This study explores imbibition dynamics and flowback of fracturing fluids in Penglaizhen sandstones by imbibition experiment and nuclear magnetic resonance analysis. A gas–water system was considered and samples were subjected to different boundary conditions and fully immersed in the wetting phase for imbibition. A close range of the pore radius was observed between 0.1 nm and 9.3 μm for all samples. Basic features of the imbibition phases were represented by the imbibition profile, and imbibition parameters were quantified in terms of gas recovery, imbibition capacity, imbibition index and pore connectivity, diffusion ability, and imbibition potential under the respective boundary conditions. The study reveals that capillary imbibition (mainly in all-face-open (AFO) and two-end-closed (TEC)) and diffusion imbibition (mainly in two-end-open (TEO) and one-end-open (OEO)) are the major mechanisms by which fracture fluids flow into the formation matrix. There is high gas recovery in all samples (>79%), which can be attributed to the high water-wet nature of the samples, while the residual gas saturation is very low with the highest value in OEO at 0.17 and the lowest in AFO at 0.01. The strong capillary force in AFO and TEC boundary conditions is the key factor for higher gas recovery than TEO and OEO samples. However, the low diffusion ability of AFO and TEC samples depict a low gas displacement effect. On the other hand, TEO and OEO boundary conditions show an increased gas displacement effect through increased diffusion ability. The imbibition index (which represents pore connection) and imbibition potential from the highest to the lowest also correspond to the diffusion ability from the highest to the lowest.