Recovery, storage, and economics of CO2/N2-enhanced coalbed methane recovery from cleated coal

Abstract

[Abstract] Enhanced coalbed methane recovery (ECBM) by CO2/N2 injection is governed by competing objectives involving CO2 retention, methane recovery, ECBM efficiency, and economic performance. This study examines how injection composition reorganizes that trade-off in dry fractured coal when cleat connectivity and the main coupled gas–solid mechanisms are represented explicitly. A two-dimensional numerical model is developed in COMSOL Multiphysics for single-phase multicomponent gas transport in an explicit matrix–cleat domain. The formulation combines competitive Langmuir adsorption kinetics, Peng–Robinson thermodynamic pressure reconstruction, Wilke mixture viscosity, and sorption-driven porosity/permeability updates. Five injection scenarios, from pure N2 to pure CO2, are compared through spatial gas-distribution maps, CO2 retention, cumulative methane recovery, ECBM efficiency, and net present value (NPV). The results show that increasing the CO2 fraction systematically shifts the system from a recovery-dominated regime to a storage-dominated one. CO2-rich injection strengthens adsorption buffering, increases retained CO2, and produces the highest and latest NPV peaks under the adopted economic assumptions, but at the cost of lower cumulative methane recovery and lower methane produced per unit of net retained CO2. N2-rich injection promotes faster cleat-scale sweep, earlier breakthrough, and the highest cumulative CH4 recovery, but yields lower CO2 retention and the smallest, earliest NPV peak. Intermediate mixtures define compromise regimes between these two extremes, with the CO2 = 25%, N2 = 75% case providing the highest ECBM efficiency, but not the highest cumulative recovery, storage, or peak NPV. Across all scenarios, NPV reaches an early maximum and then declines, indicating a finite value-maximizing operating window and negative marginal discounted value after the peak under the benchmark assumptions. These results show that there is no universal best CO2/N2 mixture for dry ECBM. Instead, CO2 retention, methane recovery, ECBM efficiency, and economic performance are competing objectives, and mixture selection must be tied to the project objective and operating window.