U.S. patent application number 14/130224 was filed with the patent office on 2014-07-24 for downhole tool for determining laterals.
This patent application is currently assigned to WELLTEC A/S. The applicant listed for this patent is Jorgen Hallundb.ae butted.k, Jimmy Kj.ae butted.rsgaard-Rasmussen. Invention is credited to Jorgen Hallundb.ae butted.k, Jimmy Kj.ae butted.rsgaard-Rasmussen.
Application Number | 20140202242 14/130224 |
Document ID | / |
Family ID | 46466461 |
Filed Date | 2014-07-24 |
United States Patent
Application |
20140202242 |
Kind Code |
A1 |
Hallundb.ae butted.k; Jorgen ;
et al. |
July 24, 2014 |
DOWNHOLE TOOL FOR DETERMINING LATERALS
Abstract
The present invention relates to a downhole tool (1) for
determining laterals in a borehole wall (3) or a borehole casing
(4), comprising a tool housing (5) extending along a longitudinal
axis (6) and having a circumference perpendicular to the
longitudinal axis and adapted to be lowered into a well, and a
plurality of sonic transceivers (7), each sonic transceiver
transmitting sonic signals (8) from the housing and receiving sonic
signals reflected from the borehole wall or borehole casing in a
pre-defined angular segment (9), wherein the plurality of sonic
transceivers are arranged along the circumference of the tool
housing having a mutual distance and are capable of transmitting
sonic signals radially away from the tool housing in an entire
central angle of 360 degrees towards the borehole wall or borehole
casing and wherein, during use, one sonic transceiver, during a
pulse time, transmits a sonic signal in the predefined angular
segment of that sonic transmitter, and wherein one sonic
transceiver, during a subsequent echo time, receives a reflected
sonic signal from the borehole wall or borehole casing, and wherein
an absence of the received reflected sonic signal, during the
subsequent echo time, indicates a lateral. Furthermore, the
invention relates to a downhole system and a method of determining
a position of a lateral.
Inventors: |
Hallundb.ae butted.k; Jorgen;
(Graested, DK) ; Kj.ae butted.rsgaard-Rasmussen;
Jimmy; (Bikerod, DK) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Hallundb.ae butted.k; Jorgen
Kj.ae butted.rsgaard-Rasmussen; Jimmy |
Graested
Bikerod |
|
DK
DK |
|
|
Assignee: |
WELLTEC A/S
Allerod
DK
|
Family ID: |
46466461 |
Appl. No.: |
14/130224 |
Filed: |
June 27, 2012 |
PCT Filed: |
June 27, 2012 |
PCT NO: |
PCT/EP2012/062419 |
371 Date: |
March 24, 2014 |
Current U.S.
Class: |
73/152.58 |
Current CPC
Class: |
E21B 47/09 20130101;
E21B 41/0035 20130101; E21B 47/092 20200501; E21B 47/085 20200501;
E21B 4/18 20130101 |
Class at
Publication: |
73/152.58 |
International
Class: |
E21B 47/09 20060101
E21B047/09 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 30, 2011 |
EP |
11172151.0 |
Claims
1. A downhole tool (1) for determining laterals in a borehole wall
(3) or a borehole casing (4), comprising: a tool housing (5)
extending along a longitudinal axis (6) and having a circumference
perpendicular to the longitudinal axis and adapted to be lowered
into a well, and a plurality of sonic transceivers (7), each sonic
transceiver transmitting sonic signals from the housing and
receiving sonic signals (8) reflected from the borehole wall or
borehole casing in a predefined angular segment (9), wherein the
plurality of sonic transceivers are arranged along the
circumference of the tool housing having a mutual distance and are
capable of transmitting sonic signals radially away from the tool
housing in an entire central angle of 360 degrees towards the
borehole wall or borehole casing and wherein, during use, one sonic
transceiver, during a pulse time, transmits a sonic signal in the
predefined angular segment of that sonic transmitter, and wherein
one sonic transceiver, during a subsequent echo time, receives a
reflected sonic signal from the borehole wall or borehole casing,
and wherein an absence of the received reflected sonic signal,
during the subsequent echo time, indicates a lateral, and wherein
the downhole tool further comprises a magnetic profiler for
measuring a magnetic profile of the borehole casing.
2. A downhole tool according to claim 1, wherein the magnetic
profiler is capable of applying a magnetic field and measuring a
change in the magnetic field.
3. A downhole tool according to claim 2, wherein the change in the
magnetic field is measured as a function of an interaction between
the borehole casing and the magnetic field.
4. A downhole tool according to claim 1, wherein the sonic
transceivers are arranged equidistantly along the circumference of
the tool housing, having a fixed mutual distance.
5. A downhole tool according to claim 1, wherein the sonic
transceivers are arranged along the circumference of the tool
housing in a regular pattern.
6. A downhole tool according to claim 1, wherein more than one
transceiver are receiving during the echo time.
7. A downhole tool according to claim 1, wherein more than one
transceiver are transmitting during the pulse time.
8. A downhole tool according to claim 1 wherein the downhole tool
comprises at least four sonic transceivers, each transceiver being
capable of transmitting sonic signals covering at least one forth
of the entire central angle such as at least eight sonic
transceivers, each transceiver being capable of transmitting sonic
signals covering at least one eighth of the entire central
angle.
9. A downhole tool according to claim 1, wherein the downhole tool
comprises an array of sonic transceivers capable of transmitting
sonic signals covering the entire central angle.
10. A downhole tool according to claim 1, wherein a plurality of
sonic signals can be transmitted during the pulse time in different
predefined angular segments.
11. A downhole tool according to claim 1, wherein the sonic
transceivers are capable of transmitting sonic signals having
different predefined amplitudes and phases.
12. A downhole tool according to claim 1, further comprising a
plurality of second sonic transceivers (10) arranged at a
longitudinal distance away from the plurality of sonic transceivers
and arranged along the circumference of the tool housing having a
mutual distance and being capable of transmitting sonic signals
radially away from the tool housing in an entire central angle of
360 degrees towards the borehole wall or borehole.
13. A downhole system (200) comprising: a wireline (14), a tool
string (100), a driving unit (11), a lateral locator (12), and an
operational tool (13) for operating in a lateral, wherein the
system further comprises a downhole tool for determining laterals
according to claim 1.
14. A downhole system according to claim 13, further comprising a
magnetic profiler.
15. A downhole system according to claim 13 the operational tool is
a logging tool, a key tool, a milling tool or a drilling tool.
16. A downhole system according to claim 13, further comprising a
positioning tool (not shown), such as a casing collar locator.
17. A method of determining a position of a lateral comprising the
steps of: moving the downhole tool according to claim 1 to a first
position in the borehole, conducting a series of pulse/echo
measurements comprising: transmitting a sonic signal by a sonic
transceiver in a first angular segment during a first pulse time,
recording if a reflected sonic signal is received by a sonic
transceiver during a first echo time, transmitting a sonic signal
by a neighbouring sonic transceiver in a second angular segment
during a second pulse time, and recording if a reflected sonic
signal is received by a sonic transceiver during a second echo
time, continuing the series of pulse/echo measurements at the first
position until all angular segments along the entire circumference
of the tool housing has been investigated using the plurality of
sonic transceivers, moving the downhole tool to a second position
in the borehole, conducting a second series of pulse/echo
measurements at the second position in the borehole, determining
the position of the lateral from the absence of received reflected
sonic signals in a subset of the measurements, indicating the
position of the lateral, and recording a magnetic profile for each
recording by the sonic transceiver.
18. A method according to claim 17, further comprising the step of
performing a plurality of measurements using the method according
to claim 17 and subsequently combining several recordings by the
sonic transceiver having matching recorded magnetic profiles.
19. A method according to claims 17, further comprising the step of
inserting an operational tool into the lateral.
20. A method according to claim 17, further comprising a step of
forcing the downhole tool into the lateral with a lateral locator
tool.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a downhole tool for
determining laterals in a borehole wall or a borehole casing,
comprising a tool housing extending along a longitudinal axis and
having a circumference perpendicular to the longitudinal axis and
adapted to be lowered into a well, and a plurality of sonic
transceivers, each sonic transceiver transmitting sonic signals
from the housing and receiving sonic signals reflected from the
borehole wall or borehole casing in a predefined angular segment.
Furthermore, the invention relates to a downhole system and a
method of determining a position of a lateral.
BACKGROUND ART
[0002] Wellbores with multiple forked branches and laterals reduce
overall costs, increase production and improve reservoir drainage.
These types of wells can increase recoverable reserves, make
reservoirs easier to manage, and are growing in popularity.
However, constructing complicated well profiles is challenging and
risky. The latest applications and system developments are
convincing operators that multilateral advantages outweigh the
disadvantages and therefore the need for navigating tools in
multilateral wells are currently increasing. Because of the
capability to more thoroughly drain reservoirs vertically and
horizontally, recoverable reserves per well and per field are
increased considerably while both capital and operating costs per
well and per field are minimised. In fact, the cost of achieving
the same degree of drainage with conventional wells would be
prohibitive in most cases, especially in the case of e.g. deepwater
subsea developments. Multilateral wells allow costs to be amortised
over several reservoir penetrations and have in some cases
eliminated the need for infill drilling. In heterogeneous
reservoirs with layers, compartments or randomly oriented natural
fractures, more pockets of oil and gas can be exploited and an
increased number of fractures can be intersected by drilling
multilateral wells. Visual representations using light, lasers,
infrared light etc. have the disadvantage of being limited during
drilling due to mud flow or during production due to oil-bearing
liquids. The use of sonic measurements for positioning and
measurement of fluid velocities etc. are therefore increasingly
being developed for these purposes. However, determination of
laterals is problematic since sonic measurements typically take
advantage of repeating pattern measurements with knowledge of
Doppler effects, which cannot be applied to the determination of
laterals.
SUMMARY OF THE INVENTION
[0003] It is an object of the present invention to wholly or partly
overcome the above disadvantages and drawbacks of the prior art.
More specifically, it is an object to provide an improved downhole
tool capable of determining downhole laterals in multilateral
wells.
[0004] The above objects, together with numerous other objects,
advantages, and features, which will become evident from the below
description, are accomplished by a solution in accordance with the
present invention by a downhole tool for determining laterals in a
borehole wall or a borehole casing, comprising: [0005] a tool
housing extending along a longitudinal axis and having a
circumference perpendicular to the longitudinal axis and adapted to
be lowered into a well, and [0006] a plurality of sonic
transceivers, each sonic transceiver transmitting sonic signals
from the housing and receiving sonic signals reflected from the
borehole wall or borehole casing in a predefined angular segment,
wherein the plurality of sonic transceivers are arranged along the
circumference of the tool housing having a mutual distance and are
capable of transmitting sonic signals radially away from the tool
housing in an entire central angle of 360 degrees towards the
borehole wall or borehole casing and wherein, during use, one sonic
transceiver, during a pulse time, transmits a sonic signal in the
predefined angular segment of that sonic transmitter, and wherein
one sonic transceiver, during a subsequent echo time, receives a
reflected sonic signal from the borehole wall or borehole casing,
and wherein an absence of the received reflected sonic signal,
during the subsequent echo time, indicates a lateral.
[0007] The downhole tool according to the present invention may
further comprise a magnetic profiler for measuring a magnetic
profile of the borehole casing
[0008] Said magnetic profiler may be capable of applying a magnetic
field and measuring a change in the magnetic field.
[0009] Further, the change in the magnetic field may be measured as
a function of an interaction between the borehole casing and the
magnetic field.
[0010] In one embodiment of the invention, the sonic transceivers
may be arranged equidistantly along the circumference of the tool
housing, having a fixed mutual distance.
[0011] Further, the sonic transceivers may be arranged along the
circumference of the tool housing in a regular pattern.
[0012] Said sonic transceivers may be arranged along the
circumference of the tool housing in a regular pattern, such as a
zigzag pattern.
[0013] In another embodiment, more than one transceiver may be
receiving during the echo time.
[0014] Also, more than one transceiver may be transmitting during
the pulse time.
[0015] Furthermore, the downhole tool may comprise at least four
sonic transceivers, each transceiver being capable of transmitting
sonic signals covering at least one forth of the entire central
angle such as at least eight sonic transceivers, each transceiver
being capable of transmitting sonic signals covering at least one
eighth of the entire central angle.
[0016] Moreover, the downhole tool may comprise an array of sonic
transceivers capable of transmitting sonic signals covering the
entire central angle.
[0017] Additionally, a plurality of sonic signals may be
transmitted during the pulse time in different predefined angular
segments.
[0018] Also, a plurality of sonic signals may be transmitted having
different predefined amplitudes and phases.
[0019] The downhole tool according to the present invention may
further comprise a plurality of second sonic transceivers arranged
at a longitudinal distance away from the plurality of sonic
transceivers and arranged along the circumference of the tool
housing having a mutual distance and being capable of transmitting
sonic signals radially away from the tool housing in an entire
central angle of 360 degrees towards the borehole wall or
borehole.
[0020] The present invention also relates to a downhole system
comprising: [0021] a wireline, [0022] a tool string, [0023] a
driving unit, [0024] a lateral locator, and an operational tool for
operating in a lateral, wherein the system further comprises a
downhole tool for determining laterals as described above.
[0025] The downhole system as described above may further comprise
a magnetic profiler.
[0026] In another embodiment, the operational tool may be a logging
tool, a key tool, a milling tool or a drilling tool.
[0027] Said downhole system may further comprise a positioning
tool, such as a casing collar locator.
[0028] Also, the present invention relates to a method of
determining a position of a lateral, comprising the steps of:
[0029] moving the downhole tool to a first position in the
borehole, [0030] conducting a series of pulse/echo measurements
comprising: [0031] transmitting a sonic signal by a sonic
transceiver in a first angular segment during a first pulse time,
[0032] recording if a reflected sonic signal is received by a sonic
transceiver during a first echo time, [0033] transmitting a sonic
signal by a neighbouring sonic transceiver in a second angular
segment during a second pulse time, and [0034] recording if a
reflected sonic signal is received by a sonic transceiver during a
second echo time, [0035] continuing the series of pulse/echo
measurements at the first position until all angular segments along
the entire circumference of the tool housing has been investigated
using the plurality of sonic transceivers, [0036] moving the
downhole tool to a second position in the borehole, [0037]
conducting a second series of pulse/echo measurements at the second
position in the borehole, and [0038] determining the position of
the lateral from the absence of received reflected sonic signals in
a subset of the measurements, indicating the position of the
lateral.
[0039] Said method may further comprise the step of recording a
magnetic profile for each recording by the sonic transceiver.
[0040] Additionally, the method described above may further
comprise the step of performing a plurality of measurements using
said method and subsequently combining several recordings by the
sonic transceiver having matching recorded magnetic profiles.
[0041] The method described above may further comprise the step of
inserting an operational tool into the lateral.
[0042] Finally, the method as described above may further comprise
a step of forcing the downhole tool into the lateral with a lateral
locator tool.
BRIEF DESCRIPTION OF THE DRAWINGS
[0043] The invention and its many advantages will be described in
more detail below with reference to the accompanying schematic
drawings, which for the purpose of illustration show some
non-limiting embodiments and in which
[0044] FIG. 1 shows a downhole tool string with a tool for
determining laterals,
[0045] FIG. 2 shows a cross-sectional view of a tool for
determining laterals,
[0046] FIG. 3a shows graphical representations of the data coming
from a tool for determining laterals to visualise the position of a
lateral having three different numbers of transceivers with
tool,
[0047] FIG. 3b shows a tool corresponding to the graphical
representations of the data in
[0048] FIG. 3a having a low number of transmitters,
[0049] FIG. 3c shows graphical representations of the data coming
from a tool having more transceivers than the tool of FIG. 3b,
[0050] FIG. 3d shows a tool corresponding to the graphical
representations of the data in FIG. 3c,
[0051] FIG. 3e shows graphical representations of the data coming
from a tool having more transceivers than the tool of FIG. 3d,
[0052] FIG. 3f shows a tool corresponding to the graphical
representations of the data in FIG. 3e,
[0053] FIG. 4 shows a downhole tool string with a tool for
determining laterals comprising a second set of sonic transceivers,
and
[0054] FIG. 5 shows a downhole tool string with a tool for
determining laterals comprising a driving unit and a lateral
locator tool.
[0055] All the figures are highly schematic and not necessarily to
scale, and they show only those parts which are necessary in order
to elucidate the invention, other parts being omitted or merely
suggested.
DETAILED DESCRIPTION OF THE INVENTION
[0056] FIG. 1 shows a downhole tool 1 for determining laterals 2 in
a borehole wall 3 in an openhole well or a borehole casing 4 in a
cased well. The downhole tool comprises a tool housing 5 extending
along a longitudinal axis 6 and a plurality of sonic transceivers
7, each sonic transceiver transmitting sonic signals 8 from the
housing 5 towards the borehole wall 3 or the borehole casing 4 and
receiving sonic signals 8 reflected from the borehole wall 3 or
borehole casing 4. In order to obtain knowledge of the exact
position of the downhole tool in the well, a magnetic profiler 15
may be placed next to the sonic transceivers 7. The magnetic
profiler 15 is a sensor tool designed to help position the downhole
tool. The signal comes from measuring a magnetic field which is
distorted by the steel casing. Characteristic spikes or signatures
show when the tool passes any significant feature. These
characteristics are repeatable and can be used to recognise and
compare features, resulting in the provision of knowledge of the
exact position of the downhole tool in the well. Simultaneously,
the velocity of the tool may be calculated by correlating buffered
signals from neighbouring magnetic sensors. The calculated
velocities may be combined to a single velocity estimate and then
integrated to obtain the position. In some downhole environments,
it may be crucial to gain precise knowledge of the position of the
downhole tool in order to be able to extract useful information
using the sonic transceivers, since several passes of a site of
interest may be required to achieve sufficient information from the
sonic transceivers which again requires precise knowledge of the
position to ensure correlating values of the recordings made by the
sonic transceivers. The correlation between magnetic and sonic
measurements may be referred to as data fusion between the magnetic
and sonic data. Data fusion between signals recorded by the sonic
transceivers 7 and the magnetic profiler 15 may be used for
alignment of consecutive measurements, thereby minimising errors
resulting from differences in a measured depth of the well.
[0057] FIG. 2 shows how the plurality of sonic transceivers 7 are
arranged along the circumference of the tool housing 5 having a
mutual distance and are capable of transmitting sonic signals 8
radially away from the tool housing 5 in a predefined angular
segment 9 in an entire central angle of 360 degrees towards the
borehole wall 3 or borehole casing 4. During use, one transceiver 7
transmits a sonic signal 8 during a pulse time in the predefined
angular segment 9 of that sonic transceiver 7, and during a
subsequent echo time, one sonic transceiver 7 receives a reflected
sonic signal 8 from the borehole wall 3 or borehole casing 4, and
an absence of the received reflected sonic signal 8 during the
subsequent echo time indicates the lateral 2. This is due to the
fact that when there is no casing wall, the signal cannot be
reflected and the signal fades.
[0058] The sonic transceivers 7 may be arranged equidistantly along
the circumference of the tool housing 5 to provide for symmetric
measurements of the surrounding borehole wall 3 or borehole casing
4 and divide the annular space between the tool housing and the
casing into predefined angular segments 9 of equal size.
[0059] To increase the reliability of the measurements, several of
the transceivers may be receiving during the echo time to ensure
that reflected sonic signals will always be received by at least
one of the transceivers. Sonic signals are highly scattered in a
downhole environment due to the symmetry of nearby hard surfaces,
rough surfaces of the casing or borehole wall and other effects
altering the paths of the sonic signals. The reflected signals are
therefore received by transceivers located both near by and far
away from the transmitting transceiver, e.g. on the other side of
the housing. By receiving reflected signals using several
transceivers, the redundancy of the system will be improved.
Receiving with several transceivers may be done by simultaneous
"listening" with several transceivers or by doing a series of
measurements in which one transceiver is used to transmit signals
and several transceivers are used to receive signals one at a
time.
[0060] Furthermore, if the physical conditions in the borehole
allow, several transceivers may be used simultaneously to transmit
signals. Physical conditions which may be appropriate for these
types of measurements may be a situation in which the downhole tool
1 fills up nearly the entire annular space in the borehole. Under
these conditions, it may be possible to do isolated measurements on
several sides of the tool housing without interfering with the
other measurements. In this way, measurement time may be
drastically decreased, e.g. reduced by half by running two
simultaneous series of measurements with transceivers placed
diagonally on two sides of the tool housing 5.
[0061] Since space is very limited when working downhole and
information stream towards the surface is also limited, the
computational power needed to conduct useful measurements downhole
is normally minimised as much as possible. As both spatial
requirements for computational power decreases and the ability to
send information to the surface increases, the problem of
computational power downhole is becoming less and less problematic.
However, the electronic circuits of the transducers in itself
represent a problem due to their volume and the related spatial
requirements. The use of four to ten transceivers placed
equidistantly along the circumference has therefore proven, by
experiments, to be sufficient for the determination of laterals
without generating too much data, which has to be processed
downhole or sent to the surface and furthermore without taking up
too much space downhole for electronic circuits of the transducers.
Sonic transceivers covering smaller angular segments are placed
appropriately to cover the full centre angle of 360 degrees of the
borehole.
[0062] Due to increasing performances and minimisation of computers
as well as due to computational power in general today, arrays of
transceivers covering the entire central angle and even being
resolved along the longitudinal axis of the tool may prospectively
be advantageous if the electronic circuits of the transducers or
transducer arrays may be reduced in size and thereby also provide
increased resolution in the determination of laterals.
[0063] FIG. 3a shows a graphical cylindrical representation of the
data coming from a corresponding downhole tool 1 shown in FIG. 3b
for determining laterals 2 to visualise the position of the lateral
2 for the user. Each square on the cylindrical representation
corresponds to one measurement at one given depth in the well. The
downhole tool 1 is moved down through the well along the
longitudinal axis 6. At a given depth in the well, a series of
measurements using the plurality of sonic transceivers 7 are made
to investigate the surroundings of the downhole tool at that
specific depth. Each of the series of measurements corresponds to
one ring of squares on the cylindrical representation.
[0064] FIG. 3c shows a graphical cylindrical representation of the
data coming from a corresponding downhole tool 1 shown in FIG. 3d,
which is capable of determining laterals 2 with an increased
resolution. The increased resolution may be obtained by placing a
greater number of sonic transceivers along the circumference of the
housing, which increases the resolution by decreasing the
resolvable angular segments. Furthermore the resolution may be
increased along the longitudinal axis by moving the downhole tool
in smaller steps along the longitudinal axis.
[0065] FIG. 3e shows a graphical cylindrical representation of the
data coming from a corresponding downhole tool 1 shown in FIG. 3f,
which is capable of determining laterals 2 with an even higher
resolution. The increased resolution may be obtained by arranging
arrays of transceivers such as an ultrasonic transducer array along
the circumference of the tool housing. The resolution along the
longitudinal axis may be equally increased by such arrays if
two-dimensional arrays are arranged along the circumference of the
tool housing providing resolution in both an angular direction and
the longitudinal direction.
[0066] FIG. 4 shows a downhole tool 1 in a borehole wall 3
comprising a lateral 2 without a borehole casing 4. The downhole
tool further comprises a second plurality of sonic transceivers 10.
Arranging a second plurality of sonic receivers 10 has the
advantage of helping the redundancy of the system, such that the
tool may still function in case of breakdown of the plurality of
sonic transceivers. Furthermore, the second plurality of
transceivers may provide a downhole tool 1 capable of determining a
lateral faster, since two series of measurements may be made per
movement of the downhole tool. Adding even more pluralities of
transceivers may further increase the redundancy of the tool, the
resolution of a determination of a lateral and/or further increase
a speed of the determination of a lateral.
[0067] FIG. 5 shows a downhole system 200 for determining laterals
comprising a driving unit 11 for conveying the downhole tool 1
along the longitudinal axis deeper into the borehole or retracting
the downhole tool 1 by a wireline 14. Furthermore, the downhole
tool comprises a lateral locator tool 12 for engaging the lateral 2
after the position of the lateral 2 has been determined by the
plurality of sonic transceivers 7. By engaging the lateral 2 with a
lateral locator tool 12, the downhole tool 1 may be forced to enter
the lateral, thereby allowing any operational tools 13 comprised in
the tool to enter the lateral 2 and perform the function of the
operational tool 13 in the lateral 2. The operational tool 13 may
be a logging tool, a key tool, a milling tool or a drilling tool.
The system may further comprise a positioning tool using magnets
and magnetometers, such as a casing collar locator.
[0068] In a method of determining a position of a lateral according
to the invention, the downhole tool 1 is moved to a first position
in the borehole for initialising the measurement for determining
the position of a lateral 2. At this first position of the downhole
tool 1, a series of pulse/echo measurements is conducted, i.e.
transmitting a sonic signal referred to as a pulse and receiving
the reflected sonic signal referred to as an echo. The pulse signal
is initially transmitted by a sonic transceiver in a first angular
segment 9 radially away from the downhole tool towards the borehole
or borehole casing during a first pulse time. The sonic signal is
reflected by the borehole or borehole casing back towards the
downhole tool and recorded if the reflected sonic signal is
received by a sonic transceiver during a first echo time. If no
sonic signal is received during the first echo time, it may
indicate that the transceiver is facing the lateral in the
borehole, since the transmitted sonic signal will be propagating
into the lateral instead of being reflected by the borehole or
borehole casing. The lack of reflected signal may also be due to
the transceiver not being able to receive the reflected signal, but
if this is the case, it is confirmed in the subsequent measurement.
If there continues to be no reflected signal in a certain angular
segment 9, it indicates that a lateral is present as illustrated in
FIG. 3c. When the tool is positioned next to a lateral, the
transducers facing the lateral have a lower probability of
measuring a reflected signal. The probabilities in each direction
are different and a probability profile may therefore be used for
determining the existence and direction of a lateral. In
statistical methods used to derive visible objects, i.e. where the
object is directly visible to the observer, the transition
probabilities of the object are the only parameters. Statistical
methods for uncovering objects not directly visible to the
observer, such as a hidden Markov model (HMM), may advantageously
be used to derive the existence of a lateral from reflected sonic
signals. Since the object is not directly visible when trying to
determine laterals, but the output, dependent on the lateral, is
visible, i.e. the output near the object, here being a lateral,
having a probability distribution over the possible output,
statistical models such as the HMM is suitable.
[0069] The series of measurements continues in the first position
by transmitting a new sonic signal by a neighbouring sonic
transceiver in a second angular segment during a subsequent second
pulse time, and in the same way the reflected sonic signal is
recorded if a sonic transceiver receives the reflected signal
during a second echo time. This type of pulse/echo measurements are
continued at the first position until all angular segments along
the entire circumference of the tool housing have been
investigated. Different schemes may be set up for the sequence of
pulsing transceivers, transmitting transceivers or receiving
transceivers, duration of pulse time, duration of echo time,
frequencies, amplitudes etc. To improve redundancy of the method,
each of the transceivers receiving the reflected signal may be used
to record the reflected signal from one transmitting transceiver by
either simultaneously "listening" with all transceivers or
"listening" with only one transceiver during the echo time and
transmitting a new pulse signal before listening with the next
receiver etc. When all angular segments have been investigated, the
downhole tool is moved to a second position in the borehole and a
second series of pulse/echo measurements is conducted at the second
position in the borehole. From the conducted series of
measurements, the position of the lateral may be determined from
the absence of received reflected sonic signals in a subset of the
measurements, since the absence of a reflected signal indicates
that the measurement was conducted at a position opposite the
position of the lateral.
[0070] By placing the downhole tool for determining laterals in a
tool string 100 along with other operational tools 13 as mentioned,
the tool string 100 may effectively perform tasks in both the
borehole wall 3 or main casing and a lateral 2 of the borehole wall
3 or the casing. Furthermore, by using a casing collar locator
(CCL) or magnetic profiler, the position of the lateral may be
stored in a memory available for the user for future use, which
will allow the user to revert to the same lateral faster on
subsequent operations.
[0071] By the plurality of sonic transceivers being arranged along
the circumference of the tool housing, having a mutual distance,
and being capable of transmitting sonic signals radially away from
the tool housing in an entire central angle of 360 degrees towards
the borehole wall or borehole casing is meant that sonic
transceivers covering smaller angular segments are placed
appropriately along the circumference of the tool. The sonic
transceivers thus cover the full centre angle of 360 degrees of the
borehole when transmitting and receiving signals.
[0072] Although the invention has been described in the above in
connection with preferred embodiments of the invention, it will be
evident for a person skilled in the art that several modifications
are conceivable without departing from the invention as defined by
the following claims.
* * * * *