JP5747408B2 - Object volume change measurement method - Google Patents

Description

  The present invention relates to a method for measuring volume change and distribution of an object using a Brillouin frequency shift or Rayleigh frequency shift phenomenon of an optical fiber.

  Various measurement methods using the Brillouin scattering phenomenon and Rayleigh scattering phenomenon of optical fibers are known (for example, Patent Document 1). One example is a distributed pressure sensor that utilizes a Brillouin frequency shift and a Rayleigh frequency shift that are generated when strain (pressure) is applied to an optical fiber. Since these frequency shifts depend on the strain applied to the optical fiber, the applied pressure can be measured by measuring the frequency shift amount.

  This pressure measurement technique using an optical fiber can be applied to measurement of a volume change of an object. For example, porous sandstone changes in volume before and after being filled with a fluid, and thus becomes one of the application fields of the pressure measurement technique. In recent years, as a countermeasure against global warming, technology for storing carbon dioxide in the ground is being developed. However, the above pressure measurement technology is used to store carbon dioxide in sandstone when carbon dioxide underground storage is executed. It can contribute to the construction of a system that monitors the situation, and a system that monitors the mechanical stability and safety of the cap rock layer (such as pelitic rock) that is the upper layer.

International Publication No. 2006/001071 Pamphlet

  However, for example, a method for accurately detecting a volume change of sandstone having an unknown composition existing in the ground has not been proposed yet. If an electrical pressure sensor is used, it is possible to detect a spot pressure change. However, it cannot be specified whether the pressure change is due to the fluid being pressed into the sandstone, that is, whether the pressure change depends on the volume change of the sandstone.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a volume change measurement method for an object that can accurately measure the volume change and the distribution of an object whose volume change degree and distribution are unknown. And

In the method for measuring volume change of an object according to one aspect of the present invention, pressure is applied to the at least two types of reference members from the outside to at least two types of reference members made of different materials. as to follow the deformation of the reference member with a step of fixing the optical fibers, respectively, in a state in which the optical fiber plus the known pressure from the outside to the reference member fixed, each test light to the optical fiber Performing at least one of a reference Brillouin measurement for obtaining a reference Brillouin frequency shift amount based on a Brillouin scattering phenomenon and a reference Rayleigh measurement for obtaining a reference Rayleigh frequency shift amount based on a Rayleigh scattering phenomenon; and the reference Brillouin measurement or From the results of the standard Rayleigh measurement, the volume change and the per unit pressure in each measurement A step for obtaining a coefficient for Brillouin measurement or a coefficient for Rayleigh measurement based on the relationship with the number of shifts, and an external pressure applied to the test member whose volume change is unknown A step of fixing the optical fiber so as to follow the deformation of the test member, and a test light incident on the optical fiber in a state where a known pressure is applied from the outside to the test member to which the optical fiber is fixed. Performing at least one of a test Brillouin measurement for obtaining a test Brillouin frequency shift amount based on a Brillouin scattering phenomenon and a test Rayleigh measurement for obtaining a test Rayleigh frequency shift amount based on a Rayleigh scattering phenomenon; From the Brillouin frequency shift amount or the test Rayleigh frequency shift amount and the Brillouin measurement coefficient or the Rayleigh measurement coefficient, the test Comprising determining a volume change of the timber, the (claim 1).

  According to this measurement method, the relationship between the volume change of the reference member and the frequency shift amount per unit pressure is grasped by the reference Brillouin measurement or the reference Rayleigh measurement. From this result, a Brillouin measurement coefficient or a Rayleigh measurement coefficient as calibration data is obtained. Then, a similar test Brillouin measurement or test Rayleigh measurement is performed on the test member to obtain a frequency shift amount, and by applying a Brillouin measurement coefficient or a Rayleigh measurement coefficient to this, Volume change can be determined.

In the above configuration, the step of performing the reference Brillouin measurement or the reference Rayleigh measurement is performed by causing test light to enter an optical fiber that is not fixed to the reference member, and calculating a reference Brillouin frequency shift amount or a reference Rayleigh frequency shift amount. A first measurement to be measured, and a second measurement to measure the reference Brillouin frequency shift amount or the reference Rayleigh frequency shift amount for optical fibers fixed to at least two kinds of reference members made of different materials, The step of obtaining the coefficient for Brillouin measurement or the coefficient for Rayleigh measurement includes the step of calculating the frequency shift amount per unit pressure for the reference Brillouin frequency shift amount or the reference Rayleigh frequency shift amount obtained in the first measurement and the second measurement by volume. The slope and intercept of the linear function obtained by plotting on the axis of change It is desirable that Mel step (claim 2).

  According to this configuration, at least three reference Brillouin frequency shift amounts or reference Rayleigh frequency shift amounts are obtained from the optical fiber not fixed to the reference member and the optical fiber fixed to at least two different reference members. Is required. Therefore, the relationship between the volume change and the frequency shift amount per unit pressure can be converted into a linear function, and the Brillouin measurement coefficient or the Rayleigh measurement coefficient can be obtained as the slope and intercept of the linear function. Thereby, the derivation of the coefficient can be performed easily and reliably.

  In the above configuration, the step of fixing the optical fiber to the reference member winds a part of one optical fiber around a cylindrical reference member and separates the other part of the optical fiber from the reference member. Forming the free fiber portion, and the step of performing the reference Brillouin measurement or the reference Rayleigh measurement includes, in a pressure vessel, a winding portion of the optical fiber around the reference member, and the free fiber portion. It is desirable to include an encapsulating step (claim 3).

  According to this configuration, it is possible to execute the first measurement and the second measurement in one measurement, and the measurement efficiency can be improved.

  Said structure WHEREIN: The said test member is a heterogeneous porous member, The step which calculates | requires the volume change of the said test member is a step which calculates | requires the volume elastic modulus and volume expansion coefficient of the said test member. (Claim 4).

  ADVANTAGE OF THE INVENTION According to this invention, the volume change measurement method of the object which can measure the volume change of an object whose volume change degree and its distribution are unknown, and its distribution exactly can be provided. Therefore, for example, it is possible to obtain the filling rate and distribution of fluid for porous sandstone, etc., and contribute to the construction of a system for monitoring the storage status of carbon dioxide in sandstone when carbon dioxide underground storage is executed. Furthermore, the volume change distribution of the cap lock layer functioning as a seal layer can also be measured.

It is a flowchart which shows roughly the procedure of the volume change measuring method which concerns on this invention. It is a schematic diagram which shows the measurement system about a reference member performed in a calibration process. It is a figure which shows the pressure application pattern in the measurement system of FIG. It is a graph which shows the result of standard Brillouin measurement. It is a graph which shows the result of standard Rayleigh measurement. It is a graph for demonstrating the coefficient for Brillouin measurement calculated | required by reference | standard Brillouin measurement. It is a graph for demonstrating the coefficient for Rayleigh measurement calculated | required by reference | standard Rayleigh measurement. It is a schematic diagram which shows the measurement system about a test member performed in a measurement process. It is a schematic diagram which shows the fixed condition of the optical fiber to a test member. It is a figure which shows the pressure application pattern in the measurement system of FIG. It is a graph which shows the relationship between a pressure and a frequency shift in a test Brillouin measurement. It is a graph which shows the relationship between a pressure and a frequency shift in a test Rayleigh measurement. It is a graph which shows the relationship between a pressure and volume distortion in a test Brillouin measurement. It is a graph which shows the relationship between a pressure and volume distortion in a test Rayleigh measurement. It is a graph which shows the relationship between a pressure and a bulk elastic modulus in a test Brillouin measurement. It is a graph which shows the relationship between a pressure and a volume elastic modulus in a test Rayleigh measurement. It is a schematic diagram showing a test apparatus for injecting supercritical carbon dioxide into a sandstone sample. It is a surrounding surface expanded view of the sandstone sample which shows the measuring point of volume change. It is a graph which shows the volume change of each measuring point. It is the graph which evaluated the progress rate of the interface of supercritical carbon dioxide and pure water. 1 is a schematic diagram of a carbon dioxide underground storage and storage status monitoring system. FIG. It is an enlarged view of the arrow F part of FIG. It is sectional drawing of a sensor cable.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a flowchart schematically showing the entire procedure of a volume change measuring method according to an embodiment of the present invention. The volume change measurement method of the present embodiment roughly includes a “calibration step” in which a Brillouin measurement and Rayleigh measurement are performed by fixing an optical fiber to a reference member whose volume change is known, and a measurement coefficient is obtained. It is equipped with a “test member measurement process” for deriving the volume change of the test member by fixing the optical fiber to the test member whose volume change is unknown using the operational coefficient and performing Brillouin measurement and Rayleigh measurement. .

  The calibration step includes a step of fixing an optical fiber to a reference member (step S1), a step of executing reference Brillouin measurement and reference Rayleigh measurement (step S2), and a step of obtaining a reference Brillouin frequency shift amount and a reference Rayleigh frequency shift amount. (Step S3) and a step of obtaining a Brillouin measurement coefficient or a Rayleigh measurement coefficient (Step S4). Moreover, the measurement process of the test member includes a step of fixing the optical fiber to the test member (step S5), a step of executing the test Brillouin measurement and the test Rayleigh measurement (step S6), the test Brillouin frequency shift amount, and A step of obtaining a test Rayleigh frequency shift amount (step S7) and a step of deriving a volume change of the test member (step S8) are included. Hereinafter, the measurement principle of the volume change measurement method of the present embodiment and the above steps will be described in detail.

<Measurement principle>
When light is incident on an optical fiber and the scattered light is subjected to frequency analysis, Rayleigh scattered light having substantially the same frequency as the incident light, Raman scattered light having a frequency significantly different from that of the incident light, and several to several tens of GHz from the incident light. Brillouin scattered light with different frequencies is observed. The Brillouin scattering phenomenon is a phenomenon in which power moves through an acoustic phonon in an optical fiber when light is incident on the optical fiber. The difference in frequency between incident light and Brillouin scattered light is called the Brillouin frequency shift, which is proportional to the speed of sound in the optical fiber, and the speed of sound depends on the strain and temperature of the optical fiber. Thus, by measuring the Brillouin frequency shift, the strain and / or temperature applied to the optical fiber can be measured.

  The Rayleigh scattering phenomenon is a scattering phenomenon that occurs because light is scattered due to the fluctuation of the refractive index in the optical fiber. The difference in frequency between the incident light and the Rayleigh scattered light is the Rayleigh frequency shift. This Rayleigh frequency shift also varies with strain and / or temperature applied to the optical fiber.

(Calibration process)
The Brillouin frequency shift amount Δν _B and the Rayleigh frequency shift amount Δν _R of the optical fiber subjected to the pressure change ΔP under a constant temperature are given by the following equations (1) and (2).

In the above formulas (1) and (2), the subscripts “B” and “R” indicate Brillouin measurement and Rayleigh measurement, respectively. Δe indicates an increase in volume strain (volume expansion coefficient). Α _B is a coefficient for Brillouin measurement, α _R is a coefficient for Rayleigh measurement, β ′ is a coefficient commonly applied to both Brillouin measurement and Rayleigh measurement, and a calibration test for a material having a known bulk modulus. Is a coefficient determined by. Further, C ₁₁ is a Brillouin measurement distortion sensitivity coefficient (= 0.0507 MHz / με), and C ₂₁ is a Rayleigh measurement distortion sensitivity coefficient (= −0.155 GHz / με).

  The following equation (3) is established for a linear material in which the bulk modulus K is kept constant with respect to the pressure change ΔP.

In this case, the Brillouin frequency shift amount Δν _B and the Rayleigh frequency shift amount Δν _R shown in the above formulas (1) and (2) are transformed as the following formulas (4) and (5). Can do.

Therefore, as the calibration test, a pressure load test is performed on a plurality of materials whose bulk modulus K is known. Then, the values of Δν _B / ΔP and Δν _R / ΔP are plotted against the inverse 1 / K of the bulk modulus. These results are linearly approximated, and a Brillouin measurement coefficient α _B , a Rayleigh measurement coefficient α _R , and a common coefficient β ′ are determined from the slope and intercept of the approximate line.

  Here, the volumetric strain increment Δe and the bulk modulus K are physical properties of the base material (reference member or test member described later) to which the optical fiber is fixed. As a premise that pressure is applied to the optical fiber, the optical fiber is fixed to the base material so as to be able to follow and change, and when the base material is placed in a pressure fluctuation environment and the base material contracts or expands, It is assumed that the optical fiber is also deformed following.

(Measurement process of specimens)
If the Brillouin measurement coefficient α _B , the Rayleigh measurement coefficient α _R , and the common coefficient β ′ are obtained in advance by the above calibration test, the volume change of the base material (test member) whose volume modulus is unknown, that is, The volume strain increment Δe and the bulk modulus K can be obtained by performing Brillouin measurement and / or Rayleigh measurement. It should be noted that for a material whose volume modulus K is unknown, the volume modulus K is not always kept constant with respect to the pressure change ΔP, and therefore an equation derived as follows is used.

  From the above formulas (1) to (5), the volume strain increment Δe can be expressed by the following formula (6).

It is assumed that the pressure in the reference state is P ₀ and changes from this P ₀ to a certain pressure P. When the above equation (6) is integrated from the pressure P ₀ to the pressure P, the following equation (7) is obtained.

  Since the definition of the volume strain is K = − (dP / de), the bulk modulus K is given by the following equation (8).

In the above equation (7), dν _B / dP and dν _R / dP are calculated by functionally fitting the pressure applied to the test member and the measurement results of the Rayleigh and Brillouin frequency shifts (for example, a cubic polynomial). It can be obtained by differentiating.

  For example, in the case of porous sandstone, the volume modulus of elasticity changes depending on whether the sandstone is filled with fluid or not. Moreover, since sandstone is generally a heterogeneous material, it is considered that unevenness occurs in the actual storage state of carbon dioxide. This unevenness appears as a spatial distribution of the bulk modulus of sandstone. Therefore, it is possible to monitor the storage state of carbon dioxide in the storage formation by arranging an optical fiber in the storage formation of carbon dioxide and performing Brillouin measurement and / or Rayleigh measurement on the optical fiber.

<Step S1>
FIG. 2 is a schematic diagram showing the measurement system M1 for the reference member 1 that is executed in the calibration process. In step S1, the measurement optical fiber 2 is fixed to the reference member 1 whose volume change is known. As the reference member 1, a cylindrical metal member can be used. In this embodiment, an aluminum (Al) circular tube and a stainless steel (SUS) circular tube are used. As the optical fiber 2, a fiber core wire having a UV coat layer on a quartz fiber strand having a core and a clad is used.

  A part of the measurement optical fiber 2 is spirally wound on the circumference of the reference member 1, and the other part is separated from the reference member 1 so as not to be affected by the pressure from the reference member 1. Is done. Thereby, the winding part 21 and the free fiber part 22 not restrained by the reference member 1 are formed in the measurement optical fiber 2. A first lead optical fiber 25 is fused to the first end 23 of the measurement optical fiber 2, and a second lead optical fiber 26 is fused to the second end 24 at the fused portion 2 </ b> A.

  In the winding portion 21, the measurement optical fiber 2 has a predetermined tension on the peripheral surface of the reference member 1 so as to follow the deformation of the reference member 1 due to external pressure applied to the reference member 1. While being wound, it is firmly bonded using an epoxy adhesive or the like. The outer diameter of the reference member 1 is sufficiently larger than the outer diameter of the measurement optical fiber 2. Thereby, when the outer diameter when the reference member 1 is placed under high pressure contracts, the winding portion 21 similarly contracts, and the winding inner diameter becomes equal to the outer diameter of the reference member 1 after contraction. By forming such a winding part 21, the pressure accompanying the volume change of the reference member 1 accurately acts on the measurement optical fiber 2, and the pressure change is accurately reflected in the Brillouin measurement and Rayleigh measurement. Become so. On the other hand, the free fiber portion 22 is in a state in which no tension is applied so that the Brillouin frequency shift amount and the Rayleigh frequency shift associated with the external pressure that purely acts on the measurement optical fiber 2 can be measured.

  The reference member 1 and the measurement optical fiber 2 are enclosed in a cylindrical pressure vessel 3. The pressure vessel 3 is a chamber capable of forming a pressure environment in a range of 1 to 50 MPa, for example, and a pressure control device 3P for controlling the pressure in the chamber is attached. A pressure partition wall 31 is provided at the opening of the pressure vessel 3. A feedthrough 32 that holds the first and second lead optical fibers 25 and 26 is passed through the pressure partition wall 31.

<Step S2>
In the pressure vessel 3, the reference member 1 and the measurement optical fiber 2 with a part (the winding portion 21) fixed thereto are sealed, and a known pressure is applied to the measurement optical fiber 2, and the measurement optical fiber 2 is applied to the measurement optical fiber 2. The test light is incident, and the reference Brillouin measurement for obtaining the reference Brillouin frequency shift amount based on the Brillouin scattering phenomenon and the reference Rayleigh measurement for obtaining the reference Rayleigh frequency shift amount based on the Rayleigh scattering phenomenon are executed. The temperature is constant at 30 ° C.

  As shown in FIG. 2, for the reference Brillouin measurement and the reference Rayleigh measurement, a pump light source 27 and a detector 29 are connected to the end portion of the first lead optical fiber 25 via a circulator 251, and the second lead optical fiber. A probe light source 28 is connected to the end of 26. The measurement optical fiber 2 is shared by both the reference Brillouin measurement and the reference Rayleigh measurement.

  The pump light source 27 uses a semiconductor laser or the like as a light source, and generates pump light composed of pulsed light. This pump light is incident on the first end 23 of the measurement optical fiber 2 through the first lead optical fiber 25. Similarly, the probe light source 28 uses a semiconductor laser or the like as a light source, and generates probe light composed of continuous light. This pump light is incident on the second end 24 of the measurement optical fiber 2 via the second lead optical fiber 26.

  The detector 29 includes a light receiving element, and light subjected to an effect of stimulated Brillouin scattering phenomenon (stimulated Brillouin scattering light) or light subjected to an effect of Rayleigh scattering phenomenon (Rayleigh backscattered light) in the measurement optical fiber 2. Is received. The detector 29 includes an arithmetic processing unit, and performs an operation for obtaining a reference Brillouin frequency shift amount and an operation for obtaining a reference Rayleigh frequency shift amount by analyzing the spectra of stimulated Brillouin scattered light and Rayleigh backscattered light. . The circulator 251 separates the return light from the measurement optical fiber 2 side to the first lead optical fiber 25 and makes it incident on the detector 29.

  When the reference Brillouin measurement is performed by the measurement system M1, the pump light source 27 generates a main light pulse and a sub light pulse, and makes these light pulses enter the first end portion 23 of the measurement optical fiber 2 as pump light. Further, the probe light source 28 causes the probe light composed of continuous light to enter the second end 24 of the measurement optical fiber 2. The main light pulse functions to pass the energy scattered by the acoustic phonon in the measurement optical fiber 2 to the probe light, and the sub light pulse excites the acoustic phonon for the main light pulse. Function. In this case, the detector 29 detects the stimulated Brillouin scattered light.

  On the other hand, when the reference Rayleigh measurement is performed by the measurement system M1, the pump light source 27 generates one type of pulsed light and makes it incident on the first end 23 of the measurement optical fiber 2. Note that the probe light source 28 is not used. In this case, the detector 29 detects Rayleigh backscattered light.

  When the reference Brillouin measurement and the reference Rayleigh measurement are performed, the internal pressure of the pressure vessel 3 is operated by the pressure control device 3P. FIG. 3 shows an example of the pressure operation, FIG. 3A shows a pressure change pattern, and FIG. 3B shows the pressure applied in each period. As shown in FIG. 3A, an example of a preferable pressure operation is that a low pressure period A, an intermediate pressure period B, and a high pressure period C are defined, each unit period is 90 minutes, and the low pressure period A → the middle In this pattern, pressure period B → high pressure period C → medium pressure period B → low pressure period A. As shown in FIG. 3 (B), the pressure in the low pressure period A is 1 MPa, and the medium pressure period B and the high pressure period C are different from each other on the first day, the second day, and the third day. To do. Thereby, the reference Brillouin measurement and the reference Rayleigh measurement can be performed in increments of 5 MPa between 1 MPa and 30 MPa. The reason why the maintenance time of 90 minutes is taken in each unit period is to stabilize the pressure response of the optical fiber 2.

  The above-mentioned reference Brillouin measurement and reference Rayleigh measurement are preferably performed a plurality of times using different materials as the reference member 1. In the present embodiment, the above-mentioned reference Brillouin measurement and reference Rayleigh measurement were performed using two types of aluminum tubes and stainless steel tubes.

<Step S3>
The detector 29 obtains the Brillouin spectrum of each region in the longitudinal direction of the measurement optical fiber 2 by detecting the stimulated Brillouin scattered light in each reference Brillouin measurement under a known pressure. Then, based on the Brillouin spectrum of each region portion, the Brillouin frequency shift amount of each region portion is obtained. The detector 29 obtains a Rayleigh spectrum of each region portion in the longitudinal direction of the measurement optical fiber 2 based on the Rayleigh backscattered light in the reference Rayleigh measurement under each pressure. Then, based on the Rayleigh spectrum of each area portion, the Rayleigh frequency shift amount of each area portion is obtained.

  FIG. 4 shows the Brillouin frequency shift amount between 1 MPa and 30 MPa obtained in the reference Brillouin measurement. In FIG. 4, the plot of “Al” is the Brillouin frequency shift amount in the winding portion 21 of the measurement optical fiber 2 when an aluminum circular tube is used as the reference member 1. Further, the plot of “SUS” is a Brillouin frequency shift amount in the winding portion 21 of the measurement optical fiber 2 when a stainless steel tube is used as the reference member 1. Further, the “free” plot represents the Brillouin frequency shift amount of the free fiber portion 22 of the measurement optical fiber 2. As apparent from FIG. 4, the response to the pressure of the Brillouin frequency shift amount changes to a “negative” linearity in both the winding portion 21 on the aluminum circular tube, the winding portion 21 on the stainless steel circular tube, and the free fiber portion 22. . It can also be seen that the free fiber portion 22 has the greatest response to pressure.

  FIG. 5 shows the Rayleigh frequency shift amount between 1 MPa and 30 MPa obtained in the reference Rayleigh measurement. In FIG. 5, the plots of “Al”, “SUS”, and “free” are the same as those in FIG. As apparent from FIG. 5, the free fiber portion 22 and the winding portion 21 to the aluminum circular tube change the response to the pressure of the Rayleigh frequency shift amount to a “positive” linear shape, while winding to the stainless steel circular tube. In the part 21, the response of the Rayleigh frequency shift amount to the pressure changes in a “negative” linear manner. Also in this reference Rayleigh measurement, it can be seen that the free fiber portion 22 has the greatest response to pressure. Depending on the material of the reference member 1, there are cases where the relationship between the pressure and the Rayleigh frequency shift changes to “positive” and cases where the relationship changes to “negative”. This suggests that volume changes of various objects can be identified by detecting both Rayleigh frequency shift amounts.

<Step S4>
From the results of the reference Brillouin measurement and the reference Rayleigh measurement in step 3, the Brillouin measurement coefficient or the Rayleigh measurement coefficient based on the relationship between the volume change in each measurement and the frequency shift amount per unit pressure is obtained. In the present embodiment, the coefficient is calculated using the relationship between the three types of pressure and the Brillouin and Rayleigh frequency shifts of the winding part 21 on the aluminum circular pipe, the winding part 21 on the stainless steel pipe, and the free fiber part 22. Ask.

  FIG. 6 is a graph for obtaining a Brillouin measurement coefficient obtained from the result of the reference Brillouin measurement. The horizontal axis of the graph of FIG. 6 is the reciprocal of the bulk modulus K, and the vertical axis is the frequency shift amount per unit pressure derived from the graph of FIG. Incidentally, the bulk elastic modulus of aluminum is 75.5 [GPa], stainless steel is 160 [GPa], and quartz that is a constituent material of the measurement optical fiber 2 is 36.9 [GPa]. Therefore, the plot P11 in FIG. 6 is a plot corresponding to the winding portion 21 around the stainless steel circular tube, P12 is the winding portion 21 around the aluminum circular tube, and P13 is a plot corresponding to the free fiber portion 22.

  FIG. 7 is a graph for obtaining the Rayleigh measurement coefficient obtained from the result of the reference Rayleigh measurement. The horizontal axis of the graph of FIG. 7 is the reciprocal of the bulk modulus, and the vertical axis is the frequency shift amount per unit pressure derived from the graph of FIG. Similarly to FIG. 6, a plot P21 in FIG. 7 is a plot corresponding to the winding portion 21 on the stainless steel circular tube, P22 is a winding portion 21 on the aluminum circular tube, and P23 is a plot corresponding to the free fiber portion 22.

From the results of FIGS. 6 and 7, the Brillouin measurement coefficient α _B , the Rayleigh measurement coefficient α _R , and the common coefficient β ′ are derived. An approximate line R1 indicating the correlation between the reference Brillouin frequency shift amount per unit pressure and the reciprocal of the bulk modulus K shown in FIG. 6 is the following equation (9), and the reference Rayleigh frequency shift per unit pressure shown in FIG. The approximate line R2 indicating the correlation between the quantity and the reciprocal of the bulk modulus K can be expressed as a linear function in the following equation (10).

y = −9.719x−0.475 (9)
y = 31.606x−0.205 (10)
The Brillouin measurement coefficient α _B is the intercept of the equation (9) = − 0.475, and the Rayleigh measurement coefficient α _R is the intercept of the equation (9) = − 0.205. The common coefficient β ′ is obtained by dividing the slope of the above equation (10) = − 9.719 by the distortion sensitivity coefficient C ₁₁ (= 0.0507 MHz / με) of the Brillouin measurement, and the slope of the above equation (10) = 31.606 is divided by the distortion sensitivity coefficient C ₂₁ (= 0.155 GHz / με) of Rayleigh measurement. When calculated,
Brillouin measurement common coefficient β ′ = 0.192 × 10 ⁶ ,
Rayleigh measurement common coefficient β ′ = 0.204 × 10 ⁶ ,
It becomes. Note that “10 ⁶ ” of the common coefficient β ′ is a multiplier for unit matching, and can be treated as “0.2” in both cases.

  The common coefficient β ′ is a ratio of strain that the measurement optical fiber 2 senses from the reference member 1. According to the above calculation example, the measurement optical fiber 2 senses 20% of the volume strain of the reference member 1. The volume strain e is represented by the sum of three strains of the x-axis, y-axis, and z-axis (e = εx + εy + εz). On the other hand, quartz glass-based optical fibers respond only to strain in the direction along the longitudinal direction. Therefore, if the reference member 1 is an isotropic material and the measurement optical fiber 2 is also deformed isotropically, theoretically, 1/3 of the volume strain e of the reference member 1 is reduced to the measurement optical fiber 2. Should be perceived (β ′ = 0.333). However, according to the experiments by the present inventors, it has been found that the ratio of the perceived distortion is 1/5 instead of 1/3, and is almost the same in both Brillouin measurement and Rayleigh measurement. These factors have yet to be determined.

From the above, all the constants of the above equation (8) are obtained. That is,
Brillouin measurement coefficient α _B = −0.475 [MHz / MPa]
Rayleigh measurement coefficient α _R = −0.205 [GHz / MPa]
Common coefficient β ′ = 0.2 × 10 ⁶
Brillouin measurement strain sensitivity coefficient C ₁₁ = 0.0507 [MHz / με]
Rayleigh measurement distortion sensitivity coefficient C ₂₁ = −0.155 [GHz / με]
It becomes. Therefore, when the known pressure P is applied and the Brillouin frequency shift amount Δν _B and the Rayleigh frequency shift amount Δν _R are obtained, the volume strain e and the bulk modulus K of the test member whose volume change is unknown are obtained. be able to.

<Step S5>
Subsequently, the process proceeds to an actual measurement process for the test member. FIG. 8 is a schematic diagram showing the measurement system M2 for the sample member 10 that is executed in the actual measurement process. Here, the measurement optical fiber 4 is arranged so as to follow the deformation of the test member 10 due to the external pressure applied to the test member 10 whose volume change is unknown. Fixed. The optical fiber 4 used is the same as the measurement optical fiber 2 used in the calibration process. In the present embodiment, the test member 10 is a sandstone sample that is cut out in a cylindrical shape and has voids inside.

  A part of the measurement optical fiber 4 is spirally wound on the circumference of the test member 10, thereby forming a winding portion 41. FIG. 9 is a schematic view showing a state of fixing the measurement optical fiber 4 to the test member 10, and shows the cylindrical test member 10 developed in a plane. In the present embodiment, the turn pitch of the optical fiber 4 is 25 mm, and the distance between the turn edge and the axial end edge of the test member 10 is 10 mm. In the winding part 41, the measurement optical fiber 4 is firmly fixed to the peripheral surface of the test member 10 using an epoxy adhesive or the like. In the fixing mode, when the outer diameter of the test member 10 placed under high pressure contracts, the winding part 41 contracts in the same manner, and the winding inner diameter becomes the outer diameter of the test member 10 after contraction. It is an aspect which becomes equal.

  The other part of the measurement optical fiber 4 is bundled at a position where it is not affected by the pressure from the test member 10 to form a first bundled portion 42. The first bundling section 42 is, like the free fiber section 22 shown in FIG. 2, a tension so that it can measure the Brillouin frequency shift amount and the Rayleigh frequency shift due to the external pressure acting on the measurement optical fiber 4 purely. Is not given. A first through optical fiber 451 is fused to the first end 43 of the measurement optical fiber 4 and a second through optical fiber 452 is fused to the second end 44 at the fused portion 4A.

  The test member 10 and the measurement optical fiber 4 are accommodated in the pressure tester 5. The pressure tester 5 is a chamber capable of forming a pressure environment in the range of 1 to 50 MPa, for example, using hydraulic pressure, and the chamber is filled with oil. The pressure in the pressure tester 5 is controlled by a pressure control device 5P. A pressure bulkhead 51 is provided at the opening of the pressure tester 5. A feedthrough 52 for holding the first and second through optical fibers 451 and 452 is passed through the pressure partition wall 51.

  One end portions of the first and second reference optical fibers 461 and 462 are fused to the other end portions of the first and second through optical fibers 451 and 452, respectively, at the fusion portion 4B. A middle portion of the first reference optical fiber 461 is provided with a second bundling portion 47 formed by bundling the optical fibers. Furthermore, one end portions of the first and second lead optical fibers 481 and 482 are fused to the other end portions of the first and second reference optical fibers 461 and 462, respectively, at the fusion portion 4C. In addition, since the 1st bundling part 42 and the 2nd bundling part 47 do not participate directly in the measurement of the volume change of the test member 10, but are provided for the purpose of temperature measurement etc., these bundles You may omit the formation of the handle.

  The other ends of the first and second lead optical fibers 481 and 482 are connected to the measuring device 6. The measuring device 6 is a measuring device having the functions of the pump light source 27, the probe light source 28 and the detector 29 shown in FIG. The measuring device 6 makes pump light incident on the first end 43 of the measuring optical fiber 4 from the first lead optical fiber 481 side. In addition, probe light is incident on the second end 44 of the measurement optical fiber 4 for Brillouin measurement from the second lead optical fiber 482 side.

<Step S6>
Using the measurement system M2 shown in FIG. 8, the test light is incident on the measurement optical fiber 4 with a known pressure applied to the measurement optical fiber 4, and the test member 10 is based on the Brillouin scattering phenomenon. The test Brillouin measurement for obtaining the test Brillouin frequency shift amount and the test Rayleigh measurement for obtaining the test Rayleigh frequency shift amount based on the Rayleigh scattering phenomenon are executed. The temperature is constant at 40 ° C. The measurement method itself of the test Brillouin measurement and the test Rayleigh measurement is the same as that of the above-mentioned reference Brillouin measurement and reference Rayleigh measurement.

  When the test Brillouin measurement and the test Rayleigh measurement are performed, the internal pressure of the pressure tester 5 is operated by the pressure control device 5P. FIG. 10 shows a pressure change pattern (pressure profile) executed in the present embodiment. Here, the pressure was changed in increments of 2 MPa from 2 MPa to 12 MPa using 0.5 MPa as reference data. After ensuring the maintenance time at each pressure level and confirming that the pressure response of the measurement optical fiber 4 was stable, the test Brillouin measurement and the test Rayleigh measurement were performed.

<Step S7>
The measurement device 6 obtains the Brillouin spectrum of each region in the longitudinal direction of the measurement optical fiber 4 by detecting the stimulated Brillouin scattered light in the test Brillouin measurement of each pressure level. Then, based on the Brillouin spectrum of each region portion, the Brillouin frequency shift amount of each region portion (particularly the winding portion 41) is obtained. Moreover, the measuring apparatus 6 calculates | requires the Rayleigh spectrum of each area | region part in the longitudinal direction of the measurement optical fiber 4 based on Rayleigh backscattered light in the test Rayleigh measurement under each pressure, respectively. Then, based on the Rayleigh spectrum of each area portion, the Rayleigh frequency shift amount of each area portion is obtained.

  FIG. 11 is a graph showing the correlation between the pressure in the test Brillouin measurement and the Brillouin frequency shift observed for the winding portion 41 of the measurement optical fiber 4 with respect to the test member 10, and FIG. 12 shows the test Rayleigh measurement. It is a graph which shows the correlation with the pressure in Rayleigh, and the amount of Rayleigh frequency shifts. Whether the test member 10 is a sandstone sample having a void or not, both the test Brillouin measurement and the test Rayleigh measurement have a non-linear relationship between pressure and frequency shift. Although not shown, the Brillouin frequency shift amount and Rayleigh frequency shift amount observed for the first bundling unit 42 are both linear.

<Step S8>
The volume change of the test member 10 is obtained from the test Brillouin frequency shift amount and the test Rayleigh frequency shift amount obtained in step S7 and the Brillouin measurement coefficient or Rayleigh measurement coefficient obtained in step S4. As shown in FIG. 11 and FIG. 12, the relationship between the pressure P and the Brillouin frequency shift amount Δν _B , the pressure P and the Rayleigh frequency shift amount Δν _R for the test member 10 by the test Brillouin measurement and the test Rayleigh measurement. The relationship is required. These values are converted into the above equation (7), the Brillouin measurement coefficient α _B , the Rayleigh measurement coefficient α _R , the common coefficient β ′, the Brillouin measurement distortion sensitivity coefficient C ₁₁ , and the Rayleigh obtained in the previous calibration step. by substituting with strain sensitivity coefficient C ₂₁ of measurement, it is possible to obtain the volume strain e of the test試部member 10.

  FIG. 13 is a graph showing the relationship between the pressure (horizontal axis) and the volume strain (vertical axis) calculated based on the results of the test Brillouin measurement. FIG. 14 is a graph showing the relationship between pressure and volumetric strain calculated based on the result of the test Rayleigh measurement. Since the test member 10 is a sandstone sample having voids therein, the volume decreases as the pressure increases. While the void strain in the sandstone is sufficiently present, the decrease rate of the volume strain is large, but it is estimated that the decrease in the volume strain is reduced when the void is decreased. The result of FIG. 13 and FIG. 14 shows the tendency well.

Further, the pressure P and the Brillouin frequency shift amount Δν _B , or the pressure P and the Rayleigh frequency shift amount Δν _R are expressed in the above equation (8) as the Brillouin measurement coefficient α _B and Rayleigh measurement obtained in the previous calibration step. By substituting with the use coefficient α _R , the common coefficient β ′, the Brillouin measurement strain sensitivity coefficient C ₁₁ , and the Rayleigh measurement strain sensitivity coefficient C ₂₁ , the bulk modulus K of the test member 10 can be obtained.

  FIG. 15 is a graph showing the relationship between the pressure (horizontal axis) and the bulk modulus (vertical axis) calculated based on the results of the test Brillouin measurement. FIG. 16 is a graph showing the relationship between pressure and bulk modulus calculated based on the results of the test Rayleigh measurement. Since the test member 10 is a sandstone sample having voids therein, it is estimated that voids in the sandstone are crushed as the pressure increases, and the rigidity gradually increases, that is, the bulk modulus increases. The tendency is shown well in FIG. 15 and FIG.

As described above, according to the volume change measuring method according to the present embodiment, the relationship between the volume change of the reference member 1 and the frequency shift amount per unit pressure is grasped by the reference Brillouin measurement or the reference Rayleigh measurement. From this result, the Brillouin measurement coefficient α _B , the Rayleigh measurement coefficient α _R, and the common coefficient β ′, which are calibration data, are obtained. Then, the same test Brillouin measurement or test Rayleigh measurement is performed on the test member 10 to obtain the Brillouin frequency shift amount Δν _{B, the} pressure P, and the Rayleigh frequency shift amount Δν _R , and this measured value and the coefficient α _B, alpha said _R and beta '(7) equation can be obtained (8) by substituting the equation, volume distortion e and bulk modulus K of the test試部member 10 (volume change).

<Measurement example of distribution of volume change>
Then, the example which applied the volume change measuring method demonstrated above to the distribution measurement of the volume change of the said sandstone sample accompanying enclosure of the carbon dioxide to a sandstone sample is shown. FIG. 17 is a schematic diagram showing a test apparatus for injecting carbon dioxide in a supercritical state into a sandstone sample 10A having voids cut out in a cylindrical shape in a state simulating the ground.

  A measurement optical fiber 61 is spirally wound around the outer peripheral surface of the cylindrical sandstone sample 10A. Although not shown, the measuring optical fiber 61 is connected to a measuring device capable of Rayleigh measurement and Brillouin measurement. The sandstone sample 10A around which the measurement optical fiber 61 is wound is accommodated in a pressure vessel 62 filled with sealing oil 621. Note that a silicon coat layer 622 is provided on the outer peripheral surface of the sandstone sample 10A in order to prevent the sealing oil 621 from entering the sandstone sample 10A. In addition, a heater 63 for keeping the temperature of the pressure vessel 62 constant is attached to the outer surface of the pressure vessel 62.

  A first syringe pump 64 for sealing pressure (hydrostatic pressure) load, a second syringe pump 65 for injecting pure water, and a third syringe pump 66 for injecting carbon dioxide are connected to the pressure vessel 62. The first syringe pump 64 is a pump for injecting sealing oil into the pressure vessel 62 in order to apply pressure from the outside of the sandstone sample 10A. The second syringe pump 65 is a pump for injecting pure water into the porous sandstone sample 10A. The third syringe pump 66 is a pump for injecting carbon dioxide into the sandstone sample 10A in a state where pure water is stored, and replacing part of the pure water with carbon dioxide.

  An example of test conditions using the test apparatus will be shown. The temperature in the pressure vessel 62 is kept constant at 40 ° C. by the heater 63. First, the first syringe pump 64 is operated, and a hydrostatic pressure of 12 MPa is applied to the sandstone sample 10A in the pressure vessel 62. Next, the second syringe pump 65 is operated, and pure water is injected into the void of the sandstone sample 10A at a water injection pressure of 10 MPa. As a result, the void of the sandstone sample 10A is filled with pure water under high pressure, and the sandstone layer in which carbon dioxide is scheduled to be stored in the ground is simulated.

  The third syringe pump 66 is operated while maintaining the 12 MPa hydrostatic pressure and the pure water injection pressure 10 MPa, and supercritical carbon dioxide is injected into the sandstone sample 10A at a pressure of 10.05 MPa. By this injection, a part of the pure water in the sandstone sample 10A is replaced with carbon dioxide in a supercritical state. This state is a state in which carbon dioxide is injected into the sandstone layer.

  Aside from the injection operation as described above, the volume change distribution of the sandstone sample 10A is measured by the measuring optical fiber 61 and its measuring device. FIG. 18 is a development of the circumferential surface of the sandstone sample 10A showing the measurement points, and FIG. 19 is a graph showing the volume change at each measurement point. In FIG. 18, circled numbers 1 to 8 indicate measurement points. These measurement points 1 to 8 are set at a pitch of 90 degrees in the circumferential direction. Since the measurement optical fiber 61 is spirally wound around the circumferential surface of the sandstone sample 10A, when the measurement points 1 to 8 are viewed in the axial direction of the sandstone sample 10A, the measurement point 1 is the most supercritical carbon dioxide injection. The measurement point 8 is the most downstream side in the upstream direction.

As shown in FIG. 19, in the time zone T ₀ before the supercritical carbon dioxide injection, the sandstone sample 10A has a volume strain (compression) of about −0.12%. Are equally detected. After the time zone T ₀ , the volume distortion increases due to carbon dioxide injection. That is, the sandstone sample 10A expands (the compressed state is relaxed). It can be seen that the time at which the volume strain starts to increase varies depending on the measurement points 1 to 8. That is, the rise of the volume distortion increase at the measurement point 1 in the uppermost stream in the carbon dioxide injection direction is the fastest, and the rise of the measurement point downstream is slower. From this, it can be seen that the interface between the supercritical carbon dioxide and pure water moves toward the downstream side in the sandstone sample 10A.

  FIG. 20 is a graph evaluating the growth rate of the interface between supercritical carbon dioxide and pure water. The vertical axis in FIG. 20 indicates the distance z from the carbon dioxide injection end to each measurement point, and the horizontal axis indicates the time at which the volume distortion starts to increase. For each measurement point, three threshold values are set and plotted. An approximate straight line L can be derived from this plot. From the slope of the approximate straight line L, the progress speed of the interface can be obtained (about 41 mm / hour).

<Significance of using Brillouin measurement and Rayleigh measurement>
In the said embodiment, the form which uses Brillouin measurement and Rayleigh measurement together was illustrated. As shown here, pressure is known at the laboratory level and it is easy to keep the temperature constant. Therefore, in the volume change measurement method of the present invention, the reference Brillouin measurement and the test Brillouin measurement, or the reference Rayleigh measurement are performed. If at least one of the test Rayleigh measurement is executed, it is possible to derive the volume change of the test member 10.

  However, for example, when measuring the volume change of sandstone whose composition is unknown in the ground, measurement in an ideal environment using a test instrument such as the pressure control device 5P exemplified above cannot be expected. That is, an environment where the pressure generation factor is unambiguous and the environmental temperature is constant cannot be expected. Therefore, if Brillouin measurement and Rayleigh measurement are used in combination and volume change data is acquired by each measurement method, pressure factors other than expansion and contraction acting on the target sandstone and temperature change factors can be excluded. Increases nature. In this respect, it is desirable to use Brillouin measurement and Rayleigh measurement in combination. Hereinafter, the advantages of this combined use will be specifically described.

  In order to separate the effects of pressure P, strain ε, and temperature T in the measured value, three or more independent measurement quantities are required. In this case, it is only necessary to prepare two types of fiber systems having different sensitivity to pressure P, strain ε, and temperature T. Such a request can be satisfied by using both Brillouin measurement and Rayleigh measurement. The relationship of the following equation (11) is established between the four frequency shifts obtained by Brillouin measurement and Rayleigh measurement and the amount of change in pressure P, strain ε, and temperature T.

  By solving the simultaneous equations of the above equation (11), the effects of pressure P, strain ε and temperature T can be separated. In laboratory experiments, the temperature T can be kept constant, and the pressure P can be freely set in the pressure vessel. Therefore, there is no need to solve the above equation (11). However, when actually measuring in the ground, all of pressure P, strain ε, and temperature T are unknown. Therefore, the volume change cannot be obtained unless hybrid measurement of Brillouin measurement and Rayleigh measurement is performed and the above simultaneous equations are solved.

  FIG. 21 is a schematic diagram of a carbon dioxide underground storage and storage status monitoring system, and FIG. 22 is an enlarged view of a portion indicated by an arrow F in FIG. Here, it is assumed that a sandstone layer serving as a carbon dioxide reservoir exists in the ground, and a cap lock layer functioning as a seal layer exists on the sandstone layer. An injection well 71 is vertically installed from the storage site 70 set up on the ground toward the sandstone layer underground. The injection well 71 includes a cylindrical casing 711 and a carbon dioxide injection tube 712 disposed therein. The injection well 71 is inserted into a bored hole 73 that has been excavated in advance, and cemented 713 is applied around it to be fixed to the underground formation.

  In order to measure the underground pressure P, strain ε, and temperature T distribution along the injection well 71, a sensor cable 72 is embedded in the layer of the cementing 713. FIG. 23 shows a cross-sectional structure of the sensor cable 72. The sensor cable 72 includes a first optical fiber 721 that is affected by pressure and a second optical fiber 722 that is cut off from the influence of pressure. The second optical fiber 722 is loosely accommodated in the metal thin tube 723 for the purpose of blocking the pressure. The first optical fiber 721 and the metal thin tube 723 are covered with a cable sheath 724 located in the outermost layer of the sensor cable 72, and an intervening layer 725 is loaded therein.

  Since the sensor cable 72 is embedded in the layer of the cementing 713, it is affected when the surrounding formation changes in volume. For example, if the sandstone layer expands due to the inclusion of carbon dioxide, the sensor cable 72 will receive pressure through the layer of cementing 713. In this case, the first optical fiber 721 is affected by the pressure, but the second optical fiber 722 accommodated in the metal thin tube 723 is not affected.

  For each of the first optical fiber 721 and the second optical fiber 722, Brillouin measurement and Rayleigh measurement are performed by the measurement device 74 installed on the ground, and Brillouin frequency shift and Rayleigh frequency shift are obtained. In the case of using the sensor cable 72, simultaneous equations for separating the effects of the pressure P, the strain ε, and the temperature T are expressed by the following equation (12). Since the second optical fiber 722 is cut off from the influence of pressure, the second optical fiber 722 is simplified from the above equation (11).

  By solving the simultaneous equations of equation (12) above, it becomes possible to eliminate factors other than the volume change of the formation, and the volume change and distribution associated with the inclusion of carbon dioxide in the sandstone layer, the soundness of the cap rock layer Monitoring can be performed.

DESCRIPTION OF SYMBOLS 1 Reference member 2 Optical fiber for measurement 21 Winding part 22 Free fiber part 3 Pressure vessel 3P Pressure control apparatus 4 Optical fiber for measurement 41 Winding part 5 Pressure tester 5P Pressure control apparatus 6 Measuring apparatus

Claims (4)

At least two reference members volume change is known of different materials, so as to follow the deformation of the said reference member associated with the external pressure is applied to at least two of the reference member, respectively the optical fiber Fixing the step,
Reference Brillouin measurement for obtaining a reference Brillouin frequency shift amount based on the Brillouin scattering phenomenon by making test light incident on each of the optical fibers in a state where a known pressure is applied from the outside to a reference member to which the optical fiber is fixed, or Performing at least one of reference Rayleigh measurements to obtain a reference Rayleigh frequency shift amount based on the Rayleigh scattering phenomenon;
From the result of the reference Brillouin measurement or reference Rayleigh measurement, obtaining a Brillouin measurement coefficient or a Rayleigh measurement coefficient based on the relationship between the volume change in each measurement and the frequency shift amount per unit pressure;
Fixing the optical fiber to the test member whose volume change is unknown so as to follow the deformation of the test member due to external pressure applied to the test member;
Test Brillouin measurement for obtaining a test Brillouin frequency shift amount based on the Brillouin scattering phenomenon by making test light incident on the optical fiber with a known pressure applied from the outside to the test member to which the optical fiber is fixed Or performing at least one of the test Rayleigh measurements to determine the test Rayleigh frequency shift amount based on the Rayleigh scattering phenomenon;
Obtaining the volume change of the test member from the test Brillouin frequency shift amount or the test Rayleigh frequency shift amount and the Brillouin measurement coefficient or the Rayleigh measurement coefficient;
Measuring method of volume change of object including Performing the reference Brillouin measurement or the reference Rayleigh measurement,
A first measurement for measuring the reference Brillouin frequency shift amount or the reference Rayleigh frequency shift amount by making the test light incident on the optical fiber not fixed to the reference member;
A second measurement for measuring the reference Brillouin frequency shift amount or the reference Rayleigh frequency shift amount with respect to optical fibers respectively fixed to at least two kinds of reference members made of different materials,
The step of obtaining the Brillouin measurement coefficient or the Rayleigh measurement coefficient includes a frequency shift amount per unit pressure with respect to the reference Brillouin frequency shift amount or the reference Rayleigh frequency shift amount obtained in the first measurement and the second measurement. The method for measuring a volume change of an object according to claim 1, which is a step of obtaining a slope and an intercept of a linear function obtained by plotting on a volume change axis. The step of fixing the optical fiber to the reference member is a free fiber in which a part of one optical fiber is wound around a cylindrical reference member and the other part of the optical fiber is separated from the reference member. Forming a part,
The step of performing the reference Brillouin measurement or the reference Rayleigh measurement includes enclosing a winding portion of the optical fiber around a reference member and the free fiber portion in a pressure vessel. Method for measuring volume change of objects. The sample member is a heterogeneous and porous member;
The method for measuring a volume change of an object according to any one of claims 1 to 3, wherein the step of obtaining a volume change of the test member is a step of obtaining a volume elastic modulus and a volume expansion coefficient of the test member.

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