Cyclical water vapor sorption-induced structural alterations of mine roof shale

Abstract

Understanding the nanoscale structural changes of shale in an unsaturated gas-water-shale system can shed light on the underlying mechanisms on water retention-induced shale deterioration. The simplified hypothesis here is that pore-scale shale structure can be passively altered by pore deformation/failure under cyclical water vapor treatments and can be described by a partially saturated effective stress law. Cyclical water vapor sorption tests were conducted on dried and water-saturated shale powders prepared by a hand-crushing (larger particle size) method and a cryogenic ball-milling (smaller particle size) method. Techniques including XRD, FESEM-EDS, and nanoindentation were used to provide evidence that the nano-porous shale is highly heterogeneous in nano-to micro-scale. The sorption curves of both samples with initial water-saturation treatments are higher than the raw samples, indicating enhanced sorption capacities in the treated samples. The degree of hysteresis among eight cycles (five cycles on raw sample and three cycles for treated sample) for both samples exhibit significant differences at low relative humidities, namely lower than ∼0.6. Very slight differences were observed among eight cycles when the relative humidity was higher than 0.6. An obvious decrease in the effective diffusion coefficients is observed in the treated samples compared to the raw samples, which is attributed to the physical alterations induced by the initial drying of water-saturated sample. In situ SANS data for shale at elevated relative humidity conditions suggest that the decrease in the scattering contrast between rock solid matrix and pores can be attributed to water capillary condensation. This occurs mainly in pores with radii smaller than ∼8.3 nm but larger than ∼1.25 nm and at a relative humidity of ∼59%. The capillary condensation continuously takes place in larger pores as the relative humidity increases to ∼87%. The results can ultimately provide the data for analyzing the interactions within an unsaturated air-water-shale system.