Water saturation-driven evolution of helium permeability in Carboniferous shale from Qaidam Basin, China: An experimental study

Abstract

Permeability is an important parameter for describing the transport properties of gas shales. This study investigates the effects of water saturation on stress-dependent permeability and the slip factor of Carboniferous shales in the Qaidam Basin, China. To analyze correlations between water saturation, effective stress, helium permeability, and slip factor, helium permeability of 12 shale samples with different water saturation at pore pressures up to 6.89 MPa and effective stresses up to 27.58 MPa at 25 °C is measured. Subsequently, shale characteristics, including mineral composition, total organic carbon (TOC) content, thermal maturity, vitrinite reflectance, and pore structure distribution (PSD; pore volume and type), are determined. The analyses reveal a consistent (exponential) relation between apparent effective gas permeability (at 6.89 MPa) and effective stress. The apparent effective helium permeability (at 4.13 MPa) decreases exponentially with increasing water saturation, particularly in shale samples with a high intrinsic permeability. The critical water saturation value of shale is 30%–40%. Two different modes of gas–water two-phase flow are observed. When water saturation is ∼30%, helium permeability decreases sharply and gas–water flow in the shale samples appear as channel flow. However, when water saturation is >40%, helium permeability slowly decreases, gas–water flow in the shale samples appear as funicular flow, gas and water simultaneously flow in all capillaries, and thin films of structured water on mineral surfaces reduce the pore-throat diameter. Thus, increased water films in narrow flow paths increase the gas slippage effect. Furthermore, the slip factor initially decreased with increasing water saturation before increasing. Helium permeability exhibits a negative correlation with TOC and clay contents and a positive correlation with quartz content. The micropores and mesopores in samples with high clay content and TOC are favorable to gas adsorption but not to gas transport. Permeability measurements of humidity oven–dried samples reveal a power–law relation between the gas slip factor and Klinkenberg-corrected permeability; the slope of the resulting curve (−0.29) is lower than that obtained for low-permeability gas sands. This study provides practical information for further studies of shale gas migration and the gas slippage effect in certain gas–water flows.