A method for identifying the coupling relationship between fracture opening and healing in the shale hydrocarbon generation and expulsion process

Abstract

The present application belongs to the field of petroleum geology and fluid geochemistry, and specifically discloses a method for identifying the coupling relationship between fracture opening and healing in the process of shale hydrocarbon generation and expulsion. By preparing shale rock slices containing natural fractures in the hydrocarbon generation stage, identifying healing mineral crystals of different structures in the fractures, observing fluid inclusions in fibrous, massive and tensile structure crystals through petrographic observation, selecting different fluid inclusion combinations, performing laser Raman, microthermometry and PVT simulation, combining the results of temperature and pressure fields of hydrocarbon-containing fluids with the hydrocarbon generation and evolution history of source rocks, the fracture opening mechanisms corresponding to the three types of structures are determined, and finally the dynamic opening and healing coupling relationship of fractures in the process of hydrocarbon generation and expulsion is effectively limited, which provides help for the related research on the formation period of fractures in the process of shale hydrocarbon generation and expulsion in oil and gas bearing basins, and provides a new idea and method for the limitation of the formation period of fractures.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of petroleum geology and fluid geochemistry, and particularly relates to a method for identifying the coupling relationship between fracture opening and healing in the process of shale hydrocarbon generation and expulsion. BACKGROUND

[0002] At present, in oil and gas bearing basins, the process of shale hydrocarbon generation and expulsion is often accompanied by the opening and healing of natural fractures, which directly affects the migration, retention efficiency and final enrichment degree of hydrocarbon fluids. Traditionally, the study of shale fracture evolution and fluid migration history mainly relies on static core observation and microscopic analysis, geochemical isotope dating and other methods. Although these methods can reveal the opening and healing process of fractures at different scales, they have the following shortcomings: first, static core observation and microscopic analysis can only obtain a cross-sectional snapshot at a certain moment, and cannot capture the whole process of fracture behavior evolution with temperature and pressure; second, isotope dating, which is costly, complicated and uncertain, cannot obtain the high pressure, high temperature and multi-field coupling conditions at the real burial depth.

[0003] Based on this, the present application proposes a method combining healed crystal petrography and inclusion analysis, which is based on the fracture opening mechanism recorded by three types of healed filling structures, i.e. fibrous, stretched and blocky structure crystals, to systematically identify the dynamic opening and healing coupling behavior of fractures in the process of hydrocarbon generation and expulsion. Through microscope, Raman spectroscopy, mineral crystal petrography identification, inclusion petrography identification and microthermometry analysis, the temperature and pressure conditions corresponding to the dynamic opening of fractures of different structure crystals are restored. From both theoretical and technical points of view, this method is feasible and has the advantages of low cost, simplicity and efficiency. SUMMARY

[0004] The present application provides a method for identifying the coupling relationship between fracture dynamic opening and healing in the process of shale hydrocarbon generation and expulsion by combining the analysis of internal healed mineral crystal petrography of natural fractures and the analysis of hydrocarbon-bearing primary fluid inclusions captured during the growth of different forms of crystals, and then simply and efficiently realizing the limitation of the opening and closing process of natural fractures in the process of shale hydrocarbon generation and expulsion, as well as the restoration of the temperature and pressure field of internal hydrocarbon-bearing fluid during the dynamic opening of fractures.

[0005] To achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0006] Collect shale rock samples containing natural fractures in the hydrocarbon generation stage (i.e. organic matter maturation-over-maturation stage), and prepare rock thin sections with a thickness of about 70-100 microns on both sides;

[0007] Through microscopic observation combined with cathodoluminescence imaging technology, the petrographic characteristics of the fracture filling minerals in the thin section are analyzed, and three types of healed mineral crystal structure are identified:

[0008] The first type is fibrous structure crystal: the fibrous structure crystals have consistent crystal morphology, large and parallel long-short axis ratio, and comb-shaped distribution, the interface between adjacent minerals is smooth, and there is no growth competition between each other, and the inside contains a clear intermediate surface, and has a back growth type growth mode;

[0009] The second type is tensile structure crystal: the tensile structure crystals have multiple solid surrounding rock trace strips inside, which are parallel to the crack wall, the interface between adjacent minerals is jagged, reflecting multiple times of small-scale crack opening and crystal healing process;

[0010] The third type is block structure mineral: the filling mineral crystals are equiaxed or elongated, the former has consistent crystal size and random strike, and the latter has significant growth competition between crystals, both of which reflect that the crystal growth occurs in an open fluid space, and has a growth type growth mode.

[0011] The above three types of crystal structure minerals capture hydrocarbon-bearing primary fluid inclusions during the precipitation process, and petrographic observation is carried out on the inclusions by using polarizing / fluorescent microscope combined with laser Raman spectrum analysis. The petrographic characteristics mainly include the types of inclusions developed in the inclusion combination (oil inclusions, gas inclusions and / or brine inclusions), the phase composition of each type of inclusion, the gas-liquid ratio, the fluorescence color of oil inclusions, and the gas phase composition of gas inclusions.

[0012] Microscopic temperature measurement analysis is carried out on the above inclusion combination, and the homogenization temperature and freezing point of brine inclusions, the homogenization temperature of oil inclusions and the homogenization temperature of gas inclusions are obtained. For the combination of oil inclusions and brine inclusions, the trapping temperature and pressure are calculated by the intersection method of the isochore lines of the two types of inclusions. For the combination of gas inclusions and brine inclusions, the final trapping temperature and pressure are determined by the extension method of the isochore line of pure gas inclusions.

[0013] The growth of fibrous structure crystals is limited by the limited crack space, and the crack opening is driven by fluid overpressure and crystal crystallization growth force, which shows that the crystal growth and crack dynamic opening have synchronicity; the tensile structure crystals record the cycle of multiple micro-scale crack opening and mineral crystallization filling process, and since the displacement amount of each crack opening is very small, the filling crystal can continue to stretch across the two walls, and also maintains the synchronicity of crack dynamic opening and crystal healing process.

[0014] The fibrous structure crystals and the tensile structure crystals both occur simultaneously due to the dynamic opening and healing of the cracks, so the trapping temperature and pressure calculated from the above primary fluid inclusions can represent the hydrocarbon-bearing fluid temperature and pressure field during the dynamic opening of the cracks in the shale hydrocarbon generation and expulsion process, and combined with the hydrocarbon generation and evolution history, the duration of the dynamic opening of the cracks can be further limited.

[0015] The blocky structure crystal represents the crystallization growth of the mineral in an open fluid space. When the fluid pressure during the crystallization of the blocky crystal calculated above is greater than the compressive stress in the vertical direction of the fracture surface, then the high-pressure fluid can actively drive the fracture to continuously open, and the crystal grows in the open space without obstruction, synchronously with the dynamic opening of the fracture; on the contrary, the fracture stops expanding, and the crystal only precipitates in the existing space, and the calculated trapping temperature and pressure cannot represent the temperature and pressure when the fracture is dynamically opened, that is, there is no coupling relationship between the dynamic opening and healing of the fracture during the shale hydrocarbon generation and expulsion process.

[0016] Through the scheme provided by the present application, natural fracture shale rock sample micro-thin sections are obtained and prepared; the filling crystal structure type in the fracture is determined through microscopic observation, including fibrous structure, tensile structure and blocky structure; the petrographic characteristics of the hydrocarbon-bearing fluid inclusions trapped by the filling minerals of different structures are analyzed by combining Raman spectrum with polarized light / fluorescent identification, and the composition of the combined hydrocarbon-bearing fluid inclusions is determined: brine inclusions, oil inclusions and gas inclusions; through micro-temperature measurement analysis, the homogenization temperature and freezing point of the brine inclusions, and the homogenization temperature of the associated oil inclusions and / or gas inclusions are obtained, and finally the trapping temperature and pressure are determined by using the isochore intersection method or the isochore extension method of pure gas inclusions; based on the mineral crystal structure analysis and fluid temperature and pressure field analysis, combined with the hydrocarbon generation and evolution history, the coupling relationship between the dynamic opening and healing process of the fracture is determined, and the period of the dynamic opening of the fracture during the hydrocarbon generation and expulsion stage of the source rock is effectively limited.

[0017] The present application has the following beneficial effects:

[0018] The method only needs to be based on the petrographic analysis of the healing minerals in the fractures of the shale rock thin sections, combined with the analysis of the primary fluid inclusions in each form of mineral, to limit the coupling relationship between the dynamic opening and healing of the fracture during the hydrocarbon generation and expulsion process of the shale, and on this basis, combined with the hydrocarbon generation history, the period of the dynamic opening of the fracture during the hydrocarbon generation and expulsion process is limited from the angle of the original hydrocarbon-bearing fluid recorded in the mineral crystallization process synchronously grown with the dynamic opening of the fracture, which is simple and efficient.

[0019] The present application can provide help for the related research on the formation period of the fracture during the hydrocarbon generation and expulsion process of the shale in the oil and gas bearing basin, and provide a new idea and method for the limitation of the formation period of the fracture. In addition, the two most important fluid condition parameters, i.e. temperature and pressure, during the hydrocarbon generation and expulsion process of the source rock can be directly recovered and obtained through the fluid inclusion analysis technology. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 The flowchart of the method for identifying the coupling relationship between the opening and healing of the fracture during the hydrocarbon generation and expulsion process of the shale provided by the present application;

[0021] Figure 2Schematic diagram of fibrous crystal structure and examples thereof;

[0022] Figure 3 Schematic diagram of stretched crystal structure and examples thereof;

[0023] Figure 4 Schematic diagram of blocky crystal structure and examples thereof. DETAILED DESCRIPTION

[0024] In order to make the purposes, technical solutions and advantages of the present application clearer, the technical solutions of the present application will be described clearly and completely below in conjunction with specific embodiments of the present application and corresponding drawings. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present specification, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the scope of protection of the present application.

[0025] The technical solutions of the present application will be described below in conjunction with the drawings. As shown in Figure 1 Figure 1 A flowchart of a method for identifying the coupling relationship between fracture opening and healing in the shale hydrocarbon expulsion process provided by the present application, comprising:

[0026] S1, preparing shale rock slices containing natural fractures in the hydrocarbon generation stage. Collect shale rock samples containing natural fractures in the organic matter maturation-over-maturation stage, and prepare rock slices with a double-face polishing thickness of 70-100 microns.

[0027] S2, microscopically identifying the crystal structure type (fibrous structure / crystal stretching structure / blocky structure) of the fracture healing mineral. Through microscopic observation, combined with cathodoluminescence technology, observe the rock slices, and identify the crystal structure of the filling according to the petrographic characteristics of the crystal of the healing mineral in the fracture.

[0028] If the crystal morphology of the filling mineral in the fracture is similar, all have a large aspect ratio, the crystal long axis direction is perpendicular to the fracture wall, the mineral boundary between adjacent crystals is smooth, there is no obvious competitive growth, and the whole is regularly arranged in a comb shape, and an obvious middle surface consistent with the strike of the fracture wall is often contained inside, the cathodoluminescence color of the same mineral on one side of the middle surface is nearly consistent, then it can be judged as a fibrous structure crystal.

[0029] If there are multiple parallel to the fracture wall in the crystal filling the fracture, and the crystal boundary is jagged, it indicates that the crystal has experienced repeated multi-stage small-scale fracture opening and healing, and the position of each new fracture opening is located in the adjacent wall rock or in the already formed crystal, then it is judged as a stretched structure crystal.

[0030] ​If the crack-filling crystals are equant and the grain sizes are roughly the same, and the strike is random, the structure is identified as blocky; if the crack-filling crystals are anisometric and the shape is mostly long columnar, i.e. the long axis is obviously larger than the short axis, and the adjacent crystals show obvious competitive growth, the structure is identified as elongated blocky. Cathodoluminescence shows that the blocky and elongated blocky structures usually have a syndiagenetic growth mode, i.e. the crystals grow from both sides of the crack to the center of the crack.

[0031] S3, Composition analysis of primary fluid inclusions and hydrocarbon-bearing fluid in each structural crystal. The composition of the hydrocarbon-bearing fluid inclusions trapped during the growth of fibrous, stretched and blocky crystals is analyzed by combining polarized light and fluorescence microscopy with laser Raman spectroscopy. First, the morphology, occurrence, size and phase distribution of the inclusions in the assembly are observed and recorded under transmitted light. Then, the fluorescence color and intensity of the inclusions are observed under fluorescence mode to identify whether there are liquid hydrocarbon inclusions. Finally, the laser Raman spectra of each phase in the inclusions are collected in the same field of view to identify the components in the inclusions by Raman spectroscopy characteristics, and to identify whether there are gaseous hydrocarbon inclusions. By combining the fluorescence response and Raman qualitative results of the inclusions, it is finally determined what types of inclusions are in the inclusion assembly, i.e. whether there are brine inclusions and their coexisting oil inclusions or gas inclusions.

[0032] S4, Determination of the temperature and pressure of the hydrocarbon-bearing fluid during the healing crystallization of each structural crystal. Microscopic temperature measurement is performed on the inclusions of each structural crystal to measure 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. According to different inclusion combinations, the conditions of the hydrocarbon-bearing fluid during the healing crystallization of the crystals are determined. For the combination of oil inclusions and brine inclusions, the temperature and pressure of the hydrocarbon-bearing fluid during the healing crystallization (i.e. the trapping temperature and trapping pressure) are obtained by the intersection method of the isochore lines of oil inclusions and brine inclusions in the P-T phase diagram. For the combination of gas inclusions and brine inclusions, the trapping pressure can be obtained by the extrapolation method of pure gas (single-phase methane) inclusion isochore line, i.e. first establish the isochore line of the inclusion according to the laser Raman shift of the gas inclusion, then extend this isochore line in the P-T space in the direction of increasing temperature and pressure until the homogenization temperature of the brine inclusion, and obtain the trapping pressure.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] The technical solution of the present invention is further described below with reference to specific embodiments:

[0037] Example 1 Figure 2The fiber-like crystal structure is shown, which is from the mature and organic-rich shale in the Paleogene Shahejie Formation in the Dongying Sag of the Bohai Bay Basin. The natural fractures developed in the shale are filled with fiber-like calcite crystals. The fluid inclusion assemblage in the fiber-like structure crystal is composed of gas-liquid two-phase oil inclusions (yellow-green fluorescence) and their associated brine inclusions (non-fluorescent). The homogenization temperature of the gas-liquid two-phase oil inclusions is between 71-96℃, the homogenization temperature of the associated gas-liquid two-phase brine inclusions is between 120-140℃, the freezing point is between -18--22℃, and the trapping temperature range obtained by the intersection of the two isochore lines is 128-152℃, and the trapping pressure is between 23-41MPa. Combined with the hydrocarbon generation and evolution history of the study area, it can be known that the temperature range corresponds to the shale liquid hydrocarbon generation and expulsion stage, and the fluid pressure is less than the normal pressure stress of the fracture, so the fluid pressure and the crystal growth force jointly drive the expansion of the fracture, resulting in the fiber-like crystal morphology of the crystal, that is, the dynamic opening and healing of the fracture are coupled, and the above values represent the fluid temperature and pressure field when the fracture is dynamically opened during the oil generation and expulsion stage. The final results are shown in Table 1.

[0038] Table 1 Mineral crystal example data and analysis results table in shale hydrocarbon generation and expulsion fractures

[0039]

[0040] Example 2 is shown as Figure 3 The crystal stretching structure is shown, which is from the high-mature and organic-rich shale in the Triassic Xujiahe Formation in the Tongnanba area of the Sichuan Basin. The natural fractures developed in the shale are filled with stretched quartz crystals. The fluid inclusion assemblage in the stretched structure crystal in the Xujiahe Formation shale in the Sichuan Basin is composed of single-phase methane inclusions and associated two-phase methane-containing brine inclusions. Laser Raman spectroscopy shows that the Raman shift of the single-phase methane inclusions is 2910.3cm -1 , the homogenization temperature of the associated brine inclusions is 169℃, and the trapping pressure obtained by the extension of the single-phase inclusion isochore is 179MPa. Combined with the hydrocarbon generation and evolution history of the study area, it can be known that the temperature range corresponds to the shale gas generation and expulsion stage. Since the scale of each fracture opening is small, the dynamic opening and healing of the fracture are synchronous, that is, the value represents the fluid temperature and pressure field when the fracture is dynamically opened during the gas generation and expulsion stage. The final results are shown in Table 1.

[0041] Example 3 is shown as Figure 4 The block-like crystal structure is shown, which is from the over-mature and organic-rich shale in the Lower Paleozoic Wufeng-Longmaxi Formation in the Northeast Sichuan Basin. The horizontal natural fractures developed in the shale are filled with block-like quartz crystals. The fluid inclusion assemblage in the block-like structure crystal is composed of single-phase methane inclusions and associated two-phase methane-containing brine inclusions. Laser Raman spectroscopy shows that the Raman shift of the single-phase methane inclusions is 2910.2cm -1The homogenization temperature of the symbiotic brine inclusion is 184 ℃, and the final capture pressure is 195 MPa by the single-phase inclusion isobaric line extension. Combined with the hydrocarbon generation history of the research area, it can be known that the temperature range corresponds to the shale gas generation and expulsion stage, and the fluid pressure is higher than the overlying rock load when the block crystal is crystallized, that is, the pressure can actively drive the horizontal fracture to be continuously and dynamically opened, which indicates that the dynamic opening of the fracture and the block healing process have coupling, so the above calculation results also represent the fluid temperature and pressure information when the fracture is dynamically opened in the shale gas generation and expulsion stage, and the final results are shown in Table 1.

[0042] Those skilled in the art should understand that the discussion of the above examples is only exemplary and is not intended to suggest that the scope of the present application is limited to these examples; under the idea of the present application, the technical features in the above examples or different examples can also be combined, the steps can be implemented in any order, and there are many other changes of different aspects of the present application as described above. In order to be brief, they are not provided in details. Any omission, modification, equivalent replacement, improvement, etc. within the spirit and principle of the present application shall be included in the protection scope of the present application.

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 double-sided polished 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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