Numerical analysis of transient conjugate heat transfer and thermal stress distribution in geothermal drilling with high-pressure liquid nitrogen jet

Abstract

This paper presents a numerical analysis of heat transfer and thermal stress in bottomhole rocks during geothermal drilling with high-pressure liquid nitrogen jet. The simulation is conducted by a three-dimensional model in transient state. The conjugate heat transfer method is employed to compute heat transfer between solid and liquid. The thermo-physical properties of liquid nitrogen and rocks are considered in detail. The results indicate that high velocity and turbulence kinetic energy of liquid nitrogen jet at impingement surface enhance heat transfer efficiency between cryogenic fluid and hot rock. Huge tensile stress is generated adjacent to the solid-liquid interface, which is favorable for rock breaking on bottomhole. The primitive rock temperature has a significant impact on maximal stress value. A set of experiments is conducted to validate the effects of thermal stresses on rock breakage. The thermal stresses diminish significantly under laboratory conditions without effective constraint on rock boundaries. In our experiments, the coal rather than other types of rock, was selected as working specimen due to its well-developed natural cracks and small tensile strength. Our results would shed light on the geothermal drilling with high-pressure nitrogen jet.