Tracking Drilling Efficiency Using Hydro-Mechanical Specific Energy

Abstract Over the last few years, different types of bits have been introduced to meet the challenges of steerable as well as rotary steerable systems; and it is imperative that bits be utilized optimally in these systems. As challenges increase with increasing depths, it becomes even more important for one to efficiently utilize the available energy. A new correlation identifying inefficient drilling conditions is presented in this paper. Mechanical Specific Energy (MSE) has been used to improve drilling rates, with mixed results. Hydro Mechanical Drilling Specific Energy (HMSE), which is introduced here, encompasses hydraulic as well as mechanical energy. HMSE quantifies the amount of energy required to drill a unit volume of rock and remove it from underneath the bit. HMSE includes axial, torsional, and hydraulic energy and is different from MSE because it includes a hydraulic term. The initial MSE correlation (Teale, 1964) was modified to accommodate the new hydraulic term. This paper attempts to better model downhole drilling by introducing the hydraulic energy term in the MSE correlation by defining it as hydro mechanical specific energy. While the majority of the drilling occurs because of the bit, it is a well known fact that some drilling occurs due to the "jet impact impingement" caused by the drilling fluid as well. A comprehensive set of equations have also been developed and introduced here for hole opening scenarios. Experimental and field data presented in this paper show that HMSE can identify inefficient drilling conditions. The new hydraulic term included in the specific energy correlation is the key to correctly match the amount of energy required to drill and overcome the strength and stresses of formation being drilled. Also, this new term illustrates how much hydraulic energy is needed to drill faster when the mechanical energy (axial and torsional) is increased. The results also show the importance of including the bit hydraulic energy term into any specific energy analysis for drilling optimization. The pump-off force for weight on the bit due to the fluid force is also included in the calculation. Field results reveal specific patterns for inefficient drilling conditions and also reveal a good correlation between calculated HMSE and the expected requirements for rock removal under existent conditions of stress at the bit face.