Observational Of Seismicity Under The Multi-period Fluid Storage

The exploitation of natural resources is an extremely important part of the development of human society,but there is a contradiction between its output and the actual needs of human beings.This contradiction is reflected in the seasonal characteristics of natural resource production(e.g.,glacial meltwater),and the seasonal changes in human demand and the stable and continuous production pattern of natural resources(such as coal,oil,natural gas,etc.).To cope with this contradiction,a "storage" that can temporarily store excess production of natural resources during the period of low demand and take it out to meet the high demand.The industrial facilities of resources storage can reduce the waste of resources capacity and improve the efficiency of natural resources application.With the accumulation of seismic observation data and the development of observation technology,seismicity related to the industrial activities has been observed close to the facilities and named as "induced earthquake".Being close to industrial facilities,highly related to the industrial production,with destructive and low depth makes the induced earthquake seriously affect the safety of industrial facilities and surrounding area.It has gain increasing attention from both the scientific community and the public.Especially for the research of induced seismicity mechanism related to the fluid storage(e.g.,reservoirs,underground gas storage).It has important scientific significance for understanding the evolution of the earth’s inner medium under repeated loading and unloading,and the interaction between fluid and earthquakes.In June 2013,the largest Underground Gas Storage(UGS)in China was opened in Hutubi,Xinjiang.About 60 km south of the UGS,the Nurga Reservoir,which is a medium-sized reservoir(Grade Ⅲ)located on a mountain stream-type river that is recharged by mixed rain and snow with significant seasonal changes,was put into service in January 2015.Both of them were well equipped with seismicity observation,which makes this region an ideal place for investigating the spatiotemporal evolution of seismicity related to multi-periodic fluid storage.Our study was based on the record of the permanent and portable seismic station in the northern Tianshan area.Firstly,we relocated the earthquakes recorded by the China Earthquake Networks Center(CENC)catalog with the waveform data and seismic phase reports.Then,we detected and located the possible missing events using the Match Filter technique(MFT)and waveform cross-correlation-based double different relocation by setting the relocated catalog as template events.Based on the detected and relocated catalog,we investigate the relationship between the seismicity and the operation of industrial facility,respectively.Although located in the Tianshan Mountains seismic zone,the study area was seismically quiet before the start of the Hutubi UGS operation.At the beginning of the operation of the Hutubi UGS,two earthquakes with M>3 occurred within 10 km from the Hutubi UGS,which aroused much discussion,but there was no consensus on its mechanism.At the same time,with the operation of UGSs and the continuation of seismic observations,the changes in the impact of multi-period cyclic storage on regional seismicity are also worthy of further research and discussion.Our result shows that the seismicity related to the operation of Hutubi UGS occurred at a shallow depth of<10 km and in close proximity(<6 km)to the boundary of the Hutubi UGS reservoir and can be divided into two clusters with conjugate occurrences.During the capacity expansion stage,seismicity was initiated from the boundary of gas injection,and we propose that the elastic and poroelastic stress changes together with injection induced strain localizations are responsible for it.The evolution of seismicity during the second injection period migrate to the shallow part with diffusivity close to 2 m2/s.Disturbed by the injection,the cracks in the hydrocarbon source formation may open and release fluids and injected into the preexisting fault will cause the redistribution of pore pressure and weaken the fault.When entering the stable operation stage,the seismicity tends to be stable and weak,occurring mainly along the major faults with relatively scattered distributed and low magnitude,which corresponds to the repeated loading/unloading stress on faults during circulating gas injection/production.For the Nurga Reservoir region,the CENC catalog show a significant increase in regional seismicity in the second year after operation.The earthquake events with M>2 increased from about 2.7 per year to more than 10 per year,which is rare reported in the medium-sized reservoirs.Our study indicates that the seismicity associated with the reservoir is mainly occurred in the shallow part within～5 km.According to its spatial distribution,it can be divided into two seismic clusters.Both of the two clusters occurred in the severely deformed active axial of the wedge structure,which was widely distributed in the North Tianshan area.The occurrence of clusters is consistent with the structures on the east and west sides of the reservoir area.Loading stress change caused by reservoir impoundment will alter stress perturbations on surrounding faults,it will lead to the tectonic stress release that may be responsible for the seismicity in the reservoir area.Comparing the two cases,we find that the earthquake sequence influence by the multi-cycle fluid storage character with high b-value,sharp increase in seismic rate,and close to the reservoir area.After several cycles of operation,the seismicity tends to be stable and weak,occurring mainly along the major faults.Compared with tectonic stress,the stress disturbance caused by industrial activities mainly plays a role in triggering.After the accumulated tectonic stress is released in a short time,the seismicity tends to be quiet.