Experimental Research on Characteristics of CO₂-CH₄ Transport in Water-Bearing Carbonate Gas Reservoirs

We investigate the transport properties of CO₂-CH₄ under high pressure conditions in both dry and variably saturated porous and fractured carbonates containing native H₂O and CH₄. The specific focus is on determining optimal CO₂ injection rates for maximizing CH₄ recovery. A series of experiments involving fluid displacement at the core scale is conducted, considering various initial H₂O and CH₄ saturations and CO₂ injection rates. Examination of the resulting residence time distribution (RTD) curves reveals several findings: (1) the larger pores support advective transfer with diffusion from the smaller pores contributing to tailing in the RTD; (2) the porous medium to free phase diffusion coefficient ratio (D') in dry samples is related to porosity (φ) as D' = φ³; (3) the presence of formation water increases heterogeneity, reduces effective porosity and decreases the Peclet number of saturated samples by two orders of magnitude and transforms D'; (4) displacement by CO₂, alleviates water blockage in the removal of CH₄ and switches transport to that of gas-phase convection. This research enhances our understanding of fundamental phenomena governing multiphase-component transport in porous and fractured media, contributing valuable insights to enhanced gas recovery and carbon storage in variably saturated carbonate reservoirs.