the system and method of the present invention permits control of down hole fluid pressures during under balanced drilling , tripping of the drill string , and well completion to substantially avoid “ killing ” of the well and thereby damaging the producing formations in the well bore . the system and method utilizes separate and interconnected fluid pathways for introducing a downwardly flowing hydrodynamic control fluid through one fluid pathway and for removing through the other fluid pathway a commingled fluid formed by mixing of the hydrodynamic control fluid and the well bore fluids flowing upwardly in the well bore .