Understand Drilling System Energy Beyond MSE
Authors: Chen Wei , Shen Yuelin , Zhang Zhengxin , Bogath Chris , Harmer Richard 
Published: September 2019

Mechanical specific energy (MSE) has been widely used in the industry to monitor drilling efficiency. However, it does not give detailed information about energy flow in the drilling system and lacks the resolution to identify the root cause of energy loss. The drilling operation is a dynamic process. Energy input may be from a surface-drive system (top drive or rotary table) or a mud motor placed downhole. In a perfect world, all of the energy is used to drill the rock. However, some of the input energy may reside in the drillstring as strain and kinetic energy due to the deformation and motion of the drillstring. Drilling energy is dissipated due to shock, vibration, fluid damping, and frictional contact between the drillstring and wellbore. A novel method has been developed to calculate the drilling energy flow in the drillstring and to enable better drilling energy management by maximizing useful energy consumption and reducing energy waste. The method provides a new way to understand and improve drilling efficiency.

The method is based on an advanced transient drilling dynamics model which simulates the full drilling system from surface to bit. The entire drillstring is meshed using 3D beam elements, and its dynamic response history is solved by the finite element method (FEM). The energy input can be calculated from surface drilling parameters, such as torque, rotation speed, flow rate, and motor differential pressure. With the simulated history of forces and dynamics of the drillstring, the corresponding strain energy and kinetic energy of the drillstring can be evaluated. The detailed cutting structure model can provide insight on the energy amount consumed by the rock cutting action of the bit and reamer. Putting all the components together leads to a holistic calculation workflow of drilling energy.

Field case studies were conducted to examine the effectiveness of this method. The studies showed the drillstring strain energy and kinetic energy are good performance indicators for drillstring reliability and stability because these energy variables reflect the severity of loading and vibration in the drillstring. The energy variables possess clear signatures for interpretation of different downhole vibration modes. Currently, the drilling efficiency is normally evaluated by MSE, which represents the amount of energy needed to remove a unit volume of rock using the surface drilling data. In this study, the energy loss is calculated to understand the percentage of input energy dissipated due to the interaction of the drillstring with the environment. In contrast to MSE, the calculation provides a more direct and detailed measurement of drilling efficiency. It gives a methodology for understanding detailed energy flow in the drilling system under different drilling vibration modes. It can be applied to bit selection, bottomhole assembly (BHA) design, and drilling parameter optimization to achieve better drilling energy management and improve drilling efficiency.

The novel approach calculates drilling energy based on the transient dynamics simulation of the full drilling system. It provides a detailed and holistic view of drilling energy input, propagation, and consumption. This method could help identify the inefficient drilling conditions and optimize drilling operation through evaluating and comparing different options.