Numerical simulation study on the coal-bed methane displacement effect of different gas injection components

The main components of gas injection to promote methane extraction are N₂, CO₂, and air, but there is currently limited research on the comparison of displacement effects for different injection components. In order to solve the above problems, a mathematical model for gas injection displacement considering fracture gas seepage and matrix pore gas diffusion is established. Based on the validation of the model, the process of injecting gas into coal samples for methane displacement is simulated. The effects of different injection components (N₂, CO₂, and air) on methane displacement are compared and studied under the same injection pressure and coal permeability conditions. The results show the following points. ① Under the same injection time, the volume fraction of injected gas gradually decreases from the injection end to the exhaust end, with the highest injected gas volume fraction near the injection end. The methane volume fraction gradually increases, with the highest methane volume fraction near the exhaust end. As the injection time increases, the area with an increase in injected gas volume fraction gradually moves towards the exhaust end until it covers the entire coal sample. The area with a decrease in methane volume fraction also gradually moves towards the exhaust end until it covers the entire coal sample. It indicates that the methane in the coal sample is gradually displaced and driven out of the whole sample. ② Within the same injection time, from the injection end to the exhaust end, the volume fractions of N₂, CO₂, and air injected gases and methane have similar changes. That is, from the injection end to the exhaust end, the volume fraction of injected gas gradually decreases and the methane volume fraction gradually increases. With the increase of injection time, the area of increase in injected gas volume fraction increases. The volume fraction of injected gas and methane at the same injection time and at the same position of the coal sample complement each other, that is, they add up to 100%. ③ The ranking of the methane displacement effects of three types of injected gases is CO₂>air> N₂. ④ The analysis of the gas volume fraction at the exhaust end shows that the gas volume fraction at the exhaust end can be divided into breakthrough stage, equilibrium stage, and displacement completion stage over time. The duration of the three stages of injecting different gases varies, with N₂ breakthrough time and displacement completion time of 30 and 90 minutes, respectively. The breakthrough time for CO₂ injection and the completion time for displacement are 20 and 80 minutes, respectively. The breakthrough time for air injection and the completion time for displacement are 28 and 87 minutes, respectively. ⑤ When applied on site, appropriate injection gases should be selected based on the adsorption and desorption capacity of specific coal seams, as well as the spontaneous combustion features of coal seams.