Research on multi-parameter collaborative control method for enhancing coalbed methane recovery by hot flue gas displacement

To investigate the influence of the interaction between geological and engineering factors on the effectiveness of hot flue gas displacement for enhancing coalbed methane recovery, a thermo-hydro-mechanical coupled model for binary gas mixture displacement was constructed. First, the spatiotemporal evolution of key physical fields was analyzed. An enhanced harvesting index (EHI) was proposed to evaluate recovery efficiency. Subsequently, the response surface method was used to explore the response patterns of pre-EHI and post-EHI to single factors and multi-factor interactions. Finally, optimization maps of key process parameters for typical regions were drawn. The main results indicate that high-permeability and high-gas regions exhibit higher pre-EHI, but EHI drops rapidly in the later stage. Increasing the N2 ratio and injection pressure effectively improves pre-EHI. As injection pressure increases in the later stage, EHI gradually shifts from N2-dominated to CO2-dominated. When the initial gas pressure is high, pre-EHI first decreases and then increases with increasing CO2 ratio. Low gas pressure and high CO2 ratio correspond to a significant increase in post-EHI. For low-permeability regions, as injection pressure increases, the CO2 ratio should follow the principle of “first increase and then decrease.” For high-permeability regions, gradually reducing injection pressure and increasing the CO2 ratio can achieve multi-objective optimization of post-gas flow rate and recovery efficiency. The research results are expected to provide theoretical guidance for precise time- and zone-specific injection of hot flue gas.