Mechanism of Structure Degradation and Permeability of Rock Mass Induced by Fluid Injection under Compression-shear Loads
Fluid injection is widely applied in engineering,such as hydraulic fracturing in unconventional oil and gas resources and geothermal resources.However,the research on the influence of mechanical properties of strata by fluid injection is mainly focused on engineering practice,and its mechanism is seldom analyzed.It is particularly important to study the effect of fluid injection on the mechanical properties of engineering rock mass through laboratory tests.Therefore,based on the engineering background of fluid injection in rock engineering,a physical test method of fluid injection coupling shear load of rock mass is put forward,and a coal-rock shear-flow test system is developed.Taking sandstone and similar materials as the object of study,tests of fluid injection into intact rock mass to form local penetration pressure,fluid to infiltrate rock mass under pressure,fluid injection or infiltration to closed or unfilled fractured rock mass,fluid injection or infiltration to filled fractured rock mass are systematically carried out.Combined with rock mechanics,fracture mechanics,groundwater dynamics and seepage theory,the mechanism of fluid injection on shear mechanical properties of rock mass is discussed.Based on the above research,the main results are as follows:(1)Based on the construction process of fluid injecting engineering rock mass,a series of test method of fluid injection coupling shear load is put forward,and a coal-rock shear-flow coupling test system is developed independently.It provides a means in laboratory for studying the evolution of mechanical parameters of engineering rock mass during fluid injection.(2)In this paper,physical simulation tests of fluid injection process under different water injection pressure and different shear stress are carried out for sandstone,and the coupling relationship between the shear mechanical properties and water injection pressure of sandstone is discussed.The mechanism of sandstone coupled with fluid injection and shear load is discussed,and the evolution law of cracks propagation and macroscopic fracture mode of sandstone is summarized.(3)Direct shear physical simulation tests were carried out for sandstone under different normal stress,different moisture state and different pore water pressure,and the evolution law of shear mechanics parameters of sandstone under hydro-mechanical coupling was analyzed.A quantitative characterization method of anisotropy of fracture structure plane is presented by using the characteristic parameters of semi-variogram,and the deterioration mechanism of shear mechanical properties of rock mass during fluid infiltration is revealed.(4)The physical simulation of direct shear tests of rock mass with different fracture modes,different shear directions and different water injection conditions were carried out by using similar materials,and the variation trend of mechanical parameters with shear displacement was discussed.Based on the effective characterization of the fracture surface by the characteristic parameters of semi-variogram,the peak shear strength criterion based on the sill and range values of the semi-variogram of the fracture surface is proposed,and the shear mechanics model of the fracture surface with two-stage roughness is established.(5)The physical simulation direct shear tests of rock mass with different filling thickness,different filling material and different filling particle size were carried out by using similar materials,and the influence of filling properties on shear mechanical properties of rock mass with fracture surface was analyzed.The evolution law of crack propagation mode with the increase of filling thickness under compression-shear load is proposed,and the deterioration mechanism of shear mechanical properties of filling plane rock mass by water injection is further discussed.