Investigating Evolution Of Mechanical Property And Permeability Of Hot Dry Rocks At Multi-scale Based On Accurate Grain-based Model

Clean and reliable energy is an important foundation for national strategic development and ensuring people’s well-being.The global warming effect caused by carbon emissions from fossil fuels continues to intensify,and the difficulty of obtaining non-renewable energy is increasing day by day.In order to achieve the goal of “dual carbon”,it is urgent to explore sustainable and clean energy and promote the national energy transformation.As a kind of multi-purpose,abundant and environmentally friendly renewable energy,geothermal energy is an important part of improving the energy structure in the future,which has attracted great attention from the industry and society.It is helpful to develop and manage geothermal energy mining projects to accurately characterize the macroscale mechanical property of hot dry rock reservoir and the mechanical property changes after hydrothermal alteration.It is of great importance to investigate the thermal effect of rocks under high temperature and the evolution law of reservoir fracture permeability after cooling shrinkage of water injection during geothermal exploitation,which is critical to the sustainability and stability of geothermal system thermal energy extraction.Based on microscale rock mechanical experiments and accurate grain-based models,this paper conducts a series of studies on the multiscale mechanical property of deep hot dry rock,the nonlinear evolution of rock mechanical property under temperature and the influence of reservoir permeability changes on engineering-scale physical fields.The main research contents and conclusions are as follows:(1)Deep reservoir rock is subjected to complex loads such as high geostress,high temperature,water softening and hydrothermal erosion,which makes it difficult for traditional macroscale rock mechanical theory to explain the complex mechanical behavior of reservoir rock under the action of multi-physical field coupling.Meanwhile,the widely distributed fracture alteration zone in the reservoir makes it difficult to obtain the complete core required for macroscale rock mechanical experiments.To solve the above problems,a microscale rock mechanical experiment system was developed in this paper.The elastic modulus and Mohr-Coulomb parameter of rock-forming minerals in fresh granite and altered granite were obtained based on nanoindentation experiment,and the elastic modulus of interphase in granite was obtained for the first time based on atomic force microscopy.On the basis of microscale rock mechanical experiments,a multi-scale analysis method of accurate grain-based model(AGBM)was proposed,and an intact lab-scale constitutive model is developed from mineral scale.Compared with the traditional grain-based model(GBM),the accurate grain-based model can truly characterize the mineral composition,shape,distribution,mechanical properties of mineral and interphases,as well as pore,microcrack and other microscale structures.Therefore,the analysis results of the accurate grain-based model are closer to the results of the traditional macroscoscale rock mechanical experiment.Additionally,the microscale rock mechanical experimental method is applicable to rock samples with arbitrary-size and-shape,the research method overcomes the engineering problem that traditional macroscale rock mechanical experiments are difficult to measure rock samples with unconventional size.The results of microscale rock mechanical experiments show that hydrothermal alteration products with poor mechanical properties(including sericite,other alteration minerals,as well as intergranular fillings)and widely distributed fracture network result in a significant decrease in the macroscale mechanical properties of altered granite compared with that of fresh granite.(2)An experimental equipment for rock microstructure observation at ultrahigh temperature was developed to carry out in-situ granite heating experiments.The real-time observation results clarified the process of initiation,propagation and colescence of microcrack in granite under high temperature.Based on the accurate grain-based model,the evolution law of rock microstructure and its influence on the nonlinear evolution law of macroscale mechanical properties of granite under high temperature are revealed.The experimental results show that,below 300℃,mineral crystals in granite only occur thermal expansion.Microcracks initiation at 300℃,and microcracks gradually propagate and coalesce between 400℃～600℃.Microcracks propagation and colescence are the main reasons for the rapid deterioration of macroscale mechanical properties of granite at 500℃～600℃.Although the traditional viewponit is that the α-β crystal transformation of quartz at around570℃ is the main cause of the rapid deterioration of the macroscale mechanical properties of granite,the real-time experimental results in this paper confirmed that the microcrack initiation temperature(300℃)is much lower than the phase transformation temperature(570℃)of quartz.(3)When the fluid passes through the deep rock fracture in geothermal reservoir,it will cause hydrothermal erosion on the fracture surface.Because the intensity of hydrothermal alteration gradually decreases from the outside to the inside,it is difficult to measure the mechanical property by traditional macroscale rock mechanical experiment and establish a unified constitutive model.In order to solve the above problems,a grain-fractured rock model was proposed.The grain-fractured rock models of fresh granite and altered granite were established to investigate the influences of hydrothermal alteration on the aperture,permeability and stress-strain evolution of granite with a single fracture during the cooling process of high-temperature rock.The results show that hydrothermal alteration of rockforming minerals in granite will lead to the deterioration of mineral mechanical properties,aggravate the uneven distribution of thermal stress,expand the cold shrinkage strain displacement,and increase aperture and permeability of the fracture.Finally,the permeability of the mineral-fractured rock model with a single fracture is extended to the hot dry rock reservoir mining at engineering scale.The influence of hydrothermal alteration on the evolution process of temperature field of geothermal system and the production well temperature of under the action of high temperature rock cooling is expounded.Hydrothermal alteration of rock and increase water injection rate will accelerate the cooling of rock around water injection well,expand the distribution range of low-temperature rock,and accelerate the cooling rate of production well temperature.