Fluid flow in porous media is the key issue in the development of oil and gas reservoirs. The traditional fluid flow mechanics of porous media, which are based on the single-scale continuum hypothesis and Darcy’s law, play an important role in the development of oil and gas reservoirs. With the increasing exploration and development of fractured vuggy carbonate and unconventional oil and gas reservoirs, there are different voids with different spatial scales, including pores, fractures, and cavities. And the scale difference is up to 10 orders of magnitude, which impacts the fluid flow in real reservoirs. Usually, there are three different scales for a reservoir rock, i.e., the pore scale, the mesoscopic scale, and the macroscopic scale. Different methods are used to discover and obtain different fluid flow mechanisms at different scales. However, connecting these mechanisms at different scales is critical for getting a systematic macroscopic fluid flow theory. As a result, upscaling theory and multiscale methods are very important for real petroleum reservoirs. It just likes to string together each pearl of different scales to form a perfect necklace. This paper reviewed the recent research progress of multiscale methods for oil and gas flow in porous media. Some remarks were made, including pore-scale flow, macroscopic unconventional oil and gas flow, large-scale fractured vuggy carbonate oil and gas flow, and upscaling theory and multiscale methods.

The displacement of residual oil by water flooding in porous media is an important mechanism of enhanced oil recovery in many sandstone reservoirs. Nonetheless, our basic understanding of the influence of complex pore geometries of natural porous media on fluid distribution is still incomplete. Herein, two-phase flow simulations were performed to investigate the pore-scale dynamics of imbibition in a heterogeneous sandstone rock sample. Furthermore, the relationship between residual oil distribution and pore structure parameters was quantitatively characterized based on a pore-throat segmentation method. The findings suggest that the pore-scale displacement and snap-off processes have a strong dependence on the coordination number and aspect ratio. The entrapment and remobilization of oil clusters were also analyzed under continuous and discontinuous displacement modes. In addition, a new quantitative method to evaluate the displacement potential and mobilization pattern of remaining oil was presented and discussed. Statistical analysis revealed that the development of sub-pathways and the suppression of snap-off are responsible for the decrease in residual oil saturation with increasing capillary number during water injection. Moreover, the connected residual oil clusters trapped in pores with high coordination number prefer to be displaced and produced. Finally, the displacement modes with different capillary numbers under different initial oil distributions were evaluated to explain the effect of pore structure. By incorporating these correlations of displacement events with pore-throat geometry, existing predictive models can be improved, which could be helpful for the fine tapping of highly disconnected remaining oil in sandstone reservoirs.