Thermal Characteristics of Borehole Stability Drilling in Hot Dry Rock.

Due to high temperature of formation and low temperature of the drilling fluid while drilling in the hot dry rock, thermal stress around the borehole is easy to appear. The thermal stress causes cracks in the borehole wall. In severe cases, the cracks caused borehole instability and lost-circulation. Based on the theory of transient heat transfer in formation, the method of Bessel infinite series is adopted to analyze the distribution of temperature around the borehole. The results showed that the specific heat of rock and the coefficient of heat conduction changing with temperature fiercely influences the transient heat conduction in the rock and had no significant effect on the heat conduction velocity in the rock. The crack propagation is related to the stress intensity factor at the crack tip and the crack opening width. Compared with the thermal conductivity coefficient and elastic modulus, the linear expansion coefficient has a stronger effect on the peak value of thermal stress. The deeper the crack around the borehole, the weaker the influence of the drilling fluid temperature on it. The temperature gradient of the borehole wall changes gently in space. The stress intensity factor of the crack tip around the borehole is far higher than the fracture toughness of the rock, and the crack opening width near the crack tip is more than 1 mm. Therefore, the rock around the borehole while drilling in the hot dry rock would almost inevitably crack and expand, influenced by the thermal stress.