Gas invasion into water-saturated, unconsolidated porous media: Implications for gas hydrate reservoirs

Gas transport through unconsolidated porous media has been a recent focus in studies of hydrate reservoir dynamics and is important in numerous other geophysical contexts. We conduct experiments to examine the controls on gas invasion through sieved sands that are suspended within the water column as air is injected slowly from below. We monitor the thickness of the free-gas column that accumulates beneath the sand to gauge the difference between the gas and liquid pressure, or overpressure, just prior to gas invading and passing through the sand layer. Comparing the threshold overpressures for initial gas invasion using different particle sizes, our experiments demonstrate the control of capillary forces when the effective stress supported by particle contacts is sufficiently large. At lower effective stresses (smaller sediment overburdens) we observe gas invasion before the capillary-controlled threshold overpressure is reached. These results demonstrate a transition from capillary invasion at high effective stresses to fracture-dominated invasion at low effective stresses. Moreover, video documentation reveals evidence for transitions from capillary to fracture-dominated behavior within single invasion episodes as the rising gas nears the sediment surface. After the initial invasion episode in each experiment, we infer that observed reductions in the overpressure needed for subsequent invasions are caused by bubble break-up and retention within the sediment column. These experiments highlight the importance of effective stress and pore size in determining the mode of gas passage through unconsolidated sediments. We discuss the implications of our findings for the dynamics of gas hydrate reservoirs and other geophysical systems.