Name: a uniaxial compression experiment with co2-bearing coal using a visualized and constant-volume gas-solid coupling test system
Uploaded: 2019-06-12T13:00:00
Description: Watch this Scientific Journal Video about A Uniaxial Compression Experiment with CO2-Bearing Coal Using a Visualized and Constant-Volume Gas-Solid Coupling Test System at JoVE.com

This work was supported by the China National Major Scientific Instruments Development Project (Grant No. 51427804) and the Shandong Province National Natural Science Foundation (Grant No. ZR2017MEE023).

1. Sample preparation
<ol>
	<li>Collect raw coal blocks from the 4671B6 working face from the Xinzhuangzi coal mine. Note that, due to the low strength and looseness of the structure, the raw coal is broken and probably mixed with impurities. To avoid the influence of these internal and external factors, as well as reduce the inhomogeneity of coal as much as possible, select large coal blocks (about 15 cm long, 10 cm wide, and 10 cm high).</li>
	<li>Use a tweezer to remove impurities mixed in the coal and scrub the cru...

<ol>
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Considering the danger of high-pressure gas, some critical steps are important during the test. The valves and O rings should be inspected and replaced regularly, and any source of ignition should not be allowed in the laboratory. When using the manual pressure-regulating valve, the experimenter should twist the valve slowly to make the pressure in the visualized vessel increase gradually. Do not disassemble the vessel during the test. When the experiment is finished, the back door of the vessel should be opened after th...

The increasing concentration of CO2 in the atmosphere is a direct factor causing the global warming effect. Due to the strong sorption capacity of coal, CO2 sequestration in a coal seam is regarded as a practical and environment-friendly means to reduce the global emission of greenhouse gas1,2,3. At the same time, the injected CO2 can replace CH4 and result in gas production promotion in coalbed methane recovery (ECBM)4,5,6. The ecological and economic prospects of CO2 sequestration have recently attracted worldwide attention among researchers, as well as among different international environmental protection groups and governmental agencies.
Coal is a heterogeneous, structurally anisotropic rock composed of a pore, fracture, and coal matrix. The pore structure&#160;has a large specific surface area, which can adsorb a large amount of gas, playing a vital role in gas sequestration, and the fracture is the main path for free gas flow7,8. This unique physical structure leads to a great gas adsorption capacity for CH4 and CO2. Mine gas is deposited in coalbed in a few forms: (1) adsorbed on the surface of micropores and larger pores; (2) absorbed in the coal molecular structure; (3) as free gas in fractures and larger pores; and (4) dissolved in deposit water. The sorption behavior of coal to CH4 and CO2 causes matrix swelling, and further studies demonstrate that it is a heterogeneous process and is related to the coal lithotypes9,10,11. In addition, gas sorption can result in damage in the constitutive relation of coal12,13,14.
The raw coal sample is generally used in coal and CO2 coupling experiments. Specifically, a large piece of raw coal from the working face in a coal mine is cut to prepare a sample. However, the physical and mechanical properties of raw coal inevitably have a high dispersion degree due to the random spatial distribution of natural pores and fractures in a coal seam. Moreover, the gas-bearing coal is soft and difficult to be reshaped. According to the principles of the orthogonal experimental method, the briquette, which is reconstituted with raw coal powder and cement, is regarded as an ideal material used in the coal sorption test15,16. Being cold-pressed with metal dies, its strength can be preset and remains stable by adjusting the quantity of cement, which benefits the comparative analysis of the single-variable effect. Additionally, although the porosity of the briquette sample is ~4-10 times, that of the raw coal sample, similar adsorption and desorption characteristics and stress-strain curve have been found in the experimental research17,18,19,20. In this paper, a scheme of a similar material for gas-bearing coal has been adopted to prepare the briquette21. The raw coal was taken from the 4671B6 working face in the Xinzhuangzi Coal Mine, Huainan, Anhui Province, China. The coal seam is approximately 450 m below ground level and 360 m below sea level, and it dips at about 15&#176; and is approximately 1.6 m in thickness. The height and diameter of the briquette sample are 100 mm and 50 mm, respectively, which is the recommended size suggested by the International Society for Rock Mechanics (ISRM)22.
The previous uniaxial or triaxial loading test instruments for gas-bearing coal experiments under laboratory conditions have some shortages and limitations, presented as fellows23,24,25,26,27,28: (1) during the loading process, the vessel volume decreases with the piston moving, causing fluctuations in gas pressure and disturbances in gas sorption; (2) the real-time image monitoring of samples, as well as circumferential deformation measurements in a high gas pressure environment, is difficult to conduct; (3) they are limited to stimulation of dynamic load disturbances on preloaded samples to analyze their mechanical response characteristics. In order to improve the instrument precision and data acquisition in the gas-solid coupling condition, a visualized and constant-volume test system29 has been developed (Figure 1), including (1) a visualized loading vessel with a constant volume chamber, which is the core component; (2) a gas filling module with a vacuum channel, two filling channels, and a releasing channel; (3) an axial loading module consisting of an electro-hydraulic servo universal testing machine and control computer; (4) a data acquisition module comprised of a circumferential displacement measurement apparatus, a gas pressure sensor, and a camera at the window of the visualized loading vessel.
The core visualized vessel (Figure 2) is specifically designed so that two adjusting cylinders are fixed on the upper plate and their pistons move simultaneously with the loading one through a beam, and the sectional area of the loading piston is equal to the sum of that of the adjusting cylinders. Flowing through an inner hole and soft pipes, the high-pressure gas in the vessel and the two cylinders is connected. Therefore, when the vessel-loading piston moves downward and compresses the gas, this structure can offset the change in volume and eliminate pressure interference. In addition, the enormous gas-induced counterforce exerting on the piston is prevented during the test, significantly improving the safety of the instrument. The windows, which are equipped with tempered borosilicate glass and situated on three sides of the vessel, provide a direct way to take a photograph of the sample. This glass has been successfully tested and proved to resist up to 10 MPa gas with a low expansion rate, high strength, light transmittance, and chemical stability29.
This paper describes the procedure to perform a uniaxial compression experiment of CO2-bearing coal with the new visualized and constant-volume gas-solid coupling test system, which includes the description of all pieces that prepare a briquette sample using raw coal powder and sodium humate, as well as the successive steps to inject high-pressure CO2 and conduct uniaxial compression. The whole sample deformation process is monitored using a camera. This experimental approach offers an alternative way to quantitively analyze the adsorption-induced damage and fracture evolution characteristic of gas-bearing coal.

<table>
	<tbody>
		<tr>
			<td>3Y-Leica MPV-SP photometer microphotometric system</td>
			<td>Leica,Germany</td>
			<td>M090063016</td>
			<td>Used for vitrinite 
			reflectance measurement</td>
		</tr>
		<tr>
			<td>Automatic isotherm adsorption instrument</td>
			<td>BeiShiDe Instrument Technology (Beijing)CO.,Ltd.</td>
			<td>3H-2000PH</td>
			<td>Isothermal adsorption test</td>
		</tr>
		<tr>
			<td>Electro hydraulic servo universal testing machine</td>
			<td>Jinan Shidaishijin testing machine CO.,Ltd</td>
			<td>WDW-100EIII</td>
			<td>Used to provide 
			axial pressure</td>
		</tr>
		<tr>
			<td>Gas pressure sensor</td>
			<td>Beijing Star Sensor Technology CO.,LTD</td>
			<td>CYYZ11</td>
			<td>Gas pressure monitoring</td>
		</tr>
		<tr>
			<td>Gas tank(carbon dioxide/helium)</td>
			<td>Heifei Henglong Gas.,Ltd</td>
			<td></td>
			<td>Gas resource</td>
		</tr>
		<tr>
			<td>high-speed camera</td>
			<td>Sony corporation</td>
			<td>FDR-AX30</td>
			<td>Image monitoring</td>
		</tr>
		<tr>
			<td>Incubator</td>
			<td>Yuyao YuanDong Digital Instrument Factory</td>
			<td>XGQ-2000</td>
			<td>Briquette drying</td>
		</tr>
		<tr>
			<td>jaw crusher</td>
			<td>Hebi Tianke Instrument CO.,Ltd</td>
			<td>EP-2</td>
			<td>Coal grinding</td>
		</tr>
		<tr>
			<td>Manual pressure reducing valve</td>
			<td>Shanghai Saergen Instrument CO.,Ltd</td>
			<td>R41</td>
			<td>Outlet gas pressure adjustment</td>
		</tr>
		<tr>
			<td>Proximate Analyzer</td>
			<td>Changsha Kaiyuan Instrument CO.,Ltd</td>
			<td>5E-MAG6700</td>
			<td>Coal industrial analysis</td>
		</tr>
		<tr>
			<td>Resistance strain gauge</td>
			<td>Jinan Sigmar Technology CO.,LTD</td>
			<td>ASMB3-16/8</td>
			<td>Poisson ratio measurement</td>
		</tr>
		<tr>
			<td>Sieve shaker (6,16mesh)</td>
			<td>Hebi Tianguan Instrument CO.,Ltd</td>
			<td>GZS-300</td>
			<td>Coal powder shelter</td>
		</tr>
		<tr>
			<td>Soft pipe</td>
			<td>Jinan Quanxing High pressure pipe CO.,Ltd</td>
			<td></td>
			<td>Inner diameter=5 mm 
			maximal pressure=30 MPa</td>
		</tr>
		<tr>
			<td>Standard rock sample circumferential deformation test apparatus</td>
			<td>Huainan Qingda Machinery CO.,Ltd</td>
			<td></td>
			<td>Circumferential deformation 
			acquisition</td>
		</tr>
		<tr>
			<td>Strain controlled 
			direct shear apparatus</td>
			<td>Beijing Aerospace Huayu Test Instrument CO.,LTD</td>
			<td>ZJ-4A</td>
			<td>Tensile strength, cohesion, internal friction 
			angle measurement</td>
		</tr>
		<tr>
			<td>Vaccum pump</td>
			<td>Fujiwara,Japan</td>
			<td>750D</td>
			<td>Used to vaccumize the vessel</td>
		</tr>
		<tr>
			<td>Valve</td>
			<td>Jiangsu Subei Valve Co.,Ltd</td>
			<td>S4 NS-MG16-MF1</td>
			<td>Gas seal</td>
		</tr>
		<tr>
			<td>Visual loading vessel</td>
			<td>Huainan Qingda Machinery CO.,Ltd</td>
			<td></td>
			<td>Instrument for sample 
			loading and real-time monitoring</td>
		</tr>
	</tbody>
</table>

a uniaxial compression experiment with co2-bearing coal using a visualized and constant-volume gas-solid coupling test system

Injecting carbon dioxide (CO2) into a deep coal seam is of great significance for reducing the concentration of greenhouse gases in the atmosphere and increasing the recovery of coalbed methane. A visualized and constant-volume gas-solid coupling system is introduced here to investigate the influence of CO2 sorption on the physical and mechanical properties of coal. Being able to keep a constant volume and monitor the sample using a camera, this system offers the potential to improve instrument accuracy and analyze fracture evolution with a fractal geometry method. This paper provides all steps to perform a uniaxial compression experiment with a briquette sample in different CO2 pressures with the gas-solid coupling test system. A briquette, cold-pressed by raw coal and sodium humate cement, is loaded in high-pressure CO2, and its surface is monitored in real-time using a camera. However, the similarity between the briquette and the raw coal still needs improvement, and a flammable gas such as methane (CH4) cannot be injected for the test. The results show that CO2 sorption leads to peak strength and elastic modulus reduction of the briquette, and the fracture evolution of the briquette in a failure state indicates fractal characteristics. The strength, elastic modulus, and fractal dimension are all correlated with CO2 pressure but not with a linear correlation. The visualized and constant-volume gas-solid coupling test system can serve as a platform for experimental research about rock mechanics considering the multifield coupling effect.

The average mass of the briquette sample was 230 g. Depending on the industrial analysis, the briquette exhibited a moisture content of 4.52% and an ash content of 15.52%. Furthermore, the volatile content was approximately 31.24%. As the sodium humate was extracted from the coal, the components of the briquette were similar to raw coal. The physical characteristics are displayed in Table 2.
The comparison of th...

Watch this Scientific Journal Video about A Uniaxial Compression Experiment with CO2-Bearing Coal Using a Visualized and Constant-Volume Gas-Solid Coupling Test System at JoVE.com

A Uniaxial Compression Experiment with CO2-Bearing Coal Using a Visualized and Constant-Volume Gas-Solid Coupling Test System

This protocol demonstrates how to prepare a briquette sample and conduct a uniaxial compression experiment with a briquette in different CO2 pressures using a visualized and constant-volume gas-solid coupling test system. It also aims to investigate changes in terms of coal&#8217;s physical and mechanical properties induced by CO2 adsorption.

A Uniaxial Compression Experiment with CO2-Bearing Coal Using a Visualized and Constant-Volume Gas-Solid Coupling Test System

Injecting carbon dioxide (CO2) into a deep coal seam is of great significance for reducing the concentration of greenhouse gases in the atmosphere and ...

This method can help answer key questions in field of carbon dioxide geological sequestration and coalbed methane recovery about the relationship between coal's properties and gas adsorption. The main advantage of this technique is that static and dynamic load can apply on briquette in constant volume state and whole experiment process is visualized by photographic monitoring. There is no particular requirement for the species of the sample, therefore this method can apply to mechanical tests of any porous rock.To begin, weigh 1000 grams and 300 grams of pulverized coal with a particle size distribution of zero to one millimeter and one to three millimeters respectively. Put them together in a beaker in a mass proportion of 0.76 to 0.24 and use a six millimeter diameter glass rod to mix them well. To prepare the cement, put four grams of sodium humate powder into a beaker and add approximately 96 millimeters of distilled water.Use a glass rod to stir them and make sure that all sodium humate is well dissolved. Then mix 230 grams of mixed coal powder and 20 grams of sodium humate solution in a beaker. To produce a standard sized briquette coat the inner surface of the shaping tools with lubricating oil.Assemble tool components Bottom plate, main body and ring and fill the hole with 250 grams of mixed material. Put Press piston on top of the mixed material and place all parts under the Piston of an electrohydraulic servo universal testing machine. Launch the software WinWdw to control the electrohydraulic servo universal testing machine.In the software, click on force range to set the maximum force to 50 kilonewtons and click on reset to clear the displacement value. Left-click on the option force loading control. Set the moving ratio to 0.1 kilonewtons per second.Set the target force value at 29.4 kilonewtons and holding time at 900 seconds. Then click on start. After that take out the shaping tools and invert them onto a rubber plate.Use a rubber hammer to disassemble tool components in the order from the bottom to the top. Put the briquette in a 40 degree Celsius incubator for 48 hours. First, fix the back door of the visualized vessel with high strength bolts.Connect the computer, data acquisition box and the embedded gas pressure sensor to the back door. To acquire the gas pressure data in the visualized vessel, launch the software Data Acquisition Sensor. On the software, click on start.Open the valve V1 and close V2, V3 and V4 to vacuum the visualized vessel chamber. After 30 minutes turn off V1 and vacuum pump. Open V2 and the gas tank with helium.Use the manual pressure reducing valve to adjust the outlet pressure of the gas tank. Carefully observe the gas pressure curve displayed on data acquisition sensor 16. When it reaches about two megapascals turn off V2 and the gas tank.Then on the computer, launch the software WinWdw to measure the friction force of the loading piston moving downward in the testing machine. In the software, click on force range to set the maximum force to five kilonewtons and click on reset to clear the displacement value. Left-click on the option displacement loading rate and set the moving ratio to one millimeter per minute.The click on start. Open V4 and discharge helium into the air. Disassemble the back door of the visualized vessel and close V4.Measure the height and diameter of the briquette using a vernier caliper with a precision of 0.02 millimeters.Weigh the mass of the briquette using an electronic scale with a precision of 0.01 grams. Install the chain roller of the circumferential deformation test apparatus around the middle position of the briquette and fix the clamp holder. Connect the sensor with the data acquisition box through the aviation connector in the visualized vessel and place them under the loading piston.To ensure the accuracy of the data acquisition, adjust the chain roller and the top surface of the briquette so that they are parallel to the loading piston. Then launch WinWdw to control the universal testing machine. In the software, left-click on the option displacement loading rate.Set the moving ratio at 10 millimeters per minute. On the remote controller, press the down button for the universal testing machine until the distance left between the piston and the briquette is around one to two millimeters. Then assemble the back door of the visualized vessel.Vacuum the visualized vessel chamber as previously. Next open V3 and the gas tank of carbon dioxide at purity 99.99%Use the manual pressure reducing value to adjust the outlet pressure of the gas tank. Carefully observe the gas pressure curve displayed in data acquisition sensor 16.When it gets close enough to the target value, close V3 and the gas tank. After 24 hours of adsorption time the gas pressure curve remains stable and the briquette has reached its adsorption and desorption dynamic equilibrium state. Place a camera with a tripod beside the window of the visualized vessel and adjust the height and angle to ensure that image of the sample is shown in the center of the camera screen.Start the software SDU Deformation Acquisition v2.0 to monitor the circumferential deformation of the briquette. Click on start. On WinWdw click on new sample and type in the height and diameter of the briquette.Click on sectional area and then click on confirm. Click on force range to set the maximum force to five kilonewtons and click on reset to clear the displacement value. Left-click on the option displacement loading rate and set the moving ratio at one millimeter per minute.Click on start to compress the sample. At the same time press the start button on the camera to begin video recording. When the sample totally fails, click on stop and data save in both WinWdw and SDU Deformation Acquisition v2.0.Then press the start button again on the camera to stop video recording. Open V4 to release carbon dioxide in the vessel chamber and disassemble the back door of the vessel. Disconnect the aviation connectors for the gas pressure sensor and circumferential deformation test apparatus.Left-click on the option displacement loading rate on WinWdw, set the moving ratio at 10 millimeters per minute. Press the up button on the remote controller of the universal testing machine. When the loading piston of the vessel is around two to three millimeters above the briquette take the briquette out and remove it from the chain roller.Dismantle the connection tool between the pistons and use a vacuum cleaner to clean the visualized vessel. In this experiment, the isothermal adsorption test proved similar capacity for methane gas adsorption between raw coal and briquette. The strength of the briquette samples used in the test had some fluctuation, but it was rather slight and had little influence on the analysis of the experimental results.When under different carbon dioxide pressures, ranging from zero to two megapascals, the stress axial strain curves showed obvious compaction, elastic and plastic deformation phases. As the carbon dioxide pressure increased the peak strength of the coal sample decreased, where it showed a nonlinear relationship. The elastic modulus decreased under the carbon dioxide saturated condition and that the relationship between the elastic modulus decreased and the gas pressure was nonlinear as well.The images obtained through the camera events the fracture's evolution on the sample's surface, under different carbon dioxide pressures. The box counting dimension method was adopted to describe the feature of fractures in failure state under different carbon dioxide pressures. The correlation coefficients between the box number and the side length were all more than 0.95.The values of the fractal dimension were proportional to those of carbon dioxide pressure and their trend indicated similarity to that of the degree of damage to the coal body. The most important thing is to have a good understanding of test procedure and some experience in experiments of rock mechanics and operation of high-pressure gas. After its improvement, this technique can provide a way for the study of porous media and gas coupling effect under uniaxial or triaxial loading state.Because high pressure gas tanks are used in this technique, people need to operate carefully during gas filling and releasing.

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