Research On High Temperature Physical Property Response And Thermal Damage Mechanism Of Shale Thermally Assisted Development

The effective exploitation of shale gas is conducive to ensuring my country’s energy security,improving the energy structure and relieving energy pressure.In the process of shale gas exploitation,heat-assisted development is an effective method to accelerate shale gas desorption,which can increase shale gas production and shorten the production cycle.Under the action of heat-assisted high temperature,shale thermophysics,permeability and pore structure change significantly.Through basic research such as shale thermophysical experiments,the variation characteristics and correlations of shale macrophysical parameters can be obtained,so as to realize the rapid identification of shale thermal damage state.The occurrence state and flow characteristics of fluids in shale pores are closely related to their pore structure,which directly affects the transmission performance of shale oil and gas.Therefore,it is of practical significance for the exploitation of shale gas to study the high temperature physical property response and thermal damage mechanism of shale thermally assisted development,and to reveal the occurrence state of shale gas under high temperature conditions.Taking the Cambrian Niutitang Formation shale and Shuigoukou Formation shale as the research object,based on the shale thermal assisted development mode of heat conduction heating and thermal radiation(microwave)heating,the changes of thermophysical properties,pore structure characteristics,permeability and mechanical parameters of shale under high temperature are studied by comprehensively using the methods of indoor test and theoretical analy sis.The thermoacoustic emission characteristics of shale under the action of real-time high temperature were analyzed,and the physical property response characteristics of shale heat conduction-assisted development and the response characteristics of microwave heat-assisted development were studied.Combined with the micro-macro response characteristics,the high temperature thermal damage mechanism of shale was revealed.The research results and conclusions are as follows:(1)Based on the thermoacoustic emission technology,the thermoacoustic emission characteristics and crack evolution of shale in the Niutitang Formation under different heating rates were studied,the thermal fatigue sensitivity of the Kaiser effect of the shale was revealed,and the thermal damage threshold temperature of shale is identified.The results show that tensile cracks and shear cracks develop together during the heating process of shale,but tensile cracks account for the main body.Near 300℃,the proportion of shear cracks begins to increase,and the increase of heating rate is conducive to the development of large-scale tensile cracks;The threshold temperature of thermal Kaiser effect of Niutitang shale is about 180℃,and the effect of heating rate on the threshold temperature is not significant;When the heating temperature is lower than 300℃,the shale still maintains the memory of the threshold temperature.When the heating temperature exceeds 300℃,the expansion of inorganic minerals and the pyrolysis of organic matter interfere with the thermal Kaiser effect,so that the shale loses the memory of the highest temperature.(2)Through the physical and mechanical tests of shale under heat conduction heating,the physical parameters,pore structure characteristics and permeability variation of Niutitang Formation shale were studied,and the temperature response characteristics of shale strength were analyzed.The results show that 300～450℃ is the threshold temperature for the change of physical properties of Niutitang Formation shale.In this temperature range,in addition to the large decrease in wave speed and capacitance,the chromaticity,hardness,gloss and impedance all increase greatly with the increase of temperature;When the heating temperature is lower than 300℃,the porosity and permeability do not change much,and when the heating temperature exceeds 300℃,the porosity and permeability increase significantly.The pore types of shale samples treated at different temperatures are divided into small holes(r<0.01 μm),middle hole(0.01μm<r<0.1μm)And large holes(r>0.1μm).After 300℃,the uniformity of pores gradually increases,and some small pores transform to mesopores;Low heating rate is conducive to the pyrolysis of organic matter,resulting in a significant reduction in the tensile strength of shale.As the heating rate increases,the shale is heated unevenly,which leads to the increase of tensile strength.When the heating rate continues to increase,the development of large-scale cracks in shale leads to the decrease of tensile strength again.(3)Based on the influencing factors of microwave thermal effect of shale,the thermal damage mechanism of shale under microwave was revealed,the microwave response of the fracture toughness of shale in the Shuigoukou Formation was studied,and the failure mode of shale with different bedding angles was clarified.The results show that moisture,dielectric constant and mineral composition are the main factors affecting the microwave thermal effect of shale.Under the action of microwave,there is a temperature threshold for the fracture toughness of shale at different bedding angles.For Crack-splitter type,Crack-arrester type and Crack-divider type shale,the threshold temperature is 50℃,125℃ and 200℃,respectively;The fracture toughness of Crack-divider type,Crack-arrester type and Crack-splitter type shale decreases in turn,and decreases gradually with the increase of microwave irradiation time,and the difference of fracture toughness of shale with different bedding angles is caused by the crack propagation mechanism during loading.(4)Combined with the micro-macro response characteristics,the microstructure changes and pyrolysis mechanism of shale after high temperature were analyzed,and the high temperature damage mechanism of the shale was revealed.The results show that after high temperature,the shale structure is thermally damaged,resulting in an increase in the number and size of pores and an increase in microcracks;Pyrolysis of organic matter and decomposition of inorganic minerals are the main causes of thermal damage to shale.Under the action of high temperature,the organic matter on the shale surface is pyrolyzed,and pores and fissures are exposed.Oxygen enters the shale along these pores and fractures.With the continuous pyrolysis of internal organic matter,more pores and cracks are produced;At a faster heating rate,the pore pressure and thermal stress in the shale increase significantly in a short time,and the shale is prone to brittle failure.