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

ABSTRACT: 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. 1. INTRODUCTION Injecting captured CO2 into depleted reservoirs is an essential technology to decrease carbon emissions and may concomitantly enhance recovery of heavy or condensed hydrocarbons (Oldenburg et al., 2001; Liu et al., 2013; Liu et al., 2019). Despite this clear utility, such reinjection methods have been seldom used in carbonate gas reservoirs due to unknown response. Currently, the large number of water-bearing gas reservoirs with low recovery urgently require more effective techniques to improve gas production (Honari et al., 2016; Mohammed et al., 2021; Pan et al., 2023). Successful implementation of the injection of CO2 for enhanced recovery of hydrocarbons and concurrent CO2 storage holds great promise if the fundamental response can be defined and thus potential benefits established (Pooladi-Darvish et al., 2008; Pentyala et al., 2021; Cheng et al., 2022). We explore the transport characteristics of CO2-CH4 at high pressure (32MPa) in both dry and variably-saturated porous and fractured carbonates containing native H2O and CH4 – in particular the role of water saturation on the CO2 breakthrough profile and subsequent gas recovery. Injection rates are varied to examine both diffusive and advective response and afterward define CO2 injection rates that optimize CH4 recovery. We conduct a series of core-scale fluid displacement experiments for a spectrum of initial H2O and CH4 saturations and CO2 injection rates. Subsequently, experimental data and analytical model are fitted.