Drilling fluids injected into geothermal and 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 due to Joule Thomason cooling effect at the drill bit. The effect of the temperature gradient on rock failure was investigated analytically and experimentally 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°C to 180°C in the Chagan Sandstone from the Tamuchage Basin, Mongolia. This explains high ROP, hole enlargement, and hole deviation in gas UBD operations. Although the test done at very small scale may not be representative to the real drilling conditions, the result supports the analytical model developed in this work, which can be used for optimizing drill bit and fluid injection rate in geothermal and oil and gas well drilling.

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