| China is a typical country with "rich coal,poor oil and little gas".Oil shale,as an important part of unconventional oil and gas resources,its large-scale exploitation will alleviate China’s dependence on foreign oil and ensure national energy security.The process of shale oil and gas production by in-situ thermal injection is a complex multi-field coupling process of solid-flow heat mass transfer.With the rise of rock temperature,on the one hand,due to the thermal stress of oil shale rock mass itself,a large number of thermal fractures occur in the rock mass,resulting in complex pore fracture evolution in the micro view and complex deformation in the macro view;On the other hand,with the pyrolysis and precipitation of organic matter,the rock mass gradually changes from dense low permeability to porous high permeability,which further leads to the change of complex stress field in the rock mass.On the one hand,these changes affect the migration of injected hot fluid and pyrolysis products in the mining area.On the other hand,the emergence of complex shear stress field is an important factor affecting the effectiveness and stability of wellbore structure.Especially in the oil shale stratum at the northern foot of Bogda mountain in Xinjiang,it exists in the form of steep inclination of 50 ° to 80 °.In the process of in-situ mining,the whole pyrolysis area is bound to be subjected to complex shear force,and the shaft and stope are subject to shear force,which is more difficult to maintain.Therefore,studying the shear mechanical properties of oil shale under high temperatures is very important to ensure the successful implementation of oil shale in-situ mining technology.At the same time,it also has great guiding significance for the in-situ modified mining of similar unconventional geological resources.In this paper,the oil shale in Balikun,Xinjiang is selected as the experimental sample,and the real-time variable angle shear experiment under high temperature is carried out by using the self-developed real-time high-temperature variable angle shear characteristic test system,combined with acoustic emission equipment and digital image correlation technology.The shear mechanical characteristics of oil shale under real-time high temperature,the evolution of acoustic emission characteristic parameters during shear and the evolution characteristics of strain field on the surface of oil shale are studied,which fully reveals the shear characteristics of oil shale under real-time high temperature.The main conclusions are as follows:(1)From the shear stress-strain curve,it can be seen that with the increase of temperature,the oil shale in different bedding directions shows the transformation from brittle failure to ductile failure.400 ℃ can be regarded as the threshold temperature of oil shale shear characteristics.(2)At the same temperature,the shear stress of oil shale in different bedding directions decreases with the increase of shear angle.At the same time,it is concluded that the shear characteristics of oil shale in different bedding directions show obvious anisotropy with temperature.(3)At the same shear angle,the shear strength of oil shale in the vertical bedding direction first decreases and then increases with the increase of temperature,and decreases to the lowest at 400 ℃,which is 76% lower than that at room temperature;At the same time,the variation trend of shear modulus and cohesion with temperature is consistent with that of shear strength with temperature;The variation of internal friction angle of oil shale in vertical bedding direction with temperature is opposite to that of cohesion;The peak shear strain increases with the increase of temperature.At 600 ℃,the peak shear strain increases by0.0431,or about 54%,compared with 100 ℃;(4)Under the same shear angle,the variation trend of shear strength of oil shale in parallel bedding direction with temperature is "V",and the minimum shear strength is2.93 mpa at 400 ℃.On the whole,the shear modulus shows a downward trend and turns at100 ℃ and 500 ℃;The cohesion of oil shale first decreases and then increases with the increase of temperature,and 400 ℃ is its threshold temperature point;The internal friction angle has three turning points in the range of normal temperature ～ 600 ℃,and the turning points are 200 ℃,400 ℃ and 500 ℃ respectively.The internal friction angle presents a "W" trend with the increase of temperature;The peak shear strain experienced a trend of slow decline-rapid increase with the increase of temperature.It decreased to the lowest at 400 ℃,and increased to the maximum at 600 ℃,which was 0.1235,an increase of 69% compared with that at room temperature.(5)The acoustic emission characteristics of oil shale in different bedding directions show obvious anisotropy,but its evolution law with temperature is similar.In the range of normal temperature ～ 400 ℃,with the increase of temperature,the acoustic emission characteristic parameters of oil shale in different bedding directions are gradually dense,indicating that the local failure characteristics of oil shale are more obvious before the peak failure,resulting in the continuous downward trend of shear strength of oil shale before 400 ℃;At 400 ℃,the acoustic emission characteristics are sparse,but its fluctuation range is large,indicating that the local instability is serious,resulting in a sudden drop in the shear strength of oil shale;After 400 ℃,the softening phenomenon of oil shale is obvious due to high temperature,resulting in active acoustic emission characteristics;Through the ultrasonic test of oil shale after different temperatures,it is concluded that the wave velocity of oil shale with different bedding decreases gradually with the increase of temperature,indicating that the increase of temperature increases the internal pores and fractures of oil shale.(6)In the range of room temperature to 600 ℃,the evolution of the strain field in the shear process of vertical bedding oil shale can be divided into two stages.From room temperature to 400 ℃,there is a strain localization zone along the bedding structure on the surface of oil shale in the crack propagation stage,and finally a macro crack is formed and connected at the strain localization zone,and the oil shale is sheared through the bedding;In the range of 400 ℃ ～ 600 ℃,the strain concentration area appears in the upper left corner and lower right corner of the oil shale during the crack propagation stage.Finally,a macro crack is generated on the surface of the oil shale at the edge of the strain concentration area,and the oil shale is subjected to compression shear failure without through bedding,resulting in the increase of shear strength;The strain field evolution of parallel bedding oil shale in the shear process is similar to the increase of temperature.In the crack propagation stage,the strain localization zone along the bedding structure appears at the shear plane on the oil shale surface.With the increase of load,the strain localization phenomenon is obvious,and the final oil shale occurs direct shear failure along the strain localization zone. |
