The present study describes the steps taken to drill the final section of an exploratory well in Santos Basin in the underbalanced (UB) condition, in 1,500-m (4921-ft) of water depth. The first well in the area reached a fractured carbonate reservoir in which massive losses of circulation caused so many operational difficulties that the formation evaluation was not conclusive. As consequence, in the third quarter of 2005, another well will be drilled, reaching the same reservoir in the UB condition. Pre-planning and planning activities, additional required equipment, riser modifications, and new procedures are presented and discussed.
Despite the fact that underbalanced drilling (UBD) has been applied for many years in the oil industry, implementing this technique from floating units is still a challenge. Several thousands of wells have been drilled underbalanced on world basis and more than 4,000 wells in Canada alone1, but almost exclusively restricted to onshore fields. Literature reports some offshore experiences that are limited to fixed platforms or jack-up rigs1–4. Operational difficulties associated to safety, logistics, and equipment placement have been keeping UBD aside from floating rigs.
At the end of 2000, Petrobras, leading a JIP with other operators, service providers and consultants, overcame an important barrier with a well in Albacora Field (297-m or 974-ft of water depth) being drilled with aerated mud from the semi-submersible rig Petrobras 17. In spite of not drilling underbalanced, the principles of operation were close because a two-phase mixture composed of nitrogen and water based mud was pumped down through the drillstring and returned to surface under pressure using a rotating control head on the top of the riser, which was decoupled from the diverter assembly, and the return taken through hoses back to a typical UBD surface system5.
After reaching that milestone, Petrobras started the development of technology to achieve the dual gradient drilling (DGD) condition by pumping gas at the bottom of marine riser in ultra-deep water. Differently from the mechanical lifting systems6, the use of gas consists of diluting the mud returns at the seafloor. From the point of the gas injection upward to the surface, the density of the drilling fluid is lighter than the effective mud density below the seafloor7. In ultra deep-water, the DGD condition provides an effective approach for managing the challenges associated with the narrow operational margin between the curves of pore pressure and formation fracture. For illustrating this capability, Fig. 1 presents two distinct curves of equivalent circulating density (ECD). The red curve represents the conventional single gradient drilling (SGD) method and the blue one stands for the DGD. In this example, which is based on an exploratory case from Capos Basin, in Brazil, the conventional SGD method needs a 9.2 lbm/gal water based mud (WBM) for reaching the final depth of the 12–1/4" section of the well at 4,200 m. Because of the friction pressure losses, the bottom hole ECD is slightly lower than 9.4 lbm/gal.