Experimental And Numerical Study Of The Mechanism Of Fault Slip Induced By Water Injection

High-pressure water injection can not only cause the microseism induced by rock fracture propagation,but also activate the fault near the water injection well point and induce large earthquake,such as the 2017 Mw 5.4 Pohang earthquake related to geothermal project in South Korea.Therefore,it is of great scientific value and engineering significance to explore the mechanism of water injection induced earthquake.From a view of rock mechanics,this paper systematically summarizes the existing mechanism of induced earthquake.And the mechanism of the influence of heterogeneity of rock,cyclic water injection and fault material(fault gouge mineral)on the characteristics of fault slip was studied.The main research work of this paper is as follows:(1)The heterogeneous numerical model of rock and fault was established using the digital image processing technique and COMSOL numerical software,which perfectly presents the rock mineral,pore and fracture.The elemental microscale parameters were determined through comparison with testing results from a homogeneous model.The damage equation of the rock based on COMSOL was established,and then the influence of fluid pressure on the behaviors of fault slip was analyzed in the established heterogeneous and homogeneous models.(2)The sealed chamber bearing high water pressure was developed and the test platform of injection-induced earthquake was set up.Laboratory earthquake tests with cyclic water injection using a biaxial compression machine was performed.The stress drop after fault sliding under different water injection conditions was studied.The energy and moment magnitude of the earthquakes induced by cyclic water injection was analyzed.The influence of rock permeability on fault slip was analyzed.Based on the COMSOL numerical model verified with the test results,the damage of surrounding rocks after the fault sliding under the condition of cycled water injection was analyzed,and and the reasonable measures to control induced earthquake was put forward.(3)To study the effect of fault materials on fault slip,laboratory earthquake tests with cyclic water injection was carried out.The fault strength under different water injection conditions was analyzed,the effect of fault materials on the fault strength was analyzed.The effect of fault materials on the stress drop and fault slip under cyclic water injection was studied.Based on the COMSOL numerical model verified with the test results,the rock damage around the fault with different fault materials was analyzed.The rock damage around the fault faults with different crossing angles,faults with 5°bends,collinear discontinuous faults and complex geese under water injection conditions were discussed.The results show that:(1)Rock heterogeneity can significantly influence injection-induced earthquakes.With increasing fluid pressure,the initial time for shear stress drop decreases,the area of secondary damage increaseand the probability of unstable slip increases in a homogeneous rock matrix.Compared with the homogeneous numerical model,the heterogeneous model has a reduced time,a higher probability of seismicityand a larger area of secondary damage.In addition,secondary damage is generated at two ends of the fault.The area of the secondary damage zone increases when unstable slip is induced,and the rate increases with slip velocity.(2)Cyclic water injection can control the energy and the moment magnitude of induced earthquake.Under identical total injection volumes and injection rates,cyclic water injection can decompose a large stress drop into several small stress drops,and the maximum stress drop decreases with the increase of the number of cycles.With an increasing number of water injection cycles,the number of induced earthquakes increases,while the maximum moment magnitude and the total energy of the induced earthquakes both decreases.The effect of cyclic water injection on reducing earthquakes increases with decreasing rock permeability.Cycled water injection can effectively reduce rock damage around the fault.Compared with non-water injection conditions,continuous water injection increases the area of rock deformation and damage around the fault;while cyclic water injection can reduce the degree of rock deformation and damage around the fault,and the reduction increases with the number of cycles.(3)The mineral content of fault material has an effect on the friction strength and sliding characteristics of fault.The stress drop increase with increasing the content of quartz in fault,but decreases with increasing the content of montmorillonite in fault.The montmorillonite with low friction strength is helpful to the stable sliding of faults,and it can reduce the permeability of faults by blocking the pores of rock particles around faults,so as to reduce the total seismic energy and increase the reduction of the total seismic energy in cyclic water injection.The fault material only affects the size of the failure area surrounding rocks.The fault geometry determines the location and size of the rock damage area around the fault.The failures surrounding the straight fault are distributed at both ends of the fault;the failure areas surrounding cross faults are mainly distributed at the joint of faults and the end of secondary faults;when the crossing angle is 45°,the rock failure around the cross fault is the lowest.The rock failure around the inflection fault,collinear discontinuous fault and Yanli fault occurs in the stress concentration area of the composite fault.