Overview
In the 1970s, when the term “deepwater” first came into common use, only the most farsighted observers could have envisioned where the industry would be within three decades. Vessels rated for drilling in up to 12,000 of water to total depths of 40,000 feet, GPS-based station keeping, sophisticated wellhead and well control systems, ROVs for operating and inspecting subsea components, advanced methods for well design and construction. This IPIM’s presentation will cover the sea and other technologies that make deep water drilling possible.
The precise definition of deep water can vary within the offshore industry but, because this presentation focuses exclusively on floating drilling rigs, we define deep water as 2,000 feet or greater and ultra deep water as greater than 7,000 feet. Most offshore oil and gas provinces have seen some deep water drilling. The bulk of deep water activity is concentrated in three areas: the Gulf of Mexico, offshore Brazil, and West Africa. The Gulf of Mexico accounts for roughly 30%of all deep water rigs currently in service. Brazil has been a major player in deep water development since the 1980sand has been a leader in developing subsea wells tied back to floating production systems. West Africa has been the scene of a number of very large development projects in the Gulf of Guinea in water depths ranging from 1,000 to 5,000 feet. While these three regions are the main focus of deep water activity, most other offshore areas have seen at least some deep water drilling, most notably the North Sea, Southeast Asia, and the Nile Delta region.
Wherever they are located, all deep water projects present unique challenges related to the surface and subsea environment. In almost all cases, deeper waters means working in open water settings, many kilometers from shore. These long distances from land-based support complicate transportation, logistics, and many other aspects of drilling operations. Depending on the location, they can also mean greater exposure to currents, wind, and wave which affect all phases of well planning and operations from rig selection to weather windows to station keeping. The subsea environment below the sea floor, or mudline can present another series of challenges, where as conventional offshore activity is generally confined to the smooth, gently sloping sea floor of the Continental Shelf, deep water drilling typically takes place along the steeper, more rugged Continental Slope and, in some cases, on the Continental Rise. These deep water settings are younger in geological age than their shallow water counterparts. They are often formed from rapid deposition in high energy environments and tend to be poorly compacted. These turbidite-dominated sequences are common and the sediments tend to be rich in clay with low permeability.
Sandy formations occur only occasionally and are discontinuous. Additionally, many of these clay-bearing shale sequences contain significant proportions of water, thus, these deep water formations have a tendency toward under compaction, resulting in higher pore pressures and lower fracture pressures than those that would be encountered in land wells or shallow offshore wells at the same depths. This leaves little room for error in well planning and design. Geohazards, which may be encountered from the sea floor to around 3,000 feet below the mudline, may include sea floor features, such as mud or debris slides, and unstable soils; incompetent formations below the sea floor; or, in the worst case, shallow sands containing water or gas under high pressure that can begin to flow into the wellbore or even around a cased wellbore to erupt at the surface. Meeting these and other deep water challenges involves a range of special requirements with respect to drilling system capacities, rig efficiencies, safety considerations, and integrating oil field and marine technologies. In this module, we will highlight the techniques and new technologies being used in the field to meet these challenges.
Deepwater Defined
The term “deepwater” has been part of the offshore lexicon for many years. But its meaning has always been relative to the state of technology in a given year (Table 1).
| Table 1: Deepwater Drilling Milestones | |
|---|---|
| 1960s | 500-ft water depths represent the practical limit of offshore drilling technology |
| 1970s | By the early part of the decade, drilling capabilities surpass 1,000 ft water depth. In 1979, the drillship Discoverer Seven Seas sets a water depth record of 4,876 ft |
| 1980s | The Discoverer Seven Seas breaks its own water depth record several times:
|
| 1990 – 2003 | The first of a new generation of rigs becomes available to push into water depths of 8,000 to 10,000 ft.
|
At the beginning of 2008, the record of 10,011 ft set by the Discoverer Deep Seas still stood—but perhaps not for long, as geologists consider new prospects in water depths of up to 12,500 ft, and the industry develops the equipment and methods needed to drill in these depths.
The Minerals Management Service (MMS), the agency of the U.S. government charged with overseeing offshore activity in U.S. federal waters, classifies water depths as follows (French et. al, 2006):
“Deepwater” starts at 1,000 feet; depths of 5,000 ft and greater may be referred to as “ultra-deepwater.”
From the standpoint of technical requirements, these thresholds are somewhat arbitrary, and do not signify abrupt changes in equipment or methods. The 1000-ft “deepwater” threshold, for example, does not preclude the use of fixed platforms, which in fact are considered viable to a depth of 2,000 ft. Beyond 2,000 ft, only floating production platforms have been installed.
Because our discussion focuses exclusively on drilling from floating rigs, we will define “deepwater” as 2,000 ft or greater, and “ultra-deepwater” as greater than 7,000 ft.
While water depth is obviously a key consideration in offshore operations, it is not the only or necessarily even the most important one. Factors such as seafloor topography, subsea well depth, and the characteristics of drilled formations are likely to be more significant in planning and designing a particular well.
