Poynting vector-guided imaging condition for imaging fractures using microseismic data

Abstract

Hydraulic fracturing has been implemented in enhanced geothermal systems and in developing unconventional reservoirs to increase the permeability of earth media. To improve safety during the hydraulic fracturing, a method is proposed for imaging pre-existing fractures adjacent to the hydro-fractured zones based on observed seismic signals during microseismic monitoring using the elastic reverse time migration with source-independent converted phase (ERTM SICP) imaging condition. The ERTM SICP imaging condition is more computationally efficient than the conventional ERTM method, and it can be used to perform migration without information on the locations of source events. It is therefore appropriate for handling microseismic data.

However, because of the difference between P-and S-wave velocities and complex geometry, ERTM SICP imaging condition produces spurious events that can be confused with fractures when applied to reflected waves. Based on the idea that the propagation directions of the P-and S-waves at mode converting points are very similar, whereas those in other regions are commonly different, we modified the imaging condition by adding a weighting function calculated from the Poynting vector of the P-and S-waves. The weighting function has different values depending on the angle between the propagation directions of the P-and S-waves. Based on the tests of the imaging performance of this modified imaging condition, we confirm that our method can successfully suppress the linear spurious events that appear in non-mode-converting regions.