Overcoming Deep and Ultra Deepwater Drilling Challenges

Abstract

Abstract The challenges found in deepwater and ultra-deepwater drilling have, in a remarkable short period, forced the oil industry to develop new significant technologies and techniques. The characteristics of the deepwater environments have pushed design criteria, normally used in onshore and shallow water wells, to values beyond their traditional limits. All drilling phases of deepwater and ultra deepwater wells face challenges. The initial phases, generally composed of soft soil or just mud, have required a lot of experience in terms of jetting the conductor pipe to avoid sinking of the wellhead. In the intermediate phases, engineers must be very careful to avoid lost circulation due to the narrow window between pore pressure and low fracture pressure gradients. Besides, well bore instability, always an issue for directional drilling, often limits the length of the deepwater well departures to values considered small if compared to those obtained in shallow waters or onshore. In addition, the drilling of permeable rocks, many times just loose and unconsolidated sands, increases the chance of differential sticking. To complete the picture, watching closely well operation and drilling parameters to keep risks under control generally is not enough. Creativity is very often necessary to overcome the ultra-deepwater challenges. This paper describes Petrobras drilling experience in deepwater and ultra-deepwater. A number of historical cases are shown to illustrate the main obstacles an Operator normally faces when drilling in deepwater. The evolution of well design and drilling practices as the result of the increase of the water depth and its related problems, are also presented. Several experiences with new tools, rig performances and drilling fluid products specific for deepwater are described. Finally, deepwater projects that include extended reach drilling and other special studies are also discussed. Introduction The discovery of important deepwater oil fields such as Marlim and Albacora has inspired Petrobras to look for new frontiers located in deeper waters. However, initially thought as simple adaptation of the work done in shallow water, a number of new challenges also followed this move. According to the oil industry, deepwater wells are those wells drilled in water depths ranging from 300 meters to 1,500 meters. Wells located in water depths higher than 1,500 meters are classified as ultra deepwater wells. Deepwater drilling with dynamic positioned (DP) rigs has been a reality in Brazil since 1984. Up to 1991, Petrobras used only these types of rigs to drill exploratory wells in the Brazilian Basins, including Campos Basin. After having drilled a number of wells in water depths higher than 1000 meters, Petrobras has started in 1990 an extensive program to drill development wells in deepwater oilfields using both, DP and anchored rigs. Fig. 1 shows a schematic map of the Brazilian Coast and the distribution of deep and ultra deepwater wells along it. Campus Basin, where the main oilfields are located, has the majority of the drilled wells. Though, Espirito Santos Basin, where a number of new discovery have been announced by Petrobras, is expected to have a considerable increase of drilling activities in the future years. Fig. 2 displays the number of wells drilled in deep and ultra deepwater versus time in years.