A method for determining the direction of the ancient maximum compressive stress when an orthogonal tensional fracture forms

Abstract

The present invention belongs to the fields of petroleum geology and tectono-diagenesis, and specifically discloses a method for determining the direction of the ancient maximum compressive stress during the formation of orthogonal tensile fractures. By combining the petrographic characteristics of fluid inclusions trapped in different minerals within orthogonal tensile fractures and the analysis of the development types of fluid inclusions, conducting laser Raman spectroscopy tests on methane-rich inclusions and microthermometric analysis of coexisting brine inclusions, using the pressure-volume-temperature simulation method to obtain the fluid pressure and temperature during the mineral crystallization process, combining the trapped temperature data with the burial history curve of this area, identifying the opening process and state of two groups of orthogonal tensile fractures, obtaining the time coupling relationship when the orthogonal tensile fractures are in the open state, and finally obtaining the direction of the ancient maximum compressive stress during the simultaneous opening process of the orthogonal tensile fractures according to tectonic stress analysis. It provides a new idea and method for judging the direction of the ancient maximum compressive stress during the formation of orthogonal tensile fractures.

Description

Technical Field

[0001] The invention belongs to the field of petroleum geology and structural-diagenetic science, and particularly relates to a method for limiting the direction of paleomaximum compressive stress when orthogonal tensile fractures are formed. Background Art

[0002] Natural tensile fractures are widespread within sedimentary basins, particularly in unconventional reservoirs such as low-porosity, low-permeability, organic-rich shales and tight sandstones. Previous research on methods for determining paleotectonic stress fields in petroliferous basins has been relatively mature. These methods primarily rely on analyzing regional tectonic evolutionary history, using statistical data on the developmental stages and patterns of various tectonic fractures, combined with mechanical models and stress inversion techniques, to define the direction of paleotectonic stresses. However, there are currently gaps in the definition of the tectonic stress field during the formation of orthogonal tensile hydraulic fracturing fractures. It is worth noting that the maximum compressive stress experienced by tensile fractures can be effectively determined through geotectonic analysis and laboratory core-scale structural simulations. However, these two analyses differ somewhat in their interpretation of paleotectonic stresses during geological history. Besides the complexity and time involved, the experimental costs are extremely high, making them unsuitable for widespread application.

[0003] Studying the paleotectonic stresses experienced by fractures—the tectonic stresses experienced when the minerals filling them formed—requires a clear understanding of the fluid field within the fractures. Fluid inclusions, as the most direct natural geological sample capable of recording information about the original fluid, play an irreplaceable role in analyzing geological processes such as diagenesis, mineralization, basin fluid evolution, and fluid migration. Therefore, we sought to identify methods based on fluid inclusions to determine the direction of paleomaximal compressive stress based on the simultaneous opening of two sets of orthogonal tensile fractures. Summary of the Invention

[0004] The present invention provides a method for limiting the direction of the ancient maximum compressive stress during the formation of orthogonal tensile cracks, which can realize a simple and efficient determination of the direction of the ancient maximum compressive stress during the formation of orthogonal tensile cracks.

[0005] To achieve the above object, the present invention adopts the following technical solutions:

[0006] S1. Hand specimens show that the rock has an orthogonal tensile fracture network. That is, there is no obvious interlacing relationship between the horizontal and vertical tensile fracture groups, and the rock shows a relatively obvious tracking and derived petrographic characteristics.

[0007] S2. Prepare fluid inclusion thin sections of rock samples containing orthogonal tensile fractures. The fluid inclusion thin sections are prepared in accordance with the petroleum industry standard SY / T 5913-2004.

[0008] S3. Determine the mineral petrological characteristics of the filling materials in the orthogonal tensile fractures. The mineral petrological characteristics include the types, combinations, distribution patterns, and growth generations of minerals in the orthogonal tensile fractures. Compare and find the same massive crystal crystallization sequences in the two sets of fractures with different strikes.

[0009] S4. Determine the petrographic characteristics included in the fluid inclusion assemblages of the above-mentioned massive minerals in the orthogonal tensile fractures. The petrographic characteristics include the size, shape, structure, occurrence, phase assemblage and phase volume ratio characteristics, and fluid inclusion stages of the fluid inclusions. Determine the types of fluid inclusions developed in the massive minerals in the orthogonal tensile fractures according to the petrographic characteristics, and identify the primary methane-rich inclusions and their coexisting brine inclusions. Conduct laser Raman tests on the methane-rich inclusions, conduct microhomogenization temperature tests on the brine inclusions, and perform pressure-volume-temperature simulations on the above results to obtain the trapping temperature and pressure of the inclusions, that is, the fluid temperature and pressure during the crystallization of the massive crystals.

[0010] S5. Project the obtained trapping temperature data onto the burial history curve of this area, so as to obtain the formation times of different massive minerals in the orthogonal tensile fractures and the hydrostatic pressure and lithostatic pressure of the strata during the massive filling period of the orthogonal tensile fractures. Compare the trapping pressure of the methane-rich inclusions obtained from the pressure-volume-temperature simulation with the lithostatic pressure at that time to clarify that the fractures in the two sets of orthogonal tensile fractures remained continuously open during the filling of the massive crystals (i.e., the natural hydraulic fracturing process).

[0011] S6. Analyze the formation and opening states of the orthogonal tensile fractures, that is, limit the precipitation sequence of the minerals in the fractures, and match the precipitation times of the same minerals in the horizontal tensile fracture group and the vertical tensile fracture group. The results show that the precipitation times of the same minerals in the two fracture groups with different occurrences are consistent, that is, the opening times of the horizontal tensile fracture group and the vertical tensile fracture group are consistent, that is, the orthogonal tensile fractures are opened simultaneously. On this basis, conduct tectonic stress analysis on the orthogonal tensile fractures, and finally obtain the direction of the paleo-maximum compressive stress when the fractures were formed and opened.

[0012] Through the solution provided by the embodiments of the present invention, it is determined that orthorhombic tensile fracture networks are developed inside the rock at the hand specimen scale, and fluid inclusion thin sections are made from rock samples containing orthorhombic tensile fractures; the petrographic characteristics of the minerals filling the orthorhombic tensile fractures are determined and the same block crystal crystallization sequences in the two sets of strike fractures are found; various types of fluid inclusions developed in the block minerals filled in the fractures are determined according to the petrographic characteristics of the fluid inclusions; the research objects we need, namely primary methane-rich inclusions and their associated brine inclusions, are found among various types of inclusions; laser Raman tests are carried out on the methane-rich inclusions and microhomogenization temperature tests are carried out on the brine inclusions; pressure-volume-temperature simulations are carried out on the above inclusions to obtain their trapping temperatures and trapping pressures; the obtained trapping temperature data are plotted on the burial history curve of this area, so as to determine the crystallization time of the block minerals of the orthorhombic tensile fractures and the fracture state of the strata during the same period of mineral precipitation, match the formation times of the same minerals in the horizontal tensile fracture group and the vertical tensile fracture group, obtain the opening state of the orthorhombic tensile fractures, and finally combine with tectonic stress analysis to obtain the direction of the ancient maximum compressive stress when the orthorhombic tensile fractures were formed.

[0013] The present invention has the following beneficial effects:

[0014] (1) This method only needs to apply petrographic observations of minerals and fluid inclusions, laser Raman spectroscopy analysis of fluid inclusions, microhomogenization temperature tests of fluid inclusions, pressure calculation by pressure-volume-temperature simulation and burial history restoration to obtain the fracture opening process and state, and then the direction of the ancient maximum compressive stress when the fractures were formed can be determined according to fracture tectonic stress analysis, providing fluid inclusion evidence required for the study of orthorhombic tensile fractures from the microscale, and avoiding the investigation and analysis of the basin-scale geotectonic background and the cumbersome rock mechanics experiments at the core scale.

[0015] (2) The present invention can help the research on the stress suffered when orthorhombic tension fractures are formed in oil and gas-bearing basins, and provides a new idea and method for the direction of the ancient maximum compressive stress when orthorhombic tensile fractures are formed. Description of the Drawings

[0016] Figure 1 It is a schematic flow chart of a method for determining the direction of the ancient maximum compressive stress when an orthorhombic tensile fracture is formed provided by the present invention;

[0017] Figure 2 It is a photo of a hand specimen of a core scale containing orthorhombic tensile fractures provided by an embodiment of the present invention;

[0018] Figure 3 It is a photo of the result of the automatic analysis system of minerals in fluid inclusion thin sections provided by an embodiment of the present invention;

[0019] Figure 4Schematic diagram of minerals filled in orthogonal tensile fractures and fluid inclusions trapped in the minerals provided by the embodiments of the present invention;

[0020] Figure 5 Photo of rich methane inclusions and associated brine inclusions trapped in minerals provided by the embodiments of the present invention;

[0021] Figure 6 Schematic diagram of tectonic stress field analysis of orthogonal tensile fracture for judging the direction of paleo-maximum compressive stress provided by the embodiments of the present invention. Specific embodiments

[0022] To make the objectives, technical solutions and advantages of the present application clearer, the technical solutions of the present application will be clearly and completely described below in conjunction with specific embodiments of the present application and the corresponding drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in this specification, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the scope of protection of the present application.

[0023] In this embodiment, for the organic-rich mud shale of the Wufeng-Longmaxi Formation in the northeastern Sichuan Basin, Well XXX with complete data is selected as the research object for detailed description. A large number of natural near-orthogonal tensile fractures are developed in this set of shale, and the mineral cementation is significant, providing a good basis for the verification of the invention.

[0024] Figure 1 Schematic diagram of the method for determining the direction of paleo-maximum compressive stress when an orthogonal tensile fracture is formed provided by the invention:

[0025] S1. Determine that the rock sample contains an orthogonal tensile fracture network at the core scale

[0026] That is, there is no obvious cross-cutting relationship between the horizontal tensile fracture group and the vertical tensile fracture group. As Figure 2 shown, Figure 2 is a hand specimen photo of the orthogonal tensile fracture network at the core scale. The horizontal tensile fracture group intersects with the vertical tensile fracture group, and the derivation relationship is significant and there is no obvious interpenetrating cutting relationship;

[0027] S2. Make a fluid inclusion thin section using the orthogonal tensile fracture rock sample

[0028] Collect the rock sample and make a fluid inclusion thin section. The thin section is polished on both sides, contains horizontal and vertical tensile fractures, and the thickness is 70-100 microns;

[0029] S3. Use the cathode luminescence and mineral automatic analysis system technology to determine the types, occurrences, and genetic relationships of the minerals filled in the fractures, and clarify the same massive crystal filling sequences in the two groups of fractures with different strike directions

[0030] By identifying and analyzing thin sections of fluid inclusions, combined with cathodoluminescence technology and the technology of the automatic mineral analysis system, the petrological characteristics of the filling minerals are determined, including the types, occurrences, generations, etc. of the minerals, and the same block crystal crystallization sequences in two sets of strike fractures are found. As Figure 3 shown, Figure 3 is the surface scan photo of the automatic mineral analysis system technology for thin sections of fluid inclusions. The results show that there are generally two stages of block filling in both sets of fractures. In the photo, the gray and yellowish-green are the wall rocks of the fractures, the blue represents block calcite, which is the early filling material of the fractures, and the bright yellow represents block quartz, and the filling time is after calcite;

[0031] S4. Petrographic observation of fluid inclusions and pressure-volume-temperature simulation

[0032] Determine the petrographic characteristics of the fluid inclusions trapped in the block minerals filled in the fractures in the prepared thin sections of fluid inclusions. Select the required primary methane-rich inclusions and their associated gas-liquid two-phase brine inclusions samples from various types of inclusions. As Figure 4 , 5 shown, Figure 4 is a schematic diagram of the block minerals filled in two sets of orthogonal tensile fractures, namely horizontal fractures and longitudinal fractures, and the primary fluid inclusions trapped in the minerals. Taking quartz and calcite as examples, the block calcite is close to the fracture wall, and the block quartz is far from the fracture wall. The occurrences of calcite close to the fracture wall in the horizontal tensile fracture group and the longitudinal tensile fracture group are consistent, and the occurrences of quartz far from the fracture wall in the horizontal tensile fracture group and the longitudinal tensile fracture group are consistent. Select the same primary methane inclusions (pure gas-phase methane inclusions and methane-rich inclusions with a large gas-liquid ratio) and their associated brine inclusions (with a small gas-liquid ratio) in different minerals as the research objects. As Figure 5 shown, Figure 5 is the primary methane-rich inclusions (with a relatively large gas-liquid ratio) trapped in the quartz filled in the orthogonal tensile fracture network and their associated brine inclusions (with a relatively small gas-liquid ratio). Laser Raman tests are carried out on the methane-rich inclusions to obtain their corresponding methane Raman peaks, microscopic temperature measurements are carried out on the brine inclusions to obtain the homogenization temperature, and pressure-volume-temperature simulations are carried out on the above inclusions to obtain the trapping temperature and trapping pressure of the inclusions, that is, the fluid conditions when the block minerals crystallize;

[0033] S5. Opening process and state identification of two sets of orthogonal tensile fractures

[0034] The trapping temperatures obtained from the inclusion pressure - volume - temperature simulation are plotted on the burial history curve to obtain the precipitation times of the massive minerals filled in the horizontal tensional fracture group and the vertical tensional fracture group, and their corresponding paleo - burial depths are obtained. The formation hydrostatic and lithostatic pressures at the time of crystallization are calculated, and the inclusion trapping pressure is compared with the paleo - formation hydrostatic and lithostatic pressures to clarify that there is no stagnation in the opening of the fractures when the massive minerals are filled in the orthogonal tensional fractures, that is, the fractures are in a continuous tensional opening state;

[0035] S6. Determination of the paleo - maximum compressive stress when two groups of tensional fractures open simultaneously

[0036] Analyze the formation times of various massive minerals obtained, match the formation times of the same minerals in the horizontal tensional fracture group and the vertical tensional fracture group to determine the period when the orthogonal tensional fractures open simultaneously, and conduct tectonic stress analysis on it to limit the paleo - maximum compressive stress of this period. As Figure 6 shown, Figure 6 is a schematic diagram of the tectonic stress analysis of the orthogonal tensional fracture network. Finally, the direction of the paleo - maximum compressive stress during the formation process of the orthogonal tensional fractures is indicated by the red arrow.

[0037] In other words, the corresponding same massive minerals in the horizontal tensional fracture group and the vertical tensional fracture group precipitate simultaneously. Taking the above quartz and calcite as examples, the calcite near the fracture wall precipitates simultaneously, and the quartz far from the fracture wall precipitates simultaneously. Therefore, when the massive minerals crystallize in the orthogonal tensional fractures, the fractures must respectively maintain the states of continuous horizontal opening and vertical continuous opening, that is, the orthogonal tensional fractures must maintain the state of simultaneous opening. Combining the tectonic stress field to conduct tectonic stress analysis on the orthogonal tensional fractures, the direction of the paleo - maximum compressive stress at the time of their formation is finally obtained.

[0038] Through the solution provided by the embodiments of the present invention, it is necessary to determine that the cracks are orthogonal tensile cracks at the hand specimen scale, and there is no obvious cross-cutting relationship between the cracks in different directions, and obtain the fluid inclusion thin sections made from the rock samples containing orthogonal tensile cracks; determine the petrological characteristics of the minerals filled in the cracks and find out the same massive crystal crystallization sequences in the two sets of strike cracks; determine the petrographic characteristics of the fluid inclusions captured by the massive minerals; determine the various types of fluid inclusions developed in the vein precipitation minerals according to the petrographic characteristics; find the research objects we need - methane-rich inclusions and brine inclusions among various types of inclusions; perform laser Raman analysis on the methane-rich inclusions and brine inclusions, and perform microscopic homogenization temperature testing on the associated brine inclusions; perform pressure-volume-temperature simulation on the above fluid inclusions to obtain their trapping temperature and trapping pressure; project the obtained trapping temperature data onto the burial history curve of this area, so as to obtain the precipitation time of the minerals, as well as the formation rupture state and paleo-hydrostatic and lithostatic pressures during the same period; analyze the precipitation time of the massive minerals filled in the orthogonal tensile cracks, clarify the opening and continuous state of the cracks during the filling process, and finally obtain the direction of the paleo-maximum compressive stress when the orthogonal tensile cracks are formed by combining tectonic stress analysis.

[0039] This method only needs to apply petromineralogical analysis, petrographic observation of fluid inclusions, laser Raman spectroscopy analysis of fluid inclusions, microscopic homogenization temperature testing of fluid inclusions, pressure-volume-temperature simulation, and the correspondence of the burial history map. Finally, through tectonic stress analysis, the direction of the paleo-maximum compressive stress when the orthogonal tensile cracks are formed can be determined, avoiding the analysis of the geotectonic background and related cumbersome rock mechanics experiments.

[0040] The present invention provides certain reference significance for the research on the stress suffered during the formation of orthogonal tensile cracks in sedimentary basins, and provides a new idea and method for the direction of the paleo-maximum compressive stress during the formation of orthogonal tensile cracks.

[0041] In summary, when orthogonal tensile cracks develop in a rock sample and there is no obvious cross-cutting relationship between the orthogonal tensile cracks, it can be directly sectioned and observed. By combining petromineralogical analysis, laser Raman spectroscopy analysis of fluid inclusions, and microscopic homogenization temperature testing of fluid inclusions, and through the correspondence between the pressure-volume-temperature simulation of fluid inclusions and the burial history map, the precipitation time of minerals is matched, and finally the direction of the maximum principal pressure when the orthogonal tensile cracks are formed is obtained through tectonic stress analysis.

[0042] Specific embodiments of this specification have been described in detail. However, the operations, steps, or modules described in other embodiments can break through the example order restrictions and be implemented in an order different from the examples, and the same still holds as long as the expected effects can be achieved. The processes shown in the accompanying drawings do not need to strictly follow a specific or continuous step order and can be flexibly adjusted according to requirements in actual applications. In some scenarios, non-sequential processing methods such as synchronous execution or parallel operations can also achieve the same technical effects or even improve efficiency.

Claims

1. A method for determining the direction of the ancient maximum compressive stress when an orthogonal tensional fracture forms, characterized in that, It includes the following steps: S1. At the core scale, it is determined that the rock sample contains an orthogonal tensile fracture network, that is, there is no obvious interpenetration relationship between the horizontal tensile fracture group and the longitudinal tensile fracture group; S2. A fluid inclusion thin section is made using the orthogonal tensile fracture rock sample; S3. Determine the petrological characteristics of the filling minerals in the orthogonal tensile fractures. Use the techniques of cathodoluminescence and mineral automatic analysis system to determine the types, occurrences, and genetic relationships of the filling minerals in the fractures, and clarify the same massive crystal filling sequences in the two sets of fractures with different strike directions; S4. Petrographic observation of fluid inclusions and pressure - volume - temperature simulation; Determine the petrographic characteristics of the fluid inclusions trapped in the filling minerals of the orthogonal tensile fractures in the prepared fluid inclusion thin section. Identify and screen out the required methane - rich fluid inclusions and their associated gas - liquid two - phase brine fluid inclusion samples among various types of inclusions. Conduct laser Raman tests on the methane - rich fluid inclusions and microscopic homogenization temperature tests on the brine fluid inclusions to obtain the homogenization temperature of the brine fluid inclusions. Conduct pressure - volume - temperature simulations on the above inclusions to obtain their trapping temperatures and pressures; S5. Identification of the opening process and state of the two sets of orthogonal tensile fractures; Combine the obtained trapping temperature data with the burial history curve of this area to obtain the precipitation time of the minerals in the orthogonal tensile fractures. Then, compare the trapping pressure of the methane - rich fluid inclusions with the paleo - hydrostatic and paleo - lithostatic pressures of the paleo - burial depth during mineral filling to obtain the opening state and duration of the fractures during the precipitation of the massive crystals in the orthogonal tensile fractures; S6. Limitation of the paleo - maximum compressive stress when the two sets of tensile fractures open simultaneously; Analyze the formation times of various minerals filled in the orthogonal tensile fractures, match the precipitation times of the same minerals, determine the time when the two sets of orthogonal tensile fractures are in the simultaneous opening state, conduct tectonic stress analysis on the orthogonal tensile fractures, and finally obtain the direction of the paleo - maximum compressive stress during the formation process of the orthogonal tensile fractures.

2. A method for determining the direction of the ancient maximum compressive stress when an orthogonal tensile crack is formed according to claim 1, characterized in that, In step S2, the fluid inclusion thin section is made with reference to the petroleum industry standard SY / T 5913 - 2004, double - sided polished, with a thickness of 70 - 100 microns.

3. A method for determining the direction of the ancient maximum compressive stress when an orthogonal tensile fracture is formed according to claim 1, characterized in that In step S4, the petrographic characteristics of fluid inclusions include the size, shape, structure, occurrence, phase combination and phase volume ratio characteristics, and the stages of fluid inclusions.

4. A method for determining the direction of the ancient maximum compressive stress when an orthogonal tensile crack is formed according to claim 1, characterized in that, In step S6, match the precipitation times of the same minerals filled in the orthogonal tensile fractures, that is, match the pressure - volume - temperature simulation results of the methane - rich fluid inclusions and their associated brine fluid inclusions trapped in the minerals filled in the horizontal tensile fracture group and the longitudinal tensile fracture group.

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