CN111028094B - Equivalent TOC calculation method based on shale scanning electron microscope image extraction - Google Patents
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Abstract
The embodiment of the invention discloses an equivalent TOC (total organic carbon) calculation method based on shale scanning electron microscope image extraction, which comprises the following steps: processing a sample, scanning and imaging, and preprocessing an electron microscope image; threshold segmentation, namely dividing the elements of the shale sample electron microscope image into four components; identifying and correcting the white edge, and extracting the content of an organic matter framework; the method has the advantages that the shale organic matter skeleton content is evaluated by a digital image processing method, the equivalent TOC content of the shale is calculated, the shale reservoir gas storage capacity and the shale reservoir transport capacity can be accurately evaluated, meanwhile, the method is higher in universality, shale samples of different types and different electron microscope scanning imaging qualities can be researched, the research is facilitated, in addition, the method is short in measurement period, simple in processing process and capable of greatly reducing the test cost.
Description
Technical Field
The embodiment of the invention relates to the technical field of petroleum and natural gas geological exploration, in particular to an equivalent TOC (total organic carbon) calculation method based on shale scanning electron microscope image extraction.
Background
Shale gas has a huge reserve as a clean unconventional energy source. The extraction of the shale pore structure characteristics is the basis for carrying out accurate reserve evaluation and pore network migration capability calculation and is also the key point of shale gas exploration and development research. The shale has complex content of components and developed pore structure, and the main components of the shale are organic matters, inorganic matters and pyrite. Organic pores and organic cracks develop in the organic matters; inorganic pores and inorganic cracks develop in the inorganic substance.
The TOC content of shale is typically obtained by combustion testing and well logging. These are TOC analyses based on field tests, which are not only highly inaccurate, but also, more importantly, have a high cost and a long measurement period. Shale structure qualitative observation and quantitative characterization based on scanning electron microscope images are hot spots which are formed in recent years. The observation of a scanning electron microscope also reaches enough precision, the shale pore structure can be directly subjected to statistical evaluation, and information contained in the image can be effectively quantitatively extracted based on a reasonable image processing technology.
However, the existing method for extracting the TOC content of the shale has the following defects:
(1) the existing extraction method for the TOC content of shale has inaccurate extraction result and insufficient effective information mining, cannot accurately evaluate the gas storage capacity and the transportation capacity of a shale reservoir, and brings great inaccuracy to research work;
(2) the existing shale TOC content extraction method is low in applicability, cannot be used for researching shale samples of different types and different electron microscope scanning imaging qualities, and is not beneficial to research;
(3) the existing method for extracting the TOC content of the shale has the disadvantages of long measurement period, complex treatment process and high cost, and causes resource waste.
Disclosure of Invention
Therefore, the equivalent TOC calculation method based on shale scanning electron microscope image extraction is provided by the embodiment of the invention, the method adopts a digital image processing method to evaluate the shale organic matter skeleton content, and calculates the equivalent TOC content of shale, so that the shale reservoir gas storage capacity and transport capacity can be accurately evaluated, meanwhile, the method has higher universality, can be used for researching shale samples of different types and different electron microscope scanning imaging qualities, and is beneficial to research, in addition, the method has the advantages of short measurement period, simple processing process and lower cost, can greatly save manpower and material resources, and can effectively solve the problems in the prior art.
In order to achieve the above object, the embodiments of the present invention provide the following technical solutions: an equivalent TOC calculation method based on shale scanning electron microscope image extraction comprises the following steps:
s100, processing a sample, scanning and imaging, and preprocessing an electron microscope image;
s200, performing threshold segmentation, and dividing elements of the shale sample electron microscope image into four components;
s300, identifying and correcting the 'white edge', and extracting the content of an organic matter framework;
s400, converting the two-dimensional plane parameters into equivalent three-dimensional parameters by using ascending dimension conversion, and calculating the equivalent TOC content of the shale sample.
Further, in step S100, the method specifically includes:
s101, selecting a sample, and performing sample cutting, surface grinding and polishing treatment to obtain a sample slice with a size required by the observation of a scanning electron microscope;
s102, obtaining a series of scanning electron microscope images of the shale sample with a representative volume scale through scanning electron microscope observation;
and S103, preprocessing the image, cutting off the black edge of the image, and filtering the electron microscope image.
Further, in step S103, the method specifically includes:
extracting four vertex angles of a rectangular region with the image gray value not being 0, reserving the region in the rectangle, and cutting out the region outside the rectangle;
and eliminating the non-smoothness of the shape scanning of the cut image by mean filtering, and then eliminating the salt and pepper noise of the image by median filtering.
Further, the step of performing mean filtering on the removed image specifically includes: generating a 3 × 3 (or 5 × 5) template, calculating an average value of 8 (or 24) pixels (excluding the target point itself) around the target point, replacing the pixel value of the target point with the average value, and then sequentially calculating the average of all pixel points of the image to obtain a filtered image;
the median filtering performed on the removed image specifically includes: and generating a 3 × 3 (or 5 × 5) template, solving a median value of 8 (or 24) pixels (excluding the target point) around the target point, replacing the pixel value of the target point with the median value, and then solving the median values of all pixel points of the image in sequence to obtain the filtered image.
Further, in step S200, the method specifically includes:
s201, selecting a representative image in the cut images, namely an image containing more pores, organic matters, inorganic matters and pyrite;
s202, respectively adjusting and distinguishing three threshold values of the four components by using a binarization method, namely a pore threshold value, a segmentation threshold value of organic matters and inorganic matters and a segmentation threshold value of inorganic matters and pyrite;
and S203, after the three thresholds are obtained, performing threshold segmentation on the scanning electron microscope image of the sample, and dividing elements of each image into four components, namely holes, organic matters (including holes), inorganic matters (including holes) and pyrite.
Further, in step S300, the identification and correction of the white edge specifically includes:
s301, sequentially marking each communicated pyrite region in the sample image, counting the area of each pyrite region, and extracting the boundary coordinate, the barycentric coordinate, the long axis length, the short axis length and the equivalent radius of each pyrite region;
s302, calculating the ratio of the long axis to the short axis of each pyrite block;
s303, counting the average gray value of 9 pixel points in the 3 multiplied by 3 area size near the center of gravity point;
s304, if the ratio of the long axis to the short axis of the pyrite block is larger than 10 or the average gray value of 9 pixel points near the gravity center is smaller than the pyrite threshold, determining that the pyrite block is actually the interference of the 'white edge' at the periphery of the hole seam, and judging the real composition of the 'white edge'.
Further, when judging the real composition of the "white border":
finding out a minimum rectangle containing the block according to the boundary coordinates, and giving out coordinates of four vertexes of the rectangle;
expanding the equivalent radius size outwards from the extracted minimum rectangle to obtain a new rectangle as the neighborhood of the hole;
when a certain outwards expanded neighborhood edge exceeds the boundary, inwards shrinking according to the step length of 3 pixels until the shrinking of the boundary-free range is not exceeded for the first time, and the shrinking processes of all edges are independent;
counting the neighborhoods of the communicating blocks, and respectively calculating the content of organic matters and inorganic matters in each neighborhood, particularly the content of the organic matters and the inorganic matters in the region contacted with the pyrite region;
judging the real composition of the white edge according to the relative content of the organic matter component and the inorganic matter component in the neighborhood, if the organic matter content is high, considering the white edge as the organic matter, and filling the white edge area as the gray value of the organic matter; if the inorganic matter content is high, the real component of the white edge is considered to be inorganic matter, and the white edge area is filled with the gray value of the inorganic matter.
Further, in step S300, the specific steps of extracting the content of the organic framework are as follows:
s305, extracting and counting the number of pixels of which all gray values are greater than a pore threshold value and less than a basic threshold value in the corrected shale sample electron microscope image;
s306, counting the number N of the pixels in the S305 o Multiplying the actual area value of the single pixel representation to obtain the area S occupied by organic matters of the shale sample o ；
S o ＝N o ×S single
S307, counting the total pixel number N of the sample s Multiplying the single pixel representation actual area value to serve as the shale sample actual area S s 。
S s ＝N s ×S single
Further, in step S400, the transformation specifically includes:
the area parameter is converted according to the following formula:
wherein s is p Is a two-dimensional area, S v Is the three-dimensional equivalent area.
Further, in step S400, when calculating the equivalent TOC content of the shale sample:
s401, calculating the three-dimensional equivalent organic matter framework area of the organic matter framework, and dividing the three-dimensional equivalent organic matter framework area by the total area of the three-dimensional equivalent sample to obtain the three-dimensional equivalent organic matter framework content gamma o ；
γ o ＝S v,o /S v,s ＝s p,o /s p,s ＝N o /N s
S402, calculating the three-dimensional equivalent TOC content of the shale sample according to the following formula:
where ρ is k For the density of kerogen, 1.05g/cm is generally taken 3 ，ρ r The density of the shale is generally 2.55g/cm 3 ，ω k The carbon content of the kerogen is 70-85% generally.
The embodiment of the invention has the following advantages:
(1) according to the method, the content of the shale organic matter framework is evaluated by adopting a digital image processing method, and the equivalent TOC content of the shale is calculated, so that the gas storage capacity and the transportation capacity of a shale reservoir can be accurately evaluated;
(2) the method has higher universality, can be used for researching shale samples of different types and different electron microscope scanning imaging qualities, and is beneficial to research;
(3) the invention has short measuring period and simple processing process, and can greatly reduce the testing cost.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below. It should be apparent that the drawings in the following description are merely exemplary, and that other embodiments can be derived from the drawings provided by those of ordinary skill in the art without inventive effort.
FIG. 1 is a schematic view of the overall flow structure of the present invention;
Detailed Description
The present invention is described in terms of particular embodiments, other advantages and features of the invention will become apparent to those skilled in the art from the following disclosure, and it is to be understood that the described embodiments are merely exemplary of the invention and that it is not intended to limit the invention to the particular embodiments disclosed. All other embodiments, which can be obtained by a person skilled in the art without making any creative effort based on the embodiments in the present invention, belong to the protection scope of the present invention.
As shown in fig. 1, the invention provides an equivalent TOC calculation method based on shale scanning electron microscope image extraction, which comprises the following steps:
and S100, processing a sample, scanning and imaging, and preprocessing an electron microscope image.
In step S100, the method specifically includes:
and S101, selecting a sample, and performing sample cutting, surface grinding and polishing treatment to obtain a sample slice with the size required by the observation of a scanning electron microscope.
And S102, observing through a scanning electron microscope to obtain a series of scanning electron microscope images of the shale sample with a representative volume scale.
Cutting the obtained shale core into blocks of 1cm multiplied by 0.5cm, selecting a surface of 1cm multiplied by 0.5cm as an observation surface, grinding, and putting into argon ion polishing equipment for polishing. After polishing, the sample is placed in an observation bin of a scanning electron microscope device for scanning electron microscope observation, and a (REV) scanning electron microscope image reaching a representative size is obtained, wherein the size of the image in this embodiment is 398.5 μm × 398.5 μm.
And S103, preprocessing the image, cutting off the black edge of the image, and filtering the electron microscope image.
In step S103, the method specifically includes:
extracting four vertex angles of a rectangular region with the image gray value not being 0, reserving the region in the rectangle, and cutting out the region outside the rectangle; and eliminating the non-smoothness of the shape scanning of the cut image by mean filtering, and then eliminating the salt and pepper noise of the image by median filtering.
The mean filtering of the clipped image specifically includes: and generating a 3 × 3 (or 5 × 5) template, calculating an average value of 8 (or 24) pixels (excluding the target point itself) around the target point, replacing the pixel value of the target point with the average value, and then sequentially calculating the average value of all pixel points of the image to obtain the filtered image.
The median filtering of the clipped image specifically includes: and generating a 3 × 3 (or 5 × 5) template, solving a median value of 8 (or 24) pixels (excluding the target point) around the target point, replacing the pixel value of the target point with the median value, and then solving the median values of all pixel points of the image in sequence to obtain the filtered image.
And S200, performing threshold segmentation, and dividing elements of the shale sample electron microscope image into four components.
In step S200, the method specifically includes:
step S201, selecting a representative image in the cut images, namely an image containing more pores, organic matters, inorganic matters and pyrite;
step S202, three threshold values for distinguishing the four components, namely a pore threshold value, a partition threshold value of organic matters and inorganic matters and a partition threshold value of inorganic matters and pyrite, are respectively adjusted by a binarization method.
Adjusting and extracting the pore threshold, taking the gray critical values with different sizes as the threshold, and judging the quality of the gray value according to two standards: first, whether the extracted wells coincide with those seen by the naked eye; and whether the obtained face porosity is close to the measured porosity or not. The best gray value is selected as the final division threshold, and the final value of the pore threshold in this embodiment is 70.
When the organic matter and inorganic matter partition threshold is selected, the grayscale critical values with different sizes larger than the pore threshold are taken for extraction test, the grayscale value of the extracted organic matter region closest to the result recognized by naked eyes is the basic threshold for distinguishing organic matter from inorganic matter, and in this embodiment, the final value of the basic threshold is 135.
When the inorganic substance and pyrite segmentation threshold is selected, the grayscale critical values with different sizes larger than the basic threshold are taken for extraction test, and the grayscale value with the closest result of the extracted pyrite region and the result identified by naked eyes is used as the pyrite threshold for distinguishing the inorganic substance and the pyrite, and the final value of the pyrite threshold in the embodiment is 205.
The main components in the shale sample in this example are organic pores, inorganic pores, organic fissures, inorganic fissures, organic matter, inorganic matter, pyrite, and other clay minerals, where the gray values of the organic pores, inorganic pores, organic fissures, and inorganic fissures are the smallest (black) and cannot be distinguished by a single gray value, the gray values of the organic matter are larger (black gray), the gray values of the inorganic matter are larger (gray), and the gray values of the pyrite are the largest (off-white or white).
And S203, after the three thresholds are obtained, performing threshold segmentation on the scanning electron microscope image of the sample, dividing elements of each image into four components of holes, organic matters (including holes), inorganic matters (including holes) and pyrite, sequentially marking the connected blocks by using a bwleael method, and marking all pixels in each connected block as the same number.
And S300, identifying and correcting the white edges, and extracting the content of the organic matter skeleton.
In step S300, the identification and correction of the white edge specifically includes:
and S301, sequentially marking each communicated pyrite region in the sample image, counting the area of each pyrite region, and extracting the boundary coordinate, the barycentric coordinate, the long axis length, the short axis length and the equivalent radius of each pyrite region.
Step S302, a ratio of a long axis to a short axis of each pyrite block is calculated.
Step S303, counting the average gray value of 9 pixel points in the area size of 3 multiplied by 3 near the center of gravity.
And S304, judging, if the ratio of the long axis to the short axis of the pyrite block is more than 10, or the average gray value of 9 pixel points near the gravity center is less than the pyrite threshold, determining that the pyrite block is substantially the interference of the 'white edge' at the periphery of the hole seam, and judging the real composition of the 'white edge'.
Judging the real composition of the white edge:
finding out the minimum rectangle containing the block according to the boundary coordinates, and giving out coordinates of four vertexes of the rectangle; expanding the equivalent radius size outwards from the extracted minimum rectangle to obtain a new rectangle as the neighborhood of the hole; when a certain outward expansion neighborhood edge meets the exceeding boundary, the contraction is inwards performed according to the step length of 3 pixels until the first time that the contraction does not exceed the boundary range is met, and the contraction processes of all edges are independent.
Counting the neighborhoods of the communicating blocks, and respectively calculating the content of organic matters and inorganic matters in each neighborhood, particularly the content of the organic matters and the inorganic matters in the region contacted with the pyrite region; judging the real composition of the white edge according to the relative content of the organic matter component and the inorganic matter component in the neighborhood, if the organic matter content is high, considering the white edge as the organic matter, and filling the white edge area as the gray value of the organic matter; if the inorganic matter content is high, the real component of the "white edge" is considered to be inorganic matter, and the "white edge" region is filled with the gray value of the inorganic matter.
In step S300, the specific steps of extracting the organic framework content are as follows:
and S305, extracting and counting the number of pixels of which all gray values are greater than a hole threshold value and less than a basic threshold value in the sample image in the corrected shale sample electron microscope image.
Step 306, counting the number N of the pixels counted in the step S305 o Multiplying the actual area value of the single pixel representation to obtain the area S occupied by organic matters of the shale sample o ；
S o ＝N o ×S single
Step S307, counting the total pixel number N of the sample s Multiplying the actual area value of the single pixel representation to be used as the actual area S of the shale sample s 。
S s ＝N s ×S single
In step S400, the transformation specifically includes:
the area parameter is converted as follows:
wherein s is p Is a two-dimensional area, S v Is the three-dimensional equivalent area.
In step S400, when calculating the equivalent TOC content of the shale sample:
step S401, calculating the three-dimensional equivalent organic matter framework area of the organic matter framework, and dividing the three-dimensional equivalent organic matter framework area by the total area of the three-dimensional equivalent sample to obtain the three-dimensional equivalent organic matter framework content gamma o ；
γ o ＝S v,o /S v,s ＝s p,o /s p,s ＝N o /N s
Step S402, calculating the three-dimensional equivalent TOC content of the shale sample according to the following formula:
where ρ is k The density of kerogen is generally 1.05g/cm 3 ，ρ r Is density of shaleDegree, typically 2.55g/cm 3 ，ω k The carbon content of the kerogen is 70-85% generally.
The calculated equivalent TOC values for the shale samples were between 2.83% and 3.44%, while the TOC of the same shale samples as measured by combustion was 4.05%. Therefore, the TOC predicted value based on the scanning electron microscope image is equivalent to the TOC value of the shale sample measured by the physicochemical technology, namely the TOC predicted value and the TOC value of the shale sample have equivalent consistency on the TOC content, and the TOC predicted value and the TOC value can be contrastively analyzed with certain accuracy.
The method has the following advantages: according to the method, the content of the shale organic matter framework is evaluated by adopting a digital image processing method, and the equivalent TOC content of the shale is calculated, so that the gas storage capacity and the transportation capacity of a shale reservoir can be accurately evaluated; the method has higher universality, can be used for researching shale samples of different types and different electron microscope scanning imaging qualities, and is beneficial to research, the processing accuracy rate of the shale samples with better electron microscope scanning imaging can reach 95 percent by the method, and the processing accuracy rate of the shale samples with poorer imaging quality can also reach more than 85 percent; the method has the advantages of short measurement period, simple processing process, low cost, great saving of manpower and material resources, and good reliability, and the difference of the test accuracy in comparison with the traditional physical and chemical technology is within 10%.
Although the invention has been described in detail above with reference to a general description and specific examples, it will be apparent to one skilled in the art that modifications or improvements may be made thereto based on the invention. Accordingly, such modifications and improvements are intended to be within the scope of the invention as claimed.
Claims (8)
1. An equivalent TOC calculation method based on shale scanning electron microscope image extraction is characterized by comprising the following steps:
s100, processing a sample, scanning and imaging, and preprocessing an electron microscope image;
s200, performing threshold segmentation, and dividing elements of the shale sample electron microscope image into four components;
s300, identifying and correcting a 'white edge', and extracting the content of an organic matter framework, wherein the 'white edge' is the phenomenon that the gray value of the edge of a pore is abnormally increased due to the edge enhancement effect of a scanning electron microscope;
s400, converting the two-dimensional plane parameters into equivalent three-dimensional parameters by using ascending dimension conversion, and calculating the equivalent TOC content of the shale sample;
calculating the three-dimensional equivalent organic matter framework area of the organic matter framework, and dividing the three-dimensional equivalent organic matter framework area by the total three-dimensional equivalent sample area to obtain the three-dimensional equivalent organic matter framework content gamma o ；
γ o ＝S v,o /S v,s ＝s p,o /s p,s ＝N o /N s ；
Wherein N is o Is the number of pixels, N s Is the total number of pixels, s p,o Representing the two-dimensional area, s, corresponding to No pixels p,s Representing the two-dimensional area, S, corresponding to Ns total pixels of the sample v,o Represents the three-dimensional equivalent area, S, corresponding to No pixels v,s Representing the three-dimensional equivalent area corresponding to Ns total pixels of the sample;
calculating the three-dimensional equivalent TOC content of the shale sample according to the following formula:
where ρ is k Taking the density of kerogen to be 1.05g/cm 3 ，ρ r Taking the density of shale as 2.55g/cm 3 ，ω k 70-85% of kerogen as carbon content.
2. The method for calculating the equivalent TOC extracted based on the shale scanning electron microscope image according to claim 1, wherein in step S100, the method specifically comprises:
s101, selecting a sample, and performing sample cutting, surface grinding and polishing treatment to obtain a sample slice with a size required by the observation of a scanning electron microscope;
s102, obtaining a series of scanning electron microscope images of the shale sample with a representative volume scale through scanning electron microscope observation;
and S103, preprocessing the image, cutting off the black edge of the image, and filtering the electron microscope image.
3. The method for calculating the equivalent TOC extracted based on the shale scanning electron microscope image according to claim 2, wherein in step S103, the method specifically comprises:
extracting four vertex angles of a rectangular region with the image gray value not being 0, reserving the region in the rectangle, and cutting out the region outside the rectangle;
and eliminating the non-smoothness of the shape scanning of the cut image by mean filtering, and then eliminating the salt and pepper noise of the image by median filtering.
4. The method for calculating equivalent TOC extracted based on shale scanning electron microscope images as claimed in claim 1, wherein in step S200, the method specifically comprises:
s201, selecting a representative image in the cut image, namely an image containing more pores, organic matters, inorganic matters and pyrite;
s202, respectively adjusting and distinguishing three threshold values of the four components by using a binarization method, namely a pore threshold value, a segmentation threshold value of organic matters and inorganic matters and a segmentation threshold value of inorganic matters and pyrite;
and S203, after the three thresholds are obtained, performing threshold segmentation on the scanning electron microscope images of the samples, and dividing elements of each image into four components, namely holes, organic matters containing the holes, inorganic matters containing the holes and pyrite.
5. The method for calculating equivalent TOC based on shale scanning electron microscope image extraction according to claim 1, wherein in step S300, the identification and correction of the white edge specifically comprises:
s301, sequentially marking each communicated pyrite region in the sample image, counting the area of each pyrite region, and extracting the boundary coordinate, the barycentric coordinate, the long axis length, the short axis length and the equivalent radius of each pyrite region;
s302, calculating the ratio of the long axis to the short axis of each pyrite block;
s303, counting the average gray value of 9 pixel points in the 3 multiplied by 3 area size near the center of gravity point;
s304, if the ratio of the long axis to the short axis of the pyrite block is larger than 10 or the average gray value of 9 pixel points near the gravity center is smaller than a pyrite threshold, determining that the pyrite block is actually white edge interference at the periphery of the hole, and judging the real composition of the white edge.
6. The equivalent TOC calculation method based on shale scanning electron microscope image extraction according to claim 5, characterized in that when the real composition of "white edge" is judged:
finding out the minimum rectangle containing the block according to the boundary coordinates of the 'white mark' and giving out coordinates of four vertexes of the rectangle;
expanding the equivalent radius size outwards from the extracted minimum rectangle to obtain a new rectangle as the neighborhood of the hole;
when a certain outwards expanded neighborhood edge exceeds the boundary, inwards shrinking according to the step length of 3 pixels until the shrinking is stopped when the first requirement that the boundary range is not exceeded, and the shrinking processes of all edges are mutually independent;
counting the neighborhoods of the communicating blocks, and respectively calculating the content of organic matters and inorganic matters in the neighborhoods;
judging the real composition of the white edge according to the relative content of the organic matter component and the inorganic matter component in the neighborhood, if the organic matter content is high, considering the white edge as the organic matter, and filling the white edge area as the gray value of the organic matter; if the inorganic matter content is high, the real component of the white edge is considered to be inorganic matter, and the white edge area is filled with the gray value of the inorganic matter.
7. The method for calculating equivalent TOC based on shale scanning electron microscope image extraction according to claim 1, wherein in step S300, the specific steps of extracting organic matter skeleton content are as follows:
s305, extracting and counting the number of pixels of which all gray values are greater than a pore threshold value and less than a basic threshold value in the corrected shale sample electron microscope image;
s306, counting the number N of the pixels in the S305 o Multiplying the actual area value of the single pixel representation to obtain the area S occupied by organic matters of the shale sample o ；
S o ＝N o ×S single
S307, counting the total pixel number N of the sample s Multiplying the actual area value of the single pixel representation to be used as the actual area S of the shale sample s ；
S s ＝N s ×S single 。
8. The method for calculating equivalent TOC extracted based on shale scanning electron microscope images according to claim 1, wherein in step S400, the transformation specifically comprises:
the area parameter is converted as follows:
wherein s is p Is a two-dimensional area, S v Is the three-dimensional equivalent area.
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