Seabed CO2 storage induced fault activation micro-seismic monitoring system and recognition method

Abstract

The invention provides a seabed CO2 storage induced fault activation micro-seismic monitoring system and an identification method. The method comprises the following steps: acquiring a data signal in a seabed CO2 injection process; calculating the distance between the monitoring position and the seismic source based on the data signal to locate the seismic source position; obtaining a seismic source moment tensor and a moment magnitude based on the seismic source position, and obtaining a seismic source mechanism solution based on the seismic source moment tensor; taking the seismic source mechanism solution as a clustering parameter, performing spatial clustering analysis on all microseismic events in the seabed CO2 injection process, and obtaining a potential hidden fault corresponding to each cluster; and for all micro-seismic events on each potential hidden fault, magnitude-frequency number linear fitting is carried out based on the moment magnitude, an earthquake b value of seismic activity on the fault surface is obtained, and fault activation is identified based on the earthquake b value. According to the invention, real-time monitoring of the fault state in seabed CO2 geological sequestration can be realized, and possible fault activation can be identified.

Description

Technical Field

[0001] The present application belongs to the field of marine CO2 geological storage technology, and in particular to the field of microseismic monitoring and identification of fault activation induced by seabed CO2 storage. Background Art

[0002] Carbon capture, utilization and storage (CCUS) is a necessary technical means to achieve the "dual carbon" goal, and carbon dioxide geological utilization and storage (CGUS) is the key link of CCUS technology, which is the process of injecting industrially captured CO2 into deep saline aquifers, depleted oil and gas fields and other geological bodies to achieve permanent CO2 emission reduction. Compared with onshore storage, offshore CO2 geological storage has huge storage potential and is the future development trend of CCUS.

[0003] However, large-scale CO2 injection will significantly change the state of the strata, bringing various geological and environmental risks. Among them, faults are the weak links in the geological structure of the strata. Due to the stress changes caused by CO2 injection, there is a risk of fault activation. Although large faults are avoided as much as possible during the engineering site selection stage of CO2 geological storage, a large number of small faults are still difficult to avoid. The fault activation caused by CO2 injection may damage the airtightness of faults, caprocks and wellbores, increase the risk of CO2 leakage, and may even induce geological disasters such as earthquakes and landslides. Therefore, the monitoring and identification of fault activation is of great significance to the safety of submarine CO2 geological storage.

[0004] At present, for the development of shallow resources on the seabed, such as hydrate mining and metal mining, the cross-sectional stress and lateral deformation of the anchor rod passing through the fault can be monitored by strain gauges and displacement gauges to obtain the stress and displacement changes of the fault, thereby identifying the fault activation. However, for the geological storage of CO2 on the seabed, such as saline layer storage and oil recovery storage, the depth of the target reservoir is often 2000-3000m, and there may be hidden faults that are difficult to accurately locate. The method of monitoring fault activation by anchor rod deformation is no longer applicable. Summary of the invention

[0005] The present application provides a microseismic monitoring system and identification method for seafloor CO2 storage-induced fault activation, so as to achieve real-time monitoring of the fault status in seafloor CO2 geological storage and identify possible fault activation.

[0006] In a first aspect, an embodiment of the present application provides a microseismic monitoring system for fault activation induced by seabed CO2 storage, comprising: a downhole optical fiber and a downhole signal acquisition unit located in a monitoring well, a seabed optical fiber and a seabed signal acquisition unit located on the seabed centered on the injection well, a control unit, a distribution unit, a signal conversion unit and a wireless transmitting unit located on an offshore platform of the injection well, and a wireless receiving unit and a data analysis unit located at a monitoring center; the downhole optical fiber is used to pick up microseismic signals propagated to the monitoring well; the downhole signal acquisition unit is used to collect the microseismic signals of the monitoring well picked up by the downhole optical fiber; the seabed optical fiber is used to pick up microseismic signals propagated to the surface of the seabed; the seabed signal acquisition unit is used to collect the microseismic signals on the surface of the seabed picked up by the seabed optical fiber; the control unit is used to power the distribution unit and to The collection unit and the seabed signal collection unit perform GPS positioning and time calibration to obtain microseismic collection positioning data and timing data, and transmit the obtained microseismic collection positioning data and timing data to the wireless transmitting unit; the distribution unit is used to distribute the optical signal to the downhole optical fiber and the seabed optical fiber, and merge the microseismic signals returned by the downhole optical fiber and the seabed optical fiber and transmit them to the signal conversion unit; the signal conversion unit converts the merged microseismic signal into a data signal and sends it to the wireless transmitting unit; the wireless transmitting unit wirelessly transmits the received data signal to the wireless receiving unit located in the monitoring center; the wireless receiving unit transmits the received data signal to the data analysis unit; the data analysis unit analyzes and processes the data signal to perform real-time monitoring and activation identification of the fault status.

[0007] In a second aspect, an embodiment of the present application provides a method for identifying fault activation induced by seabed CO2 storage, and a microseismic monitoring system for fault activation induced by seabed CO2 storage as described above is deployed in a monitoring area, and the identification method comprises: obtaining data signals during seabed CO2 injection; calculating the distance between the monitoring position and the earthquake source based on the data signal to locate the earthquake source position; obtaining the earthquake source moment tensor and moment magnitude based on the earthquake source position, and obtaining the earthquake source mechanism solution based on the earthquake source moment tensor; using the earthquake source mechanism solution as a clustering parameter, performing spatial clustering analysis on all microseismic events during seabed CO2 injection, and obtaining a potential hidden fault corresponding to each cluster; for all microseismic events on each potential hidden fault, performing magnitude-frequency linear fitting based on the moment magnitude to obtain the earthquake b value of the earthquake activity on the fault plane, and identifying fault activation based on the earthquake b value.

[0008] In one implementation of the second aspect, an analytical solution model of reservoir fluid pressure in CO2 geological storage is used to estimate the influence range of microseismic monitoring of seabed CO2 geological storage, and the monitoring area is determined based on the influence range; wherein the influence range includes a CO2 enrichment zone, a CO2 plume front transition zone, and a CO2 pressure influence zone; monitoring wells are arranged within the annular range of the CO2 plume front transition zone, and a distributed downhole optical fiber is arranged in the monitoring well to a certain depth above the reservoir; an injection well is arranged in the center of the CO2 enrichment zone, and a plurality of distributed seabed optical fibers are arranged uniformly and radially on the seabed surface with the injection well as the center; a control unit, a distribution unit, a signal conversion unit, and a wireless transmission unit are arranged on the marine platform of the injection well, and a wireless receiving unit and a data analysis unit are arranged in an offshore or onshore monitoring center.

[0009] In one embodiment of the second aspect, in acquiring the data signal during the seabed CO2 injection process, it also includes judging and eliminating abnormal microseismic signals; wherein the abnormal microseismic signals are judged by the Pearson correlation coefficient between the waveforms at different monitoring positions.

[0010] In one embodiment of the second aspect, the calculation of the distance between the monitoring position and the earthquake source based on the data signal includes: picking up the first arrival of the waveform of each monitoring position using a long-short time window energy ratio method; obtaining the arrival time difference between the P wave and the S wave and the wave velocities of the P wave and the S wave according to the waveform at the monitoring position; and calculating the distance between the monitoring position and the earthquake source based on the arrival time difference between the P wave and the S wave and the wave velocities of the P wave and the S wave.

[0011] In one implementation of the second aspect, ray tracing combined with an arithmetic optimization algorithm is used to obtain the optimal source position.

[0012] In one implementation of the second aspect, obtaining a focal moment tensor based on the focal position, and obtaining a moment magnitude and focal mechanism solution based on the focal moment tensor include: obtaining the amplitudes of P waves and S waves at the monitoring position based on the focal position; obtaining the amplitudes of the P waves and S waves at the monitoring position to form a vector w, and obtaining an inversion vector of the focal moment tensor M based on the amplitudes to form the vector w G is an overdetermined matrix; based on the inversion vector Determine the normalized focal mechanism solution.

[0013] In one implementation of the second aspect, it also includes: decomposing the eigenvalues ​​of the normalized focal mechanism solution to obtain an isotropic source tensor, a compensated linear vector source tensor, and a double-couple source tensor; and drawing a focal mechanism beach ball of all microseismic events based on the isotropic source tensor, the compensated linear vector source tensor, and the double-couple source tensor.

[0014] In one implementation of the second aspect, the minimum number k of microseismic events in a cluster is: Where m is the number of microseismic events.

[0015] In an implementation of the second aspect, when the earthquake b value is ≤1, it is determined that the fault has entered an activation stage.

[0016] The method for identifying fault activation induced by seabed CO2 storage provided in the embodiment of the present application has the following beneficial effects:

[0017] 1. This application adopts a combination of downhole monitoring and seabed monitoring to improve the redundancy of the monitoring system, making the location of the earthquake source more accurate, and can achieve real-time monitoring of the fault status in seabed CO2 geological storage and identify possible fault activation.

[0018] 2. This application estimates the monitoring range in advance based on the actual submarine CO2 geological storage project, making the layout of monitoring facilities more reasonable and targeted.

[0019] 3. In this application, cluster analysis is conducted on all microseismic events during the CO2 injection process, which has the ability to identify potential hidden faults.

[0020] 4. This application identifies fault activation based on the b-value of seismic activity on the fault plane, and can predict the tectonic activity of the fault in advance before a larger magnitude earthquake occurs. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Shown is a basic structural schematic diagram of a microseismic monitoring system for seafloor CO2 storage-induced fault activation according to an embodiment of the present application.

[0022] Figure 2 Shown is an overall flow chart of a method for identifying fault activation induced by seafloor CO2 storage according to an embodiment of the present application.

[0023] Figure 3 Shown is a schematic diagram of monitoring range estimation and monitoring facility layout in the identification method of seabed CO2 storage-induced fault activation according to an embodiment of the present application.

[0024] Figure 4 Shown is a schematic diagram of using a microseismic monitoring system to identify fault activation induced by seabed CO2 storage in one embodiment of the present application.

[0025] Figure 5 Shown is a flow chart for determining the earthquake source location in a method for identifying fault activation induced by seafloor CO2 storage according to an embodiment of the present application.

[0026] Figure 6An example diagram showing earthquake source location, earthquake source mechanism inversion and earthquake source clustering analysis in a method for identifying fault activation induced by submarine CO2 storage according to an embodiment of the present application.

[0027] Figure 7 Shown is the magnitude-frequency GR relationship fitting of seismic activity on a certain fault in the method for identifying fault activation induced by submarine CO2 storage in one embodiment of the present application and the corresponding b-value diagram.

[0028] Component number description

[0029] 100 Microseismic monitoring system for fault activation induced by submarine CO2 storage

[0030] 110 Downhole Fiber

[0031] 120 Submarine Optical Fiber

[0032] 130 Downhole signal acquisition unit

[0033] 140 Submarine signal acquisition unit

[0034] 150 Control Unit

[0035] 160 allocation units

[0036] 170 Signal conversion unit

[0037] 180 Wireless Transmitter Unit

[0038] 190 Wireless receiving unit

[0039] 1110 Data Analysis Unit

[0040] 10 Injection Wells

[0041] 20 Monitoring Wells

[0042] 30 Offshore Platform

[0043] 40 CO2 plume leading edge transition zone

[0044] 50 Fault

[0045] 60 Reservoir

[0046] S100~S500 Steps

[0047] Steps S210 to S230 DETAILED DESCRIPTION

[0048] The following describes the embodiments of the present application through specific examples, and those skilled in the art can easily understand other advantages and effects of the present application from the contents disclosed in this specification. The present application can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways according to different viewpoints and applications without departing from the spirit of the present application. It should be noted that the following embodiments and features in the embodiments can be combined with each other without conflict.

[0049] The embodiments of the present application provide a microseismic monitoring system and identification method for seafloor CO2 storage-induced fault activation, so as to achieve real-time monitoring of the fault status in seafloor CO2 geological storage and identify possible fault activation.

[0050] The method for identifying fault activation induced by submarine CO2 storage provided in the embodiments of the present application can estimate the monitoring range and reasonably arrange the monitoring equipment. After the monitoring system is initialized, the microseismic signals are collected by joint monitoring of downhole optical fiber and submarine optical fiber and transmitted to the data analysis unit for preprocessing via wireless communication. After the first arrival is picked up by the long-short time window energy ratio method, the microseismic events are accurately located by ray tracing combined with an optimization algorithm. Potential hidden faults are accurately identified by focal mechanism inversion based on amplitude information and focal clustering analysis based on density. Finally, fault activation is identified based on the b-value of seismic activity on the fault plane.

[0051] The following will be combined with the attached examples of the present application Figure 1 To Attachment Figure 7 , the technical solution in the embodiment of the present application is described in detail, so that those skilled in the art can understand and implement the microseismic monitoring system and identification method of the submarine CO2 storage-induced fault activation in the present embodiment without creative labor.

[0052] An embodiment of the present application provides a microseismic monitoring system for fault activation induced by seafloor CO2 storage. Figure 1 The following is a basic structural diagram of a microseismic monitoring system for fault activation induced by seafloor CO2 storage according to an embodiment of the present application. Figure 1 As shown, the submarine CO2 storage-induced fault activation microseismic monitoring system 100 of this embodiment includes: a downhole optical fiber 110 and a downhole signal acquisition unit 130 located in the monitoring well 20, a submarine optical fiber 120 and a submarine signal acquisition unit 140 located on the seabed of the seabed centered on the injection well 10, a control unit 150, a distribution unit 160, a signal conversion unit 170 and a wireless transmitting unit 180 located on the marine platform 30 of the injection well 10, and a wireless receiving unit 190 and a data analysis unit 1110 located in the monitoring center.

[0053] Among them, the distribution unit 160 distributes the optical signal to the downhole optical fiber 110 and the submarine optical fiber 120, and the downhole signal acquisition unit 130 and the submarine signal acquisition unit 140 collect the optical signal caused by the microseismic, which is converted into a data signal by the signal conversion unit 170, and wirelessly transmitted by the wireless transmitting unit 180 and the wireless receiving unit 190. Finally, the data analysis unit 1110 analyzes and processes the data to monitor the status of the fault 50 in real time and identify its activation.

[0054] The following is a detailed description of the microseismic monitoring system 100 for seafloor CO2 storage-induced fault activation of this embodiment.

[0055] In this embodiment, the downhole optical fiber 110 is used to pick up the microseismic signal propagated to the monitoring well 20. The downhole optical fiber 110 picks up the microseismic signal propagated to the monitoring well 20 through a distributed optical fiber microseismic sensor (DAS) located in the monitoring well 20, and uses phase-sensitive optical time domain reflectometry technology to sense the phase change of Rayleigh backscattered light caused by the microseismic signal.

[0056] In this embodiment, the downhole signal acquisition unit 130 is used to acquire the microseismic signal of the monitoring well 20 picked up by the downhole optical fiber 110 , and to separate, amplify and transmit the optical signal of the downhole optical fiber 110 .

[0057] Specifically, in this embodiment, the optical signal acquisition unit located in the monitoring well 20, including but not limited to an optical circulator and an optical amplifier, separates the input light and reflected light of the downhole optical fiber 110, and then amplifies the amplitude of the optical signal and transmits it.

[0058] In this embodiment, the submarine optical fiber 120 is used to pick up the microseismic signal propagated to the seabed surface. The submarine optical fiber 120 picks up the microseismic signal propagated to the seabed surface through a distributed optical fiber microseismic sensor (DAS) located on the seabed of the seabed, and uses phase-sensitive optical time domain reflectometry technology to sense the phase change of Rayleigh backscattered light caused by the microseismic signal.

[0059] In this embodiment, the seabed signal acquisition unit 140 is used to collect the microseismic signals on the seabed surface picked up by the seabed optical fiber 120 , and to separate, amplify and transmit the optical signals of the seabed optical fiber 120 .

[0060] Specifically, the submarine signal acquisition unit 140 includes but is not limited to an optical circulator and an optical amplifier, etc., and uses the optical circulator and the optical amplifier to separate and amplify the optical signal of the submarine optical fiber 120.

[0061] In this embodiment, the control unit 150 is used to power the distribution unit 160, perform GPS positioning and time calibration on the downhole signal acquisition unit 130 and the seabed signal acquisition unit 140, obtain microseismic acquisition positioning data and timing data, and transmit the acquired microseismic acquisition positioning data and timing data to the wireless transmission unit 180.

[0062] Specifically, in this embodiment, the control unit 150 includes a power supply, a GPS positioning device and a time calibration device, performs GPS positioning and time calibration on the signal acquisition units underground and on the seabed, and powers the distribution unit 160, and then transmits the obtained microseismic acquisition positioning and timing data to the wireless transmission unit 180.

[0063] In this embodiment, the distribution unit 160 is used to distribute the optical signal to the downhole optical fiber 110 and the submarine optical fiber 120 , and combine the microseismic signals returned by the downhole optical fiber 110 and the submarine optical fiber 120 and transmit them to the signal conversion unit 170 .

[0064] Specifically, in this embodiment, the distribution unit 160 includes but is not limited to a broadband light source, an optical switch, a wavelength division multiplexer, a distributor, etc., which performs multi-channel modulation on the optical signal of the broadband light source and distributes it to the downhole optical fiber 110 and the submarine optical fiber 120, and then merges the optical signals returned by the downhole optical fiber 110 and the submarine optical fiber 120 and transmits them to the signal conversion unit 170.

[0065] In this embodiment, the signal conversion unit 170 converts the combined microseismic signals into data signals and sends the data signals to the wireless transmission unit 180 .

[0066] Specifically, in this embodiment, the signal conversion unit 170 includes but is not limited to a coherent receiver and a dynamic demodulator, which receives the optical signal transmitted by the distribution unit 160 and implements wavelength division multiplexing of multiple optical signals, and then demodulates them into corresponding vibration signals, converts them into data signals and transmits them to the wireless transmitting unit 180.

[0067] In this embodiment, the wireless transmitting unit 180 encodes, compresses, and modulates the received data signal, and then wirelessly transmits the data signal to the wireless receiving unit 190 located in the monitoring center.

[0068] Specifically, in this embodiment, the wireless transmitting unit 180 includes but is not limited to a controller, an encoder, a radio frequency front end, an antenna, etc., which encodes, compresses, and modulates the data signal transmitted by the control unit 150 and the signal conversion unit 170, and then transmits it to the cloud in the form of a wireless signal through the 5G cellular network.

[0069] In this embodiment, the wireless receiving unit 190 filters, amplifies, and decodes the received data signal and transmits the received data signal to the data analyzing unit 1110 .

[0070] Specifically, in this embodiment, the wireless receiving unit 190 includes but is not limited to an antenna, a filter, an amplifier, a decoder, etc., and filters, amplifies, and decodes the 5G wireless signal received from the cloud, and then transmits it to the data analysis unit 1110.

[0071] In this embodiment, the data analysis unit 1110 analyzes and processes the data signal to perform real-time monitoring and activation identification of the state of the fault 50 .

[0072] Specifically, in this embodiment, the data analysis unit 1110 includes a high-performance computing workstation and a data analysis program therein, and analyzes and processes the data transmitted from the wireless receiving unit 190 .

[0073] In this embodiment, the transmission process of the microseismic signal in the system is downhole / submarine optical fiber 120 → downhole signal acquisition unit 130 / submarine signal acquisition unit 140 → distribution unit 160 → signal conversion unit 170 → wireless transmission unit 180 → wireless receiving unit 190 → data analysis unit 1110.

[0074] The submarine CO2 storage-induced fault activation microseismic monitoring system 100 of this embodiment adopts a combination of downhole monitoring and submarine monitoring to improve the redundancy of the monitoring system and make the location of the earthquake source more accurate.

[0075] The embodiment of the present application also provides a method for identifying seabed CO2 storage-induced fault activation, and the seabed CO2 storage-induced fault activation microseismic monitoring system 100 as described above is deployed in the monitoring area.

[0076] Figure 2 The flowchart of the method for identifying fault activation induced by seafloor CO2 storage in the embodiment of the present application is shown. Specifically, Figure 2 As shown, the method for identifying fault activation induced by seabed CO2 storage provided in the embodiment of the present application includes the following steps S100 to S500.

[0077] In one implementation of this embodiment, an analytical solution model of reservoir fluid pressure in CO2 geological storage is used to estimate the influence range of microseismic monitoring of seabed CO2 geological storage, and the monitoring area is determined based on the influence range.

[0078] Exemplarily, this embodiment uses an analytical solution model of reservoir fluid pressure in CO2 geological storage to estimate the range of microseismic monitoring of seabed CO2 geological storage.

[0079] Assume that the reservoir 60 is a saturated aquifer, homogeneous and isotropic, with a constant thickness H. After CO2 is injected, it forms a steady isothermal two-phase flow with salt water, and the CO2 plume is radially symmetrical with the injection well 10 as the center. Figure 3 As shown, the radii of the CO2 enrichment zone, the CO2 plume front transition zone 40, and the CO2 pressure influence zone can be estimated according to the following formula.

[0080]

[0081] Where r1 is the radius of the CO2 enrichment zone, r2 is the radius of the CO2 plume front transition zone 40, r3 is the radius of the CO2 pressure influence zone, and k w =k c =0.5; μ w and μ c In particular, the viscosity of the salt water and CO2 in the reservoir 60 is estimated based on the temperature and pressure of the reservoir 60 from the logging data; V c is the total volume of CO2 injected; φ is the porosity of the reservoir 60; S is the water storage coefficient of the reservoir 60; T is the water conductivity of the reservoir 60; t is the total time of CO2 injection; u0 is the dimensionless time, usually 0.4 to 0.6.

[0082] This embodiment estimates the monitoring range in advance based on the actual submarine CO2 geological storage project, making the layout of the monitoring facilities more reasonable and targeted.

[0083] Specifically, according to the location of the injection well 10, and the geological and logging data of the marine CO2 geological storage site, through multi-field and multi-phase numerical simulation or analytical solution of CO2 injection, the influence range of injected CO2 during the CO2 geological storage monitoring period is estimated, including the CO2 enrichment zone, the CO2 plume front transition zone 40, and the CO2 pressure influence zone, and then the monitoring range is preliminarily determined based on this range.

[0084] Figure 3 The schematic diagram of monitoring range estimation and monitoring facility layout in the identification method of fault activation induced by seabed CO2 storage in one embodiment of the present application is shown. Figure 3As shown, in this embodiment, the influence range includes the CO2 enrichment zone, the CO2 plume front transition zone 40 and the CO2 pressure influence zone; a monitoring well 20 is arranged within the annular range of the CO2 plume front transition zone 40, and a distributed downhole optical fiber 110 is arranged in the monitoring well 20 to a certain depth above the reservoir 60; an injection well 10 is arranged in the center of the CO2 enrichment zone, and a plurality of distributed submarine optical fibers 120 are uniformly radially arranged on the seabed surface with the injection well 10 as the center; a control unit 150, a distribution unit 160, a signal conversion unit 170 and a wireless transmission unit 180 are arranged on the marine platform 30 of the injection well 10, and a wireless receiving unit 190 and a data analysis unit 1110 are arranged in a monitoring center near the coast or on shore.

[0085] Specifically, the interpretation of geological and logging data of marine CO2 geological storage sites is used to evaluate the areas within the monitoring range where faults 50 may exist, such as Figure 4 As shown, a permanent monitoring well 20 is arranged through an offshore platform 30 on one side of an estimated annular range of the CO2 plume front transition zone 40 near a possible fault 50 area, a distributed acoustic fiber (DAS) is arranged in the monitoring well 20 to a certain depth above the reservoir 60, and then 8 distributed acoustic fibers (DAS) are arranged uniformly and radially on the seabed surface with the injection well 10 as the center, and the distance between the far end and the injection well 10 is about (r2+r3) / 2; a control unit 150, a distribution unit 160, a signal conversion unit 170 and a wireless transmission unit 180 are arranged on the offshore platform 30 of the injection well 10, and a wireless receiving unit 190 and a data analysis unit 1110 are arranged in a monitoring center near the coast or on shore.

[0086] After the monitoring facilities are arranged, the seabed CO2 storage-induced fault activation microseismic monitoring system 100 is initialized: the power is turned on and power is supplied to the distribution unit 160, the downhole signal acquisition unit 130 and the seabed signal acquisition unit 140 are GPS-located, and the time is calibrated.

[0087] Specifically, the monitoring system is initialized by the control unit 150, the power of the control unit 150 is turned on and the distribution unit 160 is powered, and instructions are sent to the downhole signal acquisition unit 130 and the seabed signal acquisition unit 140 through the distribution unit 160 to obtain the position, length and direction of the downhole optical fiber 110 and the seabed optical fiber 120, and returned to the GPS positioning device in the control unit 150, so as to calculate the spatial position of each measuring point on the downhole optical fiber 110 and the seabed optical fiber 120, and then the time calibration device in the control unit 150 uses a crystal oscillator to calibrate the signal acquisition start time of the downhole signal acquisition unit 130 and the seabed signal acquisition unit 140, and transmit the positioning data and time data to the data analysis unit 1110 and store them for use through the communication between the wireless transmitting unit 180 and the wireless receiving unit 190.

[0088] Step S100, obtaining data signals during the seabed CO2 injection process.

[0089] When a microseismic event occurs in the formation, seismic waves (including P waves and S waves) are transmitted to the downhole optical fiber 110 and the submarine optical fiber 120, and cause the Rayleigh backscattered light phase to change. The optical signal is transmitted to the signal conversion unit 170 through the acquisition unit and the distribution unit 160, and is converted into a data signal after separation and demodulation, and then transmitted to the data analysis unit 1110 through the communication of the wireless transmitting unit 180 and the wireless receiving unit 190. In this embodiment, the microseismic signal during the CO2 injection process is collected by the monitoring system, and the data is pre-processed, including noise suppression and data cleaning, to obtain the data signal during the submarine CO2 injection process.

[0090] Exemplarily, the empirical mode decomposition (EMD) method is used to suppress the noise of the collected microseismic signals, and the accuracy of the microseismic signals is verified and cleaned through the correlation of waveforms between different monitoring locations.

[0091] The empirical mode decomposition (EMD) method is used to suppress the noise of the collected microseismic signals as follows:

[0092] Firstly, the microseismic signal is decomposed into a series of intrinsic mode functions (IMFs), and the decomposed intrinsic mode functions are arranged from high frequency to low frequency. Then, a bandpass filter is constructed according to the empirical mode decomposition results to filter the decomposed intrinsic mode functions. Finally, the filtered microseismic signal is reconstructed through the filtered intrinsic mode functions.

[0093] In one implementation of this embodiment, in acquiring the data signal during the seabed CO2 injection process, it also includes judging and eliminating abnormal microseismic signals; wherein the abnormal microseismic signals are judged by the Pearson correlation coefficient between the waveforms at different monitoring positions.

[0094] For example, the accuracy of the microseismic signal is verified by the correlation of waveforms between different monitoring locations. The microseismic signal waveform w at location A is A (t) and the microseismic signal waveform w at position B B The Pearson correlation coefficient between (t) is:

[0095]

[0096] in, and They are waveform w A (t) and w BAfter obtaining the Pearson correlation coefficients between the microseismic signals at all monitoring locations on the same optical fiber, the microseismic signals with correlation coefficients less than 0.7 with other monitoring locations can be considered abnormal and cleaned.

[0097] Step S200, calculating the distance between the monitoring position and the earthquake source based on the data signal to locate the earthquake source.

[0098] Figure 5 The flowchart of determining the earthquake source location in the method for identifying fault activation induced by seafloor CO2 storage in one embodiment of the present application is shown. Figure 5 As shown, in one implementation of this embodiment, the calculation of the distance between the monitoring position and the earthquake source based on the data signal includes the following steps S210 to S230.

[0099] Step S210, picking up the first arrival of the waveform at each monitoring position by using the long-short time window energy ratio method;

[0100] Step S220, obtaining the arrival time difference between the P wave and the S wave and the wave speeds of the P wave and the S wave according to the waveform at the monitoring position;

[0101] Step S230, calculating the distance between the monitoring position and the earthquake source according to the arrival time difference between the P wave and the S wave and the wave speeds of the P wave and the S wave.

[0102] In this embodiment, the long-short time window energy ratio method is used to pick up the first arrival of the waveform at each monitoring position, wherein the calculation formula of the long-short time window energy ratio is:

[0103]

[0104] Among them, w i is the value of the sampling point; N0 is the sampling index of the current time point; N S N is the short-time window length (number of sampling points); L is the length of the long time window (number of sampling points). When the energy ratio of the long and short time windows STA / LTA exceeds a certain threshold, it is determined that a microseismic event has occurred and the time is picked up. In particular, the threshold here is 1.5.

[0105] Preferably, this embodiment adopts the end-point diagram method to calculate the estimated distance between the monitoring position and the earthquake source according to the arrival time difference between the P wave and the S wave and the wave speed of the P wave and the S wave. The formula is:

[0106]

[0107] Where Δt is the arrival time difference between the P wave and the S wave; c p and c sare the speeds of P waves and S waves respectively.

[0108] In one implementation of this embodiment, ray tracing combined with an arithmetic optimization algorithm is used to obtain the optimal earthquake source position.

[0109] Specifically, the root mean square average of the distance errors between all monitoring locations and the earthquake source is used as the optimization target parameter, that is:

[0110]

[0111] Where N is the number of valid monitoring point locations; is the estimated distance between the ith monitoring location and the earthquake source; d i is the actual distance between the ith monitoring position and the current designated source position. Ray tracing combined with optimization algorithm is used to obtain the optimal source location. In particular, the arithmetic optimization algorithm (AOA) is used here. The spatial distribution of the sources of all microseismic events is plotted, such as Figure 4 shown.

[0112] Step S300, obtaining a focal moment tensor and a moment magnitude based on the focal position, and obtaining a focal mechanism solution based on the focal moment tensor.

[0113] In an implementation of this embodiment, obtaining a focal moment tensor based on the focal position, and obtaining a moment magnitude and a focal mechanism solution based on the focal moment tensor includes:

[0114] Based on the source position, the amplitudes of the P wave and the S wave at the monitoring position are obtained; the amplitudes of the P wave and the S wave at the monitoring position are obtained to form a vector w, and based on the amplitude vector w, an inversion vector of the source moment tensor M is obtained. G is an overdetermined matrix; based on the inversion vector Determine the normalized focal mechanism solution.

[0115] This embodiment uses moment tensor inversion based on amplitude information to invert the focal mechanism. Specifically, according to the Green's function formula of a point source in an elastic medium, the amplitudes of the P wave and the S wave at a certain monitoring position are:

[0116]

[0117] Wherein, the subscripts i, j, and k represent the three coordinate directions of the three-dimensional space respectively; d is the distance between the earthquake source and the monitoring location; ρ is the density; n i 、n j 、n k are the three components of the unit direction vector of the monitoring position relative to the earthquake source; δ ij is the Kronecker symbol; Mjk is the moment tensor of the earthquake source. The amplitudes of P waves and S waves at all monitoring locations form a vector:

[0118] The vector reconstructed from the source moment tensor M The relationship between is constructed as follows:

[0119]

[0120] Among them, G is an overdetermined matrix. Invert the following formula to get:

[0121]

[0122] In this embodiment, the focal moment tensor M is solved by using the earthquake moment M0 and the normalized focal mechanism. The focal mechanism solution is obtained by

[0123]

[0124] According to the earthquake moment M0 (unit: N·m), the moment magnitude M is obtained. w for:

[0125]

[0126] In one implementation of this embodiment, it also includes: decomposing the eigenvalues ​​of the normalized focal mechanism solution to obtain an isotropic source tensor, a compensated linear vector source tensor, and a double-couple source tensor; and drawing the focal mechanism beach balls of all microseismic events based on the isotropic source tensors, the compensated linear vector source tensors, and the double-couple source tensors.

[0127] According to the focal location, the moment tensor inversion based on the amplitude information is carried out to obtain the corresponding moment magnitude and normalized focal mechanism solution, and the focal mechanism beach ball of the microseismic event is drawn.

[0128] Specifically, the normalized focal mechanism solution Eigenvalue decomposition to obtain the isotropic source tensor Compensated Linear Vector Source Tensor and the double-couple source tensor

[0129]

[0130] Then draw the focal mechanism beach balls of all microseismic events, e.g. Figure 4 shown.

[0131] Step S400: Using the focal mechanism solution as a clustering parameter, spatial clustering analysis is performed on all microseismic events during the seafloor CO2 injection process to obtain a potential hidden fault corresponding to each cluster.

[0132] That is, a density-based spatial clustering analysis is carried out on all microseismic events during the CO2 injection process and they are divided into several clusters, each of which corresponds to a potential hidden fault.

[0133] In one implementation of this embodiment, the minimum number k of microseismic events in a cluster is: Where m is the number of microseismic events.

[0134] Figure 6 This is an example diagram showing the earthquake source location, earthquake source mechanism inversion and earthquake source clustering analysis in the method for identifying fault activation induced by seafloor CO2 storage in one embodiment of the present application. Figure 6 As shown in the example, the normalized focal mechanism solution As the clustering parameter, density-based spatial clustering analysis (DBSCAN) is performed on all microseismic events during the CO2 injection process. Assuming that there are m microseismic events in total, the minimum number of microseismic events in a cluster is:

[0135] k = int(m / 25)

[0136] The neighborhood radius r of the cluster object a for:

[0137]

[0138] Where, Γ(*) is the gamma function; is the distance between the normalized focal mechanism solutions of microseismic events i and j; represents the maximum distance between the normalized focal mechanism solutions of all m microseismic events. For a microseismic event i, Microseismic event j is considered to fall within the neighborhood of microseismic event i. All m microseismic events are traversed. When there are at least k microseismic events in the neighborhood of a certain microseismic event, it is considered to meet the dense condition and together with all other microseismic events in the neighborhood, it constitutes a set Ω. Then all microseismic events that also meet the dense condition are found in the set Ω, and they are incorporated into Ω in a continuous cycle until no more microseismic events that meet the dense condition can be added. At this time, all microseismic events in the set Ω constitute a cluster. Finally, all m microseismic events are divided into several clusters by this method, and each cluster corresponds to a potential hidden fault, such as Figure 4 shown.

[0139] This embodiment has the ability to identify potential hidden faults by performing cluster analysis on all microseismic events during the CO2 injection process.

[0140] Step S500, for all microseismic events on each potential hidden fault, a magnitude-frequency linear fitting is performed based on the moment magnitude to obtain the seismic b-value of the seismic activity on the fault plane, and fault activation is identified based on the seismic b-value.

[0141] That is, in this embodiment, a logarithmic linear fitting is performed on the GR relationship between the magnitude and frequency of seismic activity on the fault plane to obtain the b value, and the b value is used as a basis for identifying fault activation.

[0142] This embodiment identifies fault activation based on the b-value of seismic activity on the fault plane, and can predict the tectonic activity of the fault in advance before an earthquake of a larger magnitude occurs.

[0143] Specifically, in an implementation of this embodiment, when the earthquake b value is ≤1, it is determined that the fault 50 has entered the activation stage.

[0144] Figure 7 The figure shows the magnitude-frequency GR relationship fitting and the corresponding b-value diagram of the seismic activity on a certain fault in the identification method of fault activation induced by seafloor CO2 storage in one embodiment of the present application. Figure 7 As shown in Figure 1, fault activation is identified based on the b-value of seismic activity on the fault plane. For all microseismic events on each cluster or each potential fault, a statistical analysis is performed based on the moment magnitude, and a logarithmic linear fit is performed on the GR relationship between magnitude and frequency as follows:

[0145] log 10 N=a-bM w

[0146] Where N is the moment magnitude greater than M w The number of microseismic events, such as Figure 7 Finally, the activation of the fault 50 is identified based on the b value obtained by fitting. When b≤1, it is determined that the fault 50 has entered the activation stage.

[0147] In summary, this application adopts the means of underground monitoring and seabed monitoring in combination, improves the redundancy of the monitoring system, makes the location of the earthquake source more accurate, can realize the real-time monitoring of the fault state in the seabed CO2 geological storage, and identify the possible fault activation; this application estimates the monitoring range in advance according to the actual seabed CO2 geological storage project, making the layout of the monitoring facilities more reasonable and more targeted; this application conducts cluster analysis on all microseismic events in the CO2 injection process, and has the ability to identify potential hidden faults; this application identifies fault activation according to the b-value of seismic activity on the fault plane, and can predict the tectonic activity of the fault in advance before a larger magnitude earthquake occurs. Therefore, this application effectively overcomes the various shortcomings in the prior art and has a high industrial utilization value.

[0148] The above embodiments are merely illustrative of the principles and effects of the present application and are not intended to limit the present application. Anyone familiar with the technology may modify or change the above embodiments without violating the spirit and scope of the present application. Therefore, all equivalent modifications or changes made by a person of ordinary skill in the art without departing from the spirit and technical ideas disclosed in the present application shall still be covered by the claims of the present application.

Claims

1. A microseismic monitoring system for fault activation induced by submarine CO2 storage, characterized in that: include: Downhole optical fiber and downhole signal acquisition unit located in the monitoring well, submarine optical fiber and submarine signal acquisition unit located on the seabed centered on the injection well, control unit, distribution unit, signal conversion unit and wireless transmission unit located on the offshore platform of the injection well, wireless receiving unit and data analysis unit located in the monitoring center; The downhole optical fiber is used to pick up the microseismic signal propagated to the monitoring well; The downhole signal acquisition unit is used to acquire the microseismic signal of the monitoring well picked up by the downhole optical fiber; The submarine optical fiber is used to pick up microseismic signals propagating to the seabed surface; The seabed signal acquisition unit is used to acquire the microseismic signal on the seabed surface picked up by the seabed optical fiber; The control unit is used to supply power to the distribution unit, perform GPS positioning and time calibration on the downhole signal acquisition unit and the seabed signal acquisition unit, obtain microseismic acquisition positioning data and timing data, and transmit the obtained microseismic acquisition positioning data and timing data to the wireless transmission unit; The distribution unit is used to distribute the optical signal to the downhole optical fiber and the submarine optical fiber, and to combine the microseismic signals returned by the downhole optical fiber and the submarine optical fiber and transmit them to the signal conversion unit; The signal conversion unit converts the combined microseismic signal into a data signal and sends it to the wireless transmission unit; The wireless transmitting unit wirelessly transmits the received data signal to a wireless receiving unit located in a monitoring center; The wireless receiving unit transmits the received data signal to the data analyzing unit; The data analysis unit analyzes and processes the data signal to perform real-time monitoring and activation identification of the fault status.

2. A method for identifying fault activation induced by seafloor CO2 storage, characterized in that: The microseismic monitoring system for fault activation induced by seafloor CO2 storage as claimed in claim 1 is deployed in the monitoring area, and the identification method comprises: Acquire data signals during the seabed CO2 injection process; Calculating the distance between the monitoring position and the earthquake source based on the data signal to locate the earthquake source; Obtaining a focal moment tensor and a moment magnitude based on the focal position, and obtaining a focal mechanism solution based on the focal moment tensor; The focal mechanism solution is used as a clustering parameter to perform spatial clustering analysis on all microseismic events during the seafloor CO2 injection process, and a potential hidden fault corresponding to each cluster is obtained; For all microseismic events on each potential hidden fault, a linear fit of magnitude-frequency is performed based on the moment magnitude to obtain the seismic b-value of the seismic activity on the fault plane, and fault activation is identified based on the seismic b-value.

3. The method for identifying fault activation induced by seabed CO2 storage according to claim 2, characterized in that: Using an analytical solution model of reservoir fluid pressure in CO2 geological storage, estimating the impact range of microseismic monitoring of seabed CO2 geological storage, and determining the monitoring area based on the impact range; The impact range includes the CO2 enrichment zone, the CO2 plume front transition zone and the CO2 pressure impact zone; a monitoring well is arranged within the annular range of the CO2 plume front transition zone, and a distributed downhole optical fiber is arranged in the monitoring well to a certain depth above the reservoir; an injection well is arranged in the center of the CO2 enrichment zone, and a plurality of distributed submarine optical fibers are arranged uniformly and radially on the seabed surface with the injection well as the center; a control unit, a distribution unit, a signal conversion unit and a wireless transmission unit are arranged on the marine platform of the injection well, and a wireless receiving unit and a data analysis unit are arranged in a monitoring center near the coast or on shore.

4. The method for identifying fault activation induced by seabed CO2 storage according to claim 2, characterized in that: The acquisition of data signals during the seabed CO2 injection process also includes judging and eliminating abnormal microseismic signals; wherein, the abnormal microseismic signals are judged by the Pearson correlation coefficient between the waveforms at different monitoring locations.

5. The method for identifying fault activation induced by seabed CO2 storage according to claim 4, characterized in that: The calculating the distance between the monitoring position and the earthquake source based on the data signal comprises: The long-short time window energy ratio method is used to pick up the first arrival of the waveform at each monitoring location; Obtaining the arrival time difference between the P wave and the S wave and the wave speeds of the P wave and the S wave according to the waveform at the monitoring position; The distance between the monitoring position and the earthquake source is calculated based on the arrival time difference between the P wave and the S wave and the wave speeds of the P wave and the S wave.

6. The method for identifying fault activation induced by seafloor CO2 storage according to claim 5, characterized in that: The optimal source location is obtained by using ray tracing combined with arithmetic optimization algorithm.

7. The method for identifying fault activation induced by seafloor CO2 storage according to claim 2, characterized in that: Obtaining a focal moment tensor based on the focal position, and obtaining a moment magnitude and focal mechanism solution based on the focal moment tensor includes: Acquire the amplitudes of the P wave and the S wave at the monitoring position based on the earthquake source position; The amplitude vector w of the P wave and the S wave at the monitoring position is obtained, and the inversion vector of the source moment tensor M is obtained based on the amplitude vector w. G is an overdetermined matrix; Based on the inversion vector Determine the normalized focal mechanism solution.

8. The method for identifying fault activation induced by seafloor CO2 storage according to claim 7, characterized in that: Also includes: The eigenvalue decomposition of the normalized focal mechanism solution is used to obtain the isotropic source tensor, the compensated linear vector source tensor and the double-couple source tensor; The focal mechanism beach balls of all microseismic events are drawn based on the isotropic source tensors, the compensated linear vector source tensors, and the double-couple source tensors.

9. The method for identifying fault activation induced by seafloor CO2 storage according to claim 2, characterized in that: The minimum number k of microseismic events in a cluster is: k = int(m / 25) Where m is the number of microseismic events.

10. The method for identifying fault activation induced by seafloor CO2 storage according to claim 2, characterized in that: When the earthquake b value is ≤1, the fault is judged to have entered the activation stage.

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