--> Water Vapor Condensing in Anisotropic Pores of Shales by Neutron Scattering

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Water Vapor Condensing in Anisotropic Pores of Shales by Neutron Scattering


Shale-water interaction in micro/nano-pore scale plays a crucial role in influencing the flow and mechanical properties of shale gas reservoirs. However, understanding such an interaction is quite challenging due to the complex shale anisotropy and surface heterogeneity. In this study, we report water vapor condensing behavior in multiscale shale pores based on an in situ ultra-small and small angle neutron scattering (USANS/SANS) technique under various relative humidity (RH) - one gray shale rich in phyllosilicate minerals subjected to the RH of 0%, 41% and 83%, and one black shale rich in kerogen subjected to the RH of 0%, 10%, 27%, 60%, 72% and 88%. USANS and SANS studies on the two shale slices directly show a preferential orientation of the flattened pores along the beddings over the range of pore radii from 1 nm to a few microns. The degree of the preferential orientation, as quantified by an alignment factor, varies with the studied length scales (2.5-25 nm), with smaller mesopores (2.5-6.3 nm) behaving more favorably-aligned than larger mesopores (12.5-25 nm). Surface roughness for the two shales are also characterized, with the fractal dimensions of 2.74 and 2.85 respectively at the length scale of 2.5-250 nm. At this scale, water condensing in the two shales tends to “isotropize” pores through a form of thicker condensing films at the lateral sides of the flattened pores. The continuous decrease of pore anisotropy with RH for the studied mesopores (2.5-25 nm) indicates that wetting film is formed in a stepwise manner rather than completely fills in these pore bodies at the measured RH values. Besides, the black shale behaves a decrease of fractal dimension with RH, indicating that water condenses at strongly curved surface concavities to form a capillary wetting regime. The gray shale, however, presents a parallel wetting film covered at the whole pore surface without changing the surface roughness. This fundamental study shows direct evidence on microscale water condensing behavior in anisotropic shale pores. During soaking period, unrecoverable water in fractures further migrates into shale matrix to achieve an unsaturated state in equilibrium with the high gas pressure. The form of wetting film at pore surfaces at low partial pressure of water vapor may facilitate gas desorption and thus enhance the early gas production rate.