Optimization Techniques for Minimizing Drilling Costs
Authors: Wilson David C. , Bentsen Ramon G. 
Published: December 2007

This paper was prepared for presentation at the 47th Annual Fall Meeting of the Society of Petroleum Engineers held in San Antonio, Tex., Oct. 8–11, 1972. Permission to copy is restricted to an abstract of not more than 300 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by who the paper is presented. Publication elsewhere after publication in the JOURNAL paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon request to the Editor of the appropriate journal provided agreement to give proper credit is made. provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers office. Such discussion may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines.

Abstract
The recent advent of comprehensive mathematical models of the well-drilling process has made it possible to estimate quantitatively the effect of certain key parameters involved therein. With the prime objective of reducing the cost of drilling a well, the authors investigated various optimization procedures and how they could be used, in conjunction with the mathematical model selected, to achieve this reduction.
The present knowledge of the drilling process restricted the number of parameters to process restricted the number of parameters to be optimized to two, namely, the weight on the bit and the rotary speed. Other factors, such as mud properties and bit type, were assumed to have been properly selected. Within these limits, three methods of varying complexity have been developed. The first method seeks to minimize the cost per foot drilled during a bit run. The second method minimizes the cost of a selected interval, and the third method minimizes the cost over a series of intervals. The methods are listed in order of increasing complexity.
It was found that each of the methods gave a worthwhile cost saving and that the saving increased as the complexity of the method increased. The data requirements for the methods increased with increasing method complexity.
Further improvements will depend on greater sophistication of the mathematical models used to describe the drilling process.

Introduction
The relatively recently acquired ability to model the well-drilling process has led to the proposal of several methods for determining optimum drilling techniques. The object of this paper is to examine how such techniques may be used in conjunction with published models of the drilling process in published models of the drilling process in order to minimize the drilling cost.
Rotary drilling is a complex subject involving many variables, some of which may be altered and others not. Those factors that may be controlled are mud properties, hydraulics, bit type, weight-on-bit and rotary speed. Unaltered variables include such things as crew efficiency, rig conditions, round-trip time, rock properties, formation to be drilled and depth. A complete description of how these variables affect rotary drilling and how they interact is available in the literature.
Experience and research indicate that seven variables should be considered for mathematical optimization. The pertinent variables include the five controllable ones cited earlier and two unalterable ones: formation to be drilled and depth.