Although recent experimental and numerical investigations about SI in tight oil reservoirs have recognized the critical role played by SI in controlling the ultimate oil recovery of tight reservoirs, the mechanism of how different influencing factors determine the outcome of SI remains unclear, which might differ somewhat from that of conventional reservoirs and need detailed investigations. For instance, the dependence of boundary conditions on SI has received considerable attention, since various boundary conditions of matrix blocks widely exist during the imbibition process. Traditionally, it has been reported that boundary conditions only affect the imbibition rate but do not impact the ultimate oil recovery if the porous sample and fluids used for each test are same. However, recent studies by authors show that the differences in imbibition recovery for tight oil sandstones among different boundary conditions. However, theyThey did not give any reasonable explanations for thatwhy this might be. In our work, the evident inconsistency was ascribed to variations in the directional permeabilities for different flow modes. Another significant aspect of SI is the study of gravitygravitational forces, which dominate the cocurrent imbibition of fluids in fractured formations. In general, as the tight matrix has a very high capillary force because of its extremely low permeability and porosity, the gravitygravitional effect (also namely buoyancy effect) on SI is always ignored. However, during the imbibition process, capillary forces rapidly decrease as the water saturation increases, implying the ratio of capillary pressure to gravity is not constant and the gravitygravitational effect can no longer can be neglected in certain scenarios. Several studies on SI in tight oil reservoirs have mentioned the contribution of gravitygravitational forces to imbibition recovery, but they did not provide direct evidence to prove this view. Further work is required to clarify the role of gravitygravitational forces in SI for the tight matrix.