The mechanism of miscible/ immiscible displacement in nitrogen gas injection as a part of EOR procedure has been understood. However, production of nitrogen inside the reservoir may have different process and impact for oil displacement. Simulation of EOR in sand pack column by involving denitrifying bacteria could be a close approachment to understand this mechanism. Production of nitrogen molecules within the pores possibly increases the system pressure, which may lead to repressurization of reservoir after pressure loss during primary recovery. An experiment for obtaining the evidence of system pressure increment post-incubation of bacteria was conducted within a closed piston vessel equipped with pressure gauge. Pressure within the system raised .35 MPa during 48 h incubation from initial pressure (1 MPa). This gradual pressure increment fact demanded further proves of correlation between additional pressure and effect on oil recovery. Therefore, a sand pack EOR simulation in core flooding apparatus had been performed and shown .2 MPa increment of pressure during 48 hours incubation of denitrifying bacteria with 3% oil recovery from saturated oil volume or 1.5% from OOIP. Biogas molecules generation and local pores re-pressurization in the unswept water flooded zones might become the responsible factors for the additional oil recovery. Sweep efficiency of displacing fluids in sand pack column could be improved with increasing their viscosities. This mechanism was applied by alternating nitrogen injection with surfactant, with expectation that foam formation would occur within the pores. Yet, the experiment showed that the ratio of surfactant to gas during injection did not provide any significant help for the oil recovery, whether foaming occurred or not, but only the concentration of surfactant (around 900 ppm) that did. Suggested methods to improve this EOR mechanism result were to increase biological activity of the agent through nutrition stimulation and combination of biogas product with more viscous metabolites such as natural surfactant.