We performed mesoscale simulations in order to investigate the effects that additives, such as surfactants and polymers, have in the oil displacement process by water and brine injection. A Many-Body Dissipative Particle Dynamics (MDPD) model was parameterized in order to reproduce physical properties obtained either by experiments or Classical Molecular Dynamics (MD). The MDPD model was then employed to simulate the displacement of n-dodecane by water and brine, with surfactant concentrations ranging from 1 wt% to 10 wt% SDS and polymer concentration of 0.5 wt% HPAM. We observed that while the additives may enter the capillary without clogging, specific combinations of surfactant concentrations and injection rates may lead to poor oil displacement. This result can be understood in terms of the balance between water-wall and (surface-mediated) water-oil interactions. As a consequence, our results show an effective reversal of the wettability character, from water-wet to oil-wet, as a function of the interface speed. We conclude by summarising the implications such results have on the fluid-fluid displacements involving additives.