Rock-breaking Mechanism Of High-temperature Granite By Abrasive Liquid Nitrogen Jet

Liquid nitrogen(LN₂)jet fracturing is considered as an efficient method to fracture hot dry rock(generally granite),which can solve the problems in traditional hydraulic fracturing technologeies,such as high initiation pressure,difficult isolation for wellbore,simple fracture structure.In the process of LN₂ jet fracturing,one of the most important operations is to use abrasive LN₂ jet to form several perforation cavities in formation firstly.In this paper,the theoretical analysis,numerical simulation and laboratory experiments are carried out to investigate the breaking mechanism of high-temperature granite by abrasive LN₂ jet.The retail research work and results are as follows:(1)A 3D numerical model is developed to simulate the flow field of abrasive LN₂ jet by using Discrete Phase Model.In calculation,the model considers the effect of ambient pressure and temperature on LN₂ properties,and the effect of fluid turbulent flow on particle motion.The research investigates the particle-carrying ability of LN₂ jet and the effects of several important parameters are analyzed on particle velocity.The results show that particles' trajectories at nozzle-outside flow field are random because of turbulent flow.The LN₂ has a good ability to carry out particles in high-flow-rate condition.Particle can obtain the highest velocity at nozzle exit in LN₂ jet compared to water jet and supercritical CO2 jet with same conditions.However,particle velocity decreses fastly at outside of nozzle in abrasive LN₂ jet.The parametric analysis indicates that the pressure drop,nozzle diameter and particle mass-flow-rete affect particle velocity significantly within simulation conditions.(2)A thermal-hydraulic-mechanical coupling model is developed to investigate the heat transfer rate on solid-liquid interface,the flow transport and heat transfer in porous rock and the thermal stress distribution in rock.The calculating results show that the highvelocity impingement and strong turbulent flow on solid-liquid interface enhace the heat transfer rate between LN₂ and rock.The flow transport of LN₂ in porous rock can increase the cooling rate of high-temperature rock.The thermal stress in rock induced by rapid cooling is tensile stress,which mainly affect the region near the cooling surface.(3)The impingement heat transfer experiment by low-pressure abrasive LN₂ jet is conducted to investigate the effect of abrasive particles on heat transfer efficiency between fluid and rock.The permeability and P-wave velocity of rock cooled by LN₂ jet are tested to value the thermal-shock effect on rock damage.The experimental results show that the particles in LN₂ jet can accelerate the extension of LN₂ wetting edge on rock surface,which can speed up the rock cooling rate.The damage of high-temperature rock mainly results from the rapid cooling of LN₂ jet,causing the increase of permeability and the decrease of P-wave velocity through rock.The higher rock temperature,the more damage rock has.(4)A series of rock-breaking experiments using high-pressure abrasive LN₂ jet are carried out to investigate the breaking characteristics of high-temperature granite.The results show that the cavities formed by abrasive LN₂ jet are largest with irregular shapes and rough walls compared with these formed by abrasive nitrogen gas jet and abrasive water jet.The pre-mixed abrasive LN₂ jet performs better than post-mixed abrasive LN₂ jet in form of rock breakage.The parameters of rock initial temperature,pressure drop,particle diameter and nozzle diameter all have important effects on rock breaking under experimental conditions.(5)The micro-structure and micro-fracture morphology of broken rock are observed by topography instrument and SEM.The influences of cryogenic fluid,high-speed particles,thermal shock and fluid wedge effect on rock breaking are analyzed separately.The result shows that the cavity surface induced by abrasive LN₂ jet is rough with some pits,bulges and deep grooves,which indicates that the rock is broken in the terms of large debris.The main fracture mode of high-temperature granite in experiment is brittle fracture mode,however,the ductile fractures also are found in some areas due to plastic deformation.Different regions of perforation vavity show different fracture patterns,resulting from different loads.The high-temperature granite achieves breakage by the generation,extension and convergence of cracks with the impacts of thermal shock,highspeed particle and high-pressure fluid.The research results shed light on the rock-breaking mechanism of hot dry rock impacted by abrasive LN₂ jet,which can provide theoretical supports for the development LN₂ jet fracturing.