Method for measuring coal penetration degree of superheated steam by using low-field nuclear magnetic resonance technology

Abstract

The invention relates to the technical field of permeability improvement of low-permeability coal seams, in particular to a method for measuring the degree of superheated steam penetrating through a coal body through a low-field nuclear magnetic resonance technology, which comprises the following steps: preparing a standard coal sample subjected to water saturation, drying and superheated steam treatment, and measuring the transverse relaxation spectrum of the standard coal sample by adopting a low-field nuclear magnetic resonance spectrometer; calculating the integral area of the transverse relaxation spectrum and constructing a penetration coefficient K; according to the method, lossless, quantitative and refined evaluation is realized, and a reasonable basis is provided for parameter optimization of a superheated steam anti-reflection process and coal type applicability judgment.

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] The application provides a method for determining the degree of superheated steam penetrating into coal bodies by using low-field nuclear magnetic resonance technology, comprising the following steps: S1. Preparing standard cylindrical coal samples with a diameter and a height of 50 mm, and numbering each standard cylindrical coal sample uniquely; S2. Placing the numbered standard cylindrical coal sample into a sample tank of a low-field nuclear magnetic resonance instrument, setting a unified test parameter set Q, and collecting the transverse relaxation time spectrum of the coal sample under the parameter set Q T 2, and calculating the spectrum integral area according to an integral formula T 2; S3. Immersing the coal sample into distilled water for 24 hours after vacuumizing the coal sample under a vacuum degree of less than or equal to-0.095 MPa for 2 hours, wiping off the surface-attached water after taking out the coal sample, immediately performing low-field nuclear magnetic resonance test according to the parameter set Q, obtaining the spectrum T 2 and calculating the integral area, and when the relative errors of the integral areas measured continuously for two times are less than 0.5%, recording the finally measured integral area as ; S4. Drying the coal sample treated in S3 under the condition of 40 DEG C for 24 hours, performing low-field nuclear magnetic resonance test according to the parameter set Q after drying, obtaining the spectrum T 2 and calculating the integral area, and when the relative errors of the integral areas measured continuously for two times are less than 0.5%, recording the finally measured integral area as ; S5. Placing the dried coal sample treated in S4 into a sealed reaction cavity, introducing superheated steam with a temperature of 150 DEG C and a pressure of 1±0.05 MPa into the reaction cavity and keeping for 3 hours, transferring the coal sample into a sample tank of a low-field nuclear magnetic resonance instrument within 3 minutes after the treatment, performing test according to the parameter set Q, obtaining the spectrum T 2 and calculating the integral area, and when the relative errors of the integral areas measured continuously for two times are less than 0.5%, recording the finally measured integral area as ; S6. Calculating the penetration coefficient of the superheated steam penetrating into the coal body K , and the expression is as follows:

[0007] If K >1, it is determined that the superheated steam has completely penetrated into the coal body; if K ≤ 1, it is determined that the superheated steam has not completely penetrated into the coal body.

[0008] In some embodiments, the low-field nuclear magnetic resonance instrument adopts a Carr-Purcell-Meiboom-Gill sequence for radio frequency pulse excitation, and the working magnetic field strength is 0.05-0.5 T.

[0009] In some embodiments, the parameter set Q at least includes: a repetition sampling waiting time TW of 2000 ms, a repetition sampling number NS of 16 times, an echo number NECH of 3000, and an echo time TE of 0.2 ms.

[0010] In some embodiments, the sealed reaction cavity maintains a temperature fluctuation of no more than ±2℃ and a pressure fluctuation of no more than ±0.02 MPa in the superheated steam treatment process.

[0011] In some embodiments, the sealed reaction cavity is made of high-temperature-resistant stainless steel and is equipped with a temperature and pressure real-time monitoring and feedback adjustment system.

[0012] In some embodiments, the preparation of the dry coal sample of S4 is based on a 40℃ constant-temperature drying box.

[0013] In some embodiments, the integral formula is as follows:

[0014] Wherein, S is T 2 the spectrum integral area, is T 2 the minimum relaxation time of the spectrum, is T 2 the maximum relaxation time of the spectrum, A ( T 2) is T 2 the signal amplitude corresponding to the relaxation time.

[0015] In some embodiments, 0.1 ms is taken, 1000 ms is taken.

[0016] Compared with the prior art, the present application has the following beneficial effects: 1. By standardizing the preparation of coal samples in three states of water saturation, dryness and superheated steam treatment, and obtaining the T 2 spectrum integral area under strictly controlled test parameters, the penetration coefficient K is constructed as a criterion to realize nondestructive, quantitative and refined determination of the degree of superheated steam penetration into coal bodies; 2. The penetration coefficient K of the present application can be directly used for the suitability screening of different coal types for superheated steam penetration technology, and provides experimental basis for the design of key parameters such as injection temperature, pressure and time in field engineering; 3. The present application combines T 2 spectrum multi-peak analysis to further intuitively reveal the synergistic evolution law of the micro, meso and macro pore structures of coal bodies under the action of superheated steam; 4. Compared with the prior art, the measurement result of the present application is less affected by the external environment, and the method of the present application is simple to operate. BRIEF DESCRIPTION OF DRAWINGS

[0017] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced as follows. Obviously, the drawings in the following description only constitute some embodiments of the present application, and for those skilled in the art, other drawings can also be obtained without creative labor on the basis of these drawings.

[0018] Figure 1 Flow chart of the method for determining the degree of superheated steam penetrating into coal body by the low-field nuclear magnetic resonance technology of the present application; Figure 2 Low-field nuclear magnetic resonance spectrum of the No. 1 coal sample after complete saturation, complete drying and complete superheated steam treatment T 2; Figure 3 Low-field nuclear magnetic resonance spectrum of the No. 2 coal sample after complete saturation, complete drying and complete superheated steam treatment T 2; Figure 4 Low-field nuclear magnetic resonance spectrum of the No. 3 coal sample after complete saturation, complete drying and complete superheated steam treatment T 2; Figure 5 Flow chart of the method for determining the degree of superheated steam penetrating into coal body by the low-field nuclear magnetic resonance technology of the present application.

[0019] It should be noted that: Figure 1 Formula (1) in the above table is the integral formula of the present application. DETAILED DESCRIPTION

[0020] The technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings of the embodiments of the present application. Figures 1-5 It should be noted that the described embodiments only constitute some embodiments of the present application, rather than all the embodiments.

[0021] Part of the interpretation: Long relaxation interval: refers to a specific interval in which the relaxation time is in a relatively long range (usually milliseconds, or even longer) during the process of recovering from a non-equilibrium state to a thermodynamic equilibrium state after a material or system is subjected to external disturbance (such as electric field, magnetic field, stress, light irradiation, etc.).

[0022] EMBODIMENT The present example provides a method for determining the degree of superheated steam penetrating coal body by low-field nuclear magnetic resonance technology, at least comprising: 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 sample in the sample tank of a low-field nuclear magnetic resonance instrument, setting a unified test parameter set Q, collecting the transverse relaxation time spectrum of the coal sample under the parameter set Q T 2, and calculating the integral area of the spectrum according to the integral formula T 2; S3. After vacuumizing the coal sample under a vacuum degree of ≤-0.095 MPa for 2 hours, completely immersing it in distilled water for 24 hours, removing it and wiping off the surface-attached moisture, immediately performing low-field nuclear magnetic resonance test according to the parameter set Q to obtain T 2 spectrum and calculate the integral area, when the relative error of the integral areas measured continuously for two times is less than 0.5%, recording the finally measured integral area as ; S4. drying the coal sample treated in S3 at 40℃ for 24 hours, after drying, performing low-field nuclear magnetic resonance test according to the parameter set Q to obtain T 2 spectrum and calculate the integral area, when the relative error of the integral areas measured continuously for two times is less than 0.5%, recording the finally measured integral area as ; S5. placing the dried coal sample treated in S4 in a sealed reaction cavity, introducing superheated steam with a temperature of 150℃ and a pressure of 1±0.05 MPa and keeping for 3 hours, after treatment, transferring it to the sample tank of the low-field nuclear magnetic resonance instrument within 3 minutes, completing the test according to the parameter set Q to obtain T 2 spectrum and calculate the integral area, when the relative error of the integral areas measured continuously for two times is less than 0.5%, recording the finally measured integral area as ; S6: calculating the penetration coefficient K , the expression is as follows:

[0023] If K >1, it is determined that the superheated steam has completely penetrated the coal body; if K ≤ 1, it is determined that the superheated steam has not completely penetrated the coal body.

[0024] In order to better understand the present application, the following is specifically described: First, standard cylindrical coal samples with a diameter and height of 50 mm are prepared, and each standard cylindrical coal sample is uniquely numbered, which are numbered as ①, ②, ③ in this embodiment to ensure the consistency of subsequent tests.

[0025] Then, the numbered standard cylindrical coal sample is placed in the sample tank of a low-field nuclear magnetic resonance instrument, a unified test parameter set Q is set, and the transverse relaxation time spectrum of the coal sample is collected under the parameter set Q T2. 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.

[0026] 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.

[0027] 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.

[0028] Then, the coal sample treated with S3 was dried at 40°C 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.

[0029] 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, 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... .

[0030] 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. The test result T 2 spectrum integral area is recorded as , and the final value is determined after repeatability verification . For the coal sample No. 1, .

[0031] Finally, the penetration coefficient K is calculated. K Based on the above three groups of integral area data, the calculation formula of the penetration coefficient is substituted to obtain the penetration coefficient of the coal sample No. 1 K =(7118.04-5089.17) / (5510.65-5089.17) ≈4.813. Since K >1, it is determined that the superheated steam has completely penetrated the coal sample, which reflects that the coal is suitable for superheated steam penetration technology.

[0032] Similarly, for the coal sample No. 2, Figure 3 , the same process is carried out to obtain , , , and the calculation result is K=(6236.33-4519.96) / (6501.28-4519.96) ≈ 0.866≤1. The overall signal intensity of the T 2 spectrum after steam treatment is improved, but the increase does not exceed the saturated state, which indicates that the steam only penetrates locally and does not form a penetrating new pore network, so it is determined that it is not completely penetrated, and the coal is not suitable for superheated steam penetration.

[0033] For the coal sample No. 3, Figure 4 , the measurement results are , , , K =(5615.66-3114.59) / (3658.37-3114.59) ≈4.599>1, which indicates that the steam completely penetrates the coal sample, which reflects that the coal is suitable for superheated steam penetration technology.

[0034] It should be noted that in the implementation process of the whole method, all nuclear magnetic tests are completed on the same instrument, the same probe, and the same environmental temperature and humidity (25±2℃, relative humidity <50%) to eliminate system errors caused by equipment drift and environmental fluctuations. T The multi-peak distribution of the 2 spectrum also provides microstructure evolution information: Figure 2 For example, in the T 2 spectrum after superheated steam treatment, there is obvious enhancement in the long relaxation interval ( ), which reflects that the number of large pores in the coal has increased; Figure 3 The limited increase in this region reflects the limited penetration of superheated steam in this coal.

[0035] In addition, it should be noted that the pressure and temperature control system of the sealed reaction cavity ensures that the temperature fluctuation during the treatment process does not exceed ±2℃, and the pressure fluctuation does not exceed ±0.02 MPa, thereby ensuring the stability of the superheated steam state.

[0036] Based on the above, it can be understood that the method of the present application not only can quantitatively judge the penetration degree of a single sample, but also can compare the penetration degrees of different coal types K values, thereby guiding the application of the superheated steam penetration technology in the gas extraction of low-permeability coal seams.

[0037] The above shows and describes the basic principles and main features of the present application and the advantages of the present application, and it is obvious for those skilled in the art that the present application is not limited to the details of the above exemplary embodiments, and the present application can be realized in other specific forms without departing from the spirit or basic characteristics of the present application. Therefore, from any point of view, the embodiments should be regarded as exemplary and non-limiting, and the scope of the present application is defined by the appended claims rather than the above description, and therefore all changes falling within the meaning and scope of the equivalent elements of the claims are intended to be included in the present application. Any reference signs in the claims should not be regarded as limiting the claims involved.

[0038] In addition, it should be understood that although the present specification is described in terms of embodiments, not every embodiment contains only one independent technical solution, and the description manner of the specification is only for the sake of clarity, and 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 those skilled in the art can understand.

Claims

1. A method for determining the degree of penetration of superheated steam into a coal body using low-field nuclear magnetic resonance technology, characterized in that, The method comprises the following steps: S1. Preparing standard cylindrical coal samples with a diameter and height of 50 mm, and numbering each standard cylindrical coal sample uniquely; S2. Place the numbered standard cylindrical coal sample into the sample cell of the low-field nuclear magnetic resonance instrument, set the uniform test parameter set Q, under which the transverse relaxation time spectrum of the coal sample is collected T 2, and according to the integral formula, calculate T 2 the spectrum integral area; S3. After vacuumizing the coal sample for 2 hours under the condition of vacuum degree ≤-0.095 MPa, completely immerse the coal sample in distilled water for 24 hours, wipe off the surface attached moisture after taking out, immediately perform low-field nuclear magnetic resonance test according to parameter set Q, obtain T 2 spectrum and calculate integral area, when the relative error of integral area measured for two times continuously is less than 0.5%, record the finally measured integral area as , i is test number; S4. The coal sample treated by S3 is dried at 40℃ for 24 hours, after drying, the low-field nuclear magnetic resonance test is performed according to parameter set Q to obtain T 2 spectrum and calculate the integral area, when the relative error of the integral area measured for two times continuously is less than 0.5%, the finally measured integral area is recorded as ; S5. The dried coal sample treated in S4 is placed in a sealed reaction chamber, superheated steam with a temperature of 150℃ and a pressure of 1±0.05MPa is introduced and maintained for 3 hours, after the treatment is completed, it is transferred to the sample tank of the low-field nuclear magnetic resonance instrument within 3 minutes, the test is completed according to the parameter set Q, and the T 2 spectrum is obtained and the integral area is calculated, when the relative error of the integral areas measured continuously for two times is less than 0.5%, the finally measured integral area is recorded as ; S6: Calculate the penetration coefficient of superheated steam penetrating into coal body K The expression is as follows: If K > 1, it is determined that the superheated steam has completely penetrated the coal body; if K ≤ 1, it is determined that the superheated steam has not completely penetrated the coal body.

2. The method of claim 1, wherein, The low-field nuclear magnetic resonance instrument adopts a Carr-Purcell-Meiboom-Gill sequence for radio frequency pulse excitation, and has a working magnetic field strength of 0.05-0.5 T.

3. The method of claim 1, wherein, The parameter set Q at least comprises: a repetition sampling waiting time TW of 2000 ms, a repetition sampling number NS of 16 times, an echo number NECH of 3000, and an echo time TE of 0.2 ms.

4. The method of claim 1, wherein, The sealed reaction cavity maintains a temperature fluctuation of no more than ±2 ℃ and a pressure fluctuation of no more than ±0.02 MPa in the superheated steam treatment process.

5. The method of claim 1, wherein, The sealed reaction cavity is made of high-temperature-resistant stainless steel and is equipped with a temperature and pressure real-time monitoring and feedback adjustment system.

6. The method of claim 1, wherein, The preparation of the dry coal sample of S4 is based on a 40 ℃ constant temperature drying box.

7. The method of claim 1, wherein, The integral formula is as follows: wherein, S is T 2 spectral integral area, is T 2 spectral minimum relaxation time, is T 2 spectral maximum relaxation time, A ( T 2) is T 2 signal amplitude corresponding to the relaxation time.

8. The method of claim 7, wherein, Take 0.1 ms, Take 1000 ms.

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