Causal fracture prediction: Curvature, stress, and geomechanics

Abstract

We propose scaling volume curvature measurements with material property estimates to produce a superior prediction of natural fractures. Curvature is one of many, indirect, fracture-inferring attributes. It does not detect fractures, but is causally related to them through the assumption that increasing curvature relates to increasing strain. There are many other variables that are causally related to fractures. We propose that it would be advantageous to create combinations of these causal variables with curvature. Some of the most well known and important causes of variations in natural fracture density are material properties relating to brittleness. Material properties are critical geologically at all scales, from large-scale regional studies to prospect-level inquiries because the properties may vary significantly within individual formations and between formations. These vertical and lateral changes in material properties may be important and should be considered in fracture estimation, along with curvature. There is a lack of clarity regarding exactly which material property is best from the perspective of physics and rock mechanics; however, we have chosen a combination of parameters that we argue is a starting point. Fortunately, material properties are routinely estimated with amplitude variation with offset (AVO) techniques, and there is little practical reason not to use them together with curvature to produce a more complete attribute inferring fracture density. The combination of these variables is a step in the direction of creating quantitative causal fracture prediction estimates.