A method for predicting coal maceral and methane adsorption capacity with longitudinal wave velocity

Abstract

A method for predicting coal maceral and methane adsorption capacity by longitudinal wave velocity, the process includes: collecting known experimental samples and dividing the same experimental samples into two parts, one part is powdery and the other part is prepared into a columnar shape with a diameter of 5 cm and a length of 10 cm; determining the content of coal maceral and methane adsorption parameters of each powdery coal sample; calculating the longitudinal wave velocity of the columnar coal sample by using an ultrasonic detector; fitting the relationship between the longitudinal wave velocity and the porosity, Langmuir volume, vitrinite group and inertinite group volume content respectively through experimental results; and predicting the coal maceral and methane adsorption capacity of unknown samples by using the established fitting formula. The coal maceral content and methane isothermal adsorption are predicted by the longitudinal wave velocity, which is easier, more reliable and simpler than directly obtaining the coal maceral content and methane adsorption parameters, and simultaneously provides a new valuable way for coalbed methane surface well exploration and prevention of underground coal mine gas related accidents.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of coalbed methane exploration and development using geophysical methods, and particularly relates to a method for predicting coal maceral and methane adsorption capacity by using P-wave velocity. TECHNICAL BACKGROUND

[0002] In the exploration of coalbed methane (CBM), coal petrology is a very important research field, which is closely related to industrial design and environmental treatment, especially to coal seams in terrestrial basins, pore structure, hydrocarbon potential and clean coal.

[0003] Methane adsorption parameters are greatly affected by coal petrology, which plays an important role in determining gas content, estimating and developing coal and CBM resources, and preventing gas-related accidents in underground coal mines.

[0004] P-wave velocity is an important parameter in geophysics, which can be widely used to predict the physical and mechanical properties of sedimentary rocks. At the same time, the test of P-wave velocity is easier, more reliable and more convenient than directly obtaining coal petrology and methane adsorption parameters. However, previous studies mainly focused on the relationship between P-wave velocity and mechanical properties, elastic properties and pore structure, and less on the correlation between coal petrology, adsorption parameters and P-wave velocity.

[0005] Traditional methods for measuring coal maceral content mainly include float-and-sink centrifugal separation technology, property index method, national standard point method and image analysis method. These measurement methods have the characteristics of large workload, complex process and high cost. However, directly predicting by P-wave velocity can greatly improve work efficiency and reduce cost. SUMMARY

[0006] In view of the deficiencies in the above background art, the present application provides a method for predicting coal maceral and methane adsorption capacity by using P-wave velocity. In order to minimize the influence of coalification, the coal samples selected in the present application have a relatively narrow maximum vitrinite reflectance (R o,max ). By measuring the P-wave velocity of coal samples to predict the maceral and methane isothermal adsorption capacity of coal, a more convenient method can be provided for estimating, developing coal and CBM resources, and preventing mine gas.

[0007] A method for predicting coal maceral and methane adsorption capacity by using P-wave velocity is provided, and the main steps are as follows.

[0008] Step 1: Collect low-rank coal samples, and the coal sample R o,max The range difference is controlled within 0.5%, and the collected known experimental samples are divided into two parts, one is in powder form, and the other is made into a columnar shape with a diameter of 5 cm and a length of 10 cm.

[0009] Step two: the industrial analysis of coal samples to obtain moisture, ash and other parameters, and determine the R o,max , porosity and coal maceral.

[0010] R o,max : under the microscope immersion objective, the reflection of the vertical incident light (λ = 546 nm) on the vitrinite polished surface within the defined area is measured by photoelectric converter, and compared with the reflection intensity of the standard material under the same conditions.

[0011] Moisture determination: a certain amount of general analysis test coal sample is weighed, dried in a heating furnace at 105-110°C in air or nitrogen stream until the mass is constant, and the moisture mass fraction of the coal sample is calculated according to the mass loss.

[0012] Ash determination: a certain amount of general analysis test coal sample is weighed, heated in a heating furnace according to the specified procedure to (815±10) °C, and ashed and burned in air or oxygen stream until the mass is constant, and the ash mass fraction of the coal sample is calculated according to the mass of the residue.

[0013] True (apparent) density: with sodium dodecyl sulfate solution as the wetting agent, the coal sample is wetted and settled in a density bottle, and the adsorbed gas is removed, and the true relative density of the coal is calculated according to the mass of the water of the same volume discharged by the coal sample.

[0014] Methane adsorption parameters: first, a coal sample of a certain particle size that reaches the equilibrium moisture is placed in a sealed container, and the volume of the experimental gas such as methane adsorbed by the coal sample under the same temperature and different pressure conditions is measured when it reaches adsorption equilibrium; then, according to the Langmuir monolayer adsorption theory, the adsorption constant, Langmuir volume (V L ) and Langmuir pressure (P L ), which characterize the adsorption characteristics of coal to methane and other experimental gases, are calculated theoretically.

[0015] Coal maceral: the polished section of fine coal is placed under reflected polarized light or incomplete orthogonal polarized light, and the volume fraction of various macerals and minerals is counted by the dot method based on the accurate identification of macerals and minerals.

[0016] Porosity: formula Φ = (ρ s -ρ a ) / ρ s ×100 (1)

[0017] Where Φ is the porosity, ρ s is the true density (g / cm 3 ), and ρ a is the apparent density (g / cm 3 ).

[0018] Step three: use non-metal ultrasonic detector (RS-ST01C) to calculate the longitudinal wave velocity of coal sample.

[0019] Pulse is sent from one end of the sample, and then received from the other end of the sample;

[0020] The calculation formula is W=L / T (2)

[0021] Wherein, W is the longitudinal wave velocity (m / s), L is the length of the test sample (m), and T is the elapsed time (s).

[0022] In order to reduce the unevenness of the longitudinal wave velocity, all dry samples are tested parallel to the bedding, three measurements of the longitudinal wave velocity are required for each sample, and the average value is used in the study; the test requires a cylindrical specimen with a length of about 10 cm and a diameter of about 5 cm.

[0023] Step four: the relationship between the longitudinal wave velocity and the porosity, Langmuir volume, vitrinite group and inertinite group content is fitted through the test results.

[0024] The coal maceral content, methane adsorption parameters and longitudinal wave velocity of each coal sample are calculated through steps two and three, the relationship between the longitudinal wave velocity and the coal maceral and methane adsorption capacity is inferred through the relationship between the longitudinal wave velocity and the porosity and density, and the relationship between the porosity, density and coal maceral content and methane adsorption capacity, and finally the relationship between the longitudinal wave velocity and the coal maceral and methane adsorption capacity is fitted through the experimental data.

[0025] Step five: based on the determination of the longitudinal wave velocity of coal, the fitting formula established is used to predict the coal maceral and methane adsorption capacity of unknown samples. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 It is the step flow chart of the present application.

[0027] Figure 2 It is the digital pulse test process of the longitudinal wave velocity of the sample.

[0028] Figure 3 It is the relationship diagram of the longitudinal wave velocity and the true density of the embodiment of the present application.

[0029] Figure 4 It is the relationship diagram of the longitudinal wave velocity and the apparent density of the embodiment of the present application.

[0030] Figure 5 It is the relationship diagram of the longitudinal wave velocity and the porosity of the embodiment of the present application.

[0031] Figure 6 It is the relationship diagram of the longitudinal wave velocity and the Langmuir volume of the embodiment of the present application.

[0032] Figure 7 The graph of the relationship between vitrinite content and porosity for the embodiment of the present application.

[0033] Figure 8 The graph of the relationship between longitudinal wave velocity and vitrinite content for the embodiment of the present application.

[0034] Figure 9 The graph of the relationship between inertinite content and density for the embodiment of the present application.

[0035] Figure 10 The graph of the relationship between inertinite and porosity for the embodiment of the present application.

[0036] Figure 11 The graph of the relationship between longitudinal wave velocity and inertinite content for the embodiment of the present application.

[0037] 1 - electric pulse generator; 2 - electric pulse; 3 - sending sensor; 4 - coal sample (10 cm long, 5 cm in diameter); 5 - receiving sensor; 6 - signal amplifier; 7 - digital oscilloscope; 8 - trigger signal. Specific implementation method

[0038] In order to better understand the present application, the present application will be described in conjunction with the following examples, which are descriptive and not limiting, and any modification that does not depart from the essence of the present application is within the scope of protection of the present application.

[0039] The present application mainly uses longitudinal wave velocity to predict coal maceral and methane isothermal adsorption capacity, which mainly includes the following five steps, and the flow chart is shown in Figure 1 .

[0040] Step one: experimental sampling, YQ-1 well exploits M6 and M7 coal seams formed by Damaogou in the Jurassic coal gas layer in Yuka coalfield in the northern Qaidam Basin, the coal seam is thick and stable in distribution, and is the target layer, sampling is performed every 0.15-0.2 m in M6 (5 coal samples) and M7 (7 coal samples) coal seams, including 9 coal and 3 carbonaceous mudstone, the collected known experimental samples are divided into two parts, one is powdery and the other is made into a column with a diameter of 5 cm and a length of 10 cm.

[0041] Step two: using the method of "invention content" step two, industrial analysis is performed on the powdery coal sample to obtain parameters such as moisture and ash content, and R o,max , porosity and coal maceral of the coal sample are measured (the results are shown in Tables 1 and 2).

[0042] Step three: using a non-metal ultrasonic detector (RS-ST01C), the longitudinal wave velocity of the coal sample is calculated according to formula (2) (the results are shown in Table 1), and the digital pulse test process of the longitudinal wave velocity of the sample is shown in Figure 2 .

[0043] Step four: fitting the relationship between P-wave velocity and porosity, Langmuir volume, vitrinite content and inertinite content by the test results.

[0044] Coal with high density usually has strong compaction strength, corresponding to faster P-wave velocity, so P-wave velocity is positively correlated with density Figure 3 4 , and density is negatively correlated with porosity, so P-wave velocity is negatively correlated with porosity.

[0045] According to the above speculation and the calculation results in Table 1, the parameters of P-wave velocity and porosity are fitted to obtain the relationship Y = 7842.4e -0.003X R 2 = 0.56, wherein X is P-wave velocity (m / s) Figure 5 .

[0046] Langmuir volume represents the methane limit capacity of coal, and the greater the porosity, the stronger the methane adsorption capacity, so Langmuir volume is positively correlated with porosity, and P-wave velocity is negatively correlated with Langmuir volume.

[0047] According to the above speculation and the calculation results in Table 1, the parameters of P-wave velocity and Langmuir volume are fitted to obtain the relationship Y = -0.04X + 101.24R 2 = 0.674, wherein X is P-wave velocity (m / s) Figure 6 .

[0048] P-wave velocity is negatively correlated with porosity and positively correlated with density, and vitrinite content is positively correlated with porosity Figure 7 , because coal with high vitrinite content can provide more micropores, and based on the above correlation between them, it can be speculated that P-wave velocity is negatively correlated with vitrinite content.

[0049] According to the above speculation and the calculation results in Table 2, the parameters of P-wave velocity and vitrinite content are fitted to obtain the relationship Y = -0.0003X 2 + 1.0731X - 924.09R 2 = 0.56, wherein X is P-wave velocity (m / s) Figure 8 .

[0050] Coal with high inertinite content has more clay minerals, and clay minerals can fill the pores and cracks in the sample, so inertinite content is positively correlated with density Figure 9 and negatively correlated with porosity Figure 10 , and based on the above relationship between them, it can be speculated that P-wave velocity is positively correlated with inertinite content. ​

[0051] Based on the above inferences and the calculation results in Table 2, the parameters of longitudinal wave velocity and inert content were fitted to obtain the relationship Y = 0.0003X. 2 -1.2797X+1405R 2 =0.88, where X is the P-wave velocity (m / s) ( Figure 11 ).

[0052] Step 5: Based on the measured longitudinal wave velocity of coal, the content of coal micro-components and methane adsorption capacity of unknown samples are predicted using the established fitting formula.

[0053] Table 1. R of the samples o,max Industrial analysis and porosity, longitudinal wave velocity and isothermal adsorption parameters

[0054]

[0055]

[0056] * - Carbonaceous mudstone.

[0057] Table 2 shows the microscopic component content of samples from the Yuka coalfield in the northern Qaidam Basin.

[0058]

[0059] V—Vitrin group; T—Structural vitrin; Te—Homogeneous vitrin; De—Matrix vitrin; Co—Agglomerated vitrin; Ge—Colloidal vitrin; VD—Detrital vitrin; I—Inertiate group; SF—Hymenofertiate; F—Filament; Mi—Microparticles; Ma—Coarse-grained; ID—Detrital-Inertiate; M—Mineral.

[0060] The above description is merely one feasible embodiment and not all embodiments. The scope of protection of this invention is not limited thereto. Any method that does not make creative improvements under the concept of this invention is within the scope of protection of this invention.

Claims

1. A method for predicting coal microstructure and methane adsorption capacity using longitudinal wave velocity, characterized in that: Based on the longitudinal wave velocity of coal, and using established fitting formulas, the microscopic components and methane adsorption capacity of unknown coal samples are predicted. The specific process is as follows: Step 1: Collect known experimental samples and divide them into two parts, one in powder form and the other in cylindrical form with a diameter of 5cm and a length of 10cm. Step 2: Determine the maximum vitrinite reflectance (Ro,max), porosity, coal petrographic micro-component content, and methane isothermal adsorption parameters for each powdered coal sample; Step 3: Calculate the longitudinal wave velocity of the columnar coal sample using a non-metallic ultrasonic detector; The calculation formula is W = L / T (1) Where W is the longitudinal wave velocity (m / s), L is the length of the test sample (m), and T is the elapsed time (s); Step 4: Fit the experimental results to derive the relationships between longitudinal wave velocity and porosity, Langmuir volume, vitrinite, and inertite volume content, respectively. Step 5: Based on the measured longitudinal wave velocity of coal, the established fitting formula is used to predict the coal microstructure and methane adsorption capacity of the unknown sample. This method is applicable to medium and low rank coals. In order to reduce the influence of coalification on the analysis of longitudinal wave velocity of coal microstructure and methane adsorption parameters, the maximum vitrinite reflectance range of the selected experimental coal samples is relatively narrow, with a range difference of less than 0.5%. High-density coal has strong compaction strength, which corresponds to a faster longitudinal wave velocity. Therefore, longitudinal wave velocity is positively correlated with density. In contrast, high-porosity coal has lower density, which suggests that longitudinal wave velocity is negatively correlated with porosity. Thus, the porosity of coal can be predicted based on longitudinal wave velocity.

2. The method for predicting coal microstructure and methane adsorption capacity using longitudinal wave velocity according to claim 1, characterized in that: Coal with high vitrinite content can provide more micropores, so vitrinite content is positively correlated with coal porosity, and it can be deduced that P-wave velocity is negatively correlated with vitrinite content; therefore, the vitrinite content in coal can be predicted based on P-wave velocity.

3. The method for predicting coal microstructure and methane adsorption capacity using longitudinal wave velocity according to claim 1, characterized in that: Coal with high inertinite content contains relatively more clay minerals. Clay minerals can fill the pores and cracks in the sample. Therefore, the inertinite content is negatively correlated with porosity and positively correlated with density. Consequently, it can be deduced that the longitudinal wave velocity is positively correlated with the inertinite content. Thus, the inertinite content in coal can be predicted based on the longitudinal wave velocity.

4. The method for predicting coal microstructure and methane adsorption capacity using longitudinal wave velocity according to claim 1, characterized in that: The Langmuir volume represents the limiting methane adsorption capacity of coal. The larger the porosity, the stronger the methane adsorption capacity. Therefore, the Langmuir volume is positively correlated with porosity. Consequently, it can be deduced that the longitudinal wave velocity is negatively correlated with the Langmuir volume. Thus, the Langmuir volume, a parameter of methane isothermal adsorption, can be predicted based on the longitudinal wave velocity.

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