The effect of mineral composition and rock fabric on brittleness index: an example using the Montney Formation, Canada

This study deals with brittleness index, which is a conceptual property that affects fracture behavior in rocks. Generally, more fracture networks are developed in more brittle rocks. For this reason, brittleness has been used as a guide to decide the target formation of hydraulic fracturing stimulation for shale and tight gas productions. Previous studies have been conducted to find out which factors have influences on the estimations of various brittleness indices to represent rock brittleness quantitatively. The objective of this study is to investigate the effect of rock fabric on the estimation of the mineralogy-based brittleness index. Both elastic moduliand mineralogy-based brittleness indices, which are the most commonly used indices, are obtained using available data from the Montney Formation, Canada. The former index is evaluated using the Young’s modulus and the Poisson’s ratio that are calculated from the sonic and density logs. The latter index is estimated based on the mineral composition from X-ray diffraction quantitative analysis. Comparison between the results from two different methods indicates that quartz might be the controlling mineral on the brittleness index of the lower Montney Formation. Thin section observations, however, show that the strength of the lower Montney Formation might be determined by dolomite and clay considering matrix-supported textures. This means that rock strength and brittleness may not be governed by same controlling factors. This suggests that rock fabrics, such as anisotropy due to the lamination or quantitative analysis of minerals which constitute grain, cement, and matrix through thin section observation, should be considered in addition to the mineral composition in order to evaluate mineralogy-based brittleness index more accurately.