Research Of Creep-penctration-thermal Cracking Of Granite Under High Temperature And3D Stresses And Extraction Of Geothermal Energy

Energy is the world’s hot issues related to national security, is the important strategic problem. Geothemal of Hot Dry rock is put forward in the1970s to the development and testing, in America, Japan, Britain, France and other countries has been a good development, considered almost inexhaustible green energy. geothermal of HDR appear. the granitoids are in rock mass, around a series of engineering and technology exploiting geothermal heat, need reveals3D granite rock stress condition of creep, seepage, and thermal cracking and so on, a series of basic rule for design and make geothermal mining technique scheme provides guidance, integrated with the existing results of China’s Tibet occurred in the deep geothermal heat rock geothermal mining, for example, the study of economic efficiency mining scheme of China and the world, high rock geothermal mining of great significance.By making use of the20MN servo-controlled triaxial rock testing machine with high temperature and high pressure of china university of mining and technology, we have carried out experimental study on creep property of large size granite specimens of Φ200mm×400mm under high temperature with temperature up to600℃and axial and lateral stress up to175MPa.. This article describes creep test methods and results of granite under a three-dimensional stress and high temperature. Combining theory with analysis of test results, we discover granite experiences the first and second phases of creep at300℃, axial stress of94MPa and lateral stress of75MPa.At the same time the creep deformation of granite shows clearly the characteristics of the steady-state creep and gradually stops. But at400℃, axial stress of94MPa and lateral stress of75MPa the deformation shows obvious characteristics of the non-steady-state creep. The research reveals Creep property of granite increased with the increasing of temperature and stress. There is an threshold of temperature about transition from steady-state creep at300℃to the non-steady-state creep at400℃Through analysis of experimental data,the paper discovered that hydrostatic stress can cause creep formulation of lu gray granite and we proposed constitutive equation of stable-state creep rate taking into consideration for hydrostatic stress and acquiered creep constitutive equation parameters of lu gray granite A1、 A2、△Q、m、n. We also discoverer that the creep property caused by hydrostatic strss is the same as those cased by differential stress in lu gray granite. It also can be divided into three stages transient creep, stable-state creep and accelerating creep. There is a temperature threshold value between300℃and400℃. The primary creep deformation disappeared under hydrostatic stress and temperature threshold value in lu gray granite. The creep formulation of lu gray granite is a function of temperature, differential stress and hydrostatic stress. All the higher temperature, differential stress and hydrostatic strss can accerlerate creep formulation of lu gray granite.(1) under three dimensional stress, axial creep deformation at300℃and500℃and volumetric creep deformation at300℃,500℃and600℃of lu gray granite shows three stages:primary creep stage, stable-state creep stage and accelerating creep stage.(2) Under high temperature and3-d stress, volume、length and radius of lu gray granite increased with the creep time because of fissure caused by thermal cracking in rock. We also discovered lateral deformation is more rapid than axial deformation at the same time.(3) It’s the basical hypothesis of viscosity-elastic-plastic problem under three dimensional stress that differential stress causes axial creep, hydrostatic stress causes volumetric creep deformation at the basis of experimental data and we proposed volumetric creep constitutive equation of Burgers model under three dimensional stress.(4) Through the analysis of creep curve, it’s proper to simulate axial creep at300℃and500℃and volumetric creep at300℃,500℃and600℃with Burgers model and calculate model parameters.This research reveals:(1) under three dimensional stress, there is thermal cracking phenomenon about all granites. We discover the seepage law of intact granite rock sample that permeability is exponential growth function of temperature through the process of thermal cracking.(2) The permeability of granite has a slow growth through initial stage of TCA, a rapid growth through late stage of TCA and up to permeability climax in the final with the temperature rising up.(3) The permeability and its rate of granite have a continuous growth through whole stage of TCA with increasing temperature.(4)The ratio of permeability climax value to initial value is up to93and permeability rate is up to3.5x10-07d/℃.TCA has a important role in enhancing permeability of granite.We adopted acoustic emission apparatus made by American Physical Acoustics Corporation to collect data during creep experimental period. We acquired acoustic emission law of rock sample creep deformation experiment under high temperature and3D stress. Among transient creep stage, acoustic emission signal is very strong and decreases with time. Among stable creep stage, rock sample is mainly dominated viscoplastic deformation and acoustic emission signal goes into quiet period. Because of heterogeneity of granite, stress concentrated on force skeleton formed by hard mineral with stress adjustment and redistribution. At the late stable creep stage, the inner of rock sample resumed fracturing. All parameters of acoustic emission signal become increasing. Among accelerating creep stage, All parameters of acoustic emission signal also become increasing and finally reached peak value. Because rock mass is general composed of various minerals and contains joints and fracture discontinuous structure surface, it is inevitable heterogeneous. It is heterogeneity that causes above acoustic emission phenomenon. Under high temperature and3D stress, it still needs more experiment to research whether there are the same acoustic emission phenomenon about homogeneous or close to homogeneous rock sample. Under high temperature and3-d stress, there are good similarity among all acoustic emission parameters event counts rate, ringdown counts rate, energy counts rate, amplitude counts rate and energy-event’s ratio.This paper analyzed that HDR geothermal field of Tibet formed continental dynamics environment, South slope of Tanggula Mountain, DangXiong Yangbajing basin the tectonic characteristics, in-site stress size and orientation,8-18km deep rock mass structure of melt rock mass, temperature field distribution and heat resources. the research and put forward to mining scheme of HDR geothermal, seepage analysis of the heat exchange area, artificial reservoir features, project implementation and investment analysis.Proposed sliding zone as artificial reservoir, decorate a injection Wells and production Wells with technical solutions,so get a artificial reservoir volume3x1011m3, it is360times than artificial reservoir of Cornwall British for HDR Geothemal mining. This plan, a set of geothermal extraction, can build a system of10million kilowatts, power station for100 years. HDR geothermal resources of5.4×108MKW.a of Yangbajing region is China’s huge HDR geothermal resources temperature.