Gas migration through a soft granular material involves a strong coupling between the motion of the gas and the deformation of the material. This process is relevant to a variety of natural phenomena, such as gas venting from sediments and gas exsolution from magma. Here, we study this process experimentally by injecting air into a quasi-2D packing of soft particles and measuring the morphology of the air as it invades and then rises due to buoyancy. We systematically increase the confining prestress in the packing by compressing it with a fluid-permeable piston, leading to a gradual transition in migration regime from fluidization to pathway opening to pore invasion. We find that mixed migration regimes emerge at intermediate confinement due to the spontaneous formation of a compaction layer at the top of the flow cell. By connecting these migration mechanisms with macroscopic invasion, trapping, and venting, we show that mixed regimes enable a sharp increase in the average amount of gas trapped within the packing, as well as much larger venting events. Our results suggest that the relationship between invasion, trapping, and venting could be controlled by modulating the confining stress.
Bibliographical noteFunding Information:
This research was supported by the Royal Society under the International Exchanges Scheme (Grant No. IE150885), by the European Research Council (ERC) under the European Union's Horizon 2020 Programme (Grant No. 805469), and by the European Union under the Erasmus Program (to R.L.). The authors also acknowledge support from the John Fell OUP Research Fund (Grant No. 132/012) and the Maurice Lubbock Memorial Fund. The authors thank Rob Style for helpful discussions about fracture mechanics.