Steam circulation in the early stages of Steam-Assisted Gravity Drainage (SAGD) is crucial for establishing hydraulic communication between the injector and producer well and for the future development of the steam chamber. Steam is the carrier of enthalpy to the reservoir, and thus, the evolution of pressure, temperature, and steam quality is important for heat transfer efficiency. In the simulation of the circulation phase (start-up), most companies in Alberta neglect the heat loss around the wellbore in the vertical/build section of the well and assume a steam quality for the lateral section of the well. Also, most of the simulations found in the literature assume a source-sink approach where the frictional pressure drops along the wellbore and the heat conduction between the wellbore and the reservoir are negligible. In this paper, the steam circulation phase of a SAGD well pair is examined in detail, taking into account heat loss around the wellbore in the vertical/build section and heat transfer and fluid losses in the lateral section of the well pair. In the model developed, wellbore hydraulics is also accounted for by using a discretized wellbore model within a fully implicit coupled thermal reservoir simulator. Field data from the circulation phase or warm up phase of a SAGD well pair at the Lindbergh SAGD project was history-matched to better understand the effect of wellbore hydraulics and heat loss between the dual completion string design and wellbore. This research will help Pengrowth Energy Corporation take into account new operating strategies for future SAGD well pairs.