Investigation of CO₂–CH₄ Displacement and Transport in Shale for Enhanced Shale Gas Recovery and CO₂ Sequestration

To gain a better understanding of the enhanced shale gas recovery by CO₂ gas injection (CO₂-ESGR) technique, the dynamic displacement mechanism of CO₂–CH₄, the CO₂ enhanced shale gas recovery (⁠$RCH4$⁠), and CO₂ storage capacity (⁠$VCO2$⁠) were studied based on transport properties of CO₂ and CH₄. Experiments of CO₂ injection into shale gas reservoir preadsorbed by CH₄ were performed in a fixed bed. Breakthrough curves were obtained under different test conditions and simulated by one-dimension advection-dispersion (AD) model. It was found that dispersion coefficient (K₁) rather than molecular diffusivity of CO₂ dominated its transport in shale. K₁ together with advection velocity (υ) of CO₂ during CH₄ displacement controls $RCH4$ and $VCO2$⁠. When transporting in shale gas reservoir, CO₂ had larger dynamic adsorption amount and υ, but smaller K₁ than CH₄. The competitive transport and adsorption behavior of CO₂ and CH₄ made it possible for CO₂ to store in shale reservoir and to drive the in-place CH₄ out of shale reservoir. The transfer zone of CO₂–CH₄ displacement (CCD) was very wide. High $RCH4$ and $VCO2$ were reached at low injection CO₂ gas pressure and for small shale particles. Higher injection flow rates of CO₂ and temperatures ranging from 298 K to 338 K had a little effect on $RCH4$ and $VCO2$⁠. For field conditions, high CO₂ injection pressure has to be used because the pore pressure of shale reservoir and adsorption amount of CH₄ increase with the increase in depth of shale gas reservoir, but $RCH4$ is still not high.