Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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APPARATUS AND METHODS FOR WELL-BORE PROXIMITY MEASUREMENT
WHILE DRILLING
BACKGROUND
1. Field of the Disclosure
[0001] This disclosure relates generally to apparatus and methods for
determining a
distance to a pre-existing wellbore and controlling drilling operations based
on the
determination.
2. Background of the Art
[0002] In the process of drilling wells (also referred to as wellbores or
boreholes)
for hydrocarbon production, it is common to drill a second well in a
predetermined
relationship to an existing well. An example of this may be when a blowout
occurred in the
existing well. Two approaches may be taken to control the blowout. One method
is to use
explosives at the surface and snuff out the fire in the burning well. This
procedure is fraught
with danger and requires prompt control of hydrocarbons flow in the well. The
second
method is to drill a second borehole to intersect the blowout well and pump
drilling mud
into the blowout well. This is not a trivial matter. An error of half a degree
can result in a
deviation of close to 90 feet at a depth of 10,000 feet. A typical borehole is
about 12 inches
in diameter, a miniscule target compared to the potential error zone.
[0003] Another situation in which accurate placement of a secondary well
relative
to a preexisting well is desired for secondary recovery operations. For
various reasons, such
as low formation pressure or high viscosity of hydrocarbons in the reservoir,
production
under natural conditions of hydrocarbons may be at uneconomically low rates.
In such
cases, a second borehole is drilled to be substantially parallel to the pre-
existing borehole.
Fluid such as water, CO2 is then injected into the formation from the second
borehole and
the injected fluid drives the hydrocarbons in the formation towards the
producing borehole
where it may be recovered.
[0004] In the second category are passive ranging techniques that do not
require
access to the pre-existing well while drilling the second well. The techniques
normally
utilize a relatively strong magnetism induced in the casing of the pre-
existing well by the
Earth's magnetic field. The signal due directly to the earth's magnetic field
is a problem,
limiting the accuracy of this measurement. Residual magnetism of the casing
introduces
additional uncertainties.
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[0005] The present disclosure discloses apparatus and methods for determining
distance from a pre-existing wellbore without accessing the pre-existing well.
SUMMARY
[0006] One embodiment of the disclosure is an apparatus for determining a
distance between a first borehole and a second borehole during drilling of the
second
borehole is disclosed that in one embodiment includes a magnet on a drilling
tool that
rotates in the second borehole to induce a primary magnetic field in a
magnetic object in the
first borehole, a substantially stationary sensor on the drilling tool that
detects a secondary
magnetic field from the magnetic object responsive to the induced primary
magnetic field,
and a controller that determines the distance between the first borehole and
the second
borehole from the detected magnetic field.
[0007] Another embodiment of the disclosure is a method of determining a
distance between a first borehole and a second borehole, the in one embodiment
includes
inducing a primary magnetic field in a magnetic object in the first borehole
using a rotating
magnet in the second borehole, detecting a secondary magnetic field from the
magnetic
object responsive to the induced primary magnetic field using a substantially
stationary
sensor in the second borehole, and determining the distance between the first
borehole and
the second borehole from the detected magnetic field.
[0007a] Another embodiment of the disclosure is a method of determining a
distance between a first borehole and a second borehole, the method
comprising: rotating a
magnet of a tool disposed in the second borehole to generate a primary
magnetic field at a
position of a magnetic object in the first borehole, wherein the magnet is
disposed on a
sleeve that rotates independently of the tool; detecting a secondary magnetic
field generated
by the magnetic object responsive to a current induced by the primary magnetic
field in the
magnetic object using a sensor in the second borehole that is stationary with
respect to the
tool; and deteunining the distance between the first borehole and the second
borehole from
the detected secondary magnetic field.
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[0007b] Another embodiment of the disclosure is an apparatus for determining a
distance between a first borehole and a second borehole during drilling of the
second
borehole, comprising: a magnet on a drilling tool in the second borehole,
wherein the
magnet rotates independently of the drilling tool to induce a primary magnetic
field in a
magnetic object in the first borehole; a sensor on the drilling tool that
detects a secondary
magnetic field from the magnetic object responsive to the induced primary
magnetic field,
wherein the sensor is stationary with respect to the drilling tool; and a
controller that
determines the distance between the first borehole and the second borehole
from the
detected secondary magnetic field.
[0007c] Another embodiment of the disclosure is a method of determining a
distance between a first borehole and a second borehole, the method
comprising: rotating a
magnet of a tool in the second borehole at a first rotational speed to
generate a primary
magnetic field in a magnetic object at the first borehole, wherein the magnet
rotates
independently of the tool; detecting a secondary magnetic field from a current
induced in the
magnetic object by the primary magnetic field using a magnetometer in the
second borehole
stationary with respect to the tool; and determining the distance between the
first borehole
and the second borehole from the detected secondary magnetic field.
[0007d] Another embodiment of the disclosure is an apparatus for determining a
distance between a first borehole and a second borehole, the apparatus
comprising: a drilling
tool in the second borehole having a magnet configured to rotate with respect
to the drilling
tool to generate a primary magnetic field from the second borehole into a
magnetic object in
the first borehole to induce a current in the magnetic object; a sensor on the
drilling tool
stationary with respect to the drilling tool for detecting a secondary
magnetic field from the
magnetic object responsive to the induced current; and a controller that
determines the
distance between the first borehole and the second borehole from the detected
secondary
magnetic field.
[0008] Examples of certain features of the apparatus and method disclosed
herein
are summarized rather broadly in order that the detailed description thereof
that follows may
be better understood. There are, of course, additional features of the
apparatus and method
disclosed hereinafter that will form the subject of the claims.
BRIEF DESCRIPTION OF THE FIGURES
[0009] For detailed understanding of the present disclosure, references should
be
made to the following detailed descriptions of the disclosure, taken in
conjunction with the
accompanying drawings, in which like elements have generally been given like
numerals
and wherein:
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FIG. 1 is a schematic illustration of an exemplary drilling system that
includes an
apparatus for determining distance between boreholes, according to one
embodiment of the
disclosure;
FIG. 2 shows a simplified layout of the magnetometer and the coordinate system
used
for the calculations;
FIG. 3 illustrates azimuthal dependence of the signal in the sensor coil;
FIG. 4 is a schematic illustration of implementation of the rotational
magnetometer;
FIG. 5 shows an embodiment that utilizes a pair of additional differentially
connected
coils synchronously rotating with the magnetic coil;
FIG. 6 shows an embodiment that utilizes switchable magnetic field source;
FIG. 7 shows time diagrams of the switchable magnetic field and the transient
responses (corresponds to the embodiment of FIG. 6);
FIG. 8 shows drilling a second borehole in close proximity to a cased
production
borehole;
FIG. 9 shows an embodiment for a radial magnet-coil arrangement that includes
two
identical or substantially identical receiver coils symmetrically installed on
the surface of the
magnet for estimating distance between adjacent wellbore ore boreholes;
FIG. 10 shows yet another embodiment of a radial magnet-coil arrangement that
includes two cols, each further containing a pair of radial oils, offset
equally at two sides of
the rotating magnet;
FIG. 11 depicts receiver coil magnetic flux as a function of receiver coil's
axial offset
as a function of distance between the coil and the casing having a five (5)
meter offset;
FIG. 12 shows yet another embodiment of a magnet and coils arrangement wherein
that includes three receiver coils placed with equal spacing;
FIG. 13 shows another embodiment of a hybrid configuration of coils that
includes six
identical or substantially identical receiver coils forming two pairs of axial
r or three pairs of
radial measurements;
FIG. 14 shows an embodiment for a radial magnet-senor arrangement that
includes a
rotating magnet and a stationary or substantially stationary senor for
detecting secondary
magnetic waves for determining distance and inclination of wellbore relative
to another
borehole; and
FIG. 15 shows yet another embodiment of a radial magnet-senor arrangement that
includes a magnet and a sensor that rotate at different rotational speeds for
determining
distance and inclination of a one borehole relative to another borehole.
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DETAILED DESCRIPTION OF THE DISCLOSURE
[0010] FIG. 1 shows a schematic diagram of a drilling system 100 with a
drillstring
120 carrying a drilling assembly 190 (also referred to as the bottomhole
assembly, or ABHA)
conveyed in a "wellbore" or "borehole" 126 for drilling the wellbore. The
drilling system
100 includes a conventional derrick 111 erected on a floor 112 which supports
a rotary table
114 that is rotated by a prime mover such as an electric motor (not shown) at
a desired
rotational speed. The drillstring 120 includes a tubing such as a drill pipe
122 or a coiled-
tubing extending downward from the surface into the borehole 126. The
drillstring 20 is
pushed into the wellbore 126 when a drill pipe 122 is used as the tubing. For
coiled-tubing
applications, a tubing injector, such as an injector (not shown), however, is
used to move the
tubing from a source thereof, such as a reel (not shown), to the wellbore 126.
The drill bit
150 attached to the end of the drillstring breaks up the geological formations
when it is
rotated to drill the borehole 126. If a drill pipe122 is used, the drillstring
120 is coupled to a
drawworks 30 via a Kelly joint 121, swivel, 138 and line 129 through a pulley
123. During
drilling operations, the drawworks 130 is operated to control the weight on
bit, which is an
important parameter that affects the rate of penetration. The operation of the
drawworks is
well known in the art and is thus not described in detail herein.
[0011] During drilling operations, a suitable drilling fluid 131 from a mud
pit (source)
132 is circulated under pressure through a channel in the drillstring 120 by a
mud pump 134.
The drilling fluid passes from the mud pump 134 into the drillstring 120 via a
desurger 136,
fluid line 128 and Kelly joint 121. The drilling fluid 131 is discharged at
the borehole bottom
151 through an opening in the drill bit 150. The drilling fluid 131 circulates
uphole through
the annular space 127 between the drillstring 120 and the borehole 126 and
returns to the mud
pit 132 via a return line 135. The drilling fluid acts to lubricate the drill
bit 150 and to carry
borehole cutting or chips away from the drill bit 150. A sensor Si preferably
placed in the
line 138 provides information about the fluid flow rate. A surface torque
sensor S2 and a
sensor S3 associated with the drillstring 120 respectively provide information
about the torque
and rotational speed of the drillstring. Additionally, a sensor (not shown)
associated with line
129 is used to provide the hook load of the drillstring 120.
[0012] In one embodiment of the disclosure, the drill bit 150 is rotated by
only
rotating the drill pipe 122. In another embodiment of the disclosure, a
downhole motor 155
(mud motor) is disposed in the drilling assembly 190 to rotate the drill bit
150 and the drill
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pipe 122 is rotated usually to supplement the rotational power, if required,
and to effect
changes in the drilling direction.
[0013] In the embodiment of FIG. 1, the mud motor 155 is coupled to the drill
bit 150
via a drive shaft (not shown) disposed in a bearing assembly 157. The mud
motor rotates the
drill bit 150 when the drilling fluid 131 passes through the mud motor 155
under pressure.
The bearing assembly 157 supports the radial and axial forces of the drill
bit. A stabilizer
158 coupled to the bearing assembly 157 acts as a centralizer for the
lowermost portion of the
mud motor assembly.
[0014] In one embodiment of the disclosure, a drilling sensor module 159 is
placed
near the drill bit 150. The drilling sensor module contains sensors, circuitry
and processing
software and algorithms relating to the dynamic drilling parameters. Such
parameters include
bit bounce, stick-slip of the drilling assembly, backward rotation, torque,
shocks, borehole
and annulus pressure, acceleration measurements and other measurements of the
drill bit
condition. A suitable telemetry or communication sub 172 using, for example,
two-way
telemetry, is also provided as illustrated in the drilling assembly 100. The
drilling sensor
module processes the sensor information and transmits it to the surface
control unit 140 via
the telemetry system 172.
[0015] The communication sub 172, a power unit 178 and an MWD tool 179 are all
connected in tandem with the drillstring 120. Flex subs, for example, are used
in connecting
the MWD tool 179 in the drilling assembly 190. Such subs and tools form the
bottom hole
drilling assembly 90 between the drillstring 120 and the drill bit 150. The
drilling assembly
190 makes various measurements including the pulsed nuclear magnetic resonance
measurements while the borehole 126 is being drilled. The communication sub
172 obtains
the signals and measurements and transfers the signals, using two-way
telemetry, for
example, to be processed on the surface. Alternatively, the signals can be
processed using a
downhole processor in the drilling assembly 190.
[0016] The surface control unit or processor 140 also receives signals from
other
downhole sensors and devices and signals from sensors Si-S3 and other sensors
used in the
system 100 and processes such signals according to programmed instructions
provided to the
surface control unit 140. The surface control unit 140 displays desired
drilling parameters
and other information on a display/monitor 142 utilized by an operator to
control the drilling
operations. The surface control unit 140 preferably includes a computer or a
microprocessor-
based processing system, memory for storing programs or models and data, a
recorder for
recording data, and other peripherals. The control unit 140 is preferably
adapted to activate
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alarms 144 when certain unsafe or undesirable operating conditions occur. The
system also
includes a downhole processor, sensor assembly for making formation evaluation
and an
orientation sensor. These may be located at any suitable position on the
bottomhole
assembly (BHA). The system further includes a device 180 for determining
distance between
the wellbore 126 and a pre-existing wellbore, as described in more detail in
reference to
FIGS. 2-15.
[0017] Turning now to FIG. 2, a permanent magnet 203 is shown on a drill
collar
section 201 of the secondary well. The magnet is transversely magnetized with
the flux
direction indicated by 221. The pre-existing well casing is denoted by 205.
The coordinate
axes x, y, and z are as indicated in FIG. 2. The collar section is provided
with a coil 213. The
coil rotates synchronously with the magnet, but the magnet-coil combination
need not be
synchronous with the rotation of the drill collar: this may be done by having
the magnet-coil
combination on a sleeve. The rotating magnet generates a variable magnetic
field at a
magnetic object such as the casing 205 of the pre-existing well. This variable
magnetic field
induces magnetization in the casing that, in turn, generate a variable
magnetic flux picked up
by the rotating coil 213.
[0018] The magnetic field generated by the magnet at the target well position
can be
approximated by the point dipole formula:
1 3(fin,CF) fin,
MAGNET (1
),
4R- r r3
Where fin, is the dipole moment of the magnet, and F is the distance from the
magnet center
to a point on the casing 205. When the magnet 203 rotates in the XY plane with
angular
velocity w, then
/9. = [cos(o)t) sin (o)t)ey (2),
where J., and e; are unit vectors in the x- and y- directions respectively.
The rotating coil
sensitivity function (magnetic field produced by the coil driven with a unit
current) can be
written as:
A COIL fi
COIL MAGNET (3).
P.
Here A7S; coa is the sensitivity function of the coil and Acoll is the
effective area of the coil.
The rotating magnet generates variable magnetization in the casing. The
magnetization
induces a variable magnetic flux in the coil. Based on the principle of
reciprocity, the
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corresponding voltage can be expressed as:
fõ,
V COIL ¨ 110dt CASINGV 91 ) = LI COIL V 9 t)dv (4),
CASING VOLUME
where /1-/i CASING is the magnetization of the casing, and A.'coõ is the coil
sensitivity function.
[0019] In eqn. (4) the sensitivity A.'coõ can be considered as a slowly
varying function
over the cross-sectional area of the casing. Therefore, we can introduce a
magnetization
average over the cross-sectional area of the casing as:
1
(M CASING) f MCASING (i; t)ds xeff = HMAGNET
t) + Yeff z HMAGNET Z r Cat )
xy XY a 5 =
A CASING CROSS SECTION
(5)5
Where zeff xi, and zeff z are the effective magnetic susceptibilities in the
direction
perpendicular and parallel to the casing axis respectively, ACASING is the
effective area of the
casing, and i represents points along the axis of the casing. Due to the shape
of the casing we
can use the following simlification:xy << X eff z This then gives, for the
coil voltage, the
equation:
d
V COIL PO = X eff _z = A CASING =A COIL= f1HMAGNET Z
(17;a ,t)21 dra (6).
Pm dt CASING LENGTH
This then provides the approximate result
3/-10 = X cif z = ACASING = A COIL = Pm = C6
VCOIL = COS(20) = t) (7).
6471- 2 = r:
Here ACASING is the cross-sectional area of the casing.
[0020] For practical valuesxeff z=1005 ACASING = 27E40-3 m2, = 2n 5 s-1 5
ACOIL = 0.2
=200m2, pm = 1000A = m2, and separation between wells ro =10m, the estimated
voltage
amplitude Vm = 48 nV. In case the thermal noise in the coil and the
preamplifier noise are the
only sources of noise the signal-to-noise ratio per 1 second measurement time
can be
expected to be around 20. If ro =5m, then Vm =0.75 V.
[0021] It is important to note from eqn. (7) that the voltage induced in the
rotating
coil by the rotating magnetization of the casing has a frequency which is
twice the rotation
frequency of the magnet/coil assembly. This means that the measured proximity
signal is
relatively easy to separate from a parasitic signal induced in the rotating
coil due to the
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earth's magnetic field. The parasitic signal has a frequency equal to the
magnet/coil rotation
frequency.
[0022] The main sources of error in the measurement technique is due to the
presence
of some second harmonic in the magnet/coil assembly rotation. In this case the
earth's
magnetic field related signal would appear at the frequency 2o) thus giving a
spurious signal
at the same frequency as the expected proximity signal. Fortunately, the
presence of 2o) -
component in the rotation speed can be assessed with an accelerometer and then
the data can
be used for eliminating the spurious signal from the measurement results. The
second
harmonic signal is easy to calculate from the accelerometer output, known
value and
direction of the earth's magnetic field, and measurements of borehole
inclination and
azimuth. A gyro survey may be needed to get the borehole inclination and
azimuth.
[0023] FIG. 3 illustrates azimuthal dependence of the voltage on the rotating
coil 213.
Using reference voltage
VF oc cos(2w = t), (8)
synchronized with the magnet/coil rotation, the following expression for the
voltage on the
coil 213 can be written
VF = V. = COS[2(CO = t c00)]. (9)
Here coo is the azimuth of the casing with respect to the secondary well.
Thus the phase of the signal on the coil 213 is sensitive to the azimuthal
position of the casing
205 with respect to the secondary well 201.
[0024] FIG. 4 is a block diagram illustrating an implementation of the
rotational
magnetometer. The magnetometer comprises a motor 401 rotating the magnet 203
and the
coil 213. The signal from the coil 213 transferred to the low noise
preamplifier 409 via an
adapter (e. g. sliding rings) 407. Provision is made to eliminate parasitic
signal 2o) generated
by the Earth's magnetic field in presence of rotational disturbances: the
signals from
rotational accelerometer 411 and the motor driver 403 are used to eliminate
parasitic signals
from the measurement data. Serving this purpose are also a controller 405,
analog-to-digital
converters 413, 417, 419, digital signal processor 415 and a variable gain
amplifier 419.
[0025] Those versed in the art and having benefit of the present disclosure
would
recognize that it is sufficient for the coil 213 to be able to responsive to a
component of the
magnetic flux due to the induced magnetization that is transverse to the z-
axis. The
configuration of the coil 213 shown in FIG. 2 is not the only arrangement that
would provide
a suitable signal, but it is one of the better designs. In principle, an
inclined planar coil on the
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BHA with the coil axis inclined to the z- axis would work. For a coil placed
on the magnet
201 the signal would be greatest when the coil axis is transverse to the z-
axis. Similarly, the
magnet does not have to be a transversally magnetized cylindrical magnet as
indicated by
201. The method would also work, albeit less efficiently, using a bar magnet
with its
magnetization direction having a component parallel to the z- axis. Those
versed in the art
and having benefit of the present disclosure would recognize also that a
longitudinal coil
spaced axially apart from the magnet 201 can be used to receive proximity
signal originating
from variable Z-component of the magnetization of the casing.
[0026] FIG. 5 illustrates an example of embodiment of the technique that
utilizes a
pair of additional differentially connected coils 217 synchronously rotating
with the
magnet/coil assembly. The additional coil assembly is sensitive to non-
parallel orientation of
the wells, i.e., the output will be zero if the two boreholes are parallel.
Any differential pair
of identical coils placed asymmetrically with respect to the magnet will also
be sensitive to
the DC magnetization of the casing (gives additional proximity information)
and not sensitive
to the Earth's magnetic field. This is particularly useful when it is desired
to drill the
secondary well to intersect the pre-existing well.
[0027] An important feature of the rotational magnetometer described above is
that
the source of the magnetic field producing variable magnetization in the
magnetic casing
does not induce any direct signal in the synchronously rotating coil 213. This
makes the
induction method with the source and the sensor coil placed in one well
feasible. Another
way to eliminate the direct field signal is to use transient mode of inducing
magnetization in
the target casing ¨ transient magnetometer.
[0028] FIG. 6 depicts an embodiment of the transient magnetometer. The
magnetometer comprises a source of switchable magnetic field 601 having a
switching coil
603 and a magnetic core 605. The magnetic field source 601 generates magnetic
field (the
isolines of the field are shown at 607) at a position of the target casing
205. The magnetic
core 605 preferably comprises a magnetic material with residual magnetization.
The residual
magnetization is used to provide a strong magnetic dipole without the need for
a DC current
driving the switching coil and causing a significant energy loss if a strong
magnetic field
needs to be generated (the application of the magnetic material with residual
magnetization in
a source of a strong switchable magnetic field is described in US patent
application Ser. No.
11/037,488). Disclosed therein is a magnetic core having residual
magnetization. Switching
the current in the coil results in magnetization reversal in the magnetic core
and a change in
the magnetic dipole moment. After the magnetization reversal is complete the
current is
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removed and the new vector of magnetic dipole of remains constant (steady-
state phase of the
antenna dipole) due to magnetic hysteresis of magnetic material employed for
the magnetic
core. The magnetometer also comprises a longitudinal coil 609 to pickup a
variable magnetic
flux produced by the casing magnetization transient occurring in response to
switching of the
magnetization in the magnetic core 605. The magnetometer further comprises a
transversal
coil 611, the signal induced in this coil is sensitive to the azimuthal
position of the casing
with respect to the secondary well 201 when the drill collar rotates.
[0029] FIG. 7 shows time diagrams of the switchable magnetic field and the
transient
responses in the coil 609. The switchable magnetic field 703 is generated by
switching
polarity of the residual magnetization in the magnetic core 605. The switching
polarity is
accomplished by driving the switching coil 603 with short pulses of electric
current 701.
Decaying signals 705, 707, 709 (transients) in the coil 609 are generated in
response
to a fast switching off or changing polarity of a "static" magnetic field. The
signals are
associated with direct coupling between the source and the sensing coil
(transient at 705), the
signal due to eddy currents in the surrounding rock formations and the
conductive collar of
the drill string (a conductive body) placed in the well 201 (transient at
707), and casing
proximity signal due to variable magnetization of the magnetic casing 205
(transient at 709).
It is important for the method that the proximity signal 709 is substantially
longer than the
undesired signals 705 and 707. It follows from the fact that a time constant
of the transient
decay is proportional to the effective magnetic permeability of a magnetic
conductor. It is to
be noted that unlike in the first embodiment, the direction of the magnetic
field does not
rotate¨it only switches polarity. As the coil 609 is also longitudinal, no
sinusoidal variation
will occur.
[0030] The following expression for the time constant of building up of the
average
(over the cross-sectional area) magnetization of the casing can be used [see,
for example,
Polivanov, K.M. Electrodinamika veshchestvennykh sred, 1988]
r cc 6 2 duo ( 1 0)
Here 6 is the wall thickness of the casing, ,u is the magnetic permeability,
which is about 100
for a typical casing material, and a is the conductivity of the material of
the casing. The
process of building up of the magnetic flux in the coil 609 is exponential
with the time
constant given by eqn.(10). By the time approximately equal to the time
constant of the
casing magnetization process all other transients will substantially decay.
Thus, by
measuring the signal in a time window (at 711) starting after a time
comparable with the time
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constant of building up of the casing magnetization (time window 711) one
effectively
eliminates all undesired signal. The expected time constant of the direct
coupling is of the
order of the duration of the pulses 701. In one embodiment, the area within
the window is
used as a distance indicator. Appropriate calibration is carried out. The
processes due to the
eddy current in the conductive surroundings are in the range 1-100us. The
signal from the
magnetic casing should last approximately 10-30ms. Thus practical acquisition
window may
be positioned between lms and 50 ms. Those versed in the art and having
benefit of the
present disclosure would recognize that it is sufficient that the magnet has a
longitudinal
component, and the coil is oriented so that is responsive to magnetic flux
changes in the
longitudinal direction.
[0031] FIG. 8 illustrates an embodiment of the disclosure in secondary
recovery
operations. A producing wellbore 820 has been drilled into a reservoir
interval 801 that
contains hydrocarbons. For various reasons, such as low formation pressure or
high viscosity
of the hydrocarbons in the reservoir, production under natural conditions of
hydrocarbons
may be at uneconomically low rates. In such cases, a second wellbore 822 is
drilled,
typically as a side bore from the wellbore 820 so as to be substantially
parallel to the main
wellbore within the reservoir. The producing wellbore is typically cased with
casing 830 that
has perforations 834. Fluid, such as water, CO2 or steam is then injected into
the formation
through the secondary wellbore 822 and the injected fluid drives the
hydrocarbons in the
formation towards the producing wellbore 820 where it may be recovered. Such
an operation
requires careful positioning of the secondary borehole 822 in proximity to the
production
wellbore 820. This may be done by monitoring the voltage in the coil. As can
be seen from
eqn. (7), the voltage varies inversely as the fifth power of the distance.
Thus, the voltage
measurements may be used as either relative distance indicators based on
voltage changes, or,
with proper calibration, as absolute distance indicators.
[0032] FIG. 9 shows an embodiment for a radial magnet-coil arrangement 900
that
includes two identical or substantially identical receiver coils symmetrically
installed on the
surface of the magnet for estimating distance between adjacent wellbore or
boreholes. In this
embodiment 900 the coil or coil arrangement 913 includes two coils 913a and
913b that are
symmetrically or substantially symmetrically co-located (installed or placed)
around or about
the centerline of the magnet 203 and/or drill collar 201. The radial magnetic
field or flux 221
from the magnet 203 magnetizes the offset well casing 205. The magnetic flux
223 from the
casing 205 is received by both coils 913a and 913b. This configuration, in
aspects, may
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maximize the receiver signal and minimize the size of the rotational
magnet/coil assembly.
Such a small size magnet/coil assembly may be installed on the drill bit or
just above the drill
bit and below (downhole) the mud motor. In aspects, such an arrangement is
more desirable
than attaching the magnet/coil on the drill collar because in high stick-slip
situations the
rotational speed of the drill collar 201 can induce large parasitic noise.
With the magnet/coil
assembly on the drill bit, the magnet/coil arrangement can still be rotated at
a fairly constant
speed using the mud motor. Pulling the drill bit a little uphole or back can
be useful because
the drill bit is then not be in contact with bottom of the borehole. The
configuration 900
continuously provided the distance between the drill collar 201 and the casing
205 while
drilling the second borehole, which measurement may be used to monitor
placement of the
second borehole with respect to the first borehole.
[0033] To estimate the distance between the drill collar 201 and the casing
205 (ri
from center of the drill color 201 to the center of the casing), signals from
both the coils 913a
and 913b are measured. Differential signals between coils 913a and 913b are
obtained while
rotating the drill collar 201. Due to the fact that the earth's magnetic field
is spatially
homogeneous while the signal from the rotating magnetization of the casing is
spatially
inhomogeneous, the parasitic signal from the earth's magnetic field is
substantially removed
from the differential signals, leaving a significant portion of signal from
the rotating
magnetization of the casing for further processing. A controller downhole
and/or at the
surface may be utilized for processing the coil signals for determining the
distance between
the boreholes. The controller may be a microprocessor based circuit and
includes memory
devices and programmed instructions for determining the distance. Such
circuits are known
in the art and are thus not described in detail herein. The distance from the
center of the coil
913 to the center of the casing is shown as "r"2while the distance between the
surfaces of the
drill color and the casing is shown as "d."
[0034] FIG. 10 shows yet another embodiment of a radial magnet-coil
arrangement
1000 that includes two coils, each coil further containing a pair of coils,
radially
symmetrically offset at two sides of the rotating magnet 203. In the
embodiment 1000, a first
coil 1015 containing a first pair of identical or substantially identical
coils 1015a and 1015b
are placed a distance "dl" from the center of magnet 203 away from one end
203a and a
second coil 1017 containing a second pair of coils 1017a and 1017b are placed
at the distance
dl away from the second end 203b of magnet 203. The coils 1017a and 1017b are
identical or
substantially identical to coils 1015a and 1015b. Thus in the embodiment 1000,
there are two
pairs of radial coils, offset equally at two sides of the rotating magnet. The
magnet/coil
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configuration 1000 provides two distance-to-casing measurements (one
corresponding to
each pair) whose differentials may be utilized to estimate the distances and
also determine the
angle of the drill collar 201 with respect to the casing 205. Therefore, this
configuration may
be utilized intermittently when drilling is temporarily stopped (for example
when drill pipe
sections are being added) to determine the angle between the drill collar 201
and the casing
205 or the inclination of the drill collar 201 relative to the casing 205. The
determined angle
or relative angle or the inclination may then be utilized to steer or guide
the next drilling
section, i.e. till the next angle or inclination measurements. When the
receiver coils 1015 and
1017 are axially offset from the magnet 203, the signal in the receiver coils
1015a, 1015b,
1017a and 1017b will generally be less than signals in the receiver coils 913a
and 913b
because the receiver coil voltage is now dependent also on the axial offset.
This means the
efficiency of this configuration will decrease as the axial offset increases.
[0035] As shown by the magnetic flux lines 221 and 223 in FIGS. 9 and 10, the
magnetic field from the magnetized casing 205 is inhomogeneous not only
radially from the
casing 205 but also axially along the direction parallel to the casing 205.
FIG. 11 shows an
exemplary graph 1100 of the magnetic flux 1110 at the receiver 215 or 217 as a
function of
the receiver axial offset 1120, i.e. (in this case two meters) between the
receiver and the
rotating magnet 203 shown in FIG. 10, wherein the distance between the drill
collar 201 and
the casing 205 (the casing offset) is five (5) meters. For example, the
receiver magnetic flux
at zero axial offset 1112 is 3.60e-11 Webb, while the receiver magnetic flux
at two (2) meters
axial offset 1114 is 2.49e-11 Webb. Therefore, by taking differential signals
between a pair
of receivers at these two locations provides almost 31% of the remaining
casing signal, which
is significantly greater than the remaining signal (3.75%) for the radial
signal.
[0036] FIG. 12 shows an exemplary axial coil configuration 1200. In the
particular
configuration of FIG. 12, three identical or substantially identical receiver
coils 1213, 1215
and 1217 are installed on the drill collar 201 with equal spacing d2. The
middle coil 1213 is
collocated with the rotating magnet 203, while coil 1215 is offset on one side
of the magnet
203 and coil 1217 on the other side. Two sets of differential signals between
coil pairs
1215/1213, and 1213/1217 may be generated and utilized for determining the
distance as well
as the relative angle between the drill collar 201 and the casing 205.
[0037] An advantage of the axial configuration 1200 of FIG. 12 is that there
is no
physical limitation on the axial offset between the coils, while for the
radial design the offset
is limited by the diameter of the drill collar so the effectiveness of the
radial gradiometer
cannot be made very high. A disadvantage may be that the
bending/twisting/vibration of the
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drill collar may introduce relatively strong parasitic noise when the coils
are axially
separated. As a result, it may be more difficult to balance the magnetic
moments of the coils
for the axial configuration than for the radial configuration.
[0038] FIG. 13 shows an exemplary hybrid coil configuration 1300. In the
particular
configuration of FIG. 13, three identical or substantially identical receiver
coils 1313, 1315
and 1317 are installed on the drill collar 201 with equal spacing d3. Each
such coil
arrangement is shown to include a pair of two identical or substantially
identical coils. For
example, coil 1313 includes a pair 1313a and 1313b, coil 1315 includes a pair
1315a and
1315b and coil 1317 includes a pair 1317a and 1317b. The middle coil 1313 is
collocated
with the rotating magnet 203, while coil 1315 is offset on one side of the
magnet 203 and coil
1317 on the other side. Two sets of differential signals between coil pairs
1315/1313, and
1313/1317 may be generated and utilized for determining the distance as well
as the relative
angle between the drill collar 201 and the casing 205. These measurements
provide
measurements for both the radial and axial configurations. For example, when
differential
signals are obtained, coil pairs 1313a and 1313b, 1315a and 1315b and 1317a
and 1317b may
be viewed as three radial configurations as shown in FIG. 9. However, when
signals from
each pair are combined, the three coil 3113, 1315 and 1317 would function as
an axial
configuration as, shown in FIG. 12.
[0039] Although coils, such as coils 1015, 1017, 1313, 1315 and 1317 are shown
to
include two coils, more than two coils and suitable differential measurements
may be utilized
for the purposes of this disclosure. Also, the spacing from the magnet to the
coils may be
different. Additionally, the hybrid and other configurations provide more
options and
combinations of measurements so that the tool performance may be optimized for
a particular
drilling environment. Additionally, the embodiments discussed thus show the
use of coils as
detectors, magnetometers may also be used to detect the secondary magnetic
field and the
signals do detected may be processed to determine the distance between
boreholes and the
inclination a borehole.
[0040] In the coil configurations shown in FIGS. 2 and 5, the rotating magnet
from a
second borehole produces a time varying magnetic field in a first borehole. A
coil in the
second borehole is then used to produce a signal responsive to a magnetic flux
resulting from
the magnetization of the casing from the first borehole. Since the signal
induced by the
rotating magnetization of the casing has a frequency which is twice the
rotation frequency of
the magnet/coil assembly, the measured proximity signal is separated from a
parasitic signal
induced in the rotating coil due to the earth's magnetic field, which has a
frequency equal to
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the magnet/coil rotation frequency. However, in often the rotational speed of
the magnet/coil
assembly is not perfectly stable and therefore the parasitic signal from the
earth's magnetic
field may have spectral component at twice the rotation frequency of the
magnet/coil
assembly. Such parasitic signal cannot be removed by a frequency-domain filter
and may
remain as a major source of noise in the final result.
[0041] The coil configurations shown in FIGS. 9, 10, 12 and 13 may reduce the
parasitic signal from the earth's magnetic field and therefore improve the
signal-to-noise ratio
of the measurement. In these configurations, the receiver coils with
substantially equal
magnetic moments are installed either radially-symmetrical or axially offset
along the drill
collar. The magnetic moments of the receiver coils may be stabilized at (near)
identical
through surface or down-hole real-time calibration. Differential signal
between a pair of
receivers is processed for estimating the distance and the angle between two
boreholes or
objects. Since the earth's magnetic field is spatially homogeneous, the
parasitic signal from
the earth's magnetic field in the two substantially identical receiver coils
should be
substantially equal and therefore cancel out. However, the signal induced by
the rotating
magnetization of the casing will not entirely cancel out because it is
spatially inhomogeneous.
Therefore, by taking the differential measurement, the noise from the earth's
magnetic field is
substantially removed, but a significant portion of the signal from the
magnetized casing still
remains for processing. The signal-to-noise ratio of the final measurement is
thus improved.
[0042] An example of improvement in the signal to noise ratio is provided
below.
When the receiver coils are collocated with the rotating magnet, the
approximate result of the
receiver voltage takes the following form:
V0( t) = Ve sin(cot) + vc sin(2cot + (pc,), where (11)
Ve = WACOILBe, and
45/to'Xeff_ZACASiNG'ACOWPm'co
17c = _______________________________________________ (12)
20487-T15i-1
Wherein:
Ve is the amplitude of signal from the spatially homogeneous magnetic field of
the Earth Be;
vc is the amplitude of signal from the rotating magnetization of the target
casing from the
permanent magnet in the well being drilled, which has a strong spatial
gradient;
(pc, is the angle between the Earth's magnetic field vector and the vector
pointing from the
well being drilled to the target casing, projected on the plane that is
perpendicular to the
surface of the receiver coils;
plc, is the vacuum permeability;
CA 02894512 2015-06-09
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Xeff_z is the effective magnetic susceptibility of the casing;
ACASING is the cross-sectional area of the casing;
&on, is the area of the receiver coil;
pm is the magnetic moment of the rotating magnet;
co is the angular frequency of the magnet/coil assembly;
r1 is the distance between the rotating magnet and the casing;
r2 is the distance between the casing and the receiver coil.
Assume that the magnet 203 is symmetric around the drill collar 201 so r1 does
not vary
during rotation. If the distance between the center of the drill collar 201
and the casing 205 is
five (5) meters, the diameter of the drill collar is15 centimeters, then
r2,varies approximately
in the range of 5+0.075 m as the receiver coil rotates with the drill collar.
In the radial
configuration, two substantially identical coils mounted on opposite sides of
the drill collar in
a radially symmetric manner, and signals from the two coils are differentially
combined.
According to Equation 12, the residual vc from the variation in r2 is
approximately
2r2
3.75% of K. from a single coil. Assume that the magnetic moments of the
receiver coils are
calibrated to be differed within 1%, then the amplitude of the differential Ve
signal from the
homogeneous earth's magnetic field is now only 1% of that from a single coil.
In summary,
for this particular case, by using the gradiometer-type of measurement, the
signal-to-noise
ratio can be improved by a factor around 3.75. In the axial configuration, the
difference in the
voltages of two coils may be expressed as:
v(t)= VCOILl(t) ¨ CO VCOIL2(t) = Arec(Bc(11) ¨ 13c (12))sin(2cot
+ (p0) (13)
Where, / is the axial offset between the magnet and the receiver coil.
Equation 13 shows that
the signals due to the earth's magnetic field is removed by differentiating
the measurements
of the two coil signals, while a significant part of the casing signal remains
for processing.
The signal amplitude at different axial offset / can be determined from FIG.
11 (assuming for
example 7-1 = r2 = 5m).
[0043] The signal-to-noise ratio may be further improved by better calibration
of the
receiver coil moments, and by using a drill collar with a greater diameter. It
is also possible to
implement an asymmetric magnet so r1varies in phase with r2 during rotation,
but this
generally leads to a smaller total moment of the magnet and therefore a
reduction of signal
strength.
[0044] FIG. 14 shows an embodiment for a radial magnet-sensor arrangement 1400
for estimating distance between adjacent wellbore or boreholes. The
arrangement 1400 may
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be on a drill collar 1410 of a downhole tool, such as drilling assembly 190,
FIG. 1, used for
drilling a wellbore or may be in the form of a sub of the drilling assembly. A
magnetic
member 1450, such as a metallic casing, in a preexisting wellbore is shown
proximate the sub
1400. Referring to FIGS. 1 and 9, the sub 1400 includes a magnet 1420 placed
around a
member 1410 of the sub 1400. In one configuration, the magnet 1420 may be a
permanent
magnet placed around the member 1410 that generates uniform magnetic field
1422 that is
induced the magnetic member 1450 when the drilling assembly (190, FIG. 1) is
used to drill
the wellbore. In one configuration, the magnet 1420 rotates when the sub 1400
rotates with
the drilling assembly (190, FIG. 1). In another embodiment, the magnet 1420
may be rotated
independent of the drilling assembly rotation at a desired rotational speed
that may be differ
from the rotational speed of the sub 1400. The magnet 1420, in one embodiment,
may be
rotated by an electrically- or hydraulically-operated motor. The magnet 1420,
when rotated
independent of the sub 1400, is referred to herein as a detachable or detached
magnet. When
the magnet 1420 rotates, it produces the uniform magnetic field 1422 (Pm)
around the
member 1410, which induces a primary magnetic field in the magnetic member
1450. The
sub 1400 further includes a sensor 1430 associated with the sub 1400. In one
aspect, sensor
1430 may include one or more magnetometers 1430a -1430n placed around the
member
1410. In one configuration, sensors 1430a-1430n may be equally spaced. Any
suitable
number of sensors (for example 2-16) may be utilized. In one configuration,
sensor 1430 or
sensors 1430a-1430n may be placed on a non-rotating sleeve around member 1410
so that the
sensors are stationary or substantially stationary relative to the rotation of
the magnet 1420.
In another configuration, sensor 1430 or the sensors 1430a-1430n may rotate
relative to the
magnet 1420 at a selected rotational speed that is different from the
rotational speed of the
magnet 1420. The senor 1430 or sensors 1430a-1430n may be rotated by any
suitable
device, including electrically- or hydraulically-operated motor. In another
aspect, a secondary
or bucking magnet 1440 having a magnetic field Pb may be placed between the
magnet 1420
and sensor 1430 or sensors 1430a-1430n oriented in a manner to reduce or
substantially
cancel the effect of the magnetic field Pm on the sensor 1430 or sensors 1430a-
1430n. The
distance between the sensor 1430 and the bucking magnet is shown as rb, while
the distance
between the magnet 1420 and the sensor 1430 is shown as r.
During drilling, in one embodiment, the magnet 1420 rotates, while the sensor
1430 or
sensors 1430a-143On remain stationary or substantially stationary relative to
the magnet
1420. Sensor 1430 or sensors 1430a-1430n receive secondary magnetic field 1424
responsive
to the primary magnetic field induced in the casing 1450 and provide signals
representative of
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the received secondary magnetic field. The secondary magnetic field BO
received at a
particular sensor may be designated by its relative location, such as BO, B90,
etc.
[0045] FIG. 15 shows an embodiment that utilizes electrical coils 1530a-1530n
that
provide electrical voltages V 0 (VO, V90, etc.) corresponding to the received
secondary
magnetic field. Any suitable number of equispaced coils may be provided around
the drill
coil. The coils may be stationary, substantially stationary or rotate at a
rotational speed that
differs from the rotational speed of the magnet 1420. As in the configuration
shown in FIG.
14, the magnet 1420 induces a primary magnetic field 1422 in the casing 1450.
The coils
1530a-1530n provide voltage signals corresponding to secondary magnetic field
1424. The
voltage signals are processed by a downhole and/or surface controller to
estimate the distance
D between the drilling assembly 1500 and the preexisting well 1450. In
aspects, the distance
D may ne determined as follows.
[0046] The bucked magnetic response for a rotating magnet and stationary
sensor
may be derived as follows:
Bo = A cos(wt + 0 ¨ 20) + (A + Kuesciguna91) cos(wt ¨ 0) BL cos(O ¨) + B r (0)
(14)
00 < 0 < 3600
45/to Cur-1)PmAcastng
where A =
40967D5
and wherein:
A casing is the cross area of magnetic casing,
[IT is casing magneti permeability
Pm is moment of permanent magnet
D is distance to casing
BL is lateral earth magnetic field
Kuesciguna9/ is residual effect from imperfect bucking
B r (0) is magnetic fields from residual magnetic charge distribution on
casing
O is azimuth angle of receiver moment direction
is direction angel from a first borehole to a second borehole
is direction angle of earth magnetic field
In the above derivation, it is assumed that a bucking magnet cancels or bucks
out the direct
magnetic field from the primary magnet while the casing signal from the
bucking magnet is
small enough and can be neglected.
[0047] The data received at evenly distributed times ti may be processed as
follows:
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Be'j = A cos(coti +¨ 2P) + (A B ) cos(coti ¨ + BL cos(0/ ¨ + Br
(0 i)
1bruescunagl
(15)
where 0./ = {0 45 90 135 180 225 270 315 }
Using a matrix-vector format:
= M =
where
= (Bet,. _ Beti:1),
I = 1, 2, N
= (COS(C0t, Of) ¨ COS(COti_i + Of) sin(cot, + Of) ¨ sin(coti_i + Of) cos(coti-
91) ¨ cos(coti_i ¨
= (Si s2 s3)T = (A cos 20 A sin 20 C)T
The solution vector is then may be solved as follows:
= (mT m)-i mT
Then, we have A = Jsi2 +
1s2
-t1) = tan- ¨
si
Further, the distance D can be determined from A.
[0048] The bucked voltage response for a rotating magnet and stationary coils
mat be
expressed as:
179 = V sin(cot + 0 _ 20) +(vs + vLecsvnugao
) sin(cot + 0) (16)
0 < 0 < 360
Wherein
45ilo (Pr ¨ I)[-nmAcasingAcoil
vs =
40967TD5
and wherein:
U) is angular frequency of rotation
fir is casing magnet permeability
Pm is moment of permanent magnet
'leasing is cross-section area of casing
Acoll is cross-section area of coil
D is distance to casing.
vbruecsvnuga is residual effect from imperfect bucking
The data processing relating to voltage signals to determine the distance may
be performed as
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follows:
Vet' = vs sin(coti + Oi _ 20) + (vs + vbruecsvnu9ao
) sin(coti ¨ ei) (17)
where Oi = {0 9001
ti {i = 1 ... NJ are evely distributed in a matrix-vector format
= M =
where
M = (sin(coti + ¨ cos(coti + sin(coti ¨
= (s1 s2 s3)T = (Vs cos 20 vs sin 20 C)T
The solution vector can be solved as follows:
= (mT m)i ..A4T -17)
Then, we have vs = \is; +
= tan-1-
2 si
Further D can be determined from vs.
[0049] The processing of the data may be performed by a downhole processor to
give
corrected measurements substantially in real time. Implicit in the control and
processing of
the data is the use of a computer program on a suitable machine readable
medium that
enables the processor to perform the control and processing. The machine-
readable medium
may include ROMs, EPROMs, EEPROMs, Flash Memories and Optical disks.
[0050] While the foregoing disclosure is directed to the preferred embodiments
of the
disclosure, various modifications will be apparent to those skilled in the
art. It is intended
that all variations within the scope and spirit of the appended claims be
embraced by the
foregoing disclosure.
