Micro-influencing mechanism of permeability on spontaneous imbibition recovery for tight sandstone reservoirs

Taking the Chang 8 tight sandstone reservoir of the Yanchang Formation of Fuxian area in Ordos Basin as an example, the influencing mechanism of permeability on imbibition recovery in tight sandstone was explored by spontaneous imbibition experiment, combining nuclear magnetic resonance (NMR) and CT Scanning. Results show that: (1) spontaneous imbibition played a vital role in water–flooding of the tight sandstone reservoir, the recovery by spontaneous imbibition of experimental core samples can reach 5.24% – 18.23%, and the higher the matrix permeability, the higher the recovery degree by spontaneous imbibition; (2) because of the thickness of adsorbed layer, pores above sub-micron scale made a great contribution to the imbibition recovery of tight sandstone reservoir, and nano-submicron pores made less contribution to imbibition recovery; (3) the connectivity of pore and throat was the major microscopic mechanism of the positive correlation between matrix permeability and spontaneous imbibition recovery. Samples of different permeability didn't differ much in the sizes of sub-micron to micron pores, but with the rise of permeability, the connected pores and throats and surface porosity increased exponentially, leading to significant increase of imbibition recovery. Key words: tight sandstone, oil recovery by imbibition, influence mechanism, permeability, adsorbed layer, pore–throat connectivity, micro-nano pore