Study On The Characteristics And Risk Assessment Of Induced Seismicity For EGS Development

Hydraulic fracturing has become a standard reservoir stimulation technology for EGS geothermal development and will induce seismicity.This kind of induced seismicity has been discovered for more than 30 years in the United States,Switzerland,Australia,Korea,Germany,Japan,New Zealand,Philippines,Indonesia,Kenya and other places.Most of these seismic magnitudes are less than M_L=3 without casualties and economic consequences.However,large earthquakes were also triggered in some EGS projects these years,which exceeded the public acceptance,resulting in casualties and building damage.For example,in Geysers geothermal reservoir,between 1997 and2003,nearly 1000 micro earthquakes with M_L?1.5 and 1-2 earthquakes with M_L?4.0were detected every year.During the hydraulic fracturing process of the EGS project in Basel,Switzerland,3500 micro-earthquakes occurred in 2006-2007,of which four earthquakes magnitude is larger than 3 and the maximum magnitude is 3.4,this large earthquakes resulted in the permanent closure of the EGS project and disaster compensation for residents.In Landau,Germany,an M_L=2.7 induced earthquake occurred in 2009 during geothermal energy extraction.In November 2007,Pohang,South Korea also experienced similar problems.One fracturing well was near the fault bottom and the other was passing through the fault plane.Although the injected fracturing fluid volume was small,it still caused a catastrophic earthquake of M_L=5.5.This is the second largest earthquake in South Korea since the earthquake is monitored in 1978,it causes nearly 80 people injured,more than 1100 houses damaged and more than 1800 people homeless.Induced seismic activity has become one of the most important limiting factors in the sustainable development of EGS projects around the world.This paper focuses on the induced seismicity caused by the development of EGS,and the key problems such as influencing factors,characteristics and prediction methods.Through field investigation,data collection,laboratorial experiment,numerical simulation and theoretical analysis,this paper conducts a lot of research on EGS induced seismicity.To assess the induced seismic risk and provide guidance for EGS field project,the key parameters and characteristics of induced seismicity are studied,and the maximum magnitude prediction equation is established and risk evaluation is carried out.In this paper,the 4D acoustic emission(AE)large-sized true triaxial experiment with high temperature and pressure is carried out to analyze the characteristics of hydraulic fracturing parameters and fracture distribution.The location,energy,temporal and spatial sequence of AE events during fracturing are recorded.The characteristics of AE events under different temperatures,confining stresses and injection flows are analyzed.The spatial and temporal distribution sequence,cumulative AE energy and maximum AE energy are studied.Secondly,the hydraulic fracturing process and AE activities are verified by using the discrete element water-force coupling model.The numerical granite reservoir model is constructed.The fracture growth processes are established under the same laboratorial experiment conditions.The spatial and temporal occurrence sequence,size,energy of AE events(induced earthquakes)are simulated.Thirdly,I propose a new parameter(injection energy)and an induced earthquake magnitude prediction method based on the worldwide EGS induced seismic data.Through correlation and sensitivity analysis,the maximum magnitude prediction equation is established with injection energy(engineering parameter)and fault length(reservoir parameter).Based on these two most important parameters,a statistical model based on the non-homogeneous Poisson process is established to analyze the probability of induced seismicity.This model can be used to evaluate the seismic risk related to hydraulic fracturing.Finally,for the first EGS project in China,through seismic geological survey and literature collection,the surface trace and activity of faults in the project site and its vicinity are clarified,the potential main control factors of the induced seismicity are judged.According to the investigation results of natural fractures and joints in the field granite outcrop,the reservoir model is established,the induced seismicity are simulated under different engineering conditions.According to the above proposed magnitude prediction method and risk assessment method,the induced seismic risk assessment is carried out,and the potential impact is analyzed.