Multi-objective optimization of enhanced coalbed methane recovery and CO2 sequestration by hot flue gas injection: A cross-scale modeling approach integrating real fracture networks

To address the challenges of low reservoir injectivity and limited influence range in conventional gas-injection enhanced coalbed methane (ECBM) recovery, this paper proposes a method that injects hot flue gas (HFG) into hydraulically fractured coal seams for CH 4 displacement with synergistic CO 2 sequestration. Focusing on the cross-scale transport of multicomponent multiphase fluids in fractured coal seams, a modeling approach is developed that couples an equivalent continuum model with a discrete fracture network (DFN) constrained by microseismic monitoring data, and a thermo-hydro-mechanical (THM) coupled model is established. Based on this model, the influences of geological and engineering factors on CH 4 recovery and CO 2 storage are revealed, and a multi-objective optimization is performed to determine key process parameters. Results show that CO 2 migrates preferentially along hydraulic fractures, significantly reducing CH 4 concentration near the wellbore, while N 2 spreads rapidly over extensive areas, creating CH 4 accumulation at the displacement front. Hydraulic fractures enhance fluid transport and expand the swept zone, but also increase the risk of premature HFG breakthrough. High injection pressure improves early-stage displacement and carbon storage, but tends to induce premature breakthrough and shorten the effective displacement period. High CO 2 concentration significantly enhances storage capacity and early-stage recovery, whereas moderate CO 2 concentration prolongs the effective displacement time. Both high initial CH 4 pressure and high permeability are favorable for improving recovery and storage-the former enables more thorough displacement by delaying breakthrough, while the latter enables efficient displacement in shorter time by accelerating fluid transport. Sensitivity analysis reveals that early-stage recovery is most sensitive to initial permeability, effective recovery to initial CH 4 pressure, and carbon storage capacity to CO 2 fraction in HFG; flue gas temperature exhibits the weakest influence on all three indicators. For a high-gas high-permeability coal seam, the optimized scheme increases early-stage recovery, effective recovery, and carbon storage capacity by 15.3%, 30.2%, and 572%, respectively, compared with the base case. This study provides a theoretical basis and methodological support for determining key technical parameters of HFG-ECBM and carbon sequestration.