Damage Analysis On Freeze-thaw Stress Of Shale Under Liquid Nitrogen Condition

Ultra-low temperature liquid nitrogen volume fracturing has the advantages of non-consumption of water resources,low reservoir damage,no pollution to the environment,and rapid backflow.It is expected to replace hydraulic fracturing as a key technology for the exploitation of shale oil and gas reservoirs.It has received increasing attention in recent years.Temperature is the key to the effect of liquid nitrogen fracturing.On the one hand,the larger the liquid nitrogen cryogenic freezing area,the more brittleness of the shale reservoir can be improved in a larger range,and the high brittleness is the formation of larger reservoir reformation volume.On the other hand,the cold impact of liquid nitrogen on shale causes significant frozen damage within the rock.This increases the complexity of the seam network and also facilitates the freezing of deep reservoirs by cryogenic liquid nitrogen.Therefore,studying the dynamic distribution and evolution of temperature-stress field of shale under liquid nitrogen immersion is of great significance for exploring the core mechanism of liquid nitrogen fracturing and its practical feasibility in field application.This paper first uses the Span-Wagner model and Vesovic model to calculate the physical properties of nitrogen,summarizes the change trend of the physical properties of nitrogen with temperature and pressure,and based on the calculation results,studies the phase of nitrogen fluid under laboratory or field conditions.According to the law of change,the concept of critical depth of liquid nitrogen fracturing well was proposed.Then,the temperature drop curve of rock was obtained by liquid nitrogen freezing shale temperature measurement experiment,and the boiling heat exchange coefficient between liquid nitrogen and rock frozen surface was solved by the inverse heat transfer method;the liquid nitrogen freezing shale temperature measurement experiment was performed after repeated freeze-thaw cycles.Study the effect of freezing damage on cold shock transmission in shale.Afterwards,through laboratory experiments of shale low-temperature mechanical properties,the uniaxial compressive strength,elastic modulus,and Poisson's ratio of shale from room temperature(20°C)to liquid nitrogen temperature(-196°C)were measured,and its temperature drop was analyzed.The trend of change;proposed brittle index calculation formula based on the mechanical properties of shale to evaluate the effect of liquid nitrogen on the low-temperature brittleness of shale.Finally,based on thermo-mechanical coupled constitutive model,three-region model and water-ice phase transition theory,the temperature-stress field equation under the influence of latent heat of phase change was established.The law of temperature stress field of shale under liquid nitrogen was simulated with Abaqus software.Comparing with the experimental results,based on the laboratory experiments and numerical simulation results,numerical simulation studies of the cold shock attenuation radius,the low temperature increase brittle area,and the freezing damage range of the reservoir under ultra-low temperature liquid nitrogen during the fracturing process are conducted.Field application provides theoretical guidance.The results show that:(1)the critical pressure of nitrogen is 3.40 MPa,and the liquid nitrogen vaporizes immediately after being heated in an indoor environment,and the temperature is always stable at-196°C.Therefore,the liquid nitrogen freeze damage shale is a large vessel saturated boiling heat transfer.In the fracturing construction wellbore and formation,the temperature of liquid nitrogen is affected by the formation heat transfer and changes from liquid to supercritical.Under a pressure of 50 MPa,when the nitrogen fluid rises from-196°C to 200°C,the volume increases by 5 times and the viscosity drops by 90.(2)Under the condition of liquid nitrogen freezing,the temperature of the rock drops most rapidly in the nucleate boiling phase,the maximum boiling heat transfer coefficient reaches 3056 W/(m?K),and the pore water influences the temperature field when the rock moisture content is low.Small,multiple freeze-thaw cycles can further accelerate the cold impact of liquid nitrogen on shale,and the complete freezing time of the rock decreases by 40%;(3)The shale temperature decreases from 20°C to-196°C and the uniaxial compressive strength increases by 80%,Young's modulus increased by 55%,Poisson's ratio decreased by 20%,rock brittleness increased by 40%,liquid nitrogen increased the brittleness of original shale slightly higher than that of dry shale,(4)increased with distance from frozen surface,The effect of liquid nitrogen on the cold impact of shale decays The nearer the frozen surface,the lower the temperature of the rock,the highest the shrinkage stress of the frozen rock specimen is 12 mm from the frozen surface,and the most vulnerable to damage;multiple freezing can significantly increase the cold shock attenuation radius,expand the low-temperature increase brittle area,fracturing simulation.The results show that under the condition of fixed liquid nitrogen injection,the radius of cold shock attenuation for three freezes increases by 140% compared with one freeze,the brittle area increases by 35%,and the area susceptible to freeze damage changes from 1.2m to the borehole wall 6m from the well wall.