Rotary drilling prediction

Abstract

In a previous paper the Stamp Test for rock drillability classification has been described. This test yields the rock strength parameters required for rotary drilling rate prediction with the aid of dimensional analysis, whereby a single, non-dimensional constant is fitted to the experimentally-determined drilling rate for a particular combination of rock, drill bit, thrust force and rotation rate. The drilling rate equation so obtained is then shown to predict the drilling rate for all other combinations of rocks, drill bits, thrust forces and rotation rates covered by the experiments. If the thrust force in rotary drilling is too low, then the roller bit buttons will not break the rock and the drilling rate is vanishingly small. This particular situation is more adequately described by a direct physical model (instead of the dimensional analysis mentioned above) in very close analogy with the Stamp Test itself. It is concluded that the model for rotary drilling thus described yields the accuracy required for practical applications. A simple model for rotary drilling economics is defined. With a reasonable assumption for the rotary bit life as a function of the thrust force and the rotation rate, it is shown that the drill rig should be utilized either at the maximum available rotation rate and a certain optimal thrust force, or at the maximum thrust force and a certain optimal rotation rate.