| In the process of in situ pyrolysis of oil shale,the increasing thermal stress caused by heating will increase the internal stress of oil shale.The change of stress field will not only affect the stability of the wellbore,but also cause the fracture closure inside the ore body,hindering the migration of heat transfer medium and products,which leads to the secondary transformation of the product migration channel of the ore body in the process of in situ pyrolysis of oil shale.Therefore,it is very important to study the variation law of mechanical properties of oil shale in the compression deformation process,determine the fracturing parameters and master the propagation law of crack after crack initiation under the action of real-time high temperature,so as to realize the scientific and efficient exploitation of oil shale in the in-situ state.In this paper,the oil shale of Taimu Mine in Balikun,Xinjiang is taken as the research object through laboratory experiments.In the temperature range of 20～600℃,the evolution laws of the internal mineral composition,surface morphology and internal pore and fracture microstructure of oil shale with the increase of temperature are studied respectively.The compressive strength,tensile strength and fracture toughness of oil shale were obtained by uniaxial compression,Brazilian splitting and three-point bending tests under real-time high temperature environment.The main research results are as follows :(1)Using Smartlab Rigaku2019 X-ray diffractometer,the evolution characteristics of mineral composition in oil shale under real-time high temperature were studied.The results show that the threshold temperature for the decomposition of mineral composition of oil shale is 400℃,which is mainly dominated by the decomposition of pyrite and the loss of crystal water from kaolin.The crystallinity of minerals will increase with the increase of temperature before 400℃,and the crystallinity will decrease over 400℃.(2)The surface morphology change characteristics of oil shale at the same position under different temperatures were studied by SU8010 cold field emission scanning electron microscope.The results show that in the temperature range of 20 ～ 200 ℃,increasing the temperature will cause the pores to close inward.Before the temperature reaches 400℃,the fracture structure mainly appears around the hard mineral and expands along the bedding direction.When the temperature exceeds 500℃,the cracks will penetrate the bedding plane and form cross-layer cracks.(3)Under the condition of thermal-mechanical coupling,the variation characteristics of internal pore and fissure structure with temperature rise are studied by CT scanning equipment when oil shale is compressed in perpendicular and parallel bedding directions.The results show that when the load is perpendicular to the oil shale bedding,in the range of20～200℃,the pores,small cracks and medium cracks in the oil shale will be closed inward,and the number and volume decrease.Large fracture structure will expand along the bedding direction while closing inward.At this time,the porosity and connected porosity decrease continuously.At 300℃ and 400℃,the pores and fractures in oil shale mainly expand along bedding,and the porosity increases with the increase of temperature.When the temperature reaches 500℃,the fracture structure will run through different beddings,and the porosity and connected porosity are 5.6 times and 8.8 times of those at room temperature.When the load is parallel to the oil shale bedding,the number and volume of pores,small cracks and medium cracks decrease at 20 ～ 100℃,showing a closed phenomenon;The volume of large fractures increases and mainly expands along the bedding direction.At this time,the total porosity decreases continuously.In the range of 100 ～ 200℃,with the increase of temperature,the volume and number of pores in oil shale decrease due to closure,while the volume and number of fracture structures increase.In this temperature range,cracks mainly expand along the bedding direction,and the total porosity increases slowly with the increase of temperature.In the range of 200 ～ 600℃,the pores and fractures in oil shale increase with the increase of temperature,and the porosity increases rapidly.(4)The mechanical properties of oil shale during uniaxial compression were studied by using real-time high-temperature uniaxial compression system and full-field strain observation system.The results show that the mechanical parameters of vertical and parallel oil shale bedding directions change similarly with the increase of temperature,and 400℃is the threshold temperature for the change of mechanical properties.In the temperature range of20 ～ 400℃,compressive strength and elastic modulus decrease with increasing temperature.At 400℃-600℃,the compressive strength and elastic modulus rebound to different degrees with the increase of temperature.The Poisson’s ratio in the vertical bedding direction decreases from 0.326 to 0.057 with the increase of temperature from 20℃ to 500℃.When the temperature rises to 600℃,the Poisson ’ s ratio increases to 0.094.The Poisson ’ s ratio of parallel bedding direction decreases from 0.28 to 0.185 with the increase of temperature in the range of 20℃～ 200℃,and increases gradually to 0.37 with the increase of temperature in the range of 200 ～ 600 ℃.In the compression deformation process of oil shale,the threshold values entering the elastic section,the stable fracture expansion section and the accelerated fracture expansion section are basically the same as the compressive strength with the increase of temperature.However,at 100 °C,the threshold values will increase abnormally.(5)The Brazilian splitting system and acoustic emission system at real-time high temperature were used to study the variation law of tensile strength and acoustic emission characteristics of oil shale in the direction of parallel bedding,vertical bedding and orthogonal bedding at 20～600℃.The results show that the tensile strength of parallel bedding direction decreases continuously with the increase of temperature,while the tensile strength of vertical and orthogonal bedding directions decreases first and then increases with the increase of temperature in the range of 20-200 ℃.In the range of 200～400 ℃,the tensile strength decreases rapidly with the increase of temperature;At 400 ～ 600℃,the tensile strength in the perpendicular direction increases rapidly,while the tensile strength in the orthogonal direction decreases slowly.At 20 ～ 400℃,the crack types of oil shale after splitting along the three directions are all penetrating central cracks,and they will change into penetrating non-central cracks after more than 400 ℃.In the range of 20-300 ℃,with the increase of temperature,the number and frequency of AE ringing during the tensile failure of oil shale decrease continuously.When the temperature exceeds 300 ℃,acoustic emission will run through the whole splitting process.(6)Using real-time high temperature rock fracture toughness test system and DIC method,the fracture characteristics of prefabricated cracks along different bedding directions of oil shale at different temperatures in the range of 20-600℃ were studied.Results showed that,the fracture toughness in the direction of parallel bedding decreases with the increase of temperature;When the temperature increases from room temperature to 100℃ and 200℃,the fracture toughness in the vertical and orthogonal directions decreases first and then increases.When the temperature exceeds 200℃,with the increase of temperature,the fracture toughness in the vertical and orthogonal bedding directions decreases rapidly first and then increases slowly,and the temperatures corresponding to the turning points are 400℃ and500℃,respectively.By calculating the absorption energy of the specimen from the beginning of loading to failure,it is found that the absorption energy decreases first and then increases when the pre-crack parallel bedding passes through the failure,and the absorption energy is the least at 100℃.When the prefabricated crack propagates in the direction of perpendicular bedding and orthogonal bedding,the absorption energy decreases first and then increases at20-300℃.At 300-600℃,the absorption energy decreases first and then increases again,and reaches the minimum values at 400℃ and 500℃,respectively.DIC method was used to analyze the variation law of the process zone at the crack tip with the increase of temperature,and it was found that when the prefabricated crack propagated along the bedding,the threshold value,the critical opening displacement and the length of the process zone increased first and then decreased in the range of 20-400℃,and reached the peak at 200℃.When the cracks propagate perpendicular to the oil shale bedding and perpendicular to the oil shale bedding,the threshold value of the crack tip entering the process zone decreases first and then increases in the range of 20 ～ 600℃,and the threshold value is the lowest at 100℃.The critical opening displacement and the length of the process zone increase first and then decrease with the increase of temperature,and reach the maximum at 400℃. |
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