54 / 2025-11-20 20:45:23

Simulation of CO2 Geo-Sequestration and Simultaneous Enhanced Gas Recovery in Coal Measures with Multi-lithological Super-Imposed Reservoirs

Coal measure gas,Shale gas,CO2 geo-sequestration,Enhanced gas recovery,Tight sandstone

Multi-lithological super-imposed coal measure strata offer strong potential for CO₂ geo-sequestration while simultaneously enabling significant gas recovery. This study examines multiphase fluid (adsorbed/free CH₄ and CO₂) transport dynamics and enhanced coal measure gas production concurrent with geological CO₂ sequestration. Herein, a reservoir model of stacked coal seam, shale and sandstone strata based on the in-situ geological characteristics of the Longtan Formation of the Songzao Coalfield (Sichuan Basin, China) 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. This work provides fundamental insights into multiphase transport and control for optimizing CO₂-enhanced recovery in multi-lithological super-imposed coal measure systems.