czerniak_geopressure_2011
TRANSCRIPT
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Role of PermeabilityWeighting in
Centroid Positioning
Matt CzerniakOctober 3, 2011
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Centroid Theory
Flemings, Stump et al., 2002
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Centroid Modeling Concept –
Early application of centroid theory assumes simple basin flank or mini-basin setting (not complex structure)Permeability weighting in 3D centroid modeling workflow allows for complex structuring and lateral changes in formation pressure.
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Flow into sand – Flow out of sand = 0
Sum{(map) αsand - (map) αshale}*{(map) kshale } = 0 over mapped area of sand on horizon
α : fluid potential in psik: permeability
Kilsdonk, 2003 AAPG
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Centroid Effect: FLOW IN = FLOW OUT
αsand = ∑ (αshale * kshale)∑ kshale
α: overpressure, psik: permeability, mDn: number of grid nodes
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Assumptions / Scope
Existing 3D Pore Pressure Volume calculated from a pre-conditioned 3D Interval Velocity Model
• Corrected for Anisotropy• Properties include: pseudo-density, fracture gradient, porosity-permeability
Use 3D mapped horizons as a proxy for geobodies• Zero Sand Thickness• Geobody is contained within 3D Volume• Regular grid spacing (e.g., 500 x 500 ft.)• Correct for dip error
Transfer select properties from 3D Pore Pressure Volume to mapped horizonsPerform permeability-weighted centroid calculations through standard Gocad Scripting Editor
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Pre-Conditioned Velocity Model
Pseudo-density
Porosity
Permeability
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Overpressure
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Fracture Gradient
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Өcos
Dip Correction
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Convert dip angle to Radians
surface mode > compute
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Dip corrected OP script
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K * OP_dc script
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sumKxOP_dc script
Command Terminal
= n
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Centroid OP script + hydrostatic
dc_sand_PPG.EMW
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Hydrofrac
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SUMMARY
A framework is provided for advanced centroid modeling calculations in 3D, assuming an existing pre-conditioned seismic velocity-based 3D PPFG volume;Gocad provides an excellent platform for 3D centroid calculations using simple scripting editor, 3D permeability-weighted centroid calculations, corrected for dip, are capable of providing rubust predrill pore pressure forecasts.
Note: Special thanks to Bill Kilsdonk and Keith Katahara [Hess] for inspiration and suggestions regarding the permeability-weighting scheme and geobodydip correction workflows, respectively.