The Influences of CO₂ Injection Pressure on CO₂ Dispersion and the Mechanism of CO₂–CH₄ Displacement in Shale

The effects of CO₂ injection pressure (P$CO2$⁠) on CO₂ dispersion and the mechanism of CO₂–CH₄ displacement in a shale sampled from Changning of China were studied. Results indicated that Coats–Smith dispersion–capacitance model gave a reasonable simulated result to the breakthrough curves of CO₂ under different injection pressures. The shapes of CO₂ breakthrough curves became more asymmetrical with the increase of CO₂ injection pressure. A higher CO₂ injection pressure caused early CO₂ breakthrough and reduced the recovery of CH₄ at CO₂ breakthrough (R_pipeline-$CH4$⁠), but improved the ultimate displaced CH₄ amount (R_ultimate-$CH4$⁠). With the increase of CO₂ injection pressure, dispersion coefficient (K_d) increased nearly exponentially. A larger K_d led to a lower R_pipeline-$CH4$ and a longer transition zone. With the increase of CO₂ injection pressure, the flowing fraction (F) in pore space decreased nearly linearly and more CO₂ diffused into stagnant region to replace adsorbed CH₄ in a shale, which resulted in a larger R_ultimate-$CH4$⁠. The mass transfer coefficient (K_m) between the flowing and stagnant regions increased with the increase of CO₂ injection pressure, which led to a smaller F and larger R_ultimate-$CH4$⁠. CO₂ diffusion provided major contribution to CO₂ dispersion at lower injection pressure, and mechanical mixing of CO₂–CH₄ offered predominant contribution to CO₂ dispersion at higher injection pressure. Larger mechanical mixing accelerated the mixing of CO₂–CH₄, which was unfavorable for R_pipeline-$CH4$⁠. Lower CO₂ injection pressure was conductive to gain higher R_pipeline-$CH4$⁠.