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

An evaluation method of coal rock pore anisotropy based on NMR test Download PDF

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CN114486670A
CN114486670A CN202111074021.4A CN202111074021A CN114486670A CN 114486670 A CN114486670 A CN 114486670A CN 202111074021 A CN202111074021 A CN 202111074021A CN 114486670 A CN114486670 A CN 114486670A
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唐淑玲
汤达祯
张泰源
陶树
蒲一帆
张奥博
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China University of Geosciences Beijing
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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

一种基于NMR测试的煤岩孔隙各向异性评价方法A method for evaluating coal rock pore anisotropy based on NMR test

技术领域technical field

本发明属于煤层气资源勘探开发中的储层物性评价领域,具体涉及一种针对高倾角地层煤岩核磁共振实验测试以及数据处理的基于NMR测试的煤岩孔隙各向异性评价方法。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.

背景技术Background technique

高倾角地层使得煤储层面临着与水平地层不同的应力状态。在倾角变化的情况下,高倾角地层与水平地层在地应力、储层压力梯度等方面有着很大不同。煤岩作为低杨氏模量、高泊松比、强各向异性的多孔介质,力学环境的不同会深刻影响其内部孔隙形态。高倾角使得同一煤层内也会有较大的深度变化,深度变化带来的应力变化使得其分析更为复杂。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.

很多学者对高倾角地层从渗透率变化、地层流体流动等角度进行了宏观的分析,并且都强调了各向异性的作用。与地层尺度的宏观性质相比,微米甚至是纳米级别的孔裂隙作为煤层气储集、运输的最基本单元,其构成与变化对储层性质有很大影响。但是鲜有针对倾角应力条件下的各向异性孔隙特征研究。主要原因在于倾角是较为宏观的地层特征,煤储层的微观孔隙的各向异性变化很难在应力作用下被检测。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.

而在目前常用的测试手段中,高压压汞、氮气吸附等测试手段由于测试样品(例如粉末状)和手段的限制,不具备各向异性研究的条件。渗透率测试受到各向异性影响较大,但是不能直接反应内部孔隙空间的分布情况。CT成像能够直观反应裂隙的分布,但是动态分析困难而且成本很高。扫描电镜技术能够直观观测微孔裂隙的结构形态,但是观测范围有限,难以整体评价。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.

发明内容SUMMARY OF THE INVENTION

为了解决现有技术中的不足之处,针对煤岩吸附孔隙空间的各项异性分布,本发明提供一种基于NMR测试的煤岩孔隙各向异性评价方法,该方法可定量化描述煤岩孔隙的形状、体积动态变化的各向异性性质,并据此得出不同形状种类的孔隙的体积占比,丰富核磁共振数据的解释结果。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.

为解决上述技术问题,本发明采用如下技术方案:一种基于NMR测试的煤岩孔隙各向异性评价方法,包括以下步骤: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)数据获取;(1) Data acquisition;

(2)数据处理;(2) data processing;

(3)数据模拟;(3) Data simulation;

(4)代数运算。(4) Algebraic operations.

步骤(1)中的数据获取包括先进行样品处理、再进行实验测试两道工序;在样品处理中,利用线切割技术将煤岩处理为若干个均为长5cm、直径为2.5cm的圆柱状煤样,使得圆柱轴向与层理面成0°、30°、45°、60°和90°的夹角;在实验测试时,对每个准备好的煤样,使用侧向施加围压的夹持器,附加3、6、9、12MPa的围压;压力稳定后,依据行业标准SYT 6490-2014进行低磁场核磁共振测试得到T2图谱。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.

步骤(2)中的数据处理主要包括依次进行的T2图谱峰值分割、峰面积变化计算和各项异性特征计算三个步骤:The data processing in step (2) mainly includes the following three steps: T2 spectrum peak segmentation, peak area change calculation and anisotropic feature calculation:

首先依据需求确定需要探求的T2峰,并保证这一峰在每个样品的每个压力点的T2图谱对应存在;之后依据公式1对峰面积进行计算: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:

Figure DEST_PATH_IMAGE002
公式1
Figure DEST_PATH_IMAGE002
Formula 1

式中H ip 表示i号样品压力P时峰面积,P表示不同的压力点。N ij 表示i号样品中第j个核磁信号;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;

依据前人研究,峰面积与孔隙体积大小呈正比,基于此,利用不同倾角下的峰面积-围压关系式对样品的孔隙体积变化的各项异性特征进行表征: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:

Figure DEST_PATH_IMAGE004
公式2
Figure DEST_PATH_IMAGE004
Formula 2

此公式表示视H i 为P的函数并基于最小二乘法对实验数据进行拟合,k和b都时拟合参数,其中k i 表示不同倾角方向上单位压力降低下减小的图谱面积,i=0°、30°、45°、60°和90°,具有表示孔隙体积随围压变化的物理意义;由此实现定量化描述煤岩孔隙体积动态变化各向异性性质的目的。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.

在步骤(3)数值模拟步骤中,主要对假设椭球形的单个孔隙的受力变化进行模拟,建立体积变化的孔隙形状-倾角方位相关系数矩阵;对于不同形状的某一类孔隙,在单位压力变化下的单位体积变化使用如下公式拟合: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:

Figure DEST_PATH_IMAGE006
公式3
Figure DEST_PATH_IMAGE006
Formula 3

式中T表示转置,d是常数项,表示在倾角和形状因子都是0的时候的形变量,单位MPa-1。其余式中各量如下所示: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:

Figure DEST_PATH_IMAGE008
公式4
Figure DEST_PATH_IMAGE008
Formula 4

A是两个拟合系数矩阵,其中各带有角标的小写字母a表示拟合系数,单位MPa-1在同一煤岩中为常数,可以使用经验数值;Eab和Ebc表示椭圆形孔隙的两个偏心率; 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;

经过计算,将不同形状的值按照大小相近程度大致分为球形和类球形孔隙,(Eab和Ebc均为0~0.8)、管状孔隙和喉道(Eab>0.8、Ebc<0.8)、裂隙(Ebc>0.8);三种形状的计算结果在各自范围内取均值,使用矩阵的横列角标表示不同种类孔隙,纵列角标表示不同倾角,其中的元素表示不同倾角的各类孔隙的应力-体积变化率R为: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:

Figure DEST_PATH_IMAGE010
公式5。
Figure DEST_PATH_IMAGE010
Formula 5.

步骤(4)中代数运算的具体过程为:The specific process of the algebraic operation in step (4) is:

设不同种类孔隙的占比表示为向量:

Figure DEST_PATH_IMAGE012
公式6Let the proportion of different types of pores be expressed as a vector:
Figure DEST_PATH_IMAGE012
Formula 6

其中

Figure DEST_PATH_IMAGE014
;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
Figure DEST_PATH_IMAGE016
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
Figure DEST_PATH_IMAGE018
和1-
Figure 239723DEST_PATH_IMAGE018
),其系数矩阵分别由公式5和下式给出:in
Figure DEST_PATH_IMAGE014
; 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.
Figure DEST_PATH_IMAGE016
In order to fully explore the anisotropy, there are two sets of mutually perpendicular components in the pore system, and the volume proportions are
Figure DEST_PATH_IMAGE018
and 1-
Figure 239723DEST_PATH_IMAGE018
), whose coefficient matrices are given by Equation 5 and the following, respectively:

Figure DEST_PATH_IMAGE020
公式7
Figure DEST_PATH_IMAGE020
Formula 7

则R、RTΛ不同方向总体的孔隙体积变化率K V 满足:Then the overall pore volume change rate K V in different directions of R, R T and Λ satisfies:

Figure DEST_PATH_IMAGE022
公式8
Figure DEST_PATH_IMAGE022
Formula 8

R、RT矩阵可以从公式5数值模拟计算中获得,K V 由公式2获得,可以对

Figure 28645DEST_PATH_IMAGE018
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
Figure 28645DEST_PATH_IMAGE018
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.

采用上述技术方案,核磁共振技术应用于各向异性样品的技术可行性:所有柱状样品都来自于同一块煤样,煤演化程度、煤岩成分、显微组分、原位地应力等影响孔隙空间分布的条件都是相似的。核磁共振弛豫的过程虽然具有定向性,但是饱和水的弛豫过程属于快速扩散(Latour,Kleinberg和Sezginer,1992),孔隙空间中自旋密度到处相等,导致测试结果只与孔隙的体积、表面积等宏观非定向的物理量相关。因此各向异性的样品并不会影响核磁共振结果对孔隙特征的表达,在技术理论上有可行性。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.

数值模拟计算不同形状孔隙的技术可行性:在扫描电镜的观察中,这一尺度下很多煤中的孔隙。煤岩的孔隙度都很低,分布在基质之中的小孔很多以椭圆形的形态出现,而微裂隙可以认为是短轴极短的椭球体。因此,可以对各微孔的受力情况进行简化,认为各微孔的受力情况相似,受力表现的差异是因椭球孔隙的倾向和形状产生的。椭圆模型与球形模型相比,虽然计算更为复杂,但是在分析各向异性时具有优势。在本发明中使用的用来描述不同形状和倾角椭球孔隙的体积变化规律(公式3和公式4)是基于物理模型的有限元分析计算得到的。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.

综上所述,本发明是一种能够反映应力作用下反映孔隙各向异性特征的实验与数据处理手段。NMR(低磁场核磁共振)的样品(即定向切割的柱状样品)具有探究各向异性的条件,而且对孔隙空间分布具有较为详细的反馈,相比其他测试方法具有研究孔隙空间动态变化的能力,并能从中推导出整体孔裂隙体积占比的方法。本方法聚焦于这两个参数以及他们随压力、倾角的变化情况,建立数值模型来分析各孔隙参数随应力的变化以及其中体现的各向异性性质,从而保证了方法的可重复性与适用性,对孔隙空间分布特征的表述具有明显效果。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.

附图说明Description of drawings

图1为本发明的评价流程示意图;Fig. 1 is the evaluation flow schematic diagram of the present invention;

图2为本发明中煤样在实验测试时圆柱轴向与层理面成一定夹角的布置示意图;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;

图3为P1峰面积(代表小微孔隙体积)与围压关系示意图;Figure 3 is a schematic diagram of the relationship between the P1 peak area (representing the volume of small micropores) and confining pressure;

图4为变化系数(公式2中ki)以及拟合度随倾角的变化;Figure 4 shows the variation coefficient (ki in Equation 2) and the degree of fit as a function of the inclination angle;

图5为r值的倾角变化量

Figure DEST_PATH_IMAGE024
与形状的关系示意图。Figure 5 shows the change of the inclination angle of the r value
Figure DEST_PATH_IMAGE024
Schematic diagram of the relationship with the shape.

具体实施方式Detailed ways

本发明的一种基于NMR测试的煤岩孔隙各向异性评价方法,包括以下步骤:A method for evaluating coal rock pore anisotropy based on NMR test of the present invention comprises the following steps:

(1)数据获取;(1) Data acquisition;

(2)数据处理;(2) data processing;

(3)数据模拟;(3) Data simulation;

(4)代数运算。(4) Algebraic operations.

步骤(1)中的数据获取包括先进行样品处理、再进行实验测试两道工序;在样品处理中,利用线切割技术将煤岩处理为若干个均为长5cm、直径为2.5cm的圆柱状煤样,使得圆柱轴向与层理面成0°、30°、45°、60°和90°的夹角(如图2所示);在实验测试时,对每个准备好的煤样,使用侧向施加围压的夹持器,附加3、6、9、12MPa的围压;压力稳定后,依据行业标准SYT 6490-2014进行低磁场核磁共振测试得到T2图谱。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.

步骤(2)中的数据处理主要包括依次进行的T2图谱峰值分割、峰面积变化计算和各项异性特征计算三个步骤:The data processing in step (2) mainly includes the following three steps: T2 spectrum peak segmentation, peak area change calculation and anisotropic feature calculation:

首先依据需求确定需要探求的T2峰,并保证这一峰在每个样品的每个压力点的T2图谱对应存在;之后依据公式1对峰面积进行计算: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:

Figure 408417DEST_PATH_IMAGE002
公式1
Figure 408417DEST_PATH_IMAGE002
Formula 1

式中H ip 表示i号样品压力P时峰面积,P表示不同的压力点。N ij 表示i号样品中第j个核磁信号;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;

依据前人研究,峰面积与孔隙体积大小呈正比,基于此,利用不同倾角下的峰面积-围压关系式对样品的孔隙体积变化的各项异性特征进行表征: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:

Figure DEST_PATH_IMAGE025
公式2
Figure DEST_PATH_IMAGE025
Formula 2

此公式表示视H i 为P的函数并基于最小二乘法对实验数据进行拟合,k和b都时拟合参数,其中k i 表示不同倾角方向上单位压力降低下减小的图谱面积,i=0°、30°、45°、60°和90°,具有表示孔隙体积随围压变化的物理意义;由此实现定量化描述煤岩孔隙体积动态变化各向异性性质的目的。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.

在步骤(3)数值模拟步骤中,主要对假设椭球形的单个孔隙的受力变化进行模拟,建立体积变化的孔隙形状-倾角方位相关系数矩阵;对于不同形状的某一类孔隙,在单位压力变化下的单位体积变化使用如下公式拟合: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:

Figure 873027DEST_PATH_IMAGE006
公式3
Figure 873027DEST_PATH_IMAGE006
Formula 3

式中T表示转置,d是常数项,表示在倾角和形状因子都是0的时候的形变量,单位MPa-1。其余式中各量如下所示: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:

Figure DEST_PATH_IMAGE026
公式4
Figure DEST_PATH_IMAGE026
Formula 4

A是两个拟合系数矩阵,其中各带有角标的小写字母a表示拟合系数,单位MPa-1在同一煤岩中为常数,可以使用经验数值;Eab和Ebc表示椭圆形孔隙的两个偏心率; 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;

经过计算,将不同形状的值按照大小相近程度大致分为球形和类球形孔隙,(Eab和Ebc均为0~0.8)、管状孔隙和喉道(Eab>0.8、Ebc<0.8)、裂隙(Ebc>0.8);三种形状的计算结果在各自范围内取均值,使用矩阵的横列角标表示不同种类孔隙,纵列角标表示不同倾角,其中的元素表示不同倾角的各类孔隙的应力-体积变化率R为: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:

Figure 603830DEST_PATH_IMAGE010
公式5。
Figure 603830DEST_PATH_IMAGE010
Formula 5.

步骤(4)中代数运算的具体过程为:The specific process of the algebraic operation in step (4) is:

设不同种类孔隙的占比表示为向量:

Figure DEST_PATH_IMAGE027
公式6Let the proportion of different types of pores be expressed as a vector:
Figure DEST_PATH_IMAGE027
Formula 6

其中

Figure DEST_PATH_IMAGE028
;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
Figure 294312DEST_PATH_IMAGE016
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
Figure 171001DEST_PATH_IMAGE018
和1-
Figure 681879DEST_PATH_IMAGE018
),其系数矩阵分别由公式5和下式给出:in
Figure DEST_PATH_IMAGE028
; 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.
Figure 294312DEST_PATH_IMAGE016
In order to fully explore the anisotropy, there are two sets of mutually perpendicular components in the pore system, and the volume proportions are
Figure 171001DEST_PATH_IMAGE018
and 1-
Figure 681879DEST_PATH_IMAGE018
), whose coefficient matrix is given by Equation 5 and the following, respectively:

Figure DEST_PATH_IMAGE029
公式7
Figure DEST_PATH_IMAGE029
Formula 7

则R、RTΛ不同方向总体的孔隙体积变化率K V 满足:Then the overall pore volume change rate K V in different directions of R, R T and Λ satisfies:

Figure 870284DEST_PATH_IMAGE022
公式8
Figure 870284DEST_PATH_IMAGE022
Formula 8

R、RT矩阵可以从公式5数值模拟计算中获得,K V 由公式2获得,可以对

Figure 948746DEST_PATH_IMAGE018
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
Figure 948746DEST_PATH_IMAGE018
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.

实例分析:以淮南地区煤样为例。不同倾角的测试结果中,核磁信号均随围压增加而减小。P1峰面积变化更明显。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.

如图3和图4中可以发现,k i 随倾角变化存在先降后增的规律,而在仅存在一组优势方向的情况下,应力变化率-倾角关系是单调的。因此还存在一组主方向与之正交的孔隙系统,公式7的存在是合理的。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.

基于样品的煤岩参数进行数值模拟分析时,可以看到r值的倾角变化量

Figure DEST_PATH_IMAGE031
与形状的关系如图5所示。孔隙形状越接近球(Eab和Ebc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(Eab和Ebc接近1),就越受到倾角的影响,变化就越大。因此要依据Eab和Ebc的大小对形状进行划分,即球形和类球形孔隙(Eab和Ebc均为0~0.8)、管状孔隙和喉道(Eab>0.8、Ebc<0.8))、裂隙(Ebc>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
Figure DEST_PATH_IMAGE031
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).

得到P1峰对应孔隙的各向异性分布情况为,球形和类球形孔隙体积占比28.51%,管状孔隙和喉道体积占比24.12%,微裂隙体积占比47.35%。两组方向的体积占比分别为49.92%和50.08%,两组方向几乎平分相等。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.

本实施例并非对本发明的形状、材料、结构等作任何形式上的限制,凡是依据本发明的技术实质对以上实施例所作的任何简单修改、等同变化与修饰,均属于本发明技术方案的保护范围。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:
Figure 955653DEST_PATH_IMAGE001
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:
Figure 18550DEST_PATH_IMAGE002
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:
Figure 583523DEST_PATH_IMAGE003
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:
Figure 888602DEST_PATH_IMAGE004
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; eabAnd EbcTwo 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 EbcAll 0-0.8), tubular pores and throats (E)ab>0.8、Ebc<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:
Figure 957053DEST_PATH_IMAGE005
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:
Figure RE-DEST_PATH_IMAGE006
equation 6
Wherein
Figure RE-DEST_PATH_IMAGE007
(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
Figure RE-DEST_PATH_IMAGE008
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
Figure RE-DEST_PATH_IMAGE009
And 1-
Figure RE-416683DEST_PATH_IMAGE009
) The coefficient matrices are given by equation 5 and the following equation:
Figure RE-DEST_PATH_IMAGE010
equation 7
R, RTΛTotal pore volume change rate K in different directions V Satisfies the following conditions:
Figure RE-DEST_PATH_IMAGE011
equation 8
R、RTThe matrix can be obtained from the numerical simulation calculation of equation 5, K V Obtained from equation 2, can be compared with
Figure RE-114250DEST_PATH_IMAGE009
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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