Natural fracture characterization using passive seismic illumination

Abstract

The presence of natural fractures in reservoir rock can significantly enhance gas production, especially in tight gas formations. Any general knowledge of the existence, location, orientation, spatial density, and connectivity of natural fractures, as well as general reservoir structure, that can be obtained prior to active seismic acquisition and drilling can be exploited to identify key areas for subsequent higher resolution active seismic imaging. Current practices for estimating fracture properties before the acquisition of surface seismic data are usually based on the assumed geology and tectonics of the region, and empirical or fracture mechanics-based relationships between stratigraphic curvature and fracturing.The objective of this research is to investigate the potential of multicomponent surface sensor arrays, and passive seismic sources in the form of local earthquakes to identify and characterize potential fractured gas reservoirs located near seismically active regions. To assess the feasibility of passive seismic fracture detection and characterization, we have developed numerical codes for modeling elastic wave propagation in reservoir structures containing multiple, finite-length fractures. This article describes our efforts to determine the conditions for favorable excitation of fracture converted waves, and to develop an imaging method that can be used to locate and characterize fractures using multicomponent, passive seismic data recorded on a surface array.iii

Numerical simulation of wave propagation in media with discrete distributions of fractures: effects of fracture sizes and spatial distributions

“…The representation of the wavefield is highly accurate as long as sufficient grid elements are available, and in very good agreement with the theoretical ray traveltimes. Note that in addition to our work presented in this paper, as far as we know the Coates and Schoenberg method has also been implemented by Nihei & Myer (2000) and Nihei et al (2000) on staggered grid FDs, and by Chunlin Wu on variable grid FDs (Wu et al 2002).…”

“…The first step is to compare results generated by our modelling method with those obtained by another method. This has been done by Coates & Schoenberg (1995), Nihei & Myer (2000) and Nihei et al (2000), who compared the synthetic seismograms from the Coates and Schoenberg method described with the exact solutions using boundary element methods. We assess the accuracy by comparing the synthetic seismograms generated by the modelling with the ray theoretical traveltimes.…”