Factors to be Considered in Drilling Optimization
Authors: Lummus J.L. 
Published: March 2010

ABSTRACT
Extensive drilling research, particularly in the last ten years, hasresulted in a better understanding of the effect of drilling variables andtheir interactions. The practical application of this knowledge has kepthole-making costs at the 1958 level in spite of inflation. Optimized drilling, which was first applied on a comprehensive basis in 1967, has significantlyreduced drilling costs, although it has yet to reach its full potential.
Optimized drilling is defined as the "mathematical treatment of the mostimportant controllable drilling variables to develop a comprehensiveminimum-cost drilling program". The variables involved in rotary drilling areclassified as alterable or unalterable and the variables selected formathematical optimization are herein described. A more detailed treatment willbe given to the most important variables optimized-mud, hydraulics, bits, weight and rotary speed. Rig selection and data acquisition are also discussed, as these factors play an important role in optimization. The paper includesdata which show that significant reductions in drilling costs have beenobtained when optimized drilling programs were effectively implemented. Aninteresting aspect of the data presented is that savings achieved in"second-round" optimums were significant and, in one case, more than thosesavings obtained in "first-round" optimums. This indicates that better data, more experience in applying optimized drilling principles and developingconfidence in the program are key factors in the successful use of this newdrilling approach.

INTRODUCTION
THE DEVELOPMENT OF ROTARY DRILLING can be divided into four distinct periods Conception Period, 1900-1920; Development Period, 1920-1948; Scientific Period, 1948-1968; and Automation Period, which began in 1968. The majoraccomplishments of the first three periods, and a prediction of what lies inthe future for the Automation Period, are shown in Table 1. In reviewing thesedevelopment periods, the question naturally arises as to the reason for theapproximate 30-year lapse between the end of the Conception Period and thestart of the Scientific Period. There are a number of reasons that can begiven, but undoubtedly the most significant one is that major oil fieldequipment firms, mud service companies and operators did not startappropriating the large amounts of money it takes to do high-quality drillingresearch until about 1948. When we look at the major accomplishments obtainedduring the Scientific Period, the most productive years are found to be from1958 to 1968. A measure of the impact of the drilling technology developedduring the latter part of the Scientific Period on hole-making costs, ascompared to total well costs, (can be seen from Figure 1). Total well costsincreased 14 per cent from 1958 through 1967, while hole-making costs remainedat the 1958 level; i.e., about $4.25/ft.111 Other costs, such as completion, logging and casing expenditures, increased 21 per cent. If the extensivedrilling research effort of the past 10 years had not been undertaken and hadnot been successfully reduced to practice in routine drilling operations, it isestimated that a typical 8,000–9,000foot hole would cost an additional$3.00/foot to drill today. This, would amount to a saving of about $500 millionfor 1967 alone, which is testimony to the fact that the investment in drillingresearch undertaken by many companies has paid off. Optimized drilling has beenone of the most significant accomplishments obtained during the Scientific Period, but it was not introduced on a comprehensive basis until 1967, andtherefore will not reach its full potential for several years. It is veryimportant to realize that optimized drilling would not be possible todaywithout the hard work of numerous researchers who have spent considerable timestudying the effects of drilling variables and how they relate to eachother.