Underground crustal stress measurement method based on magnetic fluid tracing and in-hole scanning imaging

Abstract

The invention relates to the technical field of mine and geological engineering measurement, in particular to an underground crustal stress measurement method based on magnetic fluid tracing and in-hole scanning imaging, which comprises the following steps of: tripping a tool string integrated with double packers and a three-axis fluxgate sensor into a drilling target section; the first time of full-circumferential three-dimensional magnetic field scanning is carried out before setting, and a background magnetic field is obtained; after setting, fracturing fluid containing magnetic particles is pumped for hydraulic fracturing, and a pressure-time curve is recorded; performing second magnetic field scanning after the crack is closed; differential processing is carried out on the two times of magnetic field data, and net magnetic abnormal signals are extracted; determining a magnetic anomaly distribution orientation through inversion, namely a maximum horizontal principal stress direction; and calculating the crustal stress according to a pressure curve. Measurement of the direction and the size of the ground stress is fused into single operation, the defects that a traditional impression method is tedious in process and prone to interference and cannot distinguish natural cracks are overcome, and the method has the advantages of being high in efficiency, objective in result, high in anti-interference capacity, safe and environmentally friendly.

Description

Technical Field

[0001] This invention relates to the field of mining and geological engineering surveying technology, specifically a method for measuring underground geostress based on magnetic fluid tracing and borehole scanning imaging. Background Technology

[0002] In deep resource extraction, energy development, and the construction of major underground engineering projects, the in-situ rock stress state is a core fundamental parameter for engineering design, stability analysis, and disaster prevention. Among numerous methods for measuring in-situ stress, hydraulic fracturing has become the primary method for deep in-situ stress measurement due to its mature theory and wide applicable depth range.

[0003] Traditional hydraulic fracturing methods, by analyzing the pressure-time curves during fracturing operations, can reliably calculate the magnitudes of the maximum and minimum horizontal principal stresses at the measuring points using classical elasticity formulas. However, this method has significant technical limitations in determining the direction of geostress. In the traditional process, after completing the fracturing operation used to measure stress magnitude, the entire downhole tool string must be retrieved, replaced with a specialized "imprint packer," and lowered back into the borehole. The rubber packer is then pressed to create an imprint on the wellbore fractures, and the packer is retrieved again for manual interpretation of the orientation. This "two-trip drilling" operation mode results in a long measurement cycle and complex process. Furthermore, in actual engineering environments, poor borehole water quality and high rock cuttings content can easily contaminate the rubber surface, leading to blurred imprints; sharp edges of the wellbore can easily damage the rubber packer; more importantly, the imprint method cannot effectively distinguish between newly generated fractures from the current fracturing operation and existing natural fractures on the wellbore, resulting in significant uncertainty in stress direction determination.

[0004] Therefore, existing geostress direction measurement techniques based on the impression method have shortcomings in terms of efficiency, reliability, and accuracy. There is an urgent need for a geostress direction determination method that can be completed simultaneously with stress magnitude measurement, has strong anti-interference capabilities, and provides objective and accurate results. Summary of the Invention

[0005] The purpose of this invention is to provide a downhole stress measurement method based on magnetic fluid tracing and borehole scanning imaging to solve the problems mentioned in the background art.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] A downhole stress measurement method based on magnetic fluid tracing and borehole scanning imaging includes the following steps:

[0008] S1. Tool string insertion: Insert the test tool string, which integrates a dual packer and a three-axis fluxgate sensor, into the target layer inside the borehole in one go.

[0009] S2. Background magnetic field scanning and setting before fracturing: Before injecting fluid into the borehole, the three-axis fluxgate sensor is activated to perform the first full-circumferential three-dimensional magnetic field scan of the target test section and its adjacent well wall, acquire and store the background magnetic field distribution data as a magnetic field reference; then the double packer is set to isolate the target test section in the borehole.

[0010] S3. Magnetic tracer fracturing and pressure monitoring: Magnetic tracer fracturing fluid is pumped into the isolated target test section for hydraulic fracturing, and the pressure-time curve during the construction process is recorded simultaneously and continuously; the magnetic tracer fracturing fluid contains base fracturing fluid and micron- or nano-sized magnetic particles, which enter and fill the newly generated hydraulic fractures with the fracturing fluid.

[0011] S4. Post-fracture magnetic field scanning: After the construction is completed and the crack is closed, control the triaxial fluxgate sensor to perform a second full-circumferential three-dimensional magnetic field scan on the same spatial path as in step S2 to obtain the post-fracture magnetic field distribution data.

[0012] S5. Differential processing and net magnetic anomaly extraction: After spatial registration of the post-fracturing magnetic field distribution data obtained in step S4 with the background magnetic field distribution data obtained in step S2, differential calculation is performed to eliminate the formation background magnetism and system noise, and the net magnetic anomaly signal generated by the magnetic particles filling the fracture is extracted.

[0013] S6. Crack Imaging and Stress Direction Determination: Perform three-dimensional imaging and inversion interpretation on the net magnetic anomaly signal obtained in step S5 to identify the dominant distribution direction of the magnetic anomaly signal. This direction is the direction of the hydraulic crack. According to the theory of hydraulic cracking, this direction is the direction of the maximum horizontal principal stress at the test point.

[0014] S7. Calculation of ground stress: Analyze the pressure-time curve recorded in step S3, identify the characteristic values ​​of rupture pressure, instantaneous closure pressure and re-tension pressure, and calculate the magnitude of the maximum and minimum horizontal principal stresses based on the hydraulic fracturing theory formula.

[0015] S8. Geostress Tensor Synthesis: Integrate the direction of the maximum horizontal principal stress determined in step S6 with the magnitude of the geostress calculated in step S7, and output the complete geostress tensor information of the test point.

[0016] As a further aspect of the present invention: in steps S2 and S4, the triaxial fluxgate sensor is an array-type magnetic sensor that can rotate 360° around its axis during the lifting or lowering of the probe, so as to achieve scanning imaging of the three-dimensional spatial magnetic field of the hole wall.

[0017] As a further aspect of the present invention: in step S3, the magnetic particles are selected from magnetite powder or ferrite powder with a particle size of 200-400 mesh, and their concentration in the fracturing fluid is 5%-15%; the fracturing fluid also contains a crosslinking agent, which is used to solidify or thicken after entering the fracture, so as to prevent the magnetic particles from flowing back with the flowback fluid.

[0018] As a further aspect of the present invention: in step S5, before performing differential calculation, a step of spatial position registration of the two consecutive scan data is included to ensure the consistency of data comparison.

[0019] As a further aspect of the present invention: In step S6, the method for identifying the dominant distribution direction of the magnetic anomaly signal is as follows: On the polar coordinate display of the net magnetic anomaly signal distribution map or the azimuth-depth profile, two magnetic anomaly high value stripes that are symmetrically distributed at 180° are identified, and the direction of the line connecting their centers is the direction of the crack.

[0020] As a further aspect of the present invention: In step S7, the theoretical formula for hydraulic fracturing is: minimum horizontal principal stress S h Equal to instantaneous shut-off pressure P s Maximum horizontal principal stress S H =3P s -P r -P0, where P r P0 is the tension pressure, and P0 is the pore pressure.

[0021] As a further aspect of the present invention, the method further includes a verification step S9: after step S4, the test section is scanned using a drilling optical imaging device, and the crack traces observed in the acquired optical image are compared and verified with the net magnetic anomaly distribution obtained in step S5.

[0022] Compared with the prior art, the beneficial effects of the present invention are:

[0023] 1. Significantly improved efficiency: By integrating the magnetic scanning probe with the fracturing packer, fracturing operations, pressure recording, and magnetic scanning can be completed simultaneously in a single well run, simplifying the traditional "two-trip drilling" process into "one-trip drilling" and significantly shortening the operation cycle.

[0024] 2. Objective and reliable results: The quantifiable magnetic field signal replaces the physical imprint that relies on subjective human interpretation, making the results objective and traceable; the differential magnetic scanning technology effectively removes background interference, making the hydraulic crack signal stand out and the direction interpretation accuracy is high.

[0025] 3. Strong anti-interference capability: The detection target is the magnetic tracer injected this time. Differential processing ensures that only newly generated hydraulic fractures produce net magnetic anomalies, effectively eliminating interference from factors such as natural fractures, rough well walls, and turbid water.

[0026] 4. Safe and environmentally friendly: The magnetic tracer used is non-toxic and non-radioactive, making it especially suitable for complex underground environments such as coal mines where safety requirements are particularly stringent.

[0027] 5. Rich in information: The obtained three-dimensional data volume of net magnetic anomaly can not only indicate the direction of the crack, but also has the potential to make a preliminary assessment of the crack geometry through inversion analysis. Attached Figure Description

[0028] Figure 1 This is a flowchart illustrating the method of the present invention.

[0029] Figure 2 This is a schematic diagram illustrating the working state of the toolchain used in the method of the present invention.

[0030] Figure 3 This is a typical pressure-time curve obtained by the hydraulic fracturing method.

[0031] In the diagram: 1. Triaxial fluxgate sensor; 2. Upper packer; 3. Lower packer; 4. Magnetic tracer fracturing fluid; 5. Hydraulic fracture; 6. Pressure sensor processing unit; 7. Pressure gauge; 8. High-pressure pump. Detailed Implementation

[0032] The technical solution of this application will be further described in detail below with reference to specific embodiments.

[0033] The embodiments of this application are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this application, and should not be construed as limiting this application.

[0034] Please see Figure 1 This invention provides a downhole in-situ stress measurement method based on magnetic fluid tracing and borehole scanning imaging, the specific implementation steps of which are as follows:

[0035] S1. Tool string insertion: The test tool string, which integrates a dual packer and a three-axis fluxgate sensor 1, is inserted into the target layer in the borehole in one go.

[0036] S2. Background magnetic field scanning and setting before fracturing: Before injecting fluid into the isolation section, the triaxial fluxgate sensor 1 is activated to perform a first full-circumferential three-dimensional magnetic field scan of the isolation section and the surrounding well wall, acquire and store background magnetic field distribution data as a magnetic field reference; then, the double packer is set to isolate the target test section in the borehole.

[0037] S3. Magnetic tracer fracturing and pressure monitoring: Magnetic tracer fracturing fluid 4 is pumped into the isolation section for hydraulic fracturing, and the pressure-time curve during the construction process is recorded continuously and synchronously; wherein, the magnetic tracer fracturing fluid 4 contains base fracturing fluid, suspending agent and micron- or nano-sized magnetic particles, and the magnetic particles enter and fill the newly generated hydraulic fractures 5 with the fracturing fluid;

[0038] The magnetic tracer particles are magnetite powder or ferrite powder with a particle size of 200-400 mesh and a concentration of 5%-15%. A crosslinking agent is added to the fracturing fluid, which can undergo slight solidification or thickening after entering the fracture to prevent the magnetic particles from flowing back into the wellbore with the backflow fluid and to ensure the magnetic signal intensity in the fracture.

[0039] S4. Magnetic field scanning after fracturing: After the construction is completed and the crack is closed so that the magnetic particles are fixed in the crack, the triaxial fluxgate sensor 1 is controlled to perform a second full-circumferential three-dimensional magnetic field scan on the same spatial path as in step S2 to obtain the magnetic field distribution data after fracturing.

[0040] The azimuth magnetic scanning probe is an array-type magnetic sensor that can rotate 360° around its axis during the lifting / lowering process, thereby achieving scanning imaging of the three-dimensional spatial magnetic field of the borehole wall.

[0041] S5. Differential processing and net magnetic anomaly extraction: Spatially register the post-fracturing magnetic field distribution data obtained in step S4 with the background magnetic field distribution data obtained in step S2, and perform differential calculation to eliminate the formation background magnetism and system noise, and extract the net magnetic anomaly signal generated by the magnetic particles filling the fracture.

[0042] Before the "difference" process, a step of spatial registration of the two consecutive scans is included to ensure the consistency of the data comparison.

[0043] S6. Crack Imaging and Stress Direction Determination: The net magnetic anomaly signal obtained in step S5 is subjected to three-dimensional imaging and inversion interpretation. The dominant distribution direction of the magnetic anomaly signal is identified by forward fitting based on magnetic dipole theory or numerical inversion algorithm based on optimization theory. This direction corresponds to the orientation of the hydraulic crack 5 filled with magnetic particles. According to the theory of hydraulic fracturing, this orientation is the direction of the maximum horizontal principal stress at the test point.

[0044] The method for determining the orientation of the induced crack is as follows: on the polar coordinate display of the net magnetic anomaly signal distribution map or the azimuth-depth profile, identify two magnetic anomaly high value stripes that are symmetrically distributed at 180°, and the direction of the line connecting their centers is the orientation of the crack.

[0045] S7. Calculation of ground stress: Analyze the pressure-time curve recorded in step S3, identify the characteristic values ​​of rupture pressure, instantaneous closure pressure and re-tension pressure, and calculate the magnitude of the maximum and minimum horizontal principal stresses based on the hydraulic fracturing theory formula.

[0046] The theoretical formula for hydraulic fracturing is: Minimum horizontal principal stress S h Equal to instantaneous shut-off pressure P s Maximum horizontal principal stress S H =3P s -P r -P0, where P r P0 is the tension pressure, and P0 is the pore pressure.

[0047] S8. Geostress Tensor Synthesis: Integrate the direction of the maximum horizontal principal stress determined in step S6 with the magnitude of the geostress calculated in step S7, and output the complete geostress tensor information of the test point.

[0048] The method further includes a verification step S9: after step S4, the test section is scanned using a borehole optical imaging device, and the crack traces observed in the acquired optical image are compared and verified with the net magnetic anomaly distribution obtained in step S5, so as to improve the accuracy of hydraulically induced crack identification.

[0049] Example

[0050] This embodiment uses a ground stress test conducted in a deep return airway drilling site of a coal mine as an example. The test borehole is a vertical borehole, the target stratum is fine sandstone, and the depth is -650 meters.

[0051] Step 1: Tool preparation and downhole deployment

[0052] Reference Figure 1 and Figure 2 In this embodiment, the integrated testing tool string is connected and lowered via a drill rod. The tool string, from top to bottom, includes: a triaxial fluxgate sensor 1, an upper packer 2, and a lower packer 3. The triaxial fluxgate sensor 1 is encapsulated within a pressure-bearing housing and rigidly connected to the upper and lower components to ensure spatial stability. Before lowering the tool string, all instruments undergo power-on checks and calibration on the ground.

[0053] Step 2: Sealing and Background Magnetic Field Scanning

[0054] The tool string was lowered to the target depth of -650 meters. First, the tool string was slowly raised at a constant speed of 5 cm / s, while the three-axis fluxgate sensor 1 rotated at a constant angular velocity of 10° / s, performing the first full-circumferential three-dimensional magnetic field scan on the packer section and the well section extending 0.5 meters above and below it (a total of 2 meters). The scan data (including the magnetic field components Bx, By, and Bz in three orthogonal directions, and the corresponding depth, azimuth, and tool attitude) was transmitted in real time to the surface workstation and stored as a "background magnetic field dataset." This dataset characterizes the background magnetic field formed by the rock strata and the geomagnetic field. Subsequently, pressure was supplied to the dual packers through the downhole control system, causing the rubber sleeves to expand and adhere to the borehole wall, isolating a test section of approximately 1 meter in length (from -649.5 meters to -650.5 meters).

[0055] Step 3: Magnetic Tracer Fracturing Construction

[0056] After background scanning is completed, fracturing operations begin. A specially formulated magnetic tracer fracturing fluid 4 is pumped into the isolation section via a downhole high-pressure pumping system. The fracturing fluid formulation is: water-based fluid + 0.3% thickener (guar gum) + 1.5% nano-ferric oxide (Fe3O4) powder (particle size D50 = 500nm, surface-dispersed). The initial pumping rate is 8L / min, with the pressure gradually increasing. The high-pressure pumping system includes a pressure sensor processing unit 6, a pressure gauge 7, and a high-pressure pump 8. Both the pressure sensor processing unit 6 and the pressure gauge 7 are mounted on the high-pressure pump 8.

[0057] Pressure sensors continuously record changes in well pressure. For example... Figure 3 As shown, when the pressure rises to the peak value P b At 32.1 MPa, a sudden pressure drop indicated rock fracturing. After pumping approximately 50 liters of magnetic tracer fracturing fluid 4, the pump was stopped, and the pressure decreased and stabilized. The instantaneous shut-off pressure, Ps, was recorded as 20.5 MPa. Subsequently, two small-volume (3 liters / minute) re-tensioning cycles were performed, and the re-tensioning pressure, Pr, was recorded as 22.8 MPa. The entire operation lasted approximately 15 minutes, and the pressure-time curve was fully recorded.

[0058] Step 4: Post-fracture magnetic field scanning

[0059] After the fracturing is completed and a 20-minute wait is allowed to ensure the cracks are essentially closed under stress, effectively trapping the magnetic particles within them. Subsequently, the control toolchain is used to perform a second 3D magnetic field scan along the exact same path as the second step (lowering back to the starting depth, then raising and rotating at the same speed) to acquire the post-fracturing magnetic field dataset. At this point, the magnetic field within the scanned area now contains the superposition of the background field and the newly generated magnetic crack volume.

[0060] Step 5: Differential Processing and Signal Extraction

[0061] The downhole workstation utilizes specialized data processing software. The software first matches the spatial trajectories of the "background magnetic field dataset" and the "post-fracturing magnetic field dataset" using a depth encoder and toolchain displacement control. It then combines data from the triaxial accelerometer and triaxial magnetometer to calculate and correct sensor attitude, ensuring a one-to-one correspondence between the spatial points of the two scans. Next, it performs a point-by-point differential calculation: ΔB = B_post - B_previous. This calculation effectively cancels out invariants such as the geomagnetic field and the inherent magnetism of the rock. The resulting net magnetic anomaly dataset (ΔB) primarily contains signals originating from the magnetic tracer introduced during this fracturing operation.

[0062] Step 6: Imaging Inversion and Stress Direction Determination

[0063] Three-dimensional imaging of the ΔB data, displayed as an azimuth-depth map, clearly reveals two distinct high-value magnetic anomaly bands extending along the depth direction near azimuths N40°E and S40°W (180° apart). These bands correspond to the outcrop of hydraulic fracture 5, filled with magnetic particles, on the wellbore and its near-wellbore extension. The software automatically identifies the center azimuth of the bands and calculates their average orientation as N40°E. Based on the theory of hydraulic fracturing, hydraulic fracture 5 extends along the direction of the maximum horizontal principal stress; therefore, the maximum horizontal principal stress S at this measuring point is determined to be... H The direction is N40°E.

[0064] Step 7: Calculation of Ground Stress Magnitude

[0065] Based on the pressure curve data recorded in step three, P has been obtained. b =32.1MPa, P s =20.5MPa, P r =22.8 MPa. According to formation data, the pore pressure P0 at this depth is approximately 6.5 MPa. Substituting into the classical hydraulic fracturing formula, the calculation is as follows:

[0066] Minimum horizontal principal stress: S h =P s =20.5MPa

[0067] Maximum horizontal principal stress: S H =3P s –P r –P0=3×20.5–22.8–6.5=31.4MPa

[0068] Step 8: Result Output and Validation

[0069] Finally, all results were combined to generate a test report: at the -650m measuring point, the maximum horizontal principal stress S H =31.4MPa, direction N40°E; minimum horizontal principal stress S h =20.5MPa.

[0070] To verify this, in this embodiment, after retrieving the test tool string, a borehole optical sight was lowered to photograph the well section near -650 meters. The optical images showed two fresh, vertical fracture traces at N40°E and S40°W azimuths on the well wall, perfectly matching the fracture locations determined by magnetic scanning, thus cross-validating the accuracy of the results of the method of this invention. The optical images also showed multiple unfilled and partially calcite-filled natural joints in this section, but these joints were not displayed in the magnetic anomaly images, proving that the method of this invention effectively eliminated the interference of natural fractures.

[0071] The parts of this invention not described in detail, such as the specific structure of the pumping system and the data acquisition circuit, all adopt commonly known technologies in the field.

[0072] This downhole stress measurement method based on magnetic fluid tracer and in-hole scanning imaging integrates the magnetic scanning probe with the fracturing packer, enabling simultaneous fracturing, pressure recording, and magnetic scanning in a single downhole operation. This simplifies the complex process of traditional "two drilling runs" to "one drilling run," shortening the operation cycle and improving measurement efficiency. It replaces physical imprints relying on subjective human interpretation with quantifiable and digitally processable magnetic field signals, resulting in objective and traceable results. Differential magnetic scanning technology effectively eliminates background interference, highlighting the signal of the detection target (hydraulic fracture 5) and achieving high direction interpretation accuracy. The detection target of this method is essentially the magnetic tracer injected during this fracturing operation. Differential technology ensures that only newly generated fractures from this operation will appear in the net magnetic anomaly signal, thus eliminating interference from unfilled natural fractures on the interpretation of stress direction. The magnetic sensor is insensitive to wellbore conditions, water quality, coal dust, etc., and is reliable in operation. The magnetic tracer used is non-toxic and non-radioactive, making it safe and environmentally friendly, and is especially suitable for complex underground environments such as coal mines where safety requirements are special. The obtained net magnetic anomaly three-dimensional data volume can not only indicate the direction of the fracture, but also has the potential to make a preliminary assessment of the geometry of the fracture through further inversion analysis, providing richer information on underground fractures than a single impression trace.

[0073] The above are merely preferred embodiments of the present invention. It should be noted that those skilled in the art can make several modifications and improvements without departing from the concept of the present invention, and these should also be considered within the scope of protection of the present invention. These will not affect the effectiveness of the implementation of the present invention or the practicality of the patent.

Claims

1. A downhole in-situ stress measurement method based on magnetic fluid tracing and borehole scanning imaging, characterized in that, Includes the following steps: S1. Tool string insertion: Insert the test tool string, which integrates a dual packer and a three-axis fluxgate sensor, into the target layer inside the borehole in one go; S2. Background magnetic field scanning and setting before fracturing: Before injecting fluid into the borehole, the triaxial fluxgate sensor is activated to perform a first full-circumferential three-dimensional magnetic field scan of the target test section and its adjacent well wall, and to acquire and store the background magnetic field distribution data; then the double packer is set to isolate the target test section in the borehole. S3. Magnetic Tracing Fracturing and Pressure Monitoring: Magnetic tracer fracturing fluid is pumped into the isolated target test section for hydraulic fracturing, and the pressure-time curve during the construction process is recorded continuously and synchronously; the magnetic tracer fracturing fluid contains base fracturing fluid and micron- or nano-sized magnetic particles, which enter and fill the newly generated hydraulic fractures with the fracturing fluid; S4. Post-fracture magnetic field scanning: After the construction is completed and the fracture is closed, control the triaxial fluxgate sensor to perform a second full-circumferential three-dimensional magnetic field scan on the same spatial path as in step S2 to obtain post-fracture magnetic field distribution data. S5. Differential processing and net magnetic anomaly extraction: After spatially registering the post-fracturing magnetic field distribution data obtained in step S4 with the background magnetic field distribution data obtained in step S2, differential calculation is performed to eliminate the formation background magnetism and system noise, and the net magnetic anomaly signal generated by the magnetic particles filling the fracture is extracted. S6. Crack Imaging and Stress Direction Determination: Perform three-dimensional imaging and inversion interpretation on the net magnetic anomaly signal obtained in step S5, identify the dominant distribution direction of the magnetic anomaly signal, which is the direction of the hydraulic crack. According to the theory of hydraulic cracking, this direction is the direction of the maximum horizontal principal stress at the test point. S7. Calculation of ground stress: Analyze the pressure-time curve recorded in step S3, identify the characteristic values ​​of rupture pressure, instantaneous closure pressure and re-tension pressure, and calculate the magnitude of the maximum and minimum horizontal principal stresses based on the hydraulic fracturing theory formula. S8. Geostress Tensor Synthesis: Integrate the direction of the maximum horizontal principal stress determined in step S6 with the magnitude of the geostress calculated in step S7, and output the complete geostress tensor information of the test point.

2. The downhole in-situ stress measurement method based on magnetic fluid tracing and borehole scanning imaging according to claim 1, characterized in that, In steps S2 and S4, the triaxial fluxgate sensor is an array-type magnetic sensor that can rotate 360° around its axis during the lifting or lowering of the probe to achieve scanning imaging of the three-dimensional spatial magnetic field of the hole wall.

3. The downhole in-situ stress measurement method based on magnetic fluid tracing and borehole scanning imaging according to claim 1, characterized in that, In step S3, the magnetic particles are selected from magnetite powder or ferrite powder with a particle size of 200-400 mesh, and their concentration in the fracturing fluid is 5%-15%; the fracturing fluid also contains a crosslinking agent, which is used to solidify or thicken after entering the fracture to prevent the magnetic particles from flowing back with the flowback fluid.

4. The downhole in-situ stress measurement method based on magnetic fluid tracing and borehole scanning imaging according to claim 1, characterized in that, In step S5, before performing the differential calculation, a step of spatial registration of the two consecutive scan data is also included to ensure the consistency of the data comparison.

5. The downhole in-situ stress measurement method based on magnetic fluid tracing and borehole scanning imaging according to claim 1, characterized in that, In step S6, the method for identifying the dominant distribution direction of the magnetic anomaly signal is as follows: on the polar coordinate display of the net magnetic anomaly signal distribution map or the azimuth-depth profile, two magnetic anomaly high value stripes that are symmetrically distributed at 180° are identified, and the direction of the line connecting their centers is the direction of the crack.

6. The downhole in-situ stress measurement method based on magnetic fluid tracing and borehole scanning imaging according to claim 1, characterized in that, In step S7, the theoretical formula for hydraulic fracturing is: minimum horizontal principal stress S h Equal to instantaneous shut-off pressure P s Maximum horizontal principal stress S H =3P s -P r -P0, where P r P0 is the tension pressure, and P0 is the pore pressure.

7. The downhole in-situ stress measurement method based on magnetic fluid tracing and borehole scanning imaging according to claim 1, characterized in that, It also includes a verification step S9: after step S4, the test section is scanned using a borehole optical imaging device, and the crack traces observed in the acquired optical image are compared and verified with the net magnetic anomaly distribution obtained in step S5.

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