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) T-2 spectra and full-scale pore-size distribution (PSD), the relationship between T-2 and pore-diameter was deduced, and T-2 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.