Wettability of CO2/brine/clay is one of the most important parameters in assessing CO2 storage capacities and containment security. Despite its importance, the literature data in this context are very limited. We thus systematically measured montmorillonite, illite, and kaolinite wettability for CO2/brine, nitrogen/brine, and nitrogen/oil systems at various pressures (5, 10, 15, and 20 MPa) and temperatures (305 and 333 K). The zeta potential of each clay mineral was also measured to investigate its link to the macroscopic contact angle. The results show that both advancing and receding water contact angles for CO2/brine, nitrogen/brine, and nitrogen/oil systems increase with an increase in pressure. However, they are only slightly reduced by increasing temperature. It was also shown that montmorillonite has a higher water contact angle in the presence of CO2, followed by illite and kaolinite. The same trend was measured for nitrogen/brine and brine/oil systems. Consequently, montmorillonite is strongly oil-wet; kaolinite and illite, however, are strongly water-wet at typical storage conditions (high pressure and elevated temperature). This has important implications for CO2 geostorage in determining the flow of CO2 and its entrapment, fluid spreading, and dynamics in the reservoir.

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Summary We present a methodology and describe a set-up that allows simultaneous acquisition of all five elastic coefficients of a transversely isotropic (TI) medium and its permeability in the direction parallel to the symmetry axis during mechanical compaction experiments. We apply the approach to synthetic shale samples and investigate the role of composition and applied stress on their elastic and transport properties. Compaction trends for the five elastic coefficients that fully characterize TI anisotropy of artificial shales are obtained for a porosity range from 40 per cent to 15 per cent. A linear increase of elastic coefficients with decreasing porosity is observed. The permeability acquired with the pressure-oscillation technique exhibits exponential decrease with decreasing porosity. Strong correlations are observed between an axial fluid permeability and seismic attributes, namely, VP/VS ratio and acoustic impedance, measured in the same direction. These correlations might be used to derive permeability of shales from seismic data given that their mineralogical composition is known.

Knowledge of anisotropic elastic properties of shales is important for understanding of shale compaction trends, improved seismic to well tie, nonhyperbolic moveout correction as well as for establishing a baseline for predicting properties of organic-rich shales. So far, however, building a predictive model of elastic properties of shales remains a difficult task. This might be explained by the multiparametric character of such modeling and the fact that the effects of some parameters cannot be measured and are thus poorly understood. The significant number of parameters required for prediction of the elastic properties of shale stems from its multicomponent nature. Shales are nanocomposite materials that comprise phyllosilicate clay particles with a substantial part of their grain size distribution smaller than 2 μm in radius and typically silt particles of quartz and feldspar with grain sizes between 2 and 60 μm (Mitchell & Soga, 2005). The complexity of the system is complemented by pores of micrometer to nanometer scale, whose shape and orientation are seldom characterized (Desbois, 2009). In the smallest of these pores, the filling water contributes to the stiffness and rigidity of the composite via electrostatic and van der Waals interactions (Holt & Kolsto, 2017). Attempts to model elastic properties of shales began with the classic work of Hornby et al. (1994) who modeled shale as a composite material that comprises anisotropic water-saturated clay blocks with a preferred orientation, silt inclusions, and free water. The orientation distribution function (ODF) of the clay crystals was estimated from scanning electron microscopy and the silt fraction from X-ray diffraction analysis or microtomographic images. The other unknown parameters, namely, elastic moduli of the clay blocks, aspect ratio of pores, and the fractions of free water and bound water in the clay blocks became fitting parameters.