Comparing the Rotary With Potential Drilling Methods

Abstract

Abstract Most new methods of oil well drilling achieve increased rates of penetration essentially by increasing the mechanical power input to the rock over the maximum practical power level obtainable by conventional rotary drilling. However, whether this would decrease the drilling cost per foot of hole depends upon other factors as well, such as the cost of operation per unit time, well depth and the distribution of rocks of various drillabilities at the well site. Introduction Rotary drilling, introduced about 1900, is today the most widely used method of drilling oil wells in this country. The past 50 years have seen great strides in improved equipment and techniques for rotary drilling. However, the limitations of rotary drilling have long been recognized and there have been many attempts to develop new drilling methods to supplement or to supplant rotary drilling. The potentialities of some of these new drilling methods for decreasing the cost of drilling oil wells are discussed herein. Drilling Rate and Mechanical Power One significant result of a study of the fundamentals of rock drilling made at Battelle is that the fracture of brittle rock from the bottom of a hole by repetitive, indexed mechanical loading involves the expenditure of a certain amount of mechanical energy per unit volume of rock fractured out. Therefore, the rate of penetration of any mechanical drilling machine would be essentially directly proportional to the mechanical power developed in the rock per unit area of hole and inversely proportional to the drilling strength, which is the energy required to fracture off a unit volume of rock. Limitations of Rotary Drilling The mechanical power output to the rock for conventional rotary drilling is proportional to the torque required to turn the bit at the bottom of the hole and to the rate of rotation of the bit. The torque reaction to rotation associated with a given amount of static weight loading on the bit is less for the rocks of greater drilling strength. Since less mechanical power is developed in rocks of greater strength, the rate of penetration decreases more rapidly than in inverse proportion to the drilling strength. There is, consequently, a wide range in the drilling rates obtained by rotary drilling, perhaps about 50 to 1 from the weakest to the strongest rock of interest in oil well drilling.