| With the shortage of petroleum resources in the world, severe environment damages have been brought by the exploit of coal resources so that the development of new green energy, such as high temperature rock geothermal, has been put forward. High dry rock geothermal resources are located in the deep stratum, so high temperature and crustal stress impact on the construction. Rock mass experiences thermal fracture when heated in which way the rock strength reduced. The speed of micro-fracture in rock adds under the actions of mechanical-thermo coupling and certain influence acts on the exploit of the high temperature rock mass. Therefore, it is very significant to research thermal fracture and Acoustic Emission (AE) characteristics of rock under the actions of mechanical-thermo coupling.This paper mainly researches on rock fracture laws and AE characteristics under the action of mechanical-thermo coupling through tests of thermal fracture AE of rock under the unconstrained and triaxial strain circumstances, the fracture AE characteristics in the high temperature creep process and fracturing with high-temperature steam under the conditions of unanxial and triaxial. Then the following results are acquired:1) Through the tests of thermal fracture AE with heating, cooling and loop alterlately under unconstrained conditions and thermal fracture AE of granite with25MPa hydrostatic stress in the heating process, some results are obtained as follows:①The thermal fracture AE of granite is obvious in the unconstrained heating process and it can be divided into3stages:In the stage of compact thermal fracture with the temperature from the normal to160℃, there is few thermal fracture AE with very low strength; In the stage of brittle thermal fracture with temperature from160to460℃, there is a rapid increase in the amount of thermal fracture AE with less strength changed; In the stage of elastic-elastic fracture with temperature above460℃, the thermal AE amount, strength and durative reach the maximum.②In the cooling process of unconstrained granite, the thermal AE can be divided into3stages:In the elastic-elastic fracture stage with the temperature from600to300℃, the amount of AEs increases with temperature decresing; In the stage of brittle fracture with the temperature from300to100℃, the strength and amount of thermal AE reach the maximum in the test; In the post brittle frature stage with temperature from100to30℃, the strength of thermal AE reduces.③In the heating process of large-sized granite (φ200×400mm) with hydrostatic stress as25MPa, thermal AE can be divided into3stages:It is the compact thermal fracture stage with the temperature from the normal to110℃, in which there are a great number of AEs, higher counts and lower energy, thermal expansion lend to the deformation and closure of the primary opening; In the stage of brittle thermal fracture stage with temperature from110to420℃, there are a great number of AEs, higher counts and higher energy; In the stage of elastic and plastic thermal fracture stage with temperature from420to500℃, there are a big number of AEs and higer counts and the peak value of energy is big but the stage energy accumulation is small. AE is made up of few high-energy AEs and many low-energy AEs. And thermal effect makes rock softening, this lend to local plastic failure.④Triaxial stress state can promote the thermal fracter to enter into the stages of elastic and partial plastic fracture. Unconstrained granite thermal facture AE happens at the temperature which is about40to50℃higher than that under hydrostatic stress as25MPa and the lagging temperature value lies on the stress state.2) Through AE experiments of big-sized granite (φ200×400mm) under the conditions of high temperature creep fracture, we can see that the creep process of granite at300℃experiences transient creep stage and the steady creep stage. There are continuous AE signals. The amount of count and energy in the first creep stage is twice of that in the second stage; The grnite creep at400℃and500℃only experiences steady creep stage in which the uniform creep AE is fewer than differential stress creep, the count is one order of magnitude lower and there are high energy AE happening at intervals. The creep AE parameter reduces with the temperature increasing.3) Through the tests on granite uniaxial steam fracture (with size of120×120×300mm) and triaxial steam fracture (with size of300x300x300mm) crack expansion and AE characteristics, it is concluded that:①Mechanical-thermo coupling effect promotes the rock fracture. The rupture pressure in the granite uniaxial hydraulic fracture is6.5MPa, the rupture pressure in the high compression rate steam fracture at430℃and350℃is respectively2.7MPa and2.2MPa, which is less than the50%of the hydraulic fracture rupture pressure. The rupture pressure in the low compression rate steam fracture at450℃is1.8MPa which is about one fourth of the hydraulic fracture rupture pressure.②Several cracks easily appear in the granite hydraulic fracture under mechanical-thermo coupling effect. There is only one crack in the low compression rate steam fracture which can run through the sample and the other cracks stop cracking before long. There is a "T"-crack when it begins to crack in the triaxial steam fracture and the cracks parallel to the minimum principal stress direction extend to the vertical minimum main stress direction.③The lateral deformation rate in the uniaxial high compression rate steam fracture is big, the deformation amount is more than the twice of that in the hydraulic frature, and the deformation amount of sample vertival rapture failure suface is about five times of that of parallel failure surface. The sample experiences compression and deformation along the axial direction in uniaxial high temperature steam fracture and the lateral deformation is swelling deformation. There are three stages in the deformation precess of triaxial steam fracture test:Steam pressure increase phase in which the axial is compressing deformation and deformation rate reduces with the steam pressure increase, while lateral is swelling deformation and the lateral swelling and deformation rate raises with the increase of steam pressure; In the pressure keeping stage, the lateral keeps swelling deformation, the deformation amount shows a basical linear relationship with time and the axial deformation gradually changes from compressing to swelling; In the stage of steam fracture, the axial and lateral deformations are both swelling deformations.④In steam fracture with unaxial high tempearature, crack expansion can be divided into pre-fracture thermal crack stage and fracture crack stage. In the stage of pre-fracture thermal crack, the distribution of cracks are randomness, thermal cracks enter on the bores and gradually develop to the periphery. There are3steps in the fracture crack stage:a. crack initiation. There appear randomly distributed cracks surrounding the fracture bore; b. expansion. Crack nucleation appears and cracks extend outward around the borehole; c. macro-cracks form. Cracks expand in the aspects of length, width and height and the sample rupture cracks stop. Hydraulic fracture cracks expansion includes3stages as crack initiation, expansion and crack formation.⑤The high temperature triaxial steam fracture crack expansion is made up of3stages. In the stage of steam pressure increasing, thermal fracture happen at the surface of drilling, rupture points are few and distribute at random. In the stage of thermal fracture with pressure keeping, rupture takes the bore as centre and expands outward. Meanwhile, the rupture density decreases along the radius. In the stage of fracture:a. Thermal fractures are concentrated, and there is crack nucleation surrounding the fracture bore; b. There are several initial cracks; c. cracks expand on length, width and height; d. The cracks parallel to the minimum principal stress direction stop; e. The cracks parallel to the minimum principal stress direction expand into the direction vertical to the minimum principal stress.⑥Cracks continue to expand and the density of rupture points around cracks increase, then the sample fractures. |
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