Ecological risk assessment of soil and water loss by thermal enhanced methane recovery: Numerical study using two-phase flow simulation

Thermal enhanced methane recovery inevitably aggravates the soil and water loss, causing severe harm to the sustainability of groundwater environment and the surrounding ecosystem. Therefore, quantitative analysis of the effect of thermal enhanced methane recovery on groundwater loss and ecological risk of coalbed methane development zone is necessary. In this study, a coupling model of gas drainage and groundwater loss is established. The model considers the dynamic gas diffusion of coal matrix, the two-phase flow of water and gas, and the influence of temperature on such flow. Based on this model, characteristics of groundwater loss of coal seam reservoir caused by enhanced methane recovery are analyzed, and the ecological risk assessment of methane recovery zone is realized. Results indicate that during heat injection, the permeability of the coal seam increases with distance from the borehole due to the competition between two-phase flow and temperature. High temperature develops the permeability, gas production, and water production of the reservoir. The change rules of water and gas productions are similar with initial increases and subsequent declines. The influence of coal gas diffusion on groundwater loss has a certain time lag. In the early stage, the dynamic attenuation of gas diffusion is not apparent. In the later stage, the supplement rate of gas from matrix to fracture decreases. The initial saturation has a significant influence on the water production rate in the early stage. A large Langmuir volume constant not only strengthens the peak value of gas drainage rate but also the gas drainage rate itself in the later declining period. Large scale coalbed methane development will face ecological risks such as water environment pollution, habitat destruction and soil degradation, which is the key aspect of ecological environment management and risk prevention.