| Hot dry rock accounts for more than 90% of geothermal resources,and the exploitable hot dry rock resources in mainland China are equivalent to 1.7 trillion tons of standard coal.Enhanced geothermal system is the main way of thermal energy extraction from hot dry rock.However,the hot dry rock reservoir is characterized by deep burial,strong anisotropy of insitu stress and prone to elastic-plastic deformation.The artificial fracture network formed by conventional hydraulic fracturing method is relatively simple,and it is easy to cause fluid short circuit during water injection heat recovery,leading to the phenomenon of "thermal breakthrough",resulting in the decrease of thermal efficiency of hot dry rock recovery.Therefore,how to enhance the complexity of artificial fractures is the key to improve the thermal efficiency of hot dry rock mining.In this study,the hot dry rock was subjected to alternating temperature loading in order to greatly improve the complexity of artificial fracture network in hot dry rock.At present,there are few researches on the physical and mechanical properties of hot dry rock under alternating temperature loading and the expansion law of artificial fracture network.By means of theoretical analysis,core testing and numerical simulation,this paper studies the physical and mechanical properties of hot dry rock and the expansion law of artificial fracture network under the action of alternating temperature load.Finally,through numerical simulation of flow and heat transfer in artificial cracks of hot dry rock,the thermal potential evaluation of artificial fracture network structure under the action of alternating temperature load is carried out.The main achievements are as follows:(1)Experimental study on physical and mechanical properties of hot dry rock under alternating temperature loading was carried out by triaxial press of rock mechanics under high temperature and high pressure.The results show that the rock mechanical parameters of hot dry rock under alternating temperature loading significantly decrease compared with that after single heat treatment.Under uniaxial compression,the three mechanical parameters decrease the most at 600°C alternating temperature loading,which are 21.6%,14.7% and 34.9%,respectively.After the treatment of alternating temperature loading,the fracture complexity of rock samples under uniaxial and triaxial loading is significantly higher than that after the single heat treatment.Compared with the single heat treatment,the porosity,permeability,thermal conductivity and thermal diffusing coefficient of hot dry rock under alternating temperature loading increased by 83%,6.1 times,8.4% and 14.5% at 600℃,indicating that alternating temperature loading can degrade the mechanical properties of hot dry rock,improve the permeability and thermal conductivity.After the treatment of alternating temperature loading,many mineral metasomatism occurs in the hot dry rock,especially quartz metasomatism perforation into plagioclase.The quartz content in the hot dry rock decreases obviously with the increase of the temperature under alternating temperature loading,which is the fundamental reason that the mechanical parameters,pore permeability parameters and thermal conductivity parameters of the hot dry rock change significantly.(2)By using the self-designed thick-walled cylinder expansion cracking device,stress concentration was generated in a finite volume space through the hydration reaction of high efficiency expansion agent mixed with water.Experimental study on expansion cracking of artificial hot dry rock fracture network under the action of alternating temperature loading was carried out.Fractal dimension was used to characterize the complexity of fracture network.The results show that: Compared with the single heat treatment,the fractal dimension of crack network caused by hot dry rock expansion under the action of alternating temperature loading is significantly improved.The fractal dimension of crack network increased by 3.86% at 600°C,which is the largest at 600°C.After the alternating temperature loading treatment,the upper and bottom cracks are mainly double-wing symmetrical structure,and the number of branch cracks increases significantly at 600°C.At the same time,the caved debris near the borehole also increased obviously,indicating that alternating thermal loading can significantly improve the complexity of artificial fracture network caused by expansion of hot dry rock.(3)An experimental study on hot dry rock hydraulic fracturing under alternating temperature loading is carried out by using the improved high temperature and high pressure hydraulic fracturing experimental system.The variation of injection pressure curve,fracture pressure and strain with injection time was analyzed.The results show that,compared with the single heat treatment,the fracture pressure and post-fracture pressure of hot dry rock hydraulic fracturing under alternating temperature loading have the largest decrease at 600°C,which are 53.4% and 23.3%,respectively.At 600°C,the maximum axial and radial strain variables are5.91% and 9.59%.Compared with the fracture morphology of pure heat treatment sample,the complexity of hydraulic fracture is obviously increased after alternating temperature loading treatment.Under the action of alternating temperature load,the hydraulic fracture propagation stage presents obvious elastoplastic failure characteristics.With the increase of temperature,the pressure curve presents a typical "smooth convex" characteristic.Hot dry rock expansion and artificial fracture propagation during hydraulic fracturing are based on thermal-mechanical coupling and thermal-fluid-mechanical coupling respectively.Hydraulic fracturing is driven forward by the joint action of water pressure and thermal stress at the crack tip.Therefore,the artificial fracture network formed by hydraulic fracturing of hot dry rock under alternating temperature load is more complex than the fracture network caused by expansion.(4)Through CFD software ANSYS Fluent,the physical models of artificial fracture network after single heat treatment and alternating temperature loading were established respectively,and the numerical simulation study on evaluation of thermal recovery potential of artificial fracture was carried out.The results show that the higher the injection velocity is,the lower the average temperature of fracture outlet section is.The outlet surface temperature of artificial fracture under alternating temperature loading treatment is obviously higher than that after single heat treatment.The smaller the angle between the branch fracture and the main fracture,the higher the heat recovery potential.When the included Angle is 60° and 30°,the fluid temperature is asymmetrically distributed along the direction of the main fracture.The research results of this paper can provide theoretical basis and technical support for enhancing the complexity of artificial fracture network and improving production thermal efficiency of hot dry rock reservoir. |
PDF Full Text Request