Pore-Scale Shale Oil Mobilization during Surfactant-Assisted CO2 Huff-n-Puff

Surfactants can enhance shale oil recovery during CO2 huff-n-puff. However, how surfactant type affects mobilization across different pore sizes remains unclear. This study compares pure CO2 with three surfactant-assisted CO2 systems over three huff-n-puff cycles using Chang 7 shale cores and crude oil. Sodium dodecyl sulfate (SDS) and two CO2-philic nonionic polyethers, C4(PO)6 and C4(PO)10, are evaluated. The latter two are polyoxypropylene butyl ethers containing six and ten propylene oxide units, respectively. High-temperature and high-pressure nuclear magnetic resonance (NMR) T2 measurements combined with one-dimensional frequency encoding are used to quantify pore-scale oil recovery and axial sweep. After three cycles, pure CO2 yields a total oil recovery of 26.11%, with recovery factors of 31.71 and 7.19% for oil in large pores and small pores, respectively. The SDS-assisted system yields only 13.25% total recovery, with corresponding recovery factors of 18.07 and 3.19%, indicating no synergistic enhancement. Both CO2-philic polyethers outperform pure CO2. The total recovery reaches 31.88% for C4(PO)6 and 28.88% for C4(PO)10, while the recovery factors for oil in large pores and small pores are 43.65 and 10.47% for C4(PO)6 and 41.17 and 7.52% for C4(PO)10, respectively. Both polyether systems extend the effective axial oil-mobilization distance to at least 100 mm, compared with 76 mm for pure CO2 and 45 mm for SDS. C4(PO)6 also maintains stronger and more sustained mobilization of oil from small pores. Its superior performance likely results from a more favorable balance between CO2 affinity and transport through confined pore throats. Compared with C4(PO)10, C4(PO)6 has a shorter PO chain and smaller molecular size, which may reduce diffusion resistance and facilitate its transport into small pores.