Effect of Fluid Temperature on Rock Failure in Borehole Drilling

Drilling fluids injected into geothermal, oil, and gas borehole create a temperature gradient in rock near the bottom hole. This temperature gradient is more pronounced in underbalanced drilling (UBD) with gaseous fluids because of the Joule-Thomason cooling effect at the drill bit. The effect of the temperature gradient on rock failure was investigated experimentally and interpreted by an analytical model in this study. The result indicates that the temperature gradient can significantly promote rock failure during drilling. The rate of penetration (ROP) increased by 22.4% on average as the temperature differential increased from 30 to 180 degrees C in the Chagan Sandstone from the Tamuchage Basin, Mongolia. This explains high ROP, hole enlargement, and hole deviation in gas UBD operations. The analytical model developed in this work can be used for optimizing drill bit and fluid injection rate in geothermal, oil, and gas well drilling.