Study On THMC Coupling Of Hydro-shearing In Hot Dry Rock In Enhanced Geothermal System

Geothermal energy is renewable,clean and green energy that is generated by continuous decay of isotopes and stored in the rocks,steam or natural fluids of the Earth's crust.There is about 42.7×106EJ geothermal resource over a depth of 3km and 80%of them from hot dry rock reservoir in enhanced geothermal system(EGS),which is about 300 times the fossil energy.The most important consideration for geothermal energy development in non-hydrothermal scenarios is the use of hydraulic fracturing technology to establish an effective network pathway to conduct fluid from injector(s)to producer(s).This hydraulic injection can create new or remobilize existing fractures for fluid circulation and heat transfer.The hot dry rock(HDR)has the high mechanical strength and the physical properties of the matrix without permeability.The natural fractures during the process of hydraulic fracturing are affected by the thermal stress,hydraulic dilatancy and chemical dissolution,which often lead to 'Hard injection' and 'Low production'.Therefore,an elastic-plastic constitutive model and thermal-hydro-mechanical and chemistry(THMC)model areestablished based on the hydro-shearing mechanism of the natural fracture.First,the case study of hydraulic fracturing in the world's key geothermal wells is carried out and the mechanics,deformation and failure characteristics of the deep hot dry rock are annlyzed combined with the granite core test.At the same time,the pre-elastic and the post-elastoplastic constitutive model are established,which respectively describes the permanent deformation characteristics of the normal dilation and tangential slip of the fractures.The results show that the fracture roughness coefficient(JRC)is the key to form the long-term conductivity of the fracture,and the plastic slip and dilation after the peak stress account for more than 90%of the total deformation.It is suggested that hydro-shearing can not only increase the self-supporting conductive opening of fractures,but also reduce the surface pumping pressure under frictional weakening.Based on the theoretical model of fracture medium flow,the heat transfer model of hot dry rock matrix is successfully coupled to analyze the characteristics of fracture opening and the temperature distribution both in fracture and rock massunder the thermo-elastic mechanism.The chemistry model was coupled,and the change of fracture aperture and fluid pressure due to silica dissolution/precipitaition was analyzed under the condition of long-tenn atably injection.Finally,acoupled thermal-hydro-mechanical-chemistry model was developed to analyze the composition of the total hydraulic aperture and primary r(ole in different fracture section.The results show that the heat induced opening is the main component of the natural fracture conductivity in the wellbore region.The fracture conductivity away from injection area is mainly formed by the hydraulic dilation,and the chemical dissolution induced opening is almost no contribution in short time hydro-shearting.Based on the theory of displacement discontinuity and total fracture aperture change,the fracture deformation induced stress model in rock mass is established.According to the heat transfer model of rock mass and circulating cold fluid,the thermal induced stress model in rock mass is established.Thus,the new stress state of unconnected cracks and rock matrix are recalculated.According to the stability theory of rock mass and fracture,the stability of the area away from the injection area is analyzed,which provides an analytical theory for the formation of underground natural fracture network.The results show that the deformation induced stress and the thermal induced stress are only valid in the range of several meters of the water injection point.In this paper,the heat transfer,fluid injection,rock deformation,silica dissolution,deformation and thermal induced stress are fully coupled,which improved the theory of underground hot dry rock hydro-shearing.The research results have certain theoretical significance for the development and optimization design of the enhanced geothermal system.