The main objective of this research is to predict the proper bit/reamer size ratio based on the rock strength weakening zone around the wellbore. Nowadays, the Reaming While Drilling (RWD) technology is gaining more and more acceptance in the petroleum industry by means of reducing drilling time and Non-Productive Time (NPT), which results in significant cost saving. The importance of this research is analyzing scenarios in which RWD would add benefit to the operation, given that the reaming process is often fraught with much inefficiency. Thus, through the process of estimation optimum reamer/size ratio, by coupling three key parameters in geo mechanics: thermo-poro-elastic, a model of rock strength is developed to analytically assess how rock strength distribution changes around the wellbores and in particular below the reamer in drilling environments. An analytical model of thermoporoelastic is used with published data (for rock properties usage purposes) in order to develop a model of rock strength below the reamer to show in how far some specific rock could be a good candidate for reamer usage. This in turn allows finding proper candidates for effective RWD applications and also can assist to determine the maximum reamer/bit size ratio for certain rock characteristics in order to optimize the drilling system. From the analysis performed, which was carried considering two different groups in terms of formation permeability, it is be conclusive that for low-permeable formation the size of reamer is function of exposure time of wellbore after making pilot hole while the reamer size for enlargement operation through the permeable formation due to fast diffusion rate is not a time dependent parameter.