Microscopic Production Characteristics of Huff-n-Puff after CO₂ Flooding in Tight Oil Sandstone Reservoirs

This study aims to investigate the impact of CO2 huff-n-puff after CO2 flooding on recovery efficiency in tight sandstone reservoirs. The experimental methodology involved the selection of three representative cores with different permeability levels to emulate class I, II, and III reservoirs. To examine immiscible, nearly miscible, and miscible conditions for different reservoir samples, a physical simulation flow system integrated with nuclear magnetic resonance technology was employed. CO2 flooding was performed followed by CO2 huff-n-puff experiments at five pressures, enabling a quantitative analysis of oil production characteristics in various pores during the flooding and huff-n-puff processes. This study provides insights into the microscopic production characteristics and oil recovery influenced by huff-n-puff in tight sandstone reservoirs subsequent to CO2 flooding. The experimental results highlight that recovery efficiency increases with higher permeability, with class I reservoir samples exhibiting the highest recovery rate at 76.47%, followed by class II reservoirs at 69.33%, and class III reservoirs at 44.43%. Huff-n-puff recovery for class I and class II reservoirs gradually declines with increasing pressure after flooding, whereas class III reservoirs display an opposite trend, with recovery gradually increasing. In addition, the microscopic oil distribution characteristics change after flooding. During the near-miscible phase, class I and class III reservoirs predominantly yield oil from medium and small pores, while class II reservoirs primarily produce oil from medium pores. Upon reaching miscibility, oil production in class I, II, and III reservoirs primarily occurs in small pores, with some contribution from medium pores. The research findings presented in this paper provide valuable theoretical support for future CO2 flooding operations in tight sandstone reservoirs and the optimization of huff-n-puff experiments subsequent to flooding.