For years attempts have been made in the drilling industry to increase the drilling efficiency and decrease the associated costs. The drilling efficiency can be evaluated by comparing applied energy, i.e., mechanical specific energy, with rock strength. The mechanical specific energy is defined as the energy required to destroy a unit volume of the rock. Over the years, this concept has been refined, and researchers proposed various models. Mechanical specific energy directly affects drilling efficiency, as excessive energy can lead to drill string vibrations and bit wear. In this study, a database was established by collecting drilling and log data from the Asmari formation in one of the oil fields of Iran. Various forms of specific energy were examined to develop the appropriate model based on operational conditions and the formation being drilled. Additionally, the confined compressive strength of the rock in the studied well was calculated. The results showed that the developed specific energy model provides a realistic energy value, as it includes all relevant parameters with an output close to the rock strength. Based on the comparison of mechanical specific energy with confined compressive strength, the optimal drilling parameters were determined: weight on bit ranges from 22.24 to 44.48 kN, flow rate ranges from 0.027 to 0.029 m3/s, torque ranges from 2522 to 3091 N·m, and rotational speed ranges from 160 to 180 rpm. Also, an inefficient drilling zone was identified in the studied well, where excessive applied energy compared to rock strength led to the drill bit damage and a significant reduction in penetration rate. The results highlighted the importance of drilling efficiency estimation in the drilling process, where an economic and technically feasible decision can be made by comparing the surface input energy with the rock strength.
