A suspended DAS borehole fracture fluid monitoring device and method

Abstract

The application discloses a kind of suspended DAS borehole fissure fluid monitoring devices, including DAS demodulator and sensing optical cable suspended in water-filled borehole, sensing optical cable bottom end fixed counterweight, sensing optical cable upper portion is wound on optical cable reel, optical cable reel is set on orifice shock isolation module, sensing optical cable and DAS demodulator are connected.A kind of suspended DAS borehole fissure fluid monitoring method is also disclosed, strain waveform data are collected, and first arrival time of event at different depths is extracted;Depth-first arrival time scatter plot is drawn, inflection point in time period is determined, linear regression is carried out to uplink wave data points and downlink wave data points, and the intersection point of two linear regression fitting straight lines determines focal depth and occurrence time, carries out solid tide modulation analysis and pressure diffusion and permeability evaluation.The application can realize the high sensitivity monitoring and quantitative dynamics analysis of fissure fluid activity, and is suitable for seismic geology research, underground energy development and major engineering safety monitoring and other fields.

Description

Technical Field

[0001] This invention belongs to the field of geophysical monitoring and hydrogeological exploration technology, specifically relating to a suspended DAS (distributed fiber optic acoustic sensing) borehole fracture fluid monitoring device and method, which can be used for in-situ and quantitative characterization of fracture fluid transport processes and mechanisms. Background Technology

[0002] DAS technology has been widely applied in recent years for borehole seismic signal acquisition, oil and gas production monitoring, and geothermal resource development. In borehole hydrological monitoring, existing methods typically involve fixing optical cables to the casing or borehole wall to identify fluid activity within the borehole. However, this rigid installation method has shortcomings: firstly, the fixing material may lead to uneven coupling between the optical cable and the borehole wall, generating mechanical friction noise and weakening the transmission of some high-frequency sound pressure signals; secondly, current research on the bending vibration response of optical cables in water-filled environments is still limited, restricting the ability to capture weak fluid signals. These problems may reduce the detection sensitivity and positioning accuracy of borehole fracture fluid events, making it difficult to meet the needs of high-precision hydrological dynamic monitoring and engineering safety early warning.

[0003] Furthermore, existing analytical methods largely remain at the level of event detection and preliminary localization, lacking in-depth exploration of fluid dynamic mechanisms. For example, there is a lack of quantitative characterization of the phase and amplitude response of solid tidal stress-modulated fracture fluid activity, and the spatiotemporal evolution analysis of event clusters does not fully utilize pore pressure diffusion and equivalent permeability. These shortcomings limit our understanding of fracture fluid-rock interaction processes and reduce the application potential of this technology in seismic geological research, energy development assessment, and safety monitoring of major engineering projects. Summary of the Invention

[0004] The purpose of this invention is to overcome the problems of insufficient sensitivity, limited event location accuracy, and lack of dynamic mechanism analysis in existing borehole fracture fluid monitoring technologies, and to propose a suspended DAS borehole fracture fluid monitoring device and method. This invention enables quantitative analysis of fluid dynamic parameters from the acquisition of highly sensitive strain waveform data, providing a reliable in-situ observation and quantitative analysis method for fracture fluid transport and its dynamic processes.

[0005] The above-mentioned objectives of the present invention are achieved through the following technical means:

[0006] A suspended DAS borehole fracture fluid monitoring device includes a DAS demodulator and a sensing optical cable suspended in a water-filled borehole. A counterweight is fixed at the bottom of the sensing optical cable, and the upper part of the sensing optical cable is wound on an optical cable drum. The optical cable drum is set on a borehole vibration isolation module, and the sensing optical cable is connected to the DAS demodulator.

[0007] As mentioned above, the sensing optical cable is a multi-core optical cable encapsulated with a non-metallic Kevlar fiber sheath.

[0008] As described above, the orifice isolation module is installed at the orifice of the water-filled borehole. The orifice isolation module includes an external support frame, inclined support dampers, and an isolation base. Multiple inclined support dampers are connected to the optical cable reel and the external support frame, and the isolation base is located at the bottom of the support frame.

[0009] A suspended DAS borehole fracture fluid monitoring device also includes a lightning protection module installed at the borehole opening of a water-filled borehole, and a remote transmission module connected to a DAS demodulator.

[0010] A suspended DAS borehole fracture fluid monitoring method includes the following steps:

[0011] Step 1: Use a DAS demodulator to continuously acquire strain waveform data excited by fracture fluid activity, coupled to the sensing optical cable through borehole water, and preprocess the strain waveform data to obtain strain data.

[0012] Step 2: Extract the first arrival time of events at different depths based on strain data;

[0013] Step 3: Based on the depth and arrival time of the event, plot the depth-arrival time scatter plot for each data point, group the data points into time periods, determine the inflection points within each time period, and divide the up-wave data points and down-wave data points into two categories using the inflection points as boundaries. Perform linear regression on the up-wave data points and down-wave data points respectively to obtain two linear regression fitted lines. If the slopes of the two linear regression fitted lines are opposite, it is determined to be a "<" shaped event, and the focal depth and occurrence time are determined by the intersection of the two linear regression fitted lines.

[0014] As described above, preprocessing strain waveform data includes removing the mean, detrending, and bandpass filtering.

[0015] As mentioned above, the arrival time is obtained based on the strain data after scanning preprocessing using a recursive short-time averaging-long-time averaging algorithm.

[0016] A suspended DAS borehole fracture fluid monitoring method also includes solid tide modulation analysis:

[0017] The time series of solid tidal body strain was calculated based on the theoretical tidal model. Events were statistically divided into frequency time series according to depth intervals and cross-spectral analysis was performed with the solid tidal body strain time series. Bandpass filters were used to extract the equal components of the lunar semi-diurnal tide and the lunar diurnal tide. The phase lag and normalized response coefficient of the frequency time series response to tidal strain were calculated.

[0018] A suspended DAS borehole fracture fluid monitoring method also includes pressure diffusion and permeability assessment:

[0019] Establish an event cluster cylindrical diffusion model: D=r 2 / 4(t-t0), where t is the occurrence time of any randomly selected event in the event cluster, t0 is the occurrence time of the earliest event in the event cluster, r is the source depth distance of the selected event relative to the earliest event in the event cluster, and D is the pore pressure diffusion coefficient. This is achieved by using the pore pressure diffusion coefficient of several selected events within the event cluster. 2 And t-t0, and perform linear regression based on the event cluster cylindrical diffusion model to obtain the pore pressure diffusion coefficient D;

[0020] Using the values ​​of r and t-t0 for several selected events within an event cluster, and performing linear regression based on v = r / (t-t0), the average migration velocity v is obtained.

[0021] Calculate the equivalent permeability K = μ × φ × C T ×D, where μ is the fluid viscosity, φ is the porosity, and C T This represents the overall compression ratio.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] 1. High sensitivity and high signal-to-noise ratio: The vertically suspended sensor cable deployment achieves efficient acoustic coupling between the sensor cable and the water in the water-filled borehole. The tensioned state enhances the sensor cable's response to water bending vibrations, improving the detection capability of weak signals; combined with the borehole isolation module design, it effectively suppresses environmental noise, making the detection performance of fracture fluid activity significantly superior to traditional fixed-type fiber optic cable deployment methods.

[0024] 2. Simple and efficient deployment: No need for fixed holes or complex hole operations, the deployment time for a single hole can be controlled within 1 to 2 hours. It is particularly suitable for deep holes, long-term observation holes and emergency monitoring scenarios, and has the advantages of high efficiency and flexibility in field applications.

[0025] 3. Possesses in-depth dynamic analysis capabilities: It can not only detect and precisely locate events, but also quantitatively estimate solid tide modulation parameters (phase lag and normalized response coefficient) and key hydraulic parameters of fracture zones (pore pressure diffusion coefficient and equivalent permeability). This allows for the revelation of the triggering and evolutionary laws of fracture fluid activity at the mechanistic level, overcoming the limitations of existing technologies that can only answer "when and where it occurs," and achieving a scientific explanation of "why it occurs and how it evolves."

[0026] 4. Wide range of applications: In addition to groundwater dynamic monitoring, this invention is also applicable to fault zone stress state assessment, oil and gas and geothermal resource development, and seepage stability evaluation of major projects (such as nuclear power plants and high dam foundations), which has both scientific research value and engineering application prospects. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the overall structural deployment of the suspended DAS borehole fracture fluid monitoring device of the present invention in a borehole.

[0028] Figure 2 This is a schematic diagram of the cross-sectional structure of the sensing optical cable used in this invention;

[0029] Figure 3 This is a schematic diagram of the orifice vibration isolation module used in this invention;

[0030] Figure 4 A schematic diagram for locating the earthquake source, where (a) is a schematic diagram of each data point in the depth-first arrival time scatter plot; and (b) is a schematic diagram of the linear regression fitting line between the up-wave data points and the down-wave data points.

[0031] Figure 5 The diagram shows the distribution of groundwater activity events in the depth-time domain, where (a) is a scatter plot of depth-first arrival time and (b) is a distribution map of depth-time domain.

[0032] Figure 6 The diagram shows the strain and event frequency sequence of solid tides and their cross-spectral analysis, where (a) is the normalized waveform of strain and events of the lunar main semi-diurnal tide M2, (b) is the schematic diagram of the phase lag of the lunar main semi-diurnal tide M2 ​​as a function of depth, (c) is the schematic diagram of the response coefficient of the lunar main semi-diurnal tide M2 ​​as a function of depth, (d) is the normalized waveform of strain and events of the lunar main diurnal tide O1, (e) is the schematic diagram of the phase lag of the lunar main diurnal tide O1 as a function of depth, and (f) is the schematic diagram of the response coefficient of the lunar main diurnal tide O1 as a function of depth.

[0033] Figure 7 The diagram shows the spatiotemporal migration characteristics of event clusters and the pore pressure diffusion analysis based on the cylindrical diffusion model. (a) Schematic diagram of the migration process of event clusters with source depth and occurrence time, (b) Schematic diagram of the statistical distribution of average migration velocity, (c) Schematic diagram of linear regression fitting under the diffusion model, and (d) Schematic diagram of the statistical distribution of pore pressure diffusion coefficient.

[0034] Figure 8 This is a schematic diagram of the process of the suspended DAS borehole fracture fluid monitoring device of the present invention.

[0035] Among them, 1-DAS demodulator; 2-orifice vibration isolation module; 3-fracture network; 4-water-filled borehole; 5-counterweight; 6-water inflow; 7-sensor optical cable; 8-drilling support;

[0036] 71-Single-mode optical fiber; 72-Kevlar sheath; 73-Non-metallic traction wire;

[0037] 21-Optical cable reel; 22-External support frame; 23-Vibration isolation base; 24-Inclined support damper; 25-Water-filled borehole opening. Detailed Implementation

[0038] To facilitate understanding and implementation of the present invention by those skilled in the art, the present invention will be further described in detail below with reference to embodiments. It should be understood that these embodiments are for illustration and explanation only and are not intended to limit the scope of protection of the present invention.

[0039] Example 1

[0040] A suspended DAS borehole fracture fluid monitoring device includes the following components:

[0041] 1. Sensor optical cable

[0042] The sensing optical cable is vertically suspended inside the water-filled borehole. It consists of a multi-core optical cable (4-6 cores) encapsulated with a high-strength non-metallic Kevlar fiber sheath and an internal non-metallic traction wire. It features lightweight (linear density ≤0.05 kg / m), small diameter (outer diameter 5-10 mm), and high water pressure resistance (≥20 MPa), making it suitable for long-term stable operation in water-filled borehole environments.

[0043] 2. Orifice vibration isolation module

[0044] The borehole isolation module is installed at the borehole opening of the water-filled borehole. Fixed to the top of the sensing optical cable, the module includes an external support frame, inclined support dampers, and an isolation base. The external support frame is a monolithic load-bearing structure. Multiple inclined support dampers connect the optical cable reel to the external support frame to absorb vertical and horizontal vibrations. The upper part of the sensing optical cable is wound around the optical cable reel. The isolation base is located at the bottom of the four legs of the support frame, allowing for fine-tuning of its horizontal position. These components work together to achieve multi-directional vibration isolation at the borehole opening, ensuring stable cable exit from the reel and vertical insertion into the borehole, effectively reducing the interference of surface vibrations on underground observations.

[0045] 3. Counterweight

[0046] The counterweight is fixed to the bottom of the sensing optical cable. Its mass ranges from 5 to 10 kg and can be adjusted according to the drilling depth and water flow conditions. It is used to keep the optical cable vertically suspended and maintain tension.

[0047] 4. DAS demodulator

[0048] The DAS demodulator connects to the top of the sensing optical cable and is based on phase-sensitive coherent optical time-domain reflectometry (φ-OTDR) technology. Key parameters include: gauge length 1–4 m, spatial sampling interval 1–4 m, and sampling frequency 1000–2000 Hz. This instrument can achieve continuous, high-fidelity acquisition of vibration signals throughout the entire length of a water-filled borehole.

[0049] The sensing optical cable hangs down to the bottom of the borehole and is connected to a counterweight, forming a continuous distributed hydrophone array. Compared with traditional fixed optical cables, this device has the following advantages: ① High-efficiency acoustic coupling (the optical cable directly contacts the water in the borehole, sensitively responding to weak fluid signals); ② Suppression of environmental noise (the borehole isolation module effectively absorbs surface disturbances); ③ Simple deployment (no need for borehole fixing, suitable for deep boreholes, long-term observation, and emergency monitoring).

[0050] 5. Auxiliary Components

[0051] It includes a lightning protection module and a remote data transmission module. The lightning protection module is installed at the borehole opening of the water-filled borehole to enhance the monitoring device's anti-interference capability in the field environment; the remote transmission module is connected to the DAS demodulator to realize real-time or near real-time data transmission and monitoring, improving the stability of long-term operation.

[0052] A suspended DAS borehole fracture fluid monitoring method, utilizing the aforementioned suspended DAS borehole fracture fluid monitoring device, includes the following steps:

[0053] Step 1: Data Collection

[0054] Strain waveform data excited by fracture fluid activity and coupled to the sensing optical cable via borehole water were continuously acquired using a DAS demodulator. The raw strain waveform data were preprocessed by removing the mean, removing the trend, and applying a 10–200 Hz bandpass filter to obtain the preprocessed strain data.

[0055] Step 2: Event Recognition

[0056] The preprocessed strain data is scanned using a recursive short-time average-long-time average (STA / LTA) algorithm. In this embodiment, the short-time window is 0.01 s and the long-time window is 0.2 s. A dual-threshold strategy is adopted for the trigger threshold: a starting threshold of 9 and an ending threshold of 6. Only events with a feature function maximum value > 10 are retained, and the arrival times of events at different depths are automatically detected and extracted.

[0057] As a preferred approach, preprocessed strain data can be input into a pre-trained deep learning model (such as CNN or YOLO) to improve detection efficiency and reliability.

[0058] Step 3: Location of the earthquake source

[0059] Based on the event's depth and arrival time, a depth-arrival-time scatter plot was created. The time intervals of the depth-arrival-time scatter plot were grouped into time segments using a 0.05 s threshold. Within each time segment, data points were sorted by depth, and the time gradient between adjacent data points was calculated. The inflection point of the "<"-shaped phase was determined by the sign change of the time gradient between adjacent data points. The inflection point was used as a boundary to divide the uphill and downhill wave data points. Linear regression was performed on both uphill and downhill wave data points to obtain two linear regression lines. If the slopes of the two linear regression lines were opposite, it was determined to be a "<"-shaped event. The focal depth and occurrence time were determined by the intersection of the two linear regression lines.

[0060] As a preferred approach, the coordinate values ​​of the data points can be normalized before fitting to reduce errors.

[0061] Step 4: Solid Moisture Modulation Analysis

[0062] The time series of solid tidal body strain was calculated based on a theoretical tidal model. Events were statistically analyzed into frequency time series according to depth intervals, and cross-spectral analysis was performed between these frequency time series and the solid tidal body strain time series (this process is an existing method). Bandpass filters were used to extract the equal components of the lunar semi-diurnal tide M2 ​​and the lunar semi-diurnal tide O1 (this process is an existing method). Furthermore, the phase lag and normalized response coefficients of the frequency time series response to tidal strain were calculated to quantitatively characterize the intensity and delay characteristics of the event's response to the solid tide.

[0063] Step 5: Pressure Diffusion and Permeability Assessment

[0064] Scatter plots of focal depth versus occurrence time for groundwater activity events are generated, and clustering algorithms (such as OPTICS) are used to identify event clusters with linear migration characteristics. In the diffusion characteristic analysis, a cylindrical diffusion model of event clusters is established: D=r 2 / 4(t-t0), where t is the occurrence time of any randomly selected event in the event cluster, t0 is the occurrence time of the earliest event in the event cluster, r is the source depth distance of the selected event relative to the earliest event in the event cluster, and D is the pore pressure diffusion coefficient. Using the (r) of several selected events within the event cluster... 2 , t-t0), and perform linear regression based on the event cluster cylindrical diffusion model. The regression slope m satisfies the relationship m=4×D, from which the pore pressure diffusion coefficient can be obtained: D=m / 4.

[0065] Similarly, for each event cluster, using the (r, t-t0) of several selected events within the event cluster, and performing a linear regression based on v=r / (t-t0), the regression slope can be used to estimate the average migration velocity v=r / (t-t0).

[0066] Based on rock physical parameters, according to the formula (K=μ×φ×C) T ×D) Calculate the equivalent permeability K within the borehole influence range, where μ is the fluid viscosity, φ is the porosity, and C T This represents the overall compression ratio.

[0067] Example 2:

[0068] This embodiment uses the DAS in-situ observation data from the water-filled borehole GQX-1 in the Dayi earthquake void of the Longmenshan fault zone as an example to further illustrate Embodiment 1.

[0069] I. Suspended DAS Borehole Fracture Fluid Monitoring Device

[0070] like Figure 1 As shown, the sensing optical cable of the present invention is vertically laid out inside the borehole GQX-1.

[0071] 1. Sensor Optical Cable: A 6-core Kevlar-sheathed sensor optical cable (containing 4 single-mode fibers and 2 multimode fibers) is selected, with an outer diameter of 6 mm, a linear density of 0.0478 kg / m, and a water pressure resistance ≥20 MPa. The cross-sectional structure is as follows: Figure 2 As shown.

[0072] 2. Borehole Isolation Module: Installed above the borehole opening, the external support frame is securely connected to the isolation base and its level is adjusted. Multiple inclined dampers connect the optical cable reel to the external support frame, forming a multi-directional damping support. The sensing optical cable is led out from the reel and lowered vertically into the borehole, achieving stable entry and effectively suppressing ground vibration interference. Its structure is as follows: Figure 3 As shown.

[0073] 3. Counterweight: A 5 kg counterweight is connected to the bottom of the sensing optical cable, allowing the cable to be vertically suspended inside the borehole and directly acoustically coupled to the water within the borehole. The total borehole depth is 1000 m, with the entry into the bedrock section beginning at 22.7 m.

[0074] 4. DAS Demodulator: Connect the sensing optical cable to the DAS demodulator (model MS-DAS2000), set the gauge length to 4m, the spatial sampling interval to 4m, and the sampling frequency to 2000Hz, to form a vertical observation array covering 22 to 259 channels with a total length of approximately 948m.

[0075] Observation conditions: The system was started one week after the hydraulic fracturing experiment and ran continuously for 13 days. During this period, the borehole was always filled with water and water continued to overflow.

[0076] Through the above deployment method, a high-density suspended DAS borehole fracture fluid monitoring device can be formed in the borehole, realizing real-time monitoring of the entire fluid activity process.

[0077] II. A Suspended DAS Method for Monitoring Borehole Fracture Fluids

[0078] like Figure 4 As shown, automatic event detection is achieved using strain waveform data:

[0079] In step 1, the mean and trend are removed, and a 10-200Hz bandpass filter is applied to suppress noise.

[0080] In step 2, the recursive short-time average-long-time average (STA / LTA) algorithm is used to scan the preprocessed strain data. The short time window is 0.01 s and the long time window is 0.2 s. The trigger threshold adopts a dual threshold strategy: the starting threshold is 9 and the ending threshold is 6. Only events with a feature function maximum value > 10 are retained. The arrival time of events at different depths is automatically detected and extracted.

[0081] In step 3, the focal depth is determined. In this embodiment, the events are mostly characterized by rapid disturbances with energy concentrated in the 10–200 Hz range, forming typical “<”-shaped up and down waves. Up waves may propagate back to the surface, and down waves may also propagate back to the bottom of the borehole.

[0082] like Figure 4 As shown, this invention uses a two-branch linear regression method to locate events based on the linear characteristics of uplink and downlink waves.

[0083] The time of the depth-first arrival time scatter plot is grouped into time periods using a time grouping threshold of 0.05 s.

[0084] For each time period, the data points are sorted by depth and the temporal gradient of adjacent data points is calculated.

[0085] The inflection point of the "<" shaped seismic phase is determined by the sign change of the time gradient between adjacent data points. The up-wave data points and down-wave data points are then divided by the inflection point.

[0086] Each time period must contain no fewer than 20 data points, with both upward and downward wave data points being greater than or equal to 5. Data points that do not meet these criteria will be removed.

[0087] Linear equations y=ax+b are established for the up-wave data points and down-wave data points respectively. The coefficients a and b are solved using the least squares method and normalized with the earliest arrival time within the time period. The first arrival time is used as the independent variable x and the depth is used as the dependent variable y.

[0088] If the slopes of the two regression fitting lines are opposite, it is determined to be a "<" shaped event, and the focal depth is determined by the intersection of the two linear regression fitting lines, retaining only the depth difference between the intersection point and the inflection point ≤ 50 m.

[0089] Three data points were selected near the intersection, and the average of their amplitude and duration was taken as the source parameters.

[0090] This method can quickly and stably determine the depth of the water inrush point and the characteristics of the event, achieving high-precision positioning.

[0091] In this embodiment, event distribution and statistical results are analyzed.

[0092] like Figure 5 As shown, statistics were compiled from approximately 190,000 events observed over 13 days:

[0093] 1. The events were mainly concentrated in the depth range of 400–700 m.

[0094] 2. The amplitude is mostly between 5 and 20 nanos of strain, and the duration is concentrated between 0.01 and 0.02 s.

[0095] 3. The location of the event concentration area coincides with the location of the fracture zone revealed by the borehole.

[0096] The results indicate that the fractured zone is the main channel for groundwater activity.

[0097] In this embodiment, solid moisture modulation analysis is performed.

[0098] like Figure 6 As shown, this invention further analyzes the modulating effect of solid tides on events:

[0099] Strain calculation: The solid tide displacement field was calculated based on the IERS 2010 standard, and the strain tensors in the east, north and vertical directions were derived to obtain the strain time series of the solid tide.

[0100] Event frequency construction: Resample events at 1-hour intervals and count the frequency in depth intervals to obtain the frequency time series.

[0101] Tidal component extraction: Butterworth filter was used to extract the lunar semi-diurnal tide M2 ​​(12.42h) and lunar semi-diurnal tide O1 (25.82h), with a bandwidth of ±20%.

[0102] Cross-spectral analysis: Fourier cross-spectral analysis is performed on the frequency time series and the strain time series of solid tides to obtain the phase difference and normalized response coefficients.

[0103] The results showed that the O1 component of the lunar main diurnal tide was significantly modulated (response coefficient 0.85–1.25), while the M2 component of the lunar main semi-diurnal tide was relatively weak (0.2–0.6). The differences in phase lag and response intensity at different depths revealed changes in rock mass permeability.

[0104] In this embodiment, pressure diffusion analysis is performed.

[0105] like Figure 7 As shown, this invention estimates pressure diffusion parameters using event migration characteristics:

[0106] Identification of event clusters with linear migration characteristics: The OPTICS clustering method was used to identify event clusters with linear migration characteristics. Typical event clusters migrate about 200 m in 10 to 40 minutes at a speed of 0.08 to 0.3 m / s.

[0107] Diffusion model fitting: using the cylindrical diffusion model r 2 = 4D(t-t0), and the fitting yields a pore pressure diffusion coefficient D = 0.09~0.8 m. 2 / s.

[0108] Equivalent permeability calculation: Substitute into the formula K=μ×φ×C T ×D, the equivalent permeability K is 10. -15 ~10 -14 m 2 The permeability is higher than that of intact rock, indicating that the fracture network in the fracture zone forms an efficient fluid channel.

[0109] Therefore, the fluid transport capacity in the shallow part of the seismic void can be quantitatively estimated, providing a basis for risk assessment and tectonic activity monitoring.

[0110] The above are merely preferred embodiments of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principle of the present invention should be considered within the scope of protection of the present invention.

Claims

1. A suspended DAS borehole fracture fluid monitoring method, utilizing a suspended DAS borehole fracture fluid monitoring device, including a DAS demodulator, and a sensing optical cable suspended in a water-filled borehole. A counterweight is fixed to the bottom end of the sensing optical cable, and the upper part of the sensing optical cable is wound around an optical cable reel. The optical cable reel is mounted on a borehole isolation module. The sensing optical cable is connected to the DAS demodulator. The sensing optical cable is a multi-core optical cable encapsulated with non-metallic Kevlar fiber sheath. The borehole isolation module is located at the borehole opening of the water-filled borehole. The borehole isolation module includes an external support frame, inclined support dampers, and an isolation base. Multiple inclined support dampers connect the optical cable reel to the external support frame. The isolation base is located at the bottom of the support frame. The device also includes a lightning protection module located at the borehole opening of the water-filled borehole, and a remote transmission module connected to the DAS demodulator. Its features are, The above method includes the following steps: Step 1: Use a DAS demodulator to continuously acquire strain waveform data excited by fracture fluid activity, coupled to the sensing optical cable through borehole water, and preprocess the strain waveform data to obtain strain data. Step 2: Extract the first arrival time of events at different depths based on strain data; Step 3: Based on the depth and arrival time of the event, plot the depth-arrival time scatter plot for each data point, group the data points into time periods, determine the inflection points within each time period, and divide the up-wave data points and down-wave data points into two categories using the inflection points as boundaries. Perform linear regression on the up-wave data points and down-wave data points respectively to obtain two linear regression fitted lines. If the slopes of the two linear regression fitted lines are opposite, it is determined to be a "<" shaped event, and the focal depth and occurrence time are determined by the intersection of the two linear regression fitted lines.

2. The suspended DAS borehole fracture fluid monitoring method according to claim 1, characterized in that, The preprocessing of the strain waveform data includes removing the mean, removing the trend, and bandpass filtering.

3. The suspended DAS borehole fracture fluid monitoring method according to claim 1, characterized in that, The first arrival time is obtained based on the strain data after scanning preprocessing using a recursive short-time average-long-time average algorithm.

4. The suspended DAS borehole fracture fluid monitoring method according to claim 1, characterized in that, It also includes solid moisture modulation analysis: The time series of solid tidal body strain was calculated based on the theoretical tidal model. Events were statistically divided into frequency time series according to depth intervals and cross-spectral analysis was performed with the solid tidal body strain time series. Bandpass filters were used to extract the equal components of the lunar semi-diurnal tide and the lunar diurnal tide. The phase lag and normalized response coefficient of the frequency time series response to tidal strain were calculated.

5. The suspended DAS borehole fracture fluid monitoring method according to claim 1, characterized in that, It also includes pressure diffusion and permeability assessment: Establish an event cluster cylindrical diffusion model: D=r 2 / 4(t-t0), where t is the occurrence time of any randomly selected event in the event cluster, t0 is the occurrence time of the earliest event in the event cluster, r is the source depth distance of the selected event relative to the earliest event in the event cluster, and D is the pore pressure diffusion coefficient. This is achieved by using the pore pressure diffusion coefficient of several selected events within the event cluster. 2 And t-t0, and perform linear regression based on the event cluster cylindrical diffusion model to obtain the pore pressure diffusion coefficient D; Using the values ​​of r and t-t0 for several selected events within an event cluster, and performing linear regression based on v = r / (t-t0), the average migration velocity v is obtained. Calculate the equivalent permeability K = μ × φ × C T ×D, where μ is the fluid viscosity, φ is the porosity, and C T This represents the overall compression ratio.

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