NMR Characterization of Movable Oil in Argillaceous-Rich Shales via High-Pressure CO2 Huff-n-Puff

While CO2 huff-n-puff (CO2 HnP) is a promising technique for shale oil recovery, the characteristics and controlling factors of microscopically movable oil in lacustrine argillaceous-rich shales remain poorly understood. Shale samples from the Qingshankou Formation in the Songliao Basin were collected, and a series of experiments, including low-pressure N2 adsorption, mercury injection porosimetry, and nuclear magnetic resonance, were conducted. High-pressure and high-temperature CO2 HnP experiments were then conducted to investigate the effects of cycle number, soaking time and changes in pore structure on movable oil distribution. The shales exhibit multi-scale pores and lamellar fractures containing substantial residual oil (41.33–52.16% saturation). CO2 HnP effectively mobilizes oil from macropores (50–1000 nm) and fractures (>1000 nm), with a limited effect in micro–mesopores (