CN-122016904-A - Coal body hole fracture evolution and damage quantitative evaluation method based on NMR and fractal theory

CN122016904ACN 122016904 ACN122016904 ACN 122016904ACN-122016904-A

Abstract

The invention discloses a coal body pore crack evolution and damage quantitative evaluation method based on an NMR and fractal theory, which comprises the following steps of S1, carrying out saturation tests on a prepared coal rock sample under different conditions through a nuclear magnetic imaging analysis system, obtaining an accumulated nuclear magnetic signal map and an NMRI internal pore crack distribution morphology map after saturation under different conditions, and S2, analyzing the accumulated nuclear magnetic signal map and the NMRI internal pore crack distribution morphology map obtained after saturation under each condition to obtain porosity, fractal dimension and pore crack area expansion and increase ratio, and comparing the changes of parameters before and after saturation under different conditions to obtain a porosity damage factor, a fractal dimension damage factor and a pore crack expansion damage factor. The method utilizes the expansion and increase of the aperture crack area, the fractal dimension and the porosity to define the damage condition of the internal pores of the coal body, adopts the damage characterization of the multi-factor pore structure, and can quantitatively describe the damage degree of the coal body.

Inventors

WANG GANG
Liang Handan
CHU XIANGYU
QIN XIANGJIE
XU HAO
HAN DONGYANG
CHEN XUECHANG

Assignees

山东科技大学

Dates

Publication Date: 20260512
Application Date: 20260120

Claims (8)

1. The coal body hole fracture evolution and damage quantitative evaluation method based on the NMR and fractal theory is characterized by comprising the following steps: S1, preparing a sample, and performing a test, namely performing a water saturation test on the prepared coal rock sample under different conditions through a nuclear magnetic imaging analysis system, and obtaining an accumulated nuclear magnetic signal map and an NMRI internal pore crack distribution morphology map after water saturation under different conditions; S2, analyzing an accumulated nuclear magnetic signal map obtained after water saturation under each condition and an NMRI internal pore crack distribution morphology map to obtain porosity, fractal dimension and pore crack area expansion increment, and comparing the changes of parameters before and after water saturation under different conditions to obtain a porosity damage factor, a fractal dimension damage factor and a pore crack expansion damage factor; s21, substituting the measured signal amplitude per unit volume of the coal sample into a marked line equation based on the accumulated nuclear magnetic signal spectrum to obtain the porosity of each stage of the coal sample, and obtaining a calculation formula of a porosity damage factor according to the porosity under different conditions; s22, according to a fractal geometric approximate calculation formula corresponding to the accumulated nuclear magnetic signal spectrum, the fractal dimension of the hole and the crack at each stage of the coal sample can be obtained, and then according to the fractal dimension of the hole and the crack under different conditions, a calculation formula of a damage factor of the fractal dimension of the hole and the crack is obtained; S23, extracting a hole-crack diagram of the same section position based on an NMRI internal hole-crack distribution morphology diagram, carrying out binarization processing on the hole-crack diagram, carrying out contour detection on the binary image by utilizing an edge detection technology, calculating pixel areas of a hole-crack area and a coal body area, defining a section hole-crack area expansion quantity by changing a difference value of the hole-crack areas of a coal rock sample before and after a condition, and obtaining hole-crack area expansion increment quantities under different conditions, thereby obtaining hole-crack expansion damage factors of the same section.
2. The quantitative evaluation method for coal body pore crack evolution and damage based on the NMR and fractal theory as recited in claim 1, wherein in the analysis process by a nuclear magnetic imaging analysis system, the NMR technology utilizes the action mechanism between the hydrogen-containing fluid and the coal rock sample, and combines the relaxation signals of the hydrogen-containing fluid in the pore cracks inside the coal rock sample to characterize the microstructure and the fluid migration characteristics of the coal rock sample, and the method is characterized by comprising the following steps: Analysis of fluid in coal rock sample hole fracture adopts transverse relaxation time It has three different relaxation mechanisms of surface relaxation, diffusion relaxation and free relaxation, and can be expressed as: Wherein, the Representing the diffusion relaxation time of the light, Representing the free relaxation time of the sample, Representing the surface relaxation time, since the contributions of both free relaxation and diffusion relaxation are much smaller than the surface relaxation, the formula can be expressed as: , wherein, In order to achieve a surface relaxation rate, Is the volume of hydrogen-containing fluid in the internal pore fissures of the coal rock sample, For the pore surface area, assuming that the pore gap inside the coal rock sample is of a simple structure, the formula can be converted into: Wherein, the In the form of a fracture geometry factor of the hole, For the pore fracture radius, a relationship between the pore characteristic parameter and the transverse relaxation time value can be established based on the above formula.
3. The quantitative evaluation method for coal body pore crack evolution and damage based on NMR and fractal theory as recited in claim 1, wherein the standard line equation is obtained in S21 by respectively placing standard oil samples with known porosities into a nuclear magnetic imaging analysis system, measuring corresponding nuclear magnetic signal quantities, and obtaining the linear relation between the porosities and the signal quantities by nuclear magnetic resonance analysis software, wherein the linear relation is the standard line equation of the porosities ; And calculating according to the accumulated nuclear magnetic signal spectrum to obtain the porosity of the coal rock sample, wherein the porosity is calculated as follows: Wherein, the Is the porosity of the coal rock sample, In order to integrate the area of the device, For volume, the damage factor due to porosity is calculated as follows: Wherein, the As a factor of the damage to the porosity, 、 To change the porosity of the coal rock samples before and after the conditions, In order to change the number of conditions, the calculation formula for obtaining the porosity damage factor is as follows: , wherein, 、 To change the integrated area of the coal rock samples before and after the conditions.
4. The quantitative evaluation method for coal body pore crack evolution and damage based on NMR and fractal theory as recited in claim 1, wherein in S22, the fractal geometric approximation calculation formula is: taking the logarithm of the two sides of the upper part to obtain: Wherein, the In order for the transverse relaxation time to be a matter of course, For the maximum transverse relaxation time, For transverse relaxation times less than The cumulative volume of pores in (a) is a percentage of the total pore volume, Is fractal dimension, can be calculated based on accumulated nuclear magnetic signal spectrum And The value of (2) is again The horizontal axis of the graph is, Establishing a coordinate system for the ordinate and connecting lines to obtain And Fitting the relation curve to obtain the slope of the relation curve Thereby obtaining fractal dimension as ; The damage factor is calculated as follows: Wherein, the 、 To change the fractal dimension of the coal rock samples before and after the conditions, In order to change the number of conditions, the calculation formula for obtaining the fractal dimension damage factor is as follows: , wherein, 、 Correspondence of coal rock samples before and after changing conditions And Slope of the relationship.
5. The quantitative evaluation method for coal body hole fracture evolution and damage based on NMR and fractal theory as recited in claim 4, wherein the method is characterized in that And In relation to the curve, according to Dividing the size of the relation curve into various areas, and carrying out piecewise linear fitting on each area, wherein the slope of the whole relation curve is the average value of the slopes of the areas.
6. The quantitative evaluation method of coal body pore crack evolution and damage based on NMR and fractal theory as recited in claim 1, wherein in S23, the pore crack duty ratio of the cross section can be obtained by performing contour detection on the binary image by using edge detection technique and calculating pixel areas of the pore crack area and the coal body area The pore fracture area expansion was calculated as follows: Wherein, the 、 The ratio of the expansion of the aperture area of the coal body to the initial coal body before and after the changing condition is defined as the expansion and increase ratio of the aperture area It is calculated as follows: ; providing the hole fracture expansion damage factor It is calculated as follows: Wherein, the 、 And the expansion and increment of the area of the hole and the crack before and after changing the conditions in the same section damage process are respectively shown in the following calculation formula of the hole and crack expansion damage factor: wherein 、 The ratio of the area of the hole crack before and after changing the condition in the same section damage process is respectively.
7. The quantitative evaluation method for fracture evolution and damage of coal body hole based on NMR and fractal theory as recited in claim 1, wherein the different conditions in S1 comprise any one of a change in temperature, a change in confining pressure and a change in injection pressure, or a change in combination of two or more of them.
8. The quantitative evaluation method for coal body hole crack evolution and damage based on NMR and fractal theory as recited in claim 1, wherein the coal rock sample is a cylindrical coal rock sample with a diameter of 25mm and a height of 100mm, and the coal rock sample is of lignite, bituminous coal or anthracite.

Description

Coal body hole fracture evolution and damage quantitative evaluation method based on NMR and fractal theory Technical Field The invention belongs to the technical field of deep gas thermal recovery and coal seam hydraulic fracturing, and particularly relates to a coal body hole fracture evolution and damage quantitative evaluation method based on NMR and fractal theory. Background Engineering fields such as deep gas thermal recovery, coal seam hydraulic fracturing and the like relate to the thermodynamic coupling effect of coal and rock, and particularly the coal and rock fracture structure and permeability are obviously changed under the action of high temperature or low temperature. When the temperature or other conditions change, the mismatched thermal expansion stress in the coal rock causes the primary fracture to develop, so that the permeability of the coal rock changes, and the quantitative characterization method for the development of the internal fracture of the coal rock caused by the conditions such as temperature change still lacks at present. Disclosure of Invention The invention aims to provide a coal body hole crack evolution and damage quantitative evaluation method based on NMR and fractal theory, which can quantify the damage degree of a coal body hole crack structure, and is favorable for accurately representing the change condition of the coal body hole crack in the temperature rise process of a low-permeability reservoir and exploring the progressive damage rule of a coal body microstructure. The technical scheme adopted by the invention is that the coal body hole fracture evolution and damage quantitative evaluation method based on the NMR and fractal theory comprises the following steps: S1, preparing a sample, and performing a test, namely performing a water saturation test on the prepared coal rock sample under different conditions through a nuclear magnetic imaging analysis system, and obtaining an accumulated nuclear magnetic signal map and an NMRI internal pore crack distribution morphology map after water saturation under different conditions; S2, analyzing an accumulated nuclear magnetic signal map obtained after water saturation under each condition and an NMRI internal pore crack distribution morphology map to obtain porosity, fractal dimension and pore crack area expansion increment, and comparing the changes of parameters before and after water saturation under different conditions to obtain a porosity damage factor, a fractal dimension damage factor and a pore crack expansion damage factor; s21, substituting the measured signal amplitude per unit volume of the coal sample into a marked line equation based on the accumulated nuclear magnetic signal spectrum to obtain the porosity of each stage of the coal sample, and obtaining a calculation formula of a porosity damage factor according to the porosity under different conditions; s22, according to a fractal geometric approximate calculation formula corresponding to the accumulated nuclear magnetic signal spectrum, the fractal dimension of the hole and the crack at each stage of the coal sample can be obtained, and then according to the fractal dimension of the hole and the crack under different conditions, a calculation formula of a damage factor of the fractal dimension of the hole and the crack is obtained; S23, extracting a hole-crack diagram of the same section position based on an NMRI internal hole-crack distribution morphology diagram, carrying out binarization processing on the hole-crack diagram, carrying out contour detection on the binary image by utilizing an edge detection technology, calculating pixel areas of a hole-crack area and a coal body area, defining a section hole-crack area expansion quantity by changing a difference value of the hole-crack areas of a coal rock sample before and after a condition, and obtaining hole-crack area expansion increment quantities under different conditions, thereby obtaining hole-crack expansion damage factors of the same section. As a preferred option in the above scheme, during the analysis by the nuclear magnetic imaging analysis system, the NMR technique utilizes the action mechanism between the hydrogen-containing fluid and the coal rock sample, and combines the relaxation signals of the hydrogen-containing fluid in the internal pore fissures of the coal rock sample to characterize the microstructure and fluid migration characteristics of the coal rock sample, specifically as follows: Analysis of fluid in coal rock sample hole fracture adopts transverse relaxation time It has three different relaxation mechanisms of surface relaxation, diffusion relaxation and free relaxation, and can be expressed as: Wherein, the Representing the diffusion relaxation time of the light,Representing the free relaxation time of the sample,Representing the surface relaxation time, since the contributions of both free relaxation and diffusion relaxation are much smaller than the surface