Methods On Acquisition Optimization,Wavefield Simulation And Inversion To Improve Microseismic Monitoring

In recent years,microseismic monitoring has been widely applied in hydraulic fracturing monitoring in tight and dense reservoirs,rock burst monitoring in mining,geological sequestration monitoring of CO2 as well as induced seismicity monitoring in the injection and production of conventional reservoirs.However,the real microseismic measurements are often affected by strong noise,which results in unreliable locations of microseismic events.Therefore,this paper carried out a comprehensive study on microseismic numerical simulation to analyze the characteristics of wave propagation with various focal mechanisms.Then,several inversion methods on microseismic events detection and location are implemented and investigated to improve the current processing techniques with more accurate event locations.Finally,the acquisition optimization methods are investigated to guide the real microseismic data acquisition.The major results are highlighted as bellows:(1)Analysis of the travel time and wavefield in microseismic monitoring using fast marching and staggered finite-difference method.Characterization of the radiation pattern of microseismic sources is obtained using moment tensor analysis.Based on the above techniques,the arrival time,synthetic waveform and traced ray for each receiver is acquired.In addition,a simulated function of microseismic sources in terms of origin time,spatial distribution and focal mechanism is established based on the field experience and rock physics experiments.Induced fractures of various geometries can be simulated by the determination of appropriate key parameters.(2)Comprehensive study on effective microseismic event detection including energy ratio,auto-aggressive Akaike information criterion(AIC),multi-scale AIC and matched filter methods.The impacts of characteristic function,window length,threshold on detection accuracy and reliability are investigated.This paper proposes an improved microseismic event detection method that based on time-frequency analysis using S transform.The real data example shows that the proposed method can detect more microseismic events in strong noise environment.(3)Comprehensive study on Geiger location,grid searching location,reverse-time location and double difference location methods.An evaluation method based on 3D location ellipsoid for location accuracy and uncertainty for reverse-time location is established in this paper.The impact of waveform load and image condition on reverse-time location has been investigated.An improved multi-scale reverse-time location method is proposed in this paper to depress the image noises and enhance image quality in location images when employing conventional reverse-time location method in low SNR data processing.(4)A quantitative acquisition optimization method based on the weighted sum of location accuracy and sensitivity is proposed in this paper.This method proposes an objective function and employs the very fast simulated annealing method to find the optimized acquisition for microseismic monitoring in a given area.The numerical and real data examples show that the main factors that influence the optimization are number of receivers,the coverage of receivers and the distances away from events to receivers.Finally,the simulated microseismic data using surface monitoring and the real field recorded microseismic data using vertical downhole monitoring from the Middle East oilfield are employed to verify the proposed methods.