Analysis of rock cracking characteristics during pyrolysis drilling

Thermal jet rock breaking technology refers to the use of high temperature medium such as supercritical water for a rapid local heating of rocks to break the rocks. Because of the low thermal conductivity of the rock matrix, thermal stress will only form on the rock surface. When the temperature stress exceeds the strength of the rock, micro-cracks will appear in the rock, and continue to expand, resulting in the thermal cracking on the rock surface, which will cause the rock surface to fall off from the body and break the rock. Based on thermal-solid coupling theory, a pyrolysis drilling model was established, and the distribution law of temperature field and temperature stress of bottom hole rock during pyrolysis was obtained by using a finite element method. The results show that during the pyrolysis drilling process, the temperature of the heated part of the rock increases rapidly, producing temperature gradients in radial and axial directions. The expansion of the heated volume is affected by compressive stress in the radial direction, buckling in the axial direction and shear stress. This is very important to the field application of pyrolysis drilling