Rock-bit drilling efficiency falls off at higher rotary speeds and at higher values of weight on bit, due to higher rate of generation of rock cuttings and the need to expend mechanical energy on their removal.

A simple relationship is shown to exist between drilling efficiency and penetration rate, whether controlled by rotary speed or by weight on bit. Factors affecting either of the two parameters of this relationship are shown to be

  1. differential pressure on bottom,

  2. mud characteristics,

  3. circulation rate,

  4. jet velocity and

  5. rock-bit design.

Examples are cited from literature.

Further analysis leads to an indication of when to expect drilling performance to depend on impact value and when on hydraulic horsepower. Two quite straightforward methods are explained for rapid determination of optimal economic rotary speed, and two for optimal economic bit weight., based on current performance. performance. Three separate methods are offered for identification of overpressured formation, using only the response of bit to standard drilling variables, thus eliminating the need to wait for a log to be run or to rely on shale determinations.


Probably the most important single realization in rotary drilling is that not only must rock be fractured on bottom, but also the fractured particles must be removed from the rock face instantly and efficiently, to provide for further fracturing and drilling progress.

For this purpose, energy is brought from the surface to the rock face and applied as efficiently as possible. This energy appears in two forms:

  1. Mechanical energy, applied through the rotary and drill string, and converted into its most useful form through the mechanism of bit cones and teeth.

  2. Hydraulic energy, applied through pump and drill string conduit, and converted into its most efficient form through correctly positioned jet nozzles, aimed at the rock face just ahead of the oncoming tooth.

It has been customary to consider the mechanical energy purely as the medium for rock fracture, and the hydraulic energy as the medium for chip removal and bottom scavenging. Further examination reveals, however, that the processes are by no means mutually exclusive. processes are by no means mutually exclusive. In fact, both types of energy play a part in both rock fracture and rock removal.


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