Spontaneous imbibition of water into the matrix blocks because of capillary forces is an important recovery mechanism for oil recovery from naturally fractured reservoirs. In modeling this process, it has been assumed classically that local equilibrium is reached and, therefore, capillary pressure and relative permeability functions are only a function of water saturation, resulting in the appearance of the self-similarity condition. In some works published in the last 2 decades, it has, however, been claimed that local equilibrium is not reached in porous media, and therefore, opposite the classical local-equilibrium/self-similar approach, non-equilibrium effects should be taken into account in modeling multi-phase flow in porous media and, in particular, for the spontaneous imbibition process. Results from laboratory works are still contradictory about the existence of such effects during the spontaneous imbibition process. In the first part of this work, existence of the non-equilibrium effects is investigated by comparing oil recoveries versus time predicted by numerical simulation based on the classical local-equilibrium approach to experimental data from the literature. There is an excellent agreement between the predictions from the numerical simulations and the observed data, indicating sufficiency of the classical method to model oil recovery versus time. In the second part, we use experimental data available in the literature and plot water saturation versus the similarity variable for different times to check the self-similarity condition. Results from this part show that, contrary to the results from part one, non-equilibrium effects exist in all imbibition modes (co-current and counter-current imbibition). The general conclusion from this work indicates that non-equilibrium effects exist during the spontaneous imbibition process but their effect is not significant enough to affect oil recovery versus time, and therefore, the classical local-equilibrium approach seems sufficient to model the spontaneous imbibition process, at least on the small core scale.