Drilling Performance Is a Function of Power at the Bit and Drilling Efficiency

Abstract

This article is a condensed version of paper IADC/SPE 151389, presented at the 2012 International Association of Drilling Contractors (IADC)/SPE Drilling Conference and Exhibition, held in San Diego, California, USA, 6-8 March 2012. Understanding of basic drilling mechanics has been greatly enhanced by the use of today’s sophisticated data acquisition systems for both surface and downhole drilling data. These systems have also helped in identifying major dysfunctions—particularly, harmful downhole vibrations. Various methods are used to optimize drilling practices, including monitoring efficiency indicators, such as mechanical specific energy (MSE), as well as preventing harmful vibrations by adjusting drilling parameters real time. However, focusing on the efficiency of components and the prevention of dysfunctions is only a partial solution. It is also necessary to focus on the magnitude of mechanical power delivered to the bit, as that defines the capacity of the system and directly affects efficiencies, operating practices, and performance. The paper shows how the power at the bit and drilling efficiency vary as a function of drillstring and bottomhole assembly (BHA) configuration, bit selection, and drilling parameter. Field examples illustrate the magnitude of parasitic losses caused by well geometry and the downhole environment. In many cases it is striking to learn how little power is left for the drill bit, which in the end dictates the potential rate of penetration (ROP) and thus the cost per foot and economics of the drilling process. Particular emphasis has been put on new and intuitive ways to visually display the power at the surface and downhole. These make it possible to quantify system limits and identify key dysfunctions in one simple graph that can be used in real time as well as in both pre- or post-well analysis. Better understanding of the entire drilling system and the functional relationship of its parts can be a strong driver for both short- and long-term improvements in system design, drilling practices, and bit selection, ultimately resulting in higher productivity and lower well construction cost. Introduction The oil and gas industry, unlike others, seldom focuses on power requirements for the rotating drilling system and their effect on performance and efficiency. This might be explained in part by the fact that for about 100 years roller cone bits have been the industry’s preferred drilling tool. Roller cone bits require relatively little torque due to their low aggressiveness, and, due to the limited load capacity of the bearings, they act as perfect torque limiters. The rotating power system therefore utilized only a small fraction of the total installed power of a rotary drill rig; lack of power was not considered a limiting factor.