Pre-salt Seafloor Construction/Remote Operations Challenges Part 2

According to Petrobras production engineers, secondary recovery is increasingly being implemented to improve oil recovery in the pre-salt carbonates, where reservoir rocks are usually oil wet, and this characteristic affects the performance of water injection. Another problem concerning water injection is related to rock-fluid interaction, which is more important and complex in carbonates. In order to assess the risks involved, as well as to define mitigation actions, rock-fluid interaction tests are being carried out in the reservoir rock and the salt cap rock. Alternative recovery methods are being implemented in the pre-salt reservoirs. “Gas injection is already being tested at the Lula field and the promising water alternating gas method (WAG) is also being tested in the field,” said Carlos Tadeu Fraga, Petrobras’ General Manager of the Pre-salt during a Topical Luncheon at the recent Rio Oil & Gas conference. In order to inject water or gas back into reservoirs, a large amount of equipment is needed. Pumps, usually ESP pumps need to be installed on the seabed along with pipes, flowlines, subsea distribution units, gas and water distribution units, gas injection risers.

With depths ranging from 1,500 to 3,000 meters, the seafloor installation processes are complex and time consuming. Subsea construction vessels and their dedicated ROVs are vital assets which are also costly, so these construction operations are orchestrated to the last detail in order to allow a seamless operation from the surface launching of the equipment through powerful cranes to the placement of the equipment at the exact location it needs to be, at a depth of possibly 2km. In terms of these surface cranes, the trend is towards the use of heave compensated cranes to facilitate operations is harsh weather conditions. Recently Petrobras revised its specification on deepwater construction vessels and PLSVs, trending toward more powerful propulsion systems and more powerful offshore cranes. The final placement of the equipment is done with the support of Work Class ROVs, which film the equipment descent from the surface, which requires at least a DP2 capable vessel, in order to assure station-keeping precision. The ROVs also have the capability to actually grab the equipment with their manipulators and place them over the exact spot they need to be in, along with connecting the equipment to units already installed on the seafloor.