Volume 65

In-Situ Characterization of COâ‚‚-CHâ‚„ Displacement Dynamics in Carbonate and Sandstone Reservoirs for Enhanced Gas Recovery Lianlin Deng, Miao Zhang, Lirong Dong, Zijun Xie

Abstract

High-temperature and high-pressure in-situ NMR displacement experiments were conducted on natural carbonate and tight sandstone cores from the Wolonghe and Sulige gas reservoirs to investigate the pore-scale gas displacement behavior and heterogeneity effects during CO₂ injection. Core pairs with varying permeability ratios were assembled to simulate different degrees of planar heterogeneity, and CH₄ was used as the initial saturated gas. The CO₂ displacement process was monitored continuously using T₂ relaxation spectra and magnetic resonance imaging (MRI). Results show that carbonate cores mainly consist of macropores, microfractures, and mesopores, while tight sandstone cores are dominated by micropores and small pores. In both lithologies, gas in large pores was first displaced, followed by that in smaller pores, whereas nanoscale pores remained largely trapped. Increasing heterogeneity strongly restricts CH₄ mobilization from small pores and reduced overall recovery, especially in tight sandstones. The NMR-based recovery factor reached 50.70%-61.15% for carbonates and 18.03%–56.70% for tight sandstones, indicating that CO₂-EGR efficiency is significantly influenced by pore structure and heterogeneity.

Keywords COâ‚‚-EGR, NMR, tight sandstone, carbonate, heterogeneity, gas recovery