|Theme||Visible||Selectable||Appearance||Zoom Range (now: 0)|
3D anisotropic waveform inversion (WI) could provide highresolution velocity models and improved event locations for microseismic surveys. Here we extend our previously developed 2D WI methodology for microseismic borehole data to 3D anisotropic models. This extension allows us to invert multicomponent data recorded in multiple boreholes and properly account for vertical and lateral heterogeneity. Synthetic examples illustrate the performance of the inversion for layer-cake and “hydraulically fractured” VTI (transversely isotropic with a vertical symmetry axis) models. In both cases, the algorithm is able to reconstruct parts of the model which are sufficiently illuminated for the employed source-receiver geometry. We also present preliminary results including moment-tensor estimation for a field data set recorded during hydraulic stimulation of a shale reservoir
Presentation Date: Wednesday, October 17, 2018
Start Time: 1:50:00 PM
Location: 205A (Anaheim Convention Center)
Presentation Type: Oral
ABSTRACT The accuracy of velocity models built from microseismic data can be increased by employing 3D anisotropic waveform inversion (WI). Here, we apply our previously developed elastic WI algorithm to velocity analysis of synthetic and field microseismic data from VTI (transversely isotropic with a vertical symmetry axis) media. A synthetic test demonstrates that WI may be able to delineate hydraulic fractures if they are properly illuminated for the employed source-receiver geometry. We evaluate the sensitivity of the inversion results to errors in the source locations and to band-limited noise in the input displacements. The WI algorithm is also applied to a microseismic data set acquired during hydraulic fracturing of a shale reservoir. The inversion of the P-wave direct arrivals helps refine the 3D fields of the horizontal velocity and anellipticity coefficient . Further improvement is expected from joint inversion of the P and S data. Presentation Date: Wednesday, September 18, 2019 Session Start Time: 8:30 AM Presentation Time: 11:25 AM Location: 221C Presentation Type: Oral
Accurate estimation of the source parameters is a major task in microseimic monitoring. Here, we employ elastic waveform inversion (WI) to estimate the location, origin time, and seismic moment tensor of microseismic sources embedded in 2D VTI (transversely isotropic with a vertical symmetry axis) media. Forward modeling is carried out with a finite difference code that generates P- and SV-waves from dislocation-type sources. In addition to minimization of the objective function with a constant step length, we apply the nonlinear conjugate gradient method (NCG) for model updating. The WI algortihm is shown to be stable in the presence of moderate random Gaussian noise. We also present preliminary results of testing the WI methodology on a data set from Bakken field, North Dakota.
Microseismic data are used to find the spatial and temporal distribution of hypocenters of events triggered during hydraulic stimulation and to estimate the source mechanisms (moment tensors). Conventional event-location techniques are based on picking the arrival times of the direct P- and S-waves in a borehole or at the surface (Rutledge and Scott, 2003). Also, microseismic events can be located without traveltime picking, by employing stacking (Anikiev et al., 2014), migration-based methods (Zhang and Zhang, 2013; Zhebel and Eisner, 2015), and waveform inversion (Kim et al., 2011; Jarillo Michel and Tsvankin, 2014a,b).
Event locations are usually determined by assuming the velocity model to be known, for example from perforation shots and borehole data. However, this approach may produce inaccurate results if there are errors in the velocity parameters, particularly those related to seismic anisotropy. Grechka and Yaskevich (2013, 2014) demonstrate that it is possible to construct anisotropic velocity models from traveltimes while locating microseismic events. Their method yields more accurate source locations than those produced by conventional isotropic techniques.
Most existing algorithms invert for the seismic moment tensor M under the assumption that the source position xs and origin time t0 are known. However, waveform inversion has the advantage of resolving the tensor Mand location xs simultaneously. Here, we refine the WI algorithm for estimating the source parameters presented by Jarillo Michel and Tsvankin (2014b) and apply it to a microseismic data set recorded in a near-vertical well at Bakken field.
Waveform inversion (WI), which has been extensively used in reflection seismology, could provide improved velocity models and event locations for microseismic surveys. Here we present an elastic, anisotropic WI algorithm for microseismic data designed to estimate the 3D velocity field along with the source locations. The gradient of the objective function with respect to the source and medium parameters is obtained using the adjoint-state method, with only two modeling simulations performed at each iteration. In the current implementation for VTI (transversely isotropic with a vertical symmetry axis) models, the source coordinates and velocity parameters are estimated sequentially at each stage of the inversion to minimize trade-offs and improve the convergence. Synthetic examples illustrate the performance of the algorithm for a 3D cloud of dislocation sources embedded in a layered VTI medium.
Presentation Date: Monday, September 25, 2017
Start Time: 1:50 PM
Presentation Type: ORAL
Pérez, Daniel O. (formerly CONICET and Y-TEC) | Lagos, Soledad R. (presently FCAG-UNLP) | Velis, Danilo R. (Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata (FCAG-UNLP) and CONICET) | Soldo, Juan C. (Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata (FCAG-UNLP) and CONICET)
We provide well-calibrated VTI velocity models useful to locate microseismic events in the Vaca Muerta shale formation, Neuquén, Argentina. Assuming layered models with weak anisotropy, we make use of the information provided by well logs and perforation shots of known position to estimate the layer velocities, depths and anisotropy. This leads to a constrained nonlinear inverse problem that consists in minimizing the discrepancies between the observed and calculated P and S-wave arrival time differences. To avoid local minima and other convergence issues, we minimize the resulting objective function using very fast simulated annealing (VFSA). We test the proposed strategy on field data and estimate a set of velocity models that honor the observed data, which we validate carrying out a simulated microseismic event location. The results show that the proposed strategy is capable of estimating layered VTI velocity models suitable to accurately locate microseismic events during a hydraulic stimulation in the VacaMuerta shale formation.
Presentation Date: Wednesday, October 17, 2018
Start Time: 9:20:00 AM
Location: Poster Station 15
Presentation Type: Poster