Experimental Study On Fracture Characteristics Of Oil Shale Under Thermal-mechanical Coupling Field

Since reforming and opening, China’s high-speed growth of the national economy is on a fast track and solid energy security is the basis of sustainable rapid economic growth. Energy structure with coal as the major energy, however, is not easy to change in the short term. The huge energy imports became the main channel of supplementary energy consumption. Around the world, however, especially the oil-producing and oil imports of transport corridor area are not peace, therefore, it has been necessary to look for a fossil fuel with a good foreground and substantial reserves in China. China’s oil shale reserves are large, widely distributed. On the basis of the reference on shale gas and similar energy’s advanced mining technology from the US and other developed countries, we can completely develop in-situ modification mining method of oil shale to improve the level of energy independence and security of our country.Oil Shale In-situ Conversion technology in China has been developed and achieved some results and mainly has two steps: reservoir reconstruction and oil and gas drainage. This article focuses on the problems about reservoir reconstruction. Oil shale belongs to the typical types of sedimentary rocks with clear bedding、hole and a lot of natural fracture. At the macro level, it has obvious transversely isotropic and mechanical performance is affected by water and temperature significantly. Therefore, the further research on the influence on the shale fracture characteristics from temperature and the bedding will have an important guiding significance to oil shale in in-situ conversion mining undoubtedly. After summarizing the research achievements of predecessors, considering the influence factors of shale mechanical properties, this paper mainly includes the following researches, and the corresponding conclusions:(1) Oil shale of Three-point bending(SCB) test. Fracture toughness of three different direction of bedding specimens after pyrolysis, between 20 ℃ and 500℃, are tested, it is concluded that law of the variation of load displacement curve slope from the specimens. Comprehensive water, organic matter and mineral composition were studied under different temperature on the influence law of fracture toughness. The results show that thermal damage and thermal cracking is the root causes of the sharp decline in oil shale macro fracture toughness.(2) The macroscopic fracture toughness of Fushun oil shale has obvious anisotropy. The maximum value of the fracture toughness of the crack-arrester and crack-divider specimens are similar and that of the crack-splitter specimen is only about 1%-2% of their maximum value. The crack-arrester and crack-divider specimens have different "threshold temperatures" Tc, the fracture toughness increased slightly before Tc, and the fracture toughness decreased sharply after Tc, which are connected with rock mass bedding direction and hot processing of mineral composition and microstructure changes. At last, the law from the changing trends and value of fracture toughness of crack-arrester and crackdivider type specimens were obtained.(3) Study on failure characteristics of oil shale rock. The reason of oil shale block cracking and damage under thermal-mechanical coupling field are analyzed. Influence of stratification and temperature, in the SCB test process, on the fracture mode and fracture morphology were studied and the law were obtained.(4)Hydraulic fracturing numerical simulation experiment. When cracks began to lose the steady expansion, larger hydraulic pressure is need and accompanied by more fracturing fluid leak-off. The peak pressure and the peak fluid leak-off have reached 2 times of value when the crack is expanding stably. period value about. It is effective and easy fracture toughness is used as a judge of the crack extension direction. simulation of hydraulic fracture propagation process and result is consistent with the project construction data.(5)On the temperature field, stress field and displacement field of studies, the patent method of multilayer thermal pyrolysis of oil shale after staged fracturing have a great advantage in the efficiency and effectiveness of mining insitu oil shale. In the production process, the good mechanical conditions for the cross bedding cracks exist in the target layer two wings. Intermittent injection of heat production once again to open the pump required greater pressure. The wellbore is vulnerable to the threat of thermal stress and should strengthen the safety and maintenance.The above research can be used for reference and guidance to the engineering practice of oil shale in situ modification of oil and gas reservoir.