Abstract
About 20% of the world's known offshore reserves of natural gas and oil are economically undrillable with today's conventional tools and technology. Major barriers to conventional methods and equipment must be overcome to economically drill for these vast reserves. The barriers to conventional deepwater drilling methods are:
Narrow margin between the formation fracture and pore pressures requiring excessive casing programs and larger, more expensive rigs to drill.
Depleted reservoirs relative to water depth where hydrostatic head of conventional mud and cuttings cause gross overbalanced and resulting damage to well productivity.
Shallow water flow hazards.
Limited ability to adjust equivalent circulating density while drilling ahead.
The tools and technology associated with underbalanced (UB) drilling are seen as key to economically and safely drilling into these reserves. Further, it is expected that by the end of this first decade of this millennium, offshore applications of underbalanced drilling tools and technology will play a profound role in the industry, particularly in deeper waters.
For example, to deal with the conventional drilling barrier described in Item 1., above: The ability to drill offshore with lightweight fluids, perhaps while incurring annulus pressure at the surface in the marine riser (if drilling from a floating rig) or surface BOP stack (if drilling from a jack up), is seen as a viable and attractive method for overcoming this conventional overbalanced drilling barrier. The objective in many cases may not be to achieve true underbalance, but to drill nearer balanced than one would wish to do if drilling conventionally.
For dealing with the barrier described in Item 2., above: Dual density (aka dual gradient) deepwater drilling methods provide an additional attractive option. In the application of this technology, a state of underbalanced is not achieved or desired, but the tools and technology one normally associated with underbalanced drilling are required to practice this technology.
For dealing with the barrier described in Item 3., above: Use of a subsea rotating control head is an option. In this case, one considers shallow water flow hazards as a well control issue and tools up to deal with it accordingly.
For dealing with conventional drilling barrier described in Item 4., above: Underbalanced operations require specialized surface equipment. In the case of dual density drilling, specialized subsea equipment. E.g., Rotating diverter control head, choke manifold, fluids separation and a means of dealing with produced hydrocarbons. The addition of this equipment usually provides at least one and sometimes several additional methods of adjusting equivalent circulating density while drilling ahead.
