Full-waveform Based Complete Moment Tensor Inversion and Stress Estimation from Downhole Microseismic Data for Hydrofracture Monitoring

Abstract

Downhole microseismics has gained increasing popularity in recent years as a way to characterize hydraulic fracturing and to estimate in-situ stress state. Conventional approaches only utilize part of the information contained in the microseismic waveforms such as the P/S far-field amplitudes to determine the focal mechanisms and infer stress state. The situation becomes more serious for downhole monitoring where only limited azimuthal coverage is available. In this study, we developed a full-waveform based approach to invert for complete moment tensor. We use the discrete wavenumber integration method as the fast forward modeling tool to calculate the full wavefield in the layered medium. By matching the waveforms across the array, a stable moment tensor solution can be obtained without imposing additional constraints. We show that by using full waveforms, the resolution of the full seismic moment tensor is improved even with data from a single monitoring well. We also determine the stress drop from the S-wave displacement spectrum. We test our method using a downhole microseismic dataset from hydraulic fracturing treatments in East Texas. The results indicate the existence of non-double-couple components in the moment tensor. The derived fracture plane direction also agrees with that derived from multiple event location.