Method for identifying fracture opening and healing coupling relation in shale hydrocarbon generation and expulsion process

Abstract

The invention belongs to the field of petroleum geology and fluid geochemistry, and particularly discloses a method for judging a fracture opening and healing coupling relation in the shale hydrocarbon generation and expulsion process. The method comprises the following steps: preparing shale rock slices which contain natural fractures and are in a hydrocarbon generation stage, identifying healing mineral crystals with different structures in the fractures, carrying out lithofacies observation on fluid inclusions in fibrous, blocky and tensile structure crystals, selecting different fluid inclusion combinations, and carrying out laser Raman, microscopic temperature measurement and PVT simulation. A hydrocarbon-containing fluid temperature and pressure field result is combined with hydrocarbon source rock hydrocarbon generation evolution history, crack opening mechanisms corresponding to the three types of structures are defined, finally, the dynamic opening and healing coupling relation of cracks in the hydrocarbon generation and expulsion process is effectively limited, assistance is provided for related research of the crack forming period in the hydrocarbon generation and expulsion process of shale in an oil-gas-containing basin, and the development of the oil-gas-containing basin is promoted. And a new thought and method are provided for limiting the formation period of the crack.

Description

Technical Field

[0001] The present invention belongs to the fields of petroleum geology and fluid geochemistry, and in particular relates to a method for identifying the coupling relationship between crack opening and healing in the process of shale hydrocarbon generation and expulsion. Background Art

[0002] Currently, in oil and gas basins, the hydrocarbon generation and expulsion process of shale is often accompanied by the opening and healing of natural fractures, a process that directly affects the migration, retention efficiency, and ultimate enrichment of hydrocarbon fluids. Traditionally, research on the evolution of shale fractures and the history of fluid migration has relied primarily on methods such as static core observation and microscopic analysis, and geochemical isotope dating. While these methods can reveal the process of fracture opening and healing at various scales, they suffer from the following limitations: First, static core observation and microscopic analysis often only provide a snapshot of a specific moment in time, failing to capture the full evolution of fracture behavior with temperature and pressure. Second, isotope dating is expensive, cumbersome, and subject to significant uncertainty, making it difficult to assess the high-pressure, high-temperature, and multi-field coupling conditions at actual burial depths.

[0003] Based on this, this study proposes a method that combines healing crystal petrography with inclusion analysis. Based on the crack opening mechanism recorded by three types of healing filling structures, namely fibrous, tensile and blocky structure crystals, the dynamic opening and healing coupling behavior of cracks during hydrocarbon generation and expulsion are systematically identified; through microscopy and Raman spectroscopy, mineral crystal petrography identification, inclusion petrography identification, microthermometry and other analyses are carried out to restore the temperature and pressure conditions corresponding to the dynamic opening of cracks in crystals with different structures. From both theoretical and technical perspectives, this method is feasible and has the advantages of low cost, simplicity and high efficiency. Summary of the Invention

[0004] The present invention provides a method for determining the coupling relationship between the dynamic opening and healing of fractures during the hydrocarbon generation and expulsion stages by combining the petrographic characteristic analysis of mineral crystals that heal inside natural fractures during the shale hydrocarbon (oil and gas) generation and expulsion process with the analysis of hydrocarbon-bearing primary fluid inclusions captured during the growth process of crystals of different morphologies. This method can then simply and efficiently achieve the limitation of the opening and closing process of natural fractures during the shale hydrocarbon generation and expulsion process, as well as the restoration of the temperature and pressure field of the internal hydrocarbon-bearing fluid when the fractures are dynamically opened.

[0005] To achieve the above object, the present invention adopts the following technical solutions: Collect shale rock samples containing natural fractures at the hydrocarbon generation stage (i.e., organic matter mature-overmature stage) and prepare double-sided polished rock thin sections with a thickness of approximately 70-100 microns; Through microscopic observation combined with cathodoluminescence imaging technology, petrographic characteristics of shale fracture filling minerals in thin sections were analyzed to identify three types of healing mineral crystal structures: First, fibrous structure crystals: The crystals in the fibrous structure have the same morphology, a large ratio of the length to the short axis, and are arranged in a comb-like pattern with parallel distribution. The interfaces between adjacent minerals are smooth, and there is no growth competition between them. There is a clear middle surface inside, and it has an antigenetic growth pattern. The second is tensile structure crystals: the interior of the tensile structure crystals shows multiple solid rock relic strips parallel to the crack walls, and the interfaces between adjacent minerals are jagged, reflecting the multi-stage repetitive small-scale crack opening and crystal healing process; The third is massive structure minerals: the filling mineral crystals are equiaxed blocks or elongated blocks. The former have uniform crystal size and random orientation, while the latter show more significant growth competition between crystals, both reflecting that crystal growth occurs in an open fluid space and has a tropic growth pattern.

[0006] Polarized light / fluorescence microscopy combined with laser Raman spectroscopy was used to conduct petrographic observations on hydrocarbon-bearing primary fluid inclusions captured during the precipitation of the above three types of crystal structure minerals. The petrographic characteristics mainly include the types of inclusions developed within the inclusion assemblage (oil inclusions, gas inclusions and / or brine inclusions), the phase composition of each type of inclusions, the gas-liquid ratio, the fluorescence color of the oil inclusions, the gas phase composition of the gas inclusions, etc.

[0007] Microthermometry analysis was performed on the above-mentioned inclusion combination to obtain the uniform temperature and freezing point of the brine inclusions, the uniform temperature of the oil inclusions, and the uniform temperature of the gas inclusions. For the combination of oil inclusions and brine inclusions, the capture temperature and pressure were calculated by the intersection method of the isochoric lines of the two types of inclusions; for the combination of gas inclusions and brine inclusions, the pure gas inclusion isochoric line epitaxy method was used to determine the final capture temperature and pressure.

[0008] The growth of fibrous structure crystals is restricted by the limited crack space, and the crack opening is driven by the fluid overpressure and crystal growth force, indicating that the crystal growth and the dynamic crack opening are synchronous; the tensile structure crystals record multiple cycles of microscale crack opening and mineral crystallization filling process. Since the displacement of each crack opening is extremely small, the filling crystals are able to continuously stretch across the two walls, which also maintains the synchronization of the dynamic crack opening and the crystal healing process.

[0009] Since the dynamic opening and healing of cracks in fibrous and tensile structure crystals occur simultaneously, the captured temperature and pressure calculated from the above primary fluid inclusions can represent the temperature and pressure field of the hydrocarbon-bearing fluid when the cracks are dynamically opened during the shale generation and expulsion process. Combined with the hydrocarbon generation evolution history, the duration of the dynamic opening of the cracks can be further limited.

[0010] The crystallization growth of minerals represented by blocky crystals occurs in an open fluid space. When the fluid pressure during crystallization, calculated above, is greater than the compressive stress perpendicular to the fracture surface, the high-pressure fluid can actively drive the fracture to continue opening, allowing the crystals to grow unimpeded in the open space, synchronized with the dynamic opening of the fracture. Conversely, when the fracture stops expanding, the crystals settle only in the existing space. The calculated capture temperature and pressure cannot represent the temperature and pressure during the dynamic opening of the fracture, indicating that there is no coupling between the dynamic opening and healing of the fracture during shale hydrocarbon generation and expulsion.

[0011] Through the scheme provided by the present invention, microscopic thin sections of naturally fractured shale rock samples are obtained and produced; microscopic observation is used to clarify the types of filling crystal structures in the fractures, including fibrous structure, tensile structure, and massive structure; petrographic characteristics of hydrocarbon-containing fluid inclusions captured by filling minerals with different structures are analyzed by combining Raman spectroscopy and polarization / fluorescence identification, and the composition of the hydrocarbon-containing fluid inclusions in the combination is clarified: brine inclusions, oil inclusions, and gas inclusions; through microscopic temperature measurement analysis, the uniform temperature and freezing point of the brine inclusions, as well as the uniform temperature of the co-existing oil inclusions and / or gas inclusions, are obtained, and finally the capture temperature and pressure are determined by the isochoric line intersection method or the pure gas inclusion isochoric line epitaxy method; based on the analysis of mineral crystal structure and fluid temperature and pressure field, combined with the hydrocarbon generation evolution history, the coupling relationship between the dynamic opening and healing process of the fractures is clarified, and the dynamic opening period of the fractures in the hydrocarbon generation and expulsion stage of the source rock is effectively limited.

[0012] The present invention has the following beneficial effects: This method, based on petrographic analysis of healing minerals within cracks in shale rock thin sections, combined with analysis of primary fluid inclusions in various mineral forms, can define the coupling relationship between dynamic crack opening and healing during shale hydrocarbon generation and expulsion. Furthermore, combined with the hydrocarbon generation history, the method uses the original hydrocarbon-bearing fluids recorded in the crystallization of minerals that grew synchronously with the dynamic crack opening to define the period of dynamic crack opening during the hydrocarbon generation and expulsion process. This method is simple and efficient. The present invention can provide assistance for research related to the formation period of cracks in the process of hydrocarbon generation and expulsion of shale in oil and gas basins, and provides a new idea and method for limiting the formation period of cracks. In addition, the two most important fluid condition parameters in the process of hydrocarbon generation and expulsion of source rocks, namely temperature and pressure, can also be directly recovered through fluid inclusion analysis technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 A schematic flow chart of a method for identifying the coupling relationship between fracture opening and healing during shale hydrocarbon generation and expulsion provided by the present invention; Figure 2 Schematic diagram of fibrous crystal structure and its examples; Figure 3Schematic diagram of stretched crystal structure and its examples; Figure 4 Schematic diagram of bulk crystal structure and its example. DETAILED DESCRIPTION

[0014] To make the purpose, technical solutions, and advantages of this application more clear, the technical solutions of this application will be clearly and completely described below in conjunction with the specific embodiments of this application and the corresponding drawings. Obviously, the embodiments described are only part of the embodiments of this application, not all of them. Based on the embodiments in this specification, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0015] The following describes the solution of this application in conjunction with the accompanying drawings. Figure 1 As shown, Figure 1 The present invention provides a flow chart of a method for identifying the coupling relationship between fracture opening and healing during shale hydrocarbon expulsion, including: S1. Preparation of thin sections of shale rock containing natural fractures in the hydrocarbon generation stage. Shale rock samples containing natural fractures in the hydrocarbon generation stage, i.e., the organic matter mature to overmature stage, are collected and double-sided polished thin sections with a thickness of 70-100 microns are prepared.

[0016] S2. Microscopic identification of the crystal structure type of fracture healing minerals (fibrous structure / stretched crystal structure / massive structure). Microscopic observation combined with cathodoluminescence observation of rock thin sections is used to identify the crystal structure of the filling based on the petrographic characteristics of the healing mineral crystals in the fractures.

[0017] If the crystals of the minerals filling the cracks have similar morphologies, large aspect ratios, the long axis of the crystals is perpendicular to the crack wall, the mineral boundaries between adjacent crystals are smooth, there is no obvious competitive growth, the overall arrangement is regular, comb-like, and there is often an obvious middle plane in the interior that is consistent with the direction of the crack wall. The cathode luminescence color of the same mineral on one side of the middle plane is almost the same, then it can be judged as a fibrous structure crystal.

[0018] If there are multiple bands of surrounding rock relics parallel to the crack wall inside the crystal filling the crack, and the crystal boundary is jagged, it means that the crystal has experienced repeated multiple stages of small-scale crack opening and healing, and the location of each new crack opening is either in the adjacent surrounding rock or in the already formed crystal. In this case, it is judged to be a tensile structure crystal.

[0019] If the crystals filling the cracks are equiaxed, with euhedral and semi-euhedral morphologies, roughly uniform particle size, and random orientation, they are identified as blocky crystals. If the filling crystals are non-equiaxed, with long columnar morphologies (i.e., with the major axis significantly larger than the minor axis), and if adjacent crystals exhibit significant competitive growth, they are identified as extended blocky crystals. Cathodoluminescence analysis reveals that blocky and extended blocky structures typically exhibit a synapomorphic growth pattern, with crystals growing from both sides of the crack toward its center.

[0020] S3. Analysis of primary fluid inclusions and hydrocarbon-bearing fluid composition in crystals of various structures. Using polarizing and fluorescence microscopy combined with laser Raman spectroscopy, a systematic petrographic and chemical composition analysis was conducted on the hydrocarbon-bearing fluid inclusion assemblages captured during the growth of fibrous, stretched, and massive crystals. First, the morphology, occurrence, size, and phase distribution of the inclusions within the assemblage were observed and recorded under transmitted light. Then, the fluorescence mode was switched to observe the fluorescence color and luminescence intensity of the inclusions to identify the presence of liquid hydrocarbon inclusions. Finally, laser Raman spectra of each phase of the inclusions were collected within the same field of view. The Raman spectral characteristics were used to identify the internal components of the inclusions and determine the presence of gaseous hydrocarbon inclusions. By combining the fluorescence response of the inclusions with the qualitative Raman results, the various inclusion types within the assemblage were determined, namely, whether brine inclusions and their associated oil or gas inclusions were present.

[0021] S4. Determine the temperature and pressure field of the hydrocarbon-containing fluid during crystal healing for each structure. Microthermometry analysis is performed on the inclusions in each structure, measuring the homogenization temperature and freezing point of the brine inclusions, as well as the homogenization temperature of the coexisting gas inclusions or the homogenization temperature of the oil inclusions. The hydrocarbon-containing fluid conditions during crystal healing are determined based on the different inclusion combinations. For combinations of oil and brine inclusions, the temperature and pressure (i.e., the capture temperature and capture pressure) of the hydrocarbon-containing fluid during crystal healing are determined by intersecting the isochoric lines of the oil and brine inclusions in the PT phase diagram. For combinations of gas and brine inclusions, the isochoric line extension method of pure gas (single-phase methane) inclusions is employed. This isochoric line is first established based on the laser Raman shift of the gas inclusions. This isochoric line is then extended along the PT space in the direction of increasing temperature and pressure until the homogenization temperature of the brine inclusions is reached, and the capture pressure is then determined.

[0022] S5. Analysis of the coupling between the healing process of fibrous and tensile crystal structures and the dynamic opening of cracks. Fibrous crystal structures exhibit an anterior growth pattern, where corresponding growth interfaces develop at the interface between the two crystals and the surrounding rock. Because the crystal growth space is limited, their growth is not subject to the constraints of normal mineral growth theory. In other words, crack opening is controlled by crystal growth, driven by both fluid overpressure and crystal growth forces. Therefore, the growth of fibrous crystals is synchronized with the dynamic opening of cracks. Tensile crystals undergo multiple stages of crack opening and healing. Because the scale of each crack opening is relatively small, the growing crystals can span the crack walls. Therefore, the growth of tensile crystals is also synchronized with the dynamic opening of cracks.

[0023] The growth of fibrous and tensile crystals is spatiotemporally synchronized with the dynamic opening of fractures. The temperature-pressure conditions captured by the fluid inclusion reconstructions above can be used to determine the temperature and pressure field of the hydrocarbon-bearing fluid during the dynamic opening of fractures during the hydrocarbon generation and expulsion phase of the shale. By combining these temperature-pressure reconstructions with the regional hydrocarbon generation history, the timing of dynamic fracture opening can be effectively constrained.

[0024] S6. Determination of the representativeness of the healing process of massive crystals for the dynamic opening of fractures. For massive crystals, if the fluid pressure at the time of precipitation, as reconstructed by fluid inclusions, exceeds the normal compressive stress of the fracture (for example, in horizontal fractures, where the fluid pressure is greater than the load of the overlying rock), the overpressure fluid can actively drive the fracture to continue expanding, thereby maintaining an open fluid space and allowing crystals to grow synchronously under unobstructed conditions to form massive crystals. At this time, the fracture healing and the dynamic opening process of the fracture are synchronized. In this case, based on the capture temperature and capture pressure obtained from fluid inclusions, combined with hydrocarbon generation history analysis, the temperature and pressure of the hydrocarbon-bearing fluid at the time of dynamic fracture opening can be obtained, thereby effectively limiting its dynamic opening time. In contrast, massive crystal precipitation occurs after the dynamic opening of the fracture has stopped. At this time, the capture temperature and pressure recorded by the inclusions cannot represent the temperature and pressure at the time of dynamic fracture opening, that is, the fracture opening and healing are not coupled.

[0025] The technical solution of the present invention is further described below with reference to specific embodiments: Example 1 Figure 2The fibrous crystal structure shown here comes from the mature organic-rich shale of the Paleogene Shahejie Formation in the Dongying Depression of the Bohai Bay Basin. The natural fractures developed within the shale are filled with fibrous calcite crystals. The fluid inclusion assemblage in the fibrous crystals consists of gas-liquid two-phase oil inclusions (yellow-green fluorescence) and their associated brine inclusions (non-fluorescent). The homogenization temperatures of the gas-liquid two-phase oil inclusions range from 71°C to 96°C, and those of the associated gas-liquid two-phase brine inclusions range from 120°C to 140°C, with freezing points ranging from -18°C to -22°C. The capture temperature range, determined by the biisochoric intersection method, is 128°C to 152°C, and the capture pressure range is 23 MPa to 41 MPa. Combined with the hydrocarbon generation and expulsion history of the study area, this temperature range corresponds to the shale liquid hydrocarbon generation and expulsion stage. The fluid pressure is less than the compressive stress in the normal direction of the fracture. Therefore, the fluid pressure and crystal growth force jointly drive fracture expansion, resulting in a fibrous crystal morphology. This indicates that the dynamic opening and healing of the fracture are coupled. The above values represent the fluid temperature and pressure fields during the dynamic opening of the fracture during the oil generation and expulsion stage. The final results are shown in Table 1.

[0026] Table 1 Example data and analysis results of mineral crystals in shale hydrocarbon generation and expulsion fractures

[0027] Example 2 Figure 3 The image shows a tensile crystal structure from the highly mature, organic-rich shale of the Triassic Xujiahe Formation in the Tongnanba area of the Sichuan Basin. Natural fractures developed within the structure are filled with tensile quartz crystals. The assemblage of fluid inclusions in the tensile crystals within the Xujiahe Formation shale in the Sichuan Basin consists of single-phase methane inclusions and coexisting two-phase methane-bearing brine inclusions. Laser Raman spectroscopy shows that the Raman shift of the single-phase methane inclusions is 2910.3 cm -1 The homogenization temperature of the symbiotic brine inclusion is 169°C. The capture pressure is finally obtained by isochoric extension of the single-phase inclusion to be 179 MPa. Combined with the hydrocarbon generation evolution history of the study area, it can be seen that this temperature range corresponds to the shale generation and exhaust hydrocarbon stage. Since the scale of each crack opening is small, the dynamic opening and healing of the cracks are synchronized, that is, this value represents the fluid temperature and pressure field when the cracks are dynamically opened during the exhaust gas stage. The final results are shown in Table 1.

[0028] Example 3 Figure 4 The massive crystal structure is shown in this example from the overmature, organic-rich shale of the Wufeng-Longmaxi Formation of the Lower Paleozoic in the northeastern Sichuan Basin. Horizontal natural fractures developed within the massive quartz crystals are filled. Fluid inclusions within the massive crystals consist of single-phase methane inclusions and coexisting two-phase methane-bearing brine inclusions. Laser Raman spectroscopy shows that the Raman shift of the single-phase methane inclusions is 2910.2 cm -1The homogenization temperature of the symbiotic brine inclusion is 184°C, and the capture pressure is finally obtained by isochoric extension of the single-phase inclusion to be 195 MPa. Combined with the hydrocarbon generation evolution history of the study area, it can be seen that this temperature range corresponds to the shale generation and exhaust hydrocarbon stage, and the fluid pressure is higher than the overlying rock load during the crystallization of the block crystals, that is, the pressure can actively drive the continuous dynamic opening of horizontal fractures, indicating that the dynamic opening of the fracture is coupled with the block healing process. Therefore, the above calculation results also represent the fluid temperature and pressure information during the dynamic opening of the fracture in the shale gas generation and exhaust stage. The final results are shown in Table 1.

[0029] Those skilled in the art will understand that the discussion of the above embodiments is merely illustrative and is not intended to limit the scope of the present invention to these examples. Within the spirit and principles of the present invention, the technical features of the above embodiments or different embodiments may be combined, the steps may be implemented in any order, and many other variations exist for the various aspects of the present invention described above, which are not provided in detail for the sake of clarity. Any omissions, modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A method for identifying the coupling relationship between fracture opening and healing during shale hydrocarbon generation and expulsion, characterized in that: The following steps are involved: S1. Collect shale rock samples containing natural fractures at the hydrocarbon generation stage, i.e., the organic matter mature-overmature stage, and prepare rock thin sections; S2. Microscopic identification of fracture healing mineral crystal structure types: Using microscopic observation combined with cathodoluminescence technology to observe rock thin sections, the petrographic characteristics of the healing mineral crystals in the fractures were used to identify fibrous structures, tensile crystal structures, and massive structures. S3. Use polarizing and fluorescence microscopy combined with laser Raman spectroscopy to systematically analyze the petrography and chemical composition of hydrocarbon-bearing fluid inclusion assemblages captured during the growth of fibrous, elongated, and massive crystals; S4. Conduct microthermometry analysis of inclusions in crystals of various structures: measure the homogenization temperature and freezing point of brine inclusions, as well as the homogenization temperature of coexisting gas and oil inclusions. Based on the different inclusion combinations, determine the hydrocarbon-bearing fluid conditions during crystal healing. For combinations of oil and brine inclusions, the temperature and pressure of the hydrocarbon-bearing fluid during healing crystallization are determined by intersecting the isochore lines of the oil and brine inclusions in the PT phase diagram. For combinations of gas and brine inclusions, the pure gas inclusion isochore epitaxy method is used. This method first establishes the isochore lines of the inclusions based on the laser Raman shift of the gas inclusions. These isochore lines are then extended along the PT space in the direction of increasing temperature and pressure until the homogenization temperature of the brine inclusions is reached, and the trapping pressure is determined. S5. Analysis of the coupling between the healing process of fibrous and crystalline tensile structures and the dynamic opening of fractures: By reconstructing the captured temperature-pressure conditions based on fluid inclusions, the temperature and pressure fields of hydrocarbon-bearing fluids during the dynamic opening of fractures during the hydrocarbon generation and expulsion phase of shale were determined. By combining the temperature and pressure reconstruction results with the regional hydrocarbon generation evolution history, the timing of dynamic fracture opening was effectively limited. S6. Representative evaluation of the dynamic opening of fractures by the healing process of blocky crystals: When the fluid pressure during precipitation reconstructed by fluid inclusions exceeds the normal compressive stress of the fracture, the temperature and pressure of the hydrocarbon-containing fluid at the time of dynamic opening of the fracture are obtained based on the capture temperature and capture pressure obtained from the fluid inclusions and combined with the analysis of the hydrocarbon generation history, thereby effectively limiting its dynamic opening time.

2. The method for identifying the coupling relationship between fracture opening and healing during shale hydrocarbon generation and expulsion according to claim 1, characterized in that: In step S1, the rock slice is polished on both sides and has a thickness of 70-100 microns.

3. The method for identifying the coupling relationship between fracture opening and healing during shale hydrocarbon generation and expulsion according to claim 1, characterized in that: The specific steps of step S3 are as follows: first, observe and record the morphology, occurrence, size and phase distribution of the inclusions in the combination under transmitted light; then switch to fluorescence mode, observe the fluorescence color and luminescence intensity of the inclusions, and identify whether there are liquid hydrocarbon inclusions; finally, collect laser Raman spectra of each phase in the inclusions in the same field of view, identify the internal components of the inclusions through Raman spectral characteristics, identify the presence of gaseous hydrocarbon inclusions, and comprehensively analyze the fluorescence response of the inclusions and the Raman qualitative results to finally determine the types of inclusions in the inclusion combination.

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