A common Pre salt offshore well from Santos Basin in Brazil has a water depth superior to 2000m (6500 ft) about 2000m of salt and produce from a carbonate reservoir. The Pre-salt reservoir is a major driver on future production development for Petrobras.
After several cases of severe circulation losses in the Pre-salt wells of Santos Basin, including the area of the well RJS-708, the previous drilling strategy was not proving to be reliable or effective. Loss control pills were ineffective while cement pills were time consuming and not providing satisfactory results. In this scenario arose the opportunity to apply the techniques of MPD (Managed Pressure Drilling) and MCD (Mud Cap drilling) as a contingency on the well RJS-708.
The well had prediction of fractures or caves along the carbonate reservoir with high probability of circulation losses during the drilling of the reservoir phase. The potential benefits of the MPD technique were large due to the possibility of minimizing the volume of fluid lost to the formation through the management of downhole pressure. The MCD technique was also considered as contingency plan, for a scenario of total losses because of the inexistence of a drilling window, even considering the downrole pressure management near balanced.
The work performed during 2013 included: Modification of the rig, training of the teams involved, preparation of MPD and MCD procedures for the drilling rig (First DP rig to use the system at Petrobras) and discussions of new procedures with the contractor. From the list of new procedures included the Dynamical Pore Pressure test, which proved to be a useful tool.
The results obtained with MPD technique were excellent. In order to remain inside operational window, an underbalance mud weight with management of the surface back pressure was adopted to remain in an overbalance condition in front of the reservoir. The MPD system managed to be inside an operating window of 0.25 ppg without significant losses, and a time deviation of less than 4% from the initial schedule and no severe losses were observed during the construction of the well