Study On Mechanism Of Brittle Fracture Of Shale Under Super-low Temperature Condition Of Liquid Nitrogen

Shale gas,as an important unconventional natural gas resources,is extremely popular in recent years.Due to the great burial depths,low permeability and porosity of shale gas reservoirs in China,in order to achieve the purpose of commercial exploitation,the large-scale fracturing must be carried out.However,with the application of hydraulic fracturing in unconventional gas reservoirs,the large scale fracturing operation makes the shortage of water resources and environmental pollution problems,and the effect of hydraulic fracturing is not satisfactory.These problems have become increasingly prominent and the liquid nitrogen fracturing becomes one of the key technologies to alleviate the shortage of water resources.Therefore,it is of great significance for the development of shale gas in China to study the mechanism of brittle fracture of shale under super-low temperature condition of liquid nitrogen,and to explore the feasibility of liquid nitrogen fracturing on the shale reservoirs.Based on the mechanism of brittle fracture of shale under super-low temperature condition of liquid nitrogen,the liquid nitrogen freezing tests were conducetd from the microstructure of shale and the macroscopic physical and mechanical properties.Then this study analyzed the anisotropy characteristic of shale and the effect of liquid nitrogen freezing on the physical and mechanical properties of shale in different bedding directions.What’s more,on the basis of these tests,the equivalent medium model of shale and the damage constitutive model of super-low temperature and load were established.Then the damage degree of shale was evaluated,and the damage evolution process of shale under super-low temperature was analyzed.The results show that the permeability and p-velocity of shale have obvious anisotropy,and with the increase of the angle between the shale bedding and core axis,the permeability and p-velocity of shale decrease gradually.The peak strength and elastic modulus of shale under different confining pressure also have obvious anisotropy,and with the increasing direction of bedding shale,the peak strength and tensile strength show a trend of "U" type,while the elastic modulus decreases gradually.The effect of liquid nitrogen freezing on the permeability of shale decreases gradually along with the increase of bedding angle,while the effect on the p-velocity increases gradually.At the same time,the effect leads to the peak strength,elastic modulus and tensile strength of shale in different bedding directions decrease significantly.And the water saturation plays a role of promoting in the process of liquid nitrogen freezing.What’s more,with the increase of the number of liquid nitrogen freezing,the permeability of shale in different bedding directions increases significantly,and the p-velocity decreases significantly.Through the numerical simulation analysis of shale equivalent medium model,it’s shown that with the increase of the number of liquid nitrogen freezing,the damage degree of shale in different bedding directions is gradually from 0 to 1,but the growth rate decreases gradually.Through the establishment of the damage constitutive model under super-low temperature and load,it’s shown that with the increase of axial strain,the damage evolution curves show a monotonically increasing and finally tends to be gentle.And due to the effect of liquid nitrogen freezing,the initial damage of shale samples after the liquid nitrogen freezing increases obviously.At the same time,with the increase of axial strain,the damage evolution rate increases to a peak value and then decreases gradually.