Abstract
Filling of brittle minerals such as quartz is one of the main factors affecting the initiation and propagation of reservoir fractures in shale fracturing, in order to explore the failure mode and thermal damage characteristics of quartz-filled shale under thermal-mechanical coupling. Combining the theory of damage mechanics and thermoelasticity, RFPA2D-Thermal is used to establish a numerical model that can reflect the damage evolution of shale under thermal-solid coupling, and the compression test under thermal-mechanical coupling is performed. The test results show that during the temperature loading process, there is a temperature critical value between 60°C and 75°C. When the temperature is less than the critical temperature, the test piece unit does not appear obvious damage. When the temperature is greater than the critical temperature, the specimen unit will experience obvious thermal damage, and the higher the temperature, the more serious the cracking. Under the thermal-mechanical coupling of shale, the tensile strength and elastic modulus of shale show a decreasing trend with the increase of temperature. The failure modes of shale under thermal-solid coupling can be roughly divided into three categories: “V”-shaped failure (30°C, 45°C, and 75°C), “M”-shaped failure (60°C), and inverted “λ”-shaped failure (90°C). The larger the fractal dimension, the more complex the failure mode of the specimen. The maximum fractal dimension is 1.262 when the temperature is 60°C, and the corresponding failure mode is the most complex “M” shape. The fractal dimension is between 1.071 and 1.189, and the corresponding failure mode is “V” shape. The fractal dimension is 1.231, and the corresponding failure mode is inverted “λ” shape.
Highlights
With the rapid consumption of conventional energy such as petroleum, unconventional clean energy such as shale gas has become a current research focus [1,2,3,4,5]
During the displacement loading process, the thermal-mechanical coupling effect of shale has a significant effect on the compressive strength and elastic modulus of shale filled with quartz minerals
When the temperature is greater than the critical temperature, the thermal stress is greater than the maximum tensile strength of the shale microscopic unit, and thermal damage occurs in the sample, which leads to increased nonuniformity of the shale
Summary
With the rapid consumption of conventional energy such as petroleum, unconventional clean energy such as shale gas has become a current research focus [1,2,3,4,5]. Guo et al [22] conducted uniaxial compression tests and acoustic emission tests on shale samples with different bedding surface dip angles at different temperatures and studied the effect of temperature on the physical and mechanical properties of shale. Yang et al [23] conducted dynamic compression tests on shale specimens in the temperature range of 20~220°C, studied the mechanical properties of shale at different temperatures, and analyzed the dynamic deformation and failure process of shale. Through the RFPA2D-Thermal software, the shale is subjected to numerical simulation tests under constant confining pressure and different temperature conditions, and the compressive strength, elastic modulus and failure process of shale under different temperature conditions are studied in detail. Calculate the fractal dimension of the acoustic emission distribution map and analyze the relationship between the fractal dimension and the failure mode under different temperature conditions. The research results will have an important reference value for fracture mechanism, secondary crack initiation and propagation prediction, and enhanced oil recovery in shale fracturing
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