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

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.