Risk reduction in the determination of fractures’ density and orientation through azimuthal processing and interpretation of seismic data
Alvaro Chaveste, Manager – Interpretation Services at Geokinetics
The determination of fracture density and orientation is important in plays in which reservoir permeability is associated with micro-fractures. In these reservoirs, effective porosity is dependent upon the density of open fractures and permeability has a directivity component associated to the fractures’ orientation. Accurate determination of fracture density and orientation using surface seismic helps, then, in determining locations of good reservoir storability, as well as directional drilling orientation for optimum permeability.
Conventionally processed seismic data provides a structural image of the sub-surface which helps, among other things, in defining macro-fractures or faults; nevertheless, the micro-fractures that define permeability and porosity within a formation, although possibly associated to macro-fractures, are not necessarily part of the structural picture and, hence, are not commonly observed in migrated stacks. Micro-cracks can be characterized by considering that velocities in cracked media vary as a function of micro-fractures’ azimuth and density; the velocity being faster in the fractures direction or minimum horizontal stress and slowest in a directional perpendicular to fractures. Velocity anisotropy provides, then, the means of estimating reservoir properties (micro-fractures’ density and orientation) from seismically derived velocities.
A case study in the Marcellus Shale (NE United States) is presented in which wide azimuth seismic data, acquired and processed to preserve the amplitude and velocity information of the source-receiver azimuth, is used to estimate the velocity anisotropy attributes required for fracture characterization. The Marcellus Shale is known to have two sets of micro-fractures associated to different tectonic events. Particular to this play is that one set of micro-fractures is perpendicular to the co-located macro-fractures. In this case although curvature, or other geometric attributes, reveal the strike of macro-fractures, this does not correspond to that of the micro-fractures. Anisotropy analysis through elliptical inversion identifies both sets of micro-fractures and verifies their expected position in relationship to the oroclinal belt and macro-fractures.
An added benefit of the resultant anisotropic velocity field is that it can be used for anisotropic NMO correction. In this case the correction applied to each trace is based upon the “velocity ellipse” at the trace’s Common Image Point (CIP) as well as the trace’s azimuth and offset. The NMO corrected data results in “flatter” gathers for subsequent pre-stack amplitude analysis and higher resolution stacks.
The use of wide azimuth seismic data results in added information that is used to increase seismic resolution through anisotropic NMO as well as to aide in the characterization of micro-cracks.
Alvaro has been the Manager of Interpretation Services for Gokinetics since joining GoK in 2007. Prior to this, from 2005-2006, he held the positions of Manager of Reservoir Characterization and Project Manager of Reservoir Services at Paradigm Geophysical. In 2003 he founded and was President of Traceseis Inc. From 1998-2003 Alvaro held several positions within Core Labratories, from Seismic Data Processing Technical Manager to Senior Geophysicist. Prior to 1998 Alvaro worked in various capacities with the Andrews Group International, Western Geophysical, Halliburton Geophysical all in Houston and GSI in Mexico City. Alvaro received his Bachelor of Science degree from Montana College of Mineral Science and Technology in 1981.
In addition Alvaro has completed professional training in Rock Physics, Well Log analysis for Geologists, Seismic Prospecting and Inversion, Seismic Velocities, Geophysics and Reservoir Development, Borehole Geophysics and Time Lapse Seismic