A Unique Mechanical Method to Predict the Density and Distribution of Natural Fractures from Well Logs
Welch, Michael; Davies, Russell K.; Knipe, Rob J.
An understanding of the density, distribution, height and vertical connectivity of natural fractures can be key to producing from fractured reservoirs. However it is usually not possible to determine these parameters directly, as these fractures are not visible on seismic and it is difficult to estimate fracture density, length and connectivity from 1D wellbore data. We have developed a mechanical model of fracture nucleation and propagation to predict the density and vertical extent of dilatant fractures in a mechanically layered section, based on mechanical properties derived from wireline logs. This provides a method of estimating key factors such as the height of fractures in a shale gas reservoir, for example, and the degree of fracture connectivity across the layers.
In this presentation we demonstrate a workflow to estimate fracture nucleation and propagation across layers of brittle shale and thin micrite layers in an outcrop as a calibrated example. We show how this method can be applied to the subsurface estimating the mechanical properties from well logs. Initially we calculate key mechanical properties, including Young's Modulus, UCS, crack surface energy and friction coefficient throughout the interval of study from density, porosity and sonic velocity logs, using published algorithms calibrated against mechanical property measurements from samples, and estimate a ductility index from the overconsolidation ratio. These parameters separate the brittle horizons where fractures are likely to nucleate, from the ductile layers which may act as barriers to fracture propagation. We combine the estimated total horizontal strain with the displacement on each fracture to derive the maximum fracture density required to accommodate the applied strain.
A key parameter controlling fracture development is the depth of burial at the time of fracturing, which will affect both the mechanical properties and the effective stress at the time of fracturing. Recalculating the mechanical properties based on decompacted porosity and density values, demonstrates the impact of the early deformation on fracture density, distribution and connectivity.
These models provide a simple mechanical approach to the prediction of fractures from well logs and an estimated mechanical stratigraphy.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013