Although several models to determine the formation temperature in the modelling of thermal production and injection processes have already been suggested, there is no rigorous or systematic comparison between these models predictions that can guide the choice of the most: appropriate one. Another issue in thermal wellbore simulators is the commonly used assumption of semisteady-state heat transfer from the wellbore up to the cementing/formation interface. The effect of the semisteady-state assumption vs. the unsteady-state assumption for the heat transfer from the wellbore up to the formation has not received much attention in the literature and can be important in some cases. The results of a detailed analysis of the two previously described issues can be implemented in all thermal wellbore and reservoir simulators to increase their accuracy. The previously described stated issues will be addressed in the present work by developing a numerical nonisothermal two-phase wellbore simulator coupled with tubular and cement material and surrounding formation. The first issue will be studied in detail by comparing five different models for the formation temperature treatment (FTT) plugged in the developed thermal wellbore simulator. Investigation of the second issue will be achieved by analyzing the three critical items: first, a 2D heat transfer partial differential equation (PDE) model of the formation is discretized in a general form; second, the gridding system is shifted from the formation, toward the casing; and third, an effective specific heat capacity for the casing is used. The effects of choosing different models for FTT and using either the unsteady-state or the semisteady-state assumption in the heat loss from the wellbore up to the formation will be investigated. The model will be validated against field data to show its merits in predicting the casing temperature. The entire wellbore system contains wellbore, tubing, insulation, annulus, easing, cementing and formation. A fundamental understanding of this system is still a challenging issue in the petroleum industry, and its accurate modelling and coupling with reservoirs has become increasingly significant as more energy resources are sought.