Experiment On Microfractrues Initiation And Propagation And Seepage Properties Of Sandstone Samples By Using X-ray Scan

The interactions of seepage and stress in rock are key problems in geotechnics, especially in rock mechanics. There are two difficult problems in the interactions: the occurrence and propagating of microfractures in loaded rock; and the relation of seepage and stress in the crack process of rock. Supported by the national natural science fund project, and with the devices including multislice spiral CT and real-time loading machine and seepage machine, the experiment on seepage and stress of rock has been worked out and some available data have been obtained By analyzing the experiment result data, some important results about the occurrence and propagating of microfractures in sand stone under different stress and seepage conditions, and crack model and the relation of seepage and stress in the crack process of rock have been worked out as follows:1. The dry sandstone's crack is brittle under uniaxial compressive condition. When the compressive conditions are seepage pressure and triaxial compressive, the crack is plasticity. With triaxial compressive without seepage pressure, it's crack pattern is between the two patterns above.2 Two types of sandstones have been scanned by multislice spiral CT (MSCT) with high resolution under uniaxial compressive condition. From the treatment and analysis of CT images, it can be suggested that the variances of the difference of CT images have close relations with the propagating rate of microfractures. From the relation curve between this variances and stresses, it can be indicated as the follows: Firstly, the threshold value of microfractures propagating in sandstones can be identified easily. For soft sandstone, the occurrence of microfractures is in 55% of the peak strength, and the whole propagating strength of microfractures is about 73% of the whole compressive strength of rock. For the hardy sandstone, the occurrence of microfractures are in 64.5% of the peak strength, and the whole propagating strength of microfractures is about 79% of the whole compressive strength of rock. Secondly, the propagating law of microfractures in sandstone can be found intuitively. In the initial stage of compression, the propagating rate of microfractures increases very slowly with the increase of compression. When the compression increases by just before the threshold values of microfracturing occurrence, the propagating rate of microfractures is decline rapidly. After the microfracturing occurrence, the propagating rate of microfractures increases until the occurrence of macrofractures.3. By analyzing the results of this experiment, the porosity formula of rock represented by CY number was proposed in meso scale. Based on the porosity formula, we worked out the porosity, the real flow rate in porous rock, the specific discharge, permeability and the diameters of micro-pore in the processes of stress and strain of rock. The results showed that the changing rules of seepage parameters have closely related to the processes of damage and cracking in rock. At the first stage of rock compression, with the increase of axial compression, the porosity, the real flow rate in porous rock, the specific discharge, permeability and the diameters of micro-pore in rock decrease. This stage is in dosed porous under compression. After the occurrence of microfracturing of rock sample, the porosity, the real flow rate in porous rock, the specific discharge, permeability and the diameters of micro-pore increase with the ascendant of axial compressioa According to the relation between stress and strain of rock in macro-scale, the deformation of the rock is elastic before the occurrence of macro-crack of rock. In this experiment, the microfracturing occurs at the stage of elastic deformation of rock and the stress of initial microfracturing occupies 45% of peak strength of the rock. At the point of macro-failure, the porosity, the real flow rate in porous rock, the specific discharge, permeability and the diameters of micro-pore reach the maximum values.Based on the porosity formula, we worked out the porosity, the real flow rate in porous rock, die specific discharge, permeability and the diameters of micro-pore in the processes of increasing seepage pressure. The results showed that the parameters of seepage increase with the ascendant of seepage pressure. Logarithm relations are presented between the parameters of seepage and seepage pressure. The reason for the logarithm relation is that the elastic deformation is happened on the micro-pores in rock under seepage pressure. The relation between the specific discharge and seepage pressure is linear and tally with the Darcy law.4. Integrated the theoretical analyzing of the fracturing process in sandystone and the deduced stress distribution model in the tip zone of microfracture under compressive condition, the model of the propagation of microfracture and failure with rock is presented. By analyzing the model, we can find that the propagation of fracturing is closely related to the changing of porosity. If the porosity is bigger, the offset fracture in the tip zone of original fracture is longer and the offset fractures will propagate easier. The propagating rule from the research is tally with the existing theories of fracture. With the means of CT, more advantages of using this model are than other existing models. With using the model, the strength factor of fracture and the surface energy are not necessary for calculating the model.