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 crusher chamber with absorbent cotton and acetaldehyde.</li>
	<li>Smash the coal blocks into small pieces with a jaw crusher, and shelter them in a sieve shaker equipped with standard screens of 6 and 16 mesh. Place the sorted coal powder separately according to diameter.</li>
	<li>Weigh 1,000 g and 300 g of pulverized coal with a particle size distribution of 0&#8211;1 mm and 1&#8211; 3 mm, respectively. Put them together in a beaker in a mass proportion of 0.76:0.24 and mix them well with a glass rod (with a diameter of 6 mm). 
	NOTE: According to the Gaudian-Schuman function of continuous packing theory, when the particle size distribution value (m) equals approximately 0.25 (mass of particle size is 1&#8211;3 mm: total mass = 0.24), the strength of the briquette is maximal30.</li>
	<li>To prepare the cement, put 4 g of sodium humate powder (99.99% purity) into a beaker and add approximately 96 mL of distilled water. Use a glass rod to stir them and make sure that all sodium humate is well dissolved. 
	NOTE: The concentration of cement directly affects the compressive strength of briquette. Table 1 reveals specific ratios of briquette preparation, of which the No. 2 sample has been used for the representative results.</li>
	<li>Put 230 g of mixed coal powder and 20 g of sodium humate solution into a beaker and mix them together. 
	NOTE: Based on previous experiences of making samples, a briquette produced with 250 g of material, using the cold press method, meets the size requirement of a standard rock sample22, where coal powder accounts for 92% and cement accounts for 8%.</li>
	<li>Cold-press the briquette using the shaping tools adapted to the size of the briquette (Figure 3).
	<ol>
		<li>To produce a standard-sized briquette, coat the inner surface of the shaping tools with lubricating oil. Assemble tool components #2, #3, and #4 of Figure 3, and fill the hole with 250 g of mixed material.</li>
		<li>Put component #1 of Figure 3 on top of the material, and place everything under the piston of an electro-hydraulic servo universal testing machine.</li>
		<li>Launch the software WinWdw (or equivalent) to control the electro-hydraulic servo universal testing machine. In the software, click on Force Range to set the maximum force to 50 kN, and click on Reset to clear the displacement value.</li>
		<li>Left-click on the option force loading control. Set the moving ratio at 0.1 kN/s. Set the target force value at 29.4 kN and holding time at 900 s. Then, click on Start.</li>
		<li>Take out the shaping tools and invert them onto a rubber plate. Use a rubber hammer to disassemble tool components #4, #2, #3, and #1 in that order.</li>
	</ol>
	</li>
	<li>Put the briquette in a 40 &#176;C incubator for 48 h. Then, weigh its mass with electronic scales (with a precision of 0.01 g) and measure its height and diameter with a Vernier caliper (with a precision of 0.02 mm) after drying.</li>
	<li>Measure the moisture content, ash content, and volatile content of the briquette, using a proximate analyzer (see the Table of Materials) at a temperature of 20 &#176;C and a relative humidity of 65% (per standard GB/T 212-2008). Perform a vitrinite reflectance measurement on the polished briquette, using a photometer microscope (per standard GB/T 6948-2008).</li>
	<li>Measure the uniaxial compressive strength, tensile strength, cohesion, and internal friction angle, using a universal testing machine and a strain controlled direct shear apparatus (per standard GB/T 23561-2010). Perform a Poisson ratio measurement using a resistance strain gauge (per standard GB/T 22315-2008).</li>
	<li>Conduct an adsorption test of the raw coal and the briquette, using an isotherm adsorption instrument (per standard GB/T19560-2008).</li>
</ol>
2. Experimental methods
<ol>
	<li>Laboratory setup
	<ol>
		<li>Place the test system in a quiet, vibration-free area of a clean laboratory without electromagnetic interference. The room temperature should remain stable during the test.</li>
		<li>Put the visualized vessel on the platform of the electro-hydraulic servo universal testing machine. Connect the piston of the testing machine with that of the visualized vessel with the use of a specific tool (see Figure 4).</li>
		<li>Install a manual pressure-reducing valve in the gas tank nozzle. Connect the valve with the gas filling channel at the bottom plate of the visualized vessel by soft pipe (with an inner diameter of 5 mm and a maximal pressure of 30 MPa). Link the vacuum channel and the vacuum pump with the same pipe.</li>
		<li>Fix the back door of the visualized vessel with high-strength bolts. Connect the computer, data acquisition box (DAQ box), and the embedded gas pressure sensor to the back door.</li>
	</ol>
	</li>
	<li>Air tightness test and blank measurement
	<ol>
		<li>To acquire the gas pressure data in the visualized vessel, launch the software DAQ Sensor-16 (or equivalent). On the software, click on Start.</li>
		<li>Start the vacuum pump. Open the valve V1 (Figure 2) and close V2, V3, and V4 (Figure 2). Vacuum the visualized vessel chamber. Turn off V1 and vacuum-pump it until it is under vacuum.</li>
		<li>Open V2 and the gas tank (with helium). Use the manual pressure-reducing valve to adjust the outlet pressure of the gas tank to approximate 2 MPa (relative pressure).</li>
		<li>Carefully observe the gas pressure curve displayed on DAQ Sensor-16. When it is about 2 MPa, turn off V2 and the gas tank. 
		NOTE: After 24 h, if the reduction of the gas pressure is less than 5%, the sealability of the visualized vessel is good.</li>
		<li>To measure the friction force of the loading piston moving downward, launch the software WinWdw to control the electro-hydraulic servo universal testing machine.</li>
		<li>In the software, click on Force Range to set the maximum force to 5 kN and click on Reset to clear the displacement value. Left-click on the option Displacement Loading Rate. Set the moving ratio at 1 mm/min; then, click on Start.</li>
		<li>When the displacement displayed on WinWdw is approximately 5 mm, click on Stop. Left-click on Data Save to save the force-displacement curve.</li>
		<li>Open V4 and discharge helium into air. Disassemble the back door of the visualized vessel and close V4. 
		CAUTION: The door and windows should be open for ventilation during the gas release due to the possible suffocation hazard.</li>
	</ol>
	</li>
	<li>Uniaxial compression experiment
	<ol>
		<li>Measure the height (h) and diameter (d) of the briquette with a Vernier caliper (with a precision of 0.02 mm). Weigh the mass (m) of the briquette with electronic scales (with a precision of 0.01 g). Calculate its apparent density (<img alt="Equation 1" src="/files/ftp_upload/59405/59405eq1.jpg" />) with the following equation. 
		&#8203;<img alt="Equation 2" src="/files/ftp_upload/59405/59405eq2.jpg" /></li>
		<li>Install the chain roller of the circumferential deformation test apparatus around the middle position of the briquette (Figure 5, #1) and fix the clamp holder (Figure 5, #2). Connect the sensor (Figure 5, #3) with the DAQ box through the aviation connector in the visualized vessel (Figure 2) and place them under the loading piston. 
		NOTE: To ensure the accuracy of the data acquisition, adjust the chain roller and the top surface of the sample so that they are parallel to the loading piston.</li>
		<li>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 mm/min. Press the Down button on the remote controller of the universal testing machine until the distance left between the piston and the sample is 1&#8211;2 mm. Then, assemble the back door of the visualized vessel.</li>
		<li>Repeat steps 2.2.1&#8211;2.2.2. Open V3 and the gas tank (CO2, purity = 99.99%). Use the manual pressure-reducing valve to adjust the outlet pressure of the gas tank to a certain value.</li>
		<li>Carefully observe the gas pressure curve displayed in DAQ Sensor-16. When it gets close enough to the target value, close V3 and the gas tank (CO2). 
		NOTE: When the gas pressure curve remains stable, the briquette has reached its adsorption and desorption dynamic equilibrium state. Generally, it takes 6&#8211;8 h for the briquette to fully adsorb. In this test, the adsorption time is set at 24 h.</li>
		<li>After 24 h, place the camera with a tripod beside the window of the visualized vessel. Adjust the height and angle to ensure that the image of the sample is shown in the center of the camera screen.</li>
		<li>Start the software SDU deformation acquisition V2.0 (or equivalent) to monitor the circumferential deformation of the briquette. Click on Start.</li>
		<li>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 5 kN, and click on Reset to clear the displacement value.</li>
		<li>Left-click on the option Displacement Loading Rate and set the moving ratio at 1 mm/min. Click on Start to compress the sample. At the same time, press the Start button on the camera to begin video recording.</li>
		<li>When the sample totally fails, click on Stop and Data Save, in that order, in both WinWdw and SDU deformation acquisition V2.0. Press the Start button again on the camera to stop video recording.</li>
		<li>Repeat step 2.2.8 to release CO2 in the vessel chamber. Disconnect the aviation connectors for the gas pressure sensor and circumferential deformation test apparatus.</li>
		<li>Left-click on the option Displacement Loading Rate on WinWdw. Set the moving ratio at 10 mm/min. Press the Up button on the remote controller of the universal testing machine. When the loading piston of the vessel is around 2&#8211;3 mm above the briquette, take the briquette out and remove it from the chain roller.</li>
		<li>Dismantle the connecting tool between the pistons. Clean the visualized vessel with a vacuum cleaner.</li>
	</ol>
	</li>
	<li>Completion
	<ol>
		<li>Based on the stress-axial strain curve and circumferential strain curve obtained from WinWdw and SDU deformation acquisition V2.0, calculate the volume strain of the sample with the following equation. 
		<img alt="Equation 3" src="/files/ftp_upload/59405/59405eq3.jpg" /> 
		&#8203;Here, <img alt="Equation 4" src="/files/ftp_upload/59405/59405eq4.jpg" /> = volume strain; <img alt="Equation 5" src="/files/ftp_upload/59405/59405eq5.jpg" /> = axial strain; <img alt="Equation 6" src="/files/ftp_upload/59405/59405eq6.jpg" /> = circumferential strain.</li>
		<li>Obtain the peak strength from the stress-axial strain curve. The strength reduction rate is calculated as follows. 
		<img alt="Equation 7" src="/files/ftp_upload/59405/59405eq7.jpg" /> 
		&#8203;Here, <img alt="Equation 8" src="/files/ftp_upload/59405/59405eq8.jpg" /> = strength reduction rate; <img alt="Equation 9" src="/files/ftp_upload/59405/59405eq9.jpg" /> = peak strength of the sample under a different pressure of CO2; <img alt="Equation 10" src="/files/ftp_upload/59405/59405eq10.jpg" /> = peak strength of the sample in atmospheric air.</li>
		<li>Calculate the elastic modulus using the linear stage in the stress-axial strain curve according to the following equation. 
		<img alt="Equation 11" src="/files/ftp_upload/59405/59405eq11.jpg" /> 
		Here, <img alt="Equation 12" src="/files/ftp_upload/59405/59405eq12.jpg" /> = elastic modulus of the sample; <img alt="Equation 13" src="/files/ftp_upload/59405/59405eq13.jpg" /> = stress increment of linear the stage (in megapascal); <img alt="Equation 14" src="/files/ftp_upload/59405/59405eq14.jpg" /> = strain increment of the linear stage. Calculate the elastic modulus reduction rate as follows. 
		<img alt="Equation 15" src="/files/ftp_upload/59405/59405eq15.jpg" /> 
		&#8203;Here, <img alt="Equation 16" src="/files/ftp_upload/59405/59405eq16.jpg" /> = elastic modulus reduction rate, <img alt="Equation 12" src="/files/ftp_upload/59405/59405eq12.jpg" /> = elastic modulus of the sample under a different pressure of CO2; <img alt="Equation 17" src="/files/ftp_upload/59405/59405eq17.jpg" /> = elastic modulus of the sample in atmospheric air.</li>
		<li>Select sample photos during the test and statistics fracture covering area using a program (e.g., written in MATLAB) according to the box-counting dimension method. 
		<img alt="Equation 18" src="/files/ftp_upload/59405/59405eq18.jpg" /> 
		&#8203;Here, <img alt="Equation 19" src="/files/ftp_upload/59405/59405eq19.jpg" /> = grid number to cover the fracture area at the square grid side length of <img alt="Equation 20" src="/files/ftp_upload/59405/59405eq20.jpg" />; <img alt="Equation 21" src="/files/ftp_upload/59405/59405eq21.jpg" /> = a constant; <img alt="Equation 22" src="/files/ftp_upload/59405/59405eq22.jpg" /> = fractal dimension; <img alt="Equation 20" src="/files/ftp_upload/59405/59405eq20.jpg" /> = side length of the square grid. The minimum grid size equals the pixel size in this test.
		<ol>
			<li>Calculate the correlation coefficient according to the following equation. 
			<img alt="Equation 23" src="/files/ftp_upload/59405/59405eq23.jpg" /> 
			Here,<img alt="Equation 24" src="/files/ftp_upload/59405/59405eq24.jpg" /> = correlation coefficient; <img alt="Equation 25" src="/files/ftp_upload/59405/59405eq25.jpg" /> = covariance of <img alt="Equation 26" src="/files/ftp_upload/59405/59405eq26.jpg" /> and <img alt="Equation 27" src="/files/ftp_upload/59405/59405eq27.jpg" />; <img alt="Equation 28" src="/files/ftp_upload/59405/59405eq28.jpg" /> = variance of <img alt="Equation 26" src="/files/ftp_upload/59405/59405eq26.jpg" />; <img alt="Equation 29" src="/files/ftp_upload/59405/59405eq29.jpg" /> = variance of <img alt="Equation 27" src="/files/ftp_upload/59405/59405eq27.jpg" />.</li>
		</ol>
		</li>
	</ol>
	</li>
</ol>

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C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Development+and+application+of+a+gas-solid+coupling+test+system+in+the+visualized+and+constant+volume+loading+state.">Development and application of a gas-solid coupling test system in the visualized and constant volume loading state.</a> Journal of China University of Mining &amp; Technology. 47 (1), 104-112 (2018).</li><li>Allen, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Particle Size Measure. , (1984).</li><li>Wang, H. P., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Coal+and+gas+outburst+simulation+system+based+on+CRISO+model.">Coal and gas outburst simulation system based on CRISO model.</a> Chinese Journal of Rock Mechanics and Engineering. 34 (11), 2301-2308 (2015).</li><li>Zhang, Q. H., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Exploration+of+similar+gas+like+methane+in+physical+simulation+test+of+coal+and+gas+outburst.">Exploration of similar gas like methane in physical simulation test of coal and gas outburst.</a> Rock and Soil Mechanics. 38 (2), 479-486 (2017).</li><li>Xia, G. Z. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Study on density and refractive index of near-critical fluid. , (2009).</li><li>Ruppel, T. C., Grein, C. T., Bienstock, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Adsorption+of+methane+on+dry+coal+at+elevated+pressure.">Adsorption of methane on dry coal at elevated pressure.</a> Fuel. 53 (3), 152-162 (1974).</li><li>Ranjith, P. G., Jasinge, D., Choi, S. K., Mehic, M., Shannon, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+effect+of+CO2+saturation+on+mechanical+properties+of+Australian+black+coal+using+acoustic+emission.">The effect of CO2 saturation on mechanical properties of Australian black coal using acoustic emission.</a> Fuel. 89 (8), 2110-2117 (2010).</li><li>Viete, D. R., Ranjith, P. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+effect+of+CO2,+on+the+geomechanical+and+permeability+behaviour+of+brown+coal:+Implications+for+coal+seam+CO2+sequestration.">The effect of CO2, on the geomechanical and permeability behaviour of brown coal: Implications for coal seam CO2 sequestration.</a> International Journal of Coal Geology. 66 (3), 204-216 (2006).</li><li>Jiang, Y. D., Zhu, J., Zhao, Y. X., Liu, J. H., Wang, H. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Constitutive+equations+of+coal+containing+methane+based+on+mixture+theory.">Constitutive equations of coal containing methane based on mixture theory.</a> Journal of China Coal Society. 32 (11), 1132-1137 (2007).</li><li>Xie, H. P., Gao, F., Zhou, H. W., Zuo, J. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fractal+fracture+and+fragmentation+in+rocks.">Fractal fracture and fragmentation in rocks.</a> Journal of Seismology. 23 (4), 1-9 (2003).</li><li>Miao, T. J., Yu, B. M., Duan, Y. G., Fang, Q. T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+fractal+analysis+of+permeability+for+fractured+rocks.">A fractal analysis of permeability for fractured rocks.</a> International Journal of Heat &amp; Mass Transfer. 81 (81), 75-80 (2015).</li><li>Liu, R. C., Jiang, Y. J., Li, B., Wang, X. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+fractal+model+for+characterizing+fluid+flow+in+fractured+rock+masses+based+on+randomly+distributed+rock+fracture+networks.">A fractal model for characterizing fluid flow in fractured rock masses based on randomly distributed rock fracture networks.</a> Computers &amp; Geotechnics. 65, 45-55 (2015).</li><li>Pan, J. N., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Micro-pores+and+fractures+of+coals+analysed+by+field+emission+scanning+electron+microscopy+and+fractal+theory.">Micro-pores and fractures of coals analysed by field emission scanning electron microscopy and fractal theory.</a> Fuel. 164, 277-285 (2016).</li></ol>

The authors have nothing to disclose.

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&lt;br/&gt; 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&lt;br/&gt; 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&lt;br/&gt; 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&lt;br/&gt; acquisition</td></tr><tr><td>Strain controlled&lt;br/&gt; direct shear apparatus</td><td>Beijing Aerospace Huayu Test Instrument CO.,LTD</td><td>ZJ-4A</td><td>Tensile strength, cohesion, internal friction&lt;br/&gt; 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&lt;br/&gt; 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 ...

a-uniaxial-compression-experiment-with-co2-bearing-coal-using

Research

JoVE Journal

Environment

8.6K Views.  Shandong University. Deze methode kan helpen bij het beantwoorden van belangrijke vragen op het gebied van koolstofdioxide geologische sekwestratie en coalbed methaan terugwinning over de relatie tussen de eigenschappen van steenkool en gas adsorptie. Het belangrijkste voordeel van deze techniek is dat statische en dynamische belasting kan worden toegepast op briket in constante volumetoestand en het hele experimentproces wordt gevisualiseerd door fotografische monitoring. Er is geen bijzondere eis voor de soorte...

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

Mechanical Expansion of Steel Tubing as a Solution to Leaky Wellbores

Wellbore cement, a procedural component of wellbore completion operations, primarily provides zonal isolation and mechanical support of the metal pipe (casing), and protects metal components from corrosive fluids. These are essential for uncompromised wellbore integrity. Cements can undergo multiple forms of failure, such as debonding at the cement/rock and cement/metal interfaces, fracturing, and defects within the cement matrix. Failures and defects within the cement will ultimately lead to fluid migration, resulting in inter-zonal fluid migration and premature well abandonment. Currently, there are over 1.8 million operating wells worldwide and over one third of these wells have leak related problems defined as Sustained Casing Pressure (SCP)1. 
The focus of this research was to develop an experimental setup at bench-scale to explore the effect of mechanical manipulation of wellbore casing-cement composite samples as a potential technology for the remediation of gas leaks. 
The experimental methodology utilized in this study enabled formation of an impermeable seal at the pipe/cement interface in a simulated wellbore system. Successful nitrogen gas flow-through measurements demonstrated that an existing microannulus was sealed at laboratory experimental conditions and fluid flow prevented by mechanical manipulation of the metal/cement composite sample. Furthermore, this methodology can be applied not only for the remediation of leaky wellbores, but also in plugging and abandonment procedures as well as wellbore completions technology, and potentially preventing negative impacts of wellbores on subsurface and surface environments.

This article reports on a laboratory scale investigation of an existing field procedure and its adaptation for sealing of leaky wellbores. It consists of mechanical expansion of metal pipe, which results in an improved metal/cement bond, ultimate sealing of hydraulic pathways and prevention of gas leaks caused by the presence of a microannular channel.

Wellbore cement, a procedural component of wellbore completion operations, primarily provides zonal isolation and mechanical support of the metal pipe ...

Engineering

Reservoir Condition Pore-scale Imaging of Multiple Fluid Phases Using X-ray Microtomography

X-ray microtomography was used to image, at a resolution of 6.6 &#181;m, the pore-scale arrangement of residual carbon dioxide ganglia in the pore-space of a carbonate rock at pressures and temperatures representative of typical formations used for CO2 storage. Chemical equilibrium between the CO2, brine and rock phases was maintained using a high pressure high temperature reactor, replicating conditions far away from the injection site. Fluid flow was controlled using high pressure high temperature syringe pumps. To maintain representative in-situ conditions within the micro-CT scanner a carbon fiber high pressure micro-CT coreholder was used. Diffusive CO2 exchange across the confining sleeve from the pore-space of the rock to the confining fluid was prevented by surrounding the core with a triple wrap of aluminum foil. Reconstructed brine contrast was modeled using a polychromatic x-ray source, and brine composition was chosen to maximize the three phase contrast between the two fluids and the rock. Flexible flow lines were used to reduce forces on the sample during image acquisition, potentially causing unwanted sample motion, a major shortcoming in previous techniques. An internal thermocouple, placed directly adjacent to the rock core, coupled with an external flexible heating wrap and a PID controller was used to maintain a constant temperature within the flow cell. Substantial amounts of CO2 were trapped, with a residual saturation of 0.203 &#177; 0.013, and the sizes of larger volume ganglia obey power law distributions, consistent with percolation theory.

We present a methodology for the imaging of multiple fluid phases at reservoir conditions by the use of x-ray microtomography. We show some representative results of capillary trapping in a carbonate rock sample.

X-ray microtomography was used to image, at a resolution of 6.6 &#181;m, the pore-scale arrangement of residual carbon dioxide ganglia in the pore-space ...

Measurement of the Rheology of Crude Oil in Equilibrium with CO2 at Reservoir Conditions

A rheometer system to measure the rheology of crude oil in equilibrium with carbon dioxide (CO2) at high temperatures and pressures is described. The system comprises a high-pressure rheometer which is connected to a circulation loop. The rheometer has a rotational flow-through measurement cell with two alternative geometries: coaxial cylinder and double gap. The circulation loop contains a mixer, to bring the crude oil sample into equilibrium with CO2, and a gear pump that transports the mixture from the mixer to the rheometer and recycles it back to the mixer. The CO2 and crude oil are brought to equilibrium by stirring and circulation and the rheology of the saturated mixture is measured by the rheometer. The system is used to measure the rheological properties of Zuata crude oil (and its toluene dilution) in equilibrium with CO2 at elevated pressures up to 220 bar and a temperature of 50 &#176;C. The results show that CO2 addition changes the oil rheology significantly, initially reducing the viscosity as the CO2 pressure is increased and then increasing the viscosity above a threshold pressure. The non-Newtonian response of the crude is also seen to change with the addition of CO2.

Measurement of the Rheology of Crude Oil in Equilibrium with CO2 at Reservoir Conditions

A method for measuring the rheology of crude oil in equilibrium with carbon dioxide at reservoir conditions is presented.

A rheometer system to measure the rheology of crude oil in equilibrium with carbon dioxide (CO2) at high temperatures and pressures is described. The ...

Dynamic Pore-scale Reservoir-condition Imaging of Reaction in Carbonates Using Synchrotron Fast Tomography

Underground storage permanence is a major concern for carbon capture and storage. Pumping CO2 into carbonate reservoirs has the potential to dissolve geologic seals and allow CO2 to escape. However, the dissolution processes at reservoir conditions are poorly understood. Thus, time-resolved experiments are needed to observe and predict the nature and rate of dissolution at the pore scale. Synchrotron fast tomography is a method of taking high-resolution time-resolved images of complex pore structures much more quickly than traditional &#181;-CT. The Diamond Lightsource Pink Beam was used to dynamically image dissolution of limestone in the presence of CO2-saturated brine at reservoir conditions. 100 scans were taken at a 6.1 &#181;m resolution over a period of 2 hours. The images were segmented and the porosity and permeability were measured using image analysis and network extraction. Porosity increased uniformly along the length of the sample; however, the rate of increase of both porosity and permeability slowed at later times.

Synchrotron fast tomography was used to dynamically image dissolution of limestone in the presence of CO2-saturated brine at reservoir conditions. 100 scans were taken at a 6.1 &#181;m resolution over a period of 2 h.

Underground storage permanence is a major concern for carbon capture and storage. Pumping CO2 into carbonate reservoirs has the potential to dissolve ...

Stress Distribution During Cold Compression of Rocks and Mineral Aggregates Using Synchrotron-based X-Ray Diffraction

We report detailed procedures for performing compression experiments on rocks and mineral aggregates within a multi-anvil deformation apparatus (D-DIA) coupled with synchrotron X-radiation. A cube-shaped sample assembly is prepared and compressed, at room temperature, by a set of four X-ray transparent sintered diamond anvils and two tungsten carbide anvils, in the lateral and the vertical planes, respectively. All six anvils are housed within a 250-ton hydraulic press and driven inward simultaneously by two wedged guide blocks. A horizontal energy dispersive X-ray beam is projected through and diffracted by the sample assembly. The beam is commonly in the mode of either white or monochromatic X-ray. In the case of white X-ray, the diffracted X-rays are detected by a solid-state detector array that collects the resulting energy dispersive diffraction pattern. In the case of monochromatic X-ray, the diffracted pattern is recorded using a two-dimensional (2-D) detector, such as an imaging plate or a charge-coupled device (CCD) detector. The 2-D diffraction patterns are analyzed to derive lattice spacings. The elastic strains of the sample are derived from the atomic lattice spacing within grains. The stress is then calculated using the predetermined elastic modulus and the elastic strain. Furthermore, the stress distribution in two-dimensions allow for understanding how stress is distributed in different orientations. In addition, a scintillator in the X-ray path yields a visible light image of the sample environment, which allows for the precise measurement of sample length changes during the experiment, yielding a direct measurement of volume strain on the sample. This type of experiment can quantify the stress distribution within geomaterials, which can ultimately shed light on the mechanism responsible for compaction. Such knowledge has the potential to significantly improve our understanding of key processes in rock mechanics, geotechnical engineering, mineral physics, and material science applications where compactive processes are important.

We report detailed procedures for compression experiments on rocks and mineral aggregates within a multi-anvil deformation apparatus coupled with synchrotron X-radiation. Such experiments allow quantification of the stress distribution within samples, that ultimately sheds light on compaction processes in geomaterials.

We report detailed procedures for performing compression experiments on rocks and mineral aggregates within a multi-anvil deformation apparatus (D-DIA) ...

Experimental Study of the Relationship Between Particle Size and Methane Sorption Capacity in Shale

The amount of adsorbed shale gas is a key parameter used in shale gas resource evaluation and target area selection, and it is also an important standard for evaluating the mining value of shale gas. Currently, studies on the correlation between particle size and methane adsorption are controversial. In this study, an isothermal adsorption apparatus, the gravimetric sorption analyzer, is used to test the adsorption capacity of different particle sizes in shale to determine the relationship between the particle size and the adsorption capacity of shale. Thegravimetric method requires fewer parameters and produces better results in terms of accuracy and consistency than methods like the volumetric method. Gravimetric measurements are performed in four steps: a blank measurement, preprocessing, a buoyancy measurement, and adsorption and desorption measurements. Gravimetric measurement is presently considered to be a more scientific and accurate method of measuring the amount of adsorption; however, it is time-consuming and requires a strict measuring technique. A Magnetic Suspension Balance (MSB) is the key to verify the accuracy and consistency of this method. Our results show that adsorption capacity and particle size are correlated, but not a linear correlation, and the adsorptions in particles sieved into 40 - 60 and 60 - 80 meshes tend to be larger. We propose that the maximum adsorption corresponding to the particle size is approximately 250 &#181;m (60 mesh) in the shale gas fracturing.

We use an isothermal adsorption apparatus, the gravimetric sorption analyzer, to test the adsorption capacity of different particle sizes of shale, in order to find out the relationship between particle size and the adsorption capacity of shale.

The amount of adsorbed shale gas is a key parameter used in shale gas resource evaluation and target area selection, and it is also an important standard ...

Achieving Moderate Pressures in Sealed Vessels Using Dry Ice As a Solid CO2 Source

Herein is presented a general strategy to perform reactions under mild to moderate CO2 pressures with dry ice. This technique obviates the need for specialized equipment to achieve modest pressures, and can even be used to achieve higher pressures in more specialized equipment and sturdier reaction vessels. At the end of the reaction, the vials can be easily depressurized by opening at room temperature. In the present example CO2 serves as both a putative directing group as well as a way to passivate amine substrates, thereby preventing oxidation during the organometallic reaction. In addition to being easily added, the directing group is also removed under vacuum, obviating the need for extensive purification to remove the directing group. This strategy allows the facile &#947;-C(sp3)-H arylation of aliphatic amines and has the potential to be applied to a variety of other amine-based reactions.

Achieving Moderate Pressures in Sealed Vessels Using Dry Ice As a Solid CO2 Source

Here we present a protocol for performing reactions in simple reaction vessels under low-to-moderate pressures of CO2. The reactions can be performed in a variety of vessels simply by administering the carbon dioxide in the form of dry ice, without the need for costly or elaborate equipment or set-ups.

Herein is presented a general strategy to perform reactions under mild to moderate CO2 pressures with dry ice. This technique obviates the need for ...

Chemistry

Pore-scale Imaging and Characterization of Hydrocarbon Reservoir Rock Wettability at Subsurface Conditions Using X-ray Microtomography

In situ wettability measurements in hydrocarbon reservoir rocks have only been possible recently. The purpose of this work is to present a protocol to characterize the complex wetting conditions of hydrocarbon reservoir rock using pore-scale three-dimensional X-ray imaging at subsurface conditions. In this work, heterogeneous carbonate reservoir rocks, extracted from a very large producing oil field, have been used to demonstrate the protocol. The rocks are saturated with brine and oil and aged over three weeks at subsurface conditions to replicate the wettability conditions that typically exist in hydrocarbon reservoirs (known as mixed-wettability). After the brine injection, high-resolution three-dimensional images (2 &#181;m/voxel) are acquired and then processed and segmented. To calculate the distribution of the contact angle, which defines the wettability, the following steps are performed. First, fluid-fluid and fluid-rock surfaces are meshed. The surfaces are smoothed to remove voxel artefacts, and in situ contact angles are measured at the three-phase contact line throughout the whole image. The main advantage of this method is its ability to characterize in situ wettability accounting for pore-scale rock properties, such as rock surface roughness, rock chemical composition, and pore size. The in situ wettability is determined rapidly at hundreds of thousands of points. 
The method is limited by the segmentation accuracy and X-ray image resolution. This protocol could be used to characterize the wettability of other complex rocks saturated with different fluids and at different conditions for a variety of applications. For example, it could help in determining the optimal wettability that could yield an extra oil recovery (i.e., designing brine salinity accordingly to obtain higher oil recovery) and to find the most efficient wetting conditions to trap more CO2 in subsurface formations.

This protocol is presented to characterize the complex wetting conditions of an opaque porous medium (hydrocarbon reservoir rock) using three-dimensional images obtained by X-ray microtomography at subsurface conditions.

In situ wettability measurements in hydrocarbon reservoir rocks have only been possible recently. The purpose of this work is to present a protocol to ...

Quantitative Analysis by Thermogravimetry-Mass Spectrum Analysis for Reactions with Evolved Gases

During energy conversion, material production, and metallurgy processes, reactions often have the features of unsteadiness, multistep, and multi-intermediates. Thermogravimetry-mass spectrum (TG-MS) is seen as a powerful tool to study reaction features. However, reaction details and reaction mechanics have not been effectively obtained directly from the ion current of TG-MS. Here, we provide a method of an equivalent characteristic spectrum analysis (ECSA) for analyzing the mass spectrum and giving the mass flow rate of reaction gases as precise as possible. The ECSA can effectively separate overlapping ion peaks and then eliminate the mass discrimination and temperature-dependent effect. Two example experiments are presented: (1) the decomposition of CaCO3 with evolved gas of CO2 and the decomposition of hydromagnesite with evolved gas of CO2 and H2O, to evaluate the ECSA on single-component system measurement and (2) the thermal pyrolysis of Zhundong coal with evolved gases of inorganic gases CO, H2, and CO2, and organic gases C2H4, C2H6, C3H8, C6H14, etc., to evaluate the ECSA on multi-component system measurement. Based on the successful calibration of the characteristic spectrum and relative sensitivity of specific gas and the ECSA on mass spectrum, we demonstrate that the ECSA accurately gives the mass flow rates of each evolved gas, including organic or inorganic gases, for not only single but multi-component reactions, which cannot be implemented by the traditional measurements.

Precise determination of the evolved gases&#39; flow rate is key to study the details of reactions. We provide a novel quantitative analysis method of equivalent characteristic spectrum analysis for thermogravimetry-mass spectrum analysis by establishing the calibration system of the characteristic spectrum and relative sensitivity, for obtaining the flow rate.

During energy conversion, material production, and metallurgy processes, reactions often have the features of unsteadiness, multistep, and ...

Microfluidic Fabrication Techniques for High-Pressure Testing of Microscale Supercritical CO2 Foam Transport in Fractured Unconventional Reservoirs

Pressure limitations of many microfluidic platforms have been a significant challenge in microfluidic experimental studies of fractured media. As a result, these platforms have not been fully exploited for direct observation of high-pressure transport in fractures. This work introduces microfluidic platforms that enable direct observation of multiphase flow in devices featuring surrogate permeable media and fractured systems. Such platforms provide a pathway to address important and timely questions such as those related to CO2 capture, utilization and storage. This work provides a detailed description of the fabrication techniques and an experimental setup that may serve to analyze the behavior of supercritical CO2 (scCO2) foam, its structure and stability. Such studies provide important insights regarding enhanced oil recovery processes and the role of hydraulic fractures in resource recovery from unconventional reservoirs. This work presents a comparative study of microfluidic devices developed using two different techniques: photolithography/wet-etching/thermal-bonding versus Selective Laser-induced Etching. Both techniques result in devices that are chemically and physically resistant and tolerant of high pressure and temperature conditions that correspond to subsurface systems of interest. Both techniques provide pathways to high-precision etched microchannels and capable lab-on-chip devices. Photolithography/wet-etching, however, enables fabrication of complex channel networks with complex geometries, which would be a challenging task for laser etching techniques. This work summarizes a step-by-step photolithography, wet-etching and glass thermal-bonding protocol and, presents representative observations of foam transport with relevance to oil recovery from unconventional tight and shale formations. Finally, this work describes the use of a high-resolution monochromatic sensor to observe scCO2 foam behavior where the entirety of the permeable medium is observed simultaneously while preserving the resolution needed to resolve features as small as 10 &#181;m.

Microfluidic Fabrication Techniques for High-Pressure Testing of Microscale Supercritical CO2 Foam Transport in Fractured Unconventional Reservoirs

This paper describes a protocol along with a comparative study of two microfluidic fabrication techniques, namely photolithography/wet-etching/thermal-bonding and Selective Laser-induced Etching (SLE), that are suitable for high-pressure conditions. These techniques constitute enabling platforms for direct observation of fluid flow in surrogate permeable media and fractured systems under reservoir conditions.

Pressure limitations of many microfluidic platforms have been a significant challenge in microfluidic experimental studies of fractured media. As a ...

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

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