During the last ten years, ENI has developed the "Lean Casing Profile" concept, successfully drilling many wells with a reduced annular clearance between casing and borehole wall. Recently, drilling and completion technologies have been further improved allowing vertical wells to be drilled with an "Extreme Lean Casing Profile". Compared with the previous concept, the Extreme Lean Casing Profile provides an additional intermediate casing section which allows drilling into the reservoir with a larger hole diameter (i.e. passing from 6" to 8 ½) or intersecting deeper targets, previously unreachable by traditional casing profile, in 6" drain hole size. The first vertical well with this innovative feature was drilled in Egypt in 2005/2006, whereas the first "Deviated Extreme Lean Well" was drilled in the Val D'Agri field in Italy during 2007/2008 (Cerro Falcone 4 OR). This well was characterized by hard and abrasive formations, hole instability, severe vibrations, mud losses and employed various high technology drilling systems. This paper provides a comprehensive description of the "Extreme Lean Casing Profile" concept, explaining the most important features and its advantages in terms of cost reduction, environmental impact and drilling performance. The paper also analyzes the operational issues related to Cerro Falcone 4 Or, particularly in the 12 ¼" section opened to 12 ⅞". In this phase, rotary steerable systems in conjunction with expandable reamers (first time in the Val D'Agri area) were used in order to run an 11 ¾" liner after the 13 ⅜" casing. Through a preliminary study using the I-Drill software, the best theoretical BHA in terms of downhole vibrations was identified analyzing and comparing the dynamic behavior of many different configurations. During drilling, a downhole tool for dynamics measurements (CoPilotTM) enabled further optimization and provided valuable information to face real time problems: in particular it ensured vibrations reductions, ROP maximization, tools' life extension and drilling risks minimization. Another purpose of this paper is to provide an overview of the operational challenges encountered during drilling of the Cerro Falcone 4 Or well in the vertical phase (anti-collision problems, verticality, wellbore tortuosity reduction), the deviated phase and horizontal phase, where, by utilizing the advanced rotary steerable system, it was possible to achieve very low friction factors that are crucial for extended reach projects

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