Study And Application Of Atrificial Fracturing Mechanism Of Enhanced Geothermal Systems

Enhanced geothermal system (EGS), also known as hot dry rock (HDR), is the projectthat obtaining a lot of hot is extracted energy from a low porosity and permeability rock layers.Its study and development has more than30years history, which is confined to United States,Britain, France, Germany, Switzerland, Japan, Australia, and so on. In our country, theresearch of HDR is late start, and these study found that these problems has a very significantimpact for the normal operation and safety of engineering, which is the multi-physics coupledproblems of the temperature-seepage-stress-damage and fracture (THMD) in the process ofdesign, construction and operation.This study mainly relies on the National High Technology Research and DevelopmentProgram (863Program)(No.2012AA052801): Engineering Test of HDR Target and Study ofArtificial Fracturing Technology. In the EGS system, coupled effect of liquid and stress ismainly considered in the fracturing process. When the system is running, the injection of coldwater would result in a significant reduction of fracturing layer temperature surrounding well,and energy exchange would also happen by the hydrothermal convection within the cracksand pores, meanwhile, heat exchange continue to occur between liquid, rocks. Theseprocesses exist not only thermal conduction, but also thermal convection and need to considerthe local change of stress field caused by temperature stress. Therefore, for the effectevaluation and prediction after fracturing, this paper established fractured water flow–heattransfer model and multi-field coupled model to assess the reasonableness and applicability oftarget and fracturing technology; then, the paper proposed optimal fracturing method that bothconsiders permeability and thermal conductivity based on field conditions and rock mechanictests of laboratory to evaluate the effect of fracture and hot water output. In this paper, mainlyresearch works are as following:1. The paper studied the rules of compression, shear and tension fracture, propagation and coalescence of rock cracks under the action of seepage-temperature-stress. The damageand fracture mechanic model subjected to seepage-temperature-stress had been firstlyestablished and numerical simulating validation is also demonstrated. The plasticity failurecriterion of artificial rock geothermal reservoir structure had been established for thehydrothermal exchange. Theoretically, they were revealed for the crack propagation becauseof the rock hydraulic fracturing and the rules of second thermal cracking of artificial rockgeothermal reservoir structure.2. Comparing and analyzing numerical simulating model and laboratory test, and basedon the existing calculation module of FLAC3D, the paper developed the coupled calculatingprocedures of the temperature-seepage-stress-damage and fracture through FISH langragewith TOUGHREACT-FLAC3Dsoftware (extended FLAC3Dmodel). The procedures havepowerful numerical features, including damage and fracture mechanics module, seepagecalculation module, temperature calculation module and multi-fields coupled analysis module.Laboratory testing model of hydraulic fracturing was performed, which verified the rationalityand accuracy of the software applied in hydraulic fracturing.3. The properties of rock material were tested. The paper carried on many experiments,including rock mechanics properties, permeability and porosity, thermal physical parameters,fracture toughness, scanning electron microscopy and acoustic emission, with the samples ofXujiaweizi region in the Daqing. These tests provide the basis for choosing the granite,sandstone, rhyolite layers as the fracturing layer and covering layer (sealing layer) in the site.The paper also explored the rules of rock mechanics properties and generalizes therelationship between rock strength and every parameter after making the large rock triaxialstress test and fracture toughness test.4. Based on currently grasping the theory of fracture and hydrothermal coupled exchangethermal simulation, the paper simulated the fracture morphology and propagation rules ofhydraulic fracturing and mechanics effect of artificial thermal reservoir structure in theprocess of coupling hydrothermal exchange with the software of Stimplan, TOUGHREACTand the paper developed. Combined target geological model, dynamic reserve was evaluatedduring the mining and operation of HDR, including the assessment of the dynamic heatextraction and system contributed to the life. These aspects also were studied, namely determining technology of geostress and layer mechanical parameters, spatial expansion rulesunder the action of stress and porosity pressure and optimized fracturing parameters to formthe best distribution of fracture.5. As the high cost of forming single horizontal well with the traditionally fracturing wayof single pair well, the dual fracturing way of horizontal well was adopted to maximizeutilization and improve production efficiency and reduce the total cost without increasing thedifficulty of the fracture. Therefore, twice fracture was considered forming cracks to add thearea of exchanging heat according to the above research, simulating thermal reservoir afterstimulation. Based on the experience of oil fracturing aspects, it is high economiccost-effective of horizontal well fracturing double fracture to improve the hot water output. Inorder to analyze the problems of interaction and mutual coupling of temperature and seepagefields between the multi-channels of diversion, the paper established water flow and heattransfer model of artificial thermal reservoir structure. Multi-fields coupled rule andinfluencing factors were revealed by analyzing simulation results of the distribution andevolution trend of seepage and temperature fields, providing a theoretical basis for theapplication of the method in engineering.