Pore-domain CH4 mobilization and transport modeling during CO2-CH4 displacement in tight sandstone cores

CO 2 injection is a promising approach for enhanced gas recovery (EGR) in tight sandstone reservoirs, yet the resulting CO 2 -CH 4 transport can be strongly affected by heterogeneity and pore-domain methane mobilization. In this study, CO 2 -CH 4 displacement experiments were performed on one single tight sandstone core and three axially heterogeneous composite tight sandstone cores under the same reservoir pressure and temperature conditions. In-situ nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) were used to monitor methane mobilization, and the volume-averaged concentration responses were interpreted using the classical advection-dispersion equation (ADE) and a mobile-immobile model (MIM). Methane-saturated NMR T 2 spectra show bimodal distributions, with short-T 2 components associated with nano- and micro-pores dominating the pore volume of the tight sandstones. During CO 2 injection, methane is preferentially mobilized from the long-T 2 components, indicating earlier recovery from relatively larger or better-connected pores. Compared with the single core, the composite cores show pronounced suppression of methane mobilization in relatively-small pores, resulting in lower recovery and uneven displacement. For the transport modeling, both ADE and MIM were applied in an equivalent one-dimensional formulation, and results show that axial heterogeneity can cause non-ADE transport, but MIM is not always required. ADE remains sufficient when methane mobilization from the short- and long-T 2 components is relatively balanced, even in composite cores with high permeability contrast. Composite cores with strongly suppressed small-pore mobilization show delayed concentration evolution and long-tailing effect, which are better described by MIM. These results suggest that pore-domain methane mobilization and pore-structure-controlled domain separation should be considered, in addition to permeability contrast, when selecting transport models for CO 2 –CH 4 flow in tight sandstones. This is relevant to reservoir-scale modeling, where permeability contrast alone may not fully describe delayed gas exchange and non-ADE transport behavior.