Experimental Study on Replacing Coal Seam CH₄ with CO₂ Gas.

Area of Science:

• Geochemistry
• Petroleum Engineering
• Analytical Chemistry

Background:

• Traditional methods struggle to differentiate adsorbed and free gas during CO2-CH4 displacement in coal, hindering accurate replacement ratio calculation.
• Understanding gas behavior is crucial for optimizing enhanced coalbed methane (eCM) recovery and carbon sequestration.

Purpose of the Study:

• To apply low-field Nuclear Magnetic Resonance (NMR) technology to analyze methane (CH4) desorption and CO2/CH4 displacement efficiency.
• To accurately distinguish between free and adsorbed gas phases during CO2 injection into coal seams.
• To investigate the impact of time, temperature, and CO2 injection pressure on CH4 displacement and replacement ratios.

Main Methods:

• Utilized low-field NMR spectroscopy to analyze the T2 spectrum of coal samples.
• Monitored CH4 desorption and CO2/CH4 displacement dynamics under varying experimental conditions.
• Quantified free and adsorbed gas phases to determine CO2/CH4 replacement efficiency.

Main Results:

• The CO2-CH4 replacement process exhibits three distinct stages: initial, dominant, and weakening competitive adsorption.
• Cumulative CH4 desorption increases with replacement time and CO2 injection pressure, peaking at approximately 40°C.
• Cumulative replacement ratio correlates positively with time and CO2 pressure, stabilizing over time and reaching a maximum at 40°C.

Conclusions:

• Low-field NMR is effective for distinguishing gas phases and quantifying CO2/CH4 replacement in coal.
• Optimal CH4 displacement by CO2 occurs at around 40°C, with higher CO2 injection pressures enhancing CH4 desorption.
• Findings provide critical insights for optimizing enhanced coalbed methane recovery strategies.