U.S. patent application number 16/365177 was filed with the patent office on 2019-07-18 for emu impulse antenna with controlled directionality and improved impedance matching.
This patent application is currently assigned to Saudi Arabian Oil Company. The applicant listed for this patent is Saudi Arabian Oil Company. Invention is credited to Erika Ellis, Jesus Manuel Felix Servin, Howard K. Schmidt.
Application Number | 20190219722 16/365177 |
Document ID | / |
Family ID | 63520087 |
Filed Date | 2019-07-18 |
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United States Patent
Application |
20190219722 |
Kind Code |
A1 |
Schmidt; Howard K. ; et
al. |
July 18, 2019 |
EMU IMPULSE ANTENNA WITH CONTROLLED DIRECTIONALITY AND IMPROVED
IMPEDANCE MATCHING
Abstract
An electromagnetic energy source for emitting pulses of
electromagnetic energy includes a sonde assembly and an energy
storage capacitor. The energy storage capacitor has an electrode
mounted in the sonde assembly and operable to generate an electric
field, and a capacitive charge storage medium surrounding the
electrode. A communication cable extends through a tubular member
to the electrode. A fast-closing switch is positioned such that
when the fast-closing switch is in a closed position, a circuit is
formed that discharges the electrode.
Inventors: |
Schmidt; Howard K.;
(Dhahran, SA) ; Ellis; Erika; (Dhahran, SA)
; Felix Servin; Jesus Manuel; (Dhahran, SA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Saudi Arabian Oil Company |
Dhahran |
|
SA |
|
|
Assignee: |
Saudi Arabian Oil Company
Dhahran
SA
|
Family ID: |
63520087 |
Appl. No.: |
16/365177 |
Filed: |
March 26, 2019 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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15878123 |
Jan 23, 2018 |
|
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16365177 |
|
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15458772 |
Mar 14, 2017 |
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15878123 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01V 3/30 20130101; G01V
3/20 20130101 |
International
Class: |
G01V 3/20 20060101
G01V003/20; G01V 3/30 20060101 G01V003/30 |
Claims
1. A method for emitting pulses of electromagnetic energy with an
electromagnetic energy source, the method comprising: providing the
electromagnetic energy source having: a sonde assembly; an energy
storage capacitor including an electrode mounted in the sonde
assembly and operable to generate an electric field, and a
capacitive charge storage medium surrounding the electrode; a
communication cable extending through a tubular member to the
electrode; and a fast-closing switch positioned such that when the
fast-closing switch is in a closed position, a circuit is formed
that discharges the electrode; and charging the energy storage
capacitor to cause the fast-closing switch to close and pulses of
electromagnetic energy to be emitted from the electromagnetic
energy source.
2. The method of claim 1, further including providing a high
voltage power to the electrode with a high voltage power supply
that is in communication with the communication cable.
3. The method of claim 1, where the sonde assembly includes a
downhole section axially aligned with, and spaced from, an uphole
section, each of the downhole section and the uphole section having
an energy storage capacitor.
4. The method of claim 3, where the communication cable extends
through the uphole section of the sonde assembly.
5. The method of claim 1, where the tubular member acts as a
ground.
6. The method of claim 1, where the communication cable is in
communication with a high voltage power supply and transmits a
control signal.
7. The method of claim 1, where the electromagnetic energy source
further includes a plurality of electromagnetic energy sources and
the method further includes emitting pulses of electromagnetic
energy to travel through a subsurface hydrocarbon reservoir.
8. The method of claim 1, further including moving the
electromagnetic energy source to a succession of locations in a
well borehole for emitting the pulses of electromagnetic energy at
the succession of locations for travel through a subsurface
hydrocarbon reservoir.
9. The method of claim 1, further including selecting the
capacitive charge storage medium that has a magnetic permeability
and electric permittivity that results in an impedance of the
pulses that corresponds to an impedance of a subsurface hydrocarbon
reservoir.
10. A method for electromagnetic imaging of a subsurface
hydrocarbon reservoir, the method comprising: lowering an
electromagnetic energy source into a well borehole to a depth of
interest in the subsurface hydrocarbon reservoir, the
electromagnetic energy source including: a sonde assembly; an
energy storage capacitor including an electrode mounted in the
sonde assembly and operable to generate an electric field, and a
capacitive charge storage medium surrounding the electrode; a
communication cable extending through a tubular member to the
electrode; and a fast-closing switch positioned such that when the
fast-closing switch is in a closed position, a circuit is formed
that discharges the electrode; and emitting pulses of
electromagnetic energy with the electromagnetic energy source to
travel through the subsurface hydrocarbon reservoir.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of, and claims priority
to and the benefit of, co-pending U.S. application Ser. No.
15/878,123, filed Jan. 23, 2018, titled "EMU Impulse Antenna With
Controlled Directionality and Improved Impedance Matching," which
is a continuation in part of, and claims priority to and the
benefit of, co-pending U.S. application Ser. No. 15/458,772, filed
Mar. 14, 2017, titled "EMU Impulse Antenna," the full disclosure of
each which is incorporated in this disclosure by reference in its
entirety for all purposes.
BACKGROUND
1. Field of the Disclosure
[0002] The present disclosure relates to imaging sub-surface
structures, particularly hydrocarbon reservoirs and fluids within
the hydrocarbon reservoirs, and more particularly to
electromagnetic energy sources for electromagnetic surveying of
sub-surface structures.
2. Description of the Related Art
[0003] Some electromagnetic (EM) surveying systems used in
geophysics provide electromagnetic energy for traveling through a
subsurface hydrocarbon reservoir for electromagnetic imaging of the
subsurface hydrocarbon reservoir. Multiple sources and receivers
can be positioned either in a bore that extends to the subsurface
hydrocarbon reservoir or an earth surface over the subsurface
hydrocarbon reservoir. In this way, the direction, velocity and
saturation of injected fluids (such as during water flood) can be
monitored. The system can also be used to locate by-passed oil and
detect conductivity zones (such as fracture corridors and super-k
zones) to provide early warning of water break-through. Such
operations can assist in optimizing reservoir management and
preventing oil bypass for improving volumetric sweep efficiency and
production rates.
[0004] Some current EM systems cannot easily match impedance of the
EM system to the geological matrix of the subsurface hydrocarbon
reservoir for increasing transmission efficiency. Some current EM
systems use a cable to provide power to the EM transmitter.
However, these systems have been shown to have difficulty
transferring a crisp pulse from the power supply down a cable, and
then matching that pulse into the antenna. In addition, the cabling
can also transmit a signal, which makes the resulting measurements
unclear.
[0005] In some current systems, the system can be deployed by
wireline under gravity tension in a vertical borehole or along a
semi-rigid means, such as coiled tubing, if used in a horizontal or
lateral borehole. Such deployment systems can introduce some
asymmetry into the radiation pattern from the proximal half of the
antenna, which could negatively impact data quality and
interpretation of gathered data.
SUMMARY OF THE DISCLOSURE
[0006] Embodiments of this disclosure provide systems and methods
for impedance matching to the formation and providing a symmetrical
radiation pattern. Systems and methods of this disclosure are
particularly suited for deployment in long term sub-surface
applications.
[0007] Embodiments of this disclosure combine a slow-wave antenna
with energy storage and pulse forming elements to form a high
power, small aperture transmitting antenna that is suited for
downhole electromagnetic interrogation technologies, such as for
electromagnetic imaging of a subsurface hydrocarbon reservoir.
Systems and methods described in this disclosure provide a
transmitter that is compact, has an instantaneous high power output
and generates a clean signal that is free of material ringing or
distortion, such as a non-attenuated signal with a high signal to
noise ratio. As used in this disclosure, a "high power" is
considered to be a power in a range of a number of kilowatts to a
number of megawatts.
[0008] Systems and methods of this disclosure eliminate external
transmission lines to a switched power supply and radiate an
electromagnetic pulse with a wavelength larger than the physical
length of the antenna structure. The antenna elements of
embodiments of this disclosure are used as capacitive energy
storage elements. A fast-closing (normally open) switch, such as a
triggered spark gap, is provided to initiate pulsed transmission.
The magnitude and shape of the current pulse will depend on the
voltage, capacitance, inductance and the resistance of the antenna
elements and the resistance of the switch. As used in this
disclosure, a "massive current" is considered to be in a current in
the range of 100-1000 amperes (A) or more. Systems and methods of
this disclosure therefore combine energy storage, pulse formation
and radiating elements into a single structure, eliminating the
need for impedance matching between separate distributed components
for these respective functions.
[0009] Systems and methods of this disclosure further eliminate the
problem of load matching between a power supply, cable or
transmission-line, and antenna. With the energy storage element and
switch inside the transmitting antenna element, the cable between
the two is eliminated, minimizing reflections and losses in the
system. As used in this disclosure, an "EMU" antenna is an acronym
for an antenna having an electric permittivity (E), and a magnetic
permeability (MU).
[0010] In an embodiment of this disclosure, an electromagnetic
energy source for emitting pulses of electromagnetic energy
includes a sonde assembly and an energy storage capacitor. The
energy storage capacitor has an electrode mounted in the sonde
assembly that is operable to generate an electric field, a
capacitive charge storage medium surrounding the electrode, and a
communication cable extending through a tubular member to the
electrode. A fast-closing switch is positioned such that when the
fast-closing switch is in a closed position, a circuit is formed
that discharges the electrode.
[0011] In alternate embodiments of this disclosure, a high voltage
power supply can be in communication with the electrode and in
communication with the communication cable. The sonde assembly can
include a downhole section axially aligned with, and spaced from,
an uphole section, each of the downhole section and the uphole
section having an energy storage capacitor. The communication cable
can extend through the uphole section of the sonde assembly.
Alternately, the tubular member can act as a ground.
[0012] In other alternate embodiments, the communication cable can
be in communication with a high voltage power supply and can
transmit a control signal. The capacitive charge storage medium can
be formed of a material that includes iron particles and an epoxy
matrix. The electromagnetic energy source can further include a
plurality of electromagnetic energy sources emitting pulses of
electromagnetic energy to travel through a subsurface hydrocarbon
reservoir. The capacitive charge storage medium can have a magnetic
permeability and electric permittivity selected to result in an
impedance of the electromagnetic energy source that corresponds to
an impedance of the subsurface hydrocarbon reservoir. The
electromagnetic energy source can be movable to a succession of
locations in a well borehole for emitting the pulses of
electromagnetic energy at the succession of locations for travel
through a subsurface hydrocarbon reservoir.
[0013] In an alternate embodiment of this disclosure, an
electromagnetic energy source for emitting pulses of
electromagnetic energy includes a sonde assembly having a downhole
section axially aligned with, and spaced from, an uphole section.
An energy storage capacitor includes an electrode mounted in each
of the downhole section and the uphole section of the sonde
assembly that is operable to generate an electric field, and a
capacitive charge storage medium mounted in each of the downhole
section and the uphole section of the sonde assembly and
surrounding each electrode. A fast-closing switch is located
between the electrode of the downhole section and the electrode of
the uphole section of the sonde assembly. A communication cable
extends through the uphole section of the sonde assembly.
[0014] In another alternate embodiment of this disclosure, an
electromagnetic energy source for emitting pulses of
electromagnetic energy has a sonde assembly and an energy storage
capacitor. The energy storage capacitor includes an electrode
mounted in the sonde assembly that is operable to generate an
electric field and a capacitive charge storage medium surrounding
the electrode. A communication cable extends through a tubular
member to the electrode and the tubular member acts as a ground. A
fast-closing switch is positioned between the electrode and the
ground.
[0015] In alternate embodiments, the sonde assembly can extend from
a downhole end of the tubular member. The communication cable can
extend through the tubular member to a surface and the
communication cable can provide a signal to the electrode.
[0016] In yet another alternate embodiment of this disclosure, a
method for emitting pulses of electromagnetic energy with an
electromagnetic energy source includes providing the
electromagnetic energy source having a sonde assembly, and an
energy storage capacitor including an electrode mounted in the
sonde assembly that is operable to generate an electric field, and
a capacitive charge storage medium surrounding the electrode. A
communication cable extends through a tubular member to the
electrode. A fast-closing switch is positioned such that when the
fast-closing switch is in a closed position, a circuit is formed
that discharges the electrode. The energy storage capacitor is
charged to cause the fast-closing switch to close and pulses of
electromagnetic energy to be emitted from the electromagnetic
energy source.
[0017] In alternate embodiments the method can further include
providing a high voltage power to the electrode with a high voltage
power supply that is in communication with the communication cable.
The sonde assembly can include a downhole section axially aligned
with, and spaced from, an uphole section, each of the downhole
section and the uphole section having an energy storage capacitor.
The communication cable can extend through the uphole section of
the sonde assembly. Alternately, the tubular member can act as a
ground. The communication cable can be in communication with a high
voltage power supply and transmits a control signal.
[0018] In other alternate embodiments, the electromagnetic energy
source can further include a plurality of electromagnetic energy
sources and the method can further include emitting pulses of
electromagnetic energy to travel through a subsurface hydrocarbon
reservoir. The method can further include moving the
electromagnetic energy source to a succession of locations in a
well borehole for emitting the pulses of electromagnetic energy at
the succession of locations for travel through a subsurface
hydrocarbon reservoir. The capacitive charge storage medium can be
selected that has a magnetic permeability and electric permittivity
that results in an impedance of the pulses that corresponds to an
impedance of a subsurface hydrocarbon reservoir.
[0019] In another alternate embodiment of this disclosure, a method
for electromagnetic imaging of a subsurface hydrocarbon reservoir
includes lowering an electromagnetic energy source into a well
borehole to a depth of interest in the subsurface hydrocarbon
reservoir. The electromagnetic energy source includes a sonde
assembly, an energy storage capacitor including an electrode
mounted in the sonde assembly that is operable to generate an
electric field, and a capacitive charge storage medium surrounding
the electrode. A communication cable extends through a tubular
member to the electrode. A fast-closing switch is positioned such
that when the fast-closing switch is in a closed position, a
circuit is formed that discharges the electrode. Pulses of
electromagnetic energy are emitted with the electromagnetic energy
source to travel through the subsurface hydrocarbon reservoir.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] So that the manner in which the recited features, aspects
and advantages of the disclosure, as well as others that will
become apparent, are attained and can be understood in detail, a
more particular description of the embodiments of the disclosure
previously briefly summarized may be had by reference to the
embodiments that are illustrated in the drawings that form a part
of this specification. It is to be noted, however, that the
appended drawings illustrate only certain embodiments of the
disclosure and are, therefore, not to be considered limiting of the
disclosure's scope, for the disclosure may admit to other equally
effective embodiments.
[0021] FIG. 1 is a schematic section view of a transmitter-receiver
array for a borehole to borehole electromagnetic survey, in
accordance with an embodiment of this disclosure.
[0022] FIG. 2 is a schematic section view of an electromagnetic
energy source and storage capacitor, in accordance with an
embodiment of this disclosure.
[0023] FIG. 3 is a schematic cross section view of the
electromagnetic energy source of FIG. 2.
[0024] FIG. 4 is a schematic cross section view of the
electromagnetic energy source of FIG. 2.
[0025] FIG. 5 is a schematic section view of an electromagnetic
energy source and storage capacitor, in accordance with an
embodiment of this disclosure.
[0026] FIG. 6 is a schematic cross section view of the
electromagnetic energy source of FIG. 5.
[0027] FIG. 7 is a schematic cross section view of the
electromagnetic energy source of FIG. 5.
DETAILED DESCRIPTION OF THE DISCLOSURE
[0028] The disclosure refers to particular features, including
process or method steps. Those of skill in the art understand that
the disclosure is not limited to or by the description of
embodiments given in the specification. The subject matter of this
disclosure is not restricted except only in the spirit of the
specification and appended Claims.
[0029] Those of skill in the art also understand that the
terminology used for describing particular embodiments does not
limit the scope or breadth of the embodiments of the disclosure. In
interpreting the specification and appended Claims, all terms
should be interpreted in the broadest possible manner consistent
with the context of each term. All technical and scientific terms
used in the specification and appended Claims have the same meaning
as commonly understood by one of ordinary skill in the art to which
this disclosure belongs unless defined otherwise.
[0030] As used in the Specification and appended Claims, the
singular forms "a", "an", and "the" include plural references
unless the context clearly indicates otherwise.
[0031] As used, the words "comprise," "has," "includes", and all
other grammatical variations are each intended to have an open,
non-limiting meaning that does not exclude additional elements,
components or steps. Embodiments of the present disclosure may
suitably "comprise", "consist" or "consist essentially of" the
limiting features disclosed, and may be practiced in the absence of
a limiting feature not disclosed. For example, it can be recognized
by those skilled in the art that certain steps can be combined into
a single step.
[0032] Where a range of values is provided in the Specification or
in the appended Claims, it is understood that the interval
encompasses each intervening value between the upper limit and the
lower limit as well as the upper limit and the lower limit. The
disclosure encompasses and bounds smaller ranges of the interval
subject to any specific exclusion provided.
[0033] Where reference is made in the specification and appended
Claims to a method comprising two or more defined steps, the
defined steps can be carried out in any order or simultaneously
except where the context excludes that possibility.
[0034] Looking at FIG. 1, an example arrangement of a
transmitter-receiver array for a borehole to borehole
electromagnetic survey is shown. The transmitter can be
electromagnetic energy source 10. Electromagnetic energy source 10
can be located within well borehole 12. Well borehole 12 can extend
through subsurface hydrocarbon reservoir 14. Electromagnetic energy
source 10 can emit pulses of electromagnetic energy to travel
through subsurface hydrocarbon reservoir 14 for electromagnetic
imaging of subsurface hydrocarbon reservoir 14.
[0035] Although one electromagnetic energy source 10 is shown in
the example of FIG. 1, in alternate embodiments, multiple
electromagnetic energy sources 10 can be located within well
borehole 12. Alternately, one or more electromagnetic energy
sources 10 can be located at the earth surface 15 over the
subsurface hydrocarbon reservoir. In the example of FIG. 1, a
series of electromagnetic sensors 16 are located in sensor bore 18.
Sensor bore 18 can be a borehole that extends through subsurface
hydrocarbon reservoir 14, and spaced apart from well borehole 12.
In alternate embodiments, electromagnetic sensors 16 can be in an
array (not shown) over the earth surface 15 over subsurface
hydrocarbon reservoir 14. When electromagnetic energy source 10 is
located in well borehole 12 and electromagnetic sensors 16 are
located over the earth surface 15, the arrangement is known as a
borehole to surface array. Generally either or both of the
electromagnetic energy source 10 and electromagnetic sensors 16 are
located within a borehole so that the EM signals pass through
subsurface hydrocarbon reservoir 14 when traveling from
electromagnetic energy source 10 to electromagnetic sensors 16.
Electromagnetic sensors 16 can form a measure of the arrival time
of the emitted pulses from electromagnetic energy source 10 to
image subsurface hydrocarbon reservoir 14.
[0036] As can be seen in FIG. 1, a multitude of EM energy
measurements can be performed with different combinations of
transmitter locations 20 and receiver locations 22 in order to
sample various parts of the subterranean features from different
directions, including subsurface hydrocarbon reservoir 14. Both the
electromagnetic energy source 10 and electromagnetic sensors 16 can
be a part of a downhole tool or located in a tool and can be
movable to between a succession of locations, such as between
transmitter locations 20 and receiver locations 22.
[0037] Electromagnetic energy source 10 can be attached to
installation string 24 for travel in well borehole 12 to a depth of
interest. Installation string 24 extends from vehicle 26 at the
surface and can be, for example, a wireline or coiled tubing.
System control unit 28 can be associated with vehicle 26 and can be
used to control the pulses emitted by electromagnetic energy source
10. A second vehicle 30 can have a receiver wireline 32 for
attaching to electromagnetic sensors 16 and for moving
electromagnetic sensors 16 within sensor bore 18. Electromagnetic
energy source 10 can include a half wave or quarter wave dipole
antenna, or a monopole antenna.
[0038] Looking at FIGS. 2-3, example embodiment of electromagnetic
energy source 10 includes sonde assembly 34. Sonde assembly 34 of
such embodiment has two main sections: downhole section 34a is
axially aligned with, and spaced from, uphole section 34b.
Electromagnetic energy source 10 also includes energy storage
capacitor 40 with capacitive charge storage medium 44.
[0039] An electrode 42 is mounted in each of downhole section 34a
and uphole section 34b of sonde assembly 34. Downhole electrode 42a
is located in downhole section 34a and uphole electrode 42b is
located in uphole section 34b. Electrode 42 can be an elongated
member and have a tubular shape. Electrode 42 can be formed of
copper, and in alternate embodiments, can be formed of silver,
aluminum, gold, graphite, or other material with sufficient
conductivity, corrosion resistance and hardness suitable for use as
an electrode. Downhole electrode 42a can be part of sonde assembly
34. Downhole electrode 42a can include a downhole open central bore
43a. In alternate embodiments, downhole electrode 42a can be a
solid tubular shaped member. Uphole electrode 42b can be part of
sonde assembly 34. Uphole electrode 42b can include uphole open
central bore 43b so that uphole electrode 42b is a tubular member.
Uphole open central bore 43b can include an opening proximate to an
uphole end of uphole electrode 42b so that communication cable 45
can pass into uphole open central bore 43b of uphole electrode
42b.
[0040] Communication cable 45 can travel through uphole open
central bore 43b and exit out of uphole open central bore 43b
proximate to a downhole end of uphole electrode 42b. Communication
cable 45 can be in communication with high voltage power supply 48
and can transmit a control signal from system control unit 28 (FIG.
1) to control the pulses emitted by electromagnetic energy source
10.
[0041] Each half of the dipole is initially held at a high voltage
relative to one another. The voltage will be dependent on the
capacitance and impedance of the circuit, as well as dimensions of
the antenna, and will change with the frequency of operation of the
antenna. The voltage of one half of the dipole is equal and
opposite of the voltage of the other half of the dipole, with the
outermost end of each half therefore having the greatest magnitude
of voltage. As used in this disclosure, "high voltage" can a
voltage in a range 1000 volts (V) and greater. The pair of antennas
is biased apart by a large voltage so that the structure can
discharge in a single massive current pulse and emit a high power
transient signal.
[0042] The capacitive charge storage medium 44 is mounted in each
of the downhole section 34a and the uphole section 34b of the sonde
assembly 34. Capacitive charge storage medium 44 can be formed of a
material that is selected so that the electric permittivity and
magnetic permeability of such material optimizes transmitter
impedance to match the external medium. This increases the
capacitance and inductance of the system and decreases the group
velocity of the pulses emitted by electromagnetic energy source 10,
to define a slow-wave antenna.
[0043] A slow wave refers to the group velocity of the EM wave
travelling along the structure. The group velocity can be made
slower by changing the electromagnetic properties of the guiding
structure or in particular the cladding around the guidewire
according to the formula:
V .varies. 1 r .mu. r ##EQU00001##
where V=wave velocity, .epsilon.r is the dielectric constant, and
.mu.r is the relative magnetic permeability.
[0044] The slower group velocity of the EM waves allows a
proportional reduction in the physical length of the antenna. A
slower group velocity is especially important for lower frequency
applications, such as applications with frequencies in the 10 kHz-1
MHz range, with very large wavelengths, such as where the length of
the antenna corresponding to one quarter of the resonant wavelength
of the non-cladded antenna is hundreds of meters.
[0045] Providing a capacitive charge storage medium 44 of
embodiments of this disclosure can materially decrease the length
of an antenna structure for a given wavelength emitted. As an
example, capacitive charge storage medium 44 can be formed of a
material that includes ferrite, steel, permalloy, TiO2, lead
zirconate titanate, magnetite, other iron particles, or a mix of
any such materials. Such particles 44a can be mixed in an epoxy
matrix 44b. The specific composition of the mixture used for
capacitive charge storage medium 44 would depend on the properties
of the reservoir materials and the geometry of the antenna, as
further described in this disclosure. In an example embodiment,
capacitive charge storage medium 44 can have particles 44a with a
relative electric permittivity of 100 and a relative magnetic
permeability of 100 and consist of both TiO2 and magnetite. The
relative electric permittivity of a material is known as the
dielectric constant of the material divided by the dielectric
constant of free space. In other alternate embodiments, the
relative magnetic permeability of particles 44a can be in a range
of 1 to 100,000. These particles 44a can be located in a 1:1
mixture in an insulating epoxy matrix 44b. This example embodiment
would result in an overall relative electric permittivity in the
range of 40, and a magnetic permeability in the range of 40 after
considering the linear combination of the components, in accordance
with effective medium theory. This example embodiment will result
in an effective antenna that performs as though it is in the range
of forty times larger than the actual length of the antenna.
[0046] Looking at FIGS. 2-4, electromagnetic energy source 10 can
further include fast-closing switch 46, which is located between
downhole and uphole electrodes 42a, 42b of downhole and uphole
sections 34a, 34b, respectively. When fast-closing switch is
closed, such as when the spark gap is broken down, electromagnetic
energy source 10 will generate an electromagnetic pulse.
Fast-closing switch 46 is positioned such that when fast-closing
switch 46 is in a closed position, a circuit is transiently formed
that connects electrodes 42a and 42b. Fast-closing switch 46 can
be, for example, a spark gap.
[0047] As an example, when the potential difference between
downhole and uphole electrodes 42a, 42b exceeds the breakdown
voltage of a gas within the gap, an electric spark can pass between
downhole and uphole electrodes 42a, 42b. In alternate embodiments,
fast-closing switch 46 can include avalanche transistors,
thyratrons, ignitrons, silicon-controlled rectifier, and triggered
spark gaps. Fast-closing switch 46 can be selected to have
performance metrics concerning peak current, peak voltage, useful
number of shots, jitter, complexity and geometry that will suit the
environment, conditions, and performance criteria for which the
electromagnetic energy source 10 is to be used.
[0048] Electromagnetic energy source 10 can also have high voltage
power supply 48 connected between downhole and uphole electrodes
42a, 42b. High voltage power supply 48 can have, for example, a
voltage over 1,500 volts. Power can be provided to high voltage
power supply 48 from outside of electromagnetic energy source 10
with pair of leads and by way of communication cable 45.
[0049] In the example embodiment of FIGS. 2-4, capacitive charge
storage medium 44 acts as a ground. In such an embodiment,
capacitive charge storage medium 44 proximate to electrode 42 will
form energy storage capacitor 40 and capacitive charge storage
medium 44 proximate to an outer diameter of capacitive charge
storage medium 44 will act as the ground.
[0050] Current limiting resistors 50 can be located between high
voltage power supply 48 and both of the downhole electrode 42a of
the downhole section 34a and the uphole electrode 42b of the uphole
section 34b. Current limiting resistors 50 can block current pulses
from returning up the supply wire towards high voltage power supply
48. This will isolate the antenna system from high voltage power
supply 48 while the electromagnetic pulse is being emitted.
[0051] Electrode 42 is centered along axis Ax of each of downhole
section 34a and uphole section 34b of sonde assembly 34. Electrode
42 is sheathed within capacitive charge storage medium 44 so that
capacitive charge storage medium 44 surrounds electrode 42. Energy
storage capacitor 40 is formed by an electric field radiating out
from electrode 42 and through the nearby capacitive charge storage
medium 44. The amount of energy stored will vary with the square of
the electric field. If electrode 42 has a small diameter, then
almost all of the electric field potential drop will occur inside
the capacitive charge storage medium 44.
[0052] Looking at FIGS. 5-7, electromagnetic energy source 10 can
alternately have a single electrode 42 of sonde assembly 34. In
such an embodiment, communication cable 45 can extend downhole
through a tubular member such as casing or coiled tubing 52 to high
voltage power supply 48. High voltage power supply 48 is in
communication with electrode 42. Electrode 42 is sheathed within
capacitive charge storage medium 44 so that capacitive charge
storage medium 44 surrounds electrode 42. Energy storage capacitor
40 is formed by an electric field radiating out from electrode 42
and through the nearby capacitive charge storage medium 44, that
can have particles 44a that are mixed in epoxy matrix 44b.
[0053] Fast-closing switch 46, which is located between high
voltage power supply 48 and coiled tubing 52, which acts as a
ground for electromagnetic energy source 10. Fast-closing switch 46
is positioned such that when fast-closing switch 46 is in a closed
position, a circuit is formed that discharges electrode 42. When
fast-closing switch is closed, such as when the spark gap is broken
down, electromagnetic energy source 10 will generate an
electromagnetic pulse. Such a configuration can be particularly
useful for permanent deployment in vertical wells where the well
has casing to the top of the productive zone, while leaving
subsurface hydrocarbon reservoir 14 open-hole. Such a configuration
is may also be particularly useful for long term reservoir
monitoring and estimating an average remaining oil column between
adjacent wells.
[0054] Electromagnetic energy source 10 of embodiments of FIGS. 5-7
operates as a grounded half dipole antenna, or a monopole antenna.
Because, the length of the antenna affects the resonant frequency,
using the single electrode system of FIGS. 5-7 will affect the
radiation efficiency as a function of wavelength. In addition,
because the power density of the resulting pulse is a function of
the direction of travel of the pulse, the use of the embodiment of
FIGS. 5-7 will result in a radiation pattern that is different than
the radiation pattern of the dipole arrangement of FIGS. 2-4. As an
example, the vertical radiation pattern of the grounded dipole
antenna, or monopole antenna, has a smaller angle of radiation
giving it a longer-range propagation advantages at frequencies less
than about 50 MHz as compared to the half wave dipole antenna of
FIGS. 2-4.
[0055] In EM transmission systems, a factor to be considered is
matching the impedance of all components in the system to maximize
transfer efficiency and minimize reflection and ringing between
each element. By including high voltage power supply 48 and
fast-closing switch 46 as subterranean component of electromagnetic
energy source 10, a transmission line that transmits the pulse from
the surface is eliminated, which in turn eliminates a cause of
ringing and inefficiency in the system.
[0056] An additional factor to be considered for improving the
performance of EM transmission systems is matching the impedance of
electromagnetic energy source 10 to the medium through which the
pulses are transmitted. Subsurface geological structures can be
comprised of rocks with porosity ranging from zero to about 25
percent (%). The pore space can be filled with water, which may
have dissolved salts that affect the conductivity of the geological
structure. Other pore spaces can be filled with liquid or gas
hydrocarbons. Taken together, the rock matrix in and around
subsurface hydrocarbon reservoir 14 can have a dielectric constant
ranging from about 5 (such as for anhydrite or oil filled
sandstone), to about 22 (such as for water filled limestone with
20% porosity).
[0057] The impedance (Z) of a medium can be calculated as shown in
the following Equation 1:
Z = .mu. Equation 1 ##EQU00002##
[0058] where .mu. is the magnetic permeability and c is the
electric permittivity of the medium. As shown in Equation 1, the
impedance of a medium is determined by the ratio of magnetic
permeability to electric permittivity (.mu./.epsilon. ratio).
[0059] The dielectric constant (k) of a material can be calculated
as shown in Equation 2:
k = r = o Equation 2 ##EQU00003##
so then,
.epsilon.=.epsilon.r.epsilon.o=k.epsilon.o Equation 3
where .epsilon.o is the electric permittivity of free space. Using
Equation 3 with Equation 1, the relationship between impedance and
dielectric constant can be expressed as shown in Equation 4:
Z = .mu. k o Equation 4 ##EQU00004##
In Equation 4 it is shown that as the dielectric constant k
increases, impedance Z decreases. And as magnetic permeability .mu.
increases, so does the impedance Z of the medium.
[0060] Some current EM transmission systems are designed to match
the impedance of free space, which is about 377.OMEGA.. Given their
higher dielectric constant compared to free space, the impedance of
the oil and brine filled portions of the reservoir can be in a
range from about 80-170 ohms (.OMEGA.), depending on whether the
reservoir includes brine or oil filled pores. Embodiments of the
current application improve the performance of electromagnetic
energy source 10 compared to EM transmission systems with an
impedance of about 377.OMEGA. by tuning the ratio of magnetic
permeability to electric permittivity while maintaining a desired
product of magnetic permeability multiplied by electric
permittivity, for miniaturization.
[0061] The materials used for capacitive charge storage medium 44
for electromagnetic energy source 10 can be selected depending on
whether electromagnetic energy source 10 is to be used for oil or
water filled rocks. As an example, if electromagnetic energy source
10 is to be used to survey water-flooded reservoirs, .mu. and
.epsilon. for electromagnetic energy source 10 would be selected to
have a WE ratio in a range of 6400 to 100,000. Alternately, if
electromagnetic energy source 10 is to be used to survey
hydrocarbon producing reservoirs, .mu. and .epsilon. for
electromagnetic energy source 10 would be selected to have a WE
ratio of 23000 to 30000.
[0062] Considering instead the ratio of relative magnetic
permeability to relative electric permittivity, the relative
magnetic permeability of the reservoir could be about 1. Depending
on the porosity and amount of residual water, the relative electric
permittivity of the reservoir could range from 10 for an original
hydrocarbon filled reservoir, to 20 for a water filled or swept
reservoir. Therefore, to optimize transmission efficiency,
desirable ratios of relative magnetic permeability to relative
electric permittivity in the sheathing material in the EMU antenna
should be close to the ratio of the reservoir medium, such as in
the range of 0.05 to 0.1.
[0063] The ratio of magnetic permeability to electric permittivity
can be selected by using a mixture of magnetic and dielectric
materials as part of capacitive charge storage medium 44, such as
titania and magnetite as well as special ferrites that
simultaneously display both increased magnetic permeability and
electric permittivity. The ratio of magnetic permeability to
electric permittivity can be tuned by adjusting the stoichiometry
of the various metals in the ferrite. In such a way, capacitive
charge storage medium 44 can have a magnetic permeability and
electric permittivity selected to result in an impedance of
electromagnetic energy source 10 that corresponds to an impedance
of subsurface hydrocarbon reservoir 14.
[0064] As an example, a material with tunable properties can be
used as part of capacitive charge storage medium 44. Such a
material can be a ferrite that has the composition
Ti.sub.x(MFe.sub.2O.sub.4+2x/y).sub.y, where x+y=1 and 0<x<1.
The metal M can be any one or several metals selected from the
group of Mn, Ni, Cu, Mg, Li and Zn.sub.0.6. Considering a ferrite
with M=Ni(12%)+Cu(28%)+Zn(60%) as an example, such a composition is
then
Ti.sub.x(Ni.sub.0.12Cu.sub.0.28Zn.sub.0.6Fe.sub.2O.sub.4+2x/y).sub.y.
In embodiments of this application, such a ferrite with an x=0.05
fraction of Ti can be used for capacitive charge storage medium 44.
Such a composition for capacitive charge storage medium 44 can
maximize the product of magnetic permeability multiplied by
electric permittivity, while maintaining the ratio of relative
magnetic permeability to relative electric permittivity of about
0.10.
[0065] In some current EM transmission systems, the cable used to
transmit power and control signals downhole to electromagnetic
energy source 10 can be routed exterior of at least one of the
electrodes 42. The use of such an external cable will introduce
asymmetry into the radiation pattern from the electromagnetic
energy source 10, which could negatively impact data quality and
the interpretation of such data. Embodiments of this disclosure
eliminate such asymmetry by either passing communication cable 45
through a tubular member to electrode 42. The tubular member
through which communication cable 45 passes can be a central bore
of uphole electrode 42b (FIG. 2), or can be casing or coiled tubing
52 in an embodiment having only one electrode 42, which is located
downhole of communication cable 45 (FIG. 5).
[0066] In an example of operation, in order to form an
electromagnetic image of subsurface hydrocarbon reservoir 14
electromagnetic energy source 10 can be mounted to, or part of, a
well tool and lowered on installation string 24 into well borehole
12 to a depth of interest. Capacitive charge storage medium 44 of
electromagnetic energy source 10 can be selected to have a magnetic
permeability and electric permittivity selected to result in an
impedance of electromagnetic energy source 10 that corresponds to
an impedance of the subsurface hydrocarbon reservoir.
[0067] The downhole tool associated with electromagnetic energy
source 10 can have a connector section with a mechanical connector
that attaches to installation string 24, an electrical power
connection, and a synchronizing signal connection. Such connector
section and connections can be orientated similar to known current
downhole tools. The downhole tool can house sonde assembly 34.
Electromagnetic energy source 10 can be encased in a strong,
insulating polymeric material to provide structural integrity while
also allowing for the transmission of electromagnetic signals.
[0068] A single electromagnetic energy source 10 can be utilized,
as shown in the example of FIG. 1. Alternately, a plurality of
electromagnetic energy sources 10 can be lowered in well borehole
12. Pulses of electromagnetic energy can be emitted from the single
electromagnetic energy source 10, or at each of the plurality of
electromagnetic energy sources 10, as applicable, to travel through
subsurface hydrocarbon reservoir 14 and a resulting signal can be
received by electromagnetic sensors 16. An electromagnetic pulse
with known characteristics is generated from the high power, pulsed
electromagnetic energy source 10 from locations in or near
subsurface hydrocarbon reservoir 14. In order to generate the
electromagnetic pulse, high voltage power supply 48 charges up
energy storage capacitor 40 through current limiting resistor 50
until fast-closing switch 46 is closed. With the fast-closing
switch closed, electromagnetic energy source 10 will emit the pulse
of electromagnetic energy. After the electromagnetic pulse is
emitted, high voltage power supply 48 can recharge energy storage
capacitor 40.
[0069] By combining energy storage, pulse formation and radiating
elements into a single structure, the problem of impedance matching
between separate distributed components of an electromagnetic
survey system required for these respective functions is
eliminated. Systems and methods of this disclosure therefore
eliminate the problem of load matching between a power supply,
cable or transmission-line, and antenna. With the energy storage
element of energy storage capacitor 40 and fast-closing switch 46
both inside the transmitting antenna element of the pair of
disclosed self-powered impulse antennas, the need for an external
cable between the power source and the transmission element are
eliminated, and reflections and losses in the system are
minimized.
[0070] In addition, because the cable that provides a control
signal to electromagnetic energy sources 10 and is associated with
high voltage power supply 48 does not extend past an electrode 42,
the resulting radiation pattern from the electromagnetic energy
source 10 will have improved symmetry compared to systems where a
power or communication cable extends exterior of an electrode
42.
[0071] A plurality of electromagnetic sensors 16 can be mounted to
or part of a well tool and lowered in sensor bore 18 that extends
through subsurface hydrocarbon reservoir 14. Alternately, the
plurality of electromagnetic sensors 16 can be arranged in an array
over an earth surface 15 over subsurface hydrocarbon reservoir 14.
The emitted pulsed EM signal is transmitted through subsurface
hydrocarbon reservoir 14 and recorded at one or more
electromagnetic sensors 16 after travel through the subsurface
formations surrounding well borehole 12 and sensor bore 18. The EM
signal recorded by electromagnetic sensors 16 differs from the
pulsed signal emitted by electromagnetic energy source 10 in
characteristics such as time, amplitude, power spectrum, and other
characteristics that depend on the properties of the intervening
medium (such as the reservoir) and spatial variations of those
properties.
[0072] Electromagnetic energy source 10 can be moved between a
succession of locations, such as transmitter locations 20, in well
borehole 12 for emitting pulses of electromagnetic energy at such
succession of locations for travel through subsurface hydrocarbon
reservoir 14. Similarly, electromagnetic sensors 16 can be moved
between a succession of locations, such as receiver locations 22,
to receive the resulting signal at such succession of locations. In
this way, a more complete electromagnetic image can be formed of
subsurface hydrocarbon reservoir 14.
[0073] Recording and processing instrumentation associated with
system control unit 28 at the surface can receive and store
information relating to the resulting signal received by
electromagnetic sensors 16. System control unit 28 can also perform
additional functions such as computerized analysis of the resulting
signal, display certain results derived from the resulting signal,
and store the resulting signal and computerized analysis on a
computer for further processing and computerized analysis. System
control unit 28 can, as an example, be used to form a measure of
the arrival time of the emitted pulses at a plurality of
electromagnetic sensors, and to analyze the measure of arrival time
data from the plurality of electromagnetic sensors. From this
information, a representation of subsurface features of the
subsurface hydrocarbon reservoir, and an image of the
representation of subsurface features of the subsurface hydrocarbon
reservoir, can be formed.
[0074] Embodiments of this disclosure thus generate information
about the spatial distribution and composition of fluids in a
hydrocarbon reservoir. The operation can be repeated periodically
to, as an example determine the direction, velocity and saturation
of injected fluids, such as a water flood, or to visualize modified
reservoir volume as a function of time. This can assist in
optimizing reservoir management, preventing oil bypass and thereby
improving volumetric sweep efficiency and production rates.
[0075] Embodiments of this disclosure have been sufficiently
described so that a person with ordinary skill in the art may
reproduce and obtain the results mentioned in this disclosure.
Nonetheless, any skilled person in the field of technique, subject
of this disclosure, may carry out modifications not described in
this disclosure, to apply these modifications to a determined
structure, or in the manufacturing process of the same, and such
resulting structures shall be covered within the scope of this
disclosure.
[0076] It should be noted and understood that there can be
improvements and modifications made of the present embodiments
described in detail in this disclosure without departing from the
scope of the disclosure.
* * * * *