Patent Grant
9829352
The present invention relates to a system for measuring simultaneously distributions of pressure, temperature, and strain of material by mean of Brillouin frequency shift and Rayleigh frequency shift phenomena in an optical fiber, and further relates to a method of monitoring and measuring characteristics of extensive material such as geological strata using the system.
There are known various measuring techniques that use the Brillouin scattering phenomenon in an optical fiber (for example, Patent Document 1). One of the techniques is a distributed pressure sensor that utilizes a Brillouin frequency shift caused by strain applied to an optical fiber. Because the Brillouin frequency shift depends on strain applied to the optical fiber, pressure applied can be measured by analyzing the frequency shift of the optical fiber that is fixed to a material deformable by pressure.
The present inventors have already proposed a system for measuring mainly pressure and temperature distributions by mean of the Rayleigh frequency shift phenomenon as well as the Brillouin frequency shift phenomenon of an optical fiber (refer to Patent Document 2). Since this system aims to measure distributions of pressure and temperature and the optical fiber is not fixed to an object to be measured, strain measured by this system is of no use.
The pressure measurement technique using an optical fiber is applicable to volume change measurement of an object. For example, porous sandstone, because it changes in volume before and after containing liquid, is one application target of the above pressure measurement technique. Technologies for geologically sequestrating carbon dioxide are being developed recently as global warming countermeasures. In cases of implementing carbon dioxide geological sequestration, the foregoing pressure measurement technique can contribute to construction of systems for monitoring a sequestrated state of carbon dioxide in a sandstone and for monitoring geomechanical integrity and safety of a cap rock stratum (such as an argillaceous rock) that is the upper stratum on the sandstone.
Patent Document 1: WO2006/001071 A1
Patent Document 2: JP 2010-216877 A
However, a method of accurately detecting, for example, a state change of underground strata has not been proposed yet. Using electrical pressure sensors allows for detecting pressure changes at some spots. However, it remains unrevealed what relationship exists between the pressure changes and a deformation observed on the ground surface and whether the geomechanical integrity is retainable if the deformation occurs on the ground surface.
The present invention is made in light of the above-described problem, with an object of providing a system that is capable of measuring simultaneously distributions of pressure, temperature, and strain of material and thereby monitors and evaluates accurately the state of an extensive material such as under the ground.
A measurement system according to the present invention includes a scattered wave acquisition unit for acquiring optical fiber scattered waves of pulse laser light entered into optical fibers set up in or along a material; a Brillouin frequency-shift measurement unit for measuring distribution of a Brillouin frequency shift in the optical fiber from the scattered waves; a Rayleigh frequency-shift measurement unit for measuring distribution of a Rayleigh frequency shift in the optical fiber from the scattered waves; a coefficient storage unit for storing coefficients that are inherent to the optical fibers set up and correlate pressure, temperature, and strain of material with the Brillouin frequency shift and the Rayleigh frequency shift; and an analyzer unit for analyzing distributions of pressure, temperature, and strain of the material along the optical fibers at a measurement time point, using the Brillouin frequency-shift distribution measured by the Brillouin frequency-shift measurement unit, the Rayleigh frequency-shift distribution measured by the Rayleigh frequency-shift measurement unit, and the coefficients stored in the coefficient storage unit.
According to the present invention, a system can be provided that is capable of measuring accurately and simultaneously distributions of pressure, temperature, and strain of material and thereby monitors and evaluates accurately the state of an extensive material such as under the ground.
A sensor cable 2a is buried along in the column of the cementing 34 to measure distributions of pressure P, temperature T, and strain ε of the underground strata along the injection well 3a. Moreover, a sensor cable 2b may be buried along in the column of the cementing along the observation well 3b. Hereinafter, the sensor cable 2a and the sensor cable 2b are collectively referred to as a sensor cable 2.
When a volume change occurs in the surrounding strata, the volume change affects the sensor cable 2 because it is buried in the column of the cementing 34. For example, when the strata are deformed by the carbon dioxide injection, the sensor cable 2 is subjected to the deformation together with the column of the cementing 34. In that case, the first optical fiber 21 receives and detects pressure of the deformed strata while the second optical fiber 22 accommodated in the fine metal tube 24 is unaffected.
For each of the first optical fiber 21 and the second optical fiber 22 thus configured, Brillouin measurement and Rayleigh measurement are performed by a measurement system 1 installed on the ground surface, to obtain distributions of Brillouin frequency shift and Rayleigh frequency shift along the optical fibers. From these distributions of Brillouin frequency shift and Rayleigh frequency shift, distributions of pressure, temperature, and stain along the sensor cable 2 can be determined simultaneously. For that reason, the inventors named the measurement system 1 as “distributed pressure temperature strain system (DPTSS) 1”.
Here, the principle of measuring distributions of pressure, temperature, and strain by mean of an optical fiber will be described. When light is entered into an optical fiber and a frequency analysis is performed for the scattered light, there are observed Rayleigh scattered light having substantially the same frequency as the entered light, Raman scattered light having a frequency largely different from the entered light, and Brillouin scattered light having a frequency different from the entered light by a few to about several tens GHz.
The Brillouin scattering phenomenon is a phenomenon caused by power transfer via acoustic phonon in an optical fiber when light is entered thereinto. The frequency difference between the entered light and the Brillouin scattered light is referred to as Brillouin frequency. The Brillouin frequency is proportional to sound velocity in the optical fiber and the sound velocity depends on strain and temperature of the optical fiber. Hence, measurement of Brillouin frequency change allows for measurement of strain applied to and/or temperature of the optical fiber. Moreover, the present inventors have confirmed that the Brillouin frequency also changes with pressure applied to the optical fiber. Here, the Brillouin frequency change is referred to as Brillouin frequency shift.
The Rayleigh scattering phenomenon is a phenomenon caused by light scattering due to variation of the refractive index of an optical fiber. The frequency difference between the entered light and the Rayleigh scattered light is Rayleigh frequency. The Rayleigh frequency also changes with strain applied to and/or temperature of the optical fiber. Here, the Rayleigh frequency change is referred to as Rayleigh frequency shift.
Conventionally, the Rayleigh scattering phenomenon has been considered as having sensitivity to strain and temperature only. In Patent Document 2 disclosed earlier by the present inventors, a system is proposed on the basis that the Rayleigh scattering phenomenon has sensitivity to strain and temperature only. As a result of a subsequent study of the present inventors, it turned out that the Rayleigh scattering phenomenon also has sensitivity to pressure in addition to strain and temperature, as with the Brillouin scattering phenomenon. That is, the Brillouin frequency shift ΔνB and the Rayleigh frequency shift ΔνR can be expressed as equations (1) and (2), respectively, with a pressure change amount ΔP, a temperature change amount ΔT, and a strain change amount Δε.
ΔνB=C13ΔP+C12ΔT+C11Δε (1)
ΔνR=C23ΔP+C22ΔT+C21Δε (2)
where Cij are coefficients inherent to an optical fiber. By determining values of these coefficients such as through a preliminary test for an optical fiber to be used, distributions of the pressure change amount ΔP, the temperature change amount ΔT, the strain change amount Δε can be calculated as described below. Thus, introduction of the pressure term into the Rayleigh scattering shift ΔνR allows for measuring distributions of pressure, temperature, and strain with higher accuracy.
Now, it is assumed that a Brillouin frequency shift ΔνB and a Rayleigh frequency shift ΔνR are measured. In order to separate influences of pressure P, temperature T, and strain ε in the measurement values, three or more independent measurement quantities are required. Because only two independent measurement values: a Brillouin frequency shift ΔνB and a Brillouin frequency shift ΔνR are obtained from one optical fiber, four independent measurement values can be obtained by using two kinds of optical fibers each having a different sensitivity to pressure P, temperature T, and strain ε. That is, the following simultaneous equations (3) are obtained.
ΔνB1=C131ΔP+C121ΔT+C111Δε1
ΔνR1=C231ΔP+C221ΔT+C211Δε1
ΔνB2=C132ΔP+C122ΔT+C112Δε2
ΔνR2=C232ΔP+C222ΔT+C212Δε2, (3)
where the superscript numerals designate the kinds of optical fibers. Since the pressure and the temperature are those of a field where the optical fibers are located, the two kinds of optical fibers have the same values. On the other hand, the strain value depends on whether or not the optical fiber is fixed to the surrounding material. Since DPTSS needs to measure strain of material around the fiber, at least one fiber must be fixed to the surrounding material.
By solving the above simultaneous equations (3), the influences of pressure P, temperature T, and strain ε can be separated. Accordingly, by performing a hybrid measurement of Brillouin frequency shift measurement (referred to as Brillouin measurement) and Rayleigh frequency shift measurement (referred to as Rayleigh measurement) and by solving the simultaneous equations (3), there can be determined the distributions of the pressure change amount ΔP, the temperature change amount ΔT, and the strain change amount Δε along the optical fibers.
Designating the first optical fiber 21 and the second optical fiber 22 in
ΔνB1=C131ΔP+C121ΔT+C111Δε1
ΔνR1=C231ΔP+C221ΔT+C211Δε1
ΔνB2=C122ΔT+C112Δε2
ΔνR2=C222ΔT+C212Δε2, (4)
because the second optical fiber 22 is free from influence of pressure.
Also in the equations (4), since the pressure and the temperature are those of the field where the optical fibers are located, the two kinds of optical fibers have the same values. As for the strain, on the other hand, strain ε1 experienced by the first optical fiber fixed to the surrounding material is different from strain ε2 experienced by the second optical fiber accommodated in the fine metal tube. Although there are four unknown quantities: ΔP, ΔT, Δε1, Δε2, there are also the four equations. Hence, these four unknown quantities can be determined. Note that a value useful as strain is the strain ε1 of the first optical fiber directly experiencing the strain of the surrounding material.
Each coefficient Cij in equations (4) are beforehand determined for the first and second optical fibers such as through a preliminary test. Then, the hybrid measurement of Brillouin measurement and Rayleigh measurement are performed and the simultaneous equations (4) are solved. In this way, the distributions of the pressure change amount ΔP, the temperature change amount ΔT, and the strain change amount Δε along the optical fibers can be determined. Since the hybrid measurement of Brillouin measurement and Rayleigh measurement can be performed simultaneously at any given time point, not only one dimensional distributions of the pressure change amount εP, the temperature change amount ΔT, and the strain change amount Δε along the optical fibers but also their time-series data can be obtained.
It is needed to take a notice that the equations (4) are for increment quantities. That is, determination of a Brillouin frequency shift and a Rayleigh frequency shift in the left hand side of the equations requires two measurements: a reference measurement in an initial state and an actual measurement after a change of the state. Note that respective quantities determined by solving the equations (4) are the amounts of pressure, temperature, and strain changes from the reference initial state. In a case of requiring absolute quantities of pressure, temperature, and strain, each absolute quantity distribution of pressure, temperature, and strain is measured beforehand in the initial measurement in some way.
The initial state may be chosen arbitrarily. As for monitoring the carbon dioxide geological sequestration, the initial measurement may be performed in a constant temperature room on the ground before the cables are set up in the boreholes (the injection well 3a and the observation well 3b). In that case, a state of uniform and constant distributions of pressure and temperature may be employed as the initial state.
Otherwise, a state after set up of the cables in the boreholes and before injection of carbon dioxide may also be employed as the initial state. In this case, the change amounts of pressure, temperature, and strain caused by injection of carbon dioxide can be directly obtained by solving the equations (4). In a case of requiring absolute quantities of pressure, temperature, and strain, absolute quantity distributions of pressure, temperature that are measured such as by electrical sensors at a quiescent state of the borehole before carbon dioxide injection may be used. The initial data measured in the constant temperature room on the ground allows for obtaining also absolute quantity distributions of pressure and temperature from a measurement performed before carbon dioxide injection.
Thus, acquiring absolute-quantity-distribution change data of or change amount distribution data of pressure P, temperature T, and strain ε allows for monitoring changes and distributions of pressure, temperature, and strain associated with carbon dioxide injection into the sandstone stratum 100. Thereby, for example, the state of injected carbon dioxide and leakage thereof from the cap rock stratum 150 can be monitored.
A coefficient storage unit 14 stores beforehand the coefficients Cij in the equations (4) determined in the preliminary test or the like. Using the measured Brillouin frequency shift and Rayleigh frequency shift and the coefficients stored in the coefficient storage unit 14, a pressure change amount ΔP, a temperature change amount ΔT, and a strain change amount Δε are analyzed in an analyzer unit 15 through the use of the equations (4), and stored in a distribution data storage unit 16. The above measurement and analysis are executed at predetermined time intervals, and their results are stored in the distribution data storage unit 16 as time interval change-amount distribution data of pressure, temperature, and strain. In a case where initial absolute quantity distributions of pressure, temperature, and stain are measured in the initial measurement, storage of these data in the distribution data storage unit 16 allows for obtaining absolute quantity distribution data at each time point in addition to the change amount distribution data. An evaluation calculation unit 17 evaluates the state of the sandstone stratum 100 such as from the time interval change amounts of pressure, temperature, and stain. Thereby, for example, the state of carbon dioxide storage and the like are monitored.
An example process of evaluating a sequestrated state of carbon dioxide by the system of
After completion of the setting up of the sensor cable 2, an initial measurement 2 is performed for a reference Brillouin spectrum and a reference Rayleigh spectrum, which are another references of the Brillouin frequency shift and the Rayleigh frequency shift (ST4). In a case of requiring absolute quantities of pressure and temperature, a Brillouin frequency shift and a Rayleigh frequency shift are determined from the measurement data of the initial measurement 1 and the initial measurement 2, to calculate distributions of a pressure change amount ΔP and a temperature change amount ΔT using the simultaneous equations (4). Subsequently, absolute quantity distributions of pressure and temperature in the initial measurement 2 are calculated using the pressure and the temperature of the constant temperature room in the initial measurement 1.
When carbon dioxide injection is started, a Brillouin spectrum and a Rayleigh spectrum are measured. Then, Brillouin frequency shifts Δν1B, Δν2B are determined (ST5) and Rayleigh frequency shifts Δν1R, Δν2R are determined (ST6) by taking the difference between the measured spectrum data and the measurement data of the initial measurement 2. Distributions of a pressure change amount ΔP, a temperature change amount ΔT, and a strain change amount Δε are calculated using the simultaneous equations (4) (ST7). The steps ST5, ST6, ST7 are executed in the DPTSS 1 at predetermined time intervals as described above (“NO” in ST8), and the measurement results are stored as time interval data in the distribution data storage unit 16. The pressure change amount ΔP, the temperature change amount ΔT, and the strain change amount Δε calculated here are respective amounts changed from the above-described initial measurement 2. If required time interval distributions are acquired (“YES” in ST8), the sequestrated state of carbon dioxide can be evaluated (ST9), as described later, by referencing a database of the stone properties such as porosity and permeation velocity of carbon dioxide (ST10).
A conceptual example of measurement data at some time point is illustrated in
By acquiring such data as shown in
Embodiment 2 describes an example of a laboratory experiment demonstrating that characteristics of extensive material such as geological strata can be monitored and measured by the system of the present invention for measuring distributions of pressure, temperature, and strain of material.
After the sample was placed in a pressure vessel and then applied with a confining pressure of 12 MPa, water and carbon dioxide were injected to evaluate a state change of the sample. Since the vessel was uniform in pressure and temperature and showed no distributions thereof, the state change of the sample was evaluated by determining mainly the strain distribution. In the case of the laboratory experiment, other point sensors were used to monitor the pressure and the temperature because they are uniform and constant. Thus, the unknown quantities reduce to the strain only and all of the equations (4) are unnecessary to use. Hence, the measurement can be performed with only one optical fiber.
First, water was injected to the sample to observe the water permeation process. Next, carbon dioxide was injected to the sample after the water was fully permeated, to observe the replacement process of the water with carbon dioxide.
In
Thus, it is found from the laboratory experiment described in Embodiment 2 that a strain distribution change of the sample Tago sandstone 110 can be measured using the system of the present invention for measuring distributions of pressure, temperature, and strain of material. Furthermore, by evaluating the measurement result, it is possible to monitor, for example, a geologically sequestrated state of carbon dioxide. In the laboratory experiment, there are almost no distributions in pressure and temperature, and their changes can be measured by other means. For carbon dioxide geological sequestration and the like, however, it is necessary to obtain distributions of underground pressure and temperature. The system of the present invention for measuring distributions of pressure, temperature, and strain of material using the first and the second optical fibers 21, 22 allows for obtaining distribution data of underground strain by also simultaneously measuring distributions of the pressure and temperature and using the equations (4). Therefore, the geologically sequestrated state of carbon dioxide can be observed.
Embodiment 3 describes application examples enabled by the system of the present invention for measuring distributions of pressure, temperature, and strain of material.
Embodiments 1 and 2 describes the monitoring during the carbon dioxide injection for geological sequestration. The present system can monitor abnormality after completion of the injection by continuing monitoring of the strain and the like thereafter. For example, it is conceivable that the carbon dioxide leaks through the cap rock stratum 150 from the carbon dioxide trapping zone 101 for such a reason as generation of a crack in the cap rock stratum.
A flow diagram of monitoring leakage from the carbon dioxide trapping zone 101 is shown in
Underground carbon dioxide undergoes a phase change to become a liquid state, a gas state, or a supercritical state. A change between these phase states can be monitored by measuring underground temperature changes. A process flow diagram of monitoring these phase states is shown in
Since such phase changes potentially occur, in particular, in a zone at a lesser depth than a cap rock stratum, the zone above the cap rock stratum is necessary to monitor. According to the system of the present invention, a temperature distribution from the ground surface to the sandstone stratum 100 can be measured all the time, thereby allowing for such monitoring.
A deformation of strata can be evaluated from an underground strain distribution.
As described above, according to the system of the present invention for measuring distributions of pressure, temperature, and strain of material, this measurement system alone can monitor and evaluate various underground states and ground surface states associated with carbon dioxide geological sequestration.
Furthermore, the system of the present invention for measuring distributions of pressure, temperature, and strain of material is applicable to monitoring of not only carbon dioxide geological sequestration but also, for example, an oil well and others such as a system for mining an underground resource by drilling deeply into the ground and the state of underground strata after such mines are abandoned.
The embodiments so far concern monitoring of the state of underground strata such as for carbon dioxide geological sequestration. Embodiment 4 describes a system for monitoring freezing over of a river and the like, other than that for monitoring the state of strata.
Brillouin frequency shift measurement and Rayleigh frequency shift measurement are performed by the DPTSS 1 using an optical fiber in the sensor cable 2 to determine simultaneously distributions of pressure, temperature, and strain along the sensor cable 2. The state of freezing can be checked by monitoring temporal changes of these distributions.
As has been describes above, according to the system of the present invention for measuring distributions of pressure, temperature, and strain of material, simultaneous measurement of distributions of pressure, temperature, and strain of material along an optical fiber and measurement of temporal changes of these distributions can be performed by one measurement system alone. Therefore, monitoring of the state of extensive material and the like can be performed accurately. In particular, the present invention brings about a great effect in monitoring the state of such extensive material as using a long optical fiber of more than 100 m.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2012-180725 | Aug 2012 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2013/071371 | 8/7/2013 | WO | 00 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2014/027592 | 2/20/2014 | WO | A |
| Number | Name | Date | Kind |
|---|---|---|---|
| 4427263 | Lagakos | Jan 1984 | A |
| 6380534 | Farhadiroushan | Apr 2002 | B1 |
| 6496265 | Duncan | Dec 2002 | B1 |
| 6788417 | Zumberge | Sep 2004 | B1 |
| 8699009 | Li | Apr 2014 | B2 |
| 20080068586 | Kishida et al. | Mar 2008 | A1 |
| 20100002226 | Hartog | Jan 2010 | A1 |
| 20100014071 | Hartog | Jan 2010 | A1 |
| Number | Date | Country |
|---|---|---|
| 2006-145465 | Jun 2006 | JP |
| 2009-42005 | Feb 2009 | JP |
| 2010-216877 | Sep 2010 | JP |
| 2011-053146 | Mar 2011 | JP |
| WO 2006001071 | Jan 2006 | WO |
| Entry |
|---|
| Benson, S., and Larry Myer. “Monitoring to ensure safe and effective geologic sequestration of carbon dioxide.” See Ref 105 (2002): 137-51. |
| Holloway, S. “Underground sequestration of carbon dioxide—a viable greenhouse gas mitigation option.” Energy 30.11 (2005): 2318-2333. |
| Inaudi, D., R. Belli, and D. Posenato. “Distributed sensing for damage localization.” 6th International Conference on Bridge Maintenance, Safety, and Management (IABMAS 2012). 2012. |
| International Search Report (PCT/ISA/210) dated Oct. 29, 2013, by the Japanese Patent Office as the International Searching Authority for International Application No. PCT/JP2013/071371. |
| Number | Date | Country | |
|---|---|---|---|
| 20150211900 A1 | Jul 2015 | US |
