Methods of rotary borehole drilling

Abstract

The graph of rotary borehole drilling rate versus feed pressure has four sections ( Fig. 1): I) abrasion of the rock; II) transitional; III) bulk disintegration; IV) a zone where the rise of drilling rate fails off. The greatest effect is achieved in section HI, where the pressure shears off large rock fragments. In this section the rock is crushed with least expenditure of energy.The slope a of the linear part of section III on the V = F (Pf) curve varies with the rock strength. In most cases this section obeys the equation v = k (pz=po),where k is some coefficient of proportionality depending on tan c~, and P0 is the minimum feed pressure at which bulk fragmentation of the face begins.After analyzing numerous experimental results by many workers, the present authors have come to the conclusion that for optimum drill rotation rates k is constant for each rock, while P0 can be fairly accurately represented by the equation P0 = 35f, where f is the rock hardness coefficient on the Protod'yakonov scale.