Water retention behavior of a gas shale: Wettability-controlled water saturation and anisotropic hydromechanical response

Abstract

Gas shales are fine-grained, organic-rich sedimentary geomaterials with ultra-low permeability requiring hydraulic stimulation for gas extraction. Characterizing their water retention behavior is critical for predicting hydromechanical behavior and fluid flow, yet it remains challenging due to their complex pore network and mixed wettability. This study investigates the water retention behavior of a gas shale through comprehensive characterization and laboratory tests, where water content and strains both perpendicular and parallel to the bedding plane were measured over two wetting-drying cycles. The results suggest that the coexistence of hydrophilic clay minerals and hydrophobic organic matter limits water access to parts of the pore and microcrack network, resulting in incomplete saturation even at a null suction. Three water retention models were modified by introducing an additional parameter to account for this wettability effect, among which the van Genuchten model provided the best overall fit. The fitted curves revealed a surprisingly low air entry value, underscoring the role of percolated hydrophobic networks in facilitating gas flow. The swelling strains indicated irreversible opening of bedding-parallel microcracks. The shrinkage strains were reversible, better representing the elastic hydromechanical anisotropy. Comparisons with other shales revealed that shrinkage anisotropy correlates more strongly with burial depth than with clay fraction, suggesting that compaction and diagenesis may play a more critical role than the amount of clay. Wettability may reduce the impact of pores and microcracks on shrinkage anisotropy. These findings emphasize the need for advanced constitutive models for gas shales that incorporate the observed wettability-controlled water saturation and hydromechanical anisotropy.