U.S. patent application number 13/731743 was filed with the patent office on 2014-07-03 for optical feedback to monitor and control laser rock removal.
This patent application is currently assigned to Halliburton Energy Services, Inc.. The applicant listed for this patent is HALLIBURTON ENERGY SERVICES, INC.. Invention is credited to Colin John Hawthorn, Timothy Holiman Hunter, James Louis, Neal Gregory Skinner, Lloyd S. Wilkiel.
Application Number | 20140182933 13/731743 |
Document ID | / |
Family ID | 51015872 |
Filed Date | 2014-07-03 |
United States Patent
Application |
20140182933 |
Kind Code |
A1 |
Skinner; Neal Gregory ; et
al. |
July 3, 2014 |
OPTICAL FEEDBACK TO MONITOR AND CONTROL LASER ROCK REMOVAL
Abstract
Methods, systems, and devices related to downhole wellbore
operations such as drilling and completing wells in an earth
formation that include a laser device. The method includes lasing a
rock and detecting an optical response of the lased rock. It can be
determined from the optical response whether the lased rock is
responding as specified (e.g. spalling, melting, etc.) If the lased
rock is not responding as specified, one or more laser parameters
are adjusted to achieve the specified response. Spalling is
determined by the detection of sparks, or other light that
erratically changes in intensity over time, by an optical detector.
The detection of steady light may indicate other types of rock
removal mechanisms, such as melt or dissociation.
Inventors: |
Skinner; Neal Gregory;
(Lewisville, TX) ; Hunter; Timothy Holiman;
(Duncan, OK) ; Hawthorn; Colin John; (Barrington,
IL) ; Wilkiel; Lloyd S.; (Westchester, IL) ;
Louis; James; (Chicago, IL) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
HALLIBURTON ENERGY SERVICES, INC. |
Houston |
TX |
US |
|
|
Assignee: |
Halliburton Energy Services,
Inc.
Houston
TX
|
Family ID: |
51015872 |
Appl. No.: |
13/731743 |
Filed: |
December 31, 2012 |
Current U.S.
Class: |
175/16 |
Current CPC
Class: |
B23K 26/40 20130101;
B23K 26/032 20130101; B23K 26/382 20151001; B23K 26/064 20151001;
B23K 26/082 20151001; B23K 2103/52 20180801; E21B 7/15 20130101;
B23K 26/08 20130101; E21B 43/11 20130101; B23K 26/38 20130101; B23K
2103/50 20180801; B23K 26/0622 20151001 |
Class at
Publication: |
175/16 |
International
Class: |
E21B 7/14 20060101
E21B007/14 |
Claims
1. A method for removing subterranean rock with a laser, the method
comprising: from inside a well bore, lasing subterranean rock
around the well bore; detecting emissions from the lased rock;
determining whether the detected emissions from the lased rock
indicate a specified material removal mechanism; and if the
detected emissions do not indicate the specified material removal
mechanism, adjusting one or more laser cutting parameters until
emissions detected from the lased rock indicates the specified
material removal mechanism.
2. The method of claim 1, further comprising assessing an optical
profile of the emissions detected from the lased rock for
characteristic properties of the specified material removal
mechanisms.
3. The method of claim 2, wherein a characteristic property of a
specified material removal mechanism comprises a detection of a
rapidly time-varying emissions.
4. The method of claim 2, wherein a characteristic property of a
material removal mechanism comprises a detection of steady
emissions.
5. The method of claim 1, wherein the detected emissions indicate
that the lased rock is spalling.
6. The method of claim 1, wherein the emission intensity indicate a
specified material removal mechanism if the emissions fluctuate
with a frequency above a specified threshold value.
7. The method of claim 1, wherein adjusting the one or more laser
parameters comprises, in response to the emissions detected from
the lased rock, changing one or more of beam irradiance of the
laser, laser power, laser spot size, laser on time, purge time, or
delay time between laser shut-off and purge turn-on.
8. The method of claim 1, wherein detecting emissions from the
lased rock comprises receiving light from the lased rock for a
period of time.
9. The method of claim 1, wherein detecting emissions from the
lased rock comprises detecting a steady emission intensity, the
method further comprising determining that the lased rock is not
spalling based on detecting the steady emission intensity.
10. The method of claim 9, further comprising determining that the
rock is melting based on detecting the steady emissions.
11. The method of claim 10, further comprising determining that the
rock is dissociating based on detecting the steady emissions.
12. The method of claim 1, where lasing subterranean rock comprises
perforating a sidewall of the well bore.
13. The method of claim 1, where lasing subterranean rock comprises
drilling the well bore.
14. A well apparatus for rock removal, comprising: a laser tool
configured for insertion into the well and to direct laser energy
onto rock; a detector configured for insertion into the well and to
detect emissions emitted from the rock; and a controller configured
to adjust power of a laser based on emissions detected from the
rock.
15. The apparatus of claim 14, wherein the controller comprises a
processor communicatively coupled to the controller and configured
to receive signals from the detector and output emissions
information to the controller.
16. The apparatus of claim 14, wherein, when the detected emissions
from the rock indicate that the rock is not responding as
specified, the controller is configured to automatically adjust one
or more of an irradiance of the laser energy, laser power, laser
spot size, laser on time, purge time, or delay time between laser
shut-off and purge turn-on.
17. The apparatus of claim 16, wherein the detected emissions from
the rock indicates that the rock is not spalling when the emissions
have a varying intensity with respect to time below a threshold
value.
18. The apparatus of claim 14, wherein the controller is configured
to maintain the power of the laser when the emissions detected from
the rock indicate that the rock is spalling.
19. The apparatus of claim 14, wherein the controller is configured
to determine that the rock is spalling when emission intensity
detected has varying intensities with respect to time, the
variations in intensities occurring with a frequency above a
threshold value.
20. The apparatus of claim 14, further comprising a reflector
configured to reflect a laser beam towards the rock and to reflect
the emission from the rock to the detector.
21. The apparatus of claim 14, further comprising a dichroic
reflector, the dichroic reflector configured to reflect a laser
beam towards the rock and to transmit the light emitted from the
rock to the detector.
22. The apparatus of claim 14, further wherein the detector
comprises one of an optical spot detector, an optical line
detector, or a two-dimensional array detector.
23. A well laser system for use in a subterranean well comprising:
a laser apparatus configured to: produce a laser beam, and direct
the laser beam towards a subterranean rock; an optical detector
configured to detect light emitted from the rock; and a controller
communicatively coupled to the optical detector, the controller
configured to: receive a signal from the optical detector,
determine from the signal whether the rock is responding as
specified; and adjust a parameter of the laser if the rock is not
responding as specified.
24. The system of claim 23, wherein the controller comprises a
processor configured to receive signals from the optical detector
and output instructions to the controller to adjust the power of
the laser if the rock is not responding as specified.
25. The system of claim 23, wherein the controller is configured to
automatically adjust the power of the laser when the light detected
from the rock indicates that the rock is not spalling.
26. The system of claim 25, wherein the controller determines that
the rock is not spalling when no sparks are detected by the optical
detector.
27. The system of claim 26, wherein the controller further
determines that the rock is dissociating when the light detected is
a steady glow.
28. The system of claim 26, wherein the controller further
determines that the rock is melting when the light detected is a
steady glow.
29. The system of claim 23, wherein the controller is configured to
maintain the power of the laser when sparks are detected by the
optical detector.
30. The system of claim 23, wherein the laser apparatus further
comprises a dichroic reflector, the dichroic reflector configured
to reflect a laser beam towards the rock and to transmit the light
emitted from the rock to the optical detector.
31. The system of claim 23, wherein the optical detector comprises
a spot detector.
32. The system of claim 23, wherein the optical detector comprises
a line detector.
33. The system of claim 23, wherein the optical detector comprises
a two-dimensional detector array.
Description
FIELD
[0001] The present disclosure relates generally to rock removal
using a laser, and more particularly, to adjusting laser power of
rock removal based on optical feedback.
BACKGROUND
[0002] Once a wellbore has been drilled and one or more zones of
interest have been reached, a well casing is run into the wellbore
and is set in place by injecting cement or other material into the
annulus between the casing and the wellbore. The casing, cement and
formation are then perforated to enable flow of fluid from the
formation into the interior of the casing. In some cases, the
casing can be omitted.
SUMMARY
[0003] Aspects of the present disclosure are directed to systems,
apparatuses, and methods for removing subterranean rock with a
laser. Certain aspects of the implementations include, lasing
subterranean rock around a well bore from inside the well bore.
Emissions, such as optical and/or thermal emissions, can be
detected from the lased rock. It can be determined whether the
detected emissions from the lased rock indicates a specified
material removal mechanism. If the detected emissions do not
indicate the specified material removal mechanism is taking place,
one or more laser cutting parameters can be adjusted until
emissions detected from the lased rock indicates the specified
material removal mechanism.
[0004] Certain aspects of the implementations are directed to a
well laser system for use in a subterranean well. The system may
include a laser apparatus configured to produce a laser beam, and
direct the laser beam towards a subterranean rock. An optical
detector may also be included and may be configured to detect light
emitted from the rock. A controller may be communicatively coupled
to the optical detector. The controller configured to receive a
signal from the optical detector, determine from the signal whether
the rock is responding as specified, and adjust a parameter of the
laser if the rock is not responding as specified.
[0005] Certain aspects of the implementations are directed to a
well apparatus for rock removal. The well apparatus may include a
laser tool configured for insertion into the well and to direct
laser energy onto rock. The laser tool may include a laser or may
include optics that direct a laser beam produced by a
terrestrially-located laser. The apparatus may also include a
detector configured for insertion into the well and to detect
emissions emitted from the rock. A controller can be configured to
adjust power of a laser based on emissions detected from the
rock.
[0006] Certain aspects of the implementations may include assessing
an optical profile of the emissions detected from the lased rock
for characteristic properties of the specified material removal
mechanisms. A characteristic property of a material removal
mechanism may include a detection of steady emissions. In other
instances, a characteristic property of a specified material
removal mechanism may include a detection of a rapidly time-varying
emissions.
[0007] In certain aspects of the implementations, the detected
emission intensity indicates that the lased rock is spalling.
[0008] In certain aspects of the implementations, the emission
intensity indicate a specified material removal mechanism if the
emissions fluctuate with a frequency above a specified threshold
value.
[0009] In certain aspects of the implementations, adjusting the one
or more laser parameters may include changing beam irradiance of
the laser in response to the optical response profile of the
emissions detected from the lased rock.
[0010] In certain aspects of the implementations, detecting
emissions from the lased rock may include receiving light from the
lased rock for a period of time.
[0011] In certain aspects of the implementations, detecting
emissions from the lased rock may include detecting a steady
emission intensity, the method further comprising determining that
the lased rock is not spalling based on detecting the steady
emission intensity. Certain implementations also may include
determining that the rock is melting based on detecting the steady
emissions. In some implementations, it may be determined that the
rock is dissociating based on detecting the steady emissions.
[0012] In certain aspects of the implementations, lasing
subterranean rock may include perforating a sidewall of the well
bore.
[0013] In certain aspects of the implementations, lasing
subterranean rock may include drilling the well bore.
[0014] In certain aspects of the implementations, the controller
may include a processor communicatively coupled to the controller
and configured to receive signals from the detector and output
emissions information to the controller.
[0015] In certain aspects of the implementations, the controller is
configured to automatically adjust the irradiance of the laser
energy when the detected emissions from the rock indicate that the
rock is not responding as specified. In certain aspects of the
implementations, the light detected from the rock indicates that
the rock is not spalling when the emissions have a varying
intensity with respect to time below a threshold value.
[0016] In certain aspects of the implementations, the controller is
configured to maintain the power of the laser when the emissions
detected from the rock indicate that the rock is spalling.
[0017] In certain aspects of the implementations, the controller is
configured to determine that the rock is spalling when emission
intensity detected has varying intensities with respect to time,
the variations in intensities occurring with a frequency above a
threshold value.
[0018] Certain implementations may include a reflector configured
to reflect a laser beam towards the rock and to reflect the
emission from the rock to the light detector. Some implementations
may include a dichroic reflector, the dichroic reflector configured
to reflect a laser beam towards the rock and to transmit the light
emitted from the rock to the light detector.
[0019] In certain aspects of the implementations, the optical
detector may include a spot detector.
[0020] In certain aspects of the implementations, the optical
detector may include a line detector.
[0021] In certain aspects of the implementations, the optical
detector may include a two-dimensional detector array.
[0022] The details of one or more embodiments of the present
disclosure are set forth in the accompanying drawings and the
description below. Other features, objects, and advantages of the
disclosure will be apparent from the description and drawings, and
from the claims.
DESCRIPTION OF DRAWINGS
[0023] FIG. 1 is a side cross-sectional view of an example laser
tool in accordance with the present disclosure depicted perforating
a well bore.
[0024] FIG. 2 is a side cross-sectional view of an example laser
tool constructed in accordance with the present disclosure depicted
perforating a well bore.
[0025] FIG. 3 is a schematic block diagram of an example
controller.
[0026] FIG. 4A is side cross-sectional view of an example laser
tool illustrating an adjustable reflector.
[0027] FIG. 4B is a top cross-sectional view of the example laser
tool of FIG. 4a illustrating the adjustable reflector.
[0028] FIG. 4C is a side cross-sectional view of an another example
laser tool showing different trajectories of the laser beam typical
in drilling a vertical well bore
[0029] FIG. 4D is a side cross-sectional view of another example
laser tool showing different trajectories of the laser beam
achieved using a fiber optic array.
[0030] FIG. 5A is a schematic diagram of a laser beam spot and a
projection of an optical spot-detector location relative to the
laser beam spot.
[0031] FIG. 5B is an example representation of the optical response
of rock spallation.
[0032] FIG. 5C is a graphical representation of an example detector
signal indicating spallation.
[0033] FIG. 5D is a graphical representation of an example detector
signal indicating inefficient rock removal.
[0034] FIG. 6A is a schematic diagram of a laser beam spot and a
projection of an optical line-detector location off-set relative to
the laser beam spot.
[0035] FIG. 6B is a schematic diagram of a laser beam spot and a
projection of an optical line-detector location intersecting the
laser beam spot.
[0036] FIG. 6C is a schematic diagram of a laser beam spot and a
projection of a location of a two-dimensional configuration of an
optical detector.
[0037] FIG. 7 is a process flow chart for controlling laser
parameters based on the optical response of a lased subterranean
formation.
[0038] FIG. 8 is a graphical representation of laser power versus
specific energy of a rock delineating the spallation zone and the
melting zone.
[0039] Like reference symbols in the various drawings indicate like
elements.
DETAILED DESCRIPTION
[0040] High power laser technology may be used for drilling and
perforating downhole hydrocarbon formations. The amount of energy
required to remove a given volume or mass of rock is defined as
specific energy. An efficient rock removal process will result in a
low specific energy while an inefficient process will exhibit a
high specific energy. For a particular rock sample, the specific
energy required to cut or drill through it depends on the laser
parameters applied. These parameters include irradiance, laser
power, spot size, laser on time, purge time, delay time between
laser off and purge on.
[0041] Rock is often not a homogenous, isotropic material and its
properties and composition change from well to well and from one
location within a well to another and localized changes in physical
or chemical composition, saturation, cracks or veins of dissimilar
materials may require changing of the laser parameters in order to
initiate a laser drilled hole and additional changes to the laser
parameters may be required "on the fly" as the laser cut hole into
the rock progresses.
[0042] The present disclosure is directed to systems, methods, and
apparatuses for remotely detecting when laser drilling, perforating
or other rock removal processes are efficient or inefficient. If an
existing set of laser parameters results in inefficient removal,
these parameters may be adjusted while the laser is down-hole until
efficient removal is reestablished. In this way, a set of laser
parameters resulting in efficient laser rock removal can be used at
all times. This will result in quicker, more cost effective laser
rock drilling, perforating or removal with improved hole
geometry.
[0043] Several mechanisms exist for laser rock removal, including
spallation, melting, dissociation, and vaporization. FIG. 8 is a
graph 800 of specific energy versus average power (one of the laser
parameters listed above) for a representative sandstone sample. The
vertical line 802 in FIG. 8 marks the transition in laser power
where the rock removal regime changes from spallation to melting.
An abrupt increase in specific energy when the removal method
switches from spallation and melt starts. Efficient (low specific
energy) rock removal can be achieved by adjusting the laser power
to the maximum level where spallation occurs, but below the power
threshold where melting begins. Usually, it is desirable to operate
in the power region where specific energy is minimum as shown in
FIG. 8. Spallation is typically preferred to material removal by
melting because spallation has a higher energy efficiency and
results in an easier handling of debris.
[0044] Similar plots can be made where other laser parameters are
changed and the remainder are held constant, however a general
pattern emerges that specific energy increases when melting starts
occurring.
[0045] Spallation and/or melting may be determined by the optical
signature of the rock during laser illumination. The absorption of
incident laser energy can result in rapid local heating of the rock
surface. A portion of absorbed energy is re-radiated from the
target region in the form of thermal emissions governed by Planck's
law. Temperatures in excess of 800 C. are commonly attained in
tenth-second timeframes when subjected to high power laser energy.
Thermal emission's thus typically peak in the visible or near-IR
region of the electro-magnetic spectrum. Current generation of high
power lasers utilized in industrial applications are capable of
delivering very high quality beams with near Gaussian intensity
profiles, especially if delivered via a long spatially filtering
fiber optic. Thermal emission from a flat, homogeneous rock surface
impacted with such a laser beam resembles a solid circular glowing
spot. Thermal emission and incident beam intensity profiles are
highly correlated, with the region of brightest thermal emission
coinciding with the peak of the incident beam profile. In practice,
rough rock surfaces and non-uniform conductivity result in more
complex thermal emission regions.
[0046] In the absence of material removal mechanisms, a constant
incident laser power would be expected to yield increasingly bright
thermal emission at lower wavelength until the energy input and
output balance and a steady state temperature is attained. In
practice, material removal processes typically initiate prior to
obtaining steady state, provided a sufficiently high power laser is
employed.
[0047] Under the most desirable material removal conditions, the
sudden energy input resulting from laser incidence induces thermal
stresses that shear rock grains and heat pore spaces in the rock
matrix, causing grains and bits of rock matrix to be ejected from
the surface. This condition is known as spallation. The ejected
material often exhibits strong visible thermal emission signatures
resulting in easy visual tracking of these particulates. The visual
effect of observing many ejected grains rapidly moving radially
outward from a central bright region at the site of laser incidence
greatly resembles a type of burning firework known as a "sparkler".
A sparkler is a form of pyrotechnic comprised of a wire coated with
a mixture containing an oxidizer; a fuel (e.g. charcoal and
sulfur); a metal powder (e.g. iron, steel or aluminum); and a
combustible binder (e.g. starch or sugar). When ignited at one end
of the wire, this pyrotechnic burns slowly and releases a shower of
white hot sparks.
[0048] Efficient spallation results in lower than otherwise
observed thermal heating of the rock surface since much of the
heated material is quickly ejected from the rock surface.
Furthermore, this process typically maintains rock temperatures
below the melt transition temperature. As such, key thermal
emission signatures indicating spallation are a dynamic, low
average intensity, longer wavelength signal corresponding to
relatively low rock surface intensity in the presence of visible
tracers of ejecta moving outward from the point of laser-rock
interaction. FIG. 5B, discussed in more detail below, is an example
representation of the optical image of rock spallation. In FIG. 5B,
a laser is directed to the rock formation. The solidly glowing
laser spot 502 is shown at the center of the heated mass. An
optical detector can be positioned in a well tool so that it
detects the optical response of the heated rock in a specified
area. In certain instances, as shown in FIG. 5B, that specified
projection location 554 is off-center from and not encompassing the
laser spot 502. However, in other instances discussed in more
detail below, the specified area can encompass the laser spot 502
and can be of different shapes and sizes. The optical detector
(e.g., discussed herein) can be used to detect spallation by the
white hot ejecta 556 coming off of the rock--looking for an erratic
or irregular, unsteady optical response produced by the ejecta 556
as they pass in and out of the specified area, momentarily lighting
the area or a portion of the area. See FIG. 5C. In certain
instances, the optical detector can be used to look for time
dependent variations in the optical response that occur with a
frequency above a specified threshold value determined to indicate
spalling and/or a certain specified degree of spalling.
[0049] In contrast, when the laser parameters result in rock
melting, the visual effect is different from spallation. When melt
is occurring, no sparks are observed; instead, the optical response
is a steadily glowing center where melted material tends to puddle
in the center. Occasional melted rock may drip out of the hole, but
generally, the light emission observed from melting is much less
dynamic that that observed during spallation. See FIG. 5D. The
visual effect of dissociation is similar to that of melt.
[0050] Dissociation is a process by which rock is heated and then
wet. The rock material can undergo a chemical reaction, turning the
calcium carbonate into CaO.sub.2. The CaO.sub.2 is soluble in
water. In some instances, dissociation may be desirable, and the
optical response that indicates a rock that is undergoing
dissociation can be identified using a detector to detect the
emitted light from the rock.
[0051] The optical response of the lased rock can be detected and
used to adjust laser parameters while the laser is down-hole. For
example, an optical detector can receive the light emitted from the
lased rock, and the variation in light intensity over time for a
given spatial area of reference can be used to determine whether
spallation is occurring (e.g., a high variation in light intensity
over time) or whether melt is occurring (e.g., a low variation in
light intensity over time) or whether neither is occurring.
[0052] Turning to FIGS. 1 and 2, FIG. 1 is a side cross-sectional
view of an example laser tool 20 in accordance with the present
disclosure depicted perforating a well bore. FIG. 2 is a side
cross-sectional view of an example laser tool 20 constructed in
accordance with the present disclosure depicted perforating a well
bore. A cased well bore 10 in a subterranean zone 12 has a casing
14 affixed therein. A layer 16 of cement or similar material fills
an annulus between the casing 14 and the well bore 10. An
illustrative laser tool 20 is depicted in use perforating the well
bore 10. The illustrative laser tool 20 is adapted to be inserted
into the well bore 10 depending from a wireline 18 (FIG. 1) or a
tubing string 19 (FIG. 2), and direct a laser beam 26. Although
depicted as removing material from the subterranean zone 12 to form
a perforation 22, the laser tool 20 can be adapted to also or
alternatively drill a new well bore, extend an existing well bore,
or heat material to emit light for use in laser induced breakdown
spectroscopy (LIBS). As the illustrative laser tool 20 of FIGS. 1
and 2 is depicted perforating a cased well bore 10, it is directing
the laser beam 26 onto the casing 14, the cement 16 and the
subterranean zone 12. The illustrative laser tool 20 and related
concepts described herein are equally applicable to an "open hole"
well bore. An open hole well bore is one in which at least a
portion of the well bore has no casing. Furthermore, the laser tool
20 may be used in perforating or drilling through various equipment
installed in a well bore, and is not limited to perforating through
casing, cement layers, and subterranean zone. When referring to a
wall of a well bore herein, the wall can include any interior
surface in the well bore, such as a sidewall or end/bottom wall
thereof.
[0053] Power and/or signals may be communicated between the surface
and the laser tool 20. Wireline 18 may include one or more
electrical conductors which may convey electrical power and/or
communication signals. Wireline 18 may additionally or
alternatively include one or more optical fibers which may convey
light (e.g. laser) power, optical spectra, and/or optical
communication signals. Neither the communication of power, nor
signals to/from the surface, are necessary for the operation of the
implementations. In lieu of such communication, downhole batteries
and/or downhole generators may be used to supply the laser tool 20
power. A downhole processor may be employed to control the laser
tool 20, with relatively little (as compared to wireline) or no
communication from the surface. For example, instructions for
performing operations may be preprogrammed into the processor (ex.
processor 44 in FIG. 3) before running the laser tool 20 into the
well bore 10 and/or the laser tool 20 may respond to simple
commands conveyed via surface operations such as rotary on/off,
relatively low data rate mud-pulse, electromagnetic telemetry, and
acoustic telemetry communication.
[0054] In implementations incorporating a tubing string 19, the
tubing may be continuous tubing or jointed pipe and may be a
drilling string. The tubing string 19 may incorporate a wireline 18
as described above. Tubing string 19 may be "wired drill pipe,"
i.e. a tubing having communication and power pathways incorporated
therein, such as the wired drill pipe. The tubing string 19 may
contain a smaller tubing string within for conveying fluids such as
those used in the fluid based light path described below or for
conveying chemicals used by the laser.
[0055] As discussed above, the laser tool 20 may be configured for
use in analyzing material using laser-induced breakdown
spectroscopy (LIBS). In LIBS, at least a portion of the material
being sampled is heated, for example to a plasma or an incandescent
state, and the wavelength spectrum and intensity of the light it
emits is measured to determine a chemical characteristic of the
material, for example, the chemical elements of the material. The
light may be in either or both of the visible and invisible
spectrums. The laser tool 20 can also be configured to determine a
physical characteristic of the material, such as its temperature or
thermal properties. The laser tool 20 can operate to heat the rock
of the subterranean zone 12 (or other material being analyzed) in
situ, i.e. without removing the rock of the subterranean zone 12,
using laser beam 26 while the laser tool 20 is operating to remove
material (drilling or perforating) or apart from operation of the
laser tool 20 to remove material.
[0056] If configured to both remove and analyze material, the laser
tool 20 can be configured to remove material and heat the material
being removed or the remaining material to emit light 36 during the
same duty cycle or during separate cycles. For example, the laser
tool 20 can remove material during a first duty cycle and operate
to heat material, at the same location or a different location, in
a second duty cycle.
[0057] The power of the laser beam 26 can be equal from cycle to
cycle, vary from cycle to cycle, or the laser beam can be fired in
non-cyclical pulses of varying power. For example, it may be
desirable to use a multi-pulse technique to heat the subterranean
zone 12 to enable use of a lower powered laser than is necessary to
heat the subterranean zone in a single pulse. In a multi-pulse
technique, a first laser beam pulse is fired toward the material
being analyzed to generate a cavity in the material and/or the
interceding or surrounding materials, such as well fluids and
drilling mud, resulting from rapidly expanding vaporized material.
A second, higher power pulse is then fired into the material being
analyzed to heat the material to a plasma or incandescent state.
The multi-pulse technique may also encompass firing the first laser
beam in a higher power pulse than the second laser beam pulse (e.g.
for blasting away interceding material). Additional laser beam
pulses may be fired, of higher or lower power than the first and
second laser beam pulses, as is desired. For example, a third laser
beam pulse may be fired to perforate the subterranean zone
rock.
[0058] As a heated portion of the subterranean zone may continue to
emit light for a brief period of time after the laser beam has
ceased being directed at the location, the optical detector 48 can
be operated to receive emitted light 36 either (or both) while the
laser beam 26 is being directed at the location and afterwards, for
example during an off cycle of the laser beam 26 or while the laser
beam 26 is being directed to heat or remove material in a different
location. It is also within the scope of the disclosure to re-heat
the subterranean zone at some time after the laser tool 20 has been
operated to remove material at the location, and thereafter use the
optical detector 48 to receive the emitted light 36.
[0059] In FIGS. 1 and 2, the illustrative laser tool 20 includes a
laser beam device 24 that generates or relays a laser beam 26 into
the subterranean zone 12. The laser tool 20 may optionally be
provided with a focusing array 28 through which the laser beam 26
passes. The laser beam device 24 may generate the laser beam 26,
and thus may be an electrical, electro-chemical laser or chemical
laser, such as a diode laser or an excimer or pulsed Na:YAG laser,
dye laser, CO laser, CO2 laser, fiber laser, chemical oxygen iodine
laser (COIL), or electric discharge oxygen iodine laser (DOIL). The
laser beam device 24 may relay the laser beam 26 generated remotely
from the laser tool 20, such as a laser generated by a laser
generator 29 on the surface and input into the laser beam device 24
via a transmission line 27 (FIG. 2), such as an optical fiber or
light path. In some implementations it may be desirable to use a
DOIL to increase service intervals of the laser tool 20, because a
DOIL does not substantially consume the chemicals used in creating
the laser beam and the chemicals need not be replenished for an
extended duration. It is to be understood that the examples of
particular lasers disclosed herein are for illustrative purposes
and not meant to limit the scope of the disclosure.
[0060] The laser beam may be pulsed, cycled, or modulated by
pulsing, cycling, or modulating the control signal, and/or using an
optical chopper, shutter, digital micro-mirror device, Kerr cell,
or other mechanical, electrical, or photonics based light switching
device to shutter, pulse, cycle, or modulate the emitted beam. In
some implementations, the laser pulse duration may be on the order
of 10 nanoseconds. A Kerr cell is one electro-optical device that
may be used to provide shuttering on the order of such speeds.
[0061] As discussed above, the laser beam 26 is generated by a
laser 24. The laser beam 26 impinges a reflector 30 and is
reflected towards the wall of the well bore 10. The laser beam 26
is directed through a window 54 in the casing 14. The laser beam 26
then impinges the layers of the well bore, as discussed above, to
create the perforation 22. Put differently, lasing the subterranean
rock includes perforating a sidewall of the well bore 10.
[0062] When the laser beam 26 impinges on the rock, the rock may
respond optically. Light emitted by the rock can traverse a path
towards the reflector 30 and towards optical detector 48. The
optical response of the lased rock can be detected by the optical
detector 48, which can send signals to a controller 38 (described
in FIG. 3), across a communications path 50. The optical detector
48 can be configured to provide a signal representative of the
detected light to a controller 38, and, in some instances, to a
processor 44. Processor 44 can receive a signal or signals from the
optical detector 48. The processor 44 can receive, analyze,
interpret, or otherwise process the received signal(s) from the
detector, and make a determination as to whether the rock is
spalling or melting (or neither). The processor 44 can also perform
Fourier transforms on optical data and apply filters to the data.
The processor 44 can also automatically adjust the laser parameters
accordingly. The processor 44 can also display a graphical analysis
of the intensity over time to an operator, who can manually adjust
the laser parameters.
[0063] In certain implementations, the optical detector 48 resides
behind the reflector 30. As discussed above, reflector 30 may be
dichroic, allowing the laser beam 26 to reflect towards the window
54, while allowing the emitted light from the rock surface and
ejecta to transmit through the reflector 30. In other
implementations, the detector 48 may reside at other locations in
the well bore (e.g., within or outside of a down-hole tool). For
example, the reflector 30 may reflect light from the ejecta towards
the surface, and the detector can be positioned to detect light
reflected from the reflector 30.
[0064] Turning briefly to FIG. 3, FIG. 3 is a schematic block
diagram of an example controller 38. Controller 38 may reside down
hole with the laser tool 20 or can reside at the surface and be in
communication with the laser tool 20 and other components of the
laser tool 20, such as detector 48. For example, the controller can
receive optical information from the detector 48 by an optical
communications line 50 which conveys optical measurements to the
controller via either electrical and/or optical signals. If the
controller is located downhole, it may contain the optical detector
48 thus obviating the need for line 50. The controller can
communicate with the laser 24 and components on the surface across
a wireline 40. The controller 38 includes a processor 44 and
computer-readable media 46. The processor 44 can perform
spectroscopic analyses based on light received from the detector 48
across optical communications line 50. The analyses can be stored
on computer-readable media 46. The processor 44 and
computer-readable media can communicate with surface equipment
across wireline 40. In addition, the processor 44 can control the
power of laser 24 based on the spectroscopic analysis. In certain
implementations, the controller 38 can communicate with laser
generator 29, shown in FIG. 2, to instruct the laser generator 29
to vary the laser parameters, such as the laser power, based on the
spectroscopic analysis indicating a "melt" or rate of spallation
optical response. The laser adjustments can be continuous and
automatic. That is, the laser power adjustments can be made without
human intervention.
[0065] Returning to FIGS. 1 and 2, focusing array 28 may include
one or more optical elements or lenses configured to focus the
laser beam 26 at a given focal length or adjustably focus the laser
beam 26 to various focal lengths. Some examples of suitable devices
for an adjustable focusing array 28 can include one or more
electro-optic lenses that change focal length as a function of
voltage applied across the lens or one or more fixed lenses and/or
mirrors movable to change the focal length. It is understood that
there are many suitable devices for manipulating an optical beam
which can be actively manipulated, responding to mechanical,
acoustical, thermal, electrical or other forms of input energy and
numerous such devices are within the scope of the disclosure. The
focusing array 28 focuses the laser beam 26 on the material being
removed or heated.
[0066] Use of an adjustable focusing array 28 enables the laser
beam 26 to be more precisely focused on the material being removed
or heated than a fixed focusing array 28, for example, when there
is movement of the laser tool 20 relative to the subterranean zone
12. An adjustable focusing array 28 also enables the laser beam 26
to be focused on the end wall of the material being removed as the
end wall moves deeper into the subterranean zone. In removing
material, the laser beam 26 can be first focused on the closest
surface of the material to be removed then adjusted to maintain
focus as the surface from which material is being removed moves
deeper into the material. In the case of perforating a well bore
10, the laser beam 26 can be first focused on the interior of the
casing 14 and adjusted to maintain focus at an end wall of the
perforation 22 as the perforation deepens through the casing 14,
the cement 16 and into the subterranean zone 12. In heating a
material being analyzed to emit light, the laser beam 26 can be
focused on the material being analyzed. The focal length and/or
properties of the laser beam may be actively manipulated, for
example to compensate for movement of the laser tool 20 relative to
the material being heated or removed.
[0067] A length to the desired location can be determined using a
distance meter, such as an acoustic or optical distance meter,
configured to measure a distance between the laser tool 20 and the
material being removed or analyzed. That length can then be used in
determining a focal length at which to focus the adjustable
focusing array 28. Optical distance meter (or range finding)
technologies are known, for example using a laser beam and a photo
diode to detect the light returned from the subterranean zone whose
range is of interest wherein a modelable relationship exists
between the distance to be measured, the focal point of the laser
beam, and the intensity of the returns. By varying the focal point
of the beam and monitoring the intensity of the returns, the
distance to the subterranean zone may be inferred. Alternatively, a
distance, relative distance, or change in distance may be inferred
with a single focal point by correlating intensity to a model or
experimental data, or monitoring intensity decrease or increase at
different times during a process (e.g. the perforating) expected to
result in a change in such distance. As another alternative,
optical time domain reflectometry may be employed as is known to
measure the time a flight of a pulse of light to and from the
subterranean zone, from which distance may be determined. The laser
beam used by the optical distance meter 66 may be from a laser beam
device 24 used for removing or heating material, or maybe a
separate beam from a separate device.
[0068] When using a fixed focusing array 28, constraining the
relative tool/subterranean zone movement so that the distance from
the well bore 10 wall to the fixed focusing array 28 remains fixed
in relation to the focusing array's focal length ensures that the
laser beam 26 will maintain the desired focus. In an adjustable
focusing array 28, it may be desirable to constrain relative
tool/subterranean zone movement to reduce the magnitude of focal
length adjustments necessary to maintain focus. Relative laser
tool/subterranean zone movement can be reduced by sizing the
exterior of the laser tool 20 close to the diameter of the well
bore 10 or by providing the laser tool 20 with one or more
stabilizer fins that project to a diameter that is close to the
diameter of the well bore 10. Movement of the laser tool 20
relative to the subterranean zone can be further reduced by
providing one or more extendable stabilizers, that can be
selectively expanded to reside close to or in contact with the wall
of the well bore 10.
[0069] Although the laser beam device 24 can be oriented to fire
directly towards the material being removed or heated in one or
more trajectories, the illustrative laser tool 20 is configured
with the laser beam device 24 firing into a reflector 30. The
reflector 30 directs the laser beam 26 toward the subterranean zone
12 and may be operated to assist in focusing the laser beam 26 or
operate alone in (when no focusing array 28 is provided) focusing
the laser beam 26 into the material being removed. In the
illustrative laser tool 20 of FIGS. 1 and 2, the laser beam 26 is
directed substantially longitudinally through the laser tool 20 and
the reflector 30 directs the laser beam 26 substantially laterally
into the well bore 10. The laser tool 20 can be configured to fire
the laser beam 26 in other directions, for example, down.
[0070] The laser beam 26 may be directed to remove material or heat
various points around the well bore 10 and in varying patterns. In
an illustrative laser tool 20 having a reflector 30, the reflector
30 can be movable in one or more directions of movement by a
remotely controlled servo 32 to control the direction, i.e.
trajectory, of the reflected laser beam 26. In a laser tool where
the laser beam device 24 fires directly into the subterranean zone
12 or in a laser tool having a reflector 30, the laser beam device
24 can be movable by control servo to control the trajectory of the
laser. In lieu of or in combination with a reflector 30, the laser
beam can be directed into the subterranean zone 12 using a light
path (see FIGS. 4D, discussed below), such as a fiber optic, that
may optionally be movable by control servo to control the
trajectory of the laser beam. The light path may include multiple
paths, such as a fiber optic array, that each direct the laser beam
in a different trajectory. The multiple paths can be used
selectively, individually or in multiples, to direct the laser beam
in different trajectories.
[0071] In the illustrative example of FIGS. 1 and 2, the laser beam
26 is directed using the reflector 30 and control servo 32, rather
than or in combination with moving the laser tool 20. The control
servo 32 can be configured to move the reflector 30, at least one
of, about a longitudinal axis of the well bore 10 (see FIG. 4A),
about a transverse axis of the well bore 10 (see FIG. 5B), or along
at least one of the longitudinal and transverse axis of the well
bore 10. FIG. 4A depicts the laser tool 20 firing the laser beam 26
through angle .alpha. about the well bore longitudinal axis.
Depending on the application, it may be desirable to configure the
laser tool 20 so that angle .alpha. may be as much as 360.degree..
The reflector 30 can be adjusted by angle A to achieve a laser beam
trajectory .alpha.. FIG. 4B depicts the laser tool 20 firing the
laser beam 26 through angle .beta. about the well bore transverse
axis. Depending on the application, it may be desirable to
configure the laser tool 20 so that angle .beta. may be as much as
360.degree.. The reflector 30 can be adjusted by angle B to achieve
a laser beam trajectory .beta.. The laser tool 20 can be
appropriately configured so as not to fire the laser beam 26 upon
itself. FIG. 4C depicts an illustrative laser tool 20 firing in
multiple trajectories, through angle .phi., typical for drilling a
vertical well bore 10. Depending on the application, angle .phi.
may be as much as 360.degree. and may be oriented through
360.degree. polar about the longitudinal axis of the laser tool 20.
The reflector 30 can be adjusted by angle .PHI. to achieve a laser
beam trajectory .phi..
[0072] FIG. 4D depicts a illustrative laser tool 20 that uses a
light path 104 comprised of multiple optical fibers 106 each
oriented to fire in a different trajectory. The laser beam 26 may
be directed through all of the multiple optical fibers 106
substantially simultaneously, or may be multiplexed through the
multiple optical fibers 106, for example, as a function of duty
cycle as is described below. Likewise, emitted light can be
received through the multiple optical fibers 106 for use in
material analysis as is described herein. Although depicted with a
specified number of optical fibers 106 arranged vertically, the
number and pattern of the optical fibers 106 can vary. For example,
only one optical fiber 106 can be provided. In another example, the
pattern in which the optical fibers 106 are arranged can additional
or alternatively extend circumferentially about the laser tool 20
to reach circumferential positions about the well bore 10. The
arrangement of optical fibers 106 can be configured to produce
specified patterns in the material removed, heated, and/or
analyzed.
[0073] By directing the laser beam 26 relative to the laser tool
20, with reflector 30, light path 104, or otherwise, the laser tool
20 can remain in a single position (without further adjustments or
reorientation) and remove or heat material in multiple locations
around the well bore 10. Accordingly, the number of adjustments
and/or orientations of the laser tool 20 during an entire operation
are reduced. Physically moving the laser tool 20 is time-consuming
relative to adjustment of the laser trajectory using the
configurations described herein (ex. by moving reflector 30).
Therefore, the ability to reach multiple trajectories without
moving the laser tool 20 reduces the amount of time necessary to
perform operations (drilling, perforating, subterranean zone
analysis).
[0074] According to the concepts described herein, the laser beam
26 can be manipulated with multiple degrees of freedom and focal
points to remove material in many different patterns. So for
example, a slice or thin wedge can be removed from the wall of the
well bore 10, orthogonal to and along the length of the well bore
10, and orthogonal to a subterranean zone bedding plane, with a
larger thickness at its distal end from the well bore 10, and
exposing far more subterranean zone surface than traditional
perforating operations. The concepts described herein enable a
perforation hole to be shaped (such as by providing slots, rather
than tubes or pits) to minimize fluid pressure down-draw. Multiple
shapes can be envisioned within the implementations which may
promote hydrocarbon recovery rate, total recovery and
efficiency.
[0075] In the illustrative laser tool 20, the laser beam 26 can be
directed to remove or heat material circumferentially about the
well bore 10 by actuating the control servo 32 to rotate the
reflector 30 about a longitudinal axis of the well bore 10 and/or
actuating the reflector 30 to move along the transverse axis of the
well bore 10. The laser beam 26 can be directed to remove or heat
material along the axis of the well bore 10 by actuating the
control servo 32 to rotate the reflector 30 about a transverse axis
of the well bore 10 or move along the longitudinal axis of the well
bore 10. The laser beam 26 can be directed to remove or heat
material in an area that is larger than could be removed in a
single trajectory, by actuating the reflector 30 to rotate about
and/or translate along at least two axes, for example the
longitudinal and transverse axis. The laser beam 26 would then be
directed in two or more different trajectories to substantially
adjacent locations on the material being heated or removed. For
example, by directing the laser beam 26 to project on the material
being removed or heated at quadrants of a circle, the laser beam 26
can substantially remove or heat the material in a circular shape.
By directing the laser beam 26 in two or more trajectories at the
same location, the laser tool 20 can remove material to form a
conical perforation having a largest diameter at the opening or
having a smallest diameter at the opening. Also, the laser beam 26
may be directed in one or more trajectories to form a perforation
in the earth formation, and concurrently while forming the
perforation or subsequently, be directed in one or more
trajectories to widen the perforation. The laser beam 26 can also
be directed in two or more different trajectories to remove or heat
material of the earth formation in a substantially continuous area
or two or more disparate areas.
[0076] The laser being directable can be also be used to drill more
efficiently and/or with unique hole characteristics, as compared to
both the classic drill-bit drilling and prior non-directable laser
drilling. In drilling with the laser beam 26, the laser beam 26
would be directed axially rather than radially, and the laser beam
tool 20 would be conveyed on the bottom of the bottom hole assembly
in place of the drilling bit (see FIG. 5C). The beam path may also
be selected to achieve directional drilling. A circular path could
be swept by the laser beam 26, cutting (for example by spalling) a
thin annular hole, approximately equal to a desired hole diameter.
The resulting "core" sticking up in the middle would be
periodically broken off and reverse circulated up the well bore 10,
for example up the middle of the drill string 19, to the surface.
The core may be lased for removal, as well. Accordingly, the laser
energy is being used only to cut a small amount of rock (i.e. the
annular hole). The same laser beam 26 directing configurations
discussed above in the context of perforating could be applied to
drilling. Because the material removal is not resulting from a
mechanical bit being rotated, a circular cross-section hole is not
necessary. For example, the laser beam 26 could be directed to
sweep out elliptical, square, or other hole shapes of interest.
[0077] Using the directionality of the material removal allows
formation of a specified hole or perforation section shape designed
and executed for purposes of enhanced production. For example the
hole or perforation can be formed in a rectangular, oval,
elliptical, or other hole section with a longer axis aligned to
expose greater (as compared to a circular cross-section) amount of
the producing subterranean zone, or aligned to provide greater
exposure to an axis of preferred permeability, or preferential
production (or non-production) of oil, water, gas, or sand. Such
specified hole or perforation section shape may be designed and
executed for purposes of well bore or perforation stability, for
example a rectangular, oval, or elliptical shape being employed
with a longer axis aligned with the principal stress field, for
increased stability and reduced tendency of collapse as compared to
a circular cross-section.
[0078] The power of the laser beam 26 can be selected such that the
duty cycle necessary to remove the material in the desired manner
(crack, chip, spall, melt or vaporize) and/or heat the material to
emit light allows enough time during off cycles of a given
trajectory for the laser beam 26 to be directed in one or more
additional trajectories. In other words, if the duty cycle
necessary to remove and/or heat the material in the desired manner
is 10%, the 90% off cycle can be utilized by re-directing the laser
beam 26 to remove and/or heat material from one or more additional
positions in the well bore 10. The duty cycle for the various
positions can be substantially equal or one or more of the
positions can have a different duty cycle. For example, the various
positions may have a different duty cycle if one or more of the
positions are a different material, if it is desired to remove
material at a different rate in different positions, or if it is
desired to remove material in one or more positions and merely heat
material in one or more different positions to emit light. The
laser beam 26 can be cycled or pulsed to achieve the required duty
cycle or the laser beam 26 can be continuous and moved from
position to position to achieve the duty cycle for each respective
position. In either manner, the laser tool 20 operates to multiplex
removal of material in one or more positions, for example to form
one or more perforations 22, substantially concurrently. Likewise
if it is desired to drill or perforate a hole that is larger than
the laser beam 26 can form on a single trajectory or that otherwise
must be formed with two or more trajectories, the same multiplexing
technique can be used to remove material in the two or more
trajectories substantially concurrently. More so, one or more
positions on the earth formation can be heated to emit light
substantially concurrently using this multiplexing technique.
[0079] In a laser tool 20 configured to analyze material, the
optical detector 48 is provided to receive emitted light 36 from
the subterranean zone 12. In an embodiment that communicates with
the surface, the optical detector 48 is coupled to the surface by a
communication link 40. The communication link 40 can be a fiber
optic or light path for communicating data or light to the surface
or can be an electrical or other type of link. The communication
link 40 can be used to transmit wavelength spectra or signals
indicative of wavelength spectra to the surface for analysis (ex.
analysis using a surface based spectrometer and processor for
determining the chemical characteristics of the material being
analyzed). In an embodiment where the optical detector 48
determines the wavelength spectrum of the emitted light 36, the
optical detector 48 can include a pyrometer and/or spectrometer 42
(FIG. 4). In addition to the spectrometer 42, if the optical
detector 48 is configured to determine the chemical characteristics
of the subterranean zone 12 (i.e. perform the LIBS), the optical
detector 48 includes at least one processor 44. The optical
detector 48 may contain memory or other computer readable media
(hereinafter computer readable media 46) for logging the emitted
light 36 wavelength spectrum information, logging the chemical
and/or thermal characteristic information, and/or storing
instructions for the processor 44 to operate at least a portion of
the method of operation described herein.
[0080] In the illustrative embodiment of FIGS. 1-2, the reflector
30 is dichroic and configured to reflect the wavelength spectrum of
laser beam 26 while passing other wavelengths. The laser beam
device 24 is configured to emit a laser beam 26 in a wavelength
spectrum that is different than the expected wavelength spectrum of
the emitted light 36. The optical detector 48 is thus configured to
receive the emitted light 36 that passes through the reflector 30.
A lens assembly 49 can be provided behind the reflector 30 axially
aligned with the incoming emitted light 36 and adapted to focus the
emitted light 36 into a transmission path 50, such as a fiber
optic, to the optical detector 48. The optical detector 48 can
include a lens assembly 49 having one or more lenses, and
optionally a filter, as is desired to condition the emitted light
36 before transmitting to the optical detector 48. Alternatively,
the optical detector 48 can be configured to receive the emitted
light 36 from a position adjacent the laser beam 26. In such an
embodiment, the reflector 30 need not be dichroic, and the lens
assembly 49 has a filter configured to filter out the wavelength
spectrum of the laser beam 26.
[0081] Some or all of the components of the laser tool 20 can be
encased in a housing 52. The housing 52 has one or more windows 54
adapted to allow passage of the laser beam 26 out of the housing 52
and emitted light 36 into the housing 52. The size and shape of the
windows 54 accommodate the aiming capabilities of the laser beam 26
and receipt of emitted light 36. The windows 54 are further adapted
to withstand the elevated pressures and temperatures experienced in
the well bore 10. Some examples of materials for constructing the
windows 54 may be silica, sapphire, or numerous other materials of
appropriate optical and strength properties. The windows 54 may
have anti-reflection coatings applied to one or both surfaces to
maximize the transmission of optical power there-through while
minimizing reflections. The windows 54 may comprise a plurality of
optical fibers positioned to direct the laser beam 26 or collect
emitted light 36 from multiple locations about the well bore 10,
for example the optical fibers may be fanned radially about the
laser tool 20.
[0082] FIG. 5A is a schematic diagram 500 of a laser beam spot 506
on a wall of the well bore and a projection of an optical
spot-detector location 508 relative to the laser beam spot 506. The
projection of the optical spot-detector location 508 shows the
off-set position of the optical spot-detector relative to the laser
beam spot 506. The optical spot-detector itself is located in the
well bore and does not contact the wall. Diagram 500 shows an
outline of a perforation 504 in the well bore surface 502, the
outline of the perforation shown as having a radius 505. The laser
beam spot 506 is shown having a radius 507. The projection of the
spot-detector location 508 is shown to be at a specified distance
509 from the laser spot 506. The diagram 500 is not drawn to scale;
however, the diagram 500 shows the position of the projection of
the spot-detector location 708 as "off-set" from the position of
the laser beam spot 506. The projection of the optical
spot-detector location 508 is off-set from the laser beam spot 506
to avoid saturation by the laser beam spot, which allows for the
detection of variations in the intensity of light. Spallation can
be associated with light that is detected when, during laser
irradiation, the received optical signal from the spot-detector is
erratic or noisy (that is, the intensity of the light detected
varies unpredictably over time; see FIG. 5B and FIG. 5C). Melt is
associated with light that is detected when, during laser
irradiation, the received optical signal from the spot detector is
steady (that is, the detected light does not vary unpredictably
over time or the change in intensity over time is within a
threshold value). An essentially monotonic, smooth increase in
detected signal intensity is expected as target material heats to
the point of melt, where-at material temperature will plateau as
energy is consumed by the state transition. See FIG. 5D. If no
light is detected, the laser power may be too low for either
spallation or melt. The laser power can be adjusted accordingly in
that instance by receiving a "no-spallation" signal and comparing
the actual laser power to the theoretical laser power shown in FIG.
8.
[0083] FIG. 5B is an example representation 550 of the optical
response of rock spallation. The laser beam spot 552 is shown at
the center of the lased rock. The projection 554 of the optical
detector is shown offset from the laser spot 552. Also shown in
FIG. 5B is an example of sparks 556. The sparks are also shown to
intersect the optical detector projection 554. The sparks 556
represent ejecta from the lased rock formation, and would intersect
the optical detector projection 554 randomly and intermittently.
Therefore, the intensity of the light detected by the optical
detector would also be random and intermittent. Detecting such
light would indicate rock spallation.
[0084] FIG. 5C is a graphical representation 560 of an example
detector signal indicating spallation. In graphical representation
560, detector signal (y-axis) is plotted against time (x-axis).
First, the laser is activated 562. When the rock is heated, the
detector signal indicates an increase in light emitted by the rock
by an increase in the amplitude of the signal strength. If
spallation occurs, the optical signal detected can resemble the
erratic signal shown by 566, which indicates that the light emitted
by the rock is erratic and time varying. Spallation stops after the
laser is turned off 568. The optical signal then degrades slowly as
the rock cools, indicating that the light emitted from the rock is
gradually losing intensity 570. The detector level A1 572 indicates
a detector intensity level for melt conditions, which is described
in more detail below in conjunction with FIG. 5D.
[0085] FIG. 5D is a graphical representation 580 of an example
detector signal indicating inefficient rock removal. Inefficient
rock removal may include melt or dissociation. In FIG. 5D, the
laser is activated 582. The light emitted from the rock increases,
which is indicated on the plot by an increase in the amplitude of
the detector signal. In this case, the amplitude of the detector
signal exceeds the reference value A1 572. In general, the peak
amplitude for melt conditions is higher than the amplitude for
spallation. Additionally, during melt and dissociation, the peak
signal does not vary erratically with time, as it would during
spallation. The steady intensity is indicated by a relatively flat
curve 584. Though curve 584 is relatively flat, low amplitude
variations may be detected as the nature of the rock face changes.
The low amplitude variations may be below the detection sensitivity
of the detector, or may be out of scale given the peak amplitude of
the detector signal. When the laser is turned off, the signal drops
off, indicating that the intensity of the emitted light is
decreasing. As the rock cools, the intensity of the light gradually
fades, which is indicated by the gradual decline in signal
strength/amplitude.
[0086] FIG. 6A is a schematic diagram 600 of a laser beam spot 506
and a projection of an optical line-detector location 602 off-set
relative to the laser beam spot 506 by an amount 604 (from the
center of the laser beam spot 506). In some implementations, the
optical detector can be a line-detector, as opposed to a
spot-detector. The off-set allows the optical detector to detect
light from sparks emitted from the subterranean formation during
spalling with a darker baseline than if the projection of the
line-detector location were located closer to the laser spot 506.
The distance should still be within a certain distance to detect a
sufficiently high density of light at a high enough intensity to
determine that spalling is occurring. The line-detector can be
considered as a "line" of optical detectors or a one or
two-dimensional array of photo-detectors. The line-detector can
detect light across a larger area than a spot-detector. FIG. 6B is
a schematic diagram 650 of a laser beam spot 506 and a projection
of an optical line-detector location 652 relative to the laser beam
spot 506. In FIG. 6B, the projection of the line-detector location
652 is across the laser beam spot 506. In this implementation, the
line sensor will need to have a higher dynamic range when measuring
across the center of the perforating tunnel than when measuring off
center, outside of the central laser glow because of the constant
and high intensity light from the laser spot 506 and the resulting
glow emitted from the location of the subterranean formation lased
by the laser.
[0087] FIG. 6C is a schematic diagram 660 of a laser beam spot 506
and a projection of a location 662 of a two-dimensional
configuration of an optical detector relative to the laser beam
spot 506. Two-dimensional detector configurations can track
trajectories of ejecta propagating outward from the laser-rock
interaction region. Additionally, two-dimensional detector
configurations can count zero-crossings across a linear or circular
line sensor. This implementation provides a quantized rate of
spallation that can be further optimized by variation of cutting
parameters. The detector projection 662 is shown to include a
dotted box 664. The dotted box 664 is a projection of the
interaction region, which may be an artificially drawn portion of
the entire detector or detector array. Sparks that cross the
boundary of the interaction region--entering the interaction region
or leaving the interaction region--can be counted. The spark
trajectories can be extrapolated as well.
[0088] FIG. 7 is a process flow chart 700 for controlling laser
parameters based on the optical response of a lased subterranean
zone. At the outset, a counter is set to zero (C=0) (701). A set of
laser parameters is selected and applied to a laser. A laser beam
is directed to impinge on a subterranean rock formation to
perforate the subterranean zone (702). Lasing the subterranean rock
can include perforating the well bore and/or drilling the well
bore. The light emitted from the rock formation is detected for a
period of time (704). A determination can be made based on the
detected light whether the rock removal is considered to be
efficient (706). For example, if the intensity of the light
detected for a specified period of time at a specific location is
erratic, noisy, irregular, or otherwise indicative of spallation,
it can be determined that spallation is occurring, in which case,
the rock removal is occurring efficiently. If the intensity of the
light detected for a specified period of time at a specific
location is steady or does not vary over time (or the variance of
the intensity over time is below a threshold value), then it can be
determined that melt is occurring, dissociation is occurring, or,
more generally, that spallation is not occurring, and therefore,
the rock removal is not occurring efficiently. In some
implementations, spalling can be determined by detecting light that
varies in intensity with respect to time, where the variation of
intensity is detected at a frequency above a threshold value. For
example, light can be detected by the optical detector and signals
representative of the detected light sent to a controller. The data
collected can be transformed from the time domain to the frequency
domain (e.g., using a Fourier transform). A high-pass filter can be
applied to the frequency-domain data, to filter any signals at
frequencies lower than a threshold value defined by the high-pass
filter. The high-pass filter could be designed such that the
resulting data indicates the presence or absence of spalling--high
volume data above the filter cut-off indicates spalling; low volume
data above the filter cut-off indicates lack of spalling.
[0089] If efficient rock removal is detected, then the counter can
be incremented (C=C+1) (712). Laser parameters can be maintained or
further refined to optimize spallation signal strengths or rates,
or overall process efficiency. A determination can then be made
whether C=Cmax (714). Cmax is maximum number of laser shots or
pulses for a particular lasing iteration. Cmax can be used to train
the control system and can be used to ensure that sufficient sample
space of data is collected to determine whether the rock removal is
occurring efficiently. If the counter does not equal a maximum
value, then lasing continues and the perforation depth can be
monitored (716). It can be determined whether a desired depth is
achieved (716). If the depth has not been achieved, the laser
continues to perforate the well bore, and light detection continues
from (704). If the depth is achieved, the laser can stop
perforating (720), and other processes can commence (including
those that use the laser, such as spectroscopic analyses).
[0090] If the counter is equal to its maximum value (and efficient
rock removal is detected), then the counter is reset to zero (C=0)
(708). The laser parameters can be adjusted (710). In this case,
the laser parameters can be adjusted so that a new lasing area is
chosen or new laser parameters are selected to test whether
efficient rock removal can be achieved using different
parameters.
[0091] If rock removal is determined to be inefficient (e.g., no
spalling is detected), then the counter is reset to zero (C=0)
(708). The laser parameters can be adjusted (710). The laser
parameters include, among other things, laser power, irradiance
(power/unit area), purge delay, orifice size on purge lance, purge
velocity, the number of purges, etc. Laser power can be reduced to
a point below that which would cause melt (see FIG. 8) or can be
increased to enter the spallation zone. The active laser power can
be compared to theoretical values, such as those presented in FIG.
8. The laser can then be started again to continue the process
(702).
[0092] Other parameters can also be varied. For example, laser
irradiance can be varied be changing the laser power or by changing
the spot size area using lenses. The laser delay can also be
varied. Delay may allow the melted rock to cool and solidify. The
solidified rock can be removed to expose a "clean" rock surface for
further lasing and removal. Cycle delay can also be varied (that
is, the time between lasing and purging).
[0093] Various configurations of the disclosed systems, devices,
and methods are available and are not meant to be limited only to
the configurations disclosed in this specification. Even though
numerous characteristics and advantages have been set forth in the
foregoing description together with details of illustrative
implementations, the disclosure is illustrative only and changes
may be made within the principle of the disclosure. Accordingly,
other embodiments are within the scope of the following claims.
* * * * *