A method for determining the degree of superheated steam penetration into coal using low-field nuclear magnetic resonance technology.

Abstract

This invention relates to the field of permeability enhancement technology for low-permeability coal seams, and more particularly to a method for determining the degree of superheated steam penetration into coal seams using low-field nuclear magnetic resonance (NMR) technology. The method includes preparing standard coal samples that have been saturated with water, dried, and treated with superheated steam; measuring their transverse relaxation spectra using a low-field NMR spectrometer; calculating the integral area of ​​the transverse relaxation spectra; and constructing a penetration coefficient. K This invention enables non-destructive, quantitative, and precise evaluation, providing a reasonable basis for optimizing superheated steam permeability enhancement process parameters and determining coal type suitability.

Description

Technical Field

[0001] This invention relates to the field of permeability enhancement technology for low-permeability coal seams, and more particularly to a method for determining the degree of superheated steam penetration into coal using low-field nuclear magnetic resonance technology. Background Technology

[0002] With the increasing global demand for clean energy, coalbed methane, as an efficient and low-carbon unconventional natural gas resource, has become an important part of energy strategy. However, about 70% of coal seams in my country are low-permeability coal seams, with low natural fracture development and dense pore structure, which restricts the channels for gas desorption and migration, seriously hindering the efficient extraction and large-scale development of coalbed methane.

[0003] To improve the permeability of low-permeability coal seams, various permeability enhancement technologies have been explored, including hydraulic fracturing, conventional steam injection, and chemical permeability enhancement. However, these traditional technologies generally have limitations: while hydraulic fracturing can effectively create fractures, it consumes a large amount of water resources, easily damages the reservoir, and has poor applicability in arid or ecologically sensitive areas; conventional saturated steam, due to its low temperature and pressure, has limited thermal penetration depth, making it difficult to effectively modify deep, dense coal seams; and while chemical permeability enhancement can improve pore structure, it suffers from problems such as reagent residue, groundwater pollution risks, and short-term effects, making it difficult to achieve long-term, environmentally friendly permeability enhancement. Against this backdrop, superheated steam permeability enhancement technology has gradually become a research hotspot due to its combination of high-temperature, high-pressure characteristics and environmental friendliness. This technology injects superheated steam at 300–500℃ into the coal seam. Under the combined action of thermal stress and high pressure, it not only promotes thermal expansion of the coal seam skeleton and expands existing microfractures but also induces the formation of new fractures, while reducing gas adsorption capacity and significantly improving coal seam permeability and gas desorption efficiency.

[0004] Whether the superheated steam can completely penetrate the target coal body is the key prerequisite for its permeability enhancement effect. If the steam only acts on the surface of the coal body and fails to penetrate into the interior, it will lead to a local permeability enhancement phenomenon where the surface layer is effective but the deep layer is ineffective, which will seriously affect the overall extraction efficiency and engineering economy. Summary of the Invention

[0005] The purpose of this invention is to provide a method for determining the degree of superheated steam penetration into coal using low-field nuclear magnetic resonance (NMR) technology, through calculation... T The volume of pores in coal is obtained by integrating the area of ​​the spectral data and then applying it to the superheated steam coal sample. T 2. Spectral integral area and dry coal sample T The difference in the integral area of ​​the 2-spectrum and the water-saturated coal sample T 2. Spectral integral area and dry coal sample T By comparing the difference in the integral area of ​​the two spectra, the penetration coefficient K is obtained, and then the degree of penetration of superheated steam into the coal body is obtained, so as to solve the technical problems involved in the background art.

[0006] This invention provides a method for determining the degree of superheated steam penetration into coal using low-field nuclear magnetic resonance (NMR) technology, comprising:

[0007] S1. Prepare standard cylindrical coal samples with a diameter and height of 50 mm, and assign a unique number to each standard cylindrical coal sample;

[0008] S2. Place the numbered standard cylindrical coal sample into the sample chamber of the low-field nuclear magnetic resonance spectrometer, set a unified set of test parameters Q, and acquire the transverse relaxation time spectrum of the coal sample under the parameter set Q. T 2. And calculate according to the integral formula. T 2. Spectral integral area;

[0009] S3. After vacuuming the coal sample for 2 hours at a vacuum level ≤ -0.095 MPa, completely immerse it in distilled water for 24 hours. Remove the sample, wipe off any surface moisture, and immediately perform low-field nuclear magnetic resonance (NMR) testing according to parameter set Q to obtain... T 2. Measure the spectrum and calculate the integral area. When the relative error between two consecutive measurements of the integral area is less than 0.5%, the final measured integral area is recorded as... ;

[0010] S4. The coal sample treated in S3 is dried at 40℃ for 24 hours. After drying, low-field nuclear magnetic resonance (NMR) testing is performed according to parameter set Q to obtain... T 2. Measure the spectrum and calculate the integral area. When the relative error between two consecutive measurements of the integral area is less than 0.5%, the final measured integral area is recorded as... ;

[0011] S5. Place the dried coal sample treated in S4 into a sealed reaction chamber, introduce superheated steam at 150℃ and 1±0.05 MPa and maintain for 3 hours. After treatment, transfer the sample to the low-field nuclear magnetic resonance spectrometer sample cell within 3 minutes, complete the test according to parameter set Q, and obtain the results. T 2. Measure the spectrum and calculate the integral area. When the relative error between two consecutive measurements of the integral area is less than 0.5%, the final measured integral area is recorded as... ;

[0012] S6: Calculate the penetration coefficient of superheated steam through the coal body K The expression is as follows:

[0013]

[0014] like K If >1, it is determined that the superheated steam has completely penetrated the coal body; if K If ≤ 1, it is determined that the superheated steam has not completely penetrated the coal body.

[0015] In some embodiments, the low-field nuclear magnetic resonance spectrometer uses a Carr-Purcell-Meiboom-Gill sequence for radio frequency pulse excitation, with a working magnetic field strength of 0.05–0.5 T.

[0016] In some embodiments, the parameter set Q includes at least: a resampling wait time TW of 2000 ms, a resampling number NS of 16 times, a number of echoes NECH of 3000, and an echo time TE of 0.2 ms.

[0017] In some embodiments, the sealed reaction chamber maintains a temperature fluctuation of no more than ±2°C and a pressure fluctuation of no more than ±0.02 MPa during the superheated steam treatment process.

[0018] In some embodiments, the sealed reaction chamber is made of high-temperature resistant stainless steel and is equipped with a real-time temperature and pressure monitoring and feedback regulation system.

[0019] In some embodiments, the preparation of the dried coal sample in S4 is carried out in a 40°C constant temperature drying oven.

[0020] In some embodiments, the integral formula is as follows:

[0021]

[0022] in, S for T 2. Spectral integral area for T 2. Spectral minimum relaxation time for T 2. Maximum relaxation time of spectrum A ( T 2) For T 2. Signal amplitude corresponding to relaxation time.

[0023] In some embodiments, Take 0.1ms, Take 1000ms.

[0024] Compared with the prior art, the present invention has the following advantages:

[0025] 1. Coal samples were prepared in three standardized states: saturated with water, dried, and treated with superheated steam, and obtained under strictly controlled testing parameters. T 2. Integral area of ​​the spectrum to construct the penetration coefficient K As a criterion, it enables non-destructive, quantitative, and precise determination of the degree to which superheated steam penetrates the coal body;

[0026] 2. Penetration coefficient of the present invention KIt can be directly used to screen the applicability of superheated steam permeation technology to different coal types, and provide experimental basis for the design of key parameters such as injection temperature, pressure and time in on-site engineering.

[0027] 3. This invention combines T 2. Multi-peak analysis of the spectrum further reveals the synergistic evolution of the micro, meso and macro pore structure of coal under the action of superheated steam;

[0028] 4. Compared with the prior art, the measurement results of the present invention are less affected by the external environment, and the method of the present invention is simple to operate. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 This is a flowchart of the method for determining the degree of superheated steam penetration into coal using low-field nuclear magnetic resonance technology according to the present invention;

[0031] Figure 2 Low-field nuclear magnetic resonance (NMR) of coal sample No. 1 after it was fully saturated with water, fully dried, and fully treated with superheated steam. T 2. Spectra;

[0032] Figure 3 Low-field nuclear magnetic resonance (NMR) of coal sample No. 2 after it was fully saturated with water, fully dried, and fully treated with superheated steam. T 2. Spectra;

[0033] Figure 4 Low-field nuclear magnetic resonance (NMR) of coal sample No. 3 after it was fully saturated with water, fully dried, and fully treated with superheated steam. T 2. Spectra;

[0034] Figure 5 This is a flowchart of the method for determining the degree of superheated steam penetration into coal using low-field nuclear magnetic resonance technology according to the present invention.

[0035] It should be noted that: Figure 1 Formula (1) in the present invention is the integral formula. Detailed Implementation

[0036] The following will be based on embodiments of the present invention. Figures 1-5 The technical solutions in the embodiments of the present invention will be clearly and completely described together. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0037] Partial interpretation:

[0038] Long relaxation interval: refers to a specific range (usually milliseconds or even longer) in which the relaxation time of a material or system is relatively long during the process of recovering from a non-equilibrium state to a thermodynamic equilibrium state after being subjected to external disturbances (such as electric field, magnetic field, stress, light irradiation, etc.).

[0039] Example

[0040] This example provides a method for determining the degree of superheated steam penetration into coal using low-field nuclear magnetic resonance (NMR) technology, comprising at least the following steps: S1. Preparing standard cylindrical coal samples with a diameter and height of 50 mm, and uniquely numbering each standard cylindrical coal sample; S2. Placing the numbered standard cylindrical coal samples in the sample chamber of a low-field NMR instrument, setting a unified set of test parameters Q, and acquiring the transverse relaxation time spectrum of the coal samples under the parameter set Q. T 2. And calculate according to the integral formula. T 2. Spectral integral area; S3. After evacuating the coal sample under a vacuum of ≤-0.095 MPa for 2 hours, completely immerse it in distilled water for 24 hours. After removal, wipe off the surface moisture and immediately perform low-field nuclear magnetic resonance testing according to parameter set Q to obtain... T 2. Measure the spectrum and calculate the integral area. When the relative error between two consecutive measurements of the integral area is less than 0.5%, the final measured integral area is recorded as... S4. The coal sample treated in S3 is dried at 40℃ for 24 hours. After drying, low-field nuclear magnetic resonance (NMR) testing is performed according to parameter set Q to obtain... T 2. Measure the spectrum and calculate the integral area. When the relative error between two consecutive measurements of the integral area is less than 0.5%, the final measured integral area is recorded as... S5. Place the dried coal sample treated in S4 into a sealed reaction chamber, introduce superheated steam at 150℃ and 1±0.05 MPa and maintain for 3 hours. After treatment, transfer it to the sample cell of the low-field nuclear magnetic resonance spectrometer within 3 minutes, complete the test according to parameter set Q, and obtain the results. T 2. Measure the spectrum and calculate the integral area. When the relative error between two consecutive measurements of the integral area is less than 0.5%, the final measured integral area is recorded as... S6: Calculate the penetration coefficient K The expression is as follows:

[0041] like K If >1, it is determined that the superheated steam has completely penetrated the coal body; if K If ≤ 1, it is determined that the superheated steam has not completely penetrated the coal body.

[0042] To better understand this invention, the details are as follows:

[0043] First, standard cylindrical coal samples with a diameter and height of 50 mm were prepared, and each standard cylindrical coal sample was uniquely numbered. In this embodiment, they were numbered ①, ②, and ③ in sequence to ensure the consistency of subsequent tests.

[0044] Next, the numbered standard cylindrical coal samples were placed in the sample chamber of the low-field nuclear magnetic resonance spectrometer, and a unified set of test parameters Q was set. The transverse relaxation time spectrum of the coal samples was then acquired under the parameter set Q. T 2. And calculate according to the integral formula. T 2. Spectral integration area. The numbered coal sample is placed in the sample chamber of the low-field NMR spectrometer. In this embodiment, a PQ001 low-field NMR spectrometer manufactured by Suzhou Newmai Analytical Instruments Co., Ltd. is used. A unified set of test parameters Q is set: resampling waiting time TW is 2000 ms, the number of resampling times NS is 16, the number of echoes NECH is 3000, and the echo time TE is 0.2 ms.

[0045] The aforementioned parameter combination can effectively capture the transverse relaxation signal of hydrogen protons in coal samples while ensuring the signal-to-noise ratio. The raw CPMG spin echo train was acquired under these parameters and processed using a multi-exponential inversion algorithm to obtain... T 2. Spectrum, and based on the integral formula ,calculate T 2. Spectral integral area, where, S for T 2. Spectral integral area for T 2. Spectral minimum relaxation time for T 2. Maximum relaxation time of spectrum for T 2. Signal amplitude corresponding to relaxation time.

[0046] Next, the coal sample was vacuumed for 2 hours at a vacuum level ≤ -0.095 MPa, then completely immersed in distilled water for 24 hours. After removal, the surface moisture was wiped off, and low-field nuclear magnetic resonance (NMR) testing was immediately performed according to parameter set Q to obtain... T 2. Measure the spectrum and calculate the integral area. When the relative error between two consecutive measurements of the integral area is less than 0.5%, the final measured integral area is recorded as... Specifically, coal sample No. 1 was placed in a vacuum desiccator and evacuated for 2 hours at a vacuum level ≤ -0.095 MPa to remove trapped gases from the coal sample. Subsequently, it was completely immersed in distilled water and soaked at room temperature and pressure for 24 hours to allow water to fully diffuse into all open pores and reach saturation equilibrium. After removal, the surface moisture was gently wiped off with absorbent paper to avoid interference from external water films on the NMR signal. The sample was immediately placed in the NMR spectrometer sample cell and tested using the aforementioned parameter set Q to obtain the NMR signal under saturated conditions. T 2 spectra, such as Figure 2 The curve for complete water saturation is shown, and its integral area is calculated. .like If the integrated area S differs significantly from the original coal sample, the water saturation treatment and testing process must be repeated until the relative error between two consecutive measured integrated areas is less than 0.5%. At this point, the last measured integrated area is recorded as... For example, in the test of coal sample No. 1, the result was obtained after three repeated operations. This satisfies the repeatability requirement.

[0047] Then, the coal sample treated with S3 was dried at 40℃ for 24 hours. After drying, low-field nuclear magnetic resonance (NMR) testing was performed according to parameter set Q to obtain... T 2. Measure the spectrum and calculate the integral area. When the relative error between two consecutive measurements of the integral area is less than 0.5%, the final measured integral area is recorded as... Specifically, the No. 1 coal sample, which had undergone the water saturation test, was transferred to a constant temperature drying oven and dried at 40°C for 24 hours. This temperature is much lower than the pyrolysis initiation temperature of coal (usually >200°C), which can effectively remove free water and some bound water from the pores, while avoiding irreversible shrinkage or chemical changes in the coal structure due to high temperature.

[0048] After drying, the coal sample was quickly placed in the sample cell of the nuclear magnetic resonance spectrometer and tested using the same parameter set Q to obtain the results under dry conditions. T 2 spectra, such as Figure 2 The curve of complete drying is shown in the figure, and the integral area is calculated. Similarly, repeatability verification was performed: if the error between two consecutive tests exceeded 0.5%, the sample was dried again and tested until the data stabilized. For coal sample No. 1, the final determination was made... .

[0049] Next, the dried coal sample after the aforementioned treatment was placed in a sealed reaction chamber, and superheated steam at a temperature of 150℃ and a pressure of 1±0.05 MPa was introduced and maintained for 3 hours. After the treatment, the sample was transferred to the low-field nuclear magnetic resonance spectrometer sample cell within 3 minutes, and the test was completed according to the parameter set Q. The T2 spectrum was obtained and the integrated area was calculated. When the relative error of the integrated area measured twice consecutively is less than 0.5%, the final measured integrated area is recorded as... Specifically, the dried No. 1 coal sample was placed into a custom-designed sealed reaction chamber (made of high-temperature resistant stainless steel, equipped with a real-time temperature and pressure monitoring and feedback control system). Superheated steam at 150°C was introduced into the chamber, and the pressure was maintained at 1±0.05 MPa for 3 hours. During this process, the superheated steam permeated the coal mass through heat conduction and convection. Its high temperature caused thermal expansion of the coal matrix, generating thermal stress, which in turn created new microcracks and pore channels on top of the existing pores. Simultaneously, the steam condensed into liquid water inside the coal mass, dynamically filling these newly created pores. After the treatment, the reaction chamber was quickly opened, the coal sample was removed, and transferred to the nuclear magnetic resonance (NMR) spectrometer sample cell within 3 minutes to complete the test, preventing the condensate from evaporating or migrating due to ambient humidity, which could affect the results. T 2. Signal authenticity, such as Figure 2 The complete curve of the superheated steam treatment is shown to prevent condensate evaporation or migration caused by ambient humidity, which could affect... T 2. Signal authenticity. Test results. T The area of ​​the spectral integral is denoted as The final value was determined after repeatability verification. For coal sample No. 1, .

[0050] Finally, calculate the penetration coefficient. K Based on the above three sets of integral area data, substitute the penetration coefficient... K Calculation formula We can obtain coal sample No. 1. K =(7118.04-5089.17) / (5510.65-5089.17) ≈4.813. Since... K >1 indicates that the superheated steam has completely penetrated the coal sample, reflecting that the coal is suitable for superheated steam permeation enhancement technology.

[0051] Similarly, such as Figure 3 The same process was applied to coal sample No. 2 to obtain... , , The calculated value is K = (6236.33 - 4519.96) / (6501.28 - 4519.96) ≈ 0.866 ≤ 1, which is the value of K after superheated steam treatment. TAlthough the overall signal intensity of the spectrum 2 has increased, the increase does not exceed that of the saturated state, indicating that the steam only penetrates locally and has not formed a new, penetrating pore network. Therefore, it is determined that the penetration is incomplete, and this type of coal is not suitable for using superheated steam to enhance permeability.

[0052] As for coal sample No. 3, Figure 4 , measured , , , K = (5615.66-3114.59) / (3658.37-3114.59) ≈4.599>1, indicating that the steam completely penetrates the coal sample, reflecting that the coal is suitable for superheated steam permeation enhancement technology.

[0053] It should be noted that, throughout the entire process of implementing the method of this invention, all NMR tests were performed on the same instrument, with the same probe, and under the same ambient temperature and humidity (25±2℃, relative humidity<50%), in order to eliminate systematic errors caused by equipment drift and environmental fluctuations. T The multi-peak distribution of the spectrum also provides information on the evolution of microstructure:

[0054] like Figure 2 In the middle, after superheated steam treatment T 2. Spectrum in long relaxation interval ( The increase in the number of large pores in coal is reflected in the significant enhancement of the pores.

[0055] Figure 3 The limited increase in this region reflects the restricted penetration of superheated steam into the coal.

[0056] In addition, it should be noted that the pressure and temperature control system of the sealed reaction chamber ensures that the temperature fluctuation does not exceed ±2℃ and the pressure fluctuation does not exceed ±0.02 MPa during the process, thus ensuring the stability of the superheated steam state.

[0057] Based on the above, it can be understood that the method of the present invention can not only quantitatively determine the penetration degree of a single sample, but also make horizontal comparisons of different coal types. K This value guides the application of superheated steam permeability enhancement technology in gas extraction from low-permeability coal seams.

[0058] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. It will be apparent to those skilled in the art that the invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered illustrative and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the scope of the invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0059] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for determining the degree of superheated steam penetration into coal using low-field nuclear magnetic resonance technology, characterized in that, include: S1. Prepare standard cylindrical coal samples with a diameter and height of 50 mm, and assign a unique number to each standard cylindrical coal sample; S2. Place the numbered standard cylindrical coal sample into the sample chamber of the low-field nuclear magnetic resonance spectrometer, set a unified set of test parameters Q, and acquire the transverse relaxation time spectrum of the coal sample under the parameter set Q. T 2. And calculate according to the integral formula. T 2. Spectral integral area; S3. After vacuuming the coal sample for 2 hours at a vacuum level ≤ -0.095 MPa, completely immerse it in distilled water for 24 hours. Remove the sample, wipe off any surface moisture, and immediately perform low-field nuclear magnetic resonance (NMR) testing according to parameter set Q to obtain... T 2. Measure the spectrum and calculate the integral area. When the relative error between two consecutive measurements of the integral area is less than 0.5%, the final measured integral area is recorded as... where i is the number of tests; S4. The coal sample treated in S3 is dried at 40℃ for 24 hours. After drying, low-field nuclear magnetic resonance (NMR) testing is performed according to parameter set Q to obtain... T 2. Measure the spectrum and calculate the integral area. When the relative error between two consecutive measurements of the integral area is less than 0.5%, the final measured integral area is recorded as... ; S5. Place the dried coal sample treated in S4 into a sealed reaction chamber, introduce superheated steam at 150℃ and 1±0.05MPa and maintain for 3 hours. After treatment, transfer the sample to the low-field nuclear magnetic resonance spectrometer sample cell within 3 minutes, complete the test according to parameter set Q, and obtain the results. T 2. Measure the spectrum and calculate the integral area. When the relative error between two consecutive measurements of the integral area is less than 0.5%, the final measured integral area is recorded as... ; S6: Calculate the penetration coefficient of superheated steam through the coal body K The expression is as follows: like K If > 1, it is determined that the superheated steam has completely penetrated the coal body; if K If ≤ 1, it is determined that the superheated steam has not completely penetrated the coal body.

2. The method according to claim 1, characterized in that, The low-field nuclear magnetic resonance spectrometer uses the Carr-Purcell-Meiboom-Gill sequence for radio frequency pulse excitation, with a working magnetic field strength of 0.05–0.5 T.

3. The method according to claim 1, characterized in that, The parameter set Q includes at least the following: resampling wait time TW is 2000 ms, resampling number NS is 16 times, echo number NECH is 3000, and echo time TE is 0.2 ms.

4. The method according to claim 1, characterized in that, During the superheated steam treatment process, the temperature fluctuation inside the sealed reaction chamber is maintained at no more than ±2℃ and the pressure fluctuation is maintained at no more than ±0.02 MPa.

5. The method according to claim 1, characterized in that, The sealed reaction chamber is made of high-temperature resistant stainless steel and is equipped with a real-time temperature and pressure monitoring and feedback adjustment system.

6. The method according to claim 1, characterized in that, The preparation of the dried coal sample in S4 was carried out in a 40℃ constant temperature drying oven.

7. The method according to claim 1, characterized in that, The integral formula is as follows: in, S for T 2. Spectral integral area for T 2. Spectral minimum relaxation time for T 2. Maximum relaxation time of spectrum A ( T 2) For T 2. Signal amplitude corresponding to relaxation time.

8. The method according to claim 7, characterized in that, Take 0.1ms, Take 1000ms.

Images