U.S. patent application number 12/912725 was filed with the patent office on 2011-02-17 for in-situ detection and analysis of methane in coal bed methane formations with spectrometers.
Invention is credited to John Herries, John Pope.
Application Number | 20110036146 12/912725 |
Document ID | / |
Family ID | 27393586 |
Filed Date | 2011-02-17 |
United States Patent
Application |
20110036146 |
Kind Code |
A1 |
Pope; John ; et al. |
February 17, 2011 |
In-Situ Detection and Analysis of Methane in Coal Bed Methane
Formations with Spectrometers
Abstract
A measuring system for in-situ measurements down a well (1) by a
spectrometer (4) is provided. The spectrometer (4) includes a
radiation source (5) and a detector (6). A probe (15) optically
connected to the spectrometer (4) and includes an optical pathway
(7) for transmission of a radiation from the radiation source (5)
and at least a second optical pathway for transmission of a
characteristic radiation from a sample to the detector (6). A
positioner is provided to position the probe (15) near a side
surface (11) of the borehole (3) and to optically couple the
optical pathways (7) to the side surface (11), wherein the probe
(15) is traversable up and down the well (1) by way of a guide
operatively connected to the probe (15) and to a fixed location at
the wellhead. By use of the apparatus and method a concentration of
methane or other substance of interest is obtained, and thereby, a
potential production of a coal bed methane formation is
obtained.
Inventors: |
Pope; John; (Laramie,
WY) ; Herries; John; (Laramie, WY) |
Correspondence
Address: |
CROWELL & MORING LLP;INTELLECTUAL PROPERTY GROUP
P.O. BOX 14300
WASHINGTON
DC
20044-4300
US
|
Family ID: |
27393586 |
Appl. No.: |
12/912725 |
Filed: |
October 26, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10266638 |
Oct 9, 2002 |
6678050 |
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12912725 |
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PCT/US01/11563 |
Apr 11, 2001 |
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10266638 |
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60196620 |
Apr 11, 2000 |
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60196182 |
Apr 11, 2000 |
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60196523 |
Apr 11, 2000 |
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60196000 |
Apr 11, 2000 |
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Current U.S.
Class: |
73/31.05 |
Current CPC
Class: |
G01N 2021/3155 20130101;
G01N 21/8507 20130101; G01V 8/02 20130101; G01V 8/10 20130101; G01N
21/27 20130101; E21B 49/00 20130101; G01N 21/65 20130101; G01N
21/359 20130101; G01J 3/28 20130101 |
Class at
Publication: |
73/31.05 |
International
Class: |
G01N 29/02 20060101
G01N029/02 |
Claims
1-76. (canceled)
77. A method for measuring methane concentration in a coal bed
methane well containing water, comprising: (a) providing a housing
element, wherein the housing element comprises a detector and a
sample interface; (b) lowering the housing element into the coal
bed methane well, whereby the sample interface is ion contact with
the water; (c) irradiating the water in contact with the sample
interface; (d) detecting a signal radiation pattern of methane with
the detector from the water in contact with the sample interface;
and (e) processing the signal radiation pattern of methane to
calculate a concentration of methane.
78. A measuring system for introduction into a well having water or
an aqueous medium, and suspected of containing a substance,
comprising: (a) a housing element that is traversable up and down
the well and comprising a spectrometer, a radiation source, a
sample interface, and a detector; (b) a guide extending down the
well and that is operatively connected to the housing element; and
(c) a signal processor to process a signal from the detector, and
capable of calculating a concentration of the substance in the
water.
79. A measuring system for in situ measurement of a well having a
bore hole, comprising: (a) a housing element that is traversable up
and down the well within the bore hole, and comprising a
spectrometer, a radiation source, a sample interface, and a
detector; (b) a guide extending down the well and that is
operatively connected to the housing element; and (c) a signal
processor to process a signal from the detector, and capable of
calculating a concentration of the substance in the water.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of International Patent
Application No. PCT/US01/11563, filed Apr. 11, 2001, designating
the United States of America, and published as WO 01/77628, the
entire disclosure of which is incorporated herein by reference.
Priority is claimed based on Provisional Application Nos.
60/196,620, 60/196,182, 60/196,523 and 60/196,000 filed Apr. 11,
2000.
TECHNICAL FIELD
[0002] This invention relates to in-situ methods of measuring or
analyzing dissolved, free, or embedded substances with a
spectrometer and an apparatus to carry out the method. In
particular this invention relates to a method and apparatus of
analyzing substances down a well. More particularly, this invention
relates to a method and apparatus to detect, analyze and measure
methane or related substances in subsurface coal bed formations
using a portable optical spectrometer to thereby predict a
potential methane production of the well.
BACKGROUND AND SUMMARY OF THE INVENTION
[0003] Coal bed methane is methane that is found in coal seams.
Methane is a significant by-product of coalification, the process
by which organic matter becomes coal. Such methane may remain in
the coal seam or it may move out of the coal seam. If it remains in
the coal seam, the methane is typically immobilized on the coal
face or in the coal pores and cleat system. Often the coal seams
are at or near underground water or aquifers, and coal bed methane
production is reliant on manipulation of underground water tables
and levels. The underground water often saturates the coal seam
where methane is found, and the underground water is often
saturated with methane. The methane may be found in aquifers in and
around coal seams, whether as a free gas or in the water, adsorbed
to the coal or embedded in the coal itself.
[0004] Methane is a primary constituent of natural gas. Recovery of
coal bed methane can be an economic method for production of
natural gas. Such recovery is now pursued in geologic basins around
the world. However, every coal seam that produces coal bed methane
has a unique set of reservoir characteristics that determine its
economic and technical viability. And those characteristics
typically exhibit considerable stratigraphic and lateral
variability.
[0005] In coal seams, methane is predominantly stored as an
immobile, molecularly adsorbed phase within micropores of the bulk
coal material. The amount of methane stored in the coal is
typically termed the gas content.
[0006] Methods of coal bed methane recovery vary from basin to
basin and operator to operator. However, a typical recovery
strategy is a well is drilled to the coal seam, usually a few
hundred to several thousand feet below the surface; casing is set
to the seam and cemented in place in order to isolate the water of
the coal from that of surrounding strata; the coal is drilled and
cleaned; a water pump and gas separation device is installed; and
water is removed from the coal seam at a rate appropriate to reduce
formation pressure, induce desorption of methane from the coal, and
enable production of methane from the well.
[0007] Assessment of the economic and technical viability of
drilling a coal bed methane well in a particular location in a
particular coal seam requires evaluation of a number of reservoir
characteristics. Those characteristics include the gas content and
storage capability of the coal; the percent gas saturation of the
coal; the gas desorption rate and coal density, permeability, and
permeability anisotropy; and gas recovery factor.
[0008] While industry has developed methods to enhance production
from formations that exhibit poor physical characteristics such as
permeability and density, currently no practical methods are
available to increase the gas content of a coal seam. Thus,
identifying coal seams that contain economic amounts of methane is
a critical task for the industry. The primary issue in identifying
such coal seams involves developing a method and apparatus to
quickly and accurately analyze coal seams for gas content.
[0009] Currently accepted methods of measuring gas content involve
extracting a sample of the coal from the seam and measuring the
amount of gas that subsequently desorbs, either by volume or with a
methane gas sensor. However, collection of the coal sample usually
changes its gas content to a significant extent before gas
desorption is monitored. This degradation of sample integrity leads
to degradation of the data collected. That degradation of data
creates significant doubt in the results of those common methods.
As well, because these methods hinge on waiting for the methane to
desorb from the coal, they require inordinate amounts of time and
expense before the data is available.
[0010] Downhole sensing of chemicals using optical spectroscopy is
known for oil wells. For example, Smits et. al., "In-Situ Optical
Fluid Analysis as an Aid to Wireline Formation Sampling", 1993 SPE
26496, developed an ultraviolet/visible spectrometer that could be
placed in a drill string. That spectrometer was incorporated in a
formation fluid sampling tool whereby formation fluids could be
flowed through the device and analyzed by the spectrometer. That
spectrometer was largely insensitive to molecular structure of the
samples, although it was capable of measuring color of the liquids
and a few vibrational bond resonances. The device only
differentiates between the O--H bond in water and the C--H bond in
hydrocarbons and correlates the color of the analyte to predict the
composition of the analyte. The composition obtained by the device
is the phase constituents of the water, gas and hydrocarbons. By
correlating observation of gas or not gas with observation of
water, hydrocarbon, and/or crude oil, the instrument can
distinguish between separate phases, mixed phases, vertical size of
phases, etc. By correlating the gas, hydrocarbon, and crude oil
indicators, the instrument can presumably indicate if a hydrocarbon
phase is gaseous, liquid, crude, or light hydrocarbon. A coal bed
methane well with varying hydrocarbons from coal to methane and,
possibly, bacterial material, provides an environment too complex
for such a device to differentiate methane and the other substances
of interest. The device is not capable of resolving signals from
different hydrocarbons to a useful extent, and the device is not
capable of accurate measurements needed for coal bed methane wells.
Furthermore, the requirements that the sample be fluid, that
analysis occur via optical transmission through the sample, and
that the sample be examined internal to the device precludes its
use for applications such as accurately measuring gas content of
coal seams.
[0011] In other apparatuses known in U.S. Pat. No. 4,802,761 (Bowen
et. al.) and U.S. Pat. No. 4,892,383 (Klainer, et. al.), a fiber
optic probe is positioned to transmit radiation to a chemically
filtered cell volume. Fluid samples from the surrounding
environment are drawn into the cell through a membrane or other
filter. The fiber-optic probe then provides an optical pathway via
which optical analysis of the sample volume can be affected. In the
method from Bowen et. al., a Raman spectrometer at the wellhead is
used to chemically analyze the samples via the fiber optic probe.
The method allows purification of downhole fluid samples using
chromotographic filters and subsequent analysis of the fluid and
its solutes using Raman spectroscopy. However, the stated
requirement that the Raman spectrometer be remote from the samples
of interest and that it employ fiber-optic transmission devices for
excitation and collection ensures that the sensitivity of the
device is limited. The device further does not consider the
conditions present in subsurface wells when analyzing, the samples.
Furthermore, as in the Smits et. al. case, the requirements in
Bowen et. al. and Klainer et. al. that the sample be fluid and that
the sample be examined internal to the device significantly
decrease the utility of the device for applications such as
measuring gas content of coal seams.
[0012] Methods of sample preparation and handling for well tools
have been described, as well. In U.S. Pat. No. 5,293,931 (Nichols
et. al.), an apparatus is disclosed for isolating multiple zones of
a well bore. The isolation allows isolated pressure measurements
through the well bore or wellhead collection of samples of fluids
from various positions in the wellbore. However, such wellhead
sample collection degrades sample integrity and does not provide a
practical method or apparatus for assessment of gas content in coal
seams. The apparatus shown significantly affects any sample
collected and is basically a collection device set down a well.
[0013] An object of the invention is to provide a method and system
to accurately measure substances in wells using optical
analysis.
[0014] Another object of the invention is to provide a method and
measuring system capable of measuring methane in a coal bed methane
well.
[0015] Another object of the invention is to provide a method and
measuring system which utilizes a spectrometer to analyze methane
and other substances with emitted, reflected or scattered radiation
from the substances and thereby allow a measurement of a side
surface of the well.
[0016] Another object of the invention is to provide a method and
measuring system to accurately measure a concentration of methane
in a coal bed methane well and calculate a concentration versus
depth for a single well and calculate concentrations versus depth
for other wells to thereby predict a potential production of a coal
bed methane field.
[0017] The objects are achieved by a measuring system for
introduction into a well with a housing traversable up and down the
well, a guide extending down the well from a fixed location and
being operatively connected to the housing, a spectrometer being
located inside the housing and including a radiation source, a
sample interface to transmit a radiation from the radiation source
to a sample, and a detector to detect a characteristic radiation
emitted, reflected or scattered from the sample and to output a
signal, and a signal processor to process the signal from the
detector and calculate a concentration of a substance in the
sample.
[0018] Another aspect of the invention is a measuring system for
in-situ measurements down a well by a spectrometer. The
spectrometer includes a radiation source and a detector. A probe is
provided optically connected to the spectrometer and including an
optical pathway for transmission of a radiation from the radiation
source and at least a second optical pathway for transmission of a
characteristic radiation from a sample to the detector. A
positioner is provided to position the probe near a side surface of
the borehole and to optically couple the optical pathways to the
side surface of the borehole, wherein the probe is traversable up
and down the well by way of a guide operatively connected to the
probe and to a fixed location at the wellhead.
[0019] Another aspect of the invention is a method of measuring
methane in at least one coal bed methane well. An instrument
package is provided in a housing, and the housing is lowered a
distance down the well. A radiation source is positioned to
irradiate a sample, and a detector is positioned to detect the
characteristic radiation from the interaction between the sample
and the incident radiation from the radiation source. The sample is
irradiated to produce the characteristic radiation. The
concentration of methane in the sample is measured by detecting the
characteristic radiation with the detector. The detector transmits
a signal representative of the concentration of methane to a signal
processor, and the signal processor processes the signal to
calculate the concentration of methane in the sample.
[0020] In another aspect of the invention, a method of measuring a
side surface of a borehole using optical spectrometers is provided.
An optical spectrometer with a radiation source and a detector is
provided. The side surface of the borehole is optically connected
to the radiation source and the detector. The radiation source
irradiates the side surface of the borehole, and the emitted,
reflected or scattered characteristic radiation from the side
surface of the borehole is collected. The collected characteristic
radiation is transmitted to the detector to output or produce a
signal. The signal is transmitted to a signal processor and the
concentration of a substance on the side surface of the borehole is
calculated.
[0021] The side surface is usually a solid material such as coal,
sandstone, clay or other deposit. The side surface has been
affected by the drill bit. The side surface may also have a film of
drilling "mud" or some other contaminant (introduced or naturally
found) that has been distributed by the drill bit. The measurement
system analyzes the surface of that material, or the material is
penetrated to analyze its interior. The surface may be treated
(i.e. by washing it with water) before being analyzed. The material
of interest is characterized along with any other materials
adsorbed or absorbed to the material. These could include gases,
liquids, or solids. Preferably, the methane adsorbed to the coal
surface and in its pores is identified. The amount of methane on
the surface and in the pores is measured.
[0022] The samples of interest may be a face of the coal seam, the
coal itself, a bacterium or bacterial community which may indicate
methane, the water in the well, methane entrained in the coal or
water, methane dissolved in the water, or free gas. A free gas may
be examined in-situ by providing a pressure change to the water or
to the coal and collecting the resultant gas by way of a
head-space. The sample or substance of interest may be physically,
biologically or chemically treated in-situ before measuring to
enhance detection or measurement.
[0023] The radiation source is of particular concern and is
selected depending on the well environment, the substance to be
measured and the background of the sample. Coal shows inordinate
fluorescence, and often bacteria and other organic material are
present near the coal seams. These substances tend to produce
fluorescence which interferes with measurements of other
substances. Unless the fluorescence is measured, the radiation
source and wavelength are selected to minimize these effects. Coal
tends to fluoresce between 600 nm and 900 nm with a significant
drop in fluorescence under 600 nm. A radiation source which takes
into account these ranges is preferred for measuring the methane,
especially the methane adsorbed to or embedded in the coal. Thus,
the methane signature relative to the other components is
maximized. In some instances a signature of the fluorescence is
maximized to characterize the methane indirectly.
[0024] The measurements lead to establishing a concentration of
methane in the coal bed formation and to the potential production
or capacity of the coal bed. The methane is analyzed by obtaining
through spectrometers a series of spectra representative of
scattered, emitted or reflected radiation from methane in the well.
The captured spectra are used to determine the concentration at
varying depths of methane present in the coal bed formation. The
spectra are manipulated and analyzed to produce the concentrations
of methane represented in the well. The use of filters which are
designed to eliminate or reduce radiation from sources present in
the well is needed to accurately determine the methane
concentration or other parameters of the coal bed methane well.
Other parameters may include a predictor element or compound that
is natural or introduced to the coal bed or well. The filters are
chosen depending on the chemical which is of interest. Raman
spectrometers are used in most testing, however, near infrared
lasers and detectors may be employed to avoid the difficulties
associated with fluorescence from material or substances in the
water or well. The measuring system in this invention is based on
high sensitivity. One factor that is used to maintain high
sensitivity of the system is the reduction or elimination of moving
parts throughout the measuring system.
[0025] Other objects, advantages and novel features of the present
invention will become apparent from the following detailed
description of the invention when considered in conjunction with
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 shows a side plan view of an embodiment of the
invention and a coal bed methane well with the spectrometer located
at the wellhead and transmission of optical radiation using fibers
to a downhole probe;
[0027] FIG. 2 shows a side plan view of another embodiment of the
invention and a coal bed methane well with the spectrometer located
in a housing lowered down the well;
[0028] FIG. 3 shows a sectional view of an embodiment of the
housing with a flow passage for liquid or gas analysis;
[0029] FIG. 4 shows a sectional view of an embodiment of the
housing with a non-contacting sample interface;
[0030] FIG. 5 shows a sectional view of an embodiment of the
housing with a head-space for gas analysis;
[0031] FIG. 6 shows a sectional view of an embodiment of the
housing with an off axis sample interface pressing to a side of the
borehole;
[0032] FIG. 7 shows a sectional view of an embodiment of the probe
with a fiber optics;
[0033] FIG. 8 shows a sectional view of an embodiment of the probe
with a sample interface pressed against the side of the
borehole;
[0034] FIG. 9 shows a sectional view of an embodiment of the probe
with the spectrometer located downhole and a sample interface as a
fiber-optic bundle pressed against the side of the borehole;
[0035] FIG. 10 shows a sectional view of an embodiment of the probe
with a flow passage and fiber-optic tip as the sample interface;
and
[0036] FIG. 11 shows a sectional view of an embodiment of the probe
with a fiber-optic optical pathway.
DETAILED DESCRIPTION OF THE DRAWINGS
[0037] FIG. 1 shows a coal bed methane well 1 with a borehole 3
extending from a well head to a coal seam 10 with an aquifer fed
water level 9. The spectrometer 4 is located at or near the
wellhead and includes a radiation source 5 for producing a
radiation to transmit down the borehole 3 to a sample interface 25.
The radiation from the radiation source is transmitted by way of at
least one optical pathway 7. The sample, in this case being water,
interacts with the radiation transmitted from the radiation source
5, and a characteristic radiation for the sample is produced by the
interaction. The characteristic radiation is then transmitted by an
optical pathway 7 to a detector 6 located in the spectrometer 4 at
the surface. A suitable optical pathway 7 for transmission is
optical fiber 8. Similar elements are represented by the same
reference numeral in the drawings.
[0038] The optical fiber 8 extends down the borehole 3 to the
housing 12 and feeds into the housing through a high-pressure
feed-through jacket 18. The jacket 18 allows the fiber 8 to enter
the housing 12 without subjecting the housing to the conditions
down the well, such as high pressure, particles and the water. The
housing protects any filter 14 or other instrumentation enclosed by
the housing. The fiber 8 may extend out of the housing through
another jacket 18 to optically couple the sample or substance of
interest. A tip 15 of the fiber 8 supplies the radiation from the
radiation source 5 and collects the characteristic radiation.
[0039] The optical fiber 8 may be a bundle of fibers where the
center fiber transmits the radiation from the radiation source 5
and the other fibers transmit the characteristic radiation. A
single collection fiber for the characteristic radiation may also
be used. The fiber 8 may also include a lens. The fibers use a
polished tip or fused tip.
[0040] The sample interface includes an inlet 16 and an outlet 17
for the water in the well. The water flows into the inlet when the
housing is positioned down the well at a depth and flows around the
tip 15 of the fiber to thereby interact with the radiation from the
radiation source 5.
[0041] In a preferred embodiment shown in FIG. 2, the spectrometer
4 is located down the well 1 in a housing 12, thus reducing the
effects of the long distance transmission of the radiation. The
spectrometer 4 is lowered down the borehole 3 by a guide wire 21 to
a depth, and the depth is controlled by a guide controller 20 at
the surface 2.
[0042] This embodiment shows the radiation source 5 providing
radiation by an optical pathway 7 which is not a fiber. The
radiation is directed to a beam splitter 23 and through a window 24
to interact with the sample or substance of interest. The emitted,
reflected or scattered radiation is then transmitted through the
window 24 into the interior and through the beam splitter 23 to the
detector 6.
[0043] In this embodiment, no moving parts are present in the
housing 12. This allows for increased sensitivity and accuracy.
[0044] The guide wire 21 may be a wireline, a slick line, coiled
tubing, drill stem or other type of guide. The guide wire is
provided for positioning the housing down the well and may also
transmit a signal to a data recorder or other processor at the
surface. If the signal is not transmitted by the guide wire, a
signal or data storage device is needed in the housing. The guide
wire may also furnish electrical power to the instrumentation
located in the housing, or a battery may be located in the
housing.
[0045] FIGS. 3-6 show embodiments of the housing 12 with the
spectrometer 4 enclosed therein, when used with a guide wire 21.
FIG. 3 shows a flow passage for the sample interface where the
radiation source 5 provides an incident radiation through a window
24 to interact with water. The characteristic radiation is
transmitted through another window 24 to the detector 6. The
characteristic radiation passes through filters 14 before the
detector 6. The housing 12 itself may be streamlined 26 to provide
for smooth passage of the housing down the well.
[0046] FIG. 4 shows a housing 12 designed for a non-contacting
sample interface at the tip of the housing. Here the radiation
source 5 produces radiation which is transmitted by an optical
pathway 7 to a reflector or grating 27 to direct the radiation
through a window 24 at the tip of the housing. The radiation
interacts with the sample or substance of interest a distance away
from the window 24. The characteristic radiation is then
transmitted through the window 24 and to a reflector or grating 27
to direct the characteristic radiation to the detector 6.
[0047] FIG. 5 shows a confocal arrangement for the housing 12. The
radiation source 5 provides radiation directed to a beam splitter
23 which reflects the radiation to a lens 30 and through a window
24 into a head-space 31. The characteristic radiation travels to
the beam splitter 23 and to another filter 14 and other lens 30 to
the detector 6.
[0048] The sample interface includes the head-space 31 which
entraps gas produced by a depressurization of water in the flow
passage. A plunger 33 or other device is used to depressurize the
water. The head-space 31 collects the gas for measurement and
analysis. Gates 32 are provided which allow the water to flow into
the housing and then isolate the water from the well to allow for
depressurization.
[0049] FIG. 6 shows an off-axis spectrometer 4 configuration. The
radiation source 5 is off-axis from the well and face of the
borehole 3. The radiation source 5 provides a radiation down an
optical pathway 7 through a lens 30 and window 24 onto a sample or
substance of interest. The characteristic radiation travels through
the window 24, another lens 30 and a filter 14 to the detector 6.
The housing 12 has an adjustable device to press the housing to the
side surface of the borehole. An extendable leg 36 is provided that
by a controller 37 moves out from the housing 12 and contacts the
side surface of the borehole opposite the window 24 and thereby
moves the housing 12 towards the opposite side of the borehole. The
confocal, off axis and non-contacting optics arrangements may be
interchanged.
[0050] FIGS. 7-11 show embodiments of the housings 12 where fiber
optics 8 are employed as at least a portion of the optical pathway
7. FIG. 7 shows a housing 12 as a probe where the spectrometer is
not located in the housing. An optical fiber 8 supports the probe
and positions the probe along the wellbore. A high-pressure
feed-through jacket 18 is used to allow the fiber 8 to enter the
housing 12 where filters 14 or other dispersive elements are
arranged. The fiber 8 exits the housing and the sample interface is
a tip 15 of the fiber 8.
[0051] FIG. 8 shows the use of fiber 8 with an adjustable device
for pressing the sample interface against the side surface 11 of
the wellbore. A bag 40 is expanded by a controller 41 against the
opposite side surface of the borehole to thereby press the tip 15
of the fiber 8 against or into the side surface of the
borehole.
[0052] FIG. 9 shows the use of fibers where the spectrometer 4 is
located in the housing 12. The radiation source 5 provides
radiation to the fiber 8 which transmits it to the sample by way of
a jacket 18. A return fiber 8 is adjacent or abutting the first
fiber at the sample interface and extends through the jacket 18 to
the detector 6. The housing 12 also has an extendable leg 36 and
controller 37 for pressing the housing 12 to the side surface
11.
[0053] FIG. 10 shows a fiber optic extending down the well and
entering a housing 12 with a flow passage. A filter 14 or other
dispersive elements are enclosed in the housing 12 and protected
from the well environment. The fiber-optic tip 15 protrudes through
a jacket 18 into the flow passage. The flow passage includes an
inlet 16 with a filter 45 to filter particulates and other
entrained material in the water and an outlet 17.
[0054] FIG. 11 shows a fiber 8 optical pathway which enters the
housing 12 and provides the transmitted radiation to a filter 14 or
other dispersive element, lens 30 and window 24.
[0055] Optical spectrometers of utility for this method include,
but are not limited to, Raman spectrometers, Fourier Transform
Raman spectrometers, infrared (IR) spectrometers, Fourier Transform
infrared spectrometers, near and far infrared spectrometers,
Fourier. Transform near and far infrared spectrometers, ultraviolet
and visible absorption spectrometers, fluorescence spectrometers,
and X-Ray spectrometers. All other spectroscopies which operate by
observing the interactions and/or consequences of the interactions
between naturally-occurring, deliberately-induced, and/or
accidentally-induced light and matter are also of utility for this
method.
[0056] For the spectrometer employing reflected, emitted or
scattered characteristic radiation, a Raman spectrometer, a near IR
spectrometer, a IR spectrometer, a UV/Vis spectrometer or
fluorimeter is suitable for characterizing the side surface of the
borehole.
[0057] Previously, using spectrometers to measure dissolved methane
in water or embedded methane at a remote location like a wellhead
was not thought possible. With the advent of portable and
inexpensive yet highly accurate spectrometers, the measurement of
dissolved methane in water is possible. In some cases the spectrum
used to analyze the material of interest may be obscured or blocked
to some extent by the medium in which it is found. In the case of
coal bed methane, the water and entrained particles may cause
significant interference with any measurement of the dissolved or
embedded methane. Certain steps may be taken to ensure a more
accurate analysis of the methane.
[0058] Data correction, filters and steps to improve the signal of
the spectrometer and methane may be used to accurately measure the
methane concentrations. Methane has a characteristic peak or peaks
in the scattered or returned optical spectrum. By adjusting filters
and any data correction equipment to the expected methane peak, the
dissolved methane may be more accurately measured. Another way of
correcting for the interference of water or other entrained
material is to adjust or select the wavelength of the radiation
used to decrease the effects of the water and entrained material
and increase, the returned signal due to the methane. The
wavelength may also be adjusted or selected to alleviate the
effects of the length of the optical pathway. The length of the
optical pathway from the spectrometer to the coal bed formation may
be 10,000 feet. The great length of pathway will result in
increased errors associated with the optical pathway. Means to
adjust or correct the laser radiation or returned radiation from
the sample may be employed at any location in the measurement
system.
[0059] In an embodiment of this method, the spectrometers are
physically located outside of the water, while sampling probes are
introduced into the samples of interest. Such probes provide
optical pathways via which interactions between light and matter
are observed. In some cases, such probes also deliver the photons
which interact with the matter. The probes used may have a lens to
focus the source or characteristic radiation or filters to adjust
the return spectrum radiation for any flaws in the system or
extraneous signals. The probes may need armoring or other means for
protecting the probe due to the pressure and other conditions of
the well. The optical pathway or fiber optics may also need
protection from the conditions of the well.
[0060] When the probe is located extreme distances from the
spectrometer, such as down a well, corrections must be employed to
correct for the inherent errors due to the distance the source
radiation and spectrum radiation must travel. One way is to allow
for longer periods of sampling in order to receive several
spectrums added together to analyze the methane present. Another
way is to adjust the signal or radiation through a filter or
correction device to allow correction feedback to adjust the return
spectrum for flaws and errors associated with the radiation
traveling such distances.
[0061] In another embodiment of this method, the spectrometers are
physically introduced into the water so as to be near the samples
of interest. This manifestation provides an unexpected benefit in
that delivery of photons to the samples and observation of
interactions between light and matter are facilitated by the
physical proximity of the spectrometers and the samples.
[0062] Both embodiments may also use error correction devices such
as dark current subtractions of the return signal to correct for
inherent system noise and errors. The systems may also use a
technique of calibrating the source radiation and spectrum signal
to assure an accurate methane concentration measurement. Such
techniques may include data processing for comparing the signals to
known spectrum signals. In order to calculate the concentration of
methane any of the known techniques of calculating the
concentration from a spectrum may be used. A preferred method is
partial least squares or PLS to calculate concentrations.
[0063] In order to realize a preferred embodiment of this method,
it is necessary to interface the spectrometers to the samples of
interest. Interfacing the spectrometers and the samples can occur
in several ways. Examples of those ways include, but are not
limited to: direct optical coupling of the spectrometers and
samples using light-guide devices; optical coupling of the
spectrometers and chemicals which result from physically treating
the samples; optically coupling of the spectrometers and chemicals
which result from chemically treating the samples; and optically
coupling of the spectrometers and chemicals which result from
biologically treating the samples.
[0064] One manner of direct optical coupling of the spectrometers
and samples using light-guide devices includes, but is not limited
to, optical coupling of the interactions between light and matter
via fiber optic devices. This manifestation provides an unexpected
benefit in that delivery of photons to the samples and observation
of interactions between light and matter occur with high throughput
directly to the samples in some cases.
[0065] A preferred manner of optical coupling is by way of direct
transmission of the radiation from the spectrometer to the sample
via lenses, filters and/or windows, and the direct transmission of
the characteristic radiation from the sample to the detector by way
of filters, windows and/or lenses. This reduces the effects of long
distance transmission through fiber optics and facilitates the
close proximity of a spectrometer and sample.
[0066] The filters used may be placed along the optical pathways of
the spectrometer. The filters or dispersive elements, collectively
filters, may be wavelength selectors, bandpass filters, notch
filters, linear variable filters, dispersive filters, gratings,
prisms, transmission gratings, echelle gratings, photoacoustic
slits and apertures.
[0067] In order for the spectrometers to withstand the conditions
particular to wellbores, such as high pressure, low or high
temperature, corrosive liquids and dissolved solids, for example,
it is preferable to enclose the spectrometers in containers which
protect them from such conditions. This novel method provides
significant advantages over the prior art in that the enclosed
spectrometers can then be introduced directly into the wellbore.
This method allows, but does not require, realization of the
benefit described by the direct interfacing or coupling of the
samples and spectrometers.
[0068] In order to interface the spectrometers and the samples
using such light-guide devices in the wellbore, it is necessary to
design the interface in such a way that is suitable for the
conditions particular to sampling environment, such as high
pressure, low or high temperature, and dissolved solids, for
example. The interface must withstand those and other conditions.
One manifestation of such an interface for a fiber optic probe
includes, but is not limited to, a high pressure feed-through
jacket which interfaces between the conditions present in the
enclosed spectrometer and those present in the wellbore. Such a
jacket provides significant advantages in that using such a jacket
direct optical coupling of the spectrometers to the samples becomes
possible.
[0069] Methods of achieving optical coupling of the spectrometers
and chemicals which result from physically treating samples
includes, but is not limited to, introduction of the samples into a
portion of the enclosed spectrometers. That portion is then
physically affected so that treatment of the samples is achieved to
give a chemical suitable for gas phase analysis via an optical
pathway using one or more spectrometers. Such physical treatments
include, but are not limited to, depressurization of the samples to
release gas into a predefined head-space portion of the enclosure.
That head space is then analyzed via optical pathways using one or
more of the spectrometers. This method provides an unexpected
benefit in that gas-phase energy spectra of chemicals are typically
comprised of much higher resolution characteristics than the
corresponding liquid-phase spectra. Thus, delineation of complex
mixtures of gases, such as methane and water, is facilitated using
this method.
[0070] The water located in the coal bed formation is considered to
be stable or at equilibrium. The drilling of the well may agitate
the water and may cause clouding or fouling of the water. In some
circumstances the effects of the drilling and preparation of the
well may be to artificially effect the concentration of the methane
in the water and surrounding coal formations. Ways to correct the
analyzed water may be employed to more accurately reflect the true
methane concentration of the formation at equilibrium. A simple way
is to allow the well to come back to an equilibrium after drilling
or disturbance. Also, the probe or instrument package that contacts
the water in the coal bed formation may be streamlined or
controlled to allow for a smooth traverse in the water. The
locations of measurement in the well may also alleviate the effects
of destabilized water/methane concentrations. By analyzing the
water at the top of the formation first, and then continue with
measurements down the well will effect the water equilibrium less
when measured before traversing the probe or package in the water
to be analyzed. A filter may also be used to strain the water or
sample.
[0071] In order to accurately predict the capacity and the
production of a coal bed methane formation by optical analysis, the
well must be drilled to an appropriate depth. The depth of the
water table, if present, the depth of the top of the coal seam and
the bottom of the coal seam are recorded. The well head must be
prepared to receive the probe or instrument package. The probe must
be coupled to the fiber-optic cable. The fiber-optic cable is
coupled to the spectrometer that contains the light source,
dispersion element, detector and signal processing equipment and
ancillary devices. The computer that serves as an instrument
controller, data collection and manipulation device is connected to
the spectrometer system. The system (computer, spectrometer,
detector and laser) are powered and the laser and operation
equipment are allowed to reach an operating temperature. The
detector is then cooled to operating temperature. The probe or
instrument package is lowered into the well through the well head
until the probe or package reaches the water table. The source or
laser emits a radiation and the radiation is directed into the
optical pathway or fiber-optic cable. The fiber-optic cable
transmits the radiation down the well to the probe. The probe emits
the radiation onto the sample of interest. The probe may contain a
lens or lenses to focus the radiation onto the sample at different
distances from the probe. The radiation interacts with the sample
and causes the sample to reflect, scatter or emit a signature or
characteristic radiation or spectrum. The spectrum or
characteristic radiation is transmitted through the probe and
optical pathway to the spectrometer. The spectrometer detects the
spectrum or characteristic radiation and analyzes the spectrum for
characteristic methane peaks or peak. The spectrometer then outputs
information to the data processor to be manipulated into
information to be used to calculate the concentration and potential
production of methane.
[0072] During the analysis an initial spectrum is taken at the
depth of the water table. The fluorescence is measured and, if the
fluorescence is high, the source radiation wavelength may be
adjusted or selected to mitigate the fluorescence. If particulates
are present and the noise level from them is high, a different
focal length may be chosen to mitigate the noise level. The
integration time for the detectors is chosen to maximize the
signal. A dark current spectrum is taken with the shutter closed
such that no light reaches the detector. The dark current the noise
that is present in the system mostly due to thermal effects. This
intensity is subtracted from each spectrum to lower the noise
level. The number of co-additions is chosen to balance signal and
time constraints. The co-additions will improve the signal to noise
but will increase the time for each measurement. The probe or
package is lowered to the top of the coal seam and a spectrum is
taken. The probe is again lowered and a spectrum is taken at
regular intervals of depth until the bottom of the well is reached.
The measurements show a concentration of methane in accordance with
depth in the well. By correlating the concentration of methane in
the well with other data, the capacity of the coal bed formation or
seam can be calculated. The probe is then retracted and the well
head sealed.
[0073] This embodiment of the invention details the technical
details surrounding the use of three different optical spectrometer
systems capable of identifying and quantitatively analyzing coal
bed methane formations. This embodiment centers around development
of an instrument package capable of detecting the chemical
signatures of dissolved methane and other gases in water and
detecting embedded or trapped methane in subsurface coal seams,
both from a lowered instrument, package and from a fixed monitoring
site. Such optic-based instruments are suitable for complex
analysis of the physical and chemical properties of dissolved
methane and similar formations in the wellbore environments.
[0074] In these cases, the instruments themselves are packaged and
adapted to the conditions prevalent in these environments, and the
formations are examined in the natural state or after suitable
treatment. This provides direct access to the chemistry and geology
of the formations to an extent unavailable from core-sampling
techniques.
[0075] At least three types of spectrometers are suitable for
wellbore remote sensing of methane. The first two spectrometers,
UV/Vis and near IR, are particularly suitable for head-space
sensing of gases released after depressurization of the coal bed
samples. UV/Vis spectroscopy provides data relating to the
molecular absorption properties of the water. Depending on
experimental concerns, this data may contain information regarding
the identity and concentration of dissolved hydrocarbon gases.
Regardless, though, it contains information related to choosing the
proper laser excitation wavelength for the Raman spectrometer. Near
infrared (NIR) spectroscopy has been widely used to remotely
characterize complex gas mixtures. In this case, the NIR
spectrometer provides data related to the structure and bonding of
the gas samples. If the spectrometer resolution is sufficient, that
data contains sufficient information to allow deconvolution of very
complex samples.
[0076] Both of the above spectrometers require substantial fluid
handling to be integrated into the sensor or instrument package.
This results in slower collection times and, for the lowered
instrument package, a lower spatial resolution for the data, when
compared to directly coupled in-situ methods. On the other hand,
Raman spectroscopy is performed using state-of-the-art
high-pressure probes, allowing rapid chemical analysis of water and
methane with no additional hardware.
[0077] Raman spectroscopy detects the identity and concentration of
dissolved hydrocarbon gases and embedded hydrocarbon gases. The
Raman scattering of typical materials is quite low, producing
significant signal-to-noise problems when using this type of
spectroscopy. However, symmetric molecules including methane show
very strong scattering. This moderates signal-to-noise concerns to
some extent.
[0078] Again, all three spectrometers are refitted to suitable
pressure tube specifications. The tube-bound spectrometers will be
immersed to suitable depths on available well equipment or located
adjacent the well, and the data is collected using existing data
translation protocols. The data bandwidth for all three instruments
is relatively low ca. 50 KB per minute is a reasonable rate
(dependent to some extent on the signal-to-noise concerns).
UV/Vis Spectrometer
[0079] Because UV/Vis spectrometers are based on low intensity,
white light sources, the use of focused optic probes (such as fiber
optics) in this case is not appropriate. Such spectrometers are
more suited to gas analysis of the head space created after
depressurization of a sample. Thus, in order to use the UV/Vis
spectrometer for methane analysis, mechanized fluid controls are
preferred.
[0080] An automated fluid decompression chamber that can be filled,
depressurized, analyzed, and evacuated on a continual basis at the
well depth of interest is provided. Depressurization of the chamber
releases the dissolved hydrocarbon gases into the resultant vacuum
where they are efficiently and quickly analyzed by the UV/Vis
spectrometer. Evacuation and flushing of the chamber is followed by
another cycle.
[0081] Some issues of concern using this type of spectrometer are
developing the appropriate optical path for analysis, avoiding
fouling of the chamber and optical windows by water-borne chemicals
and bio-organisms, and establishing the appropriate
temperature/pressure conditions for data collection. Corresponding
solutions are multiple reflection collection geometries which
afford very high sensitivities, proper introduction of
anti-foulants to the chamber during flushing, and laboratory
correlation of the entire range of available pressure/temperature
collection conditions to resulting data quality.
[0082] Doing such head-space analysis also provides a convenient
method for the sensor platform to analyze chemically gas bubbles
resulting from dissolution, cavitation or mixing, which would not
otherwise be suitable for analysis. For example, diversion of
captured gas into the head-space through appropriate valves
provides the opportunity for direct UV/Vis and NIR analysis of the
emitted gases.
Near IR Spectrometer
[0083] Near IR and Raman spectrometers detect the identity (i.e.
molecular bonding) and concentration of dissolved and embedded
hydrocarbon gases. Near IR analysis, widely used for quality
control in industrial processes, typically gives moderate signals
with sufficient information (i.e. overtones of the vibrational
bands) to treat very complicated samples. Near IR spectrometers may
be used for head-space analysis. Allowing multiple reflections of
the beam through the cell (and thus multiple passes of the beam
through the sample) provides the unexpected benefit of increasing
the signal-to-noise ratio of the data. Direct optical coupling of
near IR spectrometers to the samples is also preferred.
Raman Spectrometer
[0084] Raman spectroscopy is widely used for in-situ analysis of
water-borne samples because water does not have a strong
interaction with typical Raman laser energies. The Raman
spectrometer is based on traditional grating optics, and thus
enjoys a high throughput of light.
[0085] Spectroscopic capabilities are maximized by, in some cases,
using a fiber-optic probe sampling motif based around a filtered,
six-around-one fiber-optic probe. The six-around-one fiber-optic
probe allows for a safe, fully-sealed optical feed-through from the
pressure vessel to the water. This design removes the elaborate
fluidics necessary for the other two spectrometers.
[0086] Until recently Raman spectroscopy would never have been
considered as an in-situ probe due to the large size of available
Raman systems and their high power consumption. High efficiency
diode lasers and charge-coupled device (CCD) detection, along with
better filter technology have made it possible to miniaturize Raman
spectrometers and decrease power consumption. Fiber-optic probes
have eliminated the complex sampling arrangements that once made
Raman spectroscopy difficult and tedious.
[0087] A long output wavelength often provides useful spectra from
samples that produce interfering fluorescence at lower wavelengths.
Even at these longer wavelengths, inorganic vibration shifts that
are commonly 400 to 1000 cm-1 wave numbers shifted in wavelength
are still near the peak sensitivity of CCD detectors but with the
added advantage of a significant reduction in the background
fluorescence interference present in many samples. A preferred
embodiment uses laser wavelengths which avoid to a reasonable
extent any fluorescence characteristic of the sample.
[0088] Usually fluorescence is mitigated by providing a laser with
a wavelength above the fluorescence. In a preferred embodiment a
wavelength of 450 nm to 580 nm is provided from a diode laser. This
range is below the wavelength of fluorescence of coal. The shorter
wavelength is used to decrease the radiation from the coal and
increase the relative radiation from the methane embedded or
adsorbed on the coal.
[0089] Remote sampling is accomplished in some cases using a
six-around-one probe. The epi-illumination probe incorporates one
excitation and six collection fibers. This probe allows direct
measurement of Raman of dissolved hydrocarbons in water without
having to transmit through thick, non-quality optical window ports.
High pressure feed-throughs are available for this probe.
[0090] Measurements of spectroscopic signatures of water-dissolved
hydrocarbons in the laboratory show an energy diagram of the known
spectroscopic signature regions of simple hydrocarbons, and the
regions interrogated by the three spectrometers considered herein.
Thus, all three spectroscopies provide information relevant to the
hydrocarbon identity and concentration.
[0091] However, the UV/Vis bands typical for these hydrocarbons are
NOT strongly characteristic many compounds absorb in the energy
region between 0 and 250 nm. Correlation of the UV/Vis results with
those from the Raman and/or near IR leads to detailed chemical
analysis. As well, the UV spectrometer must operate in the region
where the methane transition occurs.
[0092] The detectors used with the spectrometer system are
important. To obtain high sensitivity and reduce interference from
other substances a CCD type detector is preferred. The
charge-coupled device detector allows for only a small portion of
the spectrum to be analyzed. Other detectors include
photomultiplier tubes, photo-diode arrays, CMOS image sensors,
avalanche photo diodes and CIDs.
[0093] The measuring system may be supplied with power by the guide
wires or by internal batteries.
[0094] In order to predict or measure a potential production from a
coal bed methane field, a series of wells is measured. Taking
measurements of methane or other substances of interest at a single
well and at varying depths down the well provides a concentration
of methane versus depth for the well. This indicates the presence
and amount of methane in the subsurface zones or strata. By
similarly measuring other wells in the coal bed methane formation
or field a dimensional plot of methane is obtained. From this the
transport of methane, production zones and extent of methane
bearing zones is obtained.
[0095] The foregoing disclosure has been set forth merely to
illustrate the invention and is not intended to be limiting. Since
modifications of the disclosed embodiments incorporating the spirit
and substance of the invention may occur to persons skilled in the
art, the invention should be construed to include everything within
the scope of the appended claims and equivalents thereof.
* * * * *