The development and utilization of oil and natural gas are all related to the complex fluid flow in reservoirs. The pores of unconventional reservoirs that oil/gas exists in have more heterogenetic features when compared to conventional reservoirs. As the fluid injection rate increases for unconventional oil/gas reservoirs, the fluid flow behavior is becoming more complex. Besides, the mixing of fluids injected into the reservoirs, such as low salinity water, fracturing fluid, surfactant solution, et al., were further complicated the fluid flow behavior. Hence, there is a high demand given the understanding of the fluid flow and controlling mechanisms in unconventional oil/gas reservoirs. The efficient and economic development of unconventional oil/gas reservoirs requires a comprehensive understanding of their pore structure and fluid flow behavior in the pores.

However, these complicated characteristics are not yet fully understood, and they restrict the improvement of oil/gas recovery. Owing to the fluid showing a unique pore-scale phenomenon and macroscopic flow characteristics, understanding the complex flow behaviors, characterizing the macrostructure or micro features of unconventional oil/gas reservoirs, and deeply exploring the influence of these characteristics on flow is essential. Therefore, research on the microscopic characteristics, flow behavior, and mutual influence can improve understanding the microscopic structure and fluid flow laws in unconventional oil/gas reservoirs. The study is significant in enhancing the theory and engineering applications in underground porous media.

Thus, to promote the development of characterization of unconventional oil/gas reservoirs and understand the seepage flow in them, this Special Issue is intended to gather relevant original articles and comprehensive reviews related to the following areas. Research methods include but are not limited to experimental methods, model making, numerical simulations, solution algorithms, flow mechanisms, etc.

Potential topics include but are not limited to the following:

• Numerical or experimental methods of multiphase flow in unconventional oil/gas reservoirs
• Description of the macroscopic structure of unconventional oil/gas reservoirs
• Multiphase flow mechanism and statistical feature extraction methods in unconventional oil/gas reservoirs
• Deep learning and its application in the characterization of unconventional oil/gas reservoirs and fluid flow
• Research and application of microfluidic technology in underground porous media