Fluid flow and CBM productivity-CO2 injectivity evaluations induced by injection pressure and reservoir condition: Implications for CO2-ECBM injection process and drainage degree

In order to study the key technology of CO 2 sequestration and enhanced coalbed methane recovery (CO 2 -ECBM), a series of large-scale physical simulation experiments were conducted with varying CO 2 injection pressures P in and original CH 4 pressures P res . The fluid flow evolution of reservoir pressure, CH 4 recovery efficiency ( RE ), and CO 2 injectivity ( J ) were monitored and analyzed in real time. The fluid flow evolution indicates centrifugal flow near the injection well and centripetal flow toward the production well, and a larger P in produces a stronger pressure gradient and more pronounced flow-state transformation. Increasing P in accelerates CO 2 breakthrough and enhances CH 4 recovery increasing from 78.08 % to 90.30 % with the original CH 4 pressure 1.5 MPa. With P in increasing from 1.5 to 2.5 MPa, CO 2 breakthrough was accelerated and RE increased from 78.08 % to 90.30 % at P res of 1.5 MPa. Under P in of 2.5 MPa, RE increased from 90.30 % to 96.70 % as P res decreased from 1.5 to 0.5 MPa, indicating that a higher P in- P res differential favors more complete CH 4 depletion. The paper evaluated CO 2 injectivity ( J ) quantitatively by the injection flow rate and reservoir pressure. The J exhibits a rise-decline-stabilization trend, indicating the evolving injection dynamics and pressure-driven behavior in the reservoir. While a lower P res further boosts the early-stage injectivity. CO 2 -ECBM is governed by a dynamic shift from replacement-dominated to displacement-dominated behavior, the time for replacement ratio to equal displacement ratio shortens from 50 to 36 min as P in increases from 1.5 to 2.5 MPa. These results suggest that improving CO 2 -ECBM performance requires both an appropriately elevated injection pressure and sufficient pre-drainage, and optimizing injection timing to extend the high-efficiency replacement stage is crucial for achieving synergistic CH 4 recovery and CO 2 injectivity.