Geomechanical Models 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 horizontal stress; stress condition is a key success factor in hydraulic fracture stimulation treatments. This study uses geomechanical models 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 layers 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 horizontal stress. (4) Arch effect of an anticline/syncline will lessen vertical stress and increase horizontal stress. Paradoxically, the lessened vertical stress translates (via the Poisson's ratio effect) to lowered horizontal stress. (5) Friction between lithological layers can suppress the bending beam effect. (6) Regional tectonic forces can influence horizontal stress and the arch effect. Models 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 horizontal stress distribution through three lithological layers 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 horizontal stress may be increasing or decreasing with depth, depending on lithology type and bounding layers. This study argues that traditional conclusions from curvature analysis should be scrutinised more closely, especially in application to unconventional reservoirs.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015