Although not considered a major challenge by Petrobras, drilling two thousand meters below the water line, through about three thousand meters of sediments, and through two thousand meters of salt is done at the frontiers of technology.
Drilling at a depth of two thousand meters is not a problem. Petrobras already has several wells at this depth, in Brazil and in the Gulf of Mexico, and produces at 1,886 meters in the Campos Basin, a mark that was once the world record (Shell currently holds the record, at more than two thousand meters from the water line in the Gulf of Mexico). The biggest challenge is the layer of salt, which, under high pressure and at high temperatures, behaves like a plastic, making it difficult to ensure the stability of the rock in the thick layer of salt - which can flow and prevent drilling in the wells. The salt layer contains several kinds of salts, halite, taquidrite, carnalite, some of which are soluble. The drilling fluid can only be defined after a series of chemical analyses are made. Engineers need to define the well's lining, specific geometry and the best drill bit to use.
The main technical issue for drilling under the conditions found in this area is to ensure the stability of the thick layer of rock salt, of up to 2,000 meters in some regions, which can flow and keep the well from being drilled. Much progress has been made in recent years, allowing not only stable drilling through the layer of salt, but also reducing well-drilling time.
It is all a matter of technology. The first well Petrobras drilled in this pre-salt section took more than a year and cost $240 million to be completed. The most recent wells Petrobras drilled there took 60 days and cost, on average, $66 million. This was possible on account of the learning that has been made in well drilling in order to cut through the 2000-meter thick salt section, of well lining specifications, of improvements made to drilling fluid quality, in well geometry design, better drill bit specifications, as well as due to progress achieved with many chemical analyses made at research centers and by Petrobras' well engineering group.
Production is typically made with wells that have a horizontal section. Petrobras has already drilled over 200 horizontal wells in deep waters in shallower reservoirs above the salt layer. It will now have to drill wells below the salt layer and is working on consolidating this technology.
Photo courtesy of Petrobras