Several gas-influx events have occurred during drilling ESS-107, an exploratory well in a 1286 m of water depth offshore location in the southeast coast of Brazil, state of Esp rito Santo. The 9 5/8" casing was set at 3804 m for drilling the final 8 1/2" phase of the program. Then, a sequence of gas-influx events affected substantially the overall performance of the drilling activity. The first influx occurrence was observed while drilling at 4719 m and it was initially diagnosed as over-pressured. Consequently, after using the driller's method to remove the gas, the mud weight was increased from 11.3 to 12.5 lbm/gal. After that, some other well control incidents happened in sequence while drilling from 4719 to 4826 m. Despite the suspicion of a fracture breathing formation phenomenon, the mud weight was increased up to 13.7 lbm/gal without putting an end to the flow of gas into the well. In addition, as water based mud (WBM) was routinely flowing through the choke and kill lines mixed with gas, the low temperature of the environment close to the seafloor triggered the formation of hydrate. So, both kill and choke lines were plugged, which was indeed the first occurrence of hydrate formation in deep water while drilling in Brazil. The present study analyzes the influxes based on all available data, such as daily drilling reports, daily drilling fluid reports, final well report, mud logging data, pressure while drilling (PWD) data, etc, to determine if the fracture breathing formation phenomenon was really the driven mechanism. The control procedures used are also discussed and evaluated. Finally, a proposal is presented to improve the overall drilling performance for the succeeding wells in the area. Introduction The EspÍrito Santo basin is located alongside the coast of the state of EspÍrito Santo and the southern coast of the state of Bahia, in the southeast coast of Brazil. As showed in Figure 1, this sedimentary basin borders at south the Campos basin, which is the most important oil province in Brazil. Its sediments can be found both onshore and offshore, covering 123,130 km2, out of which 17,900 km2 are onshore and another 52,860 km2 are shallower than 400 m. In spite of being the pioneer in terms of the exploratory offshore activity in Brazil, most of the producing oil fields in the Espirito Santo basin are onshore. In fact, the offshore portion of the basin has been showing good potential for the gas production. The fields of Cangoá and Peroá, which are gas producers discovered in 1988, are illustrative examples. The sandstones of the Urucutuca formation, which were deposited in different geotectonic scenarios, are usually the main offshore goals in the Espirito Santo basin. In particular, the turbidite sandstones of the Urucutuca formation are attractive because of their potential for gas production. Departing from this exploratory perspective, the well ESS-107 was planned to be drilled on a 1286-m of water depth location at the block BES-2, south of Peroá field and at about 60 km from the coast of Brazil, as indicated in Figure 2. The drilling operation was executed conventionally in accordance with the normal standards of performance until reaching the end of the 12 1/4" phase. The 9 5/8" casing was set at 3804 m and a formation integrity test (FIT) was performed, limited to 13.8 lbm/gal. While drilling ahead with the 8 1/2" bit at 4719 m, total mud volume augmented suddenly and a pit gain of 10 bbl was noticed. Simultaneously, pump pressure dropped down about 250 psi. Then, the drilling crew performed a flow-check that expanded the pit gain to 25 bbl. The well was shut in and conventional (driller's method) kill operations commenced. After raising the mud weight from 11.3 to 13.2 lbm/gal with an intermediate step at 12.5 lbm/gal, the well, which appeared to be killed, was opened and found to be static. The drilling operation was resumed, but after circulating bottoms up subsequently to connections, gas peaks and pit gains were registered. In spite of further increases in mud weight to a maximum of 13.7 lbm/gal, the entrance of gas into the borehole could not be prevented.

The biggest challenge of the aerated fluid drilling technology development was overcome with the well in Albacora being drilled with the semi-submersible platform Petrobras 17 (P-17). A previous paper1 described the stages before the drilling activity: planning, new equipment, rig modifications and all the preparation for drilling the well. This paper describes the drilling operation itself. All the important points and differences relative to the conventional drilling method are highlighted, with special emphasis to the new equipment installed at the platform: the vertical compact separator, the automatic control system, and the RiserCap™ rotating control head. The operation of the new equipment, performance and the problems observed are presented and discussed. The points of improvement are suggested and ways to move forward with this new technology also discussed in the paper. The present work also addresses the positive impact of this field test relative to the implementation of the light-weight fluids technology from floating units. This initiative has been carried out systematically by Petrobras in the form of a Joint Industry Project (JIP) and has been congregating operators, service companies and consultants. The next steps of this project to use effectively light-weight fluids for drilling in deepwater are discussed. The experienced obtained with this operation was crucial to direct and guide the most important points to be studied and developed in the next phase of the project. Introduction The injection of a liquid and gas mixture for drilling purposes is not new in the oil industry. Besides the recent trend towards the use of underbalanced drilling (UBD), the literature2–5 documents a considerable number of operations carried out for limiting loss of circulation, avoiding differential sticking or improving penetration rates in competent crystalline rocks. These traditional low-head drilling (LHD) methods, which conduct downhole pressures just above the reservoir pressure, focus on targets that are somewhat distinct from those aimed by the UBD approach. The drastic reduction or elimination of formation damage are indeed the major motivation for the use of the underbalanced concept, as discussed elsewhere6,7. However, despite the differences between LHD and UBD, they are both techniques in which the injection of a gas-liquid mixture plays an important role in the drilling process. The injection of gas and the treatment of the returning mixture certainly take part in the list of the most challenging activities to manage in the operations. These aspects are so demanding that they are responsible for having almost restricted the application of UBD and LHD to the onshore scenario. However, the widely announced success of many projects is motivating the industry to employ considerable efforts for making it available in the offshore environment where lack of space for accommodating new equipment is the main concern. Besides the demand for space, floating vessels request much more in terms of innovative design because they have a configuration of surface and subsea equipment that is completely different from land rigs, fixed platforms or jack-up units. In addition, it is always good to remember that the adequate solutions for floaters should be technically and economically viable without causing any impact to the operational risks. Petrobras, which has the majority of its reserves in water depths greater than 400 m, decided to develop competence to bring this technology to floaters. The venture successfully achieved all the goals8–13 and involved other operators and service companies in the form of a JIP. The present work reports the ultimate task of the whole project that was the LHD test performed in the well Albacora 64H in December 2000.