A. Reshetnikov

Microseismic Imaging From a Single Geophone: KTB

We have applied our approach for microseismic imaging to the data obtained from the German Continental Deep Drilling program (KTB project). This is a continuous data stream containing induced microseismicity data recorded at a single 3C geophone located at approximately 3.5 km depth. Using P and S time picks we have located 58 microseismic events using data from the borehole geophone and from near-surface stations. Since microearthquakes occur not at the same time, we have managed to separate continuous data stream recorded at the receiver to the number of 3C traces containing waveforms from different events. Using these traces we have produced seismic gather for the microseismicity cloud. We apply our approach to the produced seismic gather and construct 3D images of the seismic data between Pand Sdirect waves. There are two focused reflectors revealed within the microseismicity cloud which belong to SE2 reflector. In order to check consistency of the obtained results, we compare our results with seismic attributes obtained from the surface seismic 3D depth migrated image. Obtained result is consistent with seismic attributes by the location and dip, furthermore it provides with more detailed image of the fault zone fine structure.

Simultaneous Inversion of Anisotropic Velocity Model and Microseismic Event Location - Synthetic and Real Data Examples

In this study we develop a practical method for the location of microseismic events in anisotropic media. The velocity anisotropy is taken into account in the additional step after the conventional location procedure. In this step, the isotropic velocity model and corresponding estimated locations serve as an initial model, and the anisotropy parameters as well as resulting location misfits are inverted simultaneously due to the coupling of velocity model and hypocenter parameters in calculation. After obtaining the anisotropic velocity model, one can relocate all the events around their isotropic velocity based locations. The method is ray theory based and only consider transversely isotropic layered model. The nonlinear inversion problem is solved by using Gauss-Newton method. The results of synthetic example closely fit with the true values and event depths are better determined than radial distances. Real data test shows that the estimated Thomsen parameters are consistent with previous study, and the isotropic velocity based locations are significantly corrected with large decrease of average misfit from 52m to 6m.

Fracture Zone Characterization by Quantitative Analysis of Reflected Phases from Microseismic Waveform data

Microseismic waveform data can provide detailed information on reservoirs. In our work, we re-process recordings of selected microseismic events from the geothermal reservoir Basel-1, in order to better characterize the reservoir. Coherent reflected phases are identified by clusterwise analysis of the waveforms, since clustered events illuminate structures in a similar way. Applying the Fresnel Volume Migration to several clusters, we image zones of high reflectivity in the vicinity of the borehole. From the ratio of reflected to direct wave, we assign a value of apparent reflectivity to each imaged reflector. Correcting for effects due to different ray paths of direct and reflected waves, we map the estimated reflection coefficients. Comparing the estimated values to the frequency- and angle-dependent reflection coefficient at a single fluid layer, we map the fracture width. Some of the reflectivity values indicate complex fluid-filled fractured zones, rather than single fractures. Under linear slip condition, a value for the normal compliance of the reservoir can be determined. In the Basel reservoir, we observe reflection coefficients of about 0.1, which means effective fracture widths in the range of centimeters.

Anisotropic Source Mechanism Construction and Waveform Modeling

In this study we determine the effect of anisotropy on source mechanisms and wave propagation in a VTI-medium. We show, that the effect of anisotropy has a significant impact on the polarity and amplitude of P-wave for normal and thrust fault and cannot be neglected. Using this information we calculated synthetic seismograms which should explain our data better compared to pure isotropic synthetic seismogram modeling

Sonic Log Based Velocity Optimization with Perforation Shots in Unconventional Oil and Gas Field

Hypocenter locations are essential information for evaluation fracking area. It is necessary to build realistic velocity model to get accurate locations. Anisotropic velocity models could represent actual travel times much more than isotropic ones in shale dominant fields. Therefore, anisotropic velocity optimization using perforation shots data is important. In this case study, gamma logs and sonic logs are available. Considering 3D reflection seismic survey interpretations, geological structure is assumed as horizontal layers. In the target depth, 8 sequences are defined in gamma logs. The sequences are classified into 5 velocity layers, and linear fitting functions are applied to the velocity trends. Anisotropic velocity model optimization procedure is carried out with perforation shots data. To take into account the velocity trends, the next 3 items are set as velocity model parameters, (1) Scaling factor for Vp, (2) Vp/Vs ratio, and (3) Thomsen parameters. The total number of the parameters becomes 19 in this study. Newton’s method is applied to solve velocity model optimization problem. The Jacobian matrix is derived by finite differences of calculated travel times. The travel time residuals of the results indicate improvements of velocity model from the original velocity model. This procedure reflect much geological setting.

Common-receiver Gathers as a Tool for Analysis of Microseismic Waveforms and Quality of Time Picks

We introduce an approach for analysis of microseismic waveform data. More precisely, we propose a method to assemble recordings into common-receiver gathers sorted in a way that neighboring traces in a gather correspond to events located close to each other. We assume that microseismic sources located at a small distance are expected to produce similar traces recorded at the same receiver. Therefore, apparent inconsistency between neighboring traces can be considered as an indication of some error in data. We present an algorithm for sorting microseismic recordings in the case when events are located and for the situation when only arrival picks are defined. The algorithm is applied to a cloud of more than a thousand induced microseismic events recorded at two borehole arrays during one stage of hydraulic fracturing. We construct both arrival time based and location based gathers and show examples of wrong arrival time picks in a dataset which can hardly be revealed from the analysis of individual recordings or common-shot gathers. We demonstrate that this approach can be used for detecting secondary arrivals such as reflections. Presented algorithm is relatively simple and can be easily included into the microseismic data processing workflow as a quality control tool.

Microseismic Imaging at KTB

We have applied our approach for microseismic imaging to the data obtained from the German Continental Deep Drilling program (KTB project). This is a continuous data stream containing induced microseismicity data recorded at a single 3C geophone located at approximately 3.5 km depth. Using P and S time picks we have located 58 microseismic events using data from the borehole geophone and from near-surface stations. Since microearthquakes occur not at the same time, we have managed to separate continuous data stream recorded at the receiver to the number of 3C traces containing waveforms from different events. Using these traces we have produced seismic gather for the microseismicity cloud. We apply our approach to the produced seismic gather and construct 3D images of the seismic data between P- and S- direct waves. There are two focused reflectors revealed within the microseismicity cloud which belong to SE2 reflector. In order to check consistency of the obtained results, we compare our results with seismic attributes obtained from the surface seismic 3D depth migrated image. Obtained result is consistent with seismic attributes by the location and dip, furthermore it provides with more detailed image of the fault zone fine structure.