Salt cavern gas storage interlayer breakage seismic wave monitoring method, system and application

Abstract

The application discloses a salt cavern gas storage interlayer breakage seismic wave monitoring method and system and application, belongs to the technical field of salt cavern underground gas storage monitoring, and comprises the following steps: arranging a ring-shaped three-dimensional monitoring array with optical fibers along the circumference of a cavity, and synchronously collecting interlayer breakage waves and rockfall collapse shock waves; constructing a four-dimensional discrimination criterion of a frequency domain, time sequence, pulse quantity and rising time ratio, and automatically distinguishing two types of waveforms through a weighted comprehensive discrimination function; inversely calculating damage parameters such as interlayer breakage area, slip amount and fallen rock mass quality based on a seismic source mechanics theory; and measuring and calculating the cavity bottom sediment thickness and porosity in two stages of cavity construction and operation. Furthermore, the disturbance radius is calculated based on the wave energy attenuation law, and the safety distance of a group of cavities is determined. A modular monitoring system is matched to realize closed-loop operation of acquisition, identification and parameter inversion. The application has the advantages of non-invasive full-cycle monitoring, strong multi-dimensional identification fault tolerance and quantitative evaluation of interlayer instability risk, and can provide support for salt cavern construction and operation safety control.

Description

Technical Field

[0001] This invention belongs to the field of monitoring technology for underground gas storage facilities in salt caverns, and particularly relates to the method, system and application of monitoring seismic waves for interlayer failure in salt cavern gas storage facilities. Background Technology

[0002] Salt cavern gas storage facilities, with their advantages of good sealing, large capacity, strong peak-shaving ability, and safety and stability, are prone to cracking, slippage, and overall failure during the entire process of water-soluble cavity construction and high-pressure gas storage operation. The mudstone interlayers inside the cavity are subjected to stress redistribution, brine erosion, and alternating gas pressure. The falling rock fragments after failure can cause problems such as partial collapse of the cavity, accumulation of sediment at the bottom of the cavity, and distortion of the cavity outline, which seriously threaten the structural safety and service life of the gas storage facility.

[0003] Current monitoring technology for salt cavern gas storage facilities has many shortcomings: 1. The monitoring methods are highly invasive and cannot monitor the entire life cycle. Traditional monitoring equipment such as well logging, cavity cameras, and pressure sensors need to be placed inside the cavity, which not only interferes with the water-soluble cavity creation operation, but is also easily damaged by brine and collapsed rock masses. It is impossible to achieve continuous monitoring throughout the entire life cycle of cavity creation and operation, and it is also difficult to achieve dynamic inversion of the cavity morphology.

[0004] 2. It is impossible to distinguish between the two types of vibration signals, resulting in low identification accuracy. The physical mechanisms of elastic waves generated by interlayer fracture and shock waves generated by rock block collapse are completely different. However, existing vibration monitoring technology can only collect raw waveforms and lacks a systematic waveform classification method. It is impossible to distinguish between fracture waves and collapse shock waves, and it is difficult to analyze the mechanical characteristics of the seismic source.

[0005] 3. Lack of parameter inversion capability: Existing monitoring methods can only qualitatively determine whether vibration has occurred, and cannot quantitatively invert damage mechanical parameters such as interlayer fracture area, slippage, mass of detached rock mass, and equivalent magnitude, and cannot quantitatively evaluate the degree of interlayer damage and disaster risk.

[0006] 4. Lack of supporting calculation technology for engineering: On the one hand, the thickness of sediment at the bottom of the cavity, porosity, and void ratio are key parameters affecting the effective storage capacity of the cavity and the stability of the surrounding rock. Currently, there is a lack of dynamic calculation methods based on vibration signals. On the other hand, the multi-cavity layout is the mainstream form of large salt cavern gas storage, but the existing cavity spacing design is based only on the range of cracks and seepage, without considering the dynamic disturbance of fracture waves and collapse shock waves. The design of safe spacing between multi-cavity structures lacks scientific basis.

[0007] 5. Insufficient sensor receiving sensitivity; conventionally laid-out optical fibers have weak ability to receive elastic waves incident horizontally around the salt cave, and weak high-frequency breakage signals are easily lost, further reducing the reliability of monitoring and analysis. Summary of the Invention

[0008] In response to the above situation and to overcome the shortcomings of the prior art, this invention provides a method, system and application for monitoring seismic waves of interlayer rupture in salt cavern gas storage facilities. It aims to solve the problems of strong intrusiveness in the monitoring of existing technologies, inability to distinguish between rupture waves and collapse shock waves, and lack of quantitative inversion of mechanical parameters and calculation methods for engineering applications.

[0009] The technical solution adopted in this invention is as follows: A method for monitoring seismic waves of interlayer failure in a salt cavern gas storage facility, based on a method for dynamically detecting the cavity morphology of the cavity using interlayer failure wave dynamics, involves deploying monitoring optical fibers in a ring around the outside of the salt cavern cavity, and using the signal time difference of the secondary seismic waves generated by the interlayer failure seismic waves and the impact of falling rocks to invert the cavity geometry, including the following steps: S1. Monitoring array layout: An even number of monitoring boreholes are evenly arranged around the cavity. The borehole depth penetrates below the bottom plate elevation of the gas storage tank. Fiber optics are laid inside each borehole to form a three-dimensional spatial sensing array around the cavity. S2. Signal classification and preprocessing: The original waveform data of the fracture wave generated by the fracture of the interlayer in the cavity and the collapse shock wave generated by the rock mass falling and hitting the bottom of the cavity are collected synchronously through the sensing fiber.

[0010] The waveforms include fracture waves and collapse shock waves. The identification method includes four quantitative judgment dimensions and completes signal classification and identification through a weighted comprehensive judgment method.

[0011] The four quantitative judgment dimensions include frequency domain judgment, timing judgment, pulse number judgment, and rise time ratio judgment. The frequency domain determination step is as follows: perform frequency domain analysis and calculation on the collected rock mass vibration signal, extract the signal main frequency parameter, and if the signal main frequency is >259Hz, it is determined to be a fracture wave signal; if the signal main frequency is <200Hz, it is determined to be a collapse shock wave signal. The timing determination step is as follows: For two sets of vibration signals corresponding to the same rock mass collapse event, identify the reception time of the fracture wave signal and the collapse shock wave signal. The reception time of the fracture wave signal is earlier than the reception time of the collapse shock wave signal, and the time interval between the two sets of signals is equal to the free fall time of the rock block. These signals are then determined to be valid corresponding signals. The pulse quantity determination step is as follows: pulse feature extraction is performed on the vibration signal waveform. If the signal is a single pulse waveform, it is determined to be a broken wave signal; if the signal contains multiple continuous impact pulse waveforms with gradually decreasing amplitude, it is determined to be a collapse impact wave signal. The rise time ratio determination step is as follows: the critical threshold of the critical rise time ratio is obtained by calibration through on-site salt cavern measured data, the rise time ratio of the vibration signal to be measured is compared with the critical threshold, if the rise time ratio of the signal is less than the critical value, it is determined to be a rupture wave signal; if the rise time ratio of the signal is greater than the critical value, it is determined to be a collapse shock wave signal. The weighted comprehensive judgment step is as follows: the weight coefficients corresponding to the four judgment dimensions are pre-calibrated using on-site measured data, the sum of the four weight coefficients is 1, a weighted comprehensive judgment function is constructed based on the weight coefficients, the judgment results of each dimension are substituted into the function value, and the rupture wave and collapse shock wave are automatically distinguished and identified according to the positive and negative attributes of the function calculation results.

[0012] This invention also includes the application of the seismic wave monitoring method for interlayer failure in salt cavern gas storage in the quantitative inversion of mechanical parameters of interlayer damage, comprising the following steps: (1) Based on the measured data of waveform amplitude and signal travel time of the classified fracture wave and collapse shock wave; (2) Based on the seismic moment calculation formula, the equivalent seismic moment of the interlayer is inverted, and combined with the shear modulus of the mudstone interlayer and the average slip of the interlayer, the actual fracture area of ​​the interlayer is calculated. (3) Calculate the total mass of the collapsed and detached rock mass based on the interlayer thickness, interlayer rock mass density and fracture area, and then convert the equivalent magnitude of the interlayer fracture by combining the waveform characteristics to complete the quantitative inversion of the mechanical parameters of the interlayer damage.

[0013] This invention also includes the application of the seismic wave monitoring method for interlayer failure in salt cavern gas storage in the calculation of sediment parameters at the bottom of the cavity, wherein the calculation of sediment parameters at the bottom of the cavity includes sediment parameter calculation during the cavity operation stage and sediment parameter calculation during the cavity construction stage; The sediment parameters during the operation of the cavity are calculated by using the difference between the top elevation of the sediment corresponding to the collapse impact signal and the bottom elevation of the cavity, which is known or calculated from historical cavity construction data, to calculate the actual thickness of the sediment; and by combining the propagation time difference of the breaking wave and the collapse impact wave in the rock strata and sediment, the sediment porosity and porosity ratio are inverted. The sediment parameters during the cavity-building stage are calculated, and a coupled calculation model is established for the collapse accumulation and uplift rate, cavity erosion and sinking rate, and sediment compaction rate to dynamically characterize the sediment thickness variation law.

[0014] This invention also includes the application of the seismic wave monitoring method for interlayer failure in salt cavern gas storage in determining the safe spacing of multiple cavities. The method for determining the safe spacing of multiple cavities includes the following steps: (1) Using the energy difference and propagation distance difference of the same seismic wave collected from adjacent monitoring points, the energy attenuation coefficients of the breaking wave in the mudstone interlayer and the collapse shock wave in the salt rock matrix were respectively calibrated. (2) Calculate the effective disturbance radius of the breaking wave and the collapse shock wave according to the energy attenuation law, and determine the minimum safe layout distance between adjacent salt cavern gas storage facilities based on the maximum disturbance radius of the two types of waves, combined with the preset safety redundancy coefficient.

[0015] The optical fiber is a helical-wound distributed acoustic sensing fiber, which is continuously laid out along the drilling depth direction. The helical structure enhances the signal reception sensitivity of horizontally incident elastic waves.

[0016] The impact force of the collapse shock wave was calculated using the Hertz contact theory formula; the falling acceleration and final impact velocity of the rock block were corrected by substituting the corresponding medium density parameters into the brine immersion condition and the high-pressure gas storage condition, respectively, and the relevant medium parameters were calibrated by on-site measurement or geological survey data.

[0017] The present invention also includes a seismic wave monitoring system for interlayer failure in a salt cavern gas storage facility, comprising an optical fiber sensing acquisition unit, a waveform intelligent classification module, a mechanical parameter inversion module, and a sediment and cavity design calculation module; The fiber optic sensing acquisition unit is used to acquire raw waveform data of breaking waves and collapse shock waves. The waveform intelligent classification module is used for automatic waveform recognition; The mechanical parameter inversion module is used for inversion calculation of interlayer mechanical parameters; The sediment and cavity design calculation module is used for sediment measurement and cavity spacing calculation logic calculation. Data interaction links are established between modules to achieve closed-loop data transmission for data collection, classification, parameter inversion, and engineering parameter calculation.

[0018] The beneficial effects of the present invention after adopting the above structure are as follows: (1) Non-invasive full-cycle monitoring with wide coverage and high sensitivity By deploying monitoring boreholes and optical fibers in the surrounding rock area outside the cavity, without intruding into the cavity and without affecting water-soluble cavity construction and high-pressure gas storage operations, continuous monitoring can be achieved throughout the entire life cycle of gas storage cavity construction and operation. The use of eight sets of monitoring boreholes arranged in a ring and evenly, combined with a layout that penetrates the bottom plate and spiral-wound sensing optical fibers, significantly improves the receiving sensitivity of horizontally incident elastic waves, eliminates deep monitoring blind spots, and achieves full-height and all-round signal coverage of the cavity.

[0019] (2) Four-dimensional quantitative discrimination combined with weighted comprehensive recognition results in high waveform classification accuracy and fault tolerance. This innovative approach proposes four quantitative judgment dimensions: frequency domain, timing, pulse count, and rise time ratio. By combining these dimensions with on-site measured calibration weights, a weighted comprehensive discrimination function is constructed, overcoming the shortcomings of single discrimination rules that are prone to misjudgment. Even if some signals are distorted by signal-to-noise ratio or operating conditions, the remaining dimensions can still achieve effective identification, realizing automated and high-precision differentiation between fracture waves and collapse shock waves.

[0020] (3) Achieve quantitative inversion of mechanical parameters of interlayer damage and quantify disaster risk. Based on the classified waveform data and combined with seismic moment and rock mechanics theories, the core mechanical parameters such as the equivalent seismic moment, fracture area, average slip, total mass of detached rock mass, and equivalent magnitude of the interlayer are inverted step by step. This enables the assessment of the degree of interlayer damage from qualitative judgment to quantitative evaluation, providing data support for early warning of interlayer instability and cavity collapse risks.

[0021] (4) Calculate the sediment parameters at the bottom of the cavity in stages and dynamically monitor the cavity capacity. Differentiated calculation models are established for different operating conditions during the cavity building and operation phases: during the cavity building phase, a multi-rate coupling model is used to characterize the dynamic change law of sediment thickness; during the operation phase, sediment thickness is directly calculated, and sediment porosity and void ratio are inverted by combining elastic wave propagation time difference to accurately grasp the sediment accumulation state at the bottom of the cavity, ensuring the effective storage capacity and structural stability of the gas storage facility.

[0022] (5) Determine the safe spacing of the cavity group based on the attenuation of wave energy to improve the scientific nature of the cavity group design. By measuring and calibrating the energy attenuation coefficients of two types of fluctuations, calculating the effective disturbance influence radius, and combining the safety redundancy coefficient to determine the minimum safe distance between adjacent salt caverns, the technical gap in the design of group cavity layout under dynamic disturbance factors is filled, ensuring the overall surrounding rock safety of large group cavity gas storage facilities.

[0023] (6) Accurate calculation of impact force under multiple working conditions, adaptable to mechanical analysis of all scenarios Based on Hertz contact theory, this study distinguishes between two working conditions: brine immersion during cavity construction and high-pressure gas storage during operation. It corrects the falling acceleration and final impact velocity of the rock block, accurately calculates the impact force of the collapse shock wave, and improves the source mechanics analysis system. Attached Figure Description

[0024] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used together with the embodiments of the invention to explain the invention and do not constitute a limitation thereof.

[0025] Figure 1 The diagram shows the distribution of monitoring points and cavities proposed in this invention, illustrating a uniform and symmetrical distribution of eight monitoring points. Detailed Implementation

[0026] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0027] It should be noted that, in this document, relational terms such as "first" and "second" are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0028] Example 1: A method for monitoring seismic waves during interlayer failure in a salt cavern gas storage facility. Based on a method for dynamically detecting the cavity morphology using interlayer failure waves, a monitoring optical fiber is deployed in a ring around the outside of the salt cavern cavity. The cavity geometry is then inverted using the signal time difference between the interlayer failure seismic wave and the secondary seismic wave generated by the impact of falling rocks. The method includes the following steps: S1. Monitoring array layout: Eight monitoring boreholes are evenly arranged around the cavity. The borehole depth extends below the bottom plate elevation of the gas storage tank. Spiral wound distributed acoustic sensing optical fibers are laid inside each borehole to form a three-dimensional spatial sensing array around the cavity. A helical-wound distributed acoustic sensing fiber is deployed within the surrounding rock mass of the gas storage facility. By receiving elastic wave signals that penetrate the surrounding rock and propagate to the fiber, indirect but continuous monitoring of mechanical events within the cavity is achieved. The specific implementation plan is as follows: Eight micro-monitoring boreholes are evenly distributed circumferentially around the gas storage facility. The closest distance between each borehole and the designed cavity wall is 10 meters to ensure that the boreholes are located within a stable area of ​​the surrounding rock and do not adversely affect the cavity's sealing performance or structural integrity. The borehole depth must penetrate to a certain depth below the gas storage facility's floor elevation to achieve signal coverage across the entire height range of the cavity (including the bottom area). A helical-wound distributed optical fiber acoustic sensor (hereinafter referred to as the optical fiber) is inserted into each borehole, and the fiber's insertion depth must also exceed the gas storage facility's floor elevation. Compared to a straight-line deployment, the helical winding structure can receive waves perpendicular to the fiber's direction of deployment. The eight monitoring boreholes are evenly distributed circumferentially around the cavity, with an angle of 45° between each borehole. Combined with the continuous sensing capability of the optical fiber along the depth direction within each borehole, they together form a three-dimensional spatial sensing array surrounding the gas storage facility. This array can simultaneously receive elastic wave signals generated by mechanical events such as fractures and collapses within the cavity from multiple directions and depths.

[0029] S2. Signal classification and preprocessing: The original waveform data of the fracture wave generated by the fracture of the interlayer in the cavity and the collapse shock wave generated by the rock mass falling and hitting the bottom of the cavity are collected synchronously through the sensing fiber.

[0030] Determining the location of the fracture To achieve precise positioning in three-dimensional space, a unified coordinate reference system must first be established. For example... Figure 1As shown, with the vertical direction of monitoring borehole No. 3 as the z-axis and downward as the positive direction, the orientation of the x-axis and y-axis is shown in the figure. The surface opening of monitoring borehole No. 3 is taken as the origin of the coordinate system, and a right-hand rectangular coordinate system is established.

[0031] In this coordinate system, the coordinates of the known and unknown quantities are expressed as follows. The burial depth of the interlayer is a known geological parameter, therefore the depth coordinates of the target interlayer at monitoring borehole No. 3 can be directly determined. Eight monitoring boreholes are evenly distributed along the circumference of the cavity, with an azimuth angle of 45 degrees between adjacent boreholes. The horizontal distance of each borehole from the cavity axis is a known design parameter. Based on this, the spatial coordinates of the adjacent monitoring borehole No. 4 at the elevation of the target interlayer can also be directly given. Let point P on the interlayer be the location of the fracture to be determined, with three unknown components in its coordinates. The height of the interlayer is known, therefore the height of the interlayer at position No. 3 is ( The horizontal angle between the monitoring positions is 45°, so the coordinates of the mezzanine position at position 4 are ( (R is the radius), the coordinates of point P (the break point) are ( HelicalDAS (Helical Distributed Acoustic Sensing Fiber) is a distributed helical vibration monitoring fiber that can determine the direction and time difference of waves, thus enabling the monitoring of... And the time difference when it propagates to points 3 and 4. The coordinates of point P are ( ).

[0032] in:

[0033] in, The straight-line distance from the break point to monitoring position 3. This represents the straight-line distance from the break point to monitoring position 4. The time difference between the arrival times at the two monitoring positions. The propagation velocity of the longitudinal wave in the mudstone interlayer (unit: m / s). If When larger:

[0034] The coordinates of the break point at this time are: .

[0035] when When larger:

[0036] The coordinates of the fracture location at this time are: .

[0037] In practical applications, eight monitoring boreholes can form multiple pairs of adjacent combinations. By using multiple sets of independent observation data for joint positioning or least squares fitting, the positioning accuracy can be further improved and the results can be cross-validated.

[0038] Example 2: The waveform includes a breaking wave and a collapse shock wave. The identification method includes four quantitative judgment dimensions and completes signal classification and identification through a weighted comprehensive judgment method.

[0039] The distinguishability between fracture waves and collapse shock waves stems from their fundamentally different source mechanics: fracture waves originate from the rapid release of elastic strain energy at the instant the fracture surface is breached, while collapse shock waves originate from the instantaneous conversion of gravitational potential energy and momentum transfer when fragments impact the bottom of the cavity. This mechanism inevitably leaves differences in frequency domain characteristics, temporal structure, and waveform morphology.

[0040] We will now construct four discrimination criteria from three dimensions: frequency domain, time domain, and waveform, and on this basis, establish a weighted comprehensive evaluation system to achieve systematic identification of the two types of waves.

[0041] I. Frequency Domain Analysis

[0042] The rapid stress release under stress concentration in mudstone interlayers is essentially a rapid potential energy crack propagation process. This means that the fracture source wave has an extremely short duration and extremely high frequency. Based on the Brune disk model, we have:

[0043]

[0044] in, Frequency (Hz); Here, we take the coefficient (dimensionless) as 0.37. The longitudinal wave velocity in the interlayer is 3500 m / s. The breaking length is usually less than 5m, so the frequency of the breaking wave is greater than 259Hz.

[0045] For collapse shock waves, the frequency is determined by the impact time:

[0046]

[0047] This refers to the impact time. The impact time is usually higher than 5ms, which means that its frequency is usually less than 200Hz. When the sediment layer is thick and the buffering effect is significant, the contact time is further extended and the dominant frequency will be even lower.

[0048] The comparison shows that the dominant frequency of the rupture wave is significantly higher than that of the collapse shock wave, and there is a identifiable frequency boundary between the two. Based on this, the first discrimination criterion is established:

[0049]

[0050] There is an ambiguous range in the frequency domain, between 200 and 259 Hz, which cannot be determined by frequency domain alone. It is necessary to rely on the other three dimensions for weighted judgment.

[0051] II. Time Series Analysis

[0052] (1) The chronological order of events

[0053] The causal sequence of fracture-collapse indicates that the fracture event must precede the collapse impact event. The time interval between the two events being received by the sensor can be expressed as:

[0054]

[0055] in, This refers to the time difference received by the sensor. Time of descent; The propagation path in the salt rock; The longitudinal wave propagation velocity in the salt rock is 4500 m / s. The propagation path in mudstone; The propagation velocity of longitudinal waves in mudstone is 3500 m / s.

[0056] This time interval consists of three components: the free fall time of the fragments and the path time difference of the two types of waves as they propagate through the salt rock and mudstone media to the sensor, respectively. The longitudinal wave propagation velocity in salt rock is taken as 4500 m / s, and in mudstone as 3500 m / s. A comparative analysis of magnitude shows that the wave propagation path in the rock mass generally does not exceed the maximum diameter of the cavity; taking an extreme case of a propagation path of 100 meters, the resulting propagation time difference is approximately 22 milliseconds, on the order of milliseconds. However, the fragments typically fall over a distance exceeding 10 meters, corresponding to a free fall time of at least 1 second, on the order of seconds. Therefore, the correction for the propagation path time difference (on the order of milliseconds) is much smaller than the fragment fall time (on the order of seconds). Thus, the interval between the two events recorded by the sensor is approximately equal to the fragment fall time. This means that the fracture wave and the collapse shock wave can be clearly distinguished as a pair of events with a definite sequence on the time axis, based on which the second discrimination criterion is established:

[0057]

[0058] When time difference In this case, the preceding event of the event pair is a rupture wave, and the following event of the event pair is a collapse shock wave.

[0059] III. Pulse Quantity Analysis

[0060] After fragments impact the cavity floor, they typically bounce multiple times under the alternating effects of rebound force and their own weight. Each impact generates an impact pulse, forming a multi-peak sequence with progressively decreasing amplitude. This characteristic is an inevitable product of the collision-bounce dynamics process, and its peak decay rate depends on the magnitude of the coefficient of restitution. In contrast, the fracture wave is generated by a single crack penetration event and is essentially a single-pulse signal, lacking a systematic, repetitive peak structure. Based on this, the third discrimination criterion is established:

[0061]

[0062] in This represents the number of effective impulse pulse peaks in the waveform.

[0063] IV. Rise Time Ratio Analysis

[0064] The rise time ratio is a dimensionless parameter characterizing the steepness of the initial waveform, defined as the ratio of rise time to total duration. Rise time refers to the time from the initial movement of the signal to the first peak; total duration refers to the entire time span during which the signal amplitude is above the noise threshold.

[0065]

[0066] Rise time ratio; For the rising time, This represents the total duration.

[0067] The source process of the fracture wave is that the crack is instantly penetrated and the energy is released in a concentrated manner in a very short time. Therefore, the waveform is a pulse signal with a sharp start and a rapid peak. The rise time accounts for a very small proportion of the total duration, and the rise time ratio is low.

[0068] The collapse shock wave is a buffered contact process in which fragments impact the sediment layer at the bottom of the cavity. The contact force gradually increases to its peak value with the depth of penetration into the sediment.

[0069] Therefore, the waveform is a gradually changing signal with a relatively flat start and a long duration, and the rise time ratio is relatively high.

[0070] Based on this, the fourth discrimination criterion is established:

[0071]

[0072] Rise time ratio; The critical threshold, i.e., the critical rise time ratio, is an empirical parameter obtained by actual testing of the obtained rock samples.

[0073] The four criteria described above characterize the differences between rupture waves and collapse shock waves from the dimensions of frequency domain, time sequence, pulse number, and rise time ratio, respectively. However, a single criterion may lead to misjudgment under certain special conditions due to insufficient signal-to-noise ratio, signal truncation, or parameter degradation. To improve the reliability of identification, a weighted comprehensive discriminant function is introduced, unifying the four criteria into a quantitative evaluation system.

[0074]

[0075] in, As weight, , Represents breaking wave, It represents the shock wave from the collapse.

[0076] Table 1. Four-dimensional comprehensive discrimination criteria for fracture wave and collapse shock wave

[0077]

[0078]

[0079] The weights of each criterion reflect their relative importance and reliability in practical identification, and the sum of the weights is 1. Each criterion contributes positive and negative values ​​to the fracture wave and the collapse shock wave, respectively (Table 1). The sign of the comprehensive discrimination function indicates the type of wave: a positive value indicates a fracture wave, and a negative value indicates a collapse shock wave.

[0080] The advantage of this comprehensive discrimination method is that even if individual criteria fail due to data quality issues, the remaining criteria can still provide effective discrimination contributions, and the overall identification result has strong fault tolerance. In engineering applications, the critical thresholds and weights of each criterion can be iteratively calibrated based on field measurement data to adapt to the specific geological and operating conditions of different gas storage facilities.

[0081] Example 3: Inversion of fracture parameters

[0082] After determining the location of the fracture, the waveform amplitude and strain information recorded by the optical fiber can be used to quantitatively invert the mechanical parameters of the fracture event. The inversion process follows a step-by-step calculation logic from magnitude to seismic moment, from seismic moment to fracture area, and from fracture area to collapse mass.

[0083] Regarding magnitude, based on the classic magnitude calculation formula in seismology, the equivalent magnitude of the rupture event can be estimated using strain information recorded by optical fibers.

[0084]

[0085] in, Let be the seismic moment of the rupture event (unit: N·m), and a and b be empirical scaling coefficients. The seismic moment was established through indoor interbedded rock sample rupture tests. The relationship was obtained by fitting the logarithmic scaling of the strain energy E.

[0086] At the same time, the energy density per unit volume is

[0087]

[0088] in

[0089]

[0090] in The elastic modulus of the rock strata (mudstone or salt rock), expressed in MPa; For adaptability, dimensionless; For stress, MPa.

[0091] The strain energy E of a fracture event is defined as the integral of the strain signal measured by the optical fiber over time:

[0092]

[0093] in, The axial strain signal that varies with time and is measured by distributed optical fiber can be obtained through DAS optical fiber monitoring. t is the duration of the breakage event, the upper limit of integration is the end time of the event, and l is the distance from the breakage point to the monitoring point in meters.

[0094] The average slip u (in meters) of the fracture surface is linearly related to the strain energy characteristic value E:

[0095]

[0096] Wherein, k is a rock mechanics calibration coefficient (dimensionless constant), which can be obtained by linear fitting after establishing the strain energy-slip calibration curve through indoor direct shear tests on the target interlayer rock mass.

[0097] There are well-established empirical scaling relationships between seismic moment and mean slip, and between magnitude and seismic moment. Physically, seismic moment is equal to the product of medium shear stiffness G, fracture area A, and mean slip u, i.e., M0 = G. A u. For interbedded rock mass fracture events, the average slip, seismic moment, and equivalent magnitude of the fracture event can be estimated from the strain signal measured by optical fiber using the above formula chain.

[0098] Therefore, the fracture area is:

[0099]

[0100] Where a and b are empirical scaling coefficients, and the seismic moment is established through indoor interbedded rock sample fracture tests. The relationship between the strain energy E and the logarithmic scaling is obtained; k is the rock mechanics calibration coefficient.

[0101] Regarding the collapse mass:

[0102]

[0103] in The density of the interlayer; The breaking area; This refers to the thickness of the interlayer.

[0104] Example 4: Determination of safe spacing between multiple cavities

[0105] Due to the volume limitations of single-cavity gas storage facilities, multi-cavity gas storage schemes are becoming increasingly common in engineering practice. The rational determination of cavity spacing is a core issue in multi-cavity design. Current cavity spacing designs are mainly based on the analysis of fracture development range and gas seepage range. However, the dynamic disturbances generated by interlayer fracture and fragment impacts on the cavity floor can also adversely affect the surrounding rock stability of adjacent cavities. Therefore, quantitatively determining the effective influence range of fracture waves and collapse shock waves is of significant reference value for the safe design of multi-cavity layouts. When elastic waves propagate in rock mass, their energy continuously attenuates with increasing propagation distance due to the combined effects of geometric diffusion and medium absorption, until it decreases to a level that no longer has a meaningful impact on the stability of the surrounding rock. Therefore, there must exist a quantifiable boundary for the wave influence range.

[0106] (1) Breaking wave

[0107] By utilizing the energy difference of the fractured wave received from two different monitoring locations, combined with the distance difference between the two monitoring locations and the fracture source, the energy loss coefficient of the fractured wave propagating in the interlayer medium can be obtained. This coefficient reflects the attenuation rate of elastic wave energy per unit propagation distance. Furthermore, starting from the initial energy at the receiving point and ending with the disappearance of the surrounding rock disturbance energy, the effective influence range of the fractured wave in the interlayer medium can be calculated using the energy attenuation relationship.

[0108] The strain energy of the wave at the receiving point is:

[0109]

[0110] like Figure 1 The energy difference between monitoring sites 3 and 4 is (since the strata are evenly distributed and the propagation distance is relatively short, the energy loss here is considered to be linear):

[0111]

[0112] Therefore, the influence range of the wave in the interlayer is:

[0113]

[0114] in For energy difference, The fluctuation energy at monitoring position 3; The energy at monitoring position 4, The energy loss coefficient in the interlayer. , It represents the distance difference; This represents the range of influence of waves in the interlayer.

[0115] (2) Collapse shock wave

[0116] The propagation medium of the collapse shock wave is mainly the salt rock and sediment layer at the bottom of the cavity, and its energy attenuation characteristics differ from those of the fracture wave in the interlayer. However, as the cavity operates for a longer period, the sediment at the bottom gradually thickens, and the presence of the sediment layer significantly alters the propagation and attenuation characteristics of the shock wave. Therefore, using monitoring data from the first collapse event (when there is no sediment accumulation at the bottom of the cavity) to calibrate the energy loss coefficient in the salt rock can obtain a benchmark attenuation parameter that most closely approximates the conditions of a pure salt rock medium. The calibration method is similar to that for fracture waves: based on the energy difference of the collapse shock wave received from two adjacent monitoring positions and the corresponding distance difference, the energy loss coefficient in the salt rock is obtained by inversion, and then the effective influence range of the collapse shock wave in the salt rock is calculated.

[0117]

[0118] Therefore, the range of influence in salt rock is as follows:

[0119]

[0120] in Due to the difference in impact energy, This refers to the fluctuation energy at one of the monitoring locations; This represents the fluctuation energy at adjacent monitoring locations. This is the energy loss coefficient in salt rock. It represents the distance difference; This refers to the range of wave influence in the salt rock.

[0121] Example 5: Determination of the characteristics of sediment at the bottom of the cavity

[0122] The characteristics of sediment, including sediment thickness, porosity, and void ratio, differ from mass and fracture area. Sediment characteristics also differ between the cavity construction and operation phases. The elevation of the nearest vibration location, obtained through surrounding fiber optic vibration monitoring, represents the elevation of the impact surface after the interlayer collapses, i.e., the elevation of the sediment accumulation surface. .

[0123] I. Sediment Thickness

[0124] (1) During cavity construction, the cavity is under continuous water-soluble mining, and the bottom and surrounding rock walls are constantly eroded and retreat, resulting in a continuous decrease in the bottom elevation and a continuous expansion of the cavity cross-sectional area. Simultaneously, interlayer collapse debris continues to accumulate, raising the elevation of the top surface of the sediment layer. Furthermore, the sediment layer undergoes compaction under its own gravity, leading to a reduction in sediment volume. Therefore, the rate of change in sediment thickness during cavity construction is the superposition of three competing effects: the sediment surface uplift rate caused by the accumulation of collapse debris, the effective sediment settling rate caused by the expansion of dissolution at the bottom and sides of the cavity (the decrease in bottom elevation and expansion of cross-sectional area result in a reduction in the thickness of sediment of the same volume), and the thickness compression rate caused by self-weight compaction. Therefore:

[0125]

[0126] in The change rate of sediment thickness (unit: m / d); The rate of uplift of the top surface of sediment caused by the accumulation of debris from the collapse of the interlayer is calculated in real time from the frequency of the collapse event and the volume of the debris (unit: m / d). The sediment settling rate (in m / d) is caused by the dissolution and expansion at the bottom and sides. The thickness compression rate (in m / d) is the rate at which sediment is compacted under its own weight.

[0127] Since fiber optic monitoring can only determine the elevation of the top surface of the sediment, while the elevation of the bottom of the cavity is constantly changing during cavity construction and cannot be directly obtained by external monitoring, the absolute thickness of the sediment cannot be determined independently by relying solely on fiber optic vibration monitoring data during the cavity construction period.

[0128] (2) During the operation period, after entering the operation period, the cavity has been constructed and has reached the design parameters, the dissolution process has stopped, and the bottom elevation of the cavity has been determined. This becomes a known constant (unit: m). At this point, the epicenter of the interlayer collapse impact event is located using a fiber optic vibration monitoring system, and its elevation is taken as the top elevation of the sediment accumulation surface. (Unit: m) then the thickness of the sediment Calculated directly using the following formula:

[0129]

[0130] in, Thickness of sediment at the bottom of the cavity during operation (in meters); Elevation of the top surface of sediment obtained by fiber optic monitoring (unit: m); The design elevation of the cavity bottom (unit: m).

[0131] II. Sludge Void Ratio and Porosity

[0132] According to the standard definition in soil mechanics, void ratio and volumetric porosity satisfy the following conversion relationship:

[0133]

[0134] Therefore, the porosity can be calculated simply by determining the porosity. The propagation time per unit distance is... The propagation distance in the pore is The propagation distance in mudstone is Therefore there is When the interlayer breaks and collapses, it will deviate, but the amount of deviation is relatively small compared to the diameter. Therefore, a simplifying assumption needs to be introduced: the trajectory of the fragments after the interlayer breaks is a vertical straight line, without significant horizontal deviation. This assumption is based on the following considerations: although the fragments are inevitably disturbed by fluid resistance and uneven airflow within the cavity during their descent, the horizontal deviation of the fragments is small compared to the horizontal scale of the cavity and can be ignored.

[0135] Based on this, we have:

[0136]

[0137]

[0138]

[0139]

[0140]

[0141] so:

[0142]

[0143] so:

[0144]

[0145]

[0146] in The porosity of the sediment is dimensionless. The void ratio of the sediment is dimensionless. The actual propagation distance from the source of the break to the monitoring point (unit: m); The moment when the broken wave was received (unit: seconds); The moment when the collapse shock wave was received (unit: seconds). The propagation time of the fracture wave in the mudstone interlayer (unit: s); The free fall time (in seconds) of the fragment from the fracture site to the sediment surface. This is the ratio of the distance traveled in the sediment to the total distance, and it is dimensionless. The propagation velocity of longitudinal waves in the mudstone interlayer (unit: m / s); The propagation speed of longitudinal waves in sediment (unit: m / s); Total propagation time (in seconds); The longitudinal wave propagation velocity (unit: m / s) of the fluid (brine or high-pressure natural gas) within the pores of the sediment.

[0147] Example 6: Determination of the impact force of the collapse shock wave

[0148] The core triggering mechanism of the collapse shock wave caused by the instability and collapse of the overlying rock mass is the intense momentum transfer resulting from the high-speed impact of the unstable rock block against the underlying bearing surface (such as the tunnel floor, sedimentary deposits, or underlying rock strata). During the collapse of the overlying rock strata, enormous gravitational potential energy is converted into kinetic energy; the impact generates intense kinetic energy dissipation and momentum impact force (rather than the fracture of the interlayer itself). Essentially, this wave is a disturbance wave generated by the impact of a large-scale collapse, and its long wavelength characteristic allows it to propagate over greater distances.

[0149] The study of collapse shock waves inevitably involves the calculation of impact force. Collapse in a gas storage facility satisfies free fall motion, but the buoyancy of the brine and high-pressure gas inside the cavity cannot be ignored. During the cavity construction period, the cavity is filled with saturated brine, while during the operation period, it is filled with high-pressure gas.

[0150] The velocity of the rock block before impact was:

[0151]

[0152] (1) During the vocal cord development period:

[0153]

[0154] (2) During the operation period:

[0155]

[0156] And:

[0157]

[0158] Right now:

[0159]

[0160] so:

[0161]

[0162] Regarding impact force, based on Hertz contact theory, we have:

[0163]

[0164] in

[0165]

[0166] During the impact, the kinetic energy of the impacting body is converted into elastic energy, therefore:

[0167]

[0168] Right now:

[0169]

[0170] in:

[0171]

[0172] so:

[0173]

[0174] The initial condition is t=0. :

[0175]

[0176] when At this point, the deformation is at its maximum, and the impact force is also at its maximum.

[0177]

[0178]

[0179] in, Impact velocity; It is the acceleration due to gravity; The height of the interlayer from the bottom of the cavity or the sediment; Acceleration of collapsed structures under different environments; The density of saturated brine; The volume of the rock block; The density of the rock block; The density of the high-pressure gas; The pressure of the gas inside the cavity; For gas volume; For gas mass; It is the gas constant; The gas temperature; For impact force; For coefficients; This is the normal deformation amount; It is the equivalent elastic modulus; The equivalent radius; The mass of the collapsed block.

[0180] Example 7: A seismic wave monitoring system for interlayer failure in a salt cavern gas storage facility, comprising an optical fiber sensing acquisition unit, a waveform intelligent classification module, a mechanical parameter inversion module, and a sediment and cavity design calculation module;

[0181] The fiber optic sensing acquisition unit is used to acquire raw waveform data of breaking waves and collapse shock waves.

[0182] The waveform intelligent classification module is used for automatic waveform recognition;

[0183] The mechanical parameter inversion module is used for inversion calculation of interlayer mechanical parameters;

[0184] The sediment and cavity design calculation module is used for sediment measurement and cavity spacing calculation logic calculation.

[0185] Data interaction links are established between modules to achieve closed-loop data transmission for data collection, classification, parameter inversion, and engineering parameter calculation.

[0186] Although embodiments of the present invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents. In conclusion, if those skilled in the art, inspired by this description, design similar structural methods and embodiments without departing from the inventive spirit of the invention, such designs should fall within the protection scope of the present invention.

Claims

1. A method for monitoring seismic waves in the interlayer fracture of a salt cavern gas storage facility, characterized in that, A monitoring fiber optic cable is laid out in a ring around the outside of the salt cavern cavity, and the cavity geometry is inverted using the signal time difference between the seismic wave from the interlayer fracture and the secondary seismic wave generated by the impact of the falling rock. The steps include: S1. Monitoring array deployment: Multiple monitoring boreholes are evenly arranged around the cavity, with the borehole depth penetrating below the bottom plate elevation of the gas storage tank. Fiber optics are laid inside each borehole to form a three-dimensional spatial sensing array surrounding the cavity. S2. Signal classification and preprocessing: The original waveform data of the fracture wave generated by the fracture of the interlayer in the cavity and the collapse shock wave generated by the rock mass falling and hitting the bottom of the cavity are acquired synchronously through optical fiber. The waveforms include fracture waves and collapse shock waves; The waveform recognition method includes four quantitative judgment dimensions and completes signal classification and recognition through a weighted comprehensive judgment method. The four quantitative judgment dimensions include frequency domain judgment, timing judgment, pulse number judgment, and rise time ratio judgment. The frequency domain determination step is as follows: perform frequency domain analysis and calculation on the collected rock mass vibration signal, extract the signal main frequency parameter, and if the signal main frequency is >259Hz, it is determined to be a fracture wave signal; if the signal main frequency is <200Hz, it is determined to be a collapse shock wave signal. The timing determination step is as follows: For two sets of vibration signals corresponding to the same rock mass collapse event, identify the reception time of the fracture wave signal and the collapse shock wave signal. The reception time of the fracture wave signal is earlier than the reception time of the collapse shock wave signal, and the time interval between the two sets of signals is equal to the free fall time of the rock block. These signals are then determined to be valid corresponding signals. The pulse quantity determination step is as follows: pulse feature extraction is performed on the vibration signal waveform. If the signal is a single pulse waveform, it is determined to be a broken wave signal; if the signal contains multiple continuous impact pulse waveforms with gradually decreasing amplitude, it is determined to be a collapse impact wave signal. The rise time ratio determination step is as follows: the critical rise time ratio threshold is obtained by calibration through on-site salt cavern measured data, the rise time ratio of the vibration signal to be measured is compared with the critical threshold, if the rise time ratio of the signal is less than the critical value, it is determined to be a rupture wave signal; if the rise time ratio of the signal is greater than the critical value, it is determined to be a collapse shock wave signal. The weighted comprehensive judgment step is as follows: the weight coefficients corresponding to the four judgment dimensions are pre-calibrated through on-site measured data, the sum of the four weight coefficients is 1, a weighted comprehensive judgment function is constructed based on the weight coefficients, the judgment results of each dimension are substituted into the function value, and the rupture wave and the collapse shock wave are automatically distinguished and identified according to the positive and negative attributes of the function calculation results. The optical fiber is a spiral-wound distributed acoustic sensing fiber, which is continuously laid out along the drilling depth direction, and relies on the spiral structure to improve the signal receiving sensitivity of horizontally incident elastic waves. The impact force of the collapse shock wave was calculated using the Hertz contact theory formula; the falling acceleration and final impact velocity of the rock block were corrected by substituting the corresponding medium density parameters into the brine immersion condition and the high-pressure gas storage condition, respectively, and the relevant medium parameters were calibrated by on-site measurement or geological survey data.

2. The application of the seismic wave monitoring method for interlayer failure in salt cavern gas storage according to claim 1 in the quantitative inversion of mechanical parameters of interlayer damage, characterized in that, Includes the following steps: (1) Based on the measured data of waveform amplitude and signal travel time of the classified fracture wave and collapse shock wave; (2) Based on the seismic moment calculation formula, the equivalent seismic moment of the interlayer is inverted, and combined with the shear modulus of the mudstone interlayer and the average slip of the interlayer, the actual fracture area of ​​the interlayer is calculated. (3) Calculate the total mass of the collapsed and detached rock mass based on the interlayer thickness, interlayer rock mass density and fracture area, and then convert the equivalent magnitude of the interlayer fracture by combining the waveform characteristics to complete the quantitative inversion of the mechanical parameters of the interlayer damage.

3. The application of the seismic wave monitoring method for interlayer failure in salt cavern gas storage according to claim 1 in the calculation of sediment parameters at the bottom of the cavity, characterized in that: The calculation of sediment parameters at the bottom of the cavity includes the calculation of sediment parameters during the operation of the cavity and the calculation of sediment parameters during the cavity creation stage; The sediment parameters during the operation of the cavity are calculated by using the difference between the top elevation of the sediment corresponding to the collapse impact signal and the bottom elevation of the cavity, which is known or calculated from historical cavity construction data, to calculate the actual thickness of the sediment; and by combining the propagation time difference of the breaking wave and the collapse wave in the rock strata and sediment, the porosity and porosity ratio of the sediment are inverted. The sediment parameters during the cavity-building stage are calculated, and a coupled calculation model is established for the collapse accumulation and uplift rate, cavity erosion and sinking rate, and sediment compaction rate to dynamically characterize the sediment thickness variation law.

4. The application of the seismic wave monitoring method for interlayer failure in salt cavern gas storage according to claim 1 in determining the safe spacing of multiple cavities, characterized in that: The method for determining the safe spacing between multiple cavities includes the following steps: (1) Using the energy difference and propagation distance difference of the same seismic wave collected from adjacent monitoring points, the energy attenuation coefficients of the breaking wave in the mudstone interlayer and the collapse shock wave in the salt rock matrix were respectively calibrated. (2) Calculate the effective disturbance radius of the breaking wave and the collapse shock wave according to the energy attenuation law, and determine the minimum safe layout distance between adjacent salt cavern gas storage facilities based on the maximum disturbance radius of the two types of waves, combined with the preset safety redundancy coefficient.

5. A seismic wave monitoring system for interlayer failure in a salt cavern gas storage facility, used to execute the seismic wave monitoring method for interlayer failure in a salt cavern gas storage facility as described in claim 1, characterized in that, It includes an optical fiber sensing and acquisition unit, a waveform intelligent classification module, a mechanical parameter inversion module, and a sediment and cavity design calculation module; The fiber optic sensing acquisition unit is used to acquire raw waveform data of breaking waves and collapse shock waves. The waveform intelligent classification module is used for automatic waveform recognition; The mechanical parameter inversion module is used for inversion calculation of interlayer mechanical parameters; The sediment and cavity design calculation module is used for sediment measurement and cavity spacing calculation logic calculation. Data interaction links are established between modules to achieve closed-loop data transmission for data collection, classification, parameter inversion, and engineering parameter calculation.

Images