Experimental Study on the Lower Limit of Mobilizable Pore Size for CO2 Invasion During CO2 Pre-Fracturing in Shale Oil of the Ma 51X Well Block

Aiming to investigate the unclear lower limit of microscopic pore mobilization during CO2 pre-fracturing in the shale oil reservoirs of the Ma51X well block, this study integrates high-temperature and high-pressure (110 °C70 MPa) CO2 huff-n-puff with nuclear magnetic resonance (NMR) experiments. The results demonstrate the following: (1) under high-temperature (110 °C) and ultra-high-pressure (70 MPa) conditions, the lower limit of mobilizable pores for CO2 to displace reservoir crude oil reaches 1.7~2.2 nm; (2) the dominant mobilized pore range for CO2 is 5.1~38.5 nm, and macropore abundance directly dictates the macroscopic sweep coverage of CO2; (3) the modification effect of CO2 on pore structure is primarily concentrated within the mesopore-to-macropore systems, and with an increase in huff-n-puff cycles, crude oil in mesopores progressively migrates toward macropores; and (4) multi-cycle CO2 huff-n-puff exhibits a cyclic performance pattern characterized by dominance in the initial cycle and subsequent attenuation. This study precisely delineates the lower limit of mobilizable pores for crude oil in the shale oil reservoirs of the Ma51X well block, providing a robust theoretical foundation for the efficient development of this formation and analogous ultra-low permeability reservoirs.