Higher Rotary Speeds Applied To Insert Bits Reduce Drilling Costs
Authors: McAdams Sidney C. , Gustafson W.F. , Cooper N.F. 
Published: December 2007

Abstract
A study has been made of drilling practices in West Texas, Oklahoma and Kansas, to determine the affect of rock bit selection and operating parameters on drilling costs. Results show that parameters on drilling costs. Results show that proper selection of rotary speeds, WOB, hydraulics proper selection of rotary speeds, WOB, hydraulics and rock bits selection are major factors in reducing drilling costs.

Introduction
A large portion of the cost of drilling a well is spent in the drilling operation itself. The time spent drilling the well can be greatly influenced by the performance of rock bits selected for the well and how they are applied.
Bit performance is affected by many factors. A major factor is bit selection itself. Bit selection is considered by many people to be an art. However, in areas of people to be an art. However, in areas of development where many wells have been drilled, sufficient records and sample logs are usually available and proper bit selection may be planned in advance. When drilling an exploratory well, bit selection is more of an art with bit selection usually made at the well-site as drilling progresses. progresses. Traditionally, milled-tooth type bits are used to drill soft sedimentary formations which are normally encountered at shallow depths. These types of bits are normally operated at low weight on bit (WOB) (4,000 lb/in, 7.0 kN/cm) and at rotary speeds ranging from 60 rpm (6.3 rad/s) to 150 rpm (15.7 rad/s). The life of a milled-tooth bit is usually dependent on the life of its cutting structure. When drilling in soft formations, milled-tooth bits achieve high rates of penetration (ROP). When hard formations are encountered, they are normally replaced by more rugged insert type bits.
The normal operating parameters for insert type bits differ from milled-tooth type bits, in that, higher WOB (7500 lb/in, 13.1 kN/cm) and slower rotary speeds (45 rpm, 4.7 rad/s) are normally applied. Previously, when substituting milled-tooth bits with insert bits, the slower rotary speeds normally used with insert bits, did not allow them to achieve ROP comparable to the milled-tooth bits. In many cases, the number of trips were reduced, but the total drilling time was generally increased. This was due to slower ROP of the insert bits in the softer formations.
Insert bits, when operated at the same rotary speeds, are capable of achieving ROP comparable to milled-tooth bits. Previous limitations of insert bits prevented them from being run at high rotary speeds, for extended periods of time. Because of this, it was not economical to replace milled-tooth bits with insert bits in many soft to medium formations.
Recent advances in rock bit technology have produced rock bits that are capable of high rotary produced rock bits that are capable of high rotary speeds with the potential of reducing operating costs. The bearing design, (Figure 1) utilizes a floating, silver plated, beryllium copper bushing designed for high rotary speeds. It is complimented by a rugged cutting structure employing tungsten-carbide inserts. This design provides the versatility to drill, at reduced provides the versatility to drill, at reduced costs, a wider range of formation types. In addition, the probability of errors in bit selection are substantially reduced.

FIELD PERFORMANCE
These bits designated High Performance "S" and "SM" have been run throughout the drilling industry where 7-7/8" (20.0 cm) hole has been drilled. In this paper, the performance in areas of West Texas, Oklahoma and Kansas, will be discussed (Figure 2). Bit performance data on fifty-nine wells from these areas was analyzed, including ninety different bits of ten different types. Bits other than the "S" and "SM" types are referred to by their IADC code. The "S" type is designed to drill soft formation IADC codes ranging from 517 to 527 and the "SM" type, soft to medium, from 517 to 629. IADC codes with values less than 517 are milled-tooth type bits (Table 1).