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Seismic Reservoir Characterization of a Gas Shale Utilizing Azimuthal Data Processing, Pre-Stack Seismic Inversion, and Ant Tracking*
David Paddock1, Christian Stolte1, Lei Zhang1, Javaid Durrani1, John Young1, and Pat Kist1
Search and Discovery Article #40310 (2008)
Posted October 30, 2008
*Adapted from oral presentation at AAPG Annual Convention, San Antonio, Texas, April 20-23, 2008 and from 2007 WTGS paper of the same title by essentially the same authors.
1Schlumberger, Houston ([email protected])
Prospective hydrocarbon-bearing zones in the subject gas shale are characterized by gas entrapped in the sediment matrix with some additional open-fracture component. This gas is economically recovered by horizontal drilling and fracturing. We show the results of an integrated workflow to get a good seismic image in the presence of anisotropy, find the sweet spots, and characterize fractures and reservoir risk.
Seismic azimuthal anisotropic analysis determines the dominant direction of the fast and slow seismic velocities indicative of maximum horizontal stress and/or fracture orientation and intensity. Corrections for HTI (horizontally transversely isotropic) medium were applied prior to VTI (vertically transversely isotropic) anisotropic Kirchhoff pre-stack time migration (KPSTM). Data processing created a pre-stack volume as input for inversion and a stacked volume as input for Ant Tracking.
Simultaneous inversion of the pre-stack data determined acoustic impedance (AI) and Poisson’s ratio (PR). A blind test of the inverted attributes is very encouraging. Sweet spots may be areas of anomalously low Poisson’s ratio (PR), away from faults, with high velocity anisotropy.
Ant Tracking reduces the risk of drilling near faults (a reservoir risk due to expected mineralization) by providing a high resolution image of fractures and faults beyond what can be interpreted from conventional seismic data. Faults were expected to be mineralized, with some mineralization extending out into the surrounding shales.
The resulting integration of Poisson's ratio, fractogram, and Ant Tracking provides effective delineation of areas with superior porosity and charge, areas with open fractures, and areas with faulting, outlining likely sweet spots as well as areas to be avoided in drilling.
Integration of anisotropic data processing with pre-stack seismic inversion and Ant Tracking provides a superior tool to explore for gas in gas shale.
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n Finding the “porous” sweet spots n Drilling and completing effectively n accurate fault delineation required to avoid risk n Understanding the effects of anisotropy n reservoir rock physics and delineation of structure n Quantifying the fracture storage of gas n Understanding the fracture system n Prior to this study, two unsuccessful wells drilled
Effective seismic gas shale workflow n Azimuthal velocity analysis improved delineation and understanding of fracture and stress regimes n Prestack inversion delineated sweet spots and areas to be avoided n Ant Tracking identified subtle faults that had been overlooked on conventional seismic More effective completions New drilling locations identified
Paddock, Dave, Christian Stolte, Lei Zhang, Javaid Durrani, (Schlumberger Reservoir Seismic Services), John Young, Pat Kist, (WesternGeco), 2007, Seismic reservoir characterization of gas shale utilizing azimuthal data processing, prestack seismic inversion and Ant Tracking: WTGS publication.
Mattner, Joerg, 2002, Fractured reservoir characterization from collecting data to dynamic modeling: Course, GeoTech Consulting, slide (as part of presentation).
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