Effects of aqueous nanoparticle suspension injection on a shale’s mechanical properties

This paper presents the first effort to unravel and quantify the strengthening of a shale induced by nanoparticle injection from the perspectives of cross-scale and multi-constituent mechanical properties. After being subject to injection of pure water and an aqueous suspension of carbon black nanoparticle of 50 nm in diameter under a differential pressure of 850 kPa, the shale specimens were characterized by big data nanoindentation (BDNi) to probe the mechanical properties of both individual constituents at the microscale and the bulk rock at the macroscale, leading to comparatively assessing the effects of injecting pure water and aqueous nanoparticle suspension on the mechanical properties. Microstructural characterization by electron microscopy and X-ray computed tomography validates the successful injection of nanoparticles into the microcracks and micropores of the rock. While the nanoparticles can infiltrate to depths of up to 100 s μm in zones with densely populated microcracks, the maximum depths of injection in crack-free zones are only 2–5 μm. Moreover, the injected nanoparticles mostly act as inert fillers in the interconnected micropores and microcracks but can seldom enter the isolated micropores. Comparison of the BDNi results from pure water versus nanoparticle-injected specimens shows that the Young’s modulus of the clay matrix experiences the highest increase by 23.1