Brittleness Index: From Conventional to Hydraulic Fracturing Energy Model

Abstract

The success of a hydraulic fracturing (HF) operation is strongly dependent on brittleness of the formation. Several models based on mechanical testing, mineral composition, and sonic log data have been proposed to quantify the brittleness of formations known as brittleness index (BI). The limitations of these conventional BI models, in particular, the consistency and applicability at field scale with the complex in situ conditions are poorly understood. We therefore developed a novel BI model based on the hydraulic fracture propagation energy (HFPE) criterion. A set of hydraulic fracturing (HF) experiments was conducted on samples with different mineralogy at true tri-axial stress conditions for model definition, i.e., the scaling law was employed to ensure that hydraulic fracture propagation resembles the field conditions. To assess the performance of the proposed BI model in predicting the rock fracability, the obtained results of the model on different samples were compared with conventional models. It was shown that the predictions of the conventional BI models are predominantly related to the failure characteristics of the rock rather than its fracability. We showed that such BI models cannot assess the hydraulic fracturing feasibility where complex failure mechanisms (i.e., splitting, shear, friction, etc.), geological condition (i.e., in situ stress) and operational factors (i.e., injection rate, fluid viscosity, etc.) are involved. The new proposed BI model, however, successfully predicted the fracability of different rock types.