Correspondence between Geomechanical Prediction of Fracture Intensity with P-Wave Velocity Anisotropy

Scott J. Wilkins
Shell International Exploration and Production Inc, Houston, TX

Fault-related strain mapped from a 3D seismic survey is used to formulate a 3D mechanical model for fracture prediction within a tight-gas, fracture play in shallow marine sandstones. Interpretation of the seismic data suggests the area has experienced two distinct phases of deformation, consistent with a regional tectonic synthesis, and thus a mechanical model is run for each phase of deformation to capture the complete spectrum of observed structures. Output from the model runs are analysed with respect to a modified Griffith-Coulomb failure criterion derived from a suite of laboratory experiments on the reservoir rocks.

Where the tensile criterion is satisfied, strain is converted to joint density using outcrop relations among bed thickness, joint height and aperture. Where the shear criterion is met, Coulomb Failure Stress (CFS) is used as a proxy for both probability and density. The model results predict shear failure to occur in dilational quadrants along the faults in response to the reduction of normal stress. Tensile failure is met in a halo surrounding these regions of predicted faulting, and exhibit a mean fracture intensity of 0.1 fractures/ft. and an average Fracture Spacing Ratio (FSR = mean layer thickness/median fracture spacing) of ~7.5. These FSRs correspond well with published outcrop observations where faults interact with jointed rock.

P-wave velocity anisotropy data provides supportive evidence for the model predictions in that the spatial distribution, density, and orientations of predicted fractures correlate well with those inferred from the seismic data.