Evaluating Carbon Dioxide Flow Regimes in Micro and Nano Pore Sizes during Carbon Dioxide Injection in Hydrocarbon Reservoirs and Their Impact on Oil Recovery

Hydrocarbon reservoirs can vary significantly from one another, with pore sizes ranging from micro-sized in conventional reservoirs, and high permeability features such as natural fractures, up to nano-sized in unconventional reservoirs such as shale. When injecting carbon dioxide (CO₂) in different pore sizes, the flow regime will vary significantly. Understanding what flow regime will dominate under different thermodynamic conditions, and in different pore sizes is imperative in order to be able to select the proper flow equation and to maximize oil recovery potential, and CO₂ storage capacity. This research studies the different flow regimes that may occur in different pore sizes of hydrocarbon reservoirs, ranging from 0.2 nanometer, and up to micro-sized pores during CO₂ injection using mathematical simulation based on the definition of Knudsen Number for flow regimes. Following the mathematical simulation, experiments were conducted using composite filter membranes with 2.7 micro-meter, 100, 10, and 0.2 nano-meter pore sizes under different pressure and temperature conditions to study the impact of different flow regimes on oil recovery percentage and behavior during CO₂ injection . The mathematical modeling determined that for nanopores less than 10 nm in size, Diffusion was the main flow regimes, while at pore sizes less then 200 nanometers a transition flow regime was observed. In the micro-pore sizes the man flow regime was Viscous flow. After performing the oil recovery experiments, it was found that in the nanopore sized membranes, the CO₂ took a much longer time to breakthrough the filter membrane, and the oil recovery was considerably lower than that observed in the micro-sized membrane. Also, as the pressure increased, the flow became more turbulent, however, the oil recovery increased. Based on this research, it different flow regimes will occur in different pore size ranges based on which different oil production behaviors will be observed.