Method and system for multi-scale, multi-stage and quantitative characterization of shale gas preservation conditions

Abstract

The present invention belongs to the technical field of unconventional oil and gas geology, and discloses a method and system for multi-scale - multi-stage - quantitative characterization of shale gas preservation conditions. The method for multi-scale - multi-stage - quantitative characterization of shale gas preservation conditions includes: clarifying the development and distribution characteristics of fractures - fissures at different scales based on 3D seismic, imaging logging, core description, thin-section observation, scanning electron microscopy, and micron CT scanning, and combining the vein filling stages, inclusion temperature and salinity, as well as carbon and oxygen isotopes, so as to realize the quantitative evaluation of shale gas preservation conditions. The present invention comprehensively combines various technical means such as geology, geophysics, and geochemistry, and establishes a method for accurately evaluating shale gas preservation conditions, which is of great significance for the evaluation of shale resource potential and the commercial value of gas reservoirs. In addition, the present invention uses formation pressure and the actual production data of shale gas wells to test the predicted shale preservation conditions, making the present invention more reliable through discrimination and comparison.

Description

Technical Field

[0001] The present invention belongs to the technical field of unconventional oil and gas geology, especially in the field of shale gas preservation condition evaluation, and particularly relates to a method and system for multi-scale - multi-stage - quantitative characterization of shale gas preservation conditions. Background Art

[0002] Different from marine shales in North America, marine shale strata in China are characterized by old age, high degree of thermal evolution, and complex burial history. Preservation conditions are the key factors affecting the enrichment and high production of shale gas. Previous studies on shale gas preservation conditions have been carried out from aspects such as regional tectonic styles (such as anticlines or synclines), fault properties (such as strike-slip faults, thrust faults, or compressional-shear faults), distance from faults, fracture-fissure development density, and lithology and thickness of shale top and bottom plates. However, these indicators can only qualitatively discriminate shale gas preservation conditions to a certain extent and cannot quantitatively evaluate the preservation conditions. To address this problem, the present invention comprehensively applies geological, geophysical, and geochemical methods to establish a method for quantitatively evaluating shale gas preservation conditions.

[0003] Through the above analysis, the problems and defects of the prior art are as follows: The prior art mainly qualitatively discriminates shale gas preservation conditions through macroscopic fracture-fissure characteristics, regional caprock conditions, etc., and cannot quantitatively evaluate the preservation conditions. Summary of the Invention

[0004] In view of the problems existing in the prior art, the present invention provides a method and system for multi-scale - multi-stage - quantitative characterization of shale gas preservation conditions, and particularly relates to a method and system for characterizing shale gas preservation conditions by integrating geological - geophysical - geochemical methods.

[0005] The present invention is implemented as follows. A method for multi-scale - multi-stage - quantitative characterization of shale gas preservation conditions includes: determining the development and distribution characteristics of fractures and fissures at different scales based on 3D seismic, imaging logging, core description, thin section observation, scanning electron microscopy, and micro-CT scanning, and quantitatively analyzing shale gas preservation conditions in combination with vein filling stages, inclusion temperature and salinity, and carbon and oxygen isotopes.

[0006] Further, the method for multi-scale - multi-stage - quantitative characterization of shale gas preservation conditions includes the following steps:

[0007] Step 1, multi-scale fracture-fissure characterization by geophysical methods;

[0008] Step 2, multi-stage fracture-vein characterization by geological methods;

[0009] Step 3, quantitative evaluation of preservation conditions by geochemical methods.

[0010] Furthermore, the multi-scale fracture-crack characterization in the geophysical method of Step 1 includes:

[0011] (1) Evaluating macro-scale fractures-cracks through 3D seismic and imaging logging methods. Fractures or cracks of different scales often exhibit phenomena such as in-phase axis offset, translation, distortion, and multiple waves on seismic profiles, thus being qualitatively and quantitatively identified. Based on seismic data, faults or cracks can be identified by methods such as direct attributes (coherence, curvature, variance) that characterize seismic reflection changes or indirect bionic algorithms [ant tracking, neural network, AFE (Automatic fault extraction)]. The first type of method is more widely used, but coherence-based attributes often have a mean effect, blurring the response of small faults. The accuracy of surface curvature is affected by the accuracy of horizon interpretation and does not necessarily correspond to the actual fault structure. The actual calculation of the variance volume requires a large amount of computing resources. The AFE technology automatically extracts fault lines and fault surfaces from discontinuous data volumes (such as coherence volumes) and can improve the efficiency and accuracy of automatic horizon tracking by using the interpreted fault surfaces as constraints. By enhancing the fault line or fault surface data, the AFE technology can depict fractures more clearly than the original coherence volume. The larger the AFE value (i.e., blue and black), the larger the scale and density of the fractures-cracks developed in the area; conversely, the smaller the AFE value (i.e., white and gray), the smaller the scale and density of the fractures-cracks development ( Figure 4 ). Imaging logging is an effective method to reflect the lithology, fracture development degree, and its type near the wellbore through color changes. The darker the color, the lower the resistivity; conversely, the brighter the color, the higher the resistivity. After obtaining the imaging logging data, import it into the Techlog2015 logging interpretation software, outline different fractures through color differences, and the fracture development characteristics can be evaluated after processing. Based on the fracture response characteristics, high-conductivity fractures, high-resistivity fractures, and faults can mainly be distinguished ( Figure 5 ). High-conductivity fractures belong to open fractures. During the drilling process, due to the invasion of low-resistivity mud drilling fluid, the resistivity of such fractures is significantly lower than that of the surrounding rock, showing a continuous or semi-closed dark sine wave curve on the imaging logging map. High-resistivity fractures generally refer to fractures completely or semi-filled with high-resistivity minerals such as quartz and calcite, and their image characteristics are shown as bright sine curves. Faults are similar to high-conductivity fractures, and their image characteristics are shown as dark sine curves. The main difference is that due to the relative movement of the hanging wall and footwall, faults are usually accompanied by stratum dislocation near the fault surface; only the strata on both sides of high-conductivity fractures are fractured under external forces, and the strata are relatively continuous.

[0012] (2) Observing the characteristics of medium and small-scale fractures through cores and thin sections. Core observation is to observe and describe the cores displayed in the core library to judge the fracture development density, angle, aperture, and filling conditions, etc. ( Figure 6). Thin section observation is to grind the rock sample into a thin rock slice of about 50mm long × 25mm wide × 1mm thick, then stick it on a glass slide, and directly observe the small-scale fracture characteristics through the single polarized light of an optical microscope. Under single polarized light, the surrounding rock (i.e. the rock around the fracture vein) appears dark black, while the filled fracture vein appears grayish white to bright white ( Figure 7 ).

[0013] (3) Analyze the microscopic crack characteristics by field emission scanning electron microscopy and micrometer CT. The scanning electron microscopy analysis is to cut the core into a block of rock samples with a length of 1 cm × width of 1 cm × thickness of 0.5 cm and stick it on a T-stage. Then, the sample surface is mechanically ground and argon ion polished. After obtaining a flat sample surface, it is placed under an electron microscope for microcrack observation ( Figure 8 ). Before the micron CT experiment, a cylindrical rock sample with a diameter of about 4 mm and a length of about 1 cm was drilled on the core sample along the vertical bedding direction. After being fixed, it was placed vertically in the micron CT scanning equipment (ZEISS Versa 520). The scanning voltage was set to 60KV, the experimental temperature was 20°C, and the single exposure time was 2s for micron CT scanning. After the scan, the data body was imported into the Avizo software, and the matrix components (such as quartz, feldspar, clay minerals, etc., which appear in light gray), heavy minerals (such as pyrite, which appears in bright white) and microcracks (appear in black) were distinguished by grayscale. Furthermore, microcracks, matrix components and pyrite were represented by red, gray and yellow, respectively, highlighting the differences in the spatial distribution of each component ( Figure 9 ).

[0014] (4) Based on the characterization results of different experimental methods from (1) to (3), the development characteristics of faults and cracks at different scales are preliminarily clarified.

[0015] Furthermore, step 2 of geological method multi-stage fracture-vein characterization includes:

[0016] 1) Clarify the fracture sequence and formation temperature through cathode luminescence and fluid inclusions. Cathodoluminescence is the most intuitive and effective method to determine the fracture vein type and formation period. Its basic principle is that the energetic electron beam is emitted by the cathode, accelerated by the anode and bombards the sample surface, causing the luminescence phenomenon. Generally speaking, quartz does not emit light under single polarized light, while calcite shows dark red to orange ( Figure 10)。In addition, for the fracture veins identified by cathodoluminescence, by observing their cutting relationships, the relative timing of fracture formation can be determined (earlier-formed fractures are cut by later-formed fractures). Fluid inclusions are the diagenetic fluids trapped in the lattice defects or cavities of minerals during the crystallization and growth of minerals. The formation temperature (i.e., the homogenization temperature) can be approximately regarded as the formation temperature of the fracture veins. Generally speaking, the higher the homogenization temperature of the fluid inclusions, the earlier the fracture veins were formed during the process of the formation being uplifted from the deep to the shallow. Conversely, the lower the homogenization temperature, the later the veins were formed. The principle and process for measuring the homogenization temperature of fluid inclusions are as follows: When the formation is uplifted from the deep to the shallow, the pressure decreases, causing some of the gases dissolved inside the fluid inclusions to precipitate. Therefore, the inclusions observed today are often gas-liquid two-phase inclusions. To measure the initial formation temperature of the fluid inclusions, the current gas-liquid two-phase inclusions need to be heated to increase the pressure inside the inclusions so that the two phases become a homogeneous liquid phase again. The temperature at this moment represents the homogenization temperature( Figure 11 )。

[0017] 2) Combining the fracture-fracture development characteristics, the fracture sequence, and the formation temperature obtained in step (4) of step one, the sealing property of the shale system can be preliminarily and qualitatively judged. Generally speaking, the lower the development degree of fractures-fractures at different scales, the fewer the fracture stages, and the higher the fracture formation temperature, the better the corresponding sealing property of the shale system may be; on the contrary, the higher the development degree of fractures-fractures at different scales, the more the fracture stages, and the lower the fracture formation temperature, it means that the possibility of fractures-fractures opening the shale reservoir is higher, and the opening time is also longer. Therefore, the corresponding sealing property of the shale system is also worse.

[0018] Furthermore, the geochemical method in step three for quantitatively evaluating the preservation conditions includes:

[0019] Accurately evaluate the shale preservation conditions by synthesizing various analysis results. The carbon and oxygen isotope compositions of carbonate rocks in different diagenetic environments are significantly different. Therefore, the source of diagenetic fluids can be studied based on the carbon and oxygen isotope differences between the fracture veins and the shale wall rock. If the difference between the two is small or there is no difference, it means that the diagenetic fluids forming the veins may mainly come from the shale series itself, and the shale system is relatively closed. If the difference between the two is large, then further combine the sealing property characteristics of the shale system preliminarily determined in step 2) of step two to check whether the shale system has open conditions, so as to accurately judge the shale sealing property( Figure 12) The measurement process of carbon and oxygen isotopes is mainly as follows: Micro-drilling technology is used to take micro-drilling samples from the fracture calcite veins. The drilled samples are crushed to less than 200 mesh. An appropriate amount (about 180 μg) of the sample is weighed and put into a flask, and phosphoric acid is added to fully react with the sample. After the reaction is complete, the released CO2 is input into a MAT253 stable isotope mass spectrometer through a GasbenchⅡ multi-purpose on-line gas preparation device for carbon and oxygen isotope measurement.

[0020] Another object of the present invention is to provide a system for multi-scale - multi-stage - quantitative characterization of shale gas preservation conditions by applying the method for multi-scale - multi-stage - quantitative characterization of shale gas preservation conditions described above. The system for multi-scale - multi-stage - quantitative characterization of shale gas preservation conditions includes:

[0021] A fracture - crack characterization module, which is used to perform multi-scale fracture - crack characterization by using geophysical methods;

[0022] A crack - vein characterization module, which is used to perform multi-stage crack - vein characterization by using geological methods;

[0023] A preservation condition evaluation module, which is used to quantitatively evaluate the preservation conditions by using geochemical methods.

[0024] Another object of the present invention is to provide a computer device. The computer device includes a memory and a processor. When a computer program stored in the memory is executed by the processor, the processor performs the following steps:

[0025] Based on 3D seismic, imaging logging, core description, thin section observation, scanning electron microscopy and micro-CT scanning, clarify the development and distribution characteristics of fractures - cracks at different scales, and combine the vein filling stages, inclusion temperature and salinity, as well as carbon and oxygen isotopes, so as to realize the quantitative evaluation of shale gas preservation conditions.

[0026] Another object of the present invention is to provide a computer-readable storage medium, which stores a computer program. When the computer program is executed by a processor, the processor performs the following steps:

[0027] Based on 3D seismic, imaging logging, core description, thin section observation, scanning electron microscopy and micro-CT scanning, clarify the development and distribution characteristics of fractures - cracks at different scales, and combine the vein filling stages, inclusion temperature and salinity, as well as carbon and oxygen isotopes, so as to realize the quantitative analysis of shale gas preservation conditions.

[0028] Another object of the present invention is to provide an information data processing terminal, which is used to implement the system for multi-scale - multi-stage - quantitative characterization of shale gas preservation conditions.

[0029] Combined with the above technical solutions and the technical problems solved, the advantages and positive effects of the technical solution to be protected by the present invention are analyzed from the following aspects:

[0030] First, in view of the technical problems existing in the above-mentioned prior art and the difficulty of solving the problem, closely combined with the technical solution to be protected by the present invention and the results and data in the R & D process, etc., analyze in detail and deeply how the technical solution of the present invention solves the technical problems and the creative technical effects brought after solving the problems. The specific description is as follows:

[0031] The present invention combines various technical means such as geology, geophysics and geochemistry to establish a method for accurately evaluating the preservation conditions of shale gas, which is of great significance for evaluating the potential of shale resources and the commercial value of gas reservoirs. In addition, the present invention uses formation pressure and the actual production data of shale gas wells to test the predicted shale preservation conditions, making the present invention more reliable through discrimination and comparison.

[0032] Formation pressure: refers to the pressure acting on the fluid in the rock pores, and this data can be quantitatively calculated through well logging data.

[0033] Actual production data of shale gas wells: mainly refers to the gas production data obtained after hydraulic fracturing of shale gas wells.

[0034] Generally speaking, when the shale preservation conditions are good, the formation generally has overpressure, and high-yield shale gas flow (i.e., overpressure rich gas) can be obtained after fracturing. On the contrary, when the shale preservation conditions are poor, the shale formation system mainly shows normal pressure or negative pressure, and the shale gas production capacity is poor after fracturing (i.e., normal pressure gas-bearing or negative pressure lean gas). Therefore, the shale preservation conditions identified by the present invention can be tested through formation pressure and shale gas production data.

[0035] The present invention has been successfully applied to the Wufeng Formation-Longmaxi Formation shale in the Jiaoshiba area of a certain basin. The AFE tone of Well JYA in the first-phase production and construction area of Jiaoshiba is white on the seismic scale (see Figure 4 ), and fractures are basically not developed in the direct caprock (see Figure 5 ), indicating that the overall development degree of macroscopic fractures and fissures is relatively low. The length and width distribution range of medium and small-scale fractures is small, mainly horizontal fractures (see Figures 6 - 7 ). Microfractures are non-structural fractures formed during the hydrocarbon generation and evolution process, which can be used as the storage space for shale gas and have no influence on the shale gas preservation conditions (see Figure 8 ). Micron CT shows that the connectivity of microfractures is poor, mainly isolated dots (see Figure 9 ). The development degree of fractures and fissures of different scales in Well JYA is relatively low. The results of cathodoluminescence and inclusion homogenization temperature show that the formation temperature of calcite veins is relatively high (see Figures 10 - 11), combining fracture and fissure features at different scales, it is considered that the shale system of Well JYA does not have the prerequisite for opening to the outside world. In addition, since the carbon and oxygen isotopes of calcite veins and shale are basically the same (see Figure 12 ), it is further proved that the fluid may mainly come from the inside of the shale system. It is comprehensively determined that the preservation conditions of the shale in Well JYA are relatively good.

[0036] In the Jiaoshiba Pingqiao block, Well JYB shows an AFE tone of grayish white to grayish black at the seismic scale (see Figure 4 ), indicating an increase in the development degree of macroscopic fractures and fissures. There are certain amounts of fracture shows in imaging logging and core observation (see Figures 5 - 6 ), and microfractures related to structures can also be seen in scanning electron microscopy (see Figure 8 ). Although the veins are relatively single under optical microscopy, two different phases of calcite veins can be clearly identified by cathodoluminescence, and the temperature is generally high (see Figure 7 , see Figures 10 - 11 ). Generally speaking, the development degree of fractures and fissures at different scales in Well JYB increases significantly, having the prerequisite for opening to the outside world. However, the carbon and oxygen isotopes of calcite veins and shale are similar (see Figure 12 ), indicating that the fractures did not communicate with the external environment after formation, and the diagenetic fluid is mainly provided by the inside of the shale system. In a relatively closed shale system, a large number of developed fractures are partially filled and cemented, and the unfilled fractures can serve as good reservoir spaces for shale gas, which is conducive to the formation of high-yield fractured shale gas reservoirs.

[0037] In the Jiaoshiba Zilichang block, Well JYC shows an AFE tone of grayish black to blue. Imaging logging, core observation and thin section observation all show that fractures are highly developed and the cutting phenomenon is obvious (see Figures 4 - 7 ). Rigid fracture seams formed under tectonic action can be seen in scanning electron microscopy (see Figure 8 ). The microfractures are highly connected in space, distributed in a network structure and have good connectivity (see Figure 9 ), indicating that Well JYC is significantly affected by tectonics and fractures and fissures at different scales are all very developed. In addition, cathodoluminescence and inclusion homogenization temperature show that although the formation temperature of the veins is high, the number of phases increases significantly and the types are also more complex (see Figures 10 - 11 ). It is comprehensively considered that Well JYC has the prerequisite for opening to the outside world. The results of carbon and oxygen isotope analysis show that there are obvious differences in the carbon and oxygen isotopes between the veins and the shale (see Figure 12 ), indicating that the developed fractures and fissures may have damaged the shale preservation conditions, resulting in the entry of external fluids into the shale formation, and the overall shale preservation conditions are poor.

[0038] To further verify the reliability and applicability of the present invention, the formation pressure and shale gas production per well of the above-mentioned evaluation wells were evaluated. The results show that the shale formation pressure coefficient of Well JYA is as high as 1.55, and the gas production is more than 200,000 cubic meters per day; Well JYB is a fractured shale gas reservoir, the shale formation pressure coefficient is as high as 1.59, and the gas production is about 320,000 cubic meters per day; the shale formation pressure coefficient of Well JYC is less than 1.0, which is normal pressure or negative pressure, and the gas production is less than 0.5 million cubic meters per day. In summary, the method for evaluating shale preservation conditions established by the present invention through comprehensive geological, geophysical and geochemical methods has reliability and indicativeness.

[0039] Thirdly, as an auxiliary evidence of the creativity of the present invention, it is also reflected in the following important aspects:

[0040] The shale gas preservation condition of the present invention is one of the main factors controlling the enrichment degree of shale gas. Therefore, quantitatively evaluating the shale preservation condition has important practical significance for the production and development of shale gas. However, the existing technologies mainly qualitatively judge the shale gas preservation condition through macroscopic fracture-fissure characteristics, regional caprock conditions and other macroscopic characteristics, which are difficult to meet the efficient development requirements of shale gas. Based on comprehensive geological, geophysical and geochemical methods, the present invention has established a method for quantitatively evaluating shale preservation conditions, which solves the limitations of the existing methods.

[0041] The present invention also overcomes the possible biases of the existing technical methods. For example, in Well JYB in the case, 3D seismic, imaging logging and core observation all show the development of a certain amount of fractures and fissures, and obvious high-conductivity fractures (i.e., open fractures) can also be seen in the direct caprock. The existing technical methods are likely to judge it as a damaged shale gas reservoir. However, the present invention combines geological, geophysical and geochemical methods and comprehensively determines that the fractures in Well JYB are not open to the outside world after formation, but are filled with internal fluids of the shale (the carbon and oxygen isotope differences between the fracture veins and the shale surrounding rock are small), and the shale system has good preservation conditions. The unfilled part of the developed fractures can further serve as the reservoir space for shale gas, forming a fractured shale gas reservoir (this is also one of the advantages of the present invention: it has a certain indicative significance for the type of shale gas reservoir). Therefore, Well JYB is an overall enriched and high-yield shale gas well. Description of the Drawings

[0042] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required to be used in the embodiments of the present invention will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0043] Figure 1It is a flowchart of a method for multi-scale, multi-stage and quantitative characterization of shale gas preservation conditions provided by an embodiment of the present invention.

[0044] Figure 2 It is a schematic principle diagram of a method for multi-scale, multi-stage and quantitative characterization of shale gas preservation conditions provided by an embodiment of the present invention.

[0045] Figure 3 It is a block diagram of the system structure for multi-scale, multi-stage and quantitative characterization of shale gas preservation conditions provided by an embodiment of the present invention;

[0046] In the figure: 1. Fault-fracture characterization module; 2. Fracture-vein characterization module; 3. Preservation condition evaluation module.

[0047] Figure 4 It is a schematic diagram showing the development characteristics of faults and fractures in 3D seismic provided by an embodiment of the present invention; the larger the AFE value (i.e., blue and black), the larger the scale and density of the developed faults and fractures in this area. Conversely, the smaller the AFE value (i.e., white and gray), the smaller the scale and density of the developed faults and fractures. The predicted fault-fracture density is relatively low at Well JYA (AFE tone is white), the development intensity of faults and fractures increases at Well JYB (AFE tone is off-white to grayish-black), and the fault-fracture development density is relatively the highest at Well JYC (AFE tone is grayish-black to blue).

[0048] Figure 5 It is a schematic diagram showing the development characteristics of drilling fractures in the study area characterized by imaging logging provided by an embodiment of the present invention.

[0049] Figure 6 It is the development characteristics of small and medium-scale fractures on the core provided by an embodiment of the present invention. Horizontal fractures mainly develop at Well JYA, and the distribution ranges of the fracture length and width are relatively small; the fracture development degree is medium at Well JYB; the fracture development degree is relatively high at Well JYC, and they are mainly high-angle fractures with obvious cutting between fractures.

[0050] Figure 7 It is the characteristics of small and medium-scale fractures observed by optical microscope provided by an embodiment of the present invention. The fracture veins at Well JYA and Well JYB are relatively single, while the fracture veins at Well JYC are more complex, and the veins in different directions cut each other.

[0051] Figure 8 It is the characteristics of microscopic fractures shown under field emission scanning electron microscope provided by an embodiment of the present invention. Granular edge microfractures mainly develop at Well JYA, which are non-structural fractures formed during the hydrocarbon generation and evolution process, can be used as shale gas reservoir spaces, and have no influence on shale gas preservation conditions; microfractures related to structures can be seen at Well JYB; microfractures ruptured due to tectonic action can be seen at Well JYC.

[0052] Figure 9It is the 3D spatial distribution characteristics of microfractures provided by the embodiments of the present invention. The degree of microfracture development in Well JYA is relatively low, and the connectivity between them is poor; the degree of microfracture development in Well JYC is relatively high, and the connectivity between them is good. Gray represents shale matrix components (such as quartz, carbonate minerals, clay minerals), yellow represents high-density components (such as pyrite), and red represents microfractures; Figures (a) and (d) are three-dimensional reconstruction diagrams of the whole sample; Figures (b) and (e) are spatial distribution diagrams of microfractures; Figures (d) and (f) are the spatial connectivity of microfractures.

[0053] Figure 10 It is the characteristics of shale veins shown by cathodoluminescence provided by the embodiments of the present invention. In Well JYA, mainly one stage of calcite veins in the horizontal direction is developed; in Well JYB, two stages of veins in the nearly horizontal and vertical directions are developed; in Well JYC, at least three stages of veins are developed: low-angle, high-angle, and nearly vertical high-angle calcite veins, and high-angle and nearly vertical quartz veins. Figure 10 The photos in Figure 7 correspond to the photos in

[0054] Figure 11 It is a schematic diagram of the homogenization temperature of fluid inclusions provided by the embodiments of the present invention; the number of vein stages in Well JYA and Well JYB is less, and the number of vein stages in Well JYC is more.

[0055] Figure 12 It is a schematic diagram of the difference in carbon and oxygen isotopes between calcite veins and shale provided by the embodiments of the present invention; the carbon and oxygen isotopes of calcite veins and shale in Well JYA and Well JYB are basically the same, indicating that the fluid mainly comes from within the shale formation and the preservation conditions of the shale are good; the difference in carbon and oxygen isotopes between calcite veins and shale in Well JYC is relatively large, indicating that the fluid mainly comes from outside the shale formation and the preservation conditions of the shale are poor. Detailed implementation manners

[0056] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, the following further elaborates on the present invention in combination with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0057] Aiming at the problems existing in the prior art, the present invention provides a method and system for multi-scale - multi-stage - quantitative characterization of shale gas preservation conditions, which will be described in detail below with reference to the accompanying drawings.

[0058] I. Explanation of the embodiments. This part is an explanatory embodiment that expands the technical solution to enable those skilled in the art to fully understand how the present invention is specifically implemented.

[0059] As Figure 1As shown in the figure, the method for multi-scale, multi-stage and quantitative characterization of shale gas preservation conditions provided by the embodiments of the present invention includes the following steps:

[0060] S101, multi-scale fracture-fissure characterization by geophysical methods;

[0061] S102, multi-stage fracture-vein characterization by geological methods;

[0062] S103, quantitative evaluation of preservation conditions by geochemical methods.

[0063] The flow chart of evaluating shale gas preservation conditions by integrating geological, geophysical and geochemical means provided by the embodiments of the present invention is as Figure 2 shown.

[0064] As Figure 3 shown, the system for multi-scale, multi-stage and quantitative characterization of shale gas preservation conditions provided by the embodiments of the present invention includes:

[0065] Fracture-fissure characterization module 1, used for multi-scale fracture-fissure characterization by geophysical methods;

[0066] Fracture-vein characterization module 2, used for multi-stage fracture-vein characterization by geological methods;

[0067] Preservation condition evaluation module 3, used for quantitatively evaluating preservation conditions by geochemical methods.

[0068] The technical solution of the present invention will be further described below in conjunction with specific embodiments.

[0069] Embodiment

[0070] 1. Based on 3D seismic, imaging logging, core description, thin section observation, scanning electron microscopy and micro-CT scanning, the present invention clarifies the development and distribution characteristics of fractures and fissures at different scales, and combines vein filling stages, inclusion temperature and salinity, and carbon and oxygen isotopes, aiming to establish a method for quantitatively evaluating shale gas preservation conditions and solve the problem of uncertainty of existing evaluation indicators.

[0071] 2. The technical solution of the present invention is detailed in Figure 2 . Among them, characterizing the development and distribution characteristics of multi-scale fractures and fissures in shale and the determination of carbon and oxygen isotopes are the keys of the present invention.

[0072] 3. The specific content includes:

[0073] 3.1 Multi-scale fracture-fissure characterization by geophysical methods: The present invention uses the Automatic Fault Extraction (AFE) technology and imaging logging to characterize the development and distribution characteristics of fractures-fissures at the macroscopic scale. The larger the AFE value (i.e., blue and black), the larger the scale and density of fractures-fissures developed in the area. Conversely, the smaller the AFE value (i.e., white and gray), the smaller the scale and density of fractures-fissures development (see Figure 4 ).

[0074] The imaging logging technology reflects the fracture development characteristics through the form of a gradient color plate scale image. Generally speaking, according to the differences in fracture response characteristics, high-conductivity fractures, high-resistivity fractures, and faults can be mainly distinguished. High-conductivity fractures belong to open fractures. During the drilling process, due to the invasion of low-resistivity mud drilling fluid, the resistivity of such fractures is significantly lower than that of the surrounding rock. Therefore, on the micro-resistivity imaging logging map, it shows a continuous or semi-closed dark sine wave curve. High-resistivity fractures generally refer to fractures completely or semi-filled with high-resistivity minerals such as quartz and calcite, and their image characteristics are manifested as bright sine curves. Faults are similar to high-conductivity fractures, and their image characteristics are manifested as dark sine curves. The main difference is that due to the relative movement of the hanging wall and footwall of the fault, there is usually a dislocation phenomenon of the strata near the fault plane; only the strata on both sides of the high-conductivity fracture are fractured under external forces, and the strata are relatively continuous (see Figure 5 ). Core observation can judge the fracture development density, angle, aperture, and filling conditions, etc. ( Figure 6 ). Thin section observation is to directly observe the characteristics of small-scale fractures through the single polarized light of an optical microscope. Under single polarized light, the surrounding rock (i.e., the rock around the fracture vein body) appears dark black, while the filled fracture vein body appears grayish-white to bright white ( Figure 7 ). Scanning electron microscopy analysis is to mechanically grind and argon ion polish the sample surface. After obtaining a flat sample surface, it is placed under the electron microscope for micro-fracture observation ( Figure 8 ). Based on density differences, micro-CT imaging uses gray levels to distinguish different components of shale samples. Among them, high-density components appear bright white (such as pyrite), medium-density components appear light gray (such as quartz, carbonate minerals, clay minerals), and micro-fractures appear black. Representing the shale matrix component in gray, the high-density component in yellow, and the micro-fracture in red, on the basis of the original micro-CT slice image, further rendering can show the main micro-fractures and the final interpretation results of each component (see Figure 9 ).

[0075] 3.2 Multi-stage fracture-vein body characterization by geological methods: The fracture sequence and vein body formation temperature can be effectively discriminated through cathodoluminescence and fluid inclusions (see Figures 10 - 11 ). Combining the multi-scale fracture-fissure characteristics characterized by geophysical methods, the open conditions of the shale system are preliminarily clarified.

[0076] Cathodoluminescence is an effective method for identifying the types and formation generations of diagenetic minerals in small- and medium-scale fracture veins. The principle is that when the sample is bombarded by electrons, various luminescence phenomena are caused by the presence of electroluminescence activator impurity elements in the minerals. By observing the cathodoluminescence of fracture veins, the types of diagenetic minerals, structural characteristics, and cutting relationships of shale veins can be comprehensively analyzed, and the relative sequence of the formation of minerals such as calcite and quartz in veins with different occurrences can be determined. Calcite appears orange-red or orange-yellow under cathodoluminescence, while quartz does not luminesce (see Figure 10 ).

[0077] Fluid inclusions are part of the diagenetic and ore-forming fluids that were trapped in the lattice defects or cavities of minerals during the crystallization and growth of minerals, and are still sealed in the host minerals and have phase boundaries with the host minerals. They preserve various geochemical information of the geological environment at that time (such as pressure, temperature, salinity, etc.). By measuring the homogenization temperature of fluid inclusions inside the veins, it can approximately represent the formation temperature of the fractures. Combining with the stratigraphic burial history map, the formation time of the fractures can be determined (see Figure 11 ).

[0078] Combining multiple information such as multi-scale fracture-fracture density, distribution characteristics, fracture formation sequence, temperature, and time, the intensity of tectonic activities experienced by the shale formation system in the geological history period, the fracture cementation and sealing time, etc. can be preliminarily determined, and the preservation conditions of the shale system can be qualitatively speculated.

[0079] 3.3 Geochemical methods for quantitatively evaluating preservation conditions: Carbonate rocks of different ages have different carbon and oxygen isotope compositions, which can be used to study the sources of diagenetic fluids and the temperatures of diagenetic environments. According to the differences in carbon and oxygen isotopes between the calcite veins filled in the fractures and the shale carbonate rocks (see Figure 12 ), it can effectively distinguish whether the vein-forming fluid mainly comes from outside or inside the shale system, so as to accurately evaluate the sealing property of the shale system.

[0080] Combining the qualitative discrimination results of shale preservation conditions and the quantitative identification results of carbon and oxygen isotopes, the connectivity relationship between the shale system and the outside world is discussed, and the preservation conditions of shale during the series of processes of sedimentation, diagenesis, uplift and denudation are clarified.

[0081] 4. The present invention proposes a flow chart of a method for quantitatively evaluating preservation conditions by geochemical methods and establishing an evaluation method for shale gas preservation conditions by comprehensively using geological, geophysical and geochemical methods (see Figure 2 ).

[0082] 5. The present invention combines various technical means such as geology, geophysics, and geochemistry to establish a method for accurately evaluating the preservation conditions of shale gas, which is of great significance for evaluating the potential of shale resources and the commercial value of gas reservoirs. In addition, the present invention uses formation pressure and actual production data of shale gas wells to test the predicted shale preservation conditions, making the present invention more reliable through discrimination and comparison.

[0083] The present invention has been successfully applied to the Wufeng Formation - Longmaxi Formation shale in the Jiaoshiba area of a certain basin. Well JYA in the first - stage production and construction area of Jiaoshiba has a white AFE tone on the seismic scale (see Figure 4 ), and fractures are basically not developed in the direct caprock (see Figure 5 ), indicating that the overall development degree of macro - fractures - fissures is relatively low. The length and width distribution ranges of medium - and small - scale fractures are small, mainly horizontal fractures (see Figures 6 - 7 ). Micro - fractures are non - tectonic fractures formed during the hydrocarbon generation and evolution process, which can serve as the space for shale gas storage and have no impact on the shale gas preservation conditions (see Figure 8 ). Micro - CT shows that the connectivity of micro - fractures is poor, mainly isolated dots (see Figure 9 ). The development degree of fractures - fissures of different scales in Well JYA is relatively low. The results of cathodoluminescence and inclusion homogenization temperature show that the formation temperature of calcite veins is relatively high (see Figures 10 - 11 ). Combining the characteristics of fractures - fissures of different scales, it is considered that the shale system of Well JYA does not have the premise of opening to the outside. In addition, since the carbon and oxygen isotopes of calcite veins and shale are basically the same (see Figure 12 ), it is further proved that the fluid may mainly come from the internal of the shale system, and it is comprehensively determined that the shale preservation conditions of Well JYA are good.

[0084] In Well JYB in a certain block, the AFE tone on the seismic scale is gray - white to gray - black (see Figure 4 ), indicating that the development degree of macro - fractures - fissures increases. There are certain amounts of fracture shows in imaging logging and core observation (see Figures 5 - 6 ), and micro - fractures related to structures can also be seen on the scanning electron microscope (see Figure 8 ). Although the veins are relatively single under the optical microscope, two different stages of calcite veins can be clearly identified by cathodoluminescence, and the temperature is generally high (see Figure 7 , see Figures 10 - 11 ). Generally speaking, the development degree of fractures - fissures of different scales in Well JYB increases significantly, having the premise of opening to the outside. However, the carbon and oxygen isotopes of calcite veins and shale are similar (see Figure 12 ), indicating that the fractures have not communicated with the external environment after formation, and the diagenetic fluid is mainly provided by the internal of the shale system. In a relatively closed shale system, a large number of developed fractures are partially filled and cemented, and the unfilled fractures can serve as good storage spaces for shale gas, which is conducive to the formation of high - yield fractured shale gas reservoirs.

[0085] In a certain block, well JYC has an AFE color tone ranging from grayish-black to blue. Imaging logging, core observation, and thin-section observation all show that fractures are highly developed and the cutting phenomenon is obvious (see Figures 4 - 7 ). Rigid fracture seams formed under tectonic action can be seen under a scanning electron microscope (see Figure 8 ). The microfractures are highly connected in space, distributed in a network structure, and have good connectivity (see Figure 9 ), indicating that well JYC is significantly affected by tectonics, and fractures of different scales are all very developed. In addition, cathodoluminescence and inclusion homogenization temperature show that although the formation temperature of the vein bodies is relatively high, the stages are significantly increased and the types are also relatively complex (see Figures 10 - 11 ). It is comprehensively considered that well JYC has the premise for opening to the outside. The results of carbon and oxygen isotope analysis show that there are obvious differences in carbon and oxygen isotopes between the vein bodies and the shale (see Figure 12 ), indicating that the developed fractures may have damaged the shale preservation conditions, resulting in the entry of external fluids into the shale formation, and the overall shale preservation conditions are poor.

[0086] In order to further verify the reliability and applicability of the present invention, the formation pressure and single-well shale gas production of the above evaluation wells were evaluated. The results show that the shale formation pressure coefficient of well JYA is as high as 1.55, and the gas production is more than 200,000 cubic meters per day; well JYB is a fractured shale gas reservoir, with a shale formation pressure coefficient as high as 1.59 and a gas production of about 320,000 cubic meters per day; the shale formation pressure coefficient of well JYC is less than 1.0, being normal pressure or negative pressure, and the gas production is less than 0.5 cubic meters per day. In summary, the method for evaluating shale preservation conditions established by the present invention by integrating geological, geophysical, and geochemical methods has reliability and indicativeness. As Figure 11 is the schematic diagram of the inclusion homogenization temperature provided by the embodiment of the present invention; the number of vein stages in well JYA and well JYB is less, while the number of vein stages in well JYC is more.

[0087] Figure 12 is the schematic diagram of the difference in carbon and oxygen isotopes between calcite vein bodies and shale provided by the embodiment of the present invention; the carbon and oxygen isotopes of the calcite vein bodies and the shale in well JYA and well JYB are basically the same, indicating that the fluid mainly comes from within the shale formation and the shale preservation conditions are good; the difference in carbon and oxygen isotopes between the calcite vein bodies and the shale in well JYC is relatively large, indicating that the fluid mainly comes from outside the shale formation and the shale preservation conditions are poor.

[0088] II. Evidence of related effects of the embodiment. Some positive effects have been achieved during the research and development or use of the embodiment of the present invention, and it indeed has great advantages compared with the prior art. The following content is described in combination with experimental results, etc.

[0089] (1) In the prior art, the preservation conditions of shale gas are mainly qualitatively judged by macroscopic features such as macroscopic fracture - crack characteristics and regional caprock conditions, which are difficult to meet the requirements of efficient development of shale gas. Based on the comprehensive geological, geophysical and geochemical methods, the present invention establishes a method for quantitatively evaluating the preservation conditions of shale, solving the limitations of the existing methods.

[0090] (2) The present invention also overcomes the possible biases of the prior art methods. For example, in the case of Well JYB, 3D seismic, imaging logging and core observation all show the development of a certain amount of fractures - cracks, and obvious high - conductivity fractures (i.e., open fractures) can also be seen in the direct caprock. The existing technical methods are likely to judge it as a damaged shale gas reservoir. However, the present invention combines geological, geophysical and geochemical methods and comprehensively determines that after the formation of the fractures in Well JYB, they are not open to the outside world but are filled with internal fluids of the shale (the carbon - oxygen isotope difference between the fracture veins and the shale surrounding rock is small), and the preservation conditions of the shale system are good. The unfilled part of the developed fractures can further serve as the reservoir space for shale gas, forming a fracture - type shale gas reservoir (this is also one of the advantages of the present invention: it has a certain guiding significance for the types of shale gas reservoirs). Therefore, Well JYB is an overall rich and high - yield shale gas well.

[0091] It should be noted that the embodiments of the present invention can be implemented through hardware, software, or a combination of software and hardware. The hardware part can be implemented using dedicated logic; the software part can be stored in a memory and executed by an appropriate instruction execution system, such as a microprocessor or dedicated designed hardware. Those of ordinary skill in the art can understand that the above - mentioned devices and methods can be implemented using computer - executable instructions and / or included in processor control code, for example, such code is provided on a carrier medium such as a disk, CD or DVD - ROM, a programmable memory such as a read - only memory (firmware), or a data carrier such as an optical or electronic signal carrier. The devices and their modules of the present invention can be implemented by hardware circuits of programmable hardware devices such as very - large - scale integrated circuits or gate arrays, semiconductors such as logic chips, transistors, etc., or programmable logic devices such as field - programmable gate arrays, or can be implemented by software executed by various types of processors, or can be implemented by a combination of the above - mentioned hardware circuits and software, such as firmware.

[0092] The above - mentioned are only the specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, any modification, equivalent replacement and improvement made within the spirit and principle of the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for multi-scale, multi-stage and quantitative characterization of shale gas preservation conditions, characterized in that The method for multi-scale, multi-stage and quantitative characterization of shale gas preservation conditions includes: determining the development and distribution characteristics of fractures and fissures at different scales based on 3D seismic, imaging logging, core description, thin section observation, scanning electron microscopy and micro-CT scanning, and combining the filling stages of veins, the temperature and salinity of inclusions, and carbon and oxygen isotopes to quantitatively analyze the shale gas preservation conditions; The method for multi-scale, multi-stage and quantitative characterization of shale gas preservation conditions includes the following steps: Step 1, multi-scale fracture and fissure characterization by geophysical methods; Step 2, multi-stage fracture and vein characterization by geological methods; Step 3, quantitative evaluation of preservation conditions by geochemical methods; The multi-scale fracture and fissure characterization by geophysical methods in Step 1 includes: Using the AFE method and imaging logging to characterize the development and distribution characteristics of fractures and fissures at the macroscopic scale; the larger the AFE value, the larger the scale and density of fractures and fissures developed in the area; the smaller the AFE value, the smaller the scale and density of fracture and fissure development; Imaging logging reflects the fracture development characteristics in the form of a gradient color plate scale image; high-conductivity fractures, high-resistivity fractures and faults are distinguished according to the differences in fracture response characteristics; The multi-stage fracture and vein characterization by geological methods in Step 2 includes: combining the macroscopic scale fractures characterized by geophysical methods and the microscopic scale fissure characteristics to determine the development and distribution of multi-scale fractures and fissures in the shale reservoir; Combining the density, distribution characteristics, formation sequence, temperature and time information of multi-scale fractures and fissures, preliminarily determining the intensity of tectonic activities experienced by the shale formation system in the geological history period and the fracture cementation and sealing time, and determining the preservation conditions of the shale system; The quantitative evaluation of preservation conditions by geochemical methods in Step 3 includes: carbonate rocks of different ages have different carbon and oxygen isotope compositions, which are used to analyze the source of diagenetic fluids and the temperature of the diagenetic environment; according to the differences in carbon and oxygen isotopes between the calcite veins filled in the fractures and the shale carbonate rocks, it is judged whether the vein-forming fluid comes from outside or inside the shale system, so as to evaluate the sealing of the shale system; Combining the qualitative discrimination results of shale preservation conditions and the quantitative identification results of carbon and oxygen isotopes, analyzing the connection relationship between the shale system and the outside world, and clarifying the preservation conditions of the shale during the series of processes of sedimentation, diagenesis, uplift and denudation.

2. The method for multi-scale, multi-stage and quantitative characterization of shale gas preservation conditions according to claim 1, wherein Based on density differences, micro-CT imaging uses gray levels to distinguish different components of shale samples.

3. A multi-scale, multi-stage, quantitative characterization system for shale gas preservation conditions, which applies the method for multi-scale, multi-stage, quantitative characterization of shale gas preservation conditions according to any one of claims 1 to 2, is characterized in that The system for multi-scale, multi-stage and quantitative characterization of shale gas preservation conditions includes: A fracture and fissure characterization module for multi-scale fracture and fissure characterization using geophysical methods; A fracture and vein characterization module for multi-stage fracture and vein characterization using geological methods; A preservation condition evaluation module for quantitatively evaluating preservation conditions using geochemical methods.

4. A computer device, characterized in that, The computer device includes a memory and a processor. When the computer program stored in the memory is executed by the processor, the processor executes the method for multi-scale, multi-stage and quantitative characterization of shale gas preservation conditions according to any one of claims 1 to 2, including the following steps: Based on 3D seismic data, imaging logging, core description, thin section observation, scanning electron microscopy and micro-CT scanning, the development and distribution characteristics of fractures and fissures at different scales are clarified. Combining with the vein filling stages, the temperature and salinity of inclusions, and carbon and oxygen isotopes, the quantitative analysis of shale gas preservation conditions is realized.

5. A computer-readable storage medium stores a computer program. When the computer program is executed by a processor, the processor is caused to execute the method for multi-scale-multi-stage-quantitative characterization of shale gas preservation conditions according to any one of claims 1 to 2, including the following steps: Based on 3D seismic data, imaging logging, core description, thin section observation, scanning electron microscopy and micro-CT scanning, the development and distribution characteristics of fractures and fissures at different scales are clarified. Combining with the vein filling stages, the temperature and salinity of inclusions, and carbon and oxygen isotopes, the quantitative evaluation of shale gas preservation conditions is realized.

6. An information data processing terminal, characterized in that, The information data processing terminal is used to implement the system for multi-scale-multi-stage-quantitative characterization of shale gas preservation conditions according to claim 3.

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