CO₂-Enhanced Multiphase Flow in Heterogenous Coal Measures: Thermal-Hydraulic-Mechanic (THM) Model for Enhancing Gas Co-Production with CO₂ Geo-Sequestration

Heterogeneous coal measure strata are promising for CO₂ geo-sequestration with substantial potential for concurrent gas production. We investigate multiphase fluid (adsorbed/free CH₄ and CO₂) transport dynamics and enhanced gas production concurrent with geological CO₂ sequestration. This study is based on the in-situ geological characteristics of the Longtan Formation of the Songzao Coalfield (Sichuan Basin, China) and a reservoir model of stacked coal seam, shale and sandstone strata is developed to evaluate the enhanced gas recovery characteristics. Evaluation of concurrent CO₂ geological sequestration and CH₄ co-production reveal the initial natural depressurization-driven gas production from the reservoir before CO₂ injection subsequently elevates gas pressures. Coupled Thermal-Hydraulic-Mechanic (THM) simulation reveals three key multiphase transport mechanisms: (1) competitive adsorption-driven CH₄ displacement with a 3.2:1 CO₂/CH₄ molar replacement ratio in the coal matrix; and (2) differential phase migration velocities (V_{free_gas} > V_{adsorbed_gas}) resulting in transient saturation inversions. Continuous CO₂ injection at 7 MPa sequestered ∼ 45.4 t CO₂ with 88.61 % adsorbed-phase storage, and CH₄ recovery increased to 42.22 % (+10.16 % vs. natural depletion). A Phase-specific analysis revealed that the sequestered CO₂ exhibits distributed differently among coal, shale, and sandstone formations, in the proportions of 43.78 %, 55.36 %, and 0.86 %, respectively. Notably, the adsorbed phase dominated the CO₂ sequestration by mass, comprising 88.61 % of the total stored volume. This work provides fundamental insights into multiphase transport and control for optimizing CO₂-enhanced recovery in heterogeneous coal measure systems.