Method and device for evaluating evolution process of overpressure microfracture network

Abstract

The invention discloses an overpressure microfracture network evolution process evaluation method and device, and relates to the technical field of oil-gas exploration. The method comprises the following steps: collecting a shale sample of a research area; a small rock cylinder is drilled through a drilling machine and a hollow drill bit, is loaded into a gold tube reactor and then is sealed and put into a high-pressure kettle. In a closed system, heating experiments at different maturity stages are carried out on the autoclave, and confining pressure is applied at the same time. And after the experiment is finished, cooling and recycling the small rock cylinders to obtain samples at different mature stages. Then CT scanning is carried out on the sample to obtain shale image slices, three-dimensional reconstruction is carried out based on the shale image slices, and a 3D rendering graph is generated. And evaluating the evolution process of the hydrocarbon generation effect induced overpressure microfracture network by analyzing the change of three-dimensional geometric parameters in the 3D rendering graph. According to the invention, under the condition of high confining pressure, the shale is subjected to a heating experiment by using the small rock cylinder and the closed system gold pipe, and dynamic simulation of the parallel bedding seam and the vertical bedding seam is successfully realized.

Description

Technical Field

[0001] The present application relates to the field of oil and gas exploration technology, and in particular to a method and device for evaluating the evolution process of an overpressure micro-fracture network. Background Art

[0002] In recent years, microfractures in organic-rich shales have attracted considerable attention due to their potential role in hydrocarbon migration and recovery during unconventional shale oil and gas production and in-situ conversion of oil shales. Previous studies have primarily documented natural microfractures induced by tectonic disturbances and hydraulic fracturing that occurs with increasing depth under fluid overpressure.

[0003] In existing technologies, researchers have successfully constructed the three-dimensional geometric structure of microfracture networks in heated shale, revealing the evolutionary mechanisms of microfracture nucleation, expansion, and merging. However, since these experiments generally lack confining pressure or apply low axial stress, their experimental results often only develop parallel bedding fractures, thereby ignoring and underestimating the development characteristics of vertical microfractures, resulting in unreliable evaluation results of shale microfractures.

[0004] Therefore, there is an urgent need for an evaluation method for the evolution process of microfracture networks to focus on the development characteristics of three-dimensional microfracture networks, improve the reliability of shale microfracture evaluation results, and provide more accurate guidance for the economic exploitation of medium- and low-maturity shale oil. Summary of the Invention

[0005] Based on this, it is necessary to provide a method and device for evaluating the evolution process of overpressure microcrack networks to address the above technical problems.

[0006] The present invention adopts the following technical solutions: The present invention provides a method for evaluating the evolution process of an overpressure microcrack network, comprising: Collect shale samples from the study area; A hollow drill bit is used to drill parallel or perpendicular to the bedding plane of the shale sample using a drilling machine, and then polished with emery to obtain a small rock column; The rock cylinders were placed in a gold tube reactor and sealed, and then placed in an autoclave. Experimental parameters were set, and heating experiments at different maturity stages were conducted in a closed stainless steel autoclave. Confining pressure was applied using a water pump during the heating test. After the heating test, the autoclave was removed from the pyrolysis furnace, cooled, and the rock cylinders in the gold tube reactor were recovered to obtain rock cylinders at different maturity stages. CT scans were performed on rock columns at different maturity stages to obtain image slices of shale at different maturity stages. Based on the shale image slices, the morphology of the shale overpressure micro-fracture network was three-dimensionally reconstructed to obtain a 3D rendering. The evolution of the overpressure microfracture network induced by hydrocarbon generation was evaluated by analyzing the number of parallel bedding fractures, the number of perpendicular bedding fractures, the width of microfractures, and the pore volume of microfractures at different maturity stages in the 3D renderings.

[0007] Preferably, the rock cylinders used in the heating test are drilled from the same bedding plane of the same shale sample.

[0008] Preferably, the confining pressure is transferred to the rock cylinder through plastic deformation of the gold tube reactor, simulating the static pressure experienced by the underground shale.

[0009] Preferably, based on the shale image slices, a three-dimensional reconstruction of the shale overpressure micro-fracture network morphology is performed to obtain a 3D rendering, which specifically includes: The shale bedding plane space of the rock column in the shale image slices at different maturity stages is converted into a two-dimensional plane parallel to the XY plane; Volume cropping is performed on the spatially transformed shale image slices; Interactive threshold segmentation and interactive brightness segmentation are performed on the volume-cropped shale image slices to extract the grayscale and brightness of the rock columns in the image, which are different from the micro-cracks composed of other materials. Removing the portion of the shale image slice whose pixels are smaller than a preset value after segmentation; Perform micro-fracture parameter analysis and statistics on the shale image slices after pixel removal and mark them; 3D volume rendering is performed on the micro-fractures in the marked shale image slices to obtain a 3D rendering image.

[0010] Preferably, microcracks with an inclination angle less than or equal to a preset angle to the bedding plane are defined as parallel bedding cracks; microcracks with an inclination angle greater than a preset angle to the bedding plane or perpendicular to the bedding plane are defined as vertical bedding cracks.

[0011] The present invention provides an evaluation device for the evolution process of an overpressure microcrack network, which is characterized by comprising: The sample collection module is used to collect shale samples in the study area. The module uses a hollow drill bit to drill parallel or perpendicular to the bedding plane of the shale sample and polishes it with emery to obtain a small rock column. The experimental module is used to load the rock cylinders into the gold tube reactor and seal it, then place the sealed gold tube reactor into the autoclave. Experimental parameters are set to conduct heating experiments at different maturity stages in the stainless steel autoclave in a closed system, and confining pressure is applied using a water pump during the heating test. After the heating test, the autoclave is removed from the pyrolysis furnace, cooled, and the rock cylinders in the gold tube reactor are recovered to obtain rock cylinders at different maturity stages. The data analysis module is used to perform CT scanning on rock columns at different maturity stages to obtain image slices of shale at different maturity stages. Based on the shale image slices, the morphology of the shale overpressure microfracture network is reconstructed in three dimensions to obtain a 3D rendering. By analyzing the changes in the three-dimensional geometric parameters of the 3D rendering at different maturity stages, the evolution process of the overpressure microfracture network induced by hydrocarbon generation is evaluated.

[0012] At least one of the above technical solutions adopted by the present invention can achieve the following beneficial effects: In a method for evaluating the evolution process of an overpressure microfracture network provided by the present invention, a dynamic simulation and quantitative characterization prediction evaluation method suitable for the formation and evolution of an overpressure microfracture network in organic-rich shale hydrocarbon generation is established. A heating experiment using small rock columns and closed system gold tubes is carried out on the organic-rich shale under high confining pressure conditions. Not only is the dynamic simulation of parallel bedding fractures and vertical bedding fractures successfully achieved, but also the 3D geometric structure evolution process of the overpressure microfracture network in the organic-rich shale during thermal maturation is further constructed and quantitatively evaluated using X-ray micro-nano CT scanning technology. This has important guiding significance for pilot experiments on in-situ conversion of organic-rich shale using electric heating and for achieving economical exploitation of medium- and low-maturity shale oil. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] The drawings described herein are used to provide a further understanding of the present application and constitute a part of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings: Figure 1 A schematic flow chart of a method for evaluating the evolution process of an overpressure micro-crack network provided by the present invention; Figure 2 This is a flow chart of the experimental design of an overpressure microcrack network evolution process evaluation method provided by the present invention; Figure 3 A flow chart of microcrack network CT data processing for an overpressure microcrack network evolution process evaluation method provided by the present invention; Figure 4 This is a graph showing the experimental results of a method for evaluating the evolution process of an overpressure microcrack network provided by the present invention; Figure 5 A schematic diagram of a device for evaluating the evolution process of an overpressure microcrack network provided by the present invention. DETAILED DESCRIPTION

[0014] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions of this application will be clearly and completely described below in conjunction with specific embodiments of the present invention and corresponding drawings. Obviously, the embodiments described are only part of the embodiments of this application, not all of them. Based on the embodiments in the 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 in detail the technical solutions provided by various embodiments of the present application in conjunction with the accompanying drawings.

[0016] Figure 1 The figure is a flow chart of a method for evaluating the evolution process of an overpressure microcrack network in the present invention, which specifically includes the following steps: S101: Collect shale samples from the study area; drill parallel or perpendicular to the bedding plane of the shale sample using a hollow drill bit on a drilling machine, and grind it with emery to obtain a small rock column.

[0017] Specifically, a set of organic-rich source rocks was selected from the Chang 73rd member of the Triassic Yanchang Formation in a certain basin, with good kerogen type (mainly type I-II1), high TOC content (mainly 5%-25%, up to 38%), and located in the oil window (R o The stratum (values ​​range from 0.7% to 1.2%) is not only a target for exploration and development of continental medium-to-high-maturity shale oil, but also a key horizon for in-situ conversion experiments of medium-to-low-maturity shale oil. In this study, a black shale core was collected from the YK1 well in the southeastern part of the basin, and basic geochemical testing and analysis were performed as follows: Table 1 Basic organic geochemical parameters of shale samples

[0018] Specifically, immature or low-maturity shale samples, as natural components of source rocks in sedimentary basins, can be used to reveal the hydrocarbon generation, evolution process, and coupled evolution of pore-fracture networks in mature shale during geological history through thermal pressure simulation experiments. In this experiment, the experimental process is as follows: Figure 2 , proposed to use a small drilling machine and a hollow drill bit to drill small rock cylinders with a diameter of 5 mm and a length of 1.0-1.5 cm parallel to the bedding, which were used as the objects for the thermal pressure simulation experiment. Figure 1 The advantage of this method is that the rock cylinders retain the rock framework of the shale sample compared to traditional whole-rock powder and kerogen extracts, and the rock structure and pore structure are relatively intact, inheriting the rock physical properties. Moreover, since the drilled rock cylinders are small in size, the same core can simultaneously meet the number of rock cylinders required for preparing the hot-pressing simulation experiment, thus avoiding the potential impact of organic-inorganic heterogeneity caused by selecting multiple core samples in the same hot-pressing simulation experiment.

[0019] Specifically, the preparation of rock columns is mainly achieved by drilling with a hollow drill bit on a drilling machine. The method is simple and easy to operate, but the following points should be noted: (1) Select fresh massive shale core samples and drill rock cylinders parallel or perpendicular to the bedding plane. Rock cylinders used in the same hot-pressure simulation experiment should be drilled on the same layer of the same core sample to avoid large experimental errors caused by sample heterogeneity. It is difficult to successfully drill rock cylinders from shale samples that are severely weathered, have abnormally developed lamellae, or are loosely cemented.

[0020] (2) After drilling the rock cylinder, the edges of the rock cylinder should be polished with diamond or other materials to prevent the rock cylinder from puncturing the gold tube during the experiment, which would cause the experiment to fail.

[0021] (3) The size of the rock cylinder is limited by the size of the gold tube and the gas production capacity of the sample. Taking all aspects into consideration, the gold tube used in this experiment has a size of 5.5 mm × 6.0 cm (inner diameter × length).

[0022] S102: The rock cylinders are placed into a gold tube reactor and sealed, and the sealed gold tube reactor is placed into an autoclave; experimental parameters are set, and heating experiments at different maturity stages are carried out on the stainless steel autoclave in a closed system, and confining pressure is applied using a water pump during the heating test; after the heating test, the autoclave is removed from the pyrolysis furnace, and after cooling, the rock cylinders in the gold tube reactor are recovered to obtain rock cylinders at different maturity stages.

[0023] Alternatively, the rock cylinder used in the heating test is drilled from the same bedding plane of the same shale sample.

[0024] Alternatively, the confining pressure is transferred to the rock cylinder through plastic deformation of the gold tube reactor, simulating the static pressure experienced by the shale in the subsurface.

[0025] Specifically, the prepared rock cylinders were weighed and loaded into a gold tube reactor (length × inner diameter × wall thickness: 60 mm × 5.5 mm × 0.25 mm). The tubes were then completely sealed using argon arc welding and subsequently placed into a stainless steel autoclave. Next, heating experiments were conducted in a pyrolysis furnace, and confining pressure was applied using a water pump, depending on the heating method, maturity, and pressure conditions required for the study. In this experiment, a constant temperature heating method was selected, with temperatures set at 333°C and 355°C, each for 72 hours, and a confining pressure maintained at 10,000 psi. The high confining pressure not only prevented the gold tube from rupturing during pyrolysis but also transferred plastic deformation to the rock cylinder sample, simulating the static rock pressure experienced by underground shale. After the hot-pressure simulation experiment, the autoclave was removed from the pyrolysis furnace, cooled, and the gold tube containing the rock cylinder was recovered. Subsequent testing and analysis were performed after confirming that the gold tube had not leaked during the experiment.

[0026] S103: Perform CT scanning on rock columns at different maturity stages to obtain image slices of shale at different maturity stages; based on the shale image slices, perform three-dimensional reconstruction of the morphology of the shale overpressure micro-fracture network therein to obtain a 3D rendering.

[0027] Optionally, based on the shale image slices, the morphology of the shale overpressure microfracture network is three-dimensionally reconstructed to obtain a 3D rendering, specifically including: spatially converting the shale bedding planes of the rock columns in the shale image slices at different maturity stages into two-dimensional planes parallel to the XY plane; volumetrically cropping the spatially converted shale image slices; performing interactive threshold segmentation and interactive brightness segmentation on the volumetrically cropped shale image slices to extract microfractures whose grayscale and brightness of the rock columns in the image are different from those of other materials; removing the portion of the segmented shale image slices whose pixels are smaller than a preset value; performing microfracture parameter analysis and statistics on the shale image slices after pixel removal and marking them; and performing 3D volume rendering on the microfractures in the marked shale image slices to obtain a 3D rendering.

[0028] Specifically, after the hot-pressure simulation experiment, the original sample and two heated rock cylinder samples were subjected to X-ray micro-nano CT scanning (micro-CT, ZEISS Xradia 520 versa). It is worth noting that once the fracture network begins to form in large quantities, the rock cylinder sample is easily broken, so before the gold tube is fully opened, AB glue needs to be injected in advance to strengthen the rock skeleton. For centimeter-scale rock cylinder samples, the scanning volume is about 240 mm³, the spatial resolution is 6.15 microns, the energy is 70 kV, and about 1,000 two-dimensional image slices can be generated for each sample. Based on these image slices, Avizo or dragonfly software is used to perform three-dimensional reconstruction of the overpressure microfracture network morphology in shale. In order to distinguish other relatively low X-ray materials (such as kerogen, matrix pores and imaging artifacts), we further proposed a data processing scheme using multiple steps to achieve effective segmentation of microfractures in shale samples. For the specific process, see Figure 3 , including the following steps: The CT data from the original and simulated cylindrical samples were imported into Avizo software, and the data volumes were spatially transformed using the Transformation Editor tool to ensure that the shale bedding plane was parallel to the XY plane. Use the Crop Editor tool to crop the data volume into a cylindrical or rectangular shape, and then crop the sub-volume data to the appropriate size based on the research needs. We focus on the development of micro-cracks at the micron to millimeter scale, so we recommend a sub-volume of approximately 20-30 mm3 in advance. Interactive Thresholding and Interactive Top-Hat are used to effectively extract microcracks that are distinguished from other materials by grayscale and brightness, and then the merged images (Or Image) are selected. Use the Remove Small Spots function to remove image regions with pixels smaller than 1000 pixels. This is primarily due to resolution issues, making it difficult to identify regions smaller than 1000 pixels as microcracks or other matrix pores. Although this process results in the loss of microcrack data typically less than 20 microns thick (including all shrinkage microcracks), it ensures the quality and reliability of the analyzed microcrack data.

[0029] Next, label analysis was performed on the data volume. The parameters of the microcracks, including length (Length3d, maximum Feret diameter), thickness (Thickness3d, the largest line segment touching the object endpoints, located on a plane perpendicular to both the maximum 3D Feret diameter and the width 3D diameter), pore volume (Volume3d, defined as voxel count: cx×cy×cz), inclination (90°–OrientationPhi, in degrees [0, +90°]), and shape factor (Shape_Va3d, defined as [area3d]³ / (36×π×[volume3d]²), which is equal to 1 for a perfect sphere), were analyzed and statistically recorded one by one.

[0030] Finally, perform 3D volume rendering on the filtered and marked microcracks, save the image, and export the analyzed microcrack data.

[0031] S104: Evaluate the evolution of the overpressure microfracture network induced by hydrocarbon generation by analyzing the number of parallel bedding fractures, the number of perpendicular bedding fractures, the microfracture width, and the microfracture pore volume at different maturity stages in the 3D renderings.

[0032] Optionally, microcracks with an inclination angle less than or equal to a preset angle to the bedding plane are defined as parallel bedding fractures; microcracks with an inclination angle greater than a preset angle to the bedding plane or perpendicular to the bedding plane are defined as vertical bedding fractures.

[0033] Specifically, most bedding-parallel microfractures have dip angles close to horizontal (parallel to the XY plane) (equal to 90° – OrientationPhi), typically less than 5°. However, some subparallel microfractures may have dip angles ranging from 5° to 10° due to the influence of nodules or other factors, and a few may even have dips as high as 10° to 20°. Therefore, for ease of description, we propose to define fractures with dip angles ≤ 20° as bedding-parallel fractures (or horizontal microfractures), while other fractures perpendicular to the bedding plane or at relatively high angles (dip angles > 20°) are defined as perpendicular bedding fractures. This method is suitable for the three-dimensional characterization and quantitative evaluation of microfracture networks in shales, typically tens to hundreds of micrometers in width and hundreds of micrometers to several millimeters in length. It complements the currently popular high-resolution field emission scanning electron microscopy technique, which uses even more microscopic (primarily nanoscale) techniques to observe the development of matrix pore networks in shales.

[0034] Specifically, Figure 4The microfracture network characteristics of the organic-rich shale in the Chang 73 section before and after thermal simulation are presented. In the original sample, a small number of bedding-parallel microfractures (number = 13 / 29.35 mm³) were observed. These initial fractures have a maximum width of tens of micrometers and generally range in length from tens of micrometers to several millimeters. These fractures are primarily interpreted as shrinkage joints, related to the decompression and dehydration of the core sample after removal from the subsurface. In contrast, during the simulated peak oil generation, due to hydrocarbon generation and fluid overpressure, the number of bedding-parallel fractures in the shale sample rapidly increased to 459 / 25.70 mm³. The maximum width of the microfractures reached hundreds of micrometers, and the microfracture pore volume increased from 2.98×107 μm³ in the original sample to 1.32×109 μm³. At a higher maturity stage, vertical bedding fractures begin to appear in shale samples, forming a three-dimensional fracture network with parallel bedding fractures, promoting large-scale hydrocarbon expulsion and reducing fluid pressure within the rock skeleton, resulting in the number of microfractures decreasing to 229 / 21.62 mm3 and the microfracture pore volume decreasing to 8.79 × 108 μm3.

[0035] The technical features of the above embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of the present invention.

[0036] The above is a method for evaluating the evolution process of an overpressure microcrack network provided by one or more embodiments of the present invention. Based on the same idea, the present invention also provides a corresponding XX device, such as Figure 5 shown.

[0037] Figure 5 A schematic diagram of an overpressure microcrack network evolution process evaluation device provided by the present invention, comprising: The sample collection module 501 is used to collect shale samples in the study area; a hollow drill bit is used to drill parallel or perpendicular to the bedding plane of the shale sample through a drilling machine, and the sample is polished with emery to obtain a small rock column; Experimental module 502 is used to load the rock cylinder into a gold tube reactor and seal it, then load the sealed gold tube reactor into an autoclave; set experimental parameters, conduct heating experiments at different maturity stages in a closed system on the stainless steel autoclave, and apply confining pressure using a water pump during the heating test; after the heating test, remove the autoclave from the pyrolysis furnace, cool it, and recover the rock cylinders in the gold tube reactor to obtain rock cylinders at different maturity stages; The data analysis module 503 is used to perform CT scanning on rock columns at different maturity stages to obtain shale image slices at different maturity stages; based on the shale image slices, the morphology of the shale overpressure microfracture network therein is three-dimensionally reconstructed to obtain a 3D rendering; and by analyzing the changes in the three-dimensional geometric parameters of the 3D rendering at different maturity stages, the evolution process of the overpressure microfracture network induced by hydrocarbon generation is evaluated.

Claims

1. A method for evaluating the evolution process of an overpressure microcrack network, characterized in that: include: Collect shale samples from the study area; The shale sample is drilled parallel or perpendicular to the bedding plane using a hollow drill bit on a drilling machine and polished with emery to obtain a small rock column; The rock cylinder is placed into a gold tube reactor and sealed, and the sealed gold tube reactor is placed into an autoclave; Experimental parameters were set, and heating experiments at different maturity stages were carried out in a closed stainless steel autoclave. Confining pressure was applied using a water pump during the heating test. After the heating test, the autoclave was removed from the pyrolysis furnace, cooled, and the rock cylinders in the gold tube reactor were recovered to obtain rock cylinders at different maturity stages. CT scans were performed on rock columns at different maturity stages to obtain image slices of shale at different maturity stages. Based on the shale image slices, the morphology of the shale overpressure micro-fracture network was three-dimensionally reconstructed to obtain a 3D rendering. The evolution of the overpressure microfracture network induced by hydrocarbon generation was evaluated by analyzing the number of parallel bedding fractures, the number of perpendicular bedding fractures, the width of microfractures, and the pore volume of microfractures at different maturity stages in the 3D renderings.

2. The method for evaluating the evolution process of an overpressure microcrack network according to claim 1, wherein: The rock cylinders used in the heating test were drilled from the same bedding plane of the same shale sample.

3. The method for evaluating the evolution process of an overpressure microcrack network according to claim 1, wherein: The confining pressure is transferred to the rock column through the plastic deformation of the gold tube reactor, simulating the static pressure experienced by underground shale.

4. The method for evaluating the evolution process of an overpressure microcrack network according to claim 1, wherein: The method of performing three-dimensional reconstruction of the shale overpressure micro-fracture network morphology based on shale image slices to obtain a 3D rendering specifically includes: The shale bedding plane space of the rock column in the shale image slices at different maturity stages is converted into a two-dimensional plane parallel to the XY plane; Volume cropping is performed on the spatially transformed shale image slices; Interactive threshold segmentation and interactive brightness segmentation are performed on the volume-cropped shale image slices to extract the grayscale and brightness of the rock columns in the image, which are different from the micro-cracks composed of other materials. Removing the portion of the shale image slice whose pixels are smaller than a preset value after segmentation; Perform micro-fracture parameter analysis and statistics on the shale image slices after pixel removal and mark them; 3D volume rendering is performed on the micro-fractures in the marked shale image slices to obtain a 3D rendering image.

5. The method for evaluating the evolution process of an overpressure microcrack network according to claim 1, wherein: defining microcracks with an inclination angle less than or equal to a preset angle to the bedding plane as parallel bedding fractures; Microcracks that are at an angle greater than a preset angle to the bedding plane or are perpendicular to the bedding plane are defined as vertical bedding cracks.

6. An evaluation device for the evolution process of an overpressure microcrack network, characterized in that: include: Sample collection module, used to collect shale samples in the study area; The shale sample is drilled parallel or perpendicular to the bedding plane using a hollow drill bit on a drilling machine and polished with emery to obtain a small rock column; The experimental module is used to load the rock cylinder into the gold tube reactor and seal it, and then load the sealed gold tube reactor into the autoclave; Experimental parameters were set, and heating experiments at different maturity stages were carried out in a closed stainless steel autoclave. Confining pressure was applied using a water pump during the heating test. After the heating test, the autoclave was removed from the pyrolysis furnace, cooled, and the rock cylinders in the gold tube reactor were recovered to obtain rock cylinders at different maturity stages. The data analysis module is used to perform CT scanning on rock columns at different maturity stages to obtain image slices of shale at different maturity stages. Based on the shale image slices, the morphology of the shale overpressure microfracture network is reconstructed in three dimensions to obtain a 3D rendering. By analyzing the changes in the three-dimensional geometric parameters of the 3D rendering at different maturity stages, the evolution process of the overpressure microfracture network induced by hydrocarbon generation is evaluated.

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