Until recently, exploration drilling had been progressing gradually into deeper water, but has managed to avoid the problem of abnormally pressured formations. The industry successfully addressed the problems at hand, which were focused on the marine environment and the design of equipment, such as risers, to operate in this environment. Production developments have always lagged far behind exploration drilling in getting into deeper water. The current deepwater record for a subsea well is set at 2340 ft (750 m), whereas exploration wells have been drilled in more than 7000 ft (2133 m). Recent lease sales in the Gulf to Mexico have resulted in blocks located in greater than 10 000ft (3048 m) water depth being purchased by operators.
This deepwater activity in the Gulf of Mexico and elsewhere has resulted in a review of the state of the art deepwater drilling technique and the implication of a deepwater drilling program on an overall development. This chapter presents such a review.
This chapter addresses the two central issues of deepwater development drilling. Firstly, there is the technical challenge of drilling abnormally pressured reservoirs. Secondly, there is the economic challenge of drilling these deepwater wells at an acceptable cost and within an acceptable development timetable. The technical challenge will be addressed first.
Pressure environment can generally be divided into three categories. These are:-
Normally pressured - the formation pressure is equivalent to a seawater gradient;
Slightly abnormal pressure - with a formation pressure of up to 12 pounds per gallon (US) (ppg) (mud weight equivalent)
Highly abnormal pressure - with a formation pressure of over 12 ppg (mud weight equivalent)
The pressure environment holds the key to the type of development drilling required and can also drive the overall choice is development type.
Typical pressure profile from abnormally pressured environments, such as would be encountered in the Gulf of Mexico, are shown in Fig 1 and 2. Figure 1 shows the typical gradients seen, whereas Fig 2 shows the same information plotted in absolute pressure terms. As the degree of abnormal pressure increases for a given water depth, the number of casing strings required increases to the point where drilling progress is slow and expensive, because it is necessary to set casing every few hundred feet. Drilling progress is not limited by the rate of penetration when ‘turning to the right’, but is restricted by the length of time required to log and run and cement casing in each selection of hole drilled. Finally, the point is reached where drilling can no longer occur, because, quite simply, no more casing strings can be run.
Recent experience1 highlights the type of equipment that is being used to drill these deepwater wells. Due to the requirement for a drilling rig to operate.
Fig 1 Deepwater drilling prospect : pore and fracture pressure gradients Casing points at arrows - 4000 ft water. (available in full paper)
Fig 2 Deepwater drilling prospect : pore and fracture pressures.(available in full paper)
