CN114486670A - An evaluation method of coal rock pore anisotropy based on NMR test - Google Patents

Abstract

A coal rock pore anisotropy evaluation method based on NMR test comprises the following steps: (1) acquiring data; (2) processing data; (3) simulating data; (4) and (4) algebraic operation. In the invention, NMR (low magnetic field nuclear magnetic resonance) samples (namely directionally cut columnar samples) have anisotropic conditions, and have more detailed feedback on pore space distribution, compared with other testing methods, the method has the capability of researching the dynamic change of the pore space and can deduce the integral pore fracture volume ratio. The method focuses on the two parameters and the change conditions of the two parameters along with the pressure and the inclination angle, and establishes a numerical model to analyze the change of each pore parameter along with the stress and the anisotropic property embodied in the pore parameter, so that the repeatability and the applicability of the method are ensured, and the method has an obvious effect on the expression of the pore space distribution characteristics.

Description

A method for evaluating coal rock pore anisotropy based on NMR test

technical field

The invention belongs to the field of reservoir physical property evaluation in the exploration and development of coalbed methane resources, and in particular relates to a coal rock pore anisotropy evaluation method based on NMR testing for high dip angle stratum coal rock nuclear magnetic resonance experimental testing and data processing.

Background technique

High dip angle formations make coal reservoirs face different stress states than horizontal formations. When the dip angle changes, the high dip angle formation and the horizontal formation are very different in in-situ stress and reservoir pressure gradient. Coal rock is a porous medium with low Young's modulus, high Poisson's ratio and strong anisotropy, and the different mechanical environment will profoundly affect its internal pore morphology. The high dip angle causes great depth changes in the same coal seam, and the stress changes brought about by the depth changes make the analysis more complicated.

Many scholars have carried out macroscopic analysis of high dip angle formations from the perspective of permeability change and formation fluid flow, and have emphasized the role of anisotropy. Compared with the macroscopic properties of the formation scale, the micron or even nanoscale pores and fractures are the most basic units of coalbed methane storage and transportation, and their composition and changes have a great impact on the reservoir properties. However, there are few studies on the characteristics of anisotropic pores under dip stress conditions. The main reason is that dip angle is a relatively macroscopic formation feature, and it is difficult to detect the anisotropic changes of microscopic pores in coal reservoirs under the action of stress.

Among the currently commonly used test methods, high-pressure mercury intrusion, nitrogen adsorption and other test methods do not have the conditions for anisotropy research due to the limitations of test samples (such as powder) and methods. The permeability test is greatly affected by anisotropy, but cannot directly reflect the distribution of the internal pore space. CT imaging can directly reflect the distribution of fractures, but dynamic analysis is difficult and expensive. Scanning electron microscopy technology can visually observe the structure and morphology of micropores and fractures, but the observation range is limited and it is difficult to evaluate the overall structure.

SUMMARY OF THE INVENTION

In order to solve the deficiencies in the prior art, aiming at the anisotropic distribution of coal adsorption pore space, the present invention provides a coal rock pore anisotropy evaluation method based on NMR test, which can quantitatively describe the coal rock pores The anisotropic properties of the dynamic changes of shape and volume are obtained, and the volume ratio of pores of different shapes and types is obtained based on this, which enriches the interpretation results of NMR data.

In order to solve the above-mentioned technical problems, the present invention adopts the following technical scheme: a method for evaluating the anisotropy of coal rock pores based on NMR test, comprising the following steps:

(1) Data acquisition;

(2) data processing;

(3) Data simulation;

(4) Algebraic operations.

The data acquisition in step (1) includes two processes of sample processing first, and then experimental testing; in the sample processing, the coal rock is processed into several cylindrical shapes with a length of 5 cm and a diameter of 2.5 cm by using wire cutting technology. Coal samples, so that the axial direction of the cylinder and the bedding surface are at angles of 0°, 30°, 45°, 60° and 90°; during the experimental test, for each prepared coal sample, use lateral confining pressure After the pressure is stabilized, the low magnetic field nuclear magnetic resonance test is carried out according to the industry standard SYT 6490-2014 to obtain the T2 spectrum.

The data processing in step (2) mainly includes the following three steps: T2 spectrum peak segmentation, peak area change calculation and anisotropic feature calculation:

First, determine the T2 peak that needs to be explored according to the requirements, and ensure that this peak corresponds to the T2 spectrum of each pressure point of each sample; then calculate the peak area according to formula 1:

公式1

Formula 1

In the formula, H _ip represents the peak area of sample i under pressure P, and P represents different pressure points. N _ij represents the jth nuclear magnetic signal in sample i;

According to previous research, the peak area is proportional to the pore volume. Based on this, the anisotropic characteristics of the pore volume change of the sample are characterized by the relationship between the peak area and the confining pressure at different dip angles:

公式2

Formula 2

This formula indicates that Hi is a function of P and the experimental data is fitted based on the least squares method. Both _k and b are fitting parameters, where ki _represents the area of the map reduced by unit pressure reduction in different inclination directions, and i = 0°, 30°, 45°, 60° and 90°, which have the physical meaning of expressing the change of pore volume with confining pressure; thus achieving the purpose of quantitatively describing the anisotropic properties of the dynamic change of coal pore volume.

In step (3) numerical simulation step, the force change of a single pore assuming an ellipsoid is mainly simulated, and the pore shape-dip angle azimuth correlation coefficient matrix of volume change is established; for a certain type of pores with different shapes, the unit pressure The unit volume change under change is fitted using the following formula:

公式3

Formula 3

In the formula, T represents the transposition, and d is a constant term, which represents the deformation amount when the inclination angle and the shape factor are both 0, and the unit is MPa ^-1 . The quantities in the rest of the formula are as follows:

公式4

Formula 4

A is two fitting coefficient matrices, in which the lowercase letters a with superscripts represent fitting coefficients, and the unit MPa ^-1 is constant in the same coal rock, and empirical values can be used; E _ab and E _bc represent the elliptical pores two eccentricities;

After calculation, the values of different shapes are roughly divided into spherical and quasi-spherical pores (E _ab and E _bc are both 0-0.8), tubular pores and throats (E _ab >0.8, E _bc <0.8) , fissures (E _bc >0.8); the calculation results of the three shapes are averaged within their respective ranges, and the columns of the matrix are used to indicate different types of pores, and the column labels to indicate different inclination angles. The stress-volume change rate R of the pores is:

公式5。

Formula 5.

The specific process of the algebraic operation in step (4) is:

公式6Let the proportion of different types of pores be expressed as a vector:

Formula 6

；三类不同形状的孔隙空间在煤体内按照不同的比例组合，在相同的力学环境下发生了不同的体积变化，从而影响孔隙体积变化率-倾角变化关系，即公式2中的k _i 随倾角

的变化；为充分探讨各向异性，设存孔隙系统存在两组互相垂直的分量，体积占比分别为

和1-

），其系数矩阵分别由公式5和下式给出：in

; Three types of pore spaces with different shapes are combined in different proportions in the coal body, and different volume changes have occurred under the same mechanical environment, thus affecting the relationship between the pore volume change rate and the inclination angle, that is, k _i in Equation 2 changes with the inclination angle.

In order to fully explore the anisotropy, there are two sets of mutually perpendicular components in the pore system, and the volume proportions are

and 1-

), whose coefficient matrices are given by Equation 5 and the following, respectively:

公式7

Formula 7

Then the overall pore volume change rate K _V in different directions of R, R _T and Λ satisfies:

公式8

Formula 8

和Λ的三个分量一共4个未知数进行求解，从而得到不同方向、不同孔隙类型在煤中的分布，完成定量化描述煤岩孔隙的形状各向异性性质，得出不同形状种类的孔隙的体积占比的目的。The R and R _T matrices can be obtained from the numerical simulation calculation of Equation 5, and K _V can be obtained from Equation 2, which can be calculated for

and the three components of Λ , a total of 4 unknowns are solved, so as to obtain the distribution of different pore types in coal in different directions, complete the quantitative description of the shape anisotropy of coal pores, and obtain the volume of pores of different shapes and types. purpose of proportion.

The technical feasibility of applying NMR technology to anisotropic samples using the above technical solutions: all columnar samples come from the same coal sample, and the degree of coal evolution, coal rock composition, microscopic composition, in-situ in-situ stress, etc. affect the pores The conditions for the spatial distribution are all similar. Although the NMR relaxation process is directional, the relaxation process of saturated water belongs to rapid diffusion (Latour, Kleinberg and Sezginer, 1992), and the spin density in the pore space is equal everywhere, resulting in the test results only related to the volume and surface area of the pores. It is related to macroscopic non-directional physical quantities. Therefore, anisotropic samples will not affect the expression of pore characteristics in NMR results, which is technically feasible.

Numerical simulation to calculate the technical feasibility of pores of different shapes: in scanning electron microscope observation, pores in many coals at this scale. The porosity of coal rocks is very low, and many small pores distributed in the matrix appear in the form of ellipses, while micro-cracks can be considered as ellipsoids with extremely short short axes. Therefore, the force situation of each micropore can be simplified, and it is considered that the force situation of each micropore is similar, and the difference in force performance is caused by the tendency and shape of the ellipsoid pores. Compared with the spherical model, the elliptical model is more computationally complex, but has advantages in analyzing anisotropy. The volume change law (Equation 3 and Equation 4) used in the present invention to describe the ellipsoid pores of different shapes and inclination angles is calculated based on the finite element analysis of the physical model.

To sum up, the present invention is an experiment and data processing method that can reflect the anisotropy characteristics of pores under the action of stress. NMR (low magnetic field nuclear magnetic resonance) samples (that is, directionally cut cylindrical samples) have conditions for exploring anisotropy, and have more detailed feedback on the distribution of pore space. Compared with other testing methods, it has the ability to study the dynamic changes of pore space. And can deduce the method of the overall pore-crack volume ratio. This method focuses on these two parameters and their changes with pressure and dip angle, and establishes a numerical model to analyze the changes of pore parameters with stress and the anisotropic properties reflected in them, thus ensuring the repeatability and applicability of the method. , which has obvious effect on the expression of pore space distribution characteristics.

Description of drawings

Fig. 1 is the evaluation flow schematic diagram of the present invention;

Fig. 2 is a schematic diagram of the arrangement of the coal sample in the present invention with a certain angle between the axial direction of the cylinder and the bedding plane during the experimental test;

Figure 3 is a schematic diagram of the relationship between the P1 peak area (representing the volume of small micropores) and confining pressure;

Figure 4 shows the variation coefficient (ki in Equation 2) and the degree of fit as a function of the inclination angle;

与形状的关系示意图。Figure 5 shows the change of the inclination angle of the r value

Schematic diagram of the relationship with the shape.

Detailed ways

A method for evaluating coal rock pore anisotropy based on NMR test of the present invention comprises the following steps:

(1) Data acquisition;

(2) data processing;

(3) Data simulation;

(4) Algebraic operations.

The data acquisition in step (1) includes two processes of sample processing first, and then experimental testing; in the sample processing, the coal rock is processed into several cylindrical shapes with a length of 5 cm and a diameter of 2.5 cm by using wire cutting technology. coal samples, so that the axial direction of the cylinder and the bedding surface are at angles of 0°, 30°, 45°, 60° and 90° (as shown in Figure 2); during the experimental test, for each prepared coal sample , using a gripper that applies confining pressure laterally, with additional confining pressures of 3, 6, 9, and 12 MPa; after the pressure is stabilized, the T2 spectrum is obtained by low-field nuclear magnetic resonance testing according to the industry standard SYT 6490-2014.

The data processing in step (2) mainly includes the following three steps: T2 spectrum peak segmentation, peak area change calculation and anisotropic feature calculation:

First, determine the T2 peak that needs to be explored according to the requirements, and ensure that this peak corresponds to the T2 spectrum of each pressure point of each sample; then calculate the peak area according to formula 1:

公式1

Formula 1

In the formula, H _ip represents the peak area of sample i under pressure P, and P represents different pressure points. N _ij represents the jth nuclear magnetic signal in sample i;

According to previous research, the peak area is proportional to the pore volume. Based on this, the anisotropic characteristics of the pore volume change of the sample are characterized by the relationship between the peak area and the confining pressure at different dip angles:

公式2

Formula 2

This formula indicates that Hi is a function of P and the experimental data is fitted based on the least squares method. Both _k and b are fitting parameters, where ki _represents the area of the map reduced by unit pressure reduction in different inclination directions, and i = 0°, 30°, 45°, 60° and 90°, which have the physical meaning of expressing the change of pore volume with confining pressure; thus achieving the purpose of quantitatively describing the anisotropic properties of the dynamic change of coal pore volume.

In step (3) numerical simulation step, the force change of a single pore assuming an ellipsoid is mainly simulated, and the pore shape-dip angle azimuth correlation coefficient matrix of volume change is established; for a certain type of pores with different shapes, the unit pressure The unit volume change under change is fitted using the following formula:

公式3

Formula 3

In the formula, T represents the transposition, and d is a constant term, which represents the deformation amount when the inclination angle and the shape factor are both 0, and the unit is MPa ^-1 . The quantities in the rest of the formula are as follows:

公式4

Formula 4

A is two fitting coefficient matrices, in which the lowercase letters a with superscripts represent fitting coefficients, and the unit MPa ^-1 is constant in the same coal rock, and empirical values can be used; E _ab and E _bc represent the elliptical pores two eccentricities;

After calculation, the values of different shapes are roughly divided into spherical and quasi-spherical pores (E _ab and E _bc are both 0-0.8), tubular pores and throats (E _ab >0.8, E _bc <0.8) , fissures (E _bc >0.8); the calculation results of the three shapes are averaged within their respective ranges, and the columns of the matrix are used to indicate different types of pores, and the column labels to indicate different inclination angles. The stress-volume change rate R of the pores is:

公式5。

Formula 5.

The specific process of the algebraic operation in step (4) is:

公式6Let the proportion of different types of pores be expressed as a vector:

Formula 6

；三类不同形状的孔隙空间在煤体内按照不同的比例组合，在相同的力学环境下发生了不同的体积变化，从而影响孔隙体积变化率-倾角变化关系，即公式2中的k _i 随倾角

的变化；为充分探讨各向异性，设存孔隙系统存在两组互相垂直的分量，体积占比分别为

和1-

），其系数矩阵分别由公式5和下式给出：in

; Three types of pore spaces with different shapes are combined in different proportions in the coal body, and different volume changes have occurred under the same mechanical environment, thus affecting the relationship between the pore volume change rate and the inclination angle, that is, k _i in Equation 2 changes with the inclination angle.

In order to fully explore the anisotropy, there are two sets of mutually perpendicular components in the pore system, and the volume proportions are

and 1-

), whose coefficient matrix is given by Equation 5 and the following, respectively:

公式7

Formula 7

Then the overall pore volume change rate K _V in different directions of R, R _T and Λ satisfies:

公式8

Formula 8

和Λ的三个分量一共4个未知数进行求解，从而得到不同方向、不同孔隙类型在煤中的分布，完成定量化描述煤岩孔隙的形状各向异性性质，得出不同形状种类的孔隙的体积占比的目的。The R and R _T matrices can be obtained from the numerical simulation calculation of Equation 5, and K _V can be obtained from Equation 2, which can be calculated for

and the three components of Λ , a total of 4 unknowns are solved, so as to obtain the distribution of different pore types in coal in different directions, complete the quantitative description of the shape anisotropy of coal pores, and obtain the volume of pores of different shapes and types. purpose of proportion.

Case analysis: Take the coal sample in Huainan area as an example. In the test results of different inclination angles, the nuclear magnetic signal decreases with the increase of confining pressure. The P1 peak area changes more obviously.

It can be found in Fig. 3 and Fig. 4 that k _i first decreases and then increases with the change of inclination angle, and when there is only one set of dominant directions, the relationship between stress change rate and inclination angle is monotonic. Therefore, there is also a group of pore systems with the main direction orthogonal to it, and the existence of Equation 7 is reasonable.

与形状的关系如图5所示。孔隙形状越接近球（E_ab和E_bc接近0），因为各向异性减弱，形变越不受倾角影响。而形状越扁、越接近喉道和裂隙的情况（E_ab和E_bc接近1），就越受到倾角的影响，变化就越大。因此要依据E_ab和E_bc的大小对形状进行划分，即球形和类球形孔隙（E_ab和E_bc均为0~0.8）、管状孔隙和喉道（E_ab>0.8、E_bc<0.8））、裂隙（E_bc>0.8）。When the numerical simulation analysis is performed based on the coal and rock parameters of the sample, the change of the dip angle of the r value can be seen

The relationship with the shape is shown in Figure 5. The closer the pore shape is to a sphere (E _ab and E _bc are close to 0), the less the deformation is affected by the dip angle because the anisotropy is weakened. The flatter the shape, the closer to the throat and fissure (E _ab and E _bc are close to 1), the more affected by the dip angle, the greater the change. Therefore, the shape should be divided according to the size of E _ab and E _bc , namely spherical and quasi-spherical pores (E _ab and E _bc are both 0~0.8), tubular pores and throat (E _ab >0.8, E _bc <0.8) ), cracks (E _bc >0.8).

The anisotropic distribution of pores corresponding to the P1 peak is obtained as follows: the volume of spherical and quasi-spherical pores accounts for 28.51%, the volume of tubular pores and throats accounts for 24.12%, and the volume of microcracks accounts for 47.35%. The volume proportions of the two groups of directions are 49.92% and 50.08%, respectively, and the two groups of directions are almost equally divided.

This embodiment does not limit the shape, material, structure, etc. of the present invention in any form. Any simple modification, equivalent change and modification made to the above embodiment according to the technical essence of the present invention belong to the protection of the technical solution of the present invention. scope.

Claims (5)

1. A coal rock pore anisotropy evaluation method based on NMR test is characterized in that: the method comprises the following steps:

(1) acquiring data;

(2) processing data;

(3) simulating data;

(4) and (4) algebraic operation.

2. The coal rock pore anisotropy evaluation method based on the NMR test is characterized in that: the data acquisition in the step (1) comprises two procedures of sample treatment and experimental test; in the sample treatment, the coal rock is treated into a plurality of cylindrical coal samples with the length of 5cm and the diameter of 2.5cm by utilizing a linear cutting technology, so that the axial direction of a cylinder and the bedding surface form included angles of 0 degree, 30 degrees, 45 degrees, 60 degrees and 90 degrees; in the experimental test, 3 MPa, 6 MPa, 9 MPa and 12MPa confining pressure are added to each prepared coal sample by using a clamp for laterally applying confining pressure; after the pressure is stabilized, performing a low magnetic field nuclear magnetic resonance test according to an industry standard SYT 6490-2014 to obtain a T2 map.

3. The coal rock pore anisotropy evaluation method based on the NMR test is characterized in that: the data processing in the step (2) mainly comprises three steps of T2 atlas peak value segmentation, peak area change calculation and anisotropic feature calculation which are sequentially carried out:

firstly, determining a T2 peak to be searched according to requirements, and ensuring that the peak correspondingly exists in a T2 map of each pressure point of each sample; the peak area was then calculated according to equation 1:

equation 1

In the formulaH _ip The peak area at sample pressure P of No. i is shown,p represents different pressure points;

N _ij represents the jth nuclear magnetic signal in sample i;

according to the previous research, the peak area is in direct proportion to the pore volume, and based on the proportion, the peak area-confining pressure relational expression under different dip angles is utilized to characterize the anisotropic characteristics of the pore volume change of the sample:

equation 2

The formula is expressedH _i Fitting the experimental data as a function of P based on least squares, k and b being both fitting parameters, whereink _i The area of the pattern which shows the reduction of unit pressure in different inclination angle directions is reduced, i =0 °, 30 °, 45 °, 60 ° and 90 °, and the physical meaning of the change of pore volume along with the ambient pressure is shown; therefore, the aim of quantitatively describing the anisotropic property of the dynamic change of the pore volume of the coal rock is fulfilled.

4. The coal rock pore anisotropy evaluation method based on the NMR test is characterized in that: in the numerical simulation step in the step (3), the stress change of a single pore which is supposed to be ellipsoidal is mainly simulated, and a pore shape-dip angle azimuth correlation coefficient matrix with the changed volume is established; for a certain type of pores of different shapes, the change per volume under change per pressure is fitted using the following formula:

equation 3

Where T represents transpose, d is a constant term representing the amount of deformation in MPa when both the tilt angle and the form factor are 0^-1，

The amounts in the remaining formula are as follows:

equation 4

AIs a matrix of two fitting coefficients, where each lower case letter a with a corner mark represents a fitting coefficient in units of MPa^-1A constant in the same coal rock, empirical values can be used; e_abAnd E_bcTwo eccentricities representing elliptical apertures;

through calculation, values of different shapes are roughly divided into spherical and spheroidal pores according to the similar degree of the sizes, (E)_abAnd E_bcAll 0-0.8), tubular pores and throats (E)_ab>0.8、E_bc<0.8), fissures (E)_bc>0.8); the calculation results of the three shapes are averaged in respective ranges, the horizontal row angle marks of the matrix are used for representing different types of pores, the vertical row angle marks of the matrix are used for representing different dip angles, and the stress-volume change rate R of each type of pores with the elements representing different dip angles is as follows:

equation 5.

5. The coal rock pore anisotropy evaluation method based on the NMR test is characterized in that: the specific process of algebraic operation in step (4) is as follows:

let the occupation ratios of the different types of pores be expressed as vectors:

equation 6

Wherein

(ii) a The three types of pore spaces with different shapes are combined in different proportions in the coal body and have different volume changes under the same mechanical environment, so that the relationship between the pore volume change rate and the inclination angle change is influenced, namely the relationship in the formula 2k _i Following inclination angle

A change in (c); to fully explore the anisotropy, the pore system is designed to have two groups of components perpendicular to each other, the volume ratio of which is respectively

And 1-

) The coefficient matrices are given by equation 5 and the following equation:

equation 7

R, R_T、ΛTotal pore volume change rate K in different directions _V Satisfies the following conditions:

equation 8

R、R_TThe matrix can be obtained from the numerical simulation calculation of equation 5, K _V Obtained from equation 2, can be compared with

AndΛthe three components are solved by 4 unknowns in total, so that the distribution of different pore types in different directions in the coal is obtained, and the purposes of quantitatively describing the shape anisotropy of the coal rock pores and obtaining the volume ratio of the pores of different shapes and types are fulfilled.

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