--> --> Geomechanical Models Challenge Common Assumptions and Conclusions of Seismic Curvature Analysis for Unconventional Reservoirs
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Geomechanical Previous HitModelsNext Hit Challenge Common Assumptions and Conclusions of Seismic Curvature Analysis for Unconventional Reservoirs

Abstract

Unconventional resources such as tight sands, shales and coal seam gas reservoirs are defined by low matrix permeability. Economic production of these reservoirs relies on natural fracture networks and hydraulic fracture stimulation to enhance reservoir permeability. Seismic curvature analysis is commonly used to identify areas of higher natural fracture density and regions of lower Previous HithorizontalNext Hit stress; stress condition is a key success factor in hydraulic fracture stimulation treatments. This study uses geomechanical Previous HitmodelsNext Hit to interrogate traditional assumptions of curvature and stress distribution in extensional and compressional tectonic settings. It is an oversimplification to assume that anticlines have favourable curvature and synclines have unfavourable curvature. A more realistic view of these structural features considers the sum of multiple phenomena: bending beam forces, overburden force, the Poisson's ratio effect, the arch effect, friction between lithological Previous HitlayersNext Hit and regional tectonic forces. (1) Bending beam theory states that beams will be in compression inside the ‘neutral surface’ and in extension outside the neutral surface. (2) Weight of the overburden is self-explanatory and generates the vertical stress. (3) Poisson's ratio effect will convert vertical stress into Previous HithorizontalNext Hit stress. (4) Arch effect of an anticline/syncline will lessen vertical stress and increase Previous HithorizontalNext Hit stress. Paradoxically, the lessened vertical stress translates (via the Poisson's ratio effect) to lowered Previous HithorizontalNext Hit stress. (5) Friction between lithological Previous HitlayersNext Hit can suppress the bending beam effect. (6) Regional tectonic forces can influence Previous HithorizontalNext Hit stress and the arch effect. Previous HitModelsNext Hit presented here are generated with the poro-elastic equation and with ABAQUS, a 2D/3D finite-element simulator that incorporates stress-strain relationships, gravity, pore pressure, far field (tectonic) stress and rock mechanical properties. We calculate vertical and Previous HithorizontalNext Hit stress distribution through three lithological Previous HitlayersNext Hit and across a structural high. Results challenge common assumptions of seismic curvature: they show low stress sweet spots occur not only at the crest of an anticline but on the flanks too, and Previous HithorizontalNext Hit stress may be increasing or decreasing with depth, depending on lithology type and bounding Previous HitlayersTop. This study argues that traditional conclusions from curvature analysis should be scrutinised more closely, especially in application to unconventional reservoirs.