Convection type geothermal resource exploration method

Abstract

The invention relates to a convection type geothermal resource exploration method and relates to the technical field of geothermal exploration, the convection type geothermal resource exploration method utilizes comprehensive means such as unmanned aerial vehicle flight, geological survey and geophysical prospecting, comprehensive exploration is realized according to a specific process and a technical route, and the exploration accuracy is improved. An inversion processing method is utilized to construct an integration of accurately depicting the exploration method, so that the defect of single exploration in the prior art is overcome, a comprehensive exploration mode is formed from a shallow part to a deep part, from a point to a surface and from a plane to a three-dimensional part, and the convection type geothermal resource distribution condition is more accurately judged. The method is clear in principle, feasible in technology, capable of rapidly and accurately delineating the favorable optimal well position in the bedrock exposure area, suitable for optimal well position positioning and resource quantity evaluation of the convection type geothermal system controlled by a structure, and wide in application range.

Description

Technical Field

[0001] The present invention relates to the field of geothermal exploration technology, and in particular to a convection-type geothermal resource exploration method. Background Art

[0002] The typical characteristic of a convection-type geothermal field is that when underground water or other fluids are under specific geological structural conditions (such as well-developed pores and cracks in rocks), their density decreases due to heating (such as heat from deep magma, heat generated by the decay of radioactive elements, and other heat sources), thus forming an upwelling; the colder fluid flows downward due to its high density to replenish, thus forming a convection circulation of the fluid.

[0003] With the development of geothermal energy industry, geothermal resource exploration, as a prerequisite for geothermal resource utilization, has become a key development focus. Geothermal resource exploration involves comprehensive geological, geophysical, and geochemical surveys, as well as drilling and testing, sampling and testing, and dynamic monitoring to identify geothermal resources in a specific area. Depending on the extent of the exploration work, it can be divided into the following stages: investigation, pre-feasibility exploration, feasibility exploration, and exploitation.

[0004] The main difficulty in exploring geothermal fields lies in the detection of geothermal fluid channels and the determination of deep groundwater distribution characteristics. This type of geothermal resource exploration requires a comprehensive assessment of geothermal geological characteristics and shallow and deep recharge and drainage conditions. However, current geothermal exploration mainly relies on single means, single geophysical methods (such as MT, gravity, geomagnetism, etc.) or geological survey analysis, which have the following shortcomings:

[0005] 1. Single geophysical exploration has multiple solutions;

[0006] 2. The geological survey results are macroscopic and not detailed enough, and hidden structures are missed;

[0007] 3. Traditional remote sensing interpretation has low accuracy.

[0008] In summary, there is an urgent need for a convection-type geothermal resource exploration method to solve the above problems. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to provide a scientifically optimized method for exploring convective geothermal resources, which accurately ascertains the characteristics of the underground structural occurrence and heat-controlling and water-conducting structures in the study area through comprehensive means, and determines the optimal development well location.

[0010] To achieve the above object, the present invention provides a convection-type geothermal resource exploration method, comprising the following steps:

[0011] S1. Determine the study area;

[0012] Specifically, previous geological survey data were collected to find a suitable study area for convection-type geothermal fields;

[0013] S2. Site investigation and geophysical survey;

[0014] Specifically, it includes drone flights, geological surveys, and mid- to deep-seated geophysical exploration.

[0015] S3. Comprehensively interpret the structure and determine the structural framework of the area;

[0016] S4. Split evaluation, delineate structural intersections, and determine optimal well locations.

[0017] As a preferred embodiment of the present invention, the specific implementation of step S2 is:

[0018] S201, UAV flight

[0019] Aerial flight technology is used to create a three-dimensional visualization of regional topography and geomorphology. This 3D map is then used to delineate surface hydrogeological units, analyze surface water flow, atmospheric precipitation accumulation, and the recharge range and source for optimal well locations. Furthermore, the 3D aerial flight results are used to analyze structural development.

[0020] S202, Geological Survey

[0021] Based on the geological conditions of the study area, geological surveys should be conducted on the basis of flight surveys, including data collection, investigation of on-site geological points and hydrogeological points. The geological surveys should focus on analyzing the accuracy of the structure of the study area and the occurrence of groundwater, in preparation for subsequent geophysical exploration.

[0022] S203, medium and deep geophysical exploration

[0023] When deploying the work, based on the preliminary investigation, the survey line should be perpendicular to the main structural direction, with contact lines set up to conduct analysis in multiple directions;

[0024] Conduct magnetotelluric surveys on convective geothermal fields in complex areas.

[0025] As a preferred embodiment of the present invention, the magnetotelluric detection in the medium-deep geophysical exploration work includes calibration, MN test, acquisition time test, consistency test, and electrode pairing test;

[0026] S2031, Calibration

[0027] To ensure the normal operation of the instrument and stable data collection before field work, the instrument and magnetic rod box must be calibrated and the calibration effect must be checked. If the calibration effect is unsatisfactory, recalibration must be carried out.

[0028] After the equipment arrives on site, all acquisition instruments are calibrated. Calibration has two functions for the instrument. One is to check whether the instrument is normal. An abnormal calibration curve often indicates a fault in the instrument's recording channel. The other is to check the standard signal response of the recording instrument itself.

[0029] S2032, MN test

[0030] For the electrode distance used in construction, the three parameters of MN=40, 60, and 100 are generally adopted. The specific selection should be made by comparative test based on the on-site interference noise conditions to ensure the signal-to-noise ratio.

[0031] S2033, Collection time test

[0032] The length of the magnetotelluric acquisition time directly affects the curve fitting and the selection of the minimum effective frequency. Depending on the depth, the acquisition time is selected to be 1 hour, 3 hours, 4 hours, 6 hours, etc. The acquisition time at a great depth must exceed 10 hours.

[0033] S2034, consistency test

[0034] When two or more instruments are used in the same measurement area, instrument consistency testing should be performed to ensure that the TE resistivity, TE phase, TM resistivity, and TM phase meet the specification requirement of a relative error of no more than 5%.

[0035] A fixed point is selected as a comparison point in the measurement area. Each instrument uses the same device for observation. The observation results are edited and interpolated to calculate the mean square of the apparent resistivity and phase of the same polarization mode according to formula (1):

[0036]

[0037] Where: m′ is the mean square relative error of the instrument consistency, %;

[0038] n——number of frequency points,

[0039] h——number of instruments, units;

[0040] A ij ——apparent resistivity or phase at the frequency point of the instrument, Ω·m or rad;

[0041] ——The average value of apparent resistivity or phase at the frequency point of each instrument, Ω·m or rad.

[0042] S2035, electrode pairing test

[0043] The electrode tanks used in the construction were soaked, and electrode pairing experiments were carried out on all electrodes. The electrodes were paired in pairs to ensure that their DC potential difference was no more than 2mV.

[0044] As a preferred embodiment of the present invention, the specific implementation method in step S3 is: inversion processing, based on the experience of geophysical exploration work in convection-type basins, the OCCAM (low dependence on the initial model and stable convergence) and Bostick methods are used to invert and interpret the work results. Two-dimensional inversion uses data observed at various points along the profile, and at the same time, based on the horizontal and vertical variation patterns of the geoelectric section, the entire model section is represented by smaller sub-regions. Through model parameterization, the corresponding model response is calculated, the observation results are fitted, and the model parameters are corrected based on the fitting situation until certain iteration termination conditions are met.

[0045] During the data inversion and interpretation process, the raw data is preprocessed using curve smoothing, interpolation, curve analysis, static correction, and other processing functions. High-frequency distortion and flying spots at some measuring points are restored using phase data, and static correction is performed on the raw data to ensure that the apparent resistivity curve is not affected by static distortion. Qualitative interpretation of MT data, primarily through analysis of curve types and apparent resistivity-frequency pseudo-sections, can help understand changes in basement uplift and depression, laying a solid foundation for further data interpretation.

[0046] As a preferred embodiment of the present invention, the specific implementation of step S4 is: data interpretation; based on the electromagnetic exploration interpretation principle, combined with the comprehensive analysis of the inversion resistivity cross-section diagram of the exploration area, the interpretation steps of this area are summarized as follows:

[0047] S401. Interpretation of survey lines is carried out step by step from the starting point, striving to closely integrate with geological data and reduce interpretation errors;

[0048] S402. On the inversion resistivity cross-section, if there is a horizontal gradient change in the apparent resistivity contour lines or an abnormal resistance band in the vertical and horizontal dense areas of the contour lines, it is inferred that this is an electrical reflection of the fault fracture zone;

[0049] S403. The division of strata is based on regional geology, comparison of electrical characteristics of major electrical layers, comprehensive analysis of previous geological data, and geological laws.

[0050] S404. In the identified geological anomaly, the best well location is delineated based on the structural distribution and regional geological laws.

[0051] Compared with the existing technology, the technical solution of this application has the following beneficial effects:

[0052] The present invention demonstrates a method for prospecting convective geothermal resources, which utilizes a combination of drone flights, geological surveys, and geophysical exploration. It implements comprehensive exploration according to specific procedures and technical routes, thereby improving exploration accuracy. The integration of this exploration method, which utilizes the inversion processing method to accurately depict structural geological features, overcomes the previous single detection, and forms a comprehensive detection model from shallow to deep, from point to surface, and from plane to three-dimensional, allowing for more accurate judgment of the distribution of convective geothermal resources. The present invention has clear principles and feasible technology, and can quickly and accurately delineate favorable optimal well locations in bedrock outcrop areas. It is suitable for optimal well location positioning and resource assessment in convective geothermal systems controlled by structures, and has a wide range of applications. BRIEF DESCRIPTION OF THE DRAWINGS

[0053] Figure 1 This is a flow chart of a convection-type geothermal resource exploration method according to one embodiment of the present application;

[0054] Figure 2 This is a schematic diagram of the process of geophysical exploration in one embodiment of the present application;

[0055] Figure 3 This is a schematic diagram of the process of medium-deep geophysical exploration work in one embodiment of the present application;

[0056] Figure 4 A three-dimensional image of a drone flying in the research area in one embodiment of the present application;

[0057] Figure 5 for Figure 4 A diagram of the fault interpreted from aerial photographs taken by a Chinese drone Figure 1 ;

[0058] Figure 6 for Figure 4 A diagram of the fault interpreted from aerial photographs taken by a Chinese drone Figure 2 ;

[0059] Figure 7 A regional geological map formed by a geological survey of the study area in one embodiment of the present application;

[0060] Figure 8 A regional geological profile formed by a geological survey of the study area in one embodiment of the present application;

[0061] Figure 9 This is a geological radar and micro-motion apparent shear wave velocity profile of the study area in one embodiment of the present application;

[0062] Figure 10 A schematic diagram of the work deployment for medium-deep geophysical exploration in one embodiment of the present application;

[0063] Figure 11This is the L1 line-view inversion profile in one embodiment of the present application;

[0064] Figure 12 This is the L2 line-view inversion profile in one embodiment of the present application;

[0065] Figure 13 A planar comprehensive position diagram of the fixed hole position in one embodiment of the present application;

[0066] Figure 14 This is a schematic diagram of the cross-sectional structure of the fixed hole position in one embodiment of the present application. DETAILED DESCRIPTION

[0067] In order to make the technical means, creative features, objectives and effects achieved by the present invention easier to understand, the present invention is further described below in conjunction with specific implementation methods.

[0068] Example 1:

[0069] A convection-type geothermal resource exploration method, such as Figure 1-3 As shown, the following steps are included:

[0070] S1. Determine the study area;

[0071] Specifically, previous geological survey data were collected to find a suitable study area for convection-type geothermal fields;

[0072] S2. Site investigation and geophysical survey;

[0073] Specifically, it includes drone flights, geological surveys, and mid- to deep-seated geophysical exploration.

[0074] S3. Comprehensively interpret the structure and determine the structural framework of the area;

[0075] S4. Split evaluation, delineate structural intersections, and determine optimal well locations.

[0076] Specifically, on-site exploration and geophysical survey include:

[0077] S201, UAV flight

[0078] First, aerial flight technology is used to display the regional topography and landform features in three dimensions. Figure 4 The 3D image shown above is used to divide the surface hydrogeological units, analyze the surface water flow direction, atmospheric precipitation accumulation, and the recharge range and source of the optimal target well location. Secondly, using the 3D aerial flight results, we analyze the structural development.

[0079] The specific parameters of the drone flight are as follows:

[0080] The density of point cloud data acquired by airborne LiDAR should be sufficient to meet the requirements for interpolating digital elevation model data. Specific values ​​are specified in Table 1. Point cloud density requirements should be appropriately relaxed in flat areas and tightened in areas with fragmented terrain.

[0081] Table 1 Point cloud density requirements

[0082]

[0083] Calculate the point cloud density based on a grid spacing no greater than 1 / 2 of the digital elevation model results.

[0084] Drawing accuracy requirements:

[0085] a. The basic contour interval of the topographic map is 1 meter, and the elevation is noted to 0.01 meter.

[0086] b. The mean error of the position of the basic control point relative to the basic starting point shall not be greater than 0.1mm on the map (10cm in the field), and the mean error of the elevation shall not be greater than 1 / 10 of the basic contour interval (10cm).

[0087] c. When measuring topographic map fragments, the mean error of the position of the feature points on the map relative to the adjacent basic points shall be ≤±0.5mm (for urban built-up areas, flat land, and hilly areas), and the mean error of the distance between adjacent feature points shall be ≤±0.4mm (for urban built-up areas, flat land, and hilly areas);

[0088] d. Elevation annotation points: The mean elevation error of elevation annotation points on topographic maps of urban built-up areas relative to adjacent basic map points shall not exceed ±0.15m; the mean elevation error of elevation annotation points or contour line interpolation points in other areas shall be ≤1 / 3 of the basic contour interval (33cm) on flat land, and may be relaxed by 0.5 times the above requirements in difficult areas.

[0089] According to the above technical requirements, the following Figure 5-Figure 6 Shown is a schematic diagram of faults interpreted from aerial photographs of the study area.

[0090] S202, Geological Survey

[0091] According to the geological conditions of the study area, geological survey work should be carried out on the basis of flight, including data collection, investigation of on-site geological points and hydrogeological points. The accuracy of the structure of the study area and the distribution of groundwater should be analyzed from the geological survey work to form a Figure 7 The regional geological map produced by the geological survey shown and Figure 8 The regional geological cross section formed by the geological survey shown is in preparation for subsequent geophysical exploration work;

[0092] In geological survey work, the geological occurrence of intrusive rock and extrusive rock formations is difficult to classify. Therefore, for these geological problems, auxiliary portable geological means are needed to assist the investigation, such as Figure 9The following figure shows a deep-depth geological radar scan and a micro-motion apparent shear wave velocity profile within the study area. Using highly accurate and convenient geophysical methods, we accurately delineated shallow structural outcrops and bedrock wind-blown zones. Combined with regional geological surveys and aerial flights, this provided the foundation for mid- to deep-layer geophysical exploration.

[0093] S203, medium and deep geophysical exploration

[0094] When deploying the work, based on the preliminary investigation, the survey line should be perpendicular to the main structural direction, with connecting lines set up to analyze in multiple directions; Figure 10 The schematic diagram of the work deployment of medium-deep geophysical exploration work is shown;

[0095] For convection-type geothermal fields in complex areas, magnetotelluric detection is an effective working method. The magnetotelluric detection in the medium-deep geophysical exploration work includes calibration, MN test, acquisition time test, consistency test, and electrode pairing test.

[0096] To ensure the normal operation of the instrument and stable data collection before field work, the instrument and magnetic rod box must be calibrated and the calibration effect must be checked. If the calibration effect is unsatisfactory, recalibration must be carried out.

[0097] After the equipment arrives on site, all acquisition instruments are calibrated. Calibration has two functions for the instrument. One is to check whether the instrument is normal. An abnormal calibration curve often indicates a fault in the instrument's recording channel. The other is to check the standard signal response of the recording instrument itself.

[0098] For the electrode distance used in construction, the three parameters of MN=40, 60, and 100 are generally adopted. The specific selection should be made by comparative test based on the on-site interference noise conditions to ensure the signal-to-noise ratio.

[0099] The length of the magnetotelluric acquisition time directly affects the curve fitting and the selection of the minimum effective frequency. Depending on the depth, the acquisition time is selected to be 1 hour, 3 hours, 4 hours, 6 hours, etc. The acquisition time at a great depth must exceed 10 hours.

[0100] When two or more instruments are used in the same measurement area, instrument consistency testing should be performed to ensure that the TE resistivity, TE phase, TM resistivity, and TM phase meet the specification requirement of a relative error of no more than 5%.

[0101] A fixed point is selected as a comparison point in the measurement area. Each instrument uses the same device for observation. The observation results are edited and interpolated to calculate the mean square of the apparent resistivity and phase in the same polarization mode according to the following formula:

[0102]

[0103] in,

[0104] Where: m′ is the mean square relative error of the instrument consistency, %;

[0105] n——number of frequency points,

[0106] h——number of instruments, units;

[0107] A ij ——apparent resistivity or phase at the frequency point of the instrument, Ω·m or rad;

[0108] ——The average value of apparent resistivity or phase at the frequency point of each instrument, Ω·m or rad.

[0109] The electrode tanks used in the construction were soaked, and electrode pairing experiments were carried out on all electrodes. The electrodes were paired in pairs to ensure that their DC potential difference was no more than 2mV.

[0110] like Figure 11 、 Figure 12 As shown, a comprehensive structural interpretation of the geophysical survey results was performed to determine the structural framework within the area. Lines L1 and L2 were identified as parallel to the regional structural orientation. The two Bostick inversion maps for the cross-sections show that at milepost 2314, L1, and at elevations -1200 to -1500, L1 and L2, respectively, at milepost 2460, show a region of significant horizontal resistivity variation. Vertically, this indicates a relatively well-developed longitudinal fracture zone, suggesting a favorable well location. The Occam inversion maps for the two figures suggest a suspected dip in the stratigraphic structure, but due to the varying stratigraphic characteristics within the study area, the structural occurrence and location are difficult to discern. Vertically, the low-resistivity zone is consistent with the Bostick inversion map, suggesting a favorable location. This is presumed to be an area of ​​well-developed fault and fracture zones. The inversion cross-section of line L6 suggests a suspected large-dipping structural anomaly, with a low-resistance anomaly at elevations between -1100 and -1400, located in Tanyanggou. Therefore, the low-resistance line connecting Tanyanggou and the L1 and L2 survey lines represents a relatively good well location.

[0111] The inversion processing results are combined with data interpretation; according to the electromagnetic exploration interpretation principle, combined with the comprehensive analysis of the inversion resistivity cross-section diagram of this exploration area, the analysis is carried out based on the following interpretation steps:

[0112] S401. Due to the dispersion of survey lines, geological changes are more complex when interpreting structural morphology. Therefore, the interpretation of survey lines is gradually extended from the starting point, striving to closely integrate with geological data and reduce interpretation errors.

[0113] S402. On the inversion resistivity cross-section, if there is a horizontal gradient change in the apparent resistivity contour lines or an abnormal resistance band in the vertical and horizontal dense areas of the contour lines, it is inferred that this is an electrical reflection of the fault fracture zone;

[0114] S403. The division of strata is based on regional geology, comparison of electrical characteristics of major electrical layers, comprehensive analysis of previous geological data, and geological laws.

[0115] S404. In the identified geological anomaly, the best well location is delineated based on the structural distribution and regional geological laws.

[0116] In view of deep geothermal resources, combined with the independently innovative structural heat control theory, we have arranged magnetotelluric detection in a targeted manner, and carried out surface geological surveys using drone mapping. Through comprehensive means, we have accurately identified the characteristics of the underground structural distribution and heat-controlling and water-conducting structures in the area, determined the optimal development well location, and equipped the development of geothermal new energy with "X-ray eyes". Figure 11 It is the plane comprehensive position diagram of the fixed hole position. Figure 12 Schematic diagram of the cross-sectional structure of the fixed hole position.

[0117] Table 2 Results of well logging interpretation of fractured zone in aquifer

[0118] Serial number Layer bottom depth (m) Layer thickness (m) Geotechnical name 1 901.55 6.55 aquifer 2 1030.70 1.00 Broken Zone 3 1106.95 4.50 Broken Zone 4 1570.80 5.30 Fracture layer 5 1616.95 3.05 Fracture layer 6 1624.70 2.80 Fracture layer 7 1721.80 0.95 Fracture layer

[0119] After drilling, the fractured zone of the aquifer was found to have a relatively clear development, with a water volume of 30m 3 / h, with a temperature of 84°C. Based on the research data, the first high-temperature hot spring well in the study area was drilled, filling the gap in regional hot spring development.

[0120] The basic principles, main features, and advantages of the present invention are shown and described above. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and modifications are intended to fall within the scope of the present invention. The scope of protection claimed in the present invention is defined by the appended claims and their equivalents.

Claims

1. A convection-type geothermal resource exploration method, characterized by: The following steps are involved: S1. Determine the study area; S2. Site investigation and geophysical survey; S3. Comprehensively interpret the structure and determine the structural framework of the area; S4. Split evaluation, delineate structural intersections, and determine optimal well locations.

2. A convection-type geothermal resource exploration method according to claim 1, characterized in that: The step S2 specifically includes: S201, UAV flight; S202, geological survey; S203. Medium and deep geophysical exploration work.

3. A convection-type geothermal resource exploration method according to claim 2, characterized in that: The specific implementation method of step S201 is: using aerial flight technology to display the regional topographic features in three dimensions, dividing the ground hydrogeological units based on the three-dimensional map, analyzing the surface water flow direction, atmospheric rainfall accumulation, and the recharge range and recharge source of the target optimal well location; secondly, using the three-dimensional aerial flight results to analyze the structural development.

4. A convection-type geothermal resource exploration method according to claim 3, characterized in that: The specific implementation method of step S202 is: based on the geological conditions of the study area, geological surveys should be carried out on the basis of flight, including data collection, investigation of on-site geological points and hydrogeological points, and the accuracy of the structure of the study area and the distribution of groundwater should be analyzed from the geological survey work to prepare for subsequent geophysical exploration work.

5. A convection-type geothermal resource exploration method according to claim 4, characterized in that: The specific implementation of step S203 is: when deploying the work, based on the preliminary investigation, the survey line should be perpendicular to the main structural direction, and a connecting line should be set to perform analysis in multiple directions.

6. A convection-type geothermal resource exploration method according to claim 5, characterized in that: The specific implementation of step S203 also includes: magnetotelluric detection.

7. A convection-type geothermal resource exploration method according to claim 6, characterized in that: The magnetotelluric detection includes calibration, MN test, acquisition time test, consistency test and electrode pairing test.

8. A convection-type geothermal resource exploration method according to claim 6, characterized in that: The specific implementation of step S3 involves inversion processing. Based on experience in geophysical exploration of convective basins, the results are interpreted using the OCCAM (low dependence on the initial model and stable convergence) and Bostick methods. Two-dimensional inversion utilizes observational data from various points along the profile and, based on the lateral and longitudinal variations of the geoelectric section, represents the entire model section as smaller subregions. Through model parameterization, the corresponding model response is calculated, fitted to the observations, and the model parameters are corrected based on the fit until a specific iteration termination condition is met.

9. A convection-type geothermal resource exploration method according to claim 8, characterized in that: The specific implementation of step S3 also includes: during the data inversion and interpretation process, pre-processing the raw data using curve smoothing, interpolation, curve analysis, static correction, and other processing functions. High-frequency distortion and flying spots at some measuring points are restored using phase data, and static correction is performed on the raw data to ensure that the apparent resistivity curve is not affected by static distortion. Qualitative interpretation of MT data primarily involves analyzing curve types and apparent resistivity-frequency pseudo-sections to understand changes in basement uplift.

10. A convection-type geothermal resource exploration method according to claim 9, characterized in that: The specific implementation of step S4 is: combining the inversion processing results with data interpretation; according to the electromagnetic exploration interpretation principle, combined with the comprehensive analysis of the inversion resistivity cross-section diagram of the exploration area, the analysis is performed based on the following interpretation steps: S401. Interpretation of survey lines is carried out step by step from the starting point, striving to closely integrate with geological data and reduce interpretation errors; S402. On the inversion resistivity cross-section, if there is a horizontal gradient change in the apparent resistivity contour lines or an abnormal resistance band in the vertical and horizontal dense areas of the contour lines, it is inferred that this is an electrical reflection of the fault fracture zone; S403. The division of strata is based on regional geology, comparison of electrical characteristics of major electrical layers, comprehensive analysis of previous geological data, and geological laws. S404. In the identified geological anomaly, the best well location is delineated based on the structural distribution and regional geological laws.

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