Seismic characterization of multiple fracture sets at Rulison Field, Colorado

Author:

Vasconcelos Ivan12,Grechka Vladimir12

Affiliation:

1. Colorado School of Mines, Department of Geophysics, Golden, Colorado. .

2. Shell International Exploration and Production, Inc., Houston, Texas 77001. .

Abstract

Conventional fracture-characterization methods assume the presence of a single set of aligned, vertical cracks in the subsur-face. We relax this assumption and demonstrate the feasibility of seismic characterization of multiple fracture sets. Our technique relies on recent numerical findings indicating that multiple, differently oriented, possibly intersecting planar cracks embedded in an otherwise isotropic host rock result in a nearly orthorhombic (or orthotropic) effective medium. Here, the governing parameters of crack-induced orthotropy are estimated from 3D, wide-azimuth, multicomponent seismic reflection data acquired over the tight-gas Rulison Field in Colorado. We translate strong azimuthal variations of the normal-moveout velocities intointerval crack densities, fracture orientations, type of fluid infill, and velocities of P- and S-waves in an unfractured rock. Our inversion procedure identifies a set of cracks aligned in approximately west northwest-east southeast direction in the western part of the study area and multiple, likely intersecting fractures in its eastern part. We validate both our underlying theoretical model and the obtained estimates by two independent measurements: (1) the estimated fluid-infill parameter indicates dry cracks as expected for the gas-producing sandstones at Rulison; and (2) the obtained crack orientations are supported by well observations. As a by-product of fracture characterization, we build an anisotropic velocity model of the Rulison reservoir which, we believe, is the first orthorhombic velocity field constructed from surface seismic data.

Publisher

Society of Exploration Geophysicists

Subject

Geochemistry and Petrology,Geophysics

Reference31 articles.

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3. del Monte, A. A. , E. Angerer, C. Reiser, and C. Glass, 2004, Integrated approach for seismic fracture characterization: Summer Research Workshop, SEG/EAGE.

4. Ellison, A. I. , 2004, Numerical modeling of heterogeneity within a fluvial point-bar deposit using outcrop and lidar data: Williams Fork Formation, Piceance Basin, Colorado: MSc. thesis, University of Colorado.

5. Gaiser, J. E. , and R. R. Van Dok, 2004, Multicomponent processing and fracture characterization analysis of two Wyoming 3-D PS-wave surveys: Pinedale Field and Washakie Basin: Summer Research Workshop, SEG/EAGE.

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