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-puffafter CO2 flooding on recovery efficiency in tight sandstonereservoirs. The experimental methodology involved the selection ofthree representative cores with different permeability levels to emulateclass I, II, and III reservoirs. To examine immiscible, nearly miscible,and miscible conditions for different reservoir samples, a physicalsimulation flow system integrated with nuclear magnetic resonancetechnology was employed. CO2 flooding was performed followedby CO2 huff-n-puff experiments at five pressures, enablinga quantitative analysis of oil production characteristics in variouspores during the flooding and huff-n-puff processes. This study providesinsights into the microscopic production characteristics and oil recoveryinfluenced by huff-n-puff in tight sandstone reservoirs subsequentto CO2 flooding. The experimental results highlight thatrecovery efficiency increases with higher permeability, with classI reservoir samples exhibiting the highest recovery rate at 76.47%,followed by class II reservoirs at 69.33%, and class III reservoirsat 44.43%. Huff-n-puff recovery for class I and class II reservoirsgradually declines with increasing pressure after flooding, whereasclass III reservoirs display an opposite trend, with recovery graduallyincreasing. In addition, the microscopic oil distribution characteristicschange after flooding. During the near-miscible phase, class I andclass III reservoirs predominantly yield oil from medium and smallpores, while class II reservoirs primarily produce oil from mediumpores. Upon reaching miscibility, oil production in class I, II, andIII reservoirs primarily occurs in small pores, with some contributionfrom medium pores. The research findings presented in this paper providevaluable theoretical support for future CO2 flooding operationsin tight sandstone reservoirs and the optimization of huff-n-puffexperiments subsequent to flooding.