Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Petroleum Engineering

First Advisor

Adam T. Bourgoyne, Jr


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 study presents the results of a feasibility study on 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 that of the sea water at the sea floor. Hydrostatic control of abnormal formation pressure could still be maintained by a weighted mud system that is not gas-cut below the sea floor. Such a dual density mud system could reduce drilling costs by reducing the number of casing strings required to drill the well and so reducing the time required to drill a deep-water well. 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 dissertation 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.