Experimental study on the influence of CO₂ injection pressure on gas diffusion coefficient

Most of the coal seams in China are characterized by low pressure, low permeability and low saturation. The methane gas in the coal seam is slowly desorbed and diffused slowly, which affects the production rate of coalbed methane. The coal samples from Zhaozhuang Coal Mine of Jinmei Group were selected to study the influence of different gas injection pressure on CH₄ displacement process and the variation law of CH₄ diffusion coefficient in the process of displacement. Independently developed CO₂ displacement CH₄ test platform was used to carry out the experiments under different gas injection pressures such as 0.6, 0.8 and 1.0 MPa. The results show that the greater the displacement pressure, the shorter the time to reach the maximum CH₄ emission, the faster the breakthrough time of CO₂, the greater the displacement efficiency and the better the displacement effect. The process of CH₄ gas displacement can be divided into three stages, first increasing sharply, then slowly increasing, and finally keeping stable. Under the same gas injection pressure, the gas diffusion coefficient first increases and then decreases with time. When the gas injection pressure is 0.6, 0.8 and 1.0 MPa, the corresponding maximum values of gas diffusion coefficient are respectively 2.27×10^-5, 3.36×10^-5, 4.62×10^-5 cm²/s. According to the experimental results, CO₂ plays an important role in CH₄ displacement, displacement adsorption-desorption and dilution displacement under different gas injection pressures. The migration of CH₄ gas in each stage is different, so the injection-gas flow rate, pressure and other parameters can be reasonably adjusted according to the corresponding migration of CH₄ gas in the experimental stage to make the injection and flooding technology collocation more efficient. The research results present a theoretical guidance for deep CO₂ burial and efficient gas extraction(CBM).