U.S. patent application number 13/587333 was filed with the patent office on 2012-12-06 for methods for magnetic imaging of geological structures.
This patent application is currently assigned to William Marsh Rice University. Invention is credited to Howard K. Schmidt, James M. Tour.
Application Number | 20120306501 13/587333 |
Document ID | / |
Family ID | 40850090 |
Filed Date | 2012-12-06 |
United States Patent
Application |
20120306501 |
Kind Code |
A1 |
Schmidt; Howard K. ; et
al. |
December 6, 2012 |
METHODS FOR MAGNETIC IMAGING OF GEOLOGICAL STRUCTURES
Abstract
Methods for imaging geological structures include injecting
magnetic materials into the geological structures, placing at least
one magnetic probe in a proximity to the geological structures,
generating a magnetic field in the geological structures and
detecting a magnetic signal. The at least one magnetic probe may be
on the surface of the geological structures or reside within the
geological structures. The methods also include injecting magnetic
materials into the geological structures, placing at least one
magnetic detector in the geological structures and measuring a
resonant frequency in the at least one magnetic detector. Methods
for using magnetic materials in dipole-dipole, dipole-loop and
loop-loop transmitter-receiver configurations for geological
structure electromagnetic imaging techniques are also
disclosed.
Inventors: |
Schmidt; Howard K.;
(Cypress, TX) ; Tour; James M.; (Bellair,
TX) |
Assignee: |
William Marsh Rice
University
Houston
TX
|
Family ID: |
40850090 |
Appl. No.: |
13/587333 |
Filed: |
August 16, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12350914 |
Jan 8, 2009 |
8269501 |
|
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13587333 |
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61019765 |
Jan 8, 2008 |
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61054362 |
May 19, 2008 |
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Current U.S.
Class: |
324/345 |
Current CPC
Class: |
G01V 3/26 20130101 |
Class at
Publication: |
324/345 |
International
Class: |
G01V 3/08 20060101
G01V003/08 |
Claims
1. A method for imaging a geological structure, the method
comprising: providing a dispersion of magnetic material in a fluid,
wherein the magnetic material comprises magnetic nanoparticles;
injecting a fluid into the geological structure; placing at least
one magnetic probe in a proximity to the geological structure;
generating a magnetic field with the at least one magnetic probe;
and detecting a post-injection magnetic signal.
2. The method of claim 1, wherein the magnetic probe provides an
antenna utilizing a dipole-dipole, dipole-loop, or loop-loop
configuration.
3. The method of claim 1, wherein the magnetic probe is an
electromagnetic probe that generates an electromagnetic field.
4. The method of claim 3, further comprising detecting an
electromagnetic signal, wherein the electromagnetic signal detected
is utilized to image the geological structure.
5. The method of claim 3, further comprising detecting an
electromagnetic signal velocity.
6. The method of claim 1, wherein the post-injection magnetic
signal detected is utilized to image fractures in the geological
structure.
7. The method of claim 1, further comprising detecting a prior
magnetic signal, wherein the prior magnetic signal is detected
before injecting the dispersion of magnetic material into the
geological structure.
8. The method of claim 7, wherein the prior magnetic signal is
compared to the post-injection magnetic signal to analyze the
geological structure.
9. The method of claim 1, wherein the magnetic field is a DC field,
an AC field, a pulsed field, or a field that varies in both time
and amplitude.
10. The method of claim 1, wherein the magnetic field generated by
the magnetic probe is modulated to enable frequency-domain,
time-domain, or phase-shift detection.
11. The method of claim 1, wherein the detecting step comprises
measuring a resonant frequency.
12. The method of claim 1, wherein the dispersion of magnetic
material in the fluid provides a permeability of 50.mu..sub.o or
greater.
13. The method of claim 1, wherein the dispersion of magnetic
material comprises a ferrofluid.
14. The method of claim 1, wherein the magnetic material is
selected from the group consisting of iron, cobalt, iron (III)
oxide, magnetite, hematite, ferrites, and combinations thereof.
15. The method of claim 14, wherein the ferrites comprise a
material having a formula AM.sub.2O.sub.4; wherein A and M are
metal atoms; and wherein at least one of A and M are Fe.
16. The method of claim 15, wherein the ferrites are doped with at
least one element that is not A or M.
17. The method of claim 1, wherein the magnetic material has a
diameter of between 10 nm to 1.mu.m.
18. The method of claim 1, wherein the magnetic material is covered
with a coating selected from the group consisting of surfactants,
polymers, or combinations thereof.
19. The method of claim 1, wherein the fluid is selected from the
group consisting of water, brine, drilling mud, fracturing fluid,
and combinations thereof.
20. The method of claim 1, wherein the detecting step is conducted
with at least one SQUID detector.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional of U.S. patent application
Ser. No. 12/350,914 filed Jan. 8, 2009, which claims priority to
provisional patent applications 61/019,765 filed Jan. 8, 2008 and
61/054,362 filed May 19, 2008, which are each incorporated by
reference as if written herein in their entirety.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] Not applicable.
BACKGROUND
[0003] Most geological structures relevant to oil and gas
production retain between 70% to 90% of their original hydrocarbon
stores after primary production driven by natural reservoir
pressure release is complete. Hydraulic fracturing is often used to
increase reservoir contact and increase production rates. During
the fracturing process, proppants are typically added to a
fracturing fluid pumped into the geological structure in order to
keep the fractures from closing in upon themselves when pressure is
released. Another technique commonly used in secondary production
is displacement flooding, of which water-flooding is the most
common. In flooding techniques, a displacing fluid is introduced
from an injection well, and oil and/or gas are extracted from a
nearby production well. The displacing fluid frees oil or gas not
released during primary production and pushes the oil or gas toward
the production well. Displacing fluids include, for example, air,
carbon dioxide, foams, surfactants, and water. Hydraulic fracturing
is often applied to injection and production wells in conjunction
with displacement flooding operations.
[0004] In spite of the undisputed utility of hydraulic fracturing
and water-flooding in petroleum production processes, few methods
exist for monitoring the extent and quality of the fracturing and
flooding processes. Fractures can be monitored and approximately
mapped three-dimensionally during the fracturing process by a
`micro-seismic` technique. The micro-seismic technique detects
sonic signatures from rocks cracking during the fracturing process.
The setup of this technique is prohibitively expensive, and data
that is generated tends to be relatively inaccurate due to high
background noise. Further, the process can only be performed during
the fracturing process and cannot be repeated thereafter.
Water-flood operations can be monitored with low resolution through
four-dimensional seismic surveys. As the density difference between
water and petroleum is small, the flood front is not abruptly
distinguishable, and the imaging resolution tends to be on the
order of tens of meters. Unlike the micro-seismic technique for
monitoring fracturing, flooding operations can be measured
periodically to monitor flooding progression.
[0005] Neither of the above techniques have the capability to
accurately determine the size, structure and location of injected
materials such as, for example, injected proppants and water-flood.
Improved knowledge concerning the location of injected proppants
and water-flood in fractures and natural geological pores would aid
production engineers in tailoring production conditions to meet
local geological settings. Further, knowledge about the location of
injected proppants and fractures would significantly improve safety
in production processes by identifying potentially catastrophic
events before their occurrence. For example, vertical fractures can
rupture the strata sealing geological structures and potentially
intersect fresh water aquifers. Detecting a vertical fracture
situation would allow production wells to be sealed, thereby
preventing petroleum loss and aquifer damage.
[0006] In view of the foregoing, improved methods for imaging
geological structures are needed. Such methods would include the
capability to obtain high-resolution images of fractures and
injected materials, as well as the ability for numerous measurement
repetitions to be made. Utilizing such imaging methods solely or in
combination with existing geological assays, production engineers
could take measures to extract residual petroleum from a geological
structure if it is determined that un-extracted hydrocarbons remain
after production stimulated by fracturing and flooding operations
or a combination thereof is complete.
SUMMARY
[0007] In various embodiments, methods for assaying a geological
structure are disclosed. The methods include providing a dispersion
of magnetic material in a fluid; injecting the dispersion of
magnetic material into the geological structure; placing at least
one magnetic probe in a proximity to the geological structure;
generating a magnetic field in the geological structure with the at
least one magnetic probe; and detecting a magnetic signal.
[0008] In other various embodiments of methods for assaying a
geological structure, the methods include: a) providing a
dispersion of magnetic material in a fluid; b) injecting the
dispersion of magnetic material into the geological structure; c)
placing at least one magnetic detector into the geological
structure; and d) measuring a resonant frequency in the at least
one magnetic detector. The resonant frequency is at least partially
determined by an amount of the magnetic material injected into
geological structure and a location of the magnetic material
relative to the at least one magnetic detector.
[0009] In other various embodiments, methods are disclosed for
using magnetic materials in electromagnetic imaging techniques
utilizing transmitter-receive antenna configurations such as
dipole-dipole, dipole-loop and loop-loop configurations. An
illustrative method utilizing such transmitter-receiver antenna
configurations includes, for example, travel-time tomography.
[0010] The foregoing has outlined rather broadly various features
of the present disclosure in order that the detailed description
that follows may be better understood. Additional features and
advantages of the disclosure will be described hereinafter, which
form the subject of the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] For a more complete understanding of the present disclosure,
and the advantages thereof, reference is now made to the following
descriptions to be taken in conjunction with the accompanying
drawings describing specific embodiments of the disclosure,
wherein:
[0012] FIG. 1 presents finite-element modeling of the
radiofrequency amplitude response of a 1 Hz dipole placed over a
brine-filled rock source (FIG. 1A) and a brine-filled rock source
loaded with 50.mu..sub.o of magnetic material (FIG. 1B);
[0013] FIG. 2 presents finite-element modeling of the extent of
y-axis magnetization in the presence of a simulated 1 mG field
generated by a 1 Hz current loop, wherein the magnetic permeability
is 1.mu..sub.o (FIG. 2A), 5.mu..sub.o (FIG. 2B), 50.mu..sub.o (FIG.
2C) and 500.mu..sub.o (FIG. 2D);
[0014] FIG. 3 presents a 1:30 scale schematic model of the
simulated magnetic flux generated in a geological structure through
a magnetic well-bore in the absence of injected magnetic
material;
[0015] FIG. 4 presents a 1:30 scale schematic model of the
simulated magnetic flux generated in a geological structure through
a magnetic well-bore in the presence of 50.mu..sub.o injected
magnetic material; and
[0016] FIG. 5 presents finite-element modeling of simulated total
magnetization in a horizontal well-bore in the presence of
50.mu..sub.o injected magnetic material as determined by a resonant
frequency magnetic detector with offset (FIG. 5A) and non-offset
(FIG. 5B) detector configurations.
DETAILED DESCRIPTION
[0017] In the following description, certain details are set forth
such as specific quantities, sizes, etc. so as to provide a
thorough understanding of the various embodiments disclosed herein.
However, it will be obvious to those skilled in the art that the
present disclosure may be practiced without such specific details.
In many cases, details concerning such considerations and the like
have been omitted inasmuch as such details are not necessary to
obtain a complete understanding of the present disclosure and are
within the skills of persons of ordinary skill in the relevant
art.
[0018] Referring to the drawings in general, it will be understood
that the illustrations are for the purpose of describing particular
embodiments of the disclosure and are not intended to be limiting
thereto. Drawings are not necessarily to scale.
[0019] While most of the terms used herein will be recognizable to
those of ordinary skill in the art, the following definitions are
nevertheless put forth to aid in the understanding of the present
disclosure. It should be understood, however, that when not
explicitly defined, terms should be interpreted as adopting a
meaning presently accepted by those of skill in the art.
[0020] "COMSOL.RTM.," refers to a finite-element modeling (FEM)
software package available for various physics and engineering
applications (http://www.comsol.com). COMSOL.RTM. modeling
presented herein includes static and time-varying three-dimensional
electromagnetic modeling.
[0021] "Ferrite" as defined herein, refers to a ferromagnetic
compound formed from iron (III) oxide and another oxide.
Illustrative ferrites include materials with a general formula
AM.sub.2O.sub.4, wherein A and M are metal atoms and at least one
of A and M is Fe.
[0022] "Ferrofluid," as defined herein, refers to a liquid that
becomes polarized in the presence of a magnetic field. A ferrofluid
typically includes a paramagnetic, superparamagnetic, ferromagnetic
or ferrimagnetic material disposed as a colloidal suspension in a
carrier fluid such as, for example, an organic solvent or water.
The magnetic material disposed in the carrier fluid can be a
magnetic nanoparticle.
[0023] "Hematite," as defined herein, refers to a common mineral
form of iron (III) oxide.
[0024] "Magnetite," as defined herein, refers to a ferrimagnetic
mineral having a chemical formula Fe.sub.3O.sub.4.
[0025] "RLC circuit," as defined herein, refers to an electrical
circuit including a resistor (R), an inductor (L), and a capacitor
(C), connected in series or in parallel.
[0026] Most economically interesting geological structures such as,
for example, petroleum reservoirs, have low magnetic
permeabilities, essentially equal to that of vacuum, .mu..sub.o. In
various embodiments, the present disclosure describes injecting
magnetic materials into geological structures and then detecting
the magnetic materials within the geological structures. Detecting
the magnetic materials provides a means for imaging the locations
of fractures and injected materials within the geological
structures. Since the magnetic permeabilities of geological
structures are typically very low, any injected magnetic materials
will substantially modify detected magnetic flux response compared
to that typically seen for native rocks, natural gas, oil, water,
and brine of most geological structures. Such magnetic imaging
techniques are advantageous over methods currently in use for
monitoring petroleum production by allowing high-resolution and
repeatable imaging during production processes.
[0027] In various embodiments, methods for assaying a geological
structure are disclosed. The methods include providing a dispersion
of magnetic material in a fluid; injecting the dispersion of
magnetic material into the geological structure; placing at least
one magnetic probe in a proximity to the geological structure;
generating a magnetic field in the geological structure with the at
least one magnetic probe; and detecting a magnetic signal. In some
embodiments, the geological structure is penetrated by at least one
vertical well. In some embodiments, the geological structure is
penetrated by at least one horizontal well. One skilled in the art
will recognize that the terms vertical well and horizontal well
should not be considered limiting, and various well-bore angles
between these two extremes are common in the art and may be
utilized within the spirit and scope of this disclosure. In various
embodiments, the geological structure includes a deposit such as,
for example, oil, gas, or combinations thereof.
[0028] Geological structures have been characterized over
geologically-relevant dimensions using electromagnetic methods, but
these methods have not heretofore utilized injected magnetic
materials. Electromagnetic methods for characterizing geological
structures have typically relied upon the low conductivity and
permittivity of petroleum compared to brine, which is usually found
concurrently with petroleum in geological structures. An
illustrative electromagnetic method for characterizing geological
structures is controlled-source electromagnetic (CSEM) surveying.
In this method, variations in geological structure conductivity are
detected via the electrical component of an applied electromagnetic
field. Spatial variation in conductivity results in changes in
received signal amplitude, thus indicating a possible
petroleum-containing geological structure. CSEM surveying has
typically been used for mapping non-conductive deposits in deep
marine environments using electric dipole transmitter and receiver
antennas. The thick layer of highly conductive seawater screens the
electric dipole receiver antennas from air-path interference.
[0029] The effects of injected magnetic material may be simulated
using finite-element modeling, such as that performed using
COMSOL.RTM. software. Illustrative finite-element modeling
depicting the influence of injected magnetic materials on
radiofrequency amplitude response in the presence of a 1 Hz dipole
in proximity to a simulated geological structure are shown in FIGS.
1A and 1B. FIG. 1A presents finite-element modeling of
radiofrequency amplitude response of a 1 Hz dipole over a
brine-filled rock source. FIG. 1B presents finite-element modeling
of radiofrequency amplitude response of a 1 Hz dipole over the same
brine-filled rock source loaded with 50.mu..sub.o of magnetic
material. In the illustrative models presented in FIGS. 1A and 1B,
the target measurement zone is located on-shore with dimensions of
4 kilometers long by 200 meters wide and positioned 1000 meters
below a rock/air interface within a sphere having a radius of 5
kilometers. The target measurement zone is modeled as rock filled
with brine having a conductivity .sigma.=1.5 S (FIG. 1A) and brine
loaded with magnetic material at .mu.=50.mu..sub.o (FIG. 1B). As is
evident from comparing the magnetic flux lines in FIGS. 1A and 1B,
magnetic material in the target measurement zone significantly
spreads radiofrequency amplitude response at the air/rock
interface.
[0030] The amount of injected magnetic material significantly
influences the spread of magnetic flux lines as illustrated by the
models shown in FIG. 2. FIGS. 2A, 2B, 2C and 2D illustrate
finite-element modeling depicting the extent of y-axis
magnetization in the presence of a simulated 1 mG field strength
generated by a 1 Hz current loop. The plane of the current loop is
oriented vertically. The current loop is 2 meters in diameter and
embedded in a target plane of 30% porosity rock filled with brine.
The target plane is 1000 meters below the surface of a geological
formation, above which is air. The target plane magnetic
permeability is varied through 1, 5, 50 and 500.mu..sub.o to
illustrate the change in observed y-axis magnetization. As is
evident from FIGS. 2A, 2B, 2C and 2D, the extent of y-axis
magnetization is highly influenced by the amount of magnetic
material present. Similar changes in magnetization can be
visualized along other axes.
[0031] A number of different magnetic materials may be used in the
methods described herein. Magnetic materials of the present
disclosure typically are characterized by high magnetic
permeabilities at low applied magnetic fields. Low applied magnetic
fields typically include, for example, magnetic field strengths
less than about 0.1 Tesla. One skilled in the art will recognize,
however, that higher magnetic fields may be applied in the methods
described herein. Magnetic materials include, for example,
paramagnetic, superparamagnetic, ferromagnetic and ferrimagnetic
materials. In various embodiments, the magnetic materials are
dispersed in a fluid such as, for example, water, brine, drilling
mud, fracturing fluid and combinations thereof. In some
embodiments, the fluid includes a proppant such as, for example,
sand. Injection of magnetic materials can be conducted during
fracturing or flooding operations. Magnetic materials can be added
to injected proppants and used during fracturing to monitor the
extent of the fracturing process. Likewise, magnetic materials can
also be used during flooding operations to monitor flood front
progression through the geological structure.
[0032] In various embodiments of the methods, the dispersion of
magnetic material comprises a ferrofluid. The ferrofluid may
include a dispersion of magnetic nanoparticles, which forms the
ferrofluid. Ferrofluids may be injected directly into the
geological structures or diluted in another fluid for injection in
the geological structures. In various embodiments of the methods,
the magnetic material comprises magnetic nanoparticles. In various
embodiments of the methods, the magnetic material includes, for
example, iron, cobalt, iron (III) oxide, magnetite, hematite,
ferrites, and combinations thereof. As defined hereinabove, an
illustrative ferrite has a general chemical formula
AM.sub.2O.sub.4, where A and M are metal atoms and at least one of
A and M is Fe. In various embodiments, the ferrites are doped with
at least one element that is not A or M. Ferrofluids generally
include magnetic metal or metal oxide particles such as, for
example, iron, cobalt, iron (III) oxide, and magnetite. Several
ferrofluids are commercially available or are easily synthesized.
Most commercially-available ferrofluids are based on magnetite,
which provides a low-field permeability of about 100.mu..sub.o.
Higher permeabilities are advantageous for increased detection
sensitivities in the embodiments described herein. An illustrative
high-permeability ferrofluid is formed from a manganese- and
zinc-doped ferrite, which provides a low-field permeability of
about 25,000.mu..sub.o. Doping a barrel of brine to about
50.mu..sub.o would require about 160 grams of this doped ferrite.
Based on current prices of iron, manganese and zinc, brine doping
could be accomplished for at most a few dollars per barrel, making
the methods described herein economically viable for geological
structure assays. Iron nanoparticle suspensions and simple slurries
of iron powders having grain sizes similar to those of sand are
also commercially available and are suitable for use in the methods
described herein.
[0033] In various embodiments, the magnetic materials are modified
prior to their injection into the geological structures.
Modifications are used, for example, to prevent particle
aggregation in the injection fluid, to reduce adhesion to the
geological structures, and to maximize transport through the
geological structures. In various embodiments, the magnetic
materials are covered with a coating such as, for example,
surfactants, polymers and combinations thereof. Surfactants are
selected from neutral, anionic, or cationic surfactants.
[0034] The sizes of the injected magnetic materials are chosen to
be most compatible with the selected magnetic imaging application.
Typical proppants used in hydraulic fracturing operations nominally
resemble sand grains having diameters between about 300.mu.m to
about 1 mm. Hydraulic fractures, in comparison, can be about one
centimeter wide or greater. Naturally-occurring pores in geological
structures encompass a wide range of dimensions depending on local
rock types and degrees of cementation. Pores in typical sandstones
are in the range of about 10.mu.m to about 50.mu.m. Carbonates
typically have a wide pore size distribution ranging between about
100 nm and about 10 mm. The bulk of the pore volume in common
petroleum-producing rocks includes pores typically greater than
about 100 nm in diameter. Therefore, magnetic materials used in the
methods described herein may be varied through a wide range of
sizes to be compatible with natural pore sizes and fractures. In
various embodiments of the methods disclosed herein, the magnetic
materials have diameters between about 10 nm and about 1.mu.m. In
various embodiments of the methods disclosed herein, the magnetic
materials have diameters between about 10 nm and about 100 nm. In
various embodiments of the methods disclosed herein, the magnetic
materials have diameters between about 10 nm and about 50 nm. As
will be evident to one skilled in the art, particle size of the
magnetic material is chosen while bearing factors other than
geological structure pore size and fracture size in mind. For
example, magnetic material particle size can influence the
particle's observed magnetic properties, hydrodynamic radius,
aggregation tendency, and extractability.
[0035] Magnetic fields may be generated within geological
structures through various means by using a magnetic probe. For
example, the magnetic fields can be supplied by permanent magnets,
electromagnets, solenoids, antennas and combinations thereof. The
magnetic probe produces a magnetic field that may be a DC field, an
AC field, a pulsed field, or a field that varies in both time and
amplitude. The magnetic probe field may be modulated in a manner to
enable frequency-domain, time-domain or phase-shift detection
methods to maximize signal-to-noise ratio, and to maximize
rejection of natural background noise and 1/f noise.
[0036] Magnetic fields are projected in the geological structures
in a number of ways by using a magnetic probe. An illustrative
means for generating a magnetic field in the geological structures
involves well-bores penetrating the geological structures. In
various embodiments, the geological structures are penetrated by at
least one well comprising a ferromagnetic material, and the
ferromagnetic material is used to channel a magnetic field into the
geological structures. Steels commonly used in drill stems and
well-bore casings are typically strongly ferromagnetic with a
low-field permeability up to about 5,000.mu..sub.o. Connecting a
magnetic probe magnetization source such as, for example, a
permanent magnet or solenoid at the surface end of such well-bore
casings allows a magnetic field component, B, to be transmitted
along the well-bore casing into the geological structures. The
well-bore casing therefore provides a magnetic flux distal to the
magnetization source. When utilized in this manner, the well-bore
casings function analogously to an antenna for transmitting a
magnetic probe signal into the geological structures.
[0037] FIG. 3 presents a 1:30 scale schematic model of the
simulated magnetic flux generated in a pristine geological
structure 300 (no native or injected magnetic material) using a
magnetic well-bore 302. The logarithm of the magnetic field
intensity is indicated by color contour in the figure. A magnetic
probe magnetization source (not shown) is applied to surface end
301 of well-bore 302. The model includes empty injection zone 303
to be used for introducing magnetic material. For this illustrative
model, well-bore 302 was chosen to be 100 meters in depth, and
injection zone 303 was chosen to be 5 meters in thickness. Magnetic
flux lines 304 are measured using movable detector 305, which is
transported along surface 306 of pristine geological structure 300.
Magnetic flux lines 304 are illustrative of those obtained in the
absence of injected magnetic material.
[0038] FIG. 4 presents the same 1:30 scale schematic model of the
simulated magnetic flux generated in infiltrated geological
structure 400 after injecting sufficient magnetic material into
flooded injection zone 401 to produce a permeability of about
50.mu..sub.o. As in FIG. 3, magnetic flux lines 404 are measured
using movable detector 405, which is transported along surface 406
of infiltrated geological structure 400. Comparison of the magnetic
flux lines 404 in FIG. 4 to the magnetic flux lines 304 in FIG. 3
demonstrates that substantial changes in simulated magnetic flux
can be realized by injecting magnetic materials into the geological
structures. The models presented in FIGS. 3 and 4 are equally valid
for use in operations involving magnetic material-doped water-flood
or magnetic material-loaded proppant.
[0039] The methods illustrated by the models presented in FIGS. 3
and 4 are advantageous in being applicable to both fracturing and
water-flood stages of the petroleum production process. Further,
the methods are readily repeatable to monitor production in near
real-time, factoring into account integration time length for data
acquisition and processing. In various embodiments, the detected
magnetic signal is correlated with an internal structure of the
geological structure. For example, changes in the magnetic flux
lines are indicative of internal structure alterations of the
geological structure. Monitoring of changes to the internal
structures of the geological structure allows production
monitoring. In various embodiments, the methods include detecting a
magnetic signal in the geological structure before injecting the
dispersion of magnetic material. Measuring a magnetic signal in the
geological structure before production begins provides a baseline
for evaluating internal structure changes resulting from fracturing
or water-flood operations. For example, comparison via subtraction
can be utilized to analyze the geological structure before and
after production begins. In typical practice of the methods
described herein, production engineers will compare field
magnetization data with mathematical models such as, for example,
those presented hereinabove and related inversion techniques to
infer the size, shape, and extent of magnetic material incursion in
the geological structure.
[0040] A magnetic signal may be induced at the surface or below the
surface of the geological structure. In various embodiments, the
proximity of the at least one magnetic probe is above the
geological structure. For example, as discussed hereinabove, a
magnetic field can be projected into the geological structure
through a magnetic well-bore. In other various embodiments, the
proximity of the at least one magnetic probe is within the
geological structure. For example, a magnetic field can be
generated within the geological structure with a solenoid located
within the geological structure. Magnetic fields generated within
the geological structure are particularly useful for practicing the
methods described herein when there is no magnetic well-bore
penetrating the geological structure.
[0041] Detection of magnetic flux lines may be conducted at one or
more detection points away from the magnetic probe providing the
applied magnetic field. Detection may occur on the surface of the
geological structure or within the geological structure. Detection
may be accomplished with a single detector or an array of
detectors. Detectors may be stationary or movable to record
magnetic flux data at more than one point. In various embodiments,
the detecting step is conducted with at least one detector that is
movable. In various embodiments, the detecting step includes
detecting a magnetic signal, moving the at least one detector, and
repeating the detecting step to collect magnetic flux data at more
than one point. Detector arrays are used to record magnetic signals
at a number of points simultaneously. A single detector may be, for
example, a SQUID detector or a conventional solenoid, each of which
may be fixed or movable over the surface of the geological
structure. In various embodiments, the detecting step is conducted
with at least one SQUID detector. SQUID detectors are advantageous
for maximizing sensitivity in the methods described herein. In
other various embodiments, the detecting step is conducted with at
least one solenoid. For detector arrays, low cost conventional
solenoids or other known magnetic sensors are more advantageous. In
still other various embodiments, the detecting step includes
measuring a resonant frequency in the at least one magnetic probe.
Measurement of a resonant frequency in the at least one magnetic
probe provides a particularly sensitive means of magnetic
permeability detection and is considered in more detail
hereinbelow.
[0042] Not all drilling applications involve a vertical well-bore
as depicted in FIGS. 3 and 4. For example, horizontal drilling
techniques provide `lateral` well-bores. Such lateral well-bores
are typically used to maximize contact with a geological structure.
Lateral well-bores may be optionally fractured prior to or during
production or used in conjunction with water-flood.
[0043] The orientation of lateral well-bores may not allow
sufficient channeling of an external magnetic field into the
geological structure, even when the lateral well-bore includes a
ferromagnetic well casing. In such instances and others, at least
one magnetic detector may be placed into the well-bore to measure
the magnetic flux lines. At least one magnetic detector may also be
placed in a vertical well-bore when an external magnetic field is
not sufficiently channeled into the geological structure from above
using a magnetic probe. In other various embodiments of methods for
assaying a geological structure, the methods include: a) providing
a dispersion of magnetic material in a fluid; b) injecting the
dispersion of magnetic material into the geological structure; c)
placing at least one magnetic detector into the geological
structure; and d) measuring a resonant frequency in the at least
one magnetic detector. The resonant frequency is at least partially
determined by an amount of the magnetic material injected into
geological structure and a location of the magnetic material
relative to the at least one magnetic detector. In various
embodiments, the at least one magnetic detector is connected to an
RLC circuit. In various embodiments, the methods include measuring
a resonant frequency, moving the at least one magnetic detector,
and repeating the measuring step. A resonant frequency magnetic
detector may include, for example, a solenoid or a directional-loop
antenna placed within a well-bore. When connected to an active RLC
circuit, a resonant frequency of a solenoid is determined by the
capacitance and inductance of the C and L circuits, respectively.
Positioning of the solenoid coil within the geological structure
provides an inductance that is at least partially determined by the
amount of magnetic material that is injected into the geological
structure and the location of the magnetic material relative to the
solenoid coil.
[0044] FIG. 5 presents finite-element modeling of the simulated
total magnetization in a lateral well-bore as determined by a
resonant frequency magnetic detector with an offset (FIG. 5A) and
non-offset (FIG. 5B) detector configuration relative to a high-flow
rate path or natural fracture channel (herein referred to as a
`runner`) away from the main well-bore. Effects of runners in
petroleum production include, for example, early water
breakthrough, which impedes efficient petroleum production.
Therefore, timely detection of runners is clearly desirable. In the
finite-element models presented in FIGS. 5A and 5B, the geological
structure is injected with 50.mu..sub.o of magnetic material, and
the magnetization is determined at a detector offset of 25 meters
from the runner in FIG. 5A and zero meters from the runner in FIG.
5B. The magnetic flux line changes produced with centered and
offset detector configurations are illustrative of the changes
observable in physical geological structures penetrated with
magnetic materials during fracturing or water-flood operations.
Filling of the runner with magnetic material changes the total
magnetization, thereby changing L by about one part in 10.sup.4, as
the magnetic probe position is moved away from the runner. Such
frequency changes are typically measurable to within about one part
in 10.sup.9 or greater using electronic detectors such as, for
example, frequency counters. Thus, in geological structures
injected with magnetic materials, the location of a flood front,
including progression into runners and filled fractures, may be
detected using a moveable resonant frequency magnetic detector. In
dual well-bore systems, the movable resonant frequency magnetic
detector may be placed in the injection well-bore, production
well-bore, or both well-bores. Likewise, the movable resonant
frequency magnetic detector may be located on the surface of the
geological structure.
[0045] Dipole-dipole, dipole-loop and loop-loop configurations for
cross-well and borehole-to-surface electromagnetic imaging
techniques have been under development for some time for detecting
and imaging conductive subsurface features. Generally, frequency
(phase) domain detection and inversion have been employed in such
systems, despite their high computational intensity.
Forward-propagation and inversion algorithms for cross-well
electromagnetic propagation in diffusive media have also been under
development. A relatively new time-domain approach called
travel-time tomography potentially provides simpler electronics and
reduced computational burden relative to other techniques in this
field. Magnetic imaging techniques have not yet been applied in
these more advanced geological imaging surveys. In various
embodiments, methods are disclosed for using magnetic materials in
electromagnetic imaging techniques utilizing transmitter-receiver
antenna configurations such as, for example, dipole-dipole,
dipole-loop, and loop-loop configurations. More sensitive detectors
including, for example, flux-gates and SQUID detectors may also be
coupled to the electromagnetic imaging techniques.
[0046] The methods disclosed herein for using magnetic materials in
dipole-dipole, dipole-loop and loop-loop configurations include
development of approaches for inverting time and amplitude signals
in the presence of magnetic materials and an estimation of
computational intensity needed for each. An additional frequency
domain detection technique relevant to these approaches is referred
to as higher-order spectral analysis. The higher-order spectral
analysis techniques include the use of the coherence of multiple
frequency components to detect weaker signals in the presence of
Gaussian and non-Gaussian noise. A multi-frequency coherent source
or a scattering mechanism that is either nonlinear or
parametric-linear is used in application of the techniques.
Application of magnetic materials in these techniques is
advantageous in providing signal detection (with sufficient
averaging) in the presence higher noise levels than is possible
with more traditional frequency domain detection techniques.
Further, application of magnetic materials in the techniques allows
sensor arrays to be used to determine time of arrival for inversion
processing at lower signal to noise ratios.
[0047] A consideration concerning the use of magnetic materials in
imaging geological structures is a modulation of electromagnetic
signal transport. In a magnetically-loaded fluid, electromagnetic
signal speed decreases in accordance with the formula (I), where c
is the speed
v=c/(.di-elect cons..mu.).sup.1/2 (1)
of light, .di-elect cons. is the relative dielectric constant, and
.mu. is the relative magnetic permeability. Velocity changes in
electromagnetic signals have been used extensively in the art of
well logging, where the dielectric constant difference between
petroleum and water shifts observed signal velocity from 5 ns/m to
29 ns/m. Electromagnetic signal velocity in an aqueous ferrofluid
at 50.mu..sub.0 is calculated to be about 200 ns/m. Therefore, a
significant electromagnetic signal velocity shift is possible for
magnetically-loaded water-floods when applied to travel-time
tomography imaging. Higher-order harmonics generated upon
saturation of the magnetic materials would facilitate frequency
domain discrimination such as, for example, through higher-order
spectral analysis.
[0048] Any of the methods described hereinabove are potentially
applicable for tracking pollutants within a geological structure.
For example, leakage from a chemical storage facility could
potentially be monitored by adding a magnetic material at the
chemical storage facility source and then analyzing for the
presence of magnetic material in a nearby geological structure. An
abrupt or gradual change in magnetic signal would indicate a
leaking condition Likewise, the methods could potentially be used
to monitor pollutant migration through a geological structure such
as, for example, from agricultural runoff. Similarly, the methods
described herein could potentially be used to monitor the transport
and chemical conversion of zero-valent iron particles that are used
in ground water contamination remediation.
[0049] From the foregoing description, one skilled in the art can
easily ascertain the essential characteristics of this disclosure,
and without departing from the spirit and scope thereof, can make
various changes and modifications to adapt the disclosure to
various usages and conditions. The embodiments described
hereinabove are meant to be illustrative only and should not be
taken as limiting of the scope of the disclosure, which is defined
in the following claims.
* * * * *
References