Abstract
Seismic anisotropy in sedimentary rocks, due to either layered bedding or fracturing, may bias microseismic locations if unaccounted in velocity models. To quantitatively assess such biases, we have applied a nonlinear location method to synthetic travel time data in seven models from isotropy to different levels of VTI (Vertical Transverse Isotropy) and HTI (Horizontal Transverse Isotropy) cases. Synthetic waveforms are recorded at two vertical receiver arrays in a three-layer velocity model using a pseudo-spectral method. Both P and S wave arrivals are used to locate three events assuming an effective isotropic velocity model. The average location error is 59 m for isotropic case, about the size of the grid interval in the velocity model, and is 156 m (158 m), 237 m (244 m), and 258 m (265 m) for 5%, 10% and 15% VTI (HTI) cases, respectively. These results suggest that even 5% seismic anisotropy, if not properly accounted, can cause significant biases in microseismic event locations.
Highlights
Hydraulic fracturing has become a common technology to enhance oil and gas production from low permeability rocks
The application of microseismic images relies on the accuracy of event locations, which depend on the quality of data and velocity models
Our results show that event locations can be determined with the errors as small as the grid interval for correct isotropic velocity models
Summary
Hydraulic fracturing has become a common technology to enhance oil and gas production from low permeability rocks. Numerous microseismic events are generated during a fracturing process, and microseismic imaging has been used to monitor fracture propagation and development. Knowing how fractures grow helps engineers to determine optimal drilling directions and the number and spatial interval of fracturing stages [1]. Mapping microseismicity is important in estimating reservoir volume and production [2]. The application of microseismic images relies on the accuracy of event locations, which depend on the quality of data and velocity models. In [3] Eisner L., M. Griffin found that even 5% change in velocity model could result in significant biases in the locations of microseismic events.
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