Research of water influence on macro and homogenized fracture toughness, brittleness and relationship between different mechanical properties of Longmaxi shale outcrop

Abstract

For better understanding the water influence on shale mechanical characteristics, chip, column, powder and semi-circular bend four types of shale sample from Longmaxi outcrop processed to 5 different moisture degree, are prepared to separately conduct SEM observation for in-situ location after processing, AFM test and nanoindentation for data of fracture toughness calculation, uniaxial/triaxial compression test for internal friction angle, nitrogen adsorption test for pore volume, SCB test for macro fracture toughness. An improved Energy-based method is proposed to calculate micro fracture toughness based on results from AFM test, in comparison with micro fracture toughness calculated with energy method and nanoindentation curves, the results of improved energy-based method are acceptable. By analogy with elastic modulus, a new method is proposed to calculate macro fracture toughness changing ratio with rising saturation based on micro fracture toughness computed with AFM test data including maximum force, maximum depth, contact area, hardness and elastic modulus. This method can be applied to computing macro brittleness changing ratio based on micro brittleness calculated with AFM test data, its changing pattern fits the pattern of brittleness computed by internal friction angle. Unlike fracture toughness getting down consistently, shale brittleness gets down initially then climbs up with rising saturation. But average value cannot represent macro level, the linear relationship between fracture toughness and Young's modulus disappears due to moisture influence. A comparative analysis between nanoindentation and AFM is conducted from three aspects, AFM is a better method to detect micro mechanical properties influenced by some factor at different degree.