CN117825431B - Coal and rock pore connectivity evaluation method based on paramagnetic ion diffusion - Google Patents
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Abstract
The invention discloses a coal and rock pore connectivity evaluation method based on paramagnetic ion diffusion, which is characterized in that NMR test is carried out on a paramagnetic ion solution saturated sample of porous compact rock, because hydrogen proton signals in pore water can be shielded after paramagnetic ions enter pores of the coal and the rock, the signals shielded by the paramagnetic ions are used for representing the pores into which the paramagnetic ions enter, and the amplitude and the area of a T 2 spectrum which are correspondingly measured are reduced after the paramagnetic ions enter the pores through diffusion, so that the connectivity degree of the pores inside the coal and the rock can be evaluated according to the difficulty of the paramagnetic ions diffusing into the coal and the rock. The invention breaks through the difficult problem of multi-scale pore connectivity test, effectively avoids a plurality of defects of a centrifugal and drying method, is suitable for various rocks such as coal, shale and the like with different strength and structural characteristics, and provides data support for pore connectivity of porous compact organic rock required by unconventional natural gas development and carbon dioxide geological sequestration.
Description
Technical Field
The invention relates to a coal and rock pore connectivity evaluation method based on paramagnetic ion diffusion, and belongs to the field of coal and rock pore structure connectivity test.
Background
Pore connectivity is an important physical attribute describing the access characteristics of the internal structures of coal and rock, which is a key basis for measuring and assessing the flow characteristics of fluids inside coal and rock. In the engineering fields of gas extraction, unconventional natural gas development, carbon dioxide geological sequestration and the like, pore connectivity of coal and rock is an extremely critical parameter.
As a nondestructive testing technology, nuclear Magnetic Resonance (NMR) technology can realize the full-scale test characterization of porous rock from nano-scale micropores, ultra-micropores to macroscopic microcrack structures, and the distribution and the duty ratio of movable water pores and bound water pores in coal and rock can be determined through water saturation and centrifugation tests, but the NMR technology has a plurality of defects. On one hand, the whole pore communication degree of the rock is difficult to reflect only by water saturation and centrifugal test; on the other hand, centrifugation is poorly applicable, and low strength rock samples cannot be applied, because: because the low-rank coal has loose structure and micro-cracks are relatively developed, a core centrifuge is easy to damage a sample, meanwhile, a loose rock mass such as coal is constructed, and the measurement of the porosity of bound water cannot be carried out by adopting a centrifugal method.
For better unified evaluation and comparison of pore connectivity, many scholars replace centrifugation with a low temperature drying method, and water discharged from pores through low temperature drying can approximately represent water stored in the connected pores. However, as the low-rank coal structure presents gel-like property, the pore structure develops, the saturated water sample absorbs a large amount of water, the drying speed is too high, the large amount of water evaporation can lead to rapid shrinkage of the coal body structure, the damage of the structure is most likely to be caused, and the testing accuracy is greatly influenced. In addition, for coal body samples with low porosity and poor wettability, the moisture content is very low in a saturated state, and the drying is very easy to cause the total loss of trace moisture, so that the NMR test fails. Meanwhile, the drying time is difficult to realize scientific control, and the communication degree of the pore structure in the rock is difficult to be scientifically reflected only through T 2 distribution comparison under the saturated water and the drying state. Therefore, a set of nuclear magnetic pore connectivity evaluation methods which can be well adapted to various attribute rocks is needed to be established.
Disclosure of Invention
Aiming at the problems in the prior art, the invention provides a paramagnetic ion diffusion-based coal and rock pore connectivity evaluation method, which can accurately obtain the communication degree and pore diameter distribution condition of the pore structure in a sample without centrifuging the sample, and is suitable for various rocks such as coal, shale and the like with different strength and structural characteristics.
In order to achieve the above purpose, the technical scheme adopted by the invention is as follows: a coal and rock pore connectivity evaluation method based on paramagnetic ion diffusion comprises the following specific steps:
A. Determining a marking formula A: before nuclear magnetic resonance testing, a standard sample is subjected to a porosity calibration experiment by adopting a peak point method, and a marking formula A of the porosity is obtained;
B. Water saturation of coal rock samples: carrying out vacuum water saturation on the coal rock sample, weighing the sample at intervals until the variation of the sample mass in the interval time is less than M% of the last test mass, and determining that the sample reaches full water saturation;
C. Obtaining the total porosity of the sample: performing nuclear magnetic resonance test on the sample to obtain a T 2 spectral peak curve of the sample in a water saturation state, converting an ordinate amplitude signal into a porosity signal according to a marking formula A, obtaining a T 2 porosity curve, wherein the area enclosed by the T 2 Saturated with water ,T2 Saturated with water curve and an abscissa is the water saturation porosity of the sample, and the water saturation porosity is S Saturated with water , and taking S Saturated with water as the total porosity of the sample; in addition, for special rock with extremely poor wettability or low pore size and low permeability, the sample can be pressurized to be saturated for shortening the test time because the water saturation process of the sample is extremely long, so as to ensure full saturation, obtain the real total porosity S Saturated with water of the sample, and simultaneously ensure the experimental requirement of paramagnetic ion diffusion in the saturated sample.
D. Paramagnetic ion saturation of the sample: completely immersing the water-saturated sample after the test in a container filled with paramagnetic solution with the mass fraction of n%, placing the sample and the container into a vacuum water-saturated machine, and starting vacuum saturation;
E. Residual porosity of samples at different times was obtained: as paramagnetic ions gradually start to enter the sample under the diffusion action, H proton signals in water in the pores where the paramagnetic ions enter are shielded, the peak area of a T 2 spectrum is continuously reduced, the sample is taken out every delta T time, the surface water stain of the sample is wiped off, and then nuclear magnetic resonance test is carried out on the sample to obtain T 2 spectrum peak curves of the sample at different times; then, respectively converting the ordinate amplitude signals of the T 2 spectral peak curves at different times into porosity signals according to a marking formula A to obtain T 2 paramagnetic ion residual porosity curves, and sequentially marking the residual porosity curves as T 21、T22···T2n,T2 paramagnetic ion residual porosity curves and the areas surrounded by the abscissa as the residual porosity of the sample, and sequentially marking the residual porosity curves as S paramagnetic ion 1、S paramagnetic ion 2···S paramagnetic ion n;
F. obtaining a residual porosity change rate k t: according to the formula Calculating to obtain the residual porosity ratio alpha t according to the formula/>Calculating a residual porosity change rate k t, wherein alpha tp is the last time residual porosity ratio at t, and a curve of the calculated residual porosity change rate k t with time can be obtained along with the increase of test time;
G. Corresponding data are obtained according to different residual porosity change rates k t: when the residual porosity change rate k t is less than 5% for the first time, determining that all high-connectivity pores are diffused by paramagnetic ions, wherein the T 2 porosity curve of the sample at T D1,tD1 is marked as T 2D1, the residual porosity is marked as S paramagnetic ion D1, when the residual porosity change rate k t is less than 0.1% for the first time, determining that the residual pore paramagnetic ions are difficult to diffuse into the high-connectivity pores, the T 2 porosity curve of the sample at T D2,tD2 is marked as T 2D2, and the residual porosity is marked as S paramagnetic ion D2,tD2, and stopping the nuclear magnetic test work of the saturated paramagnetic ion solution sample;
H. Obtaining isolated pore porosity of a sample: after the nuclear magnetic testing of the saturated paramagnetic ion solution sample is finished, the sample is put into a constant-temperature drying oven for drying, the temperature is 60 ℃ for 24 hours, the last nuclear magnetic resonance test is carried out after the drying is finished, a T 2 spectrum peak curve of the sample in a dry state is obtained, a ordinate amplitude signal is converted into a porosity signal according to a marking formula A, the obtained T 2 porosity curve is marked as a T 2 Drying ,T2 Drying curve, the area enclosed by an abscissa is marked as a drying residual porosity, the drying residual porosity is marked as S Drying , and S Drying is used as isolated pore porosity S Isolation of of the sample, namely S Isolation of ＝S Drying ;
I. Obtaining pore size distribution conditions of a sample: according to the parameters obtained in the steps G and H, the connectivity of the pores is divided into accessible pores, inaccessible pores and isolated pores from good to poor in sequence, and the porosities of different pores are calculated respectively; and then converting the T 2 porosity curves corresponding to the T 2 Saturated with water 、T2D1、T2D2、T2 Drying into pore diameter-porosity curves, namely pore diameter distribution diagrams, and finally obtaining pore diameter distribution conditions of pores with different degrees of communication.
Further, the porosity calibration experiment in the step A specifically comprises the following steps: and performing nuclear magnetic resonance test on a standard sample with a certain volume and known porosity, and measuring 3-6 nuclear magnetic signals to obtain a correlation curve of nuclear magnetic signal quantity and porosity of a unit volume, thereby fitting and obtaining a marking formula A of the porosity.
Further, the interval time in the step B is 12h, and M is 0.05. Setting the parameters can ensure the accuracy of parameter testing.
Further, the specific selection process of the paramagnetic solution with the mass fraction of n% in the step D comprises the following steps: firstly, carrying out a response rule test on paramagnetic solutions with different mass fractions of paramagnetic ions through nuclear magnetic resonance signals, when the effective nuclear magnetic resonance volume is reduced to below 0.01% for the first time in the test, recording the mass fraction of the paramagnetic ions of the paramagnetic solution as m%, considering that part of the paramagnetic ions are diffused into a sample to dilute the paramagnetic ion solution, and using the paramagnetic ion solution with the mass fraction of n% for the test to enhance the shielding effect of the paramagnetic ions on H protons in water, wherein the relation between n and m satisfies (n-m) is more than or equal to 15.
Further, the paramagnetic solution in the step D is a paramagnetic solution with one of Mn 2+ ions, copper ions, iron ions and chromium ions; mn 2+ ions are preferred for their best effect.
Further, the specific process of dividing the step I into accessible holes, more difficult-to-reach holes, difficult-to-reach holes and isolated holes is as follows: before time t D1, paramagnetic ions firstly enter the pores with good connectivity and rapidly diffuse, and the partial pores are accessible pores in the sample, and the accessible pore porosity S Easy access ＝S Saturated with water -S paramagnetic ion D1; in the time t D1 and the time t D2, the paramagnetic ion diffusion speed is slowed down, which shows that the connectivity of the part of pores is poor, namely the pores which are difficult to reach are secondary communication pores in the sample, after the time of the pore porosity S Is difficult to reach ＝S paramagnetic ion D1-S paramagnetic ion D2;t D2 which is difficult to reach, the paramagnetic ion diffusion speed is extremely slow, and the residual porosity S paramagnetic ion D2 minus the isolated pore porosity S Isolation of at the moment is the pore porosity S Refractory to arrival of the sample, wherein the difficult to reach pores refer to the pores which are difficult to diffuse into and are mutually communicated in a short time, and the pore porosity S Refractory to arrival ＝S paramagnetic ion D2-S Drying which is difficult to reach; isolated pores are pores that do not communicate with other pores and have a porosity of S Isolation of .
Further, in order to reduce errors, three or more coal rock samples are randomly taken, steps B to H are repeated, and the average value of the parameters in the steps G and H is taken as final data for subsequent acquisition of the pore size distribution condition of the samples.
Compared with the prior art, when in NMR test, the existence of paramagnetic ions can extremely obviously influence the NMR relaxation response of liquid phase substances in porous rock, the existence of paramagnetic ions can obviously improve the relaxation attenuation speed of 1 H in an aqueous phase, so that the relaxation time of 1 H is greatly shortened to a level which can not be measured, based on the principle, the inventor researches out the technical scheme of the invention, namely, by carrying out NMR test on a saturated sample of a paramagnetic ion solution of compact organic rock, the distribution and the duty ratio of different communicated pores can be obtained by carrying out NMR test on the communication degree of the pores of coal and rock, since the paramagnetic ions can shield the signals of the aqueous solution in the pores after entering the pores of the coal and the rock, the signals shielded by the paramagnetic ions are used for representing the phenomena that the amplitudes and the areas of T 2 spectra obtained by the corresponding NMR test can be reduced after the paramagnetic ions enter the pores by diffusion, and the connectivity degree of the pores inside the coal and the rock can be evaluated according to the difficulty of the diffusion of the paramagnetic ions in the rock. The method breaks through the difficulty of multi-scale pore connectivity test, effectively avoids a plurality of defects of a centrifugal method and a drying method, is suitable for various rocks such as coal, shale and the like with different strength and structural characteristics, has higher universality and popularization, and provides data support for the pore connectivity degree of dense organic rocks required by coal mine gas extraction, unconventional natural gas development and carbon dioxide geological storage.
Drawings
FIG. 1 is a graph of the present invention dividing apertures of varying degrees of connectivity;
FIG. 2 is a graph of data obtained by nuclear magnetic resonance testing of a saturated paramagnetic ion sample in accordance with the present invention;
FIG. 3 is a graphical representation of data for determining a paramagnetic solution having a mass fraction of n% in the present invention.
Detailed Description
The present invention will be further described below.
The method comprises the following specific steps:
A. Determining a marking formula A: the porosity calibration experiment is carried out on the standard sample by adopting a peak point method before nuclear magnetic resonance test, and specifically comprises the following steps: and performing nuclear magnetic resonance test on a standard sample with a certain volume and known porosity, and measuring 3-6 nuclear magnetic signals to obtain a correlation curve of nuclear magnetic signal quantity and porosity of a unit volume, thereby fitting and obtaining a marking formula A of the porosity.
B. Water saturation of coal rock samples: carrying out vacuum water saturation on the coal rock sample, weighing the sample every 12h until the variation of the sample mass in the interval time is less than 0.05% of the last test mass, and determining that the sample reaches full water saturation;
C. Obtaining the total porosity of the sample: performing nuclear magnetic resonance test on the sample to obtain a T 2 spectral peak curve of the sample in a water saturation state, converting an ordinate amplitude signal into a porosity signal according to a marking formula A, obtaining a T 2 porosity curve, wherein the area enclosed by the T 2 Saturated with water ,T2 Saturated with water curve and an abscissa is the water saturation porosity of the sample, and the water saturation porosity is S Saturated with water , and taking S Saturated with water as the total porosity of the sample, wherein the total porosity is 16.13% in the embodiment; in addition, for special rock with extremely poor wettability or low pore size and low permeability, the sample can be pressurized to be saturated for shortening the test time because the water saturation process of the sample is extremely long, so as to ensure full saturation, obtain the real total porosity S Saturated with water of the sample, and simultaneously ensure the experimental requirement of paramagnetic ion diffusion in the saturated sample.
D. Paramagnetic ion saturation of the sample: completely immersing the water-saturated sample after the test in a container filled with paramagnetic solution with the mass fraction of n%, placing the sample and the container into a vacuum water-saturated machine, and starting vacuum saturation; the specific selection process of the paramagnetic solution with the mass fraction of n percent comprises the following steps: first, response rule tests are performed on paramagnetic solutions with different mass fractions of paramagnetic ions through nuclear magnetic resonance signals as shown in fig. 3, wherein the paramagnetic solution is MnCl 2 paramagnetic solution with Mn 2+ ions. Other paramagnetic ions such as copper ions, iron ions, chromium ions and the like can also be used, and Mn 2+ ions are preferred for the best effect. When the effective nmr volume is reduced below 0.01% for the first time in the experiment, the mass fraction of paramagnetic ions in the paramagnetic solution is recorded as 30%, and considering that part of paramagnetic ions diffuse into the sample and will dilute the paramagnetic ion solution, the experiment uses a paramagnetic ion solution with a mass fraction of 50% to enhance the shielding effect of paramagnetic ions on H protons in water, i.e., n=50.
E. Residual porosity of samples at different times was obtained: as paramagnetic ions gradually start to enter the sample under the diffusion action, H proton signals in water in the pores where the paramagnetic ions enter are shielded, the peak area of a T 2 spectrum is continuously reduced, the sample is taken out every delta T time, the surface water stain of the sample is wiped off, and then nuclear magnetic resonance test is carried out on the sample to obtain T 2 spectrum peak curves of the sample at different times; then, respectively converting the ordinate amplitude signals of the T 2 spectral peak curves at different times into porosity signals according to a marking formula A to obtain T 2 paramagnetic ion residual porosity curves, and sequentially marking the residual porosity curves as T 21、T22···T2n,T2 paramagnetic ion residual porosity curves and the areas surrounded by the abscissa as the residual porosity of the sample, and sequentially marking the residual porosity curves as S paramagnetic ion 1、S paramagnetic ion 2···S paramagnetic ion n;
F. obtaining a residual porosity change rate k t: according to the formula Calculating to obtain the residual porosity ratio alpha t according to the formula/>Calculating a residual porosity change rate k t, wherein alpha tp is the last time residual porosity ratio at t, and a curve of the calculated residual porosity change rate k t over time can be obtained as the test time increases as shown in fig. 2;
G. Corresponding data are obtained according to different residual porosity change rates k t: when the residual porosity change rate k t is less than 5% for the first time, it is determined that the highly interconnected pores are all diffused by paramagnetic ions, the T 2 porosity curve of the sample at time T D1,tD1 is denoted as T 2D1, the residual porosity is denoted as S paramagnetic ion D1, when the residual porosity change rate k t is less than 0.1% for the first time, it is determined that the residual pore paramagnetic ions are hardly diffused thereafter, the T 2 porosity curve of the sample at time T D2,tD2 is denoted as T 2D2, after the residual porosity is recorded as S paramagnetic ion D2,tD2 time, stopping the nuclear magnetic testing work of the saturated paramagnetic ion solution sample; as shown in fig. 2, k 5 is the slope of saturated ions for 2 days to saturated ions for 3 days equal to 4.2229%, and the residual porosity change rate is less than 5%/day for the first time, so saturated ions are selected as t D1 time for 3 days; k 4 is that the slope from 18 days of saturated ions to 40 days of saturated ions is equal to 0.011666%/day, and the residual porosity change rate is less than 0.1%/day, so the saturated ions are selected as t D2 time for 40 days. Since the diffusion rate of manganese ions is extremely slow in this stage, the nuclear magnetic test time interval of the saturated manganese sample can be prolonged appropriately, and although the time t D2 can be increased, the increase of the time t D2 has a negligible influence on the test result because the porosity change amount is extremely small in a quite long time in this stage.
H. Obtaining isolated pore porosity of a sample: after the nuclear magnetic testing of the saturated paramagnetic ion solution sample is finished, the sample is put into a constant-temperature drying oven for drying, the temperature is 60 ℃ for 24 hours, the last nuclear magnetic resonance test is carried out after the drying is finished, a T 2 spectrum peak curve of the sample in a dry state is obtained, a ordinate amplitude signal is converted into a porosity signal according to a marking formula A, the obtained T 2 porosity curve is marked as a T 2 Drying ,T2 Drying curve, the area enclosed by an abscissa is marked as a drying residual porosity, the drying residual porosity is marked as S Drying , and S Drying is used as isolated pore porosity S Isolation of of the sample, namely S Isolation of ＝S Drying =2.37%;
I. Obtaining pore size distribution conditions of a sample: according to the parameters obtained in the steps G and H, the connectivity of the pores is divided into accessible pores, inaccessible pores and isolated pores from good to poor in sequence, and the porosities of different pores are calculated respectively; the specific process is as follows: before t D1, paramagnetic ions firstly enter the pores with good connectivity and rapidly diffuse, and the partial pores are accessible pores in the sample, wherein the accessible pore porosity S Easy access ＝S Saturated with water -S Saturated manganese D1 =16.13-7.70=8.43%; in the time t D1 and the time t D2, paramagnetic ion diffusion speed is slowed down, which shows that the connectivity of the part of pores is poor, namely a more difficult-to-reach pore is a secondary communication pore in the sample, after the time of the more difficult-to-reach pore porosity S Is difficult to reach ＝S Saturated manganese D1-S Saturated manganese D2＝7.70-5.77＝1.93％;tD2, the paramagnetic ion diffusion speed is extremely slow, and the residual porosity S paramagnetic ion D2 minus the isolated pore porosity S Isolation of is the difficult-to-reach pore porosity S Refractory to arrival of the sample, wherein the difficult-to-reach pore refers to a pore which is difficult to diffuse and is communicated with each other in a short time, and the difficult-to-reach pore porosity S Refractory to arrival ＝S Saturated manganese D2-S Drying =5.77-2.37=3.40%; isolated pores, i.e., pores that do not communicate with other pores, and have a porosity S Isolation of , as shown in fig. 1;
and then, respectively converting the T 2 porosity curves corresponding to the T 2 Saturated with water 、T2 D1、T2 D2、T2 Drying into pore diameter-porosity curves, namely pore diameter distribution diagrams, by adopting a T 2 -r conversion method in a 'GB-T42035-2022 method for measuring pore diameter distribution of coal and rock', as shown in figure 1, and finally obtaining pore diameter distribution conditions of pores with different degrees of communication.
As an improvement of the invention, in order to reduce errors, five coal rock samples are randomly taken, steps B to H are repeated, and the average value of the parameters of the respective steps G and H is taken as final data for subsequent acquisition of the pore size distribution condition of the samples.
The foregoing is only a preferred embodiment of the invention, it being noted that: it will be apparent to those skilled in the art that various modifications and adaptations can be made without departing from the principles of the present invention, and such modifications and adaptations are intended to be comprehended within the scope of the invention.
Claims (7)
1. A coal and rock pore connectivity evaluation method based on paramagnetic ion diffusion is characterized by comprising the following specific steps:
A. Determining a marking formula A: before nuclear magnetic resonance testing, a standard sample is subjected to a porosity calibration experiment by adopting a peak point method, and a marking formula A of the porosity is obtained;
B. Water saturation of coal rock samples: carrying out vacuum water saturation on the coal rock sample, weighing the sample at intervals until the variation of the sample mass in the interval time is less than M% of the last test mass, and determining that the sample reaches full water saturation;
C. obtaining the total porosity of the sample: performing nuclear magnetic resonance test on the sample to obtain a T 2 spectral peak curve of the sample in a water saturation state, converting an ordinate amplitude signal into a porosity signal according to a marking formula A, obtaining a T 2 porosity curve, wherein the area enclosed by the T 2 Saturated with water ,T2 Saturated with water curve and an abscissa is the water saturation porosity of the sample, and the water saturation porosity is S Saturated with water , and taking S Saturated with water as the total porosity of the sample;
D. Paramagnetic ion saturation of the sample: completely immersing the water-saturated sample after the test in a container filled with paramagnetic solution with the mass fraction of n%, placing the sample and the container into a vacuum water-saturated machine, and starting vacuum saturation;
E. Residual porosity of samples at different times was obtained: as paramagnetic ions gradually start to enter the sample under the diffusion action, H proton signals in water in the pores where the paramagnetic ions enter are shielded, the peak area of a T 2 spectrum is continuously reduced, the sample is taken out every delta T time, the surface water stain of the sample is wiped off, and then nuclear magnetic resonance test is carried out on the sample to obtain T 2 spectrum peak curves of the sample at different times; then, respectively converting the ordinate amplitude signals of the T 2 spectral peak curves at different times into porosity signals according to a marking formula A to obtain T 2 paramagnetic ion residual porosity curves, and sequentially marking the residual porosity curves as T 21、T22···T2n,T2 paramagnetic ion residual porosity curves and the areas surrounded by the abscissa as the residual porosity of the sample, and sequentially marking the residual porosity curves as S paramagnetic ion 1、S paramagnetic ion 2···S paramagnetic ion n;
F. obtaining a residual porosity change rate k t: according to the formula Calculating to obtain the residual porosity ratio alpha t according to the formula/>Calculating a residual porosity change rate k t, wherein alpha tp is the last time residual porosity ratio at t, and a curve of the calculated residual porosity change rate k t with time can be obtained along with the increase of test time;
G. Corresponding data are obtained according to different residual porosity change rates k t: when the residual porosity change rate k t is less than 5% for the first time, determining that all high-connectivity pores are diffused by paramagnetic ions, wherein the T 2 porosity curve of the sample at T D1,tD1 is marked as T 2D1, the residual porosity is marked as S paramagnetic ion D1, when the residual porosity change rate k t is less than 0.1% for the first time, determining that the residual pore paramagnetic ions are difficult to diffuse into, the T 2 porosity curve of the sample at T D2,tD2 is marked as T 2D2, and the residual porosity is marked as S paramagnetic ion D2,tD2, and stopping the nuclear magnetic test work of the saturated paramagnetic ion sample;
H. Obtaining isolated pore porosity of a sample: after the nuclear magnetic testing of the saturated paramagnetic ion solution sample is finished, the sample is put into a constant-temperature drying oven for drying, the temperature is 60 ℃ for 24 hours, the last nuclear magnetic resonance test is carried out after the drying is finished, a T 2 spectrum peak curve of the sample in a dry state is obtained, a ordinate amplitude signal is converted into a porosity signal according to a marking formula A, the obtained T 2 porosity curve is marked as a T 2 Drying ,T2 Drying curve, the area enclosed by an abscissa is marked as a drying residual porosity, the drying residual porosity is marked as S Drying , and S Drying is used as isolated pore porosity S Isolation of of the sample, namely S Isolation of ＝S Drying ;
I. Obtaining pore size distribution conditions of a sample: according to the parameters obtained in the steps G and H, the connectivity of the pores is divided into accessible pores, inaccessible pores and isolated pores from good to poor in sequence, and the porosities of different pores are calculated respectively; and then converting the T 2 porosity curves corresponding to the T 2 Saturated with water 、T2D1、T2D2、T2 Drying into pore diameter-porosity curves, namely pore diameter distribution diagrams, and finally obtaining pore diameter distribution conditions of pores with different degrees of communication.
2. The method for evaluating the pore connectivity of coal and rock based on paramagnetic ion diffusion according to claim 1, wherein the porosity calibration experiment in the step a specifically comprises the following steps: and performing nuclear magnetic resonance test on a standard sample with a certain volume and known porosity, and measuring 3-6 nuclear magnetic signals to obtain a correlation curve of nuclear magnetic signal quantity and porosity of a unit volume, thereby fitting and obtaining a marking formula A of the porosity.
3. The method for evaluating the pore connectivity of coal and rock based on paramagnetic ion diffusion according to claim 1, wherein the interval time in the step B is 12h and m is 0.05.
4. The method for evaluating the connectivity of pores of coal and rock based on paramagnetic ion diffusion according to claim 1, wherein the specific selection process of the paramagnetic solution with the mass fraction of n% in the step D is as follows: firstly, carrying out a response rule test on paramagnetic solutions with different mass fractions of paramagnetic ions through nuclear magnetic resonance signals, when the effective nuclear magnetic resonance volume is reduced to below 0.01% for the first time in the test, recording the mass fraction of the paramagnetic ions of the paramagnetic solution as m%, considering that part of the paramagnetic ions are diffused into a sample to dilute the paramagnetic ion solution, and using the paramagnetic ion solution with the mass fraction of n% for the test to enhance the shielding effect of the paramagnetic ions on H protons in water, wherein the relation between n and m satisfies (n-m) is more than or equal to 15.
5. The method for evaluating pore connectivity of coal and rock based on paramagnetic ion diffusion according to claim 1, wherein the paramagnetic solution in the step D is a paramagnetic solution having one of Mn 2+ ion, copper ion, iron ion, and chromium ion.
6. The method for evaluating the connectivity of pores of coal and rock based on paramagnetic ion diffusion according to claim 1, wherein the specific process of dividing the pores into accessible pores, more accessible pores, difficult to reach pores and isolated pores in the step I is as follows: before time t D1, paramagnetic ions firstly enter the pores with good connectivity and rapidly diffuse, and the partial pores are accessible pores in the sample, and the accessible pore porosity S Easy access ＝S Saturated with water -S paramagnetic ion D1; in the time t D1 and the time t D2, the paramagnetic ion diffusion speed is slowed down, which shows that the connectivity of the part of pores is poor, namely the pores which are difficult to reach are secondary communication pores in the sample, after the time of the pore porosity S Is difficult to reach ＝S paramagnetic ion D1-S paramagnetic ion D2;t D2 which is difficult to reach, the paramagnetic ion diffusion speed is extremely slow, and the residual porosity S paramagnetic ion D2 minus the isolated pore porosity S Isolation of at the moment is the pore porosity S Refractory to arrival of the sample, wherein the difficult to reach pores refer to the pores which are difficult to diffuse into and are mutually communicated in a short time, and the pore porosity S Refractory to arrival ＝S paramagnetic ion D2-S Drying which is difficult to reach; isolated pores are pores that do not communicate with other pores and have a porosity of S Isolation of .
7. The method for evaluating the pore connectivity of coal and rock based on paramagnetic ion diffusion according to claim 1, wherein in order to reduce errors, three or more coal and rock samples are randomly taken and the steps B to H are repeated, and the average value of the parameters in the respective steps G and H is used as final data for subsequent acquisition of the pore size distribution of the samples.
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