Experimental Study On Thermal Cracking And Microcosmic Regularity Of Granite Under High Temperature Steam

With the increasing need of energy, the geothermal of hot dry rock is gaining more and more attention in many countries, and is on a high speed of growth in developed countries. The storage of geothermal of hot dry rock is massive that it could be used as the alternative energy of sustainable development. The exploitation of geothermal of hot dry rock involves lots of disciplines and sophisticated technologies, among which the core technology is the design and formation of artificial reservoir structure, which is directed connected with the efficiency of geothermal of hot dry rock.The construction method of current artificial reservoir structure is mainly hydrofracturing. Generally, the hydrofracturing method could only generate one vertical fissure vertical to the principal stress. Furthermore, this method, in essence, could only add to the length of fissure, and has limited capacity in increasing the density of fissure. Therefore, the conventional hydrofracturing method is a significant constraint of the appliance of hydrofracturing method. However, geothermal development is faced granite fracturing process at high temperatures due to the unevenness of the rock will produce thermal stress concentration in the rock caused by internal thermal cracking, which will result in hydraulic fracturing at high temperature to produce a lot of random factors. Secondly, how to change both the length of the crack, but also makes the crack density increases and the fracture network connectivity to increase the heat exchange area, is worthy of study. The hot dry rock in mining field simulation experiment more difficult, so often use reverse thinking, this article especially using high temperature and pressure steam injection mode to simulate non-uniform stress state under heat granite broken, the use of independent research and laboratory equipment, laboratory studied law and granite thermal cracking distribution of crack under action of high-temperature steam, the following conclusions:(1)At high temperature, the tensile strength of granite will be greatly reduced, thereby reducing the hydraulic fracturing granite fracturing pressure. In this experiment, the granite brittle fracture temperature of 483 ℃, the vapor pressure of 10.6MPa.(2)Under this experimental condition, the occurrence of granite thermal cracking process can be divided into five stages. And thermal cracking threshold temperature of 260 ℃, this temperature fracture well developed, or through the formation of a network.(3)Steam at high temperature and pressure effects, in addition to generating a main granite cracks in the direction perpendicular to the minimum principal stress, but also due to thermal cracking, causing secondary cracks in a number of randomly distributed in the other direction.In this paper, the use of special equipment to do after the granite thermal cracking permeability tests and longitudinal wave velocity testing to understand the thermal cracking process of physical and mechanical changes. Obtained from the fracture surface samples recently granite top longitudinal wave velocity and permeability changes in the most obvious, the most affected, most fractured well, change the intermediate layer is not obvious, but it can be seen that the direction of extens ion fractures offset the lower sample minimal impact. Each sample hole restores the granite velocity and permeability distribution to give its crack propagation direction.The article also fractures surface SEM experiment, from the microscopic point of view to study the mechanism of fracture granite. Get granite fracture morphology significantly brittle characteristics, the fracture morphology mainly trans-granular fracture and inter-granular fracture. Under this experimental condition, also exhibit fragmentation fracture and fatigue fracture and other injuries. From the crack density inter-granular fracture and trans-granular fracture to determine the degree of thermal cracking in different locations.Through comprehensive analys is of macroscopic experiments and theoretical and experimental data to obtain the relevant crack propagation law, the formation of cracks in different directions, forming the space fracture network of interconnected meaningful to help increase the heat exchange area of artificial retention layer help improve the efficiency of geothermal exploration.