CN117094043B

CN117094043B - Coal powder migration evaluation method and system based on 3D printing simulation coal seam hole fracture model

Info

Publication number: CN117094043B
Application number: CN202311057664.7A
Authority: CN
Inventors: 王博洋; 付晓飞; 柳波; 潘哲君; 李晶晶; 白龙辉; 迟亚奥
Original assignee: Northeast Petroleum University
Current assignee: Northeast Petroleum University
Priority date: 2023-08-22
Filing date: 2023-08-22
Publication date: 2024-02-27
Anticipated expiration: 2043-08-22
Also published as: CN117094043A

Abstract

The application relates to a pulverized coal migration evaluation method and system based on a 3D printing simulation coal seam hole fracture model, which are used for acquiring fracture and pore characteristic parameters of a raw coal sample of a coal seam to be simulated; based on the crack and pore characteristic parameters of the raw coal sample, a 3D printing technology is utilized to manufacture a simulated coal seam hole crack model; and carrying out a coal powder migration experiment on the manufactured simulated coal seam hole fracture model to obtain a coal powder migration evaluation result. The method realizes the visual characterization of the migration and sedimentation rule of the coal powder in the hole and the crack, and can repeat experiments.

Description

Coal powder migration evaluation method and system based on 3D printing simulation coal seam hole fracture model

Technical Field

The application belongs to the technical field of coalbed methane exploitation, and particularly relates to a coal powder migration evaluation method and system based on a 3D printing simulation coalbed hole fracture model.

Background

Coal fines refer to solid phase particulates that exist, remain or migrate in the reservoir, channel and drainage equipment systems during the development of coal bed gas. In view of the fact that coal beds have special mechanical properties (lower elastic modulus and higher poisson ratio), the problem of solid-phase particle production mainly of 'coal dust' penetrates through the whole coal bed methane development process, and is one of key factors for restricting coal bed methane development. How to solve the problem of reservoir damage caused by coal dust migration is a precondition for guaranteeing the efficient and stable drainage and production of a coal-bed gas well.

In the prior art, the following methods are available for researching coal dust: the patent with application number 201610320259.3 adopts an artificial coal brick mode to carry out coal dust generation and migration simulation experiments, and has the defects that the structure of a raw coal body is destroyed, and the visual representation of the coal dust in the migration process is difficult to realize. The patent with the application number of 202220188755.9 adopts two circular connecting plates and five groups of pipelines to simulate a fracture channel, and has the defect that the actual coal seam pore form is not considered, and only the migration rule of coal dust in a fracturing fracture can be simulated. The patent application numbers 202111440213.2 and 201210521199.3 adopts a rotary spiral positioner to regulate and control the width of the artificial fracture, and the defects are the same as the defects, and only the migration condition of coal dust in the fracture can be considered. The patent application numbers 201310055324.0 and 201510501351.5 adopts a real rock core from a coal bed to simulate coal dust generation, migration, sedimentation and blocking, and has the defect that the coal sample cannot be reused, so that experimental results of the same hole and crack combination under different fluid conditions cannot be compared.

At present, some achievements are obtained in three aspects of coal dust output influencing factors and factors, coal dust output characteristic differences under different geology and engineering conditions and influence of coal dust output on the productivity of a gas well. However, research on coal fines migration and sedimentation laws is very weak. The coal reservoir is taken as a black box, and although the visual characterization of the change of the aperture in the coal powder transportation process can be realized by adopting the means of nuclear magnetic online imaging, CT and the like, the cost is high, the precision is poor, the sample cannot be repeated, and the research requirement is difficult to meet; some scholars simulate the migration of the pulverized coal in the cracks by adopting means of a flat model, artificial joint making and the like, and often neglect an important factor of pores, so that the pulverized coal migration mechanism and sedimentation rule are not clear.

Disclosure of Invention

In view of the above analysis, the present invention aims to provide a pulverized coal migration evaluation method and system based on a 3D printing simulated coal seam hole fracture model, which are used for solving one or more of the above problems in the prior art.

The purpose of the invention is realized in the following way:

on the one hand, the coal powder migration evaluation method based on the 3D printing simulation coal seam hole fracture model comprises the following steps:

step S1: acquiring characteristic parameters of cracks and pores of a raw coal sample of a coal bed to be simulated;

step S2: based on the crack and pore characteristic parameters of the raw coal sample, a 3D printing technology is utilized to manufacture a simulated coal seam hole crack model;

step S3: and carrying out a coal powder migration experiment on the manufactured simulated coal seam hole fracture model to obtain a coal powder migration evaluation result.

Further, in step S1, acquiring a fracture characteristic parameter of a raw coal sample of a coal seam to be simulated, including the following steps:

preparing a raw coal sample of a coal bed to be simulated into a square coal sample with the length of 1cm multiplied by 1cm, and acquiring crack development characteristic parameters with the crack width of more than 10 mu m in a three-dimensional space of the square coal sample by a micrometer CT scanner, wherein the crack three-dimensional development characteristic parameters with the crack width of more than 10 mu m comprise crack width, crack quantity and crack development angle;

The cube coal sample is provided with a symmetrical plane perpendicular to a bedding plane, the cube coal sample is divided into a first half coal sample and a second half coal sample which are symmetrical along the symmetrical plane, argon ion polishing is carried out on the longitudinal section of the first half coal sample or the longitudinal section of the second half coal sample to obtain a polished surface to be scanned, two-dimensional imaging scanning is carried out on the polished surface by utilizing a focused ion beam-scanning electron microscope in an ultra-large area scanning mode, the number of cracks with the slit width of more than 200nm in the polished surface of 1cm multiplied by 1cm is obtained, and the number of cracks with the slit width of more than 200nm in the area range of 400 mu m multiplied by 400 mu m at the center of the polished surface is obtained;

and defining a center area of 400 mu m multiplied by 400 mu m at the center of the polished surface of the square coal sample, and carrying out two-dimensional imaging scanning on the center area by utilizing a focused ion beam-scanning electron microscope in an ultra-high precision scanning mode to obtain the number of cracks with the slit width larger than 4nm in the range of the center area of 400 mu m multiplied by 400 mu m.

Further, the micrometer CT scanner acquires the number X of cracks with the seam width larger than 10 mu m on a symmetrical plane of 1cm multiplied by 1cm of the square coal sample; the number Y of cracks with the seam width larger than 200nm on a polished surface of 1cm multiplied by 1cm obtained in the ultra-large area scanning mode; a ratio a of the number of cracks with a slit width of more than 10 mu m to the number of cracks with a slit width of 200nm-10 mu m; a=x/(Y-X); the number Y' of slits with a slit width of 200nm-10 μm in the range of 400 μm×400 μm of the center area of the polished surface obtained in the ultra-large area scanning mode; the number of cracks with the slit width of 4nm-10 μm in the range of 400 μm multiplied by 400 μm at the center of the polishing surface obtained in the ultra-high precision scanning mode is Z; the ratio of the number of cracks with the slit width of 200nm-10 mu m to the number of cracks with the slit width of 4nm-200nm in the range of 400 mu m multiplied by 400 mu m of the center area of the polishing surface is b; b=y '/(Z-Y'); the number of all cracks with the seam width larger than 10 mu m in the three-dimensional space of the square coal sample is N, and the number of the cracks with the seam width of 200nm-10 mu m in the three-dimensional space of the square coal sample is N/a; the number of cracks with the seam width of 4nm-200nm in the three-dimensional space of the square coal sample is N/(a multiplied by b).

Further, in step S2, when the 3D printing technology is used to manufacture the simulated coal seam hole fracture model, the internal fracture angle of the simulated coal seam hole fracture model is determined according to the following rule: the crack development angle with the crack width larger than 10 μm is directly according to the space development angle obtained by the micron CT scanner; setting a crack development angle of 200nm-10 mu m of the slit width by referring to a statistical average value of included angles between a crack of 200nm-10 mu m of the slit width and a crack of more than 10 mu m of the slit width in a scanning image obtained in an ultra-large area scanning mode; the crack development angle of the slit width of 4nm-200nm is set by referring to the statistical average value of the included angles between the slit width of 4nm-200nm and the slit width of more than 10 mu m in the scanning image obtained in the ultra-high precision scanning mode.

Further, in step S1, the pore morphology and pore size of the raw coal sample are obtained through nitrogen adsorption experiments; the pore morphology comprises: class I holes are open air holes; the II type holes are air holes with one end closed; class III holes are narrow necked bottle holes; in step S2, when the 3D printing technology is utilized to manufacture the simulated coal seam hole fracture model, the type of the internal pores of the simulated coal seam hole fracture model is randomly set.

Further, in step S3, a coal powder migration experiment is sequentially performed on the plurality of the same simulated coal seam hole fracture models manufactured by adopting different fluid conditions; in the experimental process, monitoring the concentration and granularity of coal dust in produced liquid, the permeability of a simulated coal seam hole fracture model and the inlet and/or outlet end pressure of a visual core holder, acquiring sedimentation pictures of the coal dust in the simulated coal seam hole fracture model under different fluid conditions, and acquiring the statistical data of the coal dust sedimentation area occupation ratio based on the sedimentation pictures; and establishing a coal powder output judging template based on statistical data of the coal powder concentration and the coal powder granularity in the produced liquid, the permeability of the simulated coal seam hole fracture model, the inlet and/or outlet end pressure of the visual core holder and the coal powder sedimentation area occupation ratio.

Further, the fluid condition parameters include flow rate, mineralization, and pH.

Further, coal powder migration experiments are carried out by adopting coal powder with different characteristics so as to obtain the influence relationship of the coal powder characteristics on the coal powder migration; the coal dust characteristics include particle size, degree of coal deterioration and mineral composition.

Further, determining coal dust output influence factor indexes and coal dust output judgment indexes; the coal powder output influencing factors comprise a plurality of crack characteristic parameters, pore characteristic parameters, mineral composition and coal deterioration degree of a raw coal sample, and flow rate, mineralization degree, pH and temperature in the experimental process; the coal powder output judging indexes comprise coal powder concentration W, holding pressure M, permeability damage rate P and coal powder sedimentation area occupation ratio S; and training the coal powder output influence factor index data serving as a training data set by adopting the training data set to obtain a prediction model, wherein the prediction model is used for obtaining a target coal powder output judgment index of a coal reservoir to be predicted according to the coal powder output influence factor index of the coal reservoir to be predicted.

On the other hand, a pulverized coal migration evaluation system based on a 3D printing simulation coal seam hole fracture model is provided, and the pulverized coal migration evaluation system based on the 3D printing simulation coal seam hole fracture model is applied to the pulverized coal migration evaluation method based on the 3D printing simulation coal seam hole fracture model; the pulverized coal migration evaluation system comprises a simulated coal seam hole fracture model, a visual core clamping device, a pulverized coal suspension injection device, a real-time high-speed imaging device, a confining pressure device and a pulverized coal-containing fluid collection device;

the visual core clamping device is provided with a transparent visual core clamping device, the core clamping device is used for clamping a simulated coal seam hole fracture model, the core clamping device is provided with an inlet and an outlet which are axially arranged, the inlet is communicated with an injection port of the pulverized coal suspension injection device through an injection pipe, and the injection pipe is provided with a pressure sensor and a first flowmeter; the outlet is connected with a liquid outlet pipe;

the real-time high-speed imaging device is used for shooting pictures of migration and sedimentation conditions of coal powder in the simulated coal seam hole fracture model or recording the migration process of the coal powder;

the confining pressure device is used for increasing confining pressure of the core holder so as to be used for simulating pulverized coal migration conditions under the set confining pressure;

the pulverized coal-containing fluid collecting device comprises a filtering component, a second flowmeter and a measuring cylinder, wherein the filtering component and the second flowmeter are arranged on the liquid outlet pipe, and the measuring cylinder is positioned below the liquid outlet of the liquid outlet pipe.

Further, the device also comprises a temperature control device, and the temperature control device can heat and cool the simulated coal seam hole fracture model in the core holder.

Further, the temperature control device comprises a refrigerator, a heater, a circulating guide pipe and a temperature sensor; the simulated coal seam hole fracture model is sleeved with a flexible first transparent sleeve, a rigid second transparent sleeve is further arranged in the core holder, the diameter of the second transparent sleeve is larger than that of the first transparent sleeve and smaller than that of the core holder, the second transparent sleeve, the first transparent sleeve and the core holder are coaxially arranged, an annular space between the first transparent sleeve and the second transparent sleeve is a confining pressure space, and an annular space between the second transparent sleeve and the core holder is a temperature adjusting space; the confining pressure space is connected with a confining pressure device; the refrigerator and the heater are communicated with a heating space in the core holder through a circulation flow guide pipe, a circulation pump is arranged on the circulation flow guide pipe, circulating liquid is fluorine oil, and the temperature sensor is arranged in the confining pressure space.

Further, the heater is communicated with the temperature adjusting space through a heating branch pipeline, the refrigerator is communicated with the temperature adjusting space through a cooling branch pipeline, the heating branch pipeline and the cooling branch pipeline are all communicated with the temperature adjusting space to form a pipeline for fluorine oil circulation, a first circulating pump and a second circulating pump are respectively arranged on the heating branch pipeline and the cooling branch pipeline, and valves are arranged on the heating branch pipeline and the cooling branch pipeline to control the communication working state of the branch pipeline and the temperature adjusting space.

Further, the device also comprises a movement track, wherein a driver is arranged on the movement track, and the real-time high-speed imaging device can move 360 degrees along the movement track under the action of the driver; the movement track is of a circular structure, the movement track is sleeved outside the visual core holder, and the center line of the movement track coincides with the axis of the visual core holder.

Compared with the prior art, the invention has at least one of the following beneficial effects:

a) According to the coal powder migration evaluation method based on the 3D printing simulation coal seam hole fracture model, provided by the invention, the basic hole fracture characteristics of the coal reservoir are reserved by utilizing a 3D printing technology, meanwhile, the migration and sedimentation characteristics of coal powder under the action of different fluids can be visually represented, the research result can provide practical basis for optimizing the development engineering design of coal bed methane and solving the solid-phase particle output problem, and the coal powder migration evaluation method is expected to promote and establish a coal reservoir protection effect evaluation method and a reservoir protection technology aiming at China, so that a beneficial hint is provided for large-scale and efficient development of coal bed methane in China.

b) According to the pulverized coal migration evaluation method based on the 3D printing simulation coal seam hole fracture model, a machine learning method is adopted, a prediction model is obtained through training the model, and a prediction evaluation result can be obtained quickly by inputting the influence factor index into the prediction model; the coal powder output influencing factors comprehensively consider the crack characteristic parameters, the pore characteristic parameters, the mineral composition and the coal deterioration degree of the raw coal sample, and the flow rate, the mineralization degree, the pH value and the temperature in the experimental process, so that the prediction result is more scientific and the result is more reliable; the pore fracture characteristic parameters comprehensively consider the proportion of the number of the cracks in the range of different crack widths to the number of all cracks, and adopt the fractal dimension of the pores to represent the pore characteristic parameters.

c) The pulverized coal migration evaluation system based on the 3D printing simulation coal seam hole fracture model provided by the invention can be used for carrying out repeated experimental research on different fluid conditions (flow velocity, mineralization degree, pH value and the like) and pulverized coal characteristics (granularity, coal with different deterioration degrees and the like) on the basis of utilizing a 3D printing technology to manufacture the simulation coal seam hole fracture model which is the same as or similar to the original coal sample hole fracture.

d) According to the pulverized coal migration evaluation system based on the 3D printing simulation coal seam hole fracture model, the circulating transparent fluorine oil is adopted to heat and cool the simulation coal seam hole fracture model, so that the temperature adjustment speed is high, real-time shooting of a camera is not affected, and the problem that a core holder needs to be taken out from an incubator to be shot again after traditional heating, so that temperature errors occur and experimental results are affected is avoided.

e) According to the pulverized coal migration evaluation system based on the 3D printing simulation coal seam hole fracture model, the real-time high-speed imaging device is arranged on the moving track, and can move in a range of 360 degrees around the axis of the visual core holder in the shooting process, so that the inside of the visual core holder can be shot at multiple angles, further simulated coal seam hole fracture model pictures with different shooting angles are obtained, when the shot images are processed in the later stage, the shot pictures with different angles can be comprehensively considered, and the result of acquiring the pulverized coal sedimentation area ratio is more accurate.

Drawings

In order to more clearly illustrate the embodiments of the present description or the technical solutions in the prior art, the drawings that are required in the embodiments or the description of the prior art will be briefly described below, and it is obvious that the drawings in the following description are only some embodiments described in the embodiments of the present description, and other drawings may be obtained according to these drawings for a person having ordinary skill in the art.

FIG. 1 is an operation flow chart of a pulverized coal migration evaluation method based on a 3D printing simulation coal seam hole fracture model;

FIG. 2 is a schematic representation of a multi-scale fracture characterization obtained using a micrometer CT and a focused ion beam-scanning electron microscope according to the present invention;

FIG. 3 is a photograph of simulated coal seam hole fracture model coal fines migration and settlement characteristics under different fluid conditions provided by the invention;

FIG. 4 is a schematic diagram showing the change of pulverized coal concentration and holding pressure along with the flow rate generated by a simulated coal seam hole fracture model under the action of differential fluid provided by the invention;

FIG. 5 is a schematic structural diagram of a pulverized coal migration evaluation system based on a 3D printing simulation coal seam hole fracture model, which is provided by the invention;

FIG. 6 is a schematic view of the cross-sectional structure A-A of FIG. 5;

fig. 7 is a schematic structural diagram of a camera provided by the invention and arranged on a moving track;

FIG. 8 is a schematic diagram of a 3D printing model with different types of hole and fissures scaled down equally.

Reference numerals:

1-simulating a coal seam hole fracture model; 2-visualizing the core holder; 3-a water suction pump; 4-piston stirrer; 5-a first pressure sensor; 6-a first flowmeter; 7-a video camera; 8-a surrounding pressure pump; 9-a filter assembly; 10-a second flowmeter; 11-measuring cylinder; 12-a second pressure sensor; 13-a freezer; 14-a heater; 15-circulating guide pipes; 16-a temperature sensor; 17-a first transparent sleeve; 18-a second transparent sleeve; 19-confining pressure space; 20-temperature adjusting space; 21-heating the branch line; 22-cooling branch pipelines; 23-a first valve; 24-a second valve; 25-motion track; 26-driver.

Detailed Description

For the purposes of making the objects, technical solutions and advantages of the embodiments of the present application more clear, the technical solutions of the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application, and it is apparent that the described embodiments are some embodiments of the present application, but not all embodiments. It should be noted that embodiments and features of embodiments in the present disclosure may be combined, separated, interchanged, and/or rearranged with one another without conflict. All other embodiments, which can be made by one of ordinary skill in the art without undue burden from the present disclosure, are within the scope of the present disclosure.

In the drawings, the size and relative sizes of elements may be exaggerated for clarity and/or descriptive purposes. While the exemplary embodiments may be variously implemented, the specific process sequences may be performed in a different order than that described. For example, two consecutively described processes may be performed substantially simultaneously or in reverse order from that described. Moreover, like reference numerals designate like parts.

The research of the applicant finds that the coal reservoir has pores with various different forms, the migration and sedimentation conditions of coal powder in the pores with different forms are not known at present, and particularly the influence of the pore combination with different forms on the migration of the coal powder is yet to be revealed; moreover, due to the influence of wettability, the dispersion and agglomeration conditions of the pulverized coal in different fluids are different, and the same pulverized coal in different fluids may cause different migration and sedimentation phenomena in the same pore fracture structure, so that the existing method cannot study the migration mechanism of the pulverized coal in the pore fracture under the action of different fluids. In addition, because the coal seam has extremely strong heterogeneity, even if parallel samples are adopted, the complete consistency of the hole and crack between samples is difficult to ensure, so that the migration condition of coal powder in the same hole and crack combination under the action of different fluids is difficult to realize if the research is required.

Example 1

Based on the problems, the invention provides a specific embodiment, and discloses a coal dust migration evaluation method based on a 3D printing simulation coal seam hole fracture model, the operation flow is shown in fig. 1, and the coal dust evaluation method comprises the following steps:

step S3: and carrying out a coal dust migration experiment on the manufactured simulated coal seam hole fracture model by using a coal dust migration evaluation system to obtain a coal dust migration evaluation result.

According to the evaluation method, a 3D printing technology is adopted to manufacture a simulated coal seam hole fracture model which is the same as or similar to the original coal sample hole fracture, and simulation of migration and settlement conditions of coal powder under different displacement conditions can be achieved.

In this embodiment, in step S1, acquiring a fracture characteristic parameter of a raw coal sample of a coal seam to be simulated includes the following steps: preparing a raw coal sample of a coal bed to be simulated into a square coal sample with the length of 1cm multiplied by 1cm, and acquiring crack development characteristic parameters with the crack width of more than 10 mu m in a three-dimensional space of the square coal sample by a micrometer CT scanner, wherein the crack three-dimensional development characteristic parameters with the crack width of more than 10 mu m comprise crack width, crack quantity and crack development angle; the cube coal sample is provided with a symmetrical plane perpendicular to a bedding plane, the cube coal sample is divided into a first half coal sample and a second half coal sample which are symmetrical along the symmetrical plane, argon ion polishing is carried out on a longitudinal section of the first half coal sample or the second half coal sample to obtain a polished surface to be scanned, two-dimensional imaging scanning is carried out on the polished surface in an oversized scanning mode by utilizing a focused ion beam-scanning electron microscope, the scanning area in the oversized scanning mode is 1cm multiplied by 1cm, the scanning precision is 200nm, the number of cracks with the slit width being larger than 200nm in the area of 1cm multiplied by 1cm of the polished surface is obtained, and the number of cracks with the slit width being larger than 200nm in the area range of 400 mu m multiplied by 400 mu m at the center of the polished surface is obtained; and defining a center area of 400 mu m multiplied by 400 mu m at the center of the polished surface of the square coal sample, performing two-dimensional imaging scanning on the center area by utilizing a focused ion beam-scanning electron microscope in an ultra-high precision scanning mode, and obtaining the number of cracks with the slit width larger than 4nm in the range of the center area of 400 mu m multiplied by 400 mu m by the scanning area of 400 mu m in the ultra-high precision scanning mode. Alternatively, a focused ion beam-scanning electron microscope model feiheios 5CX was used. A schematic representation of multi-scale crack characterization obtained using micro CT and focused ion beam-scanning electron microscopy is shown in fig. 2. Fig. 2 shows scanning accuracy in different modes of a micro CT and a focused ion beam-scanning electron microscope, for example, the micro CT scanning accuracy is 10 μm, the focused ion beam-scanning electron microscope ultra-large area scanning mode accuracy is 200nm, and the ultra-high accuracy scanning mode accuracy is 4nm. FIG. 2 shows the scanned areas of different means and the identifiable cracks with different slit widths, for example, the micro CT can obtain the crack plane with the slit width larger than 10 μm and the three-dimensional development number, the large-area scanning mode can obtain the crack plane development number with the slit width larger than 200nm, and the ultra-high-precision scanning mode can obtain the crack plane development number with the slit width larger than 4nm. It is worth to say that the proportions of the cracks with different dimensions obtained at this time are all obtained under the same area, for example, the ratio of the number of cracks with the slit width of more than 10 μm to the number of cracks with the slit width of 200nm-10 μm is the statistical quantitative ratio carried out under the same area of 1cm multiplied by 1 cm; the ratio of the number of slits having a slit width of 200nm to 10 μm to the number of slits having a slit width of 4nm to 200nm is a statistical ratio of the number performed under the same area of 400 μm×400 μm. According to the scanning results of a micrometer CT scanner and a focused ion beam-scanning electron microscope, calculating and obtaining the number of cracks with the slit width larger than 10 mu m, the number of cracks with the slit width of 200nm-10 mu m and the number of cracks with the slit width of 4nm-200nm in a three-dimensional space of a square coal sample, thereby adopting a 3D printing technology to establish a full-scale crack development model, and the established simulated coal seam hole crack model has similar crack development conditions as those of the raw coal sample. Specifically, the micrometer CT scanner acquires the number X of cracks with the seam width larger than 10 mu m on a 1cm multiplied by 1cm symmetrical plane of a square coal sample; the number Y of cracks with the seam width larger than 200nm on a polished surface of 1cm multiplied by 1cm obtained in the ultra-large area scanning mode of a focused ion beam-scanning electron microscope; a ratio a of the number of cracks with a slit width of more than 10 mu m to the number of cracks with a slit width of 200nm-10 mu m; a=x/(Y-X); a slit number Y' of 200nm-10 μm in a slit width in a range of 400 μm×400 μm in a center area of a polished surface obtained in an ultra-large area scanning mode of a focused ion beam-scanning electron microscope; the number of slits with a slit width of 4nm-10 μm in the range of 400 μm×400 μm at the center of the polished surface obtained in the ultra-high precision scanning mode of the focused ion beam-scanning electron microscope is Z; the ratio of the number of cracks with the slit width of 200nm-10 mu m to the number of cracks with the slit width of 4nm-200nm in the range of 400 mu m multiplied by 400 mu m of the center area of the polishing surface is b; b=y '/(Z-Y'); the number of all cracks with the seam width larger than 10 mu m in the three-dimensional space of the square coal sample is N, and the number of the cracks with the seam width of 200nm-10 mu m in the three-dimensional space of the square coal sample is N/a; the number of cracks with the seam width of 4nm-200nm in the three-dimensional space of the square coal sample is N/(a multiplied by b).

The applicant found that the nano-scale cracks often develop along with the main cracks above the micro-scale, so in the preparation of the simulated coal seam hole crack model in this embodiment, when 3D printing parameters are set, the cracks with the slit width of 200nm-10 μm and the crack development angles with the slit width of 4nm-200nm and the crack development angles with the slit width of more than 10 μm are determined by referring to the scan images obtained by the practical focused ion beam-scanning electron microscope. That is, in step S2, when the simulated coal seam hole fracture model is manufactured by using the 3D printing technology, the internal fracture angle of the simulated coal seam hole fracture model is determined according to the following rule: the crack development angle with the crack width larger than 10 μm is directly according to the space development angle obtained by the micron CT scanner; setting a statistical average value of included angles between a crack with a crack width of 200nm-10 mu m and a crack with a crack width of more than 10 mu m in a scanning image obtained by referring to a focused ion beam-scanning electron microscope; the crack development angle of the slit width of 4nm-200nm is set by referring to the statistical average value of the included angles between the slit width of 4nm-200nm and the slit width of more than 10 mu m in the scanning image obtained by the focused ion beam-scanning electron microscope.

In the ultra-large area scanning mode, the scanning area of the focused ion beam-scanning electron microscope is large (1 cm multiplied by 1 cm), and the scanning precision is low (200-250 nm); in the ultra-high precision scanning mode, the scanning area is small (400 μm×400 μm-0.16 mm×0.16 mm), and the scanning precision is high (4 nm-20 nm).

Although the ultra-high precision scanning mode can scan a large area, the scanning time is long, tens of times higher, and the cost is high, which is not practical. Therefore, in this embodiment, the focused ion beam-scanning electron microscope is used to perform two-dimensional imaging scanning on the polished surface in the ultra-large area scanning mode, the number of cracks with the slit width larger than 200nm in the area range of 400 μm×400 μm at the center of the polished surface is obtained first, then the number of cracks with the slit width larger than 200nm in the area range of 400 μm×400 μm at the center of the polished surface is obtained directly in the scanning result, and then the ultra-high precision scanning mode is used to perform two-dimensional imaging scanning on the area range of 400 μm×400 μm at the center of the polished surface, so as to obtain the number of cracks with the slit width larger than 4nm, thereby improving the data result obtaining efficiency and greatly reducing the cost.

In order to explore the influence of different pore type combinations and different pore sizes on coal dust migration, the practical coal reservoir pore development situation is more intuitively simulated, and in the embodiment, the pore type and pore morphology characteristics are defined according to the adsorption and desorption curve morphology of a coal rock sample by carrying out a nitrogen adsorption experiment on the raw coal sample. Specifically, in step S1, the pore morphology and pore size of the raw coal sample development are obtained through a nitrogen adsorption experiment; the pore morphology comprises class I pores, class II pores and class III pores; wherein, the I-type holes are open air holes, and can generate adsorption loop, comprising four parallel plate holes with four open sides and two cylindrical holes with two open ends; the II type holes are cylindrical holes with one closed end, parallel plate-shaped holes, wedge-shaped holes and conical holes, and one closed end is not provided with ventilation holes, and no adsorption loop is generated; the III-class hole is a narrow-necked bottle hole, also called an ink bottle hole, and the desorption branch has a sharp descending inflection point. In step S2, when the 3D printing technology is utilized to manufacture the simulated coal seam hole fracture model, the type of the internal pores of the simulated coal seam hole fracture model is randomly set. And (3) preparing simulated coal seam pore models of different types of pores and combinations thereof based on a 3D printing technology, and establishing the simulated coal seam pore models with pore development conditions which are highly similar to those of raw coal samples, so that the coal dust migration experiment is more practical, and the experiment can be repeated.

Further, considering that coal seam hole cracks can be reduced under the action of different stratum pressures, simulated coal seam hole crack models with different aperture holes and different seam width cracks can be manufactured, and coal powder migration simulation under different confining pressures can be realized.

In this embodiment, in step S3, the raw coal is crushed to obtain pulverized coal, and a vibrating screen is used to screen pulverized coal particles with different particle sizes, such as 60-80 mesh, 80-120 mesh, 120-200 mesh, 200-400 mesh, 400-800 mesh, and 800-1000 mesh. The background solution takes the analysis result of the water quality of the stratum water as a proportioning standard, the reference index comprises mineralization degree and ion type, and the influence of different fluid actions on the coal dust migration is considered, and the mineralization degree, pH and flow velocity are taken as variables.

In the embodiment, in step S3, a coal powder migration experiment is sequentially performed on a plurality of identical simulated coal seam hole fracture models manufactured by adopting different fluid conditions; among the fluid condition parameters are flow rate, mineralization and pH. In the experimental process, monitoring the concentration and granularity of coal powder in the produced liquid, the permeability of a simulated coal seam hole fracture model and the pressure of an inlet and/or outlet end of a core holder, and shooting the simulated coal seam hole fracture model in real time to obtain the migration and sedimentation pictures of the coal powder in the simulated coal seam hole fracture model under different fluid conditions, wherein the picture is shown in fig. 3; obtaining statistical data of coal dust settling area occupation ratio based on the settling photos; and establishing a coal powder output judging template based on statistical data of coal powder concentration and coal powder granularity in the produced liquid, simulated coal seam hole fracture model permeability damage rate, inlet and/or outlet end pressure of the core holder and coal powder settling area occupation ratio.

In one alternative embodiment, coal fines with different characteristics are adopted for coal fines migration experiments so as to obtain the influence relationship of the coal fines characteristics on the coal fines migration; the coal dust characteristics include particle size, degree of coal deterioration and mineral composition.

In order to obtain the coal powder migration evaluation result more accurately and rapidly, the coal powder migration evaluation method in the embodiment adopts a machine learning method, a prediction model is obtained by training the model, and the influence factor index is input into the prediction model to obtain the prediction evaluation result rapidly. Specifically, determining a coal dust output influence factor index and a coal dust output judgment index; the coal powder output influencing factors comprise a plurality of crack characteristic parameters, pore characteristic parameters, mineral composition and coal deterioration degree of a raw coal sample, and flow rate, mineralization degree, pH and temperature in the experimental process; the coal powder output judging indexes comprise coal powder concentration W, holding pressure M, permeability damage rate P and coal powder sedimentation area occupation ratio S; and training the coal powder output influence factor index data serving as a training data set by adopting the training data set to obtain a prediction model, wherein the prediction model is used for obtaining a target coal powder output judgment index of a coal reservoir to be predicted according to the coal powder output influence factor index of the coal reservoir to be predicted. In actual production, inputting coal dust output influence factor index data of a target area coal reservoir to be predicted into a trained prediction model to obtain coal dust concentration W, holding pressure M, permeability damage rate P and coal dust sedimentation area occupation ratio S of coal dust discharged from the target area coal reservoir to be predicted. FIG. 4 is a schematic diagram showing the change of the pulverized coal concentration and the holding pressure with the flow rate generated by the simulated coal seam hole fracture model under the action of the differential fluid.

Wherein the fracture characteristic parameters comprise the proportion of the number of the fractures with different width ranges to the number of all the fractures, such as: the number of cracks with the width of 4nm-200nm is equal to the ratio alpha, the number of cracks with the width of 200nm-10 mu m is equal to the ratio beta, and the number of cracks with the width of more than 10 mu m is equal to the ratio gamma. The pore characteristic parameter is characterized by pore fractal dimension, which is a parameter for measuring the complexity of a pore structure and is a dimensionless number. In general, the greater the fractal dimension of the pore, the greater the complexity of the pore system and the more complex the spatial distribution of the pore. The embodiment comprehensively considers the proportion of the number of the cracks in the width range of different cracks to the number of all the cracks and adopts the fractal dimension of the pores to represent the characteristic parameters of the pores, and has the advantages that when the influence of the pore characteristics on the migration and the settlement of coal dust is evaluated, the defect that the pore characteristics are represented only by the fractal dimension in the conventional method is overcome, the proportion of the cracks in different scales is a more direct factor for influencing the migration and the settlement of the coal dust, and the evaluation result is more accurate.

In this embodiment, a machine learning algorithm is used as classifier training data to build a prediction model, optionally, a ReLU function (modified linear unit) is used as an activation function, a gradient optimization algorithm is used to iteratively update the weights and biases in the neural network to achieve the optimum, and a root mean square error analysis model is used for error analysis.

In the experimental process, the real-time high-speed imaging device is utilized to acquire the sedimentation pictures of the pulverized coal in the model under different conditions in real time, and imageJ software can be adopted to process the sedimentation pictures of the pulverized coal in the model, and the method comprises the following specific steps: firstly, determining the number of pixel points of each length unit according to SetScale, converting a sedimentation photo into an 8-bit gray scale, carrying out Threshold segmentation by adopting Threshold, removing Noise points by Noise-Despeckle, and determining the coal dust sedimentation area occupation ratio S by Analyzepartics. For an exemplary coal dust yield determination template in this embodiment, see table 1.

Table 1 coal dust yield judging template

In the table above, clay mineral content (a), concentration of injected coal fines (B), pore fractal dimension (C), fracture characteristic parameter (D), flow rate (E), mineralization degree (F), temperature (T), coal deterioration degree (G), pH value (pH); the concentration (W) of the produced liquid pulverized coal, the holding pressure (M), the permeability damage rate (P) and the pulverized coal settling area occupation ratio (S); a1 is the clay mineral content set for the first experiment, an is the nth, B1 is the quartz content set for the first experiment, bn is the nth, and so on.

Compared with the prior art, the pulverized coal migration evaluation method based on the 3D printing simulation coal seam hole fracture model provided by the embodiment at least can realize one of the following beneficial effects:

1. The basic pore characteristics of the coal reservoir are reserved by utilizing a 3D printing technology, the influence experiments of the condition changes such as different material composition pulverized coal, different flow rate conditions, different fluid properties, different temperatures and the like on the migration and sedimentation of different types of pore pulverized coal can be carried out, and the experiments can be repeated; meanwhile, the migration and sedimentation conditions of the pulverized coal in the model under different conditions can be observed according to the real-time high-speed imaging device.

2. By adopting a machine learning method, a prediction model is obtained by training the model, and a prediction evaluation result can be obtained rapidly by inputting the influence factor index into the prediction model.

3. The coal powder output influencing factors comprehensively consider the crack characteristic parameters, the pore characteristic parameters, the mineral composition and the coal deterioration degree of the raw coal sample, and the flow rate, the mineralization degree, the pH value and the temperature in the experimental process, so that the prediction result is more scientific and the result is more reliable.

4. The pore fracture characteristic parameters comprehensively consider the proportion of the number of cracks in the range of different crack widths to the number of all cracks, and adopt the fractal dimension of the pores to represent the pore fracture characteristic parameters, and the method has the advantages that when the influence of pore fracture characteristics on coal powder migration and settlement is evaluated, the defect that the pore characteristics are represented only by the fractal dimension in the conventional method is overcome, the proportion of the cracks in different scales is a more direct factor influencing coal powder migration and settlement, and the evaluation result is more accurate.

Example 2

5-7, a pulverized coal migration evaluation system based on a 3D printing simulation coal seam hole fracture model is disclosed, and comprises a simulation coal seam hole fracture model 1, a pulverized coal suspension injection device, a visual core clamping device, a real-time high-speed imaging device, a confining pressure device and a pulverized coal-containing fluid collection device;

wherein, the simulated coal seam hole fracture model 1 is manufactured by using a 3D printing technology, and the manufacturing steps are as shown in the embodiment 1; the 3D printing material for manufacturing the simulated coal seam hole fracture model 1 is high-temperature-resistant transparent photosensitive resin, can bear the high temperature below 100 ℃, and is finished by adopting an SLA light curing 3D printing technology. And obtaining different types of pores (I type pores, II type pores and III type pores) of raw coal through a nitrogen adsorption experiment, taking a single pore type, two-two combination or three-three combination mode as a prototype, and carrying out different types of pore depiction by adopting a 3D printing technology. For the convenience of displacement experiments, the specification of the simulated coal seam pore model is a cylinder with the diameter of 2.5cm multiplied by the length of 4.5cm, and the simulated coal seam pore model can also be of a cuboid structure. In view of the increase of effective stress caused by the decrease of fluid pressure in the drainage process of an actual coal seam, in this embodiment, because the adopted high-temperature-resistant transparent photosensitive resin does not have the hole fracture compressibility of the actual coal seam, in order to simulate the compression characteristics of the hole fracture under the effective stress, the embodiment performs 3D printing on different types of pores or fractures in an equal-proportion reduction mode, for example, referring to fig. 8, the reduction multiple is 0.5 times and 0.25 times based on the hole fracture size of a model one, and from fig. 8, it can be seen that the model one represents the original hole fracture size before the coal seam is not drained, the one-way drainage stage is accompanied by the increase of the drainage degree, the fluid pressure is reduced, so that the hole fracture of the coal seam is compressed, the multiple of the hole fracture compression and the drainage accumulation time are in an exponential positive correlation, and the hole fracture in the later stage of the one-way drainage stage is assumed to be compressed to be 0.5 times, and the hole fracture in the later stage of the one-way drainage stage may be compressed to be 0.25 times. The coal bed gas drainage and production process in the unidirectional drainage stage is accompanied with coal dust production, and the model in the figure can be used for simulation of coal dust migration and sedimentation under the condition of considering hole fracture compression in different periods in the unidirectional drainage stage of the coal bed gas well. And paving quartz sand in different types of pores and combinations thereof, wherein the granularity of the quartz sand is paved in a mode of matching with the pore size, and paving the quartz sand can form small pores by taking the whole pore as a standard, so that coal dust can be settled when passing through.

The pulverized coal suspension injection device comprises a water suction pump 3 and a piston stirrer 4. The stirring function of the pulverized coal suspension injection device is achieved through the water suction pump 3 and the piston stirrer 4, the water suction pump 3 is used for injecting stratum water solution into the piston stirrer 4, the piston stirrer 4 comprises a container barrel, a piston and a stirrer, the piston is arranged in the container barrel, an upper cavity of the piston stirring container is communicated with an outlet of the water suction pump 3, a lower cavity of the piston stirring container is provided with a liquid injection port, the liquid injection port is communicated with an axial inlet of the core holder, the axis of the liquid injection port is coincident with the axis of the container barrel, and the stirrer is connected with the bottom of the lower cavity of the container barrel.

The visual core holder comprises a transparent visual core holder 2, which may be referred to below simply as core holder. The core holder is used for holding the 3D printed simulated coal seam hole fracture model 1, after the simulated coal seam hole fracture model 1 is installed in the core holder, a flexible first transparent sleeve 17 is sleeved outside the simulated coal seam hole fracture model 1, the first transparent sleeve 17 can separate fluid media forming confining pressure, the confining pressure is not influenced on the side peripheral wall of the simulated coal seam hole fracture model 1, and a confining pressure space 19 is formed between the first transparent sleeve 17 sleeved outside the simulated coal seam hole fracture model 1 and the inner wall of the core holder; the core holder is provided with an inlet and an outlet which are axially arranged, an injection port of the pulverized coal suspension injection device is communicated with the inlet of the core holder through an injection pipe, the outlet of the core holder is connected with a liquid outlet pipe, and a first pressure sensor 5 and a first flowmeter 6 are arranged on the injection pipe. Optionally, the visual core holder 2 is provided with observation glass, or the visual core holder 2 is integrally made of transparent materials such as toughened glass or Parm board;

The real-time high-speed imaging device comprises a camera 7, a camera interface and a computer with built-in program-controlled shooting software, wherein the camera 7 is an imaging camera with 300 ten thousand pixels or higher pixels and is used for shooting pictures of migration and sedimentation conditions of coal dust in pores and cracks of the simulated coal seam hole crack model 1 or recording the migration process of the coal dust.

The confining pressure device comprises a confining pressure pump 8 and a confining pressure sensor, the confining pressure pump 8 is communicated with a confining pressure space 19 of the rock core holder through a confining pressure pipe, the confining pressure sensor is arranged on the confining pressure pipe, fluid media are injected into the confining pressure space 19 of the rock core holder through the confining pressure pump 8 to increase confining pressure of the rock core holder, and pulverized coal migration conditions under different confining pressures can be simulated.

The pulverized coal-containing fluid collecting device comprises a filtering component 9, a second flowmeter 10 and a measuring cylinder 11, wherein the filtering component 9 and the second flowmeter 10 are arranged on a liquid outlet pipe connected with an outlet of the core holder, and the measuring cylinder 11 is positioned below a liquid outlet of the liquid outlet pipe; a second pressure sensor 12 is also provided on the outlet pipe for detecting the fluid pressure at the outlet of the core holder. The produced coal powder firstly passes through the filter assembly 9 along with the liquid, the coal powder is left in the filter element when passing through the filter assembly 9, and then the mass of the coal powder in the filter element is weighed by an electronic balance after being dried; the separated liquid is passed through a measuring cylinder 11 to measure its volume produced.

In order to quantitatively characterize the influence of coal powder migration on the model permeability and the like, the second pressure sensor 12 is used for monitoring the concentration of the produced coal powder and the holding pressure at the outlet end of the core holder. Wherein, the concentration (W) of the coal powder in the produced liquid can indirectly reflect the migration scale of the coal powder, and the higher the value is, the larger the migration scale of the coal powder is; the build-up pressure (M) can indirectly reflect the change condition of the flow conductivity of the model caused by the sedimentation of the pulverized coal, the larger the value of the build-up pressure (M) is, the larger the flow conductivity of the model is reduced, the nature of the build-up pressure is equal to the permeability damage rate (P), the build-up pressure is adopted to evaluate the damage degree of the reservoir when no fluid passes through, and otherwise, the damage degree of the reservoir is evaluated by the aid of the build-up pressure.

The inventors found that: the increase of temperature may cause a series of physical-chemical changes of minerals, such as dehydration, expansion and decomposition of clay minerals, spalling and crushing generated by quartz phase transition, increase of the composition of pulverized coal particles, and complicating the suspension and sedimentation rules; when the temperature is increased to a certain degree, new cracks are generated in the reservoir, and a fluid migration channel is increased; the physicochemical properties of the pulverized coal particles are changed at different temperatures, so that the dispersion and agglomeration conditions of the pulverized coal particles are influenced. The temperature is therefore critical for studying the migration of coal fines in the reservoir. However, in the related research of the existing pulverized coal migration, only the pulverized coal migration and sedimentation caused by the fluid pressure change are often considered, and the influence of a cooling process on the pulverized coal migration is considered by fresh students, particularly in the hydraulic fracturing process, a large amount of pulverized coal is generated when a coal bed is broken, and the hydraulic fracturing is actually a cooling process. The experimental displacement device in the prior art can only realize temperature rise, but can not realize rapid cooling, so that the influence of the cooling process on coal powder migration can not be realized. Based on the above, in this embodiment, the pulverized coal migration evaluation system based on the 3D printing simulated coal seam hole fracture model further includes a temperature control device, where the temperature control device can implement heating and cooling of the simulated coal seam hole fracture model 1 in the core holder.

Specifically, the temperature control device includes a refrigerator 13, a heater 14, a circulation flow guide pipe 15, and a temperature sensor 16, in this technical solution, as shown in fig. 6, a temperature adjusting space 20 is further provided outside the confining pressure space 19, and the confining pressure space 19 and the temperature adjusting space 20 are separated by a rigid second transparent sleeve 18, that is, the diameter of the second transparent sleeve 18 is larger than the diameter of the first transparent sleeve 17 and smaller than the diameter of the core holder, the second transparent sleeve 18, the first transparent sleeve 17, and the core holder are coaxially arranged, and the confining pressure space 19 and the temperature adjusting space 20 are annular spaces; the confining pressure space 19 is connected with a confining pressure device; the refrigerator 13 and the heater 14 are communicated with a heating space in the core holder through a circulating guide pipe 15, circulating liquid is fluorine oil, a circulating pump is arranged on the circulating guide pipe 15, and a temperature sensor 16 is arranged in a confining pressure space 19. The circulation flow guide pipe 15 comprises a heating branch pipeline 21 and a cooling branch pipeline 22, the heater 14 is communicated with the temperature adjusting space 20 through the heating branch pipeline 21, the refrigerator 13 is communicated with the temperature adjusting space 20 through the cooling branch pipeline 22, the heating branch pipeline 21 and the cooling branch pipeline 22 are all connected with the temperature adjusting space 20 to form pipelines for circulating fluorine oil, a first circulation pump and a second circulation pump are respectively arranged on the heating branch pipeline 21 and the cooling branch pipeline 22, the heater and the refrigerator are both provided with liquid storage containers for storing fluorine oil, a first valve 23 and a second valve 24 are respectively arranged on the heating branch pipeline 21 and the cooling branch pipeline 22, and the communication working states of the heating branch pipeline 21, the cooling branch pipeline 22 and the temperature adjusting space 20 are controlled through the first valve 23 and the second valve 24. The circulating fluorine oil is heated through the heater 14, the circulating fluorine oil is cooled through the refrigerator 13, the heating and cooling processes of the fluorine oil are independently controlled, the temperature in the confining pressure space 19 can be monitored in real time through the temperature sensor 16, and then the temperature detection of the simulated coal seam hole fracture model 1 is indirectly realized. Compared with the traditional mode of heating the core holder in the incubator, the embodiment adopts circulated transparent fluorine oil to heat and cool the simulated coal seam hole fracture model 1, so that the temperature adjustment speed is high, the real-time shooting of the camera 7 can not be influenced, the situation that the core holder needs to be taken out from the incubator to be shot again after traditional heating is avoided, temperature errors occur, and experimental results are influenced.

It should be noted that, in the present embodiment, the visualized core holder 2 and the second transparent sleeve 18 may be made of parm board, so as to ensure that no melting deformation occurs at 100 ℃, and of course, other transparent materials in the prior art may be used to meet the experimental requirements. The first transparent tube is made of transparent and flexible material, such as high temperature resistant transparent plastic film sleeve.

In the prior art, the imaging device adopted by the coal powder migration experimental device is fixedly arranged, only single-angle shooting can be realized, corresponding data statistics can be carried out on one surface, but the migration and sedimentation process of the coal powder in the hole cracking is a three-dimensional space. Therefore, the average value of the coal dust sedimentation area ratio obtained by the photos shot at multiple angles can reflect the actual situation, and the coal dust migration evaluation system based on the 3D printing simulation coal seam hole fracture model in the embodiment further comprises a moving track 25, as shown in fig. 7, a driver 26 is arranged on the moving track 25, and the real-time high-speed imaging device can move 360 degrees along the moving track 25 under the action of the driver 26. The movement track 25 is of a circular ring structure, the movement track 25 is sleeved outside the visual core holder 2, and the center line of the movement track 25 coincides with the axis of the visual core holder 2. The circulation flow guide pipe 15 is arranged outside the movement track 25, so that the movement of the high-speed imaging device is not influenced, and real-time shooting in the heating and cooling processes can be realized. According to the embodiment, the real-time high-speed imaging device is arranged on the moving track 25, and can move in a range of 360 degrees around the axis of the visual core holder 2 in the shooting process, so that the inside of the visual core holder 2 can be shot at multiple angles, further simulated coal seam hole fracture model pictures with different shooting angles are obtained, when the shot images are processed at the later stage, the shot pictures with different angles can be comprehensively considered, and the result of acquiring the coal dust sedimentation area occupation ratio is more accurate.

Compared with the prior art, the pulverized coal migration evaluation system based on the 3D printing simulation coal seam hole fracture model has at least one of the following beneficial effects:

1. on the basis of utilizing a 3D printing technology to manufacture a simulated coal seam hole fracture model which is the same as or similar to the raw coal sample hole fracture, repeated experimental researches on different fluid conditions (flow velocity, mineralization degree, pH value and the like) and coal powder characteristics (granularity, different coal deterioration degree and the like) can be carried out.

2. The simulated coal seam hole fracture model is heated and cooled by adopting the circulating transparent fluorine oil, so that the temperature adjusting speed is high, the real-time shooting of a camera is not influenced, the problem that a core holder needs to be taken out from an incubator after traditional heating and then shooting is carried out, temperature errors occur, and experimental results are influenced is avoided.

3. Through setting up real-time high-speed image device on the motion track, can move around the 360 within range of axes of visual rock core holder in shooting process, can multiple angles like this shoot the inside of visual rock core holder, and then obtain the emulation coal seam hole crack model picture of different shooting angles, when later stage is handled the shooting image, can take the picture by different angles into consideration comprehensively for obtain the coal dust settlement area and occupy more accurate result.

The foregoing embodiments have been provided for the purpose of illustrating the general principles of the present application, and are not meant to limit the scope of the invention, but to limit the scope of the invention.

Claims

1. The coal dust migration evaluation method based on the 3D printing simulation coal seam hole fracture model is characterized by comprising the following steps of:

step S3: carrying out a coal powder migration experiment on the manufactured simulated coal seam hole fracture model to obtain a coal powder migration evaluation result;

in step S1, acquiring a fracture characteristic parameter of a raw coal sample of a coal seam to be simulated, including the following steps:

defining a center area of 400 mu m multiplied by 400 mu m at the center of the polished surface of the square coal sample, and carrying out two-dimensional imaging scanning on the center area by utilizing a focused ion beam-scanning electron microscope in an ultra-high precision scanning mode to obtain the number of cracks with the slit width larger than 4nm in the range of the center area of 400 mu m multiplied by 400 mu m;

the micrometer CT scanner acquires the number X of cracks with the seam width larger than 10 mu m on a symmetrical plane of 1cm multiplied by 1cm of a square coal sample; the number Y of cracks with the seam width larger than 200nm on a polished surface of 1cm multiplied by 1cm obtained in the ultra-large area scanning mode; a ratio a of the number of cracks with a slit width of more than 10 mu m to the number of cracks with a slit width of 200nm-10 mu m; a=x/(Y-X);

The number Y' of slits with a slit width of 200nm-10 μm in the range of 400 μm×400 μm of the center area of the polished surface obtained in the ultra-large area scanning mode; the number of cracks with the slit width of 4nm-10 μm in the range of 400 μm multiplied by 400 μm at the center of the polishing surface obtained in the ultra-high precision scanning mode is Z; the ratio of the number of cracks with the slit width of 200nm-10 mu m to the number of cracks with the slit width of 4nm-200nm in the range of 400 mu m multiplied by 400 mu m of the center area of the polishing surface is b; b=y '/(Z-Y');

the number of all cracks with the seam width larger than 10 mu m in the three-dimensional space of the square coal sample is N, and the number of the cracks with the seam width of 200nm-10 mu m in the three-dimensional space of the square coal sample is N/a; the number of cracks with the seam width of 4nm-200nm in the three-dimensional space of the square coal sample is N/(a multiplied by b);

in the step S3, adopting different fluid conditions to sequentially perform coal powder migration experiments on a plurality of manufactured same simulated coal seam hole fracture models;

in the experimental process, monitoring the concentration and granularity of coal dust in produced liquid, the permeability of a simulated coal seam hole fracture model and the inlet and/or outlet end pressure of a visual core holder, acquiring sedimentation pictures of the coal dust in the simulated coal seam hole fracture model under different fluid conditions, and acquiring the statistical data of the coal dust sedimentation area occupation ratio based on the sedimentation pictures;

Establishing a coal powder output judging template based on statistical data of coal powder concentration and coal powder granularity in the produced liquid, the permeability of a simulated coal seam hole fracture model, inlet and/or outlet end pressure of a visual core holder and coal powder sedimentation area occupation ratio;

determining coal powder output influence factor indexes and coal powder output judgment indexes; the coal powder output influencing factors comprise a plurality of crack characteristic parameters, pore characteristic parameters, mineral composition and coal deterioration degree of a raw coal sample, and flow rate, mineralization degree, pH and temperature in the experimental process; the coal powder output judging indexes comprise coal powder concentration W, holding pressure M, permeability damage rate P and coal powder sedimentation area occupation ratio S;

and training the coal powder output influence factor index data serving as a training data set by adopting the training data set to obtain a prediction model, wherein the prediction model is used for obtaining a target coal powder output judgment index of a coal reservoir to be predicted according to the coal powder output influence factor index of the coal reservoir to be predicted.

2. The pulverized coal migration evaluation method based on the 3D printing simulation coal seam hole fracture model according to claim 1, wherein in step S2, when the simulation coal seam hole fracture model is manufactured by using the 3D printing technology, the internal fracture angle of the simulation coal seam hole fracture model is determined according to the following rule:

The crack development angle with the crack width larger than 10 μm is directly according to the space development angle obtained by the micron CT scanner;

setting a crack development angle of 200nm-10 mu m of the slit width by referring to a statistical average value of included angles between a crack of 200nm-10 mu m of the slit width and a crack of more than 10 mu m of the slit width in a scanning image obtained in an ultra-large area scanning mode;

the crack development angle of the slit width of 4nm-200nm is set by referring to the statistical average value of the included angles between the slit width of 4nm-200nm and the slit width of more than 10 mu m in the scanning image obtained in the ultra-high precision scanning mode.

3. The coal dust migration evaluation method based on the 3D printing simulation coal seam hole fracture model according to claim 1, wherein in the step S1, the pore morphology and the pore size of the raw coal sample development are obtained through a nitrogen adsorption experiment; the pore morphology comprises: class I holes are open air holes; the II type holes are air holes with one end closed; class III holes are narrow necked bottle holes;

in step S2, when the 3D printing technology is utilized to manufacture the simulated coal seam hole fracture model, the type of the internal pores of the simulated coal seam hole fracture model is randomly set.

4. The method for evaluating coal fines migration based on the 3D printing simulated coalbed methane fracture model according to claim 1, wherein in step S3, the fluid condition parameters include flow rate, mineralization degree, and pH.

5. The coal dust migration evaluation method based on the 3D printing simulation coal seam hole fracture model according to claim 1, wherein coal dust migration experiments are carried out by adopting coal dust with different characteristics so as to obtain the influence relationship of the coal dust characteristics on the coal dust migration;

the coal dust characteristics include particle size, degree of coal deterioration and mineral composition.

6. A pulverized coal migration evaluation system based on a 3D printing simulation coal seam hole fracture model, which is characterized by being applied to the pulverized coal migration evaluation method based on the 3D printing simulation coal seam hole fracture model according to any one of claims 1 to 5;

the pulverized coal migration evaluation system comprises a simulated coal seam hole fracture model, a visual core clamping device, a pulverized coal suspension injection device, a real-time high-speed imaging device, a confining pressure device and a pulverized coal-containing fluid collection device;