Summary

A recently published analytic technique for computing locations of microseismic events jointly with velocities of homogeneous isotropic models is extended to surface microseismic monitoring and transverse isotropy with a tilted symmetry axis (TTI). The analysis of traveltimes of the direct P-, SV-, and SHwaves, conducted under the assumptions of homogeneity and weak anisotropy, reveals that the SVwave data acquired in modern wide-azimuth surface microseismic surveys yield uniquely solvable joint inverse problems for an arbitrary symmetry-axis tilt, whereas the tilts should be close to 90 ? from the vertical for the P-waves propagating in anelliptically anisotropic media and strictly equal to 90° for the SH-waves to maintain the uniqueness of the joint inversion. These theoretical findings, confirmed on raytracing synthetic, are applied to a field microseismic data set. The P-waves excited by microseismic events are found to exhibit significantly flatter moveouts and better focused stacks when located in a constructed effective TTI model as compared to those located in a horizontally layered isotropic model provided as a part of conventional microseismic service.

Introduction

Hydraulic fracturing, routinely conducted to produce oil and gas from unconventional reservoirs, is often complemented by microseismic monitoring that helps delineate the created fracture networks and estimate the stimulated reservoir volume. Since its estimation primarily relies on hypocenters of microseismic events triggered in the course of a well treatment, it also requires a velocity model for computing those hypocenters. When building such a model, a geophysicist, typically aiming to account for the velocity heterogeneity and anisotropy, would either fix a model based on the available sonic logs and active-source data prior to locating the microseismicity (e.g., Pei et al., 2009) or construct a model simultaneously with locating the recorded microseismic events (Grechka and Yaskevich, 2014). This joint velocity-model building/event-location approach, applicable in the absence of sonic logs and active-source data, relies on the understanding of which velocityand anisotropy-related parameters are uniquely constrained in a given data-acquisition geometry.

This content is only available via PDF.
You can access this article if you purchase or spend a download.