Deepwater Environments
Deepwater operations present challenges related not only to water depth, but also to open water environments and subsurface geology. While these parameters differ from one region to another and also within the same region, certain characteristics do tend to be associated more commonly with deepwater environments than with shallow-water ones.
Open Water
Deeper waters almost always mean greater distances from land. This extra distance may not be significant in mild environments such as offshore West Africa, where the main concerns are sea swell and coastal currents. But in other environments such as the North Sea and Gulf of Mexico, it translates into harsher wind and wave conditions that can significantly impact well planning, design and construction operations.
In the Gulf of Mexico, for example, warm-water eddy currents break off from the Loop Current that enters the Gulf through the Yucatan straits and exits through the Florida straits (Figure 6 Eddy currents in the Gulf of Mexico. NASA, Jet Propulsion Laboratory). These eddy currents, which can be up to several hundred miles across and several thousand feet deep, rotate clockwise at speeds of up to four or five miles per hour. They drift slowly westward across the Gulf, along a path that takes them through the Deepwater drilling areas in the Central and Western Gulf. Obviously, this mass of rapidly moving water exerts a strong force on any structure it encounters – far greater than the shallower currents found nearer to shore.
Eddy currents also affect hurricanes. The warm water provides energy that can be drawn up by hurricanes that pass over them, allowing the hurricanes to strengthen. This seems to have happened several times in recent years. Both Katrina and Rita in the summer of 2005 strengthened as they moved across the Deepwater areas (Figure 7 Hurricane Katrina, 2005. From National Oceanic and Atmospheric Administration) then weakened as they moved over cooler water before making landfall. Both storms did more damage to Deepwater floating production platforms and drilling rigs than had been expected and alerted authorities to potential deficiencies in Deepwater specifications and requirements.
The wind and wave forces of a hurricane may combine with the strong flow of an eddy current to exert extremely heavy loads on offshore structures and drilling rigs. It may have been such a combination that tore several semisubmersible rigs loose from their moorings and set them adrift during Katrina and Rita, and caused the Typhoon mini-Tension Leg Platform production facility to capsize (Figure 8: Typhoon platform (a) before and Figure 9 (b) after Hurricane Rita.
To the extent that the deepwater wind and wave regime may be more severe than in shallower water, drillers may experience a reduced weather window for moving a rig on location and deploying the mooring system, and also for moving off location.
Subsurface Environment
Offshore drilling takes place on the continental margin, which is divided into the continental shelf, slope, and rise (Figure 10).
The shallowest segment, the continental shelf, is usually smooth and gently sloping and may extend out from shore for many miles until the water depth reaches about 650 ft. In the Gulf of Mexico off the Texas coast, the 100-ft water depth can be more than 30 miles from the beach. By contrast, in active tectonic zones such as offshore California and much of the rest of the Pacific Rim, the shelf may be only a few miles wide. Shelf sediments typically have been deposited slowly in shallow water environments over numerous cycles of high and low sea levels. Where the shelf ends, the seafloor drops off at a much steeper rate to form the continental slope. The seafloor topography of the slope is far more rugged than the smooth shelf and is characterized by canyons, gullies, and outcroppings. The slope eventually flattens somewhat to become the continental rise that extends outward and downward to the deep ocean floor, or abyssal plain. The deepest drilling in the Gulf of Mexico is now occurring on the continental rise.
The deepwater formations in the Gulf of Mexico, offshore Brazil and West Africa have certain geological characteristics in common:
- Deepwater basins tend to be younger than shallow-water basins.
- Their sediments have been deposited rapidly in a high-energy (turbid) marine environment at the base of the continental slope and tend to be poorly compacted.
- Turbidite-dominated sequences are common, and the sediments tend to be clay-rich with low permeability. Sandy formations occur only occasionally and are discontinuous. Many of the clay-bearing shale sequences contain significant proportions of water.
These characteristics give rise to one of the key challenges of deepwater drilling: the narrow “window” between formation pore pressure and fracture pressure.
Pore Pressure and Fracture Pressure
The depositional history and immaturity of deepwater basins together with the fact that much of their “overburden” corresponds to the weight of the water above a formation rather than water plus rock matrix—makes them more likely to contain sediments that are undercompacted. This tends to result in pore pressures that are higher, and fracture pressures that are lower, than those that would be encountered in land wells or shallow offshore wells at the same depths.
This combination of higher pore pressures and lower fracture pressures has important implications for deepwater drilling projects. These are addressed in the subtopic titled “Deepwater Well Planning and Design,” but are summarized as follows:
- Drillers control formation pressure by adjusting the properties—primarily the density or weight—of the drilling fluid. This function of the drilling fluid, or mud, is expressed in the well-known equation relating hydrostatic pressure to Equivalent Mud Weight (MW) and True Vertical Depth (TVD):
Phyd = 0.052 ⋅ MW ⋅ TVD
where
Phyd is in psi,
MW is in pounds per gallon (ppg) and
TVD is in feet.
- To prevent formation fluid from entering the wellbore, the mud weight must be high enough so that the pressure exerted by the mud column is higher than the formation pore pressure. At the same, it must not be so high that the pressure of the mud column begins pushing fluid into the formation (the condition known as “lost circulation” or “lost returns”) or worse, that it actually causes the formation to fracture. In normally pressured formations, it is usually possible to weight up the mud to handle any expected formation pressure without risk of lost returns and formation damage. The difference between the pore pressure and the fracture pressure is great enough to allow for some latitude in the mud weight schedule.
- The situation is less forgiving in Deepwater wells, which tend to exhibit narrower differences between pore pressure/fracture pressure, leaving little margin for error in determining mud weight (Figure 11, Comparison of pore pressures and fracture pressures for a land well and a subsea well drilled in 4000 ft of water. Note that the subsea well exhibits a much smaller difference between pore pressure and fracture pressure). The pressure that is needed to prevent intrusion of formation fluids may be very close to the pressure that will fracture the formation. The weight of the mud column also increases as the well gets deeper, and this can push the pressure beyond the fracture pressure of the formation. The narrow pore pressure/fracture pressure window is the source of many challenges in Deepwater drilling and places severe constraints on the well planning and design process.
When a deepwater well finally reaches its reservoir target, it is more likely than a well in shallow water to encounter high temperatures and high pressures that create challenges for drilling, well testing, well completion, and production operations. These reservoirs and the wells drilled into them are commonly given the designation HTHP. Blowout preventers and production trees for deepwater wells may be rated for pressures of 15,000 psi and temperatures of 350º F.
Low Seafloor Temperatures
Low seafloor temperatures are characteristic of all deepwater operations. Even in tropical regions, the water temperature below 2000 ft [610 m] will be no higher than 34 to 38 º F [1 to 3 º C]. These low temperatures affect both hydrocarbon fluid flows and the metallurgical properties of subsea equipment. The effects are most significant in production operations, where hot well fluids enter cold seafloor equipment and piping, but can also impact drilling operations.
Geohazards
Several types of geological hazards (geohazards) are encountered frequently in deepwater drilling areas. Some of the geohazards reflect the depositional history of deepwater basins, particularly mass-transport deposits (MTDs) resulting from resedimentation occurring after the period of original deposition. Deepwater geohazards are found from the seafloor to depths of about 3000 ft below the mud line (BML) and pose significant risk for a successful drilling operation, including loss of the well. Geohazards can be identified on 3D seismic records and by seafloor and shallow subsurface surveys. Identification and proper planning allow drillers to avoid or mitigate the effects of deepwater geohazards.
Deepwater geohazards include seafloor features or events such as mud or debris slides and unstable soils, as well as a variety of incompetent formations below the seafloor. While these can create difficulties for drilling, the greater threat comes from shallow formations (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. These shallow geopressured sands are usually shut off by chemical treatment delivered in the drilling mud and by casing and cementing. These measures are not always effective, however. Marathon Oil Company reported the loss to shallow water flow (SWF) of an exploration well drilled in 2001 on Garden Banks Block 515 in 3300-ft water after the geopressured formation had been cased off (West and West, 2005). In 1999, Marathon had drilled successfully through the same SWF sands.
Shallow water flow was identified as one of the five most urgent challenges for deepwater drilling (Sparkman and Smith, 1996), and has been addressed by numerous studies. SWF creates difficulties in setting conductor pipe and surface casing; and when it is occurs in a well with a narrow fracture gradient, it contributes to increased well costs through the need for additional casing strings. Severe SWF and fracture gradient conditions may, in fact, make it impossible to penetrate a potential reservoir with a wellbore of sufficient diameter to allow for economically feasible production flow rates.
HSE issues
Protection of health, safety and the environment (HSE) is paramount for onshore and offshore operations alike. Avoiding accidents and eliminating risks to personnel is a top priority, both in operational activities and equipment design, and minimizing pollution and harmful environmental impacts is a constant objective.
All of the risks associated with the hostile marine environment and the handling of volatile liquids are common to both deepwater and shallow water drilling operations. Several aspects of deepwater drilling, however, may increase the urgency of HSE concerns.
Weather and environmental forces
Deepwater operations generally involve exposure to marginally more severe and less predictable weather and environmental forces. In The Gulf of Mexico, for example, the powerful eddy currents create hazards in deepwater that to do not exist in shallower water areas. Storms may be more severe in deepwater areas, and these regions may also be more exposed to rare phenomena such as rogue waves.
Distance from shore
Transport to and from offshore facilities is inherently risky, and greater distances would seem to add to the risk. Remoteness and distance from shore also mean that evacuation of deepwater facilities in the face of hurricanes or other severe storms requires more lead time and takes longer to accomplish. Careful planning and contingency arrangements are a must.
Sensitive seafloor ecology
Certain deepwater areas in the Gulf of Mexico are home to communities of unique benthic flora and fauna, which place them off-limits to drilling. Where little is known about the deepwater seafloor environment, drillers may be required to conduct detailed underwater surveys and eliminate or minimize release of pollutants.
Geology and Geohazards
The geology and geohazards of deepwater areas may increase the risk of blowouts and require more conservative well-control procedures. In previously unexplored deepwater trends, ignorance of the underlying geology adds risk to drilling operations.
