Accurate Microseismic Event Location Inversion Using a Gradient-Based Method

Abstract

From the microseismic data collected during multi-stage hydraulic fracturing, we may pick out the first arrival times for both P-and S-waves for each event and at each geophone. With triaxial geophones, the azimuths of the events may be derived from the hodograms. The P-and S-wave arrival times contain the distance information from the event location to the geophones as well as the event initiating time. The azimuth from hodograms gives the orientation information of the event locations. Inverting these interpreted microseismic data yields the event location parameters, which can be used to characterize the hydraulic fractures for reservoir modeling. This paper presents an efficient gradient-based microseismic event location inversion method. The forward model that is used to calculate the first arrival times is the finite-difference solution of the Eikonal equation. A novel method is devised to obtain the gradient of the first arrival times to the event location parameters in addition to the first arrival times in one forward model run. The method is applied to a realistic multilayer shale gas reservoir with a horizontal well and several stages of hydraulic fractures. The results show that the first arrival times obtained from the surface geophones yield better event location parameter estimation assuming we can pick up the events from the surface geophone signal. When the geophones are placed downhole in nearby wells, the inverted event locations are less accurate than the ones with surface geophones but can have a relative good characterization fo the fracture propagation.

Locating Mine Microseismic Events in a 3D Velocity Model through the Gaussian Beam Reverse-Time Migration Technique

“…The Geiger's location method adopts a first-order gradient descent algorithm, which is fast in computation but easily affected by the initial value. Thurber [9] and Li et al [12] used the Newton and Gauss-Newton algorithm based on second-order Hessian to solve the inversion problem, improving the stability of the inversion but at the cost of longer computation time for calculating the second-order Hessian. Prugger and Gendzwill [13] and Li et al [14] introduced the simplex method into the source location problem, obtaining a higher calculation speed and better location accuracy.…”

An Efficient Method for Velocity Calibration and Prediction of Microseismic Event Location Using very Fast Simulated Annealing

“…Various linear inversion techniques (e.g. least square and Singular value decomposition (SVD)) are performed to invert velocity structure and locate microseismic events by using first arrival travel times and particle motion of seismic P and/or S waves [3,4,[6][7][8][9][10][11]. However, those linear inversion methods strongly depend on the initial models that may lead to trap in local minimum [5] due to less receiver coverage (<15 geophones in the monitoring well).…”

Assimilating Microseismic and Welltest Data Using EnKF for Accurate Reservoir Characterisation

“…The improved versions were developed by some authors (Podvin and Lecomte 1991;Sambridge et al 1992;Qin et al 1992;Zhao 1996;Zelt and Barton 1998). In this study, we will use the finite-difference solution to the Eikonal equation as the forward model for predicting the microseismic first arrival times (Zelt and Barton 1998;Li et al 2012).…”