Velocity calibration by incremental pseudomaster events for single-well microseismic monitoring

Abstract

The velocity model plays a critical role in accurately determining microseismic event locations during hydraulic fracturing. Conventional velocity optimization methods, such as updating the velocity model using perforation shots, have the drawback of weak ray coverage during single-well monitoring. The microseismic events distributed in different areas can broaden the ray coverage and help improve the accuracy of the velocity model, whereas only a few perforation shots are available for a given stage. We have developed an incremental pseudomaster (IPM) method to reduce the velocity errors. The master event location technique is used to determine the positions of representative events in each iteration of the IPM method. These events then act as new pseudomasters to further optimize the velocity model using Bayesian inference. With progressive iterations, the number of pseudomaster events increases and their distribution range expands. The accuracy of the velocity model continuously improves, which means that the final IPM-based velocity model, generated based on the pseudomasters distributed throughout the study area, more accurately locates all microseismic events than the perforation shot alone method. One synthetic example and one field test have been conducted, and the results demonstrate that the IPM method could overcome the drawbacks of weak ray coverage and improve the accuracy of the inverted velocity model and event locations.