Controlling drilling mud density in drilling operations utilizing concentric drill tubes and at least two different density fluids. The mud required at the wellhead is combined with a base fluid of a different density to produce diluted mud in a riser. By combining the appropriate quantities of drilling mud with base fluid, greater control over the pressure in the wellbore and various risers can be achieved. Concentric drill tubes or risers are disclosed, wherein a first annulus defined within one riser is utilized to carry one fluid to the wellbore injection point, while a second annulus defined within another riser is utilized to carry a combination fluid and cuttings back to the drilling rig. In one embodiment with three concentric drill tubes, a third annulus is also define, wherein the various annuli can be utilized in various combinations to deliver at least two different fluids to the wellbore and return a combination fluid from the wellbore. Blowout preventers may also be used in combination with the process to control these pressures.