Evaluation of gasification agent strategies for underground coal gasification in post-ECBM reservoirs: A coupled thermo-hydro-chemical numerical simulation approach

Implementing underground coal gasification (UCG) in post-ECBM reservoirs, namely coal reservoirs after CO 2 -enhanced coalbed methane (CO 2 -ECBM) recovery, provides a potential pathway for residual energy recovery. However, the pre-existing CO 2 -rich environment alters reservoir conditions and introduces additional constraints on gasification agent selection and process control. In this study, an in-house coupled thermo-hydro-chemical (THC) simulator was developed to investigate UCG under CO 2 -rich post-ECBM reservoir conditions. The gasification performance of different gasification agent strategies, including pure O 2 injection, CO 2 –O 2 co-injection, and water co-injection with O 2 , was systematically evaluated under various injection rates, together with a simplified screening-level cost assessment based on the assumed cost parameters. The results demonstrate that UCG can be initiated and maintained under the investigated reservoir, operating, and kinetic conditions. During gasification, the CO 2 distribution evolves dynamically with cavity development, suggesting its potential to reflect reaction-zone evolution. For pure oxygen injection, increasing the injection rate accelerates coal conversion and reduces unit gas cost, but decreases syngas lower heating value and combustible component fraction. Compared with pure oxygen injection, CO 2 –O 2 co-injection increases residual coal mass and results in lower syngas quality and higher unit gas cost under the investigated conditions. In contrast, moderate water co-injection improves syngas quality and reduces unit energy cost at low-to-moderate oxygen injection rates by promoting steam-related gasification reactions. Among the simulated cases, an oxygen injection rate of 3 × 10 −4 m 3 /s combined with a water injection rate of 3 × 10 −6 m 3 /s exhibited the most favorable overall performance within the investigated parameter range. Overall, this study provides insights into the thermochemical response of UCG under post-ECBM conditions and offers a numerical basis for comparative evaluation of gasification strategies prior to experimental and engineering-scale investigations.