Abstract: The two-phase displacement flow characteristics within complex porous rocks are pivotal to the recovery rates in unconventional oil and gas recovery as well as geothermal exploitation, garnering significant research interest. This study focuses on four types of porous media with both ordered and disordered structures, employing a self-constructed microfluidic experimental visualization setup to investigate the flow characteristics under five different flow rates and four distinct viscosities of the displaced fluid. The influence of capillary number, viscosity ratio, and the disorder of the porous structure on the two-phase displacement process is thoroughly analyzed. Findings indicate that the fractal dimension of the invading phase is correlated with the displacement pattern and positively associated with displacement efficiency, serving as a universal function of the invading phase's content. The distribution characteristics at displacement completion are intimately linked to the displacement mode. A transition from compact displacement to capillary fingering is marked by an increase in the quantity and standard deviation of the trapped phase. Conversely, the shift from capillary fingering to viscous fingering mirrors the transition from capillary fingering to ordered dendritic, characterized by a reduction in trapped phase quantity and an enlargement of standard deviation. Incorporating the impact of fluid viscosity ratios, a modified relationship between the capillary number and the displacement saturation of the invading phase is formulated, enabling the prediction of displacement efficiency that accounts for viscosity ratio effects.

Key words: complex pores, structural disorder, capillary number, viscosity ratio, displacement efficiency