Recent successful exploration efforts in deep waters have heightened interest in-developing oil and gas reservoirs on the continental slope. Leases have been obtained in water depths up to 10,000 ft with a requirement that they be drilled within the next decade. Use of current techniques to drill these leases will require extremely large floating drilling units and large diameter marine riser systems. This paper presents the results of a feasibility study of the use of an automated gas-lift system for a marine riser that will maintain the hydrostatic pressure in the subsea well-head equal to the hydrostatic pressure of the seawater at the seafloor. Hydrostatic control of abnormal formation pressure could still -be maintained by a weighted mud system that is not gas cut below the seafloor. Such a dual density mud system could reduce drilling costs by reducing the number of casing strings required to drill the well and by reducing the diameter requirements of the marine riser and subsea blowout preventers. The system would have the advantages of riserless drilling without giving up the well control advantages of a closed, weighted mud system. A steady-state numerical model was developed that can be used to determine the gas injection requirements needed to achieve a desired dual density configuration. The numerical model was verified through tests conducted in a 6,000 foot research well. Once verified, the model was used to define the gas requirements and practical limits of a marine gas-lift system based on estimated additional costs of gas compression and nitrogen membrane filters. The practical limits are presented in terms of maximum mud density, water depth, and riser diameter combinations. The paper also discusses the operational changes that would be required for various drilling procedures such as making a connection, running casing, kick detection, and well control operations. Introduction: Drilling operations in waters deeper than 3,000 ft are increasing throughout the world, and the industry is now looking into the challenges imposed by ultra-deep waters (>7,000 ft). One of these challenges comes from the fact that the current technology relies on the use of marine risers to extend the well up to the drilling vessel, so the mud and tools can get in and out of the well in much the same way that is done on land However. when drilling with mud densities in excess of sea water density, the exposed sediments in the open borehole see a pressure tending to cause formation fracture that is affected by the full column of drilling fluid all the way up to the drilling vessel. The overburden pressure of the sediments tending to help resist formation breakdown is generated by sediment densities in excess of seawater hydrostatic only from the mud line down.