Gas Shales Testing in Controlled Partially Saturated Conditions

Laboratory measurement of mechanical properties of gas shales represents a challenging topic in geomechanics due to their peculiar features such as heterogeneity, anisotropy, low permeability, and complex physical phenomena related to the hydro-mechanical coupling. Since these geomaterials are partially water saturated in their native state and in-situ changes in water saturation can occur due to drilling and fracturing procedures, the role of partial saturation has to be considered for the experimental assessment of their geomechanical parameters. The paper provides quantitative experimental data that demonstrate the impact of changing partial water saturation on the static elastic and strength (UCS) properties of gas shales from two different unconventional plays. To achieve this goal, a testing set-up and experimental methodology based on the control of total suction were specifically developed. Two core samples from different unconventional shale gas reservoirs were tested. The obtained results clearly show a significant dependence of the Young’s modulus on total suction, where a non-linear decrease of stiffness higher than 50% can be observed when suction is reduced during a wetting process. On the other hand, the Poisson’s ratio is not highly influenced by different saturation states; it can be considered constant for suction values greater than 10 MPa. A significant impact on the uniaxial compressive strength was also observed, where a 20% decrease of this strength parameter was measured when gas shales are conditioned to lower suction value. To include the observed behavior of gas shales in elastic constitutive model, an empirical relationship to express the dependence of Young’s modulus on suction is proposed. Besides a better understanding of the behavior of gas shales during the drilling procedures and hydraulic fracturing phase, the findings of this study are also of great relevance to understand the impact that exposure of core samples to non-controlled air humidity may have on the results of laboratory tests.