Rock-breaking Mechanisms Of Liquid Nitrogen Jets

As a novel and efficient rock-breaking method,liquid nitrogen(LN₂)jets show a great potential in improving the drilling rate of deep hard formations.To reveal rock-breaking mechanisms of LN₂ jets and explore its feasibility in improving drilling rates in deep formations,laboratory experiments,theoretical analysis and numerical simulation were combined in our work to address the critical scientific issues,such as the deterioration of physical properties of high-temperature rocks subjected to LN₂-cooling,the flow field and heat transfer of LN₂ jets,the rock-breaking performances of LN₂ jets and the coupled multi-physics responses of rock.Main findings are shown as follows:1.Physical properties of high-temperature rocks were tested after LN₂-cooling treatment to determine the damage characteristics of rocks.According to results,mechanical properties of rocks deteriorate significantly after LN₂-cooling,and the deterioration aggravates with increasing rock temperatures.At 150?600?,uniaxial compressive strength of rocks subjected to LN₂ cooling is 11?18%lower than those subjected to water cooling.For rocks with different lithologic features,the deterioration of physical properties induced by LN₂-cooling is different.Granite shows the most remarkable deterioration in physical properties,followed by shale and sandstone.Physical properties decay with increasing heating-cooling cycles,but the decay rate declines.After about 10 cycles,physical properties do not continue to change with increasing cycles(Final reduction value in strength is 35%for granite at 200?).Higher rock temperature and smaller particle size contribute to improving the decay rate of physical properties and intensifying the damage of rocks in the LN₂-cooling cycles.2.Based on the detached eddy simulation(DES)approach,a flow and heat transfer model was built for a cryogenic nitrogen jet by considering physical changes and compressibility of fluid.Vortex dynamics and heat transfer characteristics of the cryogenic nitrogen jet under high confining pressure conditions were determined.The cryogenic nitrogen jet is a process involving the rapid evolution and development of coherent vortices.The dominant frequency of vortex instability,vorticity and pressure oscillations grow with increasing nozzle pressure difference,which improves the heat transfer rates of cryogenic nitrogen jets.Compared to the LN₂ jet,the SC-N₂ jet has higher vorticity,dominant frequency of vortex instability and heat transfer rate.Unlike the LN₂ jet,confining pressures have a great influence on the SC-N₂ jet.Lower confining pressure is more conducive for the SC-N₂ jet to enhance vortice sacles and heat transfer performances.Within the range of h/d?6,higher standoff distance contributes to fully developing the cryogenic nitrogen jet and improving its heat transfer rate.3.Rock-breaking experiments with LN₂ jets were conducted to determine the damage characteristics of rocks and optimize jet parameters.According to experimental results,rock failure under the impact of LN₂ jet is characterized by large volumetric breakage.Compared to the water jet,the LN₂ jet shows significantly better rock-breaking performances.At nozzle pressure differences of 5?20 MPa,the rock-breaking volume of LN₂ jet is 7?32 times higher than that of water jet,while its specific energy consumption is 90.8%lower than the water jet.Macroscopic cracks are created on the rock surface after the erosion of LN₂ jet,which reduce the threshold pressure for rock-breaking significantly.Rock-breaking perforamnces of LN₂ jet improves with increasing initial temperatures of rocks.At the nozzle pressure difference of 25MPa,the threshold temperature for volumetric breakage of granite falls in the range of 150?260?during the impingement of LN₂ jet.4.A loose coupling method was adopted to build a thermal coupled fluid-structure interaction(FSI)model for the impingement of LN₂ jet.Mechanical responses of rock and the effect of thermal stress on rock damage were determined.According to results,the process of LN₂ jet impingement on rock can be divided into two stages:(1)the jet impact dominated stage and(2)the thermal dominated stage.In the former stage,stress distributions under the impact of water jet and LN₂ jet are similar.A compressive zone surrounded by a ring tensile stress zone is generated on the impact surface.In the latter stage,the compressive stress near the stagnation point transfers to tensile stress gradually under the effect of LN₂ cooling.The LN₂ jet induces high thermal stresses to intensify tensile and shear failures in rock.Higher initial temperatures of rock can improve the heat transfer rate and the magnitude of the maximum principal stresses,which aggravates the damage and makes the damage occur in advance.This paper reveals the rock-breaking mechanisms under joint actions of jet impact and cryogenic cracking,which provides theoretical basis for forming a more efficient drilling method in deep hard formations.