Variation in Permeability of Rocks due to Transient Disturbances in Axial Stress or Pore Pressure

Le résumé fourni par la source

Large earthquakes can lead to persistent variations in the groundwater level in the near field. This would be due to the persistent variations in the permeability of the rock mass by permanent change in the strain through fault movement. These persistent variations in the groundwater levels may occur in the intermediate and even in the far fields which cannot be explained by variation in permeability associated with change in strain caused by fault movement because the change in strain is transient in the intermediate and far fields. The persistent variation in the permeability of rock mass caused by transient stress disturbances by earthquakes, if it exists, could explain not only the variation in groundwater level but also increase in petroleum production due to artificial vibrations, induction of small earthquakes by seismic waves, etc. in intermediate and far fields. However, it has not been verified whether the transient stress disturbances induce a persistent increase or decrease in the rock permeability.\nTo clarify the effects of transient stress disturbances on rock permeability, the study of three rock types, Kushiro Cretaceous sandstone, Shikotsu welded tuff, and Inada granite were carried out. The permeability values of intact and triaxially fractured Kushirio Cretaceous sandstone and Shikotsu welded tuff were measured before and after transient axial stress or pore pressure disturbances; the permeability of triaxially fractured Inada granite was measured under multiple transient disturbances in axial stress.\nThe permeability of the Kushiro Cretaceous sandstone decreased at zero stress disturbances. For the intact rocks, the permeability was kept almost constant with the disturbance amplitude. For the fractured rocks, the reductions became larger as the disturbance amplitudes increased. Regarding the pore pressure disturbances, the reductions in permeability of the intact rocks decreased as the pore pressure disturbances increased, whereas the reductions increased as the disturbance amplitudes increased for the fractured rocks. \nFor the Shikotsu welded tuff, the permeability decreased by fracturing. However, the change by the disturbances was small for all conditions. \nThe permeability of fractured Inada granite decreased with time in the tests. However, the permeability increased with each series of axial stress disturbances for amplitudes of 3 MPa or larger. The degree of increase in permeability increased with the axial stress disturbance amplitudes. The increased permeability decreased with time and recovered to its value before the disturbances. The time of recovery was longer for larger axial stress disturbance amplitudes. \nThe stress disturbances showed either decreasing or increasing effects on the permeability depending on the rock type and experimental conditions as stated above. However, when focusing on the fractured rocks which would be more important in field applications rather than the intact rocks, the argillaceous Kushiro Cretaceous sandstone mainly exhibited decreasing effects. The effects were mainly due to the closure of the rupture planes and the clogging of flow pathways by fine particles. In contrast, the glassy Shikotsu welded tuff showed small change. This would be because there were a lot of water flow paths in crushed glassy matrix. Permeability may not change even some flow paths were opened/closed or clogged/unclogged. The crystalline Inada granite mainly exhibited increase in permeability. The effects were mainly due to the opening of the apertures propped by hard fine particles, the enhancement of new microcracks, and unclogging of the fines from the flow pathways. Variations in permeability due to transient stress disturbances have already been employed in seismic enhanced oil recovery (EOR) technique expecting an increase in permeability due to the movement of entrapped fluid in the reservoir. It could also be used for evaluation of the change in sealing ability of nuclear waste disposal caverns, and further employed to enhance natural gas recovery, to reroute underground water flow for various purposes, to prevent large earthquakes by inducing many small earthquakes, etc. in the future.

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