Abstract
The brittleness index (BI) is a key parameter used to identify the desirable fracturing intervals of shale gas reservoirs. Its correlation with fracability is still controversial. There have been a variety of methods proposed that can estimate BI. The brittleness evaluation method based on stress-strain curves according to the energy-balanced law is the most suitable and reliable in this study. Triaxial compression test, optical microscopy and scanning electron microscopy (SEM) observation, and X-ray diffraction analysis (XRD) were performed on nine drill core samples from well SY3 located in the peripheral regions of Sichuan Basin, China. These tests further evaluated several commonly used methods (brittleness indices based on rock elastic parameters, rock mineral compositions) and determined the relationship between brittleness, rock elastic parameters, and the content of minerals. The results obtained indicate that for sedimentary rocks, a higher Young’s modulus reduces the brittleness of rock, and Poisson’s ratio weakly correlates with brittleness. Excessive amounts of quartz or carbonate minerals can increase the cohesiveness of rock, leading to poor brittleness. Furthermore, the most suitable fracturing layers possess a high brittleness index and low minimum horizontal stress.
Highlights
Shale gas reservoirs are remarkably different compared to conventional reservoirs due to their ultra-low porosity and permeability
Optical microscopy and scanning electron microscopy (SEM) observation, and X-ray diffraction analysis (XRD) were performed on nine drill core samples from well SY3 located in the peripheral regions of Sichuan Basin, China
The brittleness index assessment methods based on rock elastic parameters and mineral compositions lack a scientific theory basis
Summary
Shale gas reservoirs are remarkably different compared to conventional reservoirs due to their ultra-low porosity and permeability. It is of utmost importance to hydraulically fracture the unconventional oil and gas reservoirs for commercial production. The identification of candidates for prospected fracturing in shale gas reservoirs is necessary to enhance gas productivity. Brittleness index is an essential mechanical parameter that has been used to evaluate whether shale gas reservoirs can form complex fracture networks [1,2]. Shales with high brittleness tend to close hydraulically created fractures slowly [3]. The quantitative evaluation of brittleness is of considerable significance to the optimization of fracturing intervals
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