Experimental study on effects of pore-scale CO2 morphology on controlling residual trapping capacity of sandstone reservoirs based on micromodel

Residual trapping is critical for ensuring the short- to medium-term security of CO2 geological storage (CGS). Conventional assessments of residual trapping capacity primarily rely on residual saturation while overlooking influences on pore-scale CO2 morphology, which would lead to uncertainties of storage efficiency and security. In this study, we conducted high-pressure and high-temperature visual micromodel experiments aimed to quantify the influence of residual CO2 morphology on residual trapping behaviors within sandstone reservoirs. Seven drainage-imbibition cycles were conducted to examine the effects of imbibition rate (0.05-0.5 mL/min), pressure (4-15 MPa), and temperature (30-50{degree sign}C) on dynamic pressure evolution, residual CO2 saturation and the morphology of residual CO2. Based on the shape factor, residual CO2 morphology was categorized into droplet, ganglion, and network types. The results indicate that although pressure evolution exhibits a similar four-stage pattern under all conditions, both residual saturation and CO2 morphology varies considerably. The increasing imbibition rate reduced residual saturation by 0.10 and raised the droplet proportion by 19.7%. Higher pressure increased saturation by 0.10 and droplet proportion by 15.4%, whereas higher temperature decreased saturation by 0.08 but increased droplets by 25.2%. Furthermore, by introducing a Morphological Index (MI) representing the combined proportion of ganglion and network structures, a morphology-saturation-coupled model (MSCM) was developed to enhance relative permeability prediction. This work quantitatively elucidates the role of residual CO2 morphology in controlling two-phase (i.e., CO2 and brine) flow behaviors, providing implications for evaluating residual trapping capacity of porous media reservoirs in CGS.