CN114486670A - Coal rock pore anisotropy evaluation method based on NMR test - Google Patents
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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
Technical Field
The invention belongs to the field of evaluation of reservoir physical properties in exploration and development of coal bed gas resources, and particularly relates to a coal rock pore anisotropy evaluation method based on NMR (nuclear magnetic resonance) tests for high-dip-angle formation coal rock nuclear magnetic resonance experimental tests and data processing.
Background
High dip formations expose coal reservoirs to different stress states than horizontal formations. Under the condition of inclination angle change, the high-inclination-angle stratum and the horizontal stratum are greatly different in the aspects of ground stress, reservoir pressure gradient and the like. Coal rock is used as a porous medium with low Young modulus, high Poisson's ratio and strong anisotropy, and the internal pore morphology of the coal rock can be deeply influenced by the difference of mechanical environments. The high dip angle causes great depth change in the same coal seam, and the stress change caused by the depth change causes the analysis to be more complicated.
Many researchers have performed macroscopic analyses of high dip formations from the perspective of permeability changes, formation fluid flow, etc., and have emphasized the effects of anisotropy. Compared with the macroscopic property of the stratum scale, the pore cracks of micron or even nanometer scale are used as the most basic unit for the storage and transportation of the coal bed gas, and the composition and the change of the pore cracks have great influence on the property of the reservoir. But there is little research on anisotropic pore characteristics under the condition of the angular stress. The main reason is that dip is a relatively macroscopic formation feature, and anisotropic changes in microscopic pores of a coal reservoir are difficult to detect under stress.
Among the currently used test means, the test means such as high-pressure mercury injection and nitrogen adsorption do not have the condition for anisotropic study due to the limitations of the test sample (e.g. powder) and means. Permeability testing is greatly affected by anisotropy, but does not directly reflect the distribution of internal pore space. CT imaging can visually reflect the distribution of fractures, but dynamic analysis is difficult and costly. The scanning electron microscope technology can visually observe the structural form of the micropore fracture, but the observation range is limited, and the integral evaluation is difficult.
Disclosure of Invention
Aiming at solving the defects in the prior art and aiming at the anisotropic distribution of the coal rock adsorption pore space, the invention provides a coal rock pore anisotropy evaluation method based on NMR test, which can quantitatively describe the anisotropic property of the dynamic change of the shape and the volume of the coal rock pore space, obtain the volume ratio of pores of different shapes and types and enrich the interpretation result of nuclear magnetic resonance data.
In order to solve the technical problems, the invention adopts the following technical scheme: 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.
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.
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:
In the formulaH ip The peak area is shown for sample pressure P, i.e., the pressure point at which P is different.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:
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.
In the numerical simulation step in the step (3), the stress change of a single pore which is assumed to be ellipsoidal is mainly simulated, and a pore shape-dip angle azimuth correlation coefficient matrix with volume change 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:
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:
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:
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:
R, RT、ΛTotal pore volume change rate K in different directions V Satisfies the following conditions:
R、RTThe 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.
By adopting the technical scheme, the technical feasibility of applying the nuclear magnetic resonance technology to the anisotropic sample is as follows: all columnar samples are from the same coal sample, and conditions influencing pore space distribution, such as coal evolution degree, coal rock components, microscopic components, in-situ crustal stress and the like, 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), and the spin density in the pore space is equal everywhere, so that the test result is only related to macroscopic non-directional physical quantities such as the volume and the surface area of the pores. Therefore, the anisotropic sample does not influence the nuclear magnetic resonance result on the expression of the pore characteristics, and is feasible in technical theory.
The technical feasibility of different shapes of pores is calculated through numerical simulation: pores in many coals at this scale are observed under scanning electron microscopy. The porosity of coal rock is very low, and many small holes distributed in the matrix appear in an elliptical shape, and the microcracks can be regarded as ellipsoids with extremely short minor axes. Therefore, the stress condition of each micropore can be simplified, the stress condition of each micropore is similar, and the difference of stress expression is generated by the tendency and the shape of the ellipsoid pores. The elliptical model has advantages in analyzing anisotropy, although more complicated in calculation than the spherical model. The volume change laws (formula 3 and formula 4) used in the present invention to describe the ellipsoid pores of different shapes and tilt angles are calculated based on finite element analysis of a physical model.
In summary, the present invention is an experimental and data processing method capable of reflecting the anisotropic characteristics of the pores under the action of stress. NMR (low magnetic field nuclear magnetic resonance) samples (i.e., directionally cut columnar samples) have the advantage of exploring anisotropy conditions and have more detailed feedback on pore space distribution, and compared with other testing methods, the method has the capability of researching the dynamic change of pore space and can deduce the integral pore fracture volume ratio from the dynamic change of the pore space. 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.
Drawings
FIG. 1 is a schematic view of an evaluation flow according to the present invention;
FIG. 2 is a schematic diagram of arrangement of a coal sample in the invention, wherein an included angle is formed between the axial direction of a cylinder and a bedding surface during experimental test;
FIG. 3 is a graph showing the relationship between P1 peak area (representing small micropore volume) and confining pressure;
FIG. 4 is a graph of the coefficient of variation (ki in equation 2) and the degree of fit as a function of tilt angle;
FIG. 5 is a graph showing the variation of the tilt angle of the r value
The relationship with the shape is shown schematically.
Detailed Description
The invention discloses a coal rock pore anisotropy evaluation method based on NMR test, which comprises the following steps:
(1) acquiring data;
(2) processing data;
(3) simulating data;
(4) and (4) algebraic operation.
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 (as shown in figure 2); 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, a low-magnetic-field nuclear magnetic resonance test is carried out according to the industry standard SYT 6490-2014 to obtain a T2 map.
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:
In the formulaH ip The peak area is shown for sample pressure P, i.e., the pressure point at which P is different.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:
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; thereby realizing the quantitative description of the coal rock poresThe purpose of the dynamic variation of the anisotropic properties of the void volume.
In the numerical simulation step in the step (3), the stress change of a single pore which is assumed to be ellipsoidal is mainly simulated, and a pore shape-dip angle azimuth correlation coefficient matrix with volume change 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:
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:
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 an elliptical aperture;
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:
The specific process of algebraic operation in step (4) is as follows:
let the 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:
R, RT、ΛTotal pore volume change rate K in different directions V Satisfies the following conditions:
R、RTThe matrix can be obtained from the numerical simulation calculation of equation 5, K V Obtained from equation 2, can be compared with
AndΛa total of 4 unknowns of the three components are solved, thereby obtainingThe distribution of different pore types in the coal in the same direction can achieve the purpose of describing the shape anisotropy of the coal rock pores quantitatively and obtaining the volume ratio of the pores of different shapes.
Example analysis: take coal sample in Huainan area as an example. In the test results of different inclination angles, the nuclear magnetic signals all decrease along with the increase of the 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 first and then increasing with the change of the inclination angle, and in the case where only one set of dominant directions exists, the stress change rate-inclination angle relationship is monotonous. There is therefore also a system of apertures with a main direction orthogonal to it, and the existence of equation 7 is reasonable.
When numerical simulation analysis is carried out on coal rock parameters based on samples, the dip angle variation of the r value can be seen
The relationship with the shape is shown in fig. 5. The closer the pore shape is to the sphere (E)abAnd EbcClose to 0) because the anisotropy decreases, the deformation is less affected by the tilt angle. And the flatter and closer the shape to the throat and crevice (E)abAnd EbcClose to 1), the more affected the tilt angle, the greater the change. Therefore, it is based on EabAnd EbcSize of (D) divides the shape, i.e. spherical and spheroidal pores (E)abAnd EbcAll 0-0.8), tubular pores and throats (E)ab>0.8、Ebc<0.8)), crevice (E)bc>0.8）。
The anisotropic distribution of the P1 peaks corresponding to the pores was found to be 28.51% spherical and spheroidal, 24.12% tubular and throat, and 47.35% microcracked. The volume ratio of the two groups of directions is 49.92% and 50.08%, and the two groups of directions are almost equally divided.
The present embodiment is not intended to limit the shape, material, structure, etc. of the present invention in any way, and any simple modification, equivalent change and modification made to the above embodiments according to the technical spirit of the present invention are within the scope of the technical solution of the present invention.
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:
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:
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:
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:
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:
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:
R, RT、ΛTotal pore volume change rate K in different directions V Satisfies the following conditions:
R、RTThe 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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