Experimental investigation of water sensitivity effects on microscale mechanical behavior of shale

Drilling and multi-stage hydraulic fracturing bring a large amount of water into clay-bearing shale reservoirs, which may lead to attenuation of fracturing effects and wellbore instability. However, a thorough understanding of changes in shale micromechanics and corresponding mechanisms when exposed to water remains unclear. In this work, samples were selected based on clay enrichment. Subsequently, the contact resonance (CR) technique based on atomic force microscopy (AFM) was performed to characterize the micromechanics of shales after exposure to water. Visual phenomena provided by environmental scanning electron microscopy (ESEM) assisted to explain the underlying mechanisms. It was found that the hydration effect lowered both the storage modulus and stiffness of samples, but with different contributions from brittle minerals and clay, as well as variations depending on bedding plane orientation. Due to the difference in composition, the investigated terrestrial shale material exhibited stronger water sensitivity and anisotropy, with a general decrease of 15%-25% in storage modulus, compared with variations of -5%-15% in modulus of marine shale samples. Moreover, microscopic observation experiments revealed that the reduction of capillary force and the interlaminar swelling of clay particles might have been active after water adsorption, which led to the decreases of modulus and stiffness values. However, the swelling-caused confining effect or void space closure during the water imbibition process might have offset this weakening effect and even increased storage modulus. At mesoscale, excessive shrinkage caused the growth of micro-cracks, which in turn significantly attenuated overall mechanical properties.