Residual oil contents of dolomicrite and sandy dolomite tight oil reservoirs after CO2 huff and puff: An experimental study

To clarify the differences in CO2 huff and puff (CO2–H–P) for reservoirs of different lithofacies, taking tight oil reservoirs located in northwestern China as object, lithofacies identification was conducted by analyzing mineral composition. By comparing low-field nuclear magnetic resonance (LF-NMR) T2 spectra and full-scale pore-size distribution (PSD), the relationship between T2 and pore-diameter was deduced, and T2 spectra obtained during CO2–H–P were converted into PSD. Based on CO2–H–P experiments combined with LF-NMR measurements, the distributions of residual oil in reservoir rocks with different lithofacies were analyzed. The results: (1) Lithofacies of the sweet spot reservoirs were dolomicrite and sandy dolomite. The former was characterized by well-developed meso-pores (MEPs) while the latter was characterized by well-developed macro-pores (MAPs); (2) The CO2–H–P efficiency of dolomicrite was lower than that of sandy dolomite; (3) The residual oil in dolomicrite was mainly distributed in MEPs, while that in sandy dolomite was in MEPs and MAPs; (4) The existence of single fracture could promote CO2–H–P efficiency of dolomicrite while it had little influence on that of sandy dolomite. The results can promote the selection of tight oil producing intervals using CO2–H–P where reservoirs of multiple lithofacies are developed.