Micro-seismic Location Based On Variable Grid Reverse Time Migration

Unconventional oil and gas reservoir exploration is a new direction in the field of oil and gas field exploration,the characteristics of such oil and gas fields with low permeability,the development process is usually used in the way of hydraulic multiple stage fracturing.Hydraulic fracturing can make the fracturing area crack under the action of strong stress,improve the permeability of oil and gas reservoir and improve the recovery rate of oil and gas.In the process of hydraulic fracturing,it is needed to locate the micro earthquake event location which is produced by the hydraulic fracturing,so as to guide the process of hydraulic fracturing.The method of micro seismic event location mainly includes inversion localization method based on arrivals,the location method by energy and waveform similarity,and the location method by reverse time migration.In this paper,we study the method of micro seismic location of reverse time migration.Position results of micro seismic location method based on reverse time migration is presented in the form of wave field reverse delay.At present,micro seismic location method based on reverse time migration is carried out on the basis of grids regular splitting of velocity model.The result of micro earthquake location is greatly influenced by the size of mesh subdivision.In order to improve the positioning accuracy of micro seismic source,the current micro seismic location method based on reverse time migration need to split all of velocity model grid small,thereby greatly increasing computing resources usage of this method and reducing computational efficiency.To solve this problem,this paper will use the wave filed reverse delay process of variable gird reverse time migration on micro seismic record,in order to achieve location of micro seismic events.This method only refine the mesh subdivision of velocity model of hydraulic fracturing key monitoring area of reservoir.It not only meets the requirement of the fine description of the local wave filed inverse time delay,but also reducesthe computation resources and improve computational efficiency.In this paper the specific contents are as follows:(1)Numerical modeling of micro seismic records.In this paper,we use the piece-wise iterative ray tracing method and staggered grid finite difference method to simulate the micro seismic record.To improve meticulous depiction ability of hydraulic fracturing area of reservoir,this paper use spatial variable staggered grid finite method to simulate the micro seismic records.(2)Micro seismic location based on variable grid reverse time migration.On the basis of micro seismic location method of conventional staggered grid reverse time migration,this paper carry out the reverse time delay of micro seismic records with spatially variable staggered grid acoustic finite difference method.Through mesh subdividing of hydraulic fracturing monitoring area of reservoir.This method achieve wave field meticulous deception of hydraulic fracturing monitoring area,to a certain extent,the location accuracy is improved.(3)model test in complex cases.This paper carry out the location of micro seismic event with spatially variable staggered grid finite difference method.In the case of complex velocity model,the location of micro seismic event is studied by the disturbance of the velocity model and the addition of strong noise to the surface records of the numerical simulation of the micro seismic forward modeling.