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

This method can help answer key questions in the shale gas field about the relationship between particle size and the adsorption capacity of shale. The main advantage of this technique is that the gravimetric method requires fewer parameters and produces better results in terms of accuracy and simulating actual underground structure. First, turn on the magnetic suspension balance computer, and start the magnetic suspension coupling controller.

Open the gas two helium cylinder, and roughly adjust it to the appropriate output pressure with the control valve. Next, confirm that the magnetic suspension coupling controller knob is in the Off position. Then, turn on the instrument temperature controller, the oil bath heater, and the vacuum pump.

Switch the coupling controller to the Zero Point position, wait one to two minutes, and confirm that the readings are normal and stable. Check the Measuring Point positions in the same way. Then, set the controller to the Zero Point position.

Start the System Control Software. Select the Blank Measurement program, and open the configuration window. Name the measurement, select gas two to use helium gas, select other fluid, and select fluid bath.

Set the maximum pressure to 70 bars with seven pressure steps, the pressure ramp to two bars per minute, and the oil bath temperature to 50 degrees Celsius. Run the program, and calculate the mass and volume of the empty sample container. After the blank test has finished, set the gas two helium pressure setpoint to one bar.

Start gas dosing to fill the sample pool with helium until it is close to atmospheric pressure. Then, stop gas dosing. Next, pull down the temperature sensor.

Disassemble the insulating cotton jacket and the double-walled thermostat. Support the double-walled thermostat when the last screw is removed. Unload the double-walled thermostat, being careful not to knock the sensor traverse tube with the wrench.

Then, disassemble the sample pool by removing the six screws symmetrically. Carefully open the sample pool. Next, check whether the instrument balance frame is level.

To adjust the balance frame, turn the magnetic coupling controller knob to Off, and carefully raise or lower the feet and support screws. Once the system is balanced, take out the sample container, and load a clean, crushed shale rock sample into the sample container. Install the sample container, and then set the coupling to the appropriate position.

Then, reinstall the sample pool, being careful to keep the edges within the flange. Install the screws symmetrically, and turn the screws with gentle, uniform force. After installing the sample pool, try installing the double-walled thermostat to ensure that it can be installed smoothly in the later buoyancy measurement.

Once this has been confirmed, remove the double-walled thermostat. Next, turn the coupling controller knob to each position to check the balance. Insert the temperature sensor, and install the electrical heating sheath and insulation cover.

Connect the electrical heating power supply, and turn on the power. Then, select the Pretreatment program in the instrument software, and open the configuration menu. Name the measurement, choose the vacuum source, and select electrical heating.

Set the sample temperature to 105 degrees Celsius, the coupling temperature to 20 degrees Celsius, and the duration to 600 minutes. Then, run the Pretreatment program. Afterwards, turn off the electrical heating, and disconnect the power supply.

Remove the electric heating sheath, insert the temperature sensor, and install the double-walled thermostat. Wrap the thermostat with an adhesive insulation cover. Next, in the software, select the Buoyancy Measurement program, and open the configuration window.

Set the oil bath temperature to 50 degrees Celsius. Set the maximum pressure to 70 bar, the pressure ramp to two bars per minute, and the number of pressure steps to seven. Then, run the process, which usually takes five to eight hours.

After that, select the Sorption Measurement program. If desorption will be evaluated, maintain the oil bath at 50 degrees Celsius, and set 19 pressure points, stepping up from zero bars to 250 bars and back to zero bars. Then, choose gas one instead of gas two for the experimental gas, and run the program.

Use a pressurized pump if needed to manually increase the experimental gas pressure to the target value. Let the process finish before removing any components from the instrument. Two shale specimens were crushed and divided by particle size.

For all samples, the absolute methane adsorption capacity linearly increased with pressure between zero and 60 bars. After that, the absolute adsorption capacity gradually plateaued. The excess adsorption capacity followed a similar trend at lower pressures, reached a maximum at about 80 to 100 bars, and then decreased with increasing pressure.

Shale samples with particle sizes of about 250 micrometers generally had the highest maximum absolute and excess adsorption capacity. This was attributed to the rock being crushed into smaller particles and to micropores and mesopores connecting to form mesopores and macropores, decreasing the total specific surface area of the shale. We first had this idea for this method when we considered how large particles should be used for isothermal adsorption experiments of shale in different regions.

Once mastered, this technique can be done in 24 hours if it is performed properly.