A method for characterizing natural fractures in a hot dry rock reservoir

Abstract

A method for characterizing natural fractures in a hot dry rock reservoir involves conducting natural fracture surveys in and around the hot dry rock mining area and performing geological statistical analysis; carrying out natural fracture analysis of drilling cores. Through the analysis of natural fractures in reservoir cores and in combination with paleomagnetism of the cores, the dip and dip angle attribute information of natural fractures in the reservoir is quantitatively determined; carrying out statistical analysis of altered minerals in cuttings. By continuously counting the amount of altered minerals in cuttings, the fracture-developed intervals are judged; carrying out logging fracture characterization based on the acoustic and electrical characteristics of fractures to identify the encountered fractures and statistically analyze the angles, properties, and density of fracture development; carrying out fine three-dimensional seismic fracture characterization, comparing the three-dimensional seismic attribute information of different fracture-developed intervals identified by logging and core logging, including ants, curvature, and maximum likelihood; comprehensively sorting out the analysis data obtained from the above steps, drawing a series of maps, and characterizing the deep hot dry rock reservoir.

Description

Technical Field

[0001] The present invention belongs to the technical field of geothermal development, and particularly relates to a method for characterizing natural fractures in a hot dry rock reservoir. Background Art

[0002] The hot dry rock project aims to develop and utilize the huge amount of energy stored in the low-permeability and high-temperature rock mass at a deep underground depth (3 - 10 km). Due to its wide distribution, the base load can be transported without storage, less land occupation, clean and pollution-free, etc., the hot dry rock resource has become an ideal sustainable energy source that can be obtained in the future. After more than fifty years of continuous exploration, more and more countries have joined the global exploration and development of hot dry rock. Establishing a large-scale, complex, and sustainable underground heat reservoir is the biggest technical challenge in the current development of hot dry rock. The existing hot dry rock engineering experiences at home and abroad have confirmed that the shear action of natural fractures constitutes the main mechanism for reservoir development and has great value for reservoir construction. Therefore, a reliable characterization of the deep natural fracture network is one of the keys to reservoir construction. At present, the development of hot dry rock in China is still in the experimental stage, and the technical method for characterizing natural fractures in the deep hot dry rock reservoir has not been established. The existing methods mostly draw on oil and gas exploration, mainly for mud shale, etc., and the accuracy of fracture characterization for hot dry rock bodies with granite as the main heat reservoir is not high, and the characterization accuracy rate is relatively low, which restricts the effective construction of the hot dry rock reservoir to a certain extent. Summary of the Invention

[0003] The purpose of the present invention is to provide a method for characterizing natural fractures in a hot dry rock reservoir, which has high accuracy in natural fracture characterization and strongly promotes the effective construction of the hot dry rock reservoir.

[0004] To achieve the above purpose, the present invention adopts the following technical solutions:

[0005] A method for characterizing natural fractures in a hot dry rock reservoir includes the following steps:

[0006] S1: Conduct a natural fracture survey in and around the hot dry rock mining area, carry out geological statistical analysis, and qualitatively judge the dominant trend and attitude information of natural fractures;

[0007] S2: Conduct an analysis of natural fractures in the drilling cores. Through the analysis of natural fractures in the reservoir cores and combining with the paleomagnetism of the cores, judge the dip and dip angle attribute information of the natural fractures in the reservoir;

[0008] S3: Conduct a statistical analysis of altered minerals in cuttings. By continuously counting the content of altered minerals in cuttings, quantitatively judge the density of the spatial development of natural fractures;

[0009] S4: Conduct logging fracture characterization based on the acoustic and electrical characteristics of natural fractures, combine the drilling time and drilling pressure to identify the natural fractures encountered during drilling, and count the angles, properties, and density of the development of natural fractures;

[0010] S5: Conduct fine characterization of three-dimensional seismic fractures, compare the three-dimensional seismic attribute information of different fracture development sections identified by well logging, cores / cuttings, including curvature, ant tracking, maximum likelihood, etc.; calibrate the attribute information of three-dimensional seismic curvature bodies, ant bodies, maximum likelihood, etc. based on the statistical analysis characteristics of natural fractures around the wellbore obtained by cores, cuttings and well logging, expand to three-dimensional space, and qualitatively characterize the location and density of reservoir fracture development sections;

[0011] S6: Comprehensively organize the analysis data obtained in S1, S2, S3, S4, and S5, draw a series of maps, and comprehensively characterize the deep geothermal reservoir; qualitatively obtain the dominant strike and dip angle of the rock mass based on the macroscopic statistical analysis of natural fractures on the surface outcrop; obtain the density distribution of natural fractures in the depth space based on continuous cuttings of the well; obtain the dominant strike and dip angle of natural fractures on the periphery of the reservoir section around the wellbore based on cores and well logging; conduct comparative analysis among the results, and if contradictions occur, select according to cores / cuttings, well logging, and surface statistics. On this basis, combine the obtained natural fracture information and the attribute information of three-dimensional seismic at this location, expand outward, and obtain the characteristics of reservoir fracture development in the three-dimensional space of the geothermal resource exploitation site.

[0012] Further, the step S1 includes:

[0013] (1) Determination of the investigation area

[0014] Select rock masses that outcrop to the surface at the periphery of the project site and are the same rock mass as the deep geothermal reservoir or have a similar tectonic evolution process, and conduct natural fracture investigations at a scale of 1:10,000.

[0015] (2) Investigation method

[0016] Refer to the technical requirements for regional geological surveys (standards of the China Geological Survey), and conduct natural fracture investigation and statistics at a scale of 1:10,000 in the investigation area.

[0017] (3) Fracture evaluation

[0018] Statistical properties of natural fractures, and obtain the dominant fracture strike and dip angle by drawing rose diagrams and density diagrams of natural fracture strikes and dip angles.

[0019] Further, the step S2 includes:

[0020] (1) Discrimination of natural fractures in cores

[0021] For the cores obtained from the geothermal reservoir section, identify natural fractures. The specific characteristics are that the natural fracture surfaces are mostly unclean, calcite and other hydrothermal sedimentary minerals are filled along the fractures, and hydrothermal alteration minerals such as chlorite and epidote appear near the fracture surfaces.

[0022] (2) Statistical analysis of natural fractures in cores

[0023] Statistically analyze the attribute information of the dip, dip angle, density, aperture, fracture nature, and development of altered minerals of natural fractures;

[0024] For non-directional coring, combined with the core paleomagnetic measurement method, obtain the fracture strike of the core;

[0025] Group according to the changes in density, dip, and dip angle, and draw rose diagrams of fracture density, dip, and dip angle changing with depth in space.

[0026] Furthermore, the step S3 includes:

[0027] (1) Determination of altered minerals

[0028] The cuttings sampling interval in the hot dry rock reservoir section is short (mostly 2 meters) and the continuity is strong. Using analysis methods such as X-ray diffraction, quantitatively analyze hydrothermal altered minerals such as chlorite and epidote in the reservoir cuttings, and use the existing core fractures to calibrate the corresponding relationship between the number of hydrothermal altered minerals and natural fractures;

[0029] (2) Evaluation of natural fractures

[0030] Draw a depth-altered mineral content diagram to quantitatively describe the densely developed section of natural fractures in the vertical space of the well.

[0031] Furthermore, the step S4 includes:

[0032] (1) Identification of natural fractures

[0033] Combined with the acoustic and electrical characteristics of natural fractures in drilling cores, use conventional acoustic imaging, resistivity logging, micro-resistivity imaging logging, and acoustic far-detection logging means to identify natural fractures and obtain information on fracture dip, dip angle, and aperture;

[0034] (2) Analysis of logging fractures

[0035] Group and statistically analyze the fracture density, dip, and dip angle information, and draw rose diagrams of fracture density, dip, and dip angle changing with depth.

[0036] Furthermore, in the step S1, the outcropping rock mass on the surface and the reservoir rock mass should be the same rock mass, or adjacent rock masses that have experienced the same geological structure evolution process.

[0037] Furthermore, in the step S3, the change in the content of altered minerals in cuttings is obtained by systematic sampling in the vertical direction of a single well and analyzing by methods such as X-ray diffraction. In the case of multiple wells, conduct lateral comparison of multiple wells.

[0038] Further, in step S4, when identifying natural fractures based on well logging, it is necessary to distinguish between natural fractures in the reservoir and drilling-induced fractures.

[0039] Further, in step S5, when characterizing fractures based on 3D seismic attributes, compared with S1 - S4, for fractures and reservoir properties, it is determined to select curvature volume, ant volume, maximum likelihood, etc.

[0040] The beneficial effects of the present invention are as follows: The method for characterizing natural fractures in a hot dry rock reservoir of the present invention takes the rock mass of the hot dry rock reservoir as the object, and conducts macroscopic statistical qualitative evaluation of natural fractures on the surface outcrop, core / cuttings analysis and imaging, acoustic well logging to quantitatively characterize the near - well fracture structure, and 3D seismic attributes to comprehensively characterize the spatial distribution characteristics of natural fractures in the deep reservoir in three - dimensional space. It has high accuracy in fracture characterization and effectively promotes the effective construction of the hot dry rock reservoir. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 It is the characterization of natural fractures in a hot dry rock reservoir according to the altered mineral content in an embodiment of the present invention.

[0042] Figure 2 It is the characterization of natural fractures in a hot dry rock reservoir according to well logging in an embodiment of the present invention.

[0043] Figure 3 It is the characterization of reservoir fractures based on 3D seismic curvature volume in an embodiment of the present invention.

[0044] Figure 4 It is the comprehensive characterization of natural fractures in a hot dry rock reservoir in an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0045] The following will illustrate the structure and the technical effects to be achieved of the present invention with specific embodiments in conjunction with the drawings. However, the selected embodiments are only for illustration and explanation, and are not intended to limit the scope of the present invention.

[0046] Please refer to Figures 1 to 4 As shown, the present invention provides a method for characterizing natural fractures in a hot dry rock reservoir. Taking a hot dry rock test and development site in a certain area as an example, the reservoir transformation depth is 3000 - 4000 m, and the lithology is granite. A large - area rock mass 10 kilometers northeast of it outcrops to the surface. Geochemical surveys and geophysical explorations show that it is the same set of rock mass as the hot dry rock reservoir. To characterize the natural fracture characteristics of the rock mass of the hot dry rock reservoir at this site, the following work is carried out:

[0047] S1: Conduct large - scale (1:10000) natural fracture geological profile surveys in the hot dry rock mining area and its periphery, obtain the strike, dip angle and nature of natural fractures for statistical analysis. After geological statistical mapping, it is obtained that the dominant strike of natural fractures is 15° northeast, the dip angle is 63°, and the nature is shear fractures;

[0048] S2: Analyze the drilling cores obtained from the reservoir section and find that the natural fractures in the reservoir section are shear fractures with an angle of 60 - 72°. Since the coring is non - directional, conduct paleomagnetic measurement on the drilling cores to obtain the strike of the natural fractures as 20° northeast, which is consistent with the surface survey. It is inferred that the natural fractures in the reservoir should be mainly northeast - trending and highly - angled developed;

[0049] S3: Conduct statistical analysis of altered minerals in cuttings. As shown by the analysis of drilling cores, it is found that alteration mostly occurs along the natural fractures in the reservoir section. The altered minerals are mainly chlorite and epidote. The content of altered minerals in the cuttings is 5%, and the corresponding natural fracture density is 3 fractures per meter. Therefore, taking the content of chlorite and epidote as the main indicators, analyze the composition of cuttings continuously obtained from the reservoir section to obtain the characteristics of the variation of the density of natural fractures with depth; judge the depth and range where the densely - developed fracture zones in the reservoir occur based on the percentage content of chlorite and epidote, and obtain data as Figure 1 ;

[0050] S4: Conduct imaging and acoustic logging fracture characterization. Based on resistivity imaging and acoustic imaging, identify the fractures encountered during drilling, and count the angles, properties, and densities of the fractures developed as Figure 2 ;

[0051] S5: Conduct fine characterization of 3D seismic fractures. Compare the 3D seismic attribute information such as ants, curvature, and maximum likelihood of different fracture - developed sections identified by well logging and core logging. According to the seismic attributes, qualitatively characterize the location and density of the fracture - developed sections in the reservoir around well TR1 as Figure 3 ;

[0052] S6: Comprehensively organize the analysis data obtained from S1, S2, S3, S4, and S5, and draw maps of natural fracture characterization as Figure 4 .

[0053] This invention is defined by the claims described above. However, based on this, those of ordinary skill in the art can make various obvious changes or modifications, which should all fall within the main spirit and protection scope of this invention.

Claims

1. A method for characterizing natural fractures in a hot dry rock reservoir, characterized in that, It includes the following steps: S1: Conduct a natural fracture investigation in the hot dry rock mining area and its periphery, carry out geological statistical analysis, and qualitatively judge the dominant strike and occurrence information of natural fractures; S2: Conduct an analysis of natural fractures in drilling cores. Through the analysis of natural fractures in reservoir cores and combined with paleomagnetism of the cores, identify the dip and dip angle attributes of natural fractures in the reservoir; S3: Conduct a statistical analysis of altered minerals in cuttings. By continuously counting the amount of altered minerals in cuttings, quantitatively judge the density of natural fractures developed in space; S4: Carry out logging fracture characterization based on the acoustic and electrical characteristics of natural fractures, identify the natural fractures encountered during drilling in combination with drilling time and drilling pressure, and count the angles, properties and density of natural fractures developed; S5: Conduct a fine characterization of 3D seismic fractures. Compare the 3D seismic attribute information of different fracture development segments identified by logging, core / cuttings, including ants, curvature, maximum likelihood; Based on the statistical analysis characteristics of natural fractures around the wellbore obtained by cores, cuttings and logging means, calibrate the attribute information of the 3D seismic curvature body, ant body, maximum likelihood, expand to 3D space, and qualitatively characterize the location and density of fracture development segments in the reservoir; S6: Comprehensively organize the analysis data obtained in S1, S2, S3, S4, and S5, draw a series of maps, and conduct a comprehensive characterization of the deep hot dry rock reservoir; Based on the macroscopic statistical analysis of natural fractures on the surface outcrop, qualitatively obtain the dominant strike and dip angle of the rock mass; Based on the continuous cuttings of the drilling, obtain the density distribution of natural fractures in the depth space; Based on the cores and logging, obtain the dominant strike and dip angle of natural fractures around the wellbore in the reservoir section; Conduct a comparative analysis among the results. If contradictions occur, select according to cores / cuttings, logging, and surface statistics. On this basis, combined with the obtained natural fracture information and the attribute information of 3D seismic at this location, obtain the fracture development characteristics of the reservoir in the 3D space of the hot dry rock resource mining site.

2. The method for characterizing natural fractures in a hot dry rock reservoir according to claim 1, wherein: The step S1 includes: (1) Determination of the investigation area Select the rock mass that outcrops to the surface at the periphery of the engineering site and is the same rock mass as the deep hot dry rock reservoir rock mass or has a similar tectonic evolution process; (2) Investigation method Refer to the technical requirements of regional geological survey and conduct a natural fracture investigation and statistics at a scale of 1:10,000 in the investigation area; (3) Fracture evaluation Statistically analyze the attributes of natural fractures, and obtain the dominant strike and dip angle of fractures by drawing rose diagrams and density diagrams of the strike and dip angle of natural fractures.

3. The method for characterizing natural fractures in a hot dry rock reservoir according to claim 1, wherein: The step S2 includes: (1) Discrimination of natural fractures in cores For the cores obtained from the hot dry rock reservoir section, identify the natural fractures. The specific characteristics are that the surface of the natural fractures is not clean, there are hydrothermal sedimentary minerals such as calcite filled along the fractures, and there are hydrothermal alterations such as chlorite and epidote near the fracture surface; (2) Statistical analysis of natural fractures in cores Statistically analyze the attribute information of the dip, dip angle, density, aperture, fracture property, and development of altered minerals of natural fractures; For non-directional coring, combine the paleomagnetic measurement method of the core to obtain the fracture strike of the core; Group according to the changes in density, dip, and dip angle, and draw rose diagrams of fracture density, dip, and dip angle changing with depth space.

4. The method for characterizing natural fractures in a hot dry rock reservoir according to claim 1, wherein: The step S3 includes: (1) Determination of altered minerals The continuous sampling interval of cuttings in the hot dry rock reservoir section is 2 m. The X-ray diffraction analysis method is used to quantitatively analyze the hydrothermal alteration minerals such as chlorite and epidote in the reservoir cuttings, and the existing core fractures are used to calibrate the corresponding relationship between the number of hydrothermal alteration minerals and natural fractures. (2) Natural fracture evaluation Draw a depth-alteration mineral content diagram to quantitatively describe the densely developed section of natural fractures in the vertical space of the well.

5. The method for characterizing natural fractures in a hot dry rock reservoir according to claim 1, wherein: The step S4 includes: (1) Natural fracture identification Combined with the acoustic and electrical characteristics of natural fractures in the drilling core, conventional acoustic imaging, resistivity logging, micro-resistivity imaging logging, and acoustic far-detection logging means are used to identify natural fractures, and information on fracture dip, dip angle, and aperture is obtained. (2) Logging fracture analysis Group and statistically analyze the fracture density, dip, and dip angle information, and draw the fracture density, dip, and dip angle rose diagrams varying with depth.

6. The method for characterizing natural fractures in a hot dry rock reservoir according to claim 1, wherein: In the step S1, the outcropping rock mass on the surface and the reservoir rock mass are the same rock mass, or adjacent rock masses that have experienced the same geological structure evolution process.

7. The method for characterizing natural fractures in a hot dry rock reservoir according to claim 1, wherein: In the step S3, the change in the content of altered minerals in cuttings is obtained by systematic sampling in the vertical direction of a single well and analyzed by the X-ray diffraction method. In the case of multiple wells, cross-well comparison is performed.

8. The method for characterizing natural fractures in a hot dry rock reservoir according to claim 1, wherein: In the step S4, when identifying natural fractures based on logging, the natural fractures in the reservoir and the drilling-induced fractures should be discriminated and distinguished.

9. The method for characterizing natural fractures in a hot dry rock reservoir according to claim 1, wherein: In the step S5, the fractures are characterized according to the 3D seismic attributes. Compared with S1-S4, the curvature body, ant body, and maximum likelihood body are determined according to the fractures and reservoir properties.

Images