STUDY ON MOVABILITY OF SPONTANEOUS IMBIBITION OIL RECOVERY FROM TIGHT RESERVOIRS BASED ON NUCLEAR MAGNETIC RESONANCE PORE CLASSIFICATION METHOD

Spontaneous imbibition is an effective method to improve the oil displacement in tight sandstone reservoirs, while the various pore classification methods may lead to variability in the refined quantitative characterization of oil movability. In this study, four typical tight sandstone cores were used to launch the spontaneous imbibition oil displacement experiments in the tight sandstone reservoir of the Yanchang Formation in Ordos Basin. With the refined pore classification method for fluid distribution pores based on nuclear magnetic resonance (NMR) fractal theory, the pore types in tight sandstone cores were distinguished, and the influence of different types of pore structures on oil mobility and spontaneous imbibition displacement rates were clarified. The results show that the oil producing degree of spontaneous imbibition in different types of cores ranges from 22.07% to 33.26%, and the oil producing degree of spontaneous imbibition in NMR T₂ spectra of bimodal cores is higher than that of unimodal core. With the NMR pore classification method, the fluid distribution pores in the typical tight sandstone cores were initially divided into P1 and P2 types, and the P1 type pores could be further classified into P1-1, P1-2 and P1-3 types. It features various spontaneous imbibition oil displacement in P1 and P2 types pores. As the dominant pore type in tight sandstone cores, the ratio of P1-2 and P1-3 type pores in P1 pores would finally determine the spontaneous imbibition oil displacement recovery in tight sandstone cores. The pore structure differences among P1-1, P1-2 and P1-3 pores play a decisive role in the spontaneous imbibition oil displacement recovery. The significance pore structure differences for pores with smaller apertures not only enhance the spontaneous imbibition oil displacement recovery, but also improve the rates of spontaneous imbibition oil displacement recovery. The tight sandstone cores of P1-2 and P1-3 pores with higher fluid mobility index show higher spontaneous imbibition oil displacement recovery.