Hydraulic fracturing is the completion method of choice in unconventional resource plays. Common laboratory protocols for measuring rock strength, Young's modulus, and Poisson's ratio typically do not account for moisture content in rocks, yet these parameters are critical in fracture designs and are greatly affected by rock moisture content. The process of water weakening is particularly complicated in shales because of the presence of both organic matter and inorganic minerals, such as clays, silica, and calcite. We study the effects of spontaneous fluid imbibition (brine and dodecane) on Young's modulus and hardness in shales using nanoindentation. The shales studied include Marcellus, Woodford, Eagle Ford, and Wolfcamp. A key objective was to compare the weakening effects of 2.5 and 7.5% KCl brine solutions vs. dodecane. Our measurements show that irrespective of the shale wettability, brine led to a greater reduction in Young's modulus (45% reduction in Marcellus, 25% in Woodford, 12% in Eagle Ford, and 21% in Wolfcamp) than dodecane (2.5% reduction in Marcellus, 17% in Woodford, 4% in Eagle Ford, and 3% in Wolfcamp). Increasing the concentration of the clay stabilizers, such as KCl, led to lower weakening. On the basis of these measurements, it seems that wettability also plays a role in water weakening. Marcellus, being strongly water-wet, experienced the greatest reduction in Young's modulus and hardness. On the other hand, the Eagle Ford samples, being predominantly oil-wet, experienced the least reduction in Young's modulus and hardness. The Wolfcamp and Woodford samples, being mixed-wet, experienced moderate reductions in Young's modulus and hardness.
