Research On Coupled Normal Stress And Fluid Flow Property Of Rock Fractures

The correlation between hydraulic permeability and mechanical properties of rock fractures is important for analysis of many geophysical problems;however,it remains uncertain in most cases due to complex fracture structures and rock mechanical properties.Subject to geological processes,natural rock fractures can be dislocated to some extent.The dislocated fractures deformed under normal stress and the geometry of the rock fractures changed,which directly affects the permeability properties of underground fluid in the fractures,and then changes the basic properties of heat and mass transfer of rock mass.The geometry of fractures are related to the type of rock and dislocation displacement,and the relationship between the specific stiffness and permeability of dislocation fracture under normal stress has not been quantitatively described and evaluated.In order to solve the above problems,the coupled deformation and fluid flow property of rock fractures are researched based on the combination of laboratory test and numerical simulation.For the experimental section of this study,granite,fine sandstone and coarse sandstone cube samples and granite cylinder sample were axially split into two halves with an identical size to generate artificial tensile fractures,four fractures were formed by splitting and lifting and horizontal dislocation.The fractures which were split by cube samples dislocated 4 mm and the loading test was carried out.High-resolution measuring instruments recorded the displacement and plastic areas of loading process which verified the validity of the fractures compression calculation model.The fracture which was split by granite cylinder sample dislocated 2 mm.The sample was put into the gripper to set the axial pressure and confining pressure,and the seepage test was carried out.The hydraulic aperture that calculated by the highprecision differential pressure gauge was compared with result of the seepage calculation model which was well verified.Based on the validity of compression calculation model and the seepage calculation model,in this study two joint planes of granite,fine sandstone and coarse sandstone were selected,and one joint plane of largesize granite was selected.In create different fractures,all joint planes were offset a series of values which applied to upper surfaces while fixing the lower surfaces.The maximum normal stress of 100 MPa was set to perform compression numerical simulation of the dislocation fractures,and the seepage simulation was carried out based on the numerical calculation results of the rock fractures.The results show that there is a good linear relationship between normal stress and contact area ratio.Under the same normal stress and the same dislocation displacement,the contact area ratio of coarse sandstone is the largest,while the contact area ratio of granite is the smallest.The contact area of the fractures is related to the roughness and Young's modulus of the fractures.The fracture aperture structures were characterized by using correlation length and relative standard deviation(Hereinafter referred to as RSD).The RSD of fracture aperture increases linearly with the increase of normal stress,while the correlation length is just the opposite.It decreases with the increase of normal stress,and the rate of decrease gradually decreases.Linear correlations between permeability and normal stiffness,and hydraulic aperture and mechanical aperture were established based on the fracture structure parameters,i.e.,correlation length and relative standard deviation,and the rock mechanical property,i.e.,Young's modulus based on the size effect of the fractures.The proposed correlation functions have clear physical meaning that can provide a reference for the problems of fracture deformation and seepage of fractured rock mass under the coupling action of multiple fields in the future safety construction and production operation of many deep underground projects,such as geological disposal of high-level radioactive waste and geothermal development.