Facts, Fallacies and Pitfalls of Using Mechanical Specific Energy (MSE) – Part 1

Abstract

Over the years, some researchers have used Mechanical Specific Energy (MSE), which is said to represent the amount of energy needed to drill a unit volume of rock, to quantify drilling efficiency. MSE was originally introduced by Teal for the mining industry in 1964. Since then, MSE has taken different forms for other reasons, based on its interpretation, and intended use. This paper provides a comprehensive review of MSE in general, discusses its different forms and narratives, and draws the readers' attention to common (and not so common) facts, pitfalls, and fallacies, of using MSE. It has been found that these specific energy concepts are held as true for all predictive purposes in drilling, amended and promoted beyond the original framework. The paper analyzes all the equations presented in the past and quantifies each component of the equations. The hydraulic terms used alongside the mechanical terms are also discussed. Extensive simulations have been carried out and will be reviewed in this paper by quantifying the energy under each term based on rate of penetration effects and implications. We aimed to demonstrate in this paper, the theoretical grounds for pitfalls and fallacies in using MSE. MSE is made up of two components: torsional energy and thrust energy. The results have shown that the thrust term is much smaller than the second torsional energy term and in most of the cases, about 2% or less. Hence, it could be neglected and thereby the equation results in the form of inverse of the rate of penetration (ROP) making the calculated MSE value redundant when the actual ROP is available. The results also have shown that when the hydraulic energy term is subtracted from the MSE equation, it results in negative rate of penetration and thereby shows a fundamental flaw in the system formulation. The purposed and merits of MSE use, by some researchers to identify drilling dysfunctions, will also be highlighted. In this process, it has been shown that nonlinear "torque wedging" causes inaccuracies in dysfunctions identification and discussions. Also, the field data presented in the paper shows that mechanical energy is not a ratio of input energy and rate of penetration. Moreover, none of the studies have accounted accurately for the effects of bit wear and motor wear on MSE. It has been found that overall, the concept relating to dysfunction quantification is a self-destructive process, which has spread from paper to paper without the required checks and verifications for accuracy. The underpinning discussions have been backed and demonstrated with numerical examples. The paper provides the pitfalls in the omissions of some of the assumptions in various MSE models used by engineers. This helps the users to carefully plan, design, engineer and construct the wells.