Fracturing index-based brittleness prediction from geophysical logging data: application to Longmaxi shale

Abstract

Understanding of brittle and ductile behavior in low permeability shale formation is essential for optimizing the completion and stimulation treatment in a shale play. For the quantification of brittleness in shale, several methods for estimating fracability index have been proposed in the literature. However, uncertainties still exist in the interpretation of fracability to decide where to place perforation clusters, what magnitude of fracability index mean to hydraulic fracture initiation and propagation, and how brittleness is best defined. This study aims to clarify these unresolved issues by introducing improved fracturing-index (FI) model after modifying Yuan et al. (SPE J 2017:1–9, 2007) fracability-evaluation model (Frac). Comparing with the use of Frac, the improved FI accurately predicts the brittle and ductile regions and provide a comparatively better idea about favorable fracturing sweet spots at TOC rich zones with added advantages of estimating efficient fracture initiation and propagation, and resistant to proppant embedment. In this study, mechanical rock properties were derived through acoustic logging data to develop mechanical earth models and designed brittleness templates for differentiating brittle and ductile regions in Longmaxi gas shale reservoir. The effectiveness of designed brittleness templates was verified through improved FI evaluation model and pre-existing fractures. The obtained result revealed that the points of high FI fall exactly into the predicted brittle regions and the points of low FI fall exactly into the predicted ductile regions. The applicability of the designed brittleness templates was further verified through the data from offset Well J-3, laboratory testing of several outcrop samples, and shale sample from different origin with varying composition.