THE TEMPERATURE FACTOR IN ROTARY DRILLING OF INDIANA LIMESTONE.

摘要

In order to investigate temperature as a factor in rotary drilling using a micro-bit drill rig, a laboratory study was conducted to determine: (a) what factors affect bearing temperature, and how they correlate with the response; (b) what are the optimum drilling conditions, based on bearing temperature and penetration rate; (c) what is the effect of rock temperatures and penetration rates, and how do these penetration rates correlate with changes in the mechanical properties of rocks due to heating.; The laboratory micro-bit drill rig was used on Indiana limestone, with air for the drilling fluid. The controllable variables were weight on bit, rotary speed, and rock temperature. Response surface methodology was used in the design of the experiments.; The main conclusions are as follows. (1) Temperature increase in bearings depends on bit weight and rotary speed. The effects are not additive, but mutually reinforcing: each increases with the increase of the level of the other. This relationship is not well understood. Also, temperature rise in the bearings increases at a decreasing rate as the bit weight is increased. (2) Rocks subjected to elevated temperatures are microfractured and drill more slowly than do intact rocks, but the magnitude of temperature (at least in the range typically encountered in geothermal drilling, 350(DEGREES)-600(DEGREES)C) has no significant effect. (3) Microcracks induced by differential thermal expansion of mineral grains in rocks subjected to elevated temperatures decrease drillability, as do macrocracks already present and/or induced by temperature-gradient thermal stresses. (4) Up to the point of inducing macrocracks, the differential temperature between the flushing air and the rock mass enhances the penetration rate. (5) Change in the compressive wave velocity, brazilian tensile strength, and uniaxial compressive strength of the rock due to exposure to elevated temperature showed no correlation with the change in penetration rates.