CN114486670B - Coal rock pore anisotropy evaluation method based on NMR test - Google Patents
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- 230000015572 biosynthetic process Effects 0.000 description 8
- 238000005755 formation reaction Methods 0.000 description 8
- 238000004458 analytical method Methods 0.000 description 6
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 description 4
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- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 2
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- QSHDDOUJBYECFT-UHFFFAOYSA-N mercury Chemical compound [Hg] QSHDDOUJBYECFT-UHFFFAOYSA-N 0.000 description 1
- 229910052753 mercury Inorganic materials 0.000 description 1
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Abstract
An evaluation method of pore anisotropy of coal rock based on NMR test comprises the following steps: (1) data acquisition; (2) data processing; (3) data simulation; (4) algebraic operation. The NMR (low magnetic field nuclear magnetic resonance) sample (namely the columnar sample subjected to directional cutting) has the condition of exploring anisotropy, has more detailed feedback on pore space distribution, has the capability of researching dynamic change of pore space compared with other testing methods, and can deduce the volume ratio of the whole pore gap. The method focuses on the two parameters and the change conditions of the parameters along with the pressure and the inclination angle, establishes a numerical model to analyze the change of each pore parameter along with the stress and the anisotropic property reflected therein, thereby ensuring the repeatability and the applicability of the method and having obvious effect on the expression of the pore space distribution characteristics.
Description
Technical Field
The invention belongs to the field of reservoir physical property evaluation in the exploration and development of coalbed methane resources, and particularly relates to a coal rock pore anisotropy evaluation method based on NMR (nuclear magnetic resonance) test aiming at high-dip-angle stratum coal rock nuclear magnetic resonance experimental test and data processing.
Background
The high dip formation places the coal reservoir face in a different stress state than the horizontal formation. In the case of a change in dip angle, the high dip angle formations are very different from the horizontal formations in terms of earth stress, reservoir pressure gradient, etc. The coal rock is used as a porous medium with low Young modulus, high Poisson ratio and strong anisotropy, and the internal pore morphology is deeply influenced by different mechanical environments. The high inclination angle causes larger depth change in the same coal layer, and the stress change caused by the depth change makes the analysis more complex.
Many students have performed macroscopic analyses of high dip formations from permeability changes, formation fluid flow, etc., and all have emphasized the role of anisotropy. In contrast to the macroscopic properties of the formation scale, microscale or even nanoscale pore fissures are used as the most basic units for coalbed methane reservoir and transportation, and the composition and changes of the pore fissures have a great influence on the reservoir properties. But there are few studies on anisotropic pore characteristics under oblique stress conditions. The main reason is that dip is a relatively macroscopic formation feature, and the anisotropic change of microscopic pores of a coal reservoir is difficult to detect under stress.
In the conventional test means, the test means such as high-pressure mercury injection and nitrogen adsorption do not have the conditions for anisotropic investigation due to the limitations of the test sample (for example, powder) and means. The permeability test is much affected by anisotropy, but cannot directly reflect the distribution of the internal pore space. CT imaging can intuitively reflect the distribution of the fissures, but dynamic analysis is difficult and costly. The scanning electron microscope technology can intuitively observe the structural morphology of the micropore crack, but has a limited observation range and is difficult to evaluate integrally.
Disclosure of Invention
Aiming at the anisotropic distribution of the coal-rock adsorption pore space, the invention provides a coal-rock pore anisotropy evaluation method based on an NMR test, which can quantitatively describe the anisotropic property of the dynamic change of the shape and the volume of the coal-rock pore, and can obtain the volume ratio of the pores of different shapes and types according to the anisotropic property, so as to enrich the interpretation result of nuclear magnetic resonance data.
In order to solve the technical problems, the invention adopts the following technical scheme: an evaluation method of pore anisotropy of coal rock based on NMR test comprises the following steps:
(1) Obtaining data;
(2) Data processing;
(3) Simulating data;
(4) Algebraic operation.
The data acquisition in the step (1) comprises two procedures of sample treatment and experimental test; in sample treatment, coal and rock are treated into a plurality of cylindrical coal samples which are 5cm long and 2.5cm in diameter by using a linear cutting technology, so that the axial direction of the cylinder forms included angles of 0 degrees, 30 degrees, 45 degrees, 60 degrees and 90 degrees with a bedding surface; during experimental tests, using a clamp for laterally applying confining pressure to each prepared coal sample, and adding confining pressures of 3, 6, 9 and 12 MPa; after the pressure is stable, a low magnetic field nuclear magnetic resonance test is carried out according to an industry standard SYT 6490-2014 to obtain a T2 map.
The data processing in the step (2) mainly comprises three steps of peak segmentation of a T2 map, calculation of peak area change and calculation of anisotropic characteristics, which are sequentially carried out:
firstly, determining a T2 peak to be searched according to requirements, and ensuring that the peak exists correspondingly in a T2 map of each pressure point of each sample; the peak area is then calculated according to equation 1:
equation 1
In the middle ofH ip The peak area at sample pressure P, i, is indicated, P representing the different pressure points.N ij Representing the j-th nuclear magnetic signal in the sample I;
according to the previous study, the peak area is in direct proportion to the pore volume, and based on the peak area-confining pressure relation at different inclinations is used for characterizing the anisotropic characteristics of the pore volume change of the sample:
equation 2
This formula represents the viewH i As a function of P and based on least squares methodFitting the data, fitting parameters at both k and b, whereink i The map areas reduced under unit pressure reduction in different inclination angles are represented by i=0°, 30 °, 45 °, 60 ° and 90 °, and the physical meaning of representing the change of pore volume along with confining pressure is provided; therefore, the purpose of quantitatively describing the anisotropic property of the dynamic change of the pore volume of the coal rock is achieved.
In the step (3) of numerical simulation, mainly simulating the stress change of a single pore with an assumed ellipsoidal shape, and establishing a pore shape-dip angle azimuth correlation coefficient matrix of volume change; for a certain class of pores of different shape, the change in unit volume at the change in unit pressure is fitted using the following formula:
equation 3
Wherein T represents a transpose, d is a constant term representing the deformation amount in MPa when both the tilt angle and the shape factor are 0 -1 . The amounts in the remaining formulae are as follows:
equation 4
AIs a matrix of two fitting coefficients, wherein each lower case letter a with an angle sign represents a fitting coefficient in MPa -1 The coal is constant in the same coal rock, and empirical values can be used; e (E) ab And E is bc Representing the two eccentricities of an elliptical aperture;
through calculation, the values of different shapes are roughly divided into spherical and spheroid pores according to the similarity of sizes, (E) ab And E is bc All 0 to 0.8), tubular pores and a throat (E) ab >0.8、E bc <0.8 Fracture (E) bc >0.8 A) is provided; the calculation results of the three shapes are averaged in the respective ranges, the row corner marks of the matrix are used for representing different types of pores, the column corner marks are used for representing different dip angles, and the stress-volume change rate R of the various pores with elements representing different dip angles is as follows:
equation 5.
The algebraic operation in the step (4) comprises the following specific processes:
let the duty cycle of the different kinds of pores be expressed as a vector:equation 6
Wherein the method comprises the steps ofThe method comprises the steps of carrying out a first treatment on the surface of the Three types of pore spaces with different shapes are combined in the coal body according to different proportions, and different volume changes occur under the same mechanical environment, so that the relation between the pore volume change rate and the inclination angle change is influenced, namely, in the formula 2k i With inclination angle->Is a variation of (2); to fully investigate the anisotropy, two sets of mutually perpendicular components are provided for the pore system, the volume ratios being +.>And 1- & lt- & gt>) The coefficient matrix is given by equation 5 and the following equation, respectively:
equation 7
R, R T 、ΛRate of pore volume change K for different directional populations V The method meets the following conditions:
equation 8
R、R T The matrix can be obtained from the numerical simulation calculation of equation 5, K V Obtained from equation 2, can be applied toAndΛthe three components of the coal rock pore are solved for 4 unknown numbers, so that the distribution of different pore types in different directions in coal is obtained, the purpose of quantitatively describing the shape anisotropy property of the coal rock pore is achieved, and the volume ratio of the pores of different shape types is obtained.
By adopting the technical scheme, the nuclear magnetic resonance technology is applied to the technical feasibility of anisotropic samples: all columnar samples are from the same coal sample, and conditions of coal evolution degree, coal rock composition, microscopic components, in-situ stress and the like affecting pore space distribution are similar. Although the process of nuclear magnetic resonance relaxation is directional, the relaxation process of saturated water belongs to rapid diffusion (Latour, kleinberg and Sezginer, 1992), the spin densities in the pore space are equal everywhere, and the test result is only related to macroscopic non-directional physical quantities such as the volume, the surface area and the like of the pores. Therefore, the anisotropic sample does not influence the expression of the nuclear magnetic resonance result on the pore characteristics, and the method is technically feasible.
Technical feasibility of numerical simulation calculation of different shaped pores: in scanning electron microscopy, many pores in coal are observed at this scale. The porosity of the coal rock is very low, many small holes distributed in the matrix appear in the form of ellipsoids, while microcracks can be considered as ellipsoids with very short axes. Therefore, the stress conditions of the micropores can be simplified, and the stress conditions of the micropores are considered to be similar, and the difference of stress performance is generated due to the tendency and the shape of the ellipsoidal pores. Elliptical models, although more complex to calculate than spherical models, have advantages in analyzing anisotropy. The volume change law (formula 3 and formula 4) used in the present invention to describe the different shapes and tilt ellipsoidal voids is calculated based on finite element analysis of a physical model.
In summary, the invention is an experimental and data processing means capable of reflecting the characteristic of pore anisotropy under the action of stress. The NMR (low magnetic field nuclear magnetic resonance) sample (i.e. the directionally cut columnar sample) has the condition of exploring anisotropy, has more detailed feedback on pore space distribution, has the capability of researching dynamic change of pore space compared with other testing methods, and can deduce the volume ratio of the integral pore gap. The method focuses on the two parameters and the change conditions of the parameters along with the pressure and the inclination angle, establishes a numerical model to analyze the change of each pore parameter along with the stress and the anisotropic property reflected therein, thereby ensuring the repeatability and the applicability of the method and having obvious effect on the expression of the pore space distribution characteristics.
Drawings
FIG. 1 is a schematic diagram of an evaluation flow of the present invention;
FIG. 2 is a schematic diagram of the arrangement of a coal sample in the present invention, wherein the axial direction of the cylinder forms a certain included angle with the bedding surface during experimental test;
FIG. 3 is a graph showing the relationship between P1 peak area (representing small micro pore volume) and confining pressure;
FIG. 4 shows the variation of the coefficient of variation (ki in equation 2) and the fitting degree with the inclination angle;
FIG. 5 shows the variation of the inclination angle of r-valueSchematic diagram of relationship to shape.
Detailed Description
The invention discloses a coal rock pore anisotropy evaluation method based on NMR test, which comprises the following steps:
(1) Obtaining data;
(2) Data processing;
(3) Simulating data;
(4) Algebraic operation.
The data acquisition in the step (1) comprises two procedures of sample treatment and experimental test; in sample treatment, the coal rock is treated into a plurality of cylindrical coal samples which are 5cm long and 2.5cm in diameter by utilizing a linear cutting technology, so that the axial direction of the cylinder forms included angles of 0 degrees, 30 degrees, 45 degrees, 60 degrees and 90 degrees with a bedding surface (shown in figure 2); during experimental tests, using a clamp for laterally applying confining pressure to each prepared coal sample, and adding confining pressures of 3, 6, 9 and 12 MPa; after the pressure is stable, a low magnetic field nuclear magnetic resonance test is carried out according to an industry standard SYT 6490-2014 to obtain a T2 map.
The data processing in the step (2) mainly comprises three steps of peak segmentation of a T2 map, calculation of peak area change and calculation of anisotropic characteristics, which are sequentially carried out:
firstly, determining a T2 peak to be searched according to requirements, and ensuring that the peak exists correspondingly in a T2 map of each pressure point of each sample; the peak area is then calculated according to equation 1:
equation 1
In the middle ofH ip The peak area at sample pressure P, i, is indicated, P representing the different pressure points.N ij Representing the j-th nuclear magnetic signal in the sample I;
according to the previous study, the peak area is in direct proportion to the pore volume, and based on the peak area-confining pressure relation at different inclinations is used for characterizing the anisotropic characteristics of the pore volume change of the sample:
equation 2
This formula represents the viewH i Fitting experimental data as a function of P and based on least squares method, fitting parameters at both k and b, whereink i The map areas reduced under unit pressure reduction in different inclination angles are represented by i=0°, 30 °, 45 °, 60 ° and 90 °, and the physical meaning of representing the change of pore volume along with confining pressure is provided; therefore, the purpose of quantitatively describing the anisotropic property of the dynamic change of the pore volume of the coal rock is achieved.
In the step (3) of numerical simulation, mainly simulating the stress change of a single pore with an assumed ellipsoidal shape, and establishing a pore shape-dip angle azimuth correlation coefficient matrix of volume change; for a certain class of pores of different shape, the change in unit volume at the change in unit pressure is fitted using the following formula:
equation 3
Wherein T represents a transpose, d is a constant term representing the deformation amount in MPa when both the tilt angle and the shape factor are 0 -1 . The amounts in the remaining formulae are as follows:
equation 4
AIs a matrix of two fitting coefficients, wherein each lower case letter a with an angle sign represents a fitting coefficient in MPa -1 The coal is constant in the same coal rock, and empirical values can be used; e (E) ab And E is bc Representing the two eccentricities of an elliptical aperture;
through calculation, the values of different shapes are roughly divided into spherical and spheroid pores according to the similarity of sizes, (E) ab And E is bc All 0 to 0.8), tubular pores and a throat (E) ab >0.8、E bc <0.8 Fracture (E) bc >0.8 A) is provided; the calculation results of the three shapes are averaged in the respective ranges, the row corner marks of the matrix are used for representing different types of pores, the column corner marks are used for representing different dip angles, and the stress-volume change rate R of the various pores with elements representing different dip angles is as follows:
equation 5.
The algebraic operation in the step (4) comprises the following specific processes:
let the duty cycle of the different kinds of pores be expressed as a vector:equation 6
Wherein the method comprises the steps ofThe method comprises the steps of carrying out a first treatment on the surface of the Three types of pore spaces with different shapes are combined in the coal body according to different proportions, and different volume changes occur under the same mechanical environment, so that the relation between the pore volume change rate and the inclination angle change is influenced, namely, in the formula 2k i With inclination angle->Is a variation of (2); to fully investigate the anisotropy, two sets of mutually perpendicular components are provided for the pore system, the volume ratios being +.>And 1- & lt- & gt>) The coefficient matrix is given by equation 5 and the following equation, respectively:
equation 7
R, R T 、ΛRate of pore volume change K for different directional populations V The method meets the following conditions:
equation 8
R、R T The matrix can be obtained from the numerical simulation calculation of equation 5, K V Obtained from equation 2, can be applied toAndΛsolving the total of 4 unknowns to obtain the distribution of different pore types in different directions in the coal, and completing quantitative description of the shape of the pores of the coal and rockThe anisotropic nature, the purpose of the volume ratio of the pores of different shape types is obtained.
Example analysis: taking a coal sample in Huainan region as an example. In the test results of different dip angles, nuclear magnetic signals are reduced along with the increase of confining pressure. The P1 peak area changes more significantly.
As can be seen in figures 3 and 4,k i there is a rule of decreasing followed by increasing with the change of the tilt angle, whereas in the case where there is only one dominant set of directions, the stress change rate-tilt angle relationship is monotonic. There is also a set of pore systems with principal directions orthogonal to them, the existence of equation 7 being reasonable.
When the numerical simulation analysis is carried out on the coal rock parameters based on the samples, the inclination angle variation of the r value can be seenThe relationship with shape is shown in fig. 5. The closer the pore shape is to the sphere (E) ab And E is bc Near 0) because the anisotropy weakens, the deformation is less affected by the tilt angle. While the flatter the shape, the closer to the throat and fissure (E ab And E is bc Approaching 1), the more affected by the tilt angle, the greater the change. Thus, take into consideration E ab And E is bc Is divided into shapes, i.e. spherical and spheroidal pores (E ab And E is bc 0 to 0.8), tubular pores and throat (E) ab >0.8、E bc <0.8 A) and a slit (E) bc >0.8）。
The anisotropic distribution of the corresponding pores of the P1 peak is obtained, wherein the volume of the spherical and spheroid pores accounts for 28.51%, the volume of the tubular pores and the throat accounts for 24.12%, and the volume of the micro-cracks accounts for 47.35%. The volume ratio of the two groups of directions is 49.92% and 50.08%, respectively, and the two groups of directions are almost equally divided.
The present embodiment is not limited in any way by the shape, material, structure, etc. of the present invention, and any simple modification, equivalent variation and modification made to the above embodiments according to the technical substance of the present invention are all included in the scope of protection of the technical solution of the present invention.
Claims (1)
1. The coal rock pore anisotropy evaluation method based on the NMR test is characterized by comprising the following steps of: the method comprises the following steps:
(1) Obtaining data;
(2) Data processing;
(3) Simulating data;
(4) Algebraic operation;
the data acquisition in the step (1) comprises two procedures of sample treatment and experimental test; in sample treatment, coal and rock are treated into a plurality of cylindrical coal samples which are 5cm long and 2.5cm in diameter by using a linear cutting technology, so that the axial direction of the cylinder forms included angles of 0 degree, 300 degrees, 45 degrees, 60 degrees and 90 degrees with a bedding surface; during experimental tests, using a clamp for laterally applying confining pressure to each prepared coal sample, and adding confining pressures of 3, 6, 9 and 12 MPa; after the pressure is stable, performing a low magnetic field nuclear magnetic resonance test according to an industry standard SYT 6490-2014 to obtain a T2 map;
the data processing in the step (2) mainly comprises three steps of peak segmentation of a T2 map, calculation of peak area change and calculation of anisotropic characteristics, which are sequentially carried out:
firstly, determining a T2 peak to be searched according to requirements, and ensuring that the peak exists correspondingly in a T2 map of each pressure point of each sample; the peak area is then calculated according to equation 1:
h in ip The peak area when the pressure P of the sample with the number i is expressed, and P represents different pressure points;
N ij representing the j-th nuclear magnetic signal in the sample I;
according to the previous study, the peak area is in direct proportion to the pore volume, and based on the peak area-confining pressure relation at different inclinations is used for characterizing the anisotropic characteristics of the pore volume change of the sample:
this formula represents view H i Fitting experimental data as a function of P and based on least squares, fitting parameters at both k and b, where k i The map areas reduced under unit pressure reduction in different inclination angles are represented by i=0°, 30 °, 45 °, 60 ° and 90 °, and the physical meaning of representing the change of pore volume along with confining pressure is provided; the purpose of quantitatively describing the anisotropic property of the dynamic change of the pore volume of the coal rock is realized;
in the step (3) of numerical simulation, mainly simulating the stress change of a single pore with an assumed ellipsoidal shape, and establishing a pore shape-dip angle azimuth correlation coefficient matrix of volume change; for a certain class of pores of different shape, the change in unit volume at the change in unit pressure is fitted using the following formula:
wherein T represents a transpose, d is a constant term representing the deformation amount in MPa when both the tilt angle and the shape factor are 0 -1 ，
The amounts in the remaining formulae are as follows:
a is a matrix of two fitting coefficients, wherein each lower case letter a with an angle sign represents a fitting coefficient in MPa -1 The coal is constant in the same coal rock, and empirical values can be used; e (E) ab And E is bc Representing the two eccentricities of an elliptical aperture;
through calculation, the values of different shapes are roughly divided into spherical and spheroid pores, tubular pores and throats according to the similar degree of the sizes; the calculation results of the three shapes are averaged in the respective ranges, the row corner marks of the matrix are used for representing different types of pores, the column corner marks are used for representing different dip angles, and the stress-volume change rate R of the various pores with elements representing different dip angles is as follows:
the algebraic operation in the step (4) comprises the following specific processes:
let the duty cycle of the different kinds of pores be expressed as a vector: Λ= [ lambda ] 1 λ 2 λ 3 ]Equation 6
Wherein lambda is 1 +λ 2 +λ 3 =1; three types of pore spaces with different shapes are combined in the coal body according to different proportions, and different volume changes occur under the same mechanical environment, so that the relation between the pore volume change rate and the inclination angle change, namely k in the formula 2, is influenced i With the change of the inclination angle; to fully investigate anisotropy, two sets of mutually perpendicular components exist in the pore system, the volume fractions are respectively equal to 1-), and the coefficient matrix is given by the formula 5 and the following formula:
r, R T Rate of pore volume change K for populations of different directions Λ V The method meets the following conditions:
K v ＝ξΛ·R+(1-ξ)Λ·R T equation 8
R、R T The matrix can be obtained from the numerical simulation calculation of equation 5, K V The method is obtained by the formula 2, and the total of 4 unknowns of the three components of the sum lambda can be solved, so that the distribution of different pore types in different directions in coal is obtained, the quantitative description of the shape anisotropy property of the coal-rock pores is completed, and the purpose of obtaining the volume ratio of the pores of different shape types is achieved.
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