Quantifying Common Drilling Problems With Mechanical Specific Energy and a Bit-Specific Coefficient of Sliding Friction

Abstract

Abstract The conventional way to assess drilling performance in the oil field is to compare actual performance to statistical standards derived from offset records. By their nature, these standards are subjective and variable. While they are ideal to monitor short-range performance and trends in well-known, older fields, they lack the power of physical models to establish absolute, technical performance standards. Several authors have proposed models in which the drilling process is treated as a mechanical, energy-balanced system. Mechanical specific energy input, drilling efficiency, and a minimum specific energy equal to the rock strength are the three key elements of these systems. Full-scale simulator tests were conducted under controlled laboratory conditions to develop and validate an energy-balanced model for drilling under hydrostatic pressure. In these tests, wide variations in drilling efficiency from a few percent to three and four times were observed in the spectrum of common drilling practices. They provided new insights into the drilling process and the nature of common drilling problems. Analysis of field data revealed good correlation between simulator and field results. Using mechanical efficiency, specific energy input, and a bit-specific coefficient of sliding friction as key indexes of drilling performance makes bit selection and the diagnosis of failures and drilling practices more accurate and less ambiguous. Introduction Teale derives the following equation for specific energy in rotary drilling:He also introduces the concept of minimum specific energy and/or maximum mechanical efficiency. The minimum specific energy is reached when the specific energy approaches, or is roughly equal to, the compressive strength of the rock being drilled, i.e.,The mechanical efficiency is thenand the maximum efficiency is reached whenTeale recognized that the specific energy cannot be represented by a single, accurate number since the drilling process is characterized by wide fluctuations of the drilling variables due to its complex dynamics and the heterogeneous nature of rock. Approximate and mean values were found to be sufficiently accurate for a model to predict and analyze drilling performance. P. 373^