Visualized investigation of transport and phase behaviors during CO2 huff-n-puff in nanomatrix-fracture tight formations

CO2 huff-n-puff is growing as a promising technique for boosting production efficiency in tight reservoirs while simultaneously minimizing the carbon footprint of hydrocarbon production. Previous research has continued to explore the knowledge involved in this process but still left significant gaps in understanding the complicated fluid behaviors and the multiscale interactions. In this study, we developed a novel multiscale-fluidic system to investigate the multiphase transport and phase transition during CO2 huff-n-puff. The multiscale model com-prises over 10(5) nanoarrays representing the matrix and is integrated into a microscale fracture network, establishing a domain that encompasses nanoconfinement to the bulk scale. Importantly, these cross-scale fluids phenomena and mechanisms are directly optically accessible. The production results in fracture-free nanomodels indicate that the limited gas diffusion and strong capillary barrier induced by inherent heterogeneity of pore size are the primary challenges for huff-n-puff operation in unstimulated reservoirs. Huff-n-puff in nanomatrix-fracture systems presents distinct fluid behaviors and spotlights the dramatic role of interplay between nano -pores and fractures. Bubble nucleation first appears in bulk fractures but experiences a notable delay in nano -pores, which indicates that bubble nucleation is still suppressed at similar to 500 nm scale. Fractures help to facilitate gas breakout within nanomatrix and eventually elevate the bubble point pressure when the matrices and fractures are interdependently coupled. The fracture network saturated with CO2 greatly accelerates the gas diffusion and shortens the soaking time required for effective huff-n-puff by expanding the oil-gas interaction area. This leads to a considerable growth in cumulative recovery compared to unfractured scenario-increasing from 59.06%/ 51.90% to 62.02%/59.47% for homogeneous and heterogeneous models. Collectively this study significantly advances our understanding of the rich physics behind huff-n-puff in multiscale systems and holds great potential for guiding practical engineering applications.

Automated summary

CO2 huff-n-puff is a promising technique for enhancing production efficiency in tight reservoirs and reducing the carbon footprint. This study developed a novel multiscale-fluidic system to investigate the multiphase transport and phase transition during CO2 huff-n-puff. The results showed that limited gas diffusion and strong capillary barrier in unstimulated reservoirs are the primary challenges for huff-n-puff operation. However, in the presence of fractures saturated with CO2, the gas diffusion is greatly accelerated and the soaking time for effective huff-n-puff is shortened.