
processor operatively
connected to the pressure pulse detector by a circuit path. The processor
comprises an internal
clock and a memory component storing a set of instructions executable by the
processor to
determine a time at which the processor receives the input signal, and store
the time in the
memory component.
[0028] In one embodiment, the downhole tool is configured by a sub that can be
lowered
downhole by a wireline.
[0029] In one embodiment, the downhole tool is powered solely by a power
source located at the
surface and used to create the firing signal.
[0030] In one embodiment, the downhole tool circuit path is further configured
to amplify the
input signal.
[0031] In one embodiment, the downhole tool circuit path is configured to
discriminate the input
signal from the firing signal by comparing a parameter of the input signal to
a predetermined
parameter associated with the firing signal, and to transmit the input signal
to the processor only
- if the input signal is discriminated from the firing signal,
[0032] In one embodiment, the set of instructions is executable by the
processor to further store
the input signal in the memory component in association with the time.
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.=
[0033] Additional aspects and advantages of the present invention will be
apparent in view of the
description, which follows. It should be understood, however, that the
detailed description and
the specific examples, while indicating preferred embodiments of the
invention, are given by
way of illustration only, since various changes and modifications within the
spirit and scope of
the invention will become apparent to those skilled in the art from this
detailed description.
Brief Description of the Drawings
[0034] The invention will now be described by way of an exemplary embodiment
with reference
to the accompanying simplified, diagrammatic, not-to-scale drawings.
[0035] Figure 1 is a schematic block diagram showing use of one embodiment of
a system of the
present invention in a field installation.
[0036] Figure 2 is a schematic block diagram of one embodiment of a downhole
tool of the
system of Figure 1.
[0037] Figure 3 is a schematic diagram of one embodiment of a circuit path of
a downhole tool
of the present invention.
[0038] Figure 4 is a schematic block diagram of one embodiment of a surface
unit of the system
of Figure 1.
[0039] Figure 5 is a schematic diagram of one embodiment of a circuit path of
a surface unit of
the present invention.
[0040] Figure 6 is a schematic diagram of an alternative embodiment of a
circuit path of a
surface unit of the present invention.
[0041] Figure 7 is a schematic depiction of one embodiment of a memory-based
downhole tool
of the present invention.
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Detailed Description of Preferred Embodiments
[0042] The present invention relates generally to systems and methods for
determining a start
time of a downhole pressure pulse created by the firing of a downhole
explosive. When
describing the present invention, all terms not defined herein have their
common art-recognized
meanings. To the extent that the following description is of a specific
embodiment or a
particular use of the invention, it is intended to be illustrative only, and
not limiting of the
claimed invention. The following description is intended to cover all
alternatives, modifications
and equivalents that are included in the spirit and scope of the invention, as
defined in the
appended claims.
[0043] As used herein, the term "explosive device" refers to any type of
device capable of
creating a "pressure pulse" in a surrounding medium upon the explosive
device's activation or
detonation. Suitable explosive devices include, but are not limited to, a
perforating gun, an
electric blasting cap, and the like. As used herein, the term "pressure pulse"
refers to a pressure
variation that propagates in a gas, liquid or solid medium, and includes
sound, vibration,
ultrasound, and infrasound waves and pulses. Sound propagates primarily as a
pressure wave or
pulse.
[0044] One embodiment of the system (10) of the present invention as shown in
Figure 1
comprises a downhole tool (12) and a surface unit (14), with such components
being operatively
connected. In one embodiment, the surface unit (14) is also operatively
connected to a firing
device (16) which generates a firing signal to detonate a downhole explosive
device (18)
positioned within a wellbore (20). As used herein, the term "operatively
connected" in
describing the relationship between components means a connection for
conveying signals
between components, such as, for example, the electric wireline (22), the
cable (24), or a
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wireless transmitter and receiver. In one embodiment, the electrical wireline
(22) is the same
transmission path that is used by the firing device (16) to transmit the
firing signal to the
explosive device (18).
[0045] In general, the downhole tool (12) detects a pressure pulse (36)
created by the detonation
of the downhole explosive device (18), using for example a pressure
transducer, and transmits a
signaling pulse (44) up-hole through the electric wireline (22) to the surface
unit (14), which can
be used to provide accurate indicator of the time at which the firing of the
explosive device (18)
occurred downhole (t=0). The surface unit (14) processes the signaling pulse
(44) to provide a
start time signal (54). The system (10) may be used in conjunction with a
remotely located
receiver (not shown) which generates a signal in response to the pressure
pulse. A computer (not
shown) equipped with a timing system is operatively connected to the surface
unit (14) to
determine the time elapsed between the start time signal (54) and the signal
subsequently
received from the receiver. The elapsed time represents the travel time of the
pressure pulse from
the downhole tool (12) to the remote receiver. This travel time can be used to
determine the
velocity of the pressure pulse if the distance between the downhole tool (12)
and the remote
receiver is known,
[0046] As shown in one embodiment in Figure 2, the downhole tool (12)
comprises a pressure
pulse detector (26) operatively connected to a circuit path comprising an
amplifier (28), a
discriminator and logic unit (30), a signal generator (32), and a power supply
(34).
[0047] The pressure pulse detector (26) detects an acoustic pressure pulse
(36) created by the
firing of the explosive device (18), and generates an input signal (38) in
response to detecting the
pressure pulse (36). The pressure pulse detector (26) may be any suitable
device known in the
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art, such as a pressure transducer, which without limitation, may include
piezoresistive,
capacitive, electromagnetic, or optical devices.
[0048] The amplifier (28) increases the amplitude of the input signal (38) to
yield a relatively
larger output signal (40). In one embodiment, the amplifier (28) modulates the
output signal (40)
to match the input signal (38) shape, but with a relatively larger amplitude.
The amplifier (28)
transmits the amplified output signal (40) to the discriminator and logic unit
(30).
[0049] In one embodiment, the pressure pulse detector (26) produces an analog
signal (38). The
discriminator of the discriminator and logic unit (30) converts the analog
input signal (38), after
amplified by amplifier (28), into a standardized output pulse (42) whenever
the amplified input
signal (38) amplitude exceeds a predetermined threshold voltage. The logic
unit of the
discriminator and logic unit (30) performs the logical operations (for
example, AND, NAND,
OR, NOR and NOT). The input signal (38) and output pulse (42) amplitude
corresponds to two
possible states: "0" (or "TRUE") or "1" ("FALSE"). In one embodiment, the
logic unit signals
are joined so that the output pulse (42) is "1" or "TRUE" only when the input
signal (38)
corresponds to a predetermined pattern. In one embodiment as shown in Figure
3, the
discriminator and logic cunit (30) comprises one or more comparators to
compare the voltage of
the input signal (38) to the predetermined threshold voltage and outputs a "1"
or "TRUE" output
pulse (42) if the input signal (38) exceeds the predetermined threshold
voltage. One comparator
may be configured for a negative signal and the other comparator may be
configured for a
positive signal to accommodate input signals (38) of either polarity. The
output pulse (42) from
the discriminator and logic unit (30) is then sent to the signal generator
(32).
[0050] The signal generator (32) receives the output pulse (42) from the
discriminator and logic
unit (30), and processes the output pulse (42) into a signaling pulse (44)
distinguishable from the
CA 02877931 2015-01-13
firing signal. In one embodiment, the signal generator (32) amplifies the
energy of the signaling
pulse (44) so that it is greater than the energy of the firing signal in order
to be detectable by the
surface unit (14). The signaling pulse (44) may have a greater voltage, or a
larger current, or
both, than the firing signal. In other embodiments, the signaling pulse (44)
may have other
distinguishable parameters in comparison with the firing signal. In one
embodiment as shown in
Figure 3, the signal generator (32) may comprise one or more integrated
circuits that amplify the
output pulse (42) to produce the signaling pulse (42). The signal generator
(32) transmits the
high energy signaling pulse (44) to the surface unit (14),
[0051] The power supply (34) provides operating power (46) to the amplifier
(28), the
discriminator and logic unit (30), and the signal generator (32). In one
embodiment, as a safety
feature, the downhole tool (12) lacks any intrinsic power source which might
accidentally
detonate the explosive device (18), The power supply (34) uses power (46) from
the same
source which operates the firing device (16) to initiate the detonation of the
explosive device
(18). In one embodiment, the operating power (46) is about 12 V DC. In one
embodiment, the
power supply (34) is "fast settling" in the sense that it quickly stabilizes
so that the amplifier (28)
and the discriminator (30) are ready to detect an input signal (38) generated
by the pressure pulse
detector (26).
[0052] By transmitting the signaling pulse (44) to the surface unit (14), the
downhole tool (12)
thus provides the surface unit (14) with a relatively accurate indicator of
the time at which the
firing of the explosive device (18) occurred. The wireline delay ¨ i.e., the
time required by the
signaling pulse (44) to travel through the electric wireline (22) to the
surface unit - may be the
major source of inaccuracy, but is practically very brief. In one embodiment,
the wireline delay
may vary by 10 fis because of transmission velocity of the signaling pulse
(44) through the
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electric wireline (22). This wireline delay depends upon the length of the
wireline or electrical
connection (22). The wireline delay is typically estimated to be about 40 u,s
for a wireline
having a length of about 5000 m. As the wireline delay is a measurable
constant delay, it can be
predicted or measured, and accounted for, if greater accuracy is required in
determining the
firing time of the downhole explosive device (18).
[0053] In one embodiment, the downhole tool (12) may be configured as a sub
which can be
lowered downhole by the electric wireline (22) to be positioned proximate to
the explosive
device (18). The sub is preferably rated to withstand elevated temperature and
pressure, in one
example, a minimum of about 100 C and about 135 MPa.
[0054] As shown in Figure 4, in one embodiment, the surface unit (14)
comprises a circuit path
comprising an analog signal filter (48), a digital filter (49), discriminator
(50), and a power
source (60). The circuit path of the surface unit (14) receives the high
energy signaling pulse
(44) from the circuit path of the downhole tool (12) through an operative
connection, such as the
electric wireline (22).
[0055] It will be appreciated that in practical implementation, the time that
elapses between the
transmission of the firing signal and transmission of the signaling pulse (44)
is very brief. If the
firing signal and signaling pulse (44) are transmitted on the same electric
wireline (22) or electric
wirelines in proximity to each other, the signaling pulse may be contaminated
with the firing
signal, and possibly interference signal from other devices. Thus, in one
embodiment, the analog
filter (48) and digital filter (49) conduct signal separation and signal
restoration on the signaling
pulse (44) received from the downhole tool (12). Signal separation is needed
when a signal has
been contaminated with interference, noise, or other signals such as
interference signals
associated with the firing signal or firing device. Signal restoration is used
when a signal has
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been distorted in some manner. The analog signal filter (48) processes the
signaling pulse (44)
received from the dovvnhole tool (12) by attenuating any electrical firing
noise. In embodiments
as shown in Figures 5 and 6, the analog filter (48) comprises one or more
resistor and capacitor
elements. The analog signal filter (48) transmits the filtered signal (52) to
the digital filter (49)
(50) for further processing.
[0056] The digital filter (49) enhances the filtered signal (52) to output a
start time signal (54) to
a computer system (not shown). In embodiments, as shown in Figures 5 and 6,
the digital filter
(49) comprises one or more integrated circuits. The integrated circuits are
programmed or
programmable with algorithms that account for known firing signals and
interference signal
generated by different firing devices (16). In this manner, the digital filter
(49) may be
customized and adapted for use with a variety of different firing devices
(16). The algorithms
subtract the known firing signal and interference signal generated by a
selected firing device
from the filtered signal (52), thus extracting only the signaling pulse (44)
for use as the start time
signal (54).
[0057] The discriminator (50) tracks the firing voltages and sets the
detection levels from the
filtered signal (52) so as to distinguish it from the firing signal or other
interference signals. In
embodiments, as shown in the Figure 5 and 6, the discriminator comprises one
or more
comparators to compare the voltage of the filtered signal (52) to a pre-
determined threshold
voltage associated with the firing signal for a standard blasting cap. If the
voltage of the filtered
signal (52) exceeds the pre-determined threshold voltage, the comparator
outputs a "1" or
"TRUE' start time signal (54). One comparator may be configured for a negative
signal and the
other comparator may be configured for a positive signal to accommodate
filtered signals (38) of
either polarity. In one embodiment as shown in Figure 6, the filtered signal
(52) may by-pass the
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discriminator (50), and proceed directly to the digital filter (49) for
processing, as described
above, In such an embodiment, the digital filter (49) effectively
discriminates the filtered signal
(52) from the firing signal or other interference signals.
[0058] The power source (60) provides operating power to the components of the
surface unit
(14).
[0059] In one embodiment, at least one noise filter (56) is included on each
of the lines (22, 24)
between the surface unit (14) and the firing device (16), respectively, to
filter such unwanted
components such as background noise or interfering signals. Optionally, a
custom firing power
supply (58) may be used to activate or detonate the explosive device (18),
which may enhance
the signal to noise ratio,
[0060] A display (not shown) which is either operatively connected to or
integral with the
surface unit (14) may display indication signals (for example, system status,
errors, alarms,
output messages, instructions, audible buzzers, indicator lights) to perform a
test of the system
(10) to ensure proper connection of all components before operation, and to
inform a user
whether firing of the downhole explosive device (18) has been detected. For
example, a lengthy
continuous beep or illumination might indicate that the explosive device (18)
has detonated
successfully, and a short fast beep or illumination might indicate that the
explosive device (18)
has misfired or a malfunction has occurred in the system (10).
[0061] The surface unit (14) may include an operational switch so that a user
can specify the
particular type of explosive device (18) from which the start time signal
pulse (54) will originate.
Types of explosive devices (18) to which the surface unit (14) may be
responsive include, but are
not limited to, non-radio frequency type detonators, PXlTM detonators
(Teledyne RISI, Inc.,
Tracy, CA), and DynaEnergeticsTM detonators (DynaEnergetics GmbH & Co. KG,
Troisdorf,
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Germany). As discussed above, the digital filter (49) may be configured with
algorithms that
account for known firing signals and interference signal generated by firing
devices (16) used
with different types of explosive devices (18),
[0062] A remote unit (not shown) may be included in the system (10) in order
to transmit the
start time signal pulse output (54) to a separate remote unit (not shown) to
be read at a different
location or site. Display means may be integral with the remote unit to
display indication signals
(for example, audible buzzers, indicator lights) to confirm firing of the
downhole explosive
device (18) in order that the user can then transmit the start time signal
pulse (54) to the remote
unit. Transmitting acoustic data remotely conveniently enables another user to
obtain the data
without having to read the display of the surface unit (14) in person, or risk
injury by being
present in a detonation area.
[0063] In use and operation, a computer which comprises a timing clock system
may be
operatively connected to both the surface unit (14) and a remote receiver
comprising
conventional acoustic measuring equipment. The computer may be a separate
physical
component, physically integrated with the surface unit (14) or the remote
receiver, or a
combination of the foregoing. The remote receiver generates a signal upon
being actuated by the
pressure pulse. Since the distance between the remote receiver and the
explosive device is
greater than the distance between the remote receiver and the downhole tool
(12), the computer
receives the signal generated by the remote receiver afier receiving the start
time signal from the
surface unit (14). The computer uses start time signal (54) transmitted from
the surface unit (14)
to accurately mark the time of detonation of the explosive device (18), and
uses the signal
transmitted from the remote receiver to accurately mark the time that the
pressure pulse reached
the remote receiver, By knowing these two times, it is possible to accurately
determine an
CA 02877931 2015-01-13
elapsed time for the acoustic pulse to travel to the receiver based on the
difference between these
two times, and accounting for wirclinc delay as necessary. The velocity of the
pressure pulse
between the explosive device (18) and the remote receiver may be determined if
the distance
between them is known.
[0064] In an alternative embodiment as shown in Figure 7, a memory-based
downhole tool (120)
is provided. In this case, the memory downhole tool (120) is configured
similarly to downhole
tool (12) shown in Figure 2 in that it comprises a pressure pulse detector
(126), an amplifier
(128), and a discriminator and logic unit (130) that operate in a similar
manner to the
corresponding components of the downhole tool (12) described above. However,
in this
embodiment, the memory-based downhole tool (120) need not be operatively
connected to a
surface unit (14). Rather than transmitting the output pulse (42), to a signal
generator (32) and
uphole to the surface unit (14), the output pulse (42) is received by a
processor (132) having an
internal clock and a memory component, which forms part of the memory-based
downhole tool
(20). The internal clock may include temperature compensation, as is well
known in the art. In
response to the output pulse (42), the processor (132) determines the timing
of the output pulse
and stores the output pulse (36) into the memory component with an associated
time tag, in a
time data file. In this embodiment, a battery power supply (150) may be used
to power the
amplifier (128), the discriminator and logic unit (130) and the processor
(132). In one
embodiment, there is no electrical connection between the electric wirclinc
(22) that is used to
power the explosive device (18) and the battery-powered circuit path of the
memory-based
downhole tool (120) to reduce the possibility of the battery power supply
(130) accidentally
activating or detonating the explosive device.
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[0065] Before use downhole, the processor (132) of the memory-based downhole
tool (120) is
operatively connected to a computer (not shown) by any conventional means
(160), such as
USB, serial port or wireless means such as Bluetooth or WiFi. The computer
includes or is
connected to a standalone timing clock system, which the computer synchronizes
to the internal
clock of the memory downhole tool (120). In this manner, the internal clock of
the memory
downhole tool (120) may be synchronized with a timing clock system that is
used to determine
when an output signal is received from a remote receiver.
[0066] The memory-based downhole tool (120) may then be deployed and used
downhole to
detect pressure pulses.
[0067] The surface timing clock system may also be connected to a receiving
device
(conventional seismic device) and receives time entries corresponding to
pressure pulses
received by the remote receiver. When the memory downhole tool (120) is
brought back to the
surface, it is re-connected to the computer and resynchronized with the
surface timing clock
system. The time difference drift from the memory downhole tool (120) can then
be determined
and corrected.
[0068] By knowing the time tagged to the output pulse (42) by the processor
(142) and the time
that the remote receiver generated the output pulse, it is possible to
accurately determine an
elapsed time for the acoustic pulse to travel to the receiver based on the
difference between these
two times. The velocity of the pressure pulse may be determined if the
distance between the
explosive device (18) and the remote receiver is known.
[0069] As will be apparent to those skilled in the art, various modifications,
adaptations and
variations of the specific disclosure herein can be made without departing
from the scope of the
invention claimed herein.
17
