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The majority of the wells which have been drilled in Iranian oilfields in Persian golf include a horizontal section through the reservoir and the most of them have been drilled by the conventional steerable mud notor assemblies. Steerable motor drilling is a relatively inefficient process with associated problems, ranging from trajectory control in unstable shales, pipe sticking in lost circulation zones and slide drilling in carbonate reservoirs. In order to minimize the effect of these issues, rotary steerable system has been deployed in a number of wells for reservoir lateral drilling over the past year. This paper looks at a range of specific applications where the change from steerable motor to rotary steerable system has been both technically and commercially successful in offshore Iranian oilfields. Through the case histories, this paper will demonstrate both tangible and intangible benefits resulting from the use of rotary steerable systems that have led to a step change in overall drilling performance.