Several experiments and field observations have demonstrated seismic waves‐induced permeability increases. However, a few experiments have revealed significant decreases in the permeability of fractured sandstone after shaking produced by dynamic stresses. In this paper, we analyze the tidal responses of the water level in two wells (Junan and Dingyuan) situated on and near, respectively, the Tan‐Lu fault in China. The results sampled by a single well indicate that seismic waves could both increase and decrease the permeability. In addition, our results indicate that the earthquakes that decreased the permeability in a single well were distributed in certain azimuthal zones. Given the azimuthal distribution of distant earthquakes, the observed permeability decrease could be attributed to the seismic wave‐induced clogging of fractures that compose the flow paths in the shallow crust. Our research contributes to an enhanced overall understanding of the effects of earthquakes on the permeability of the shallow crust.
The mechanical properties are important for the evaluation of stratum deformation and instability in hydrate exploitation. By using an integrated test apparatus for synthesis of hydrate sediment and tri-axial tests, a series of compression tests is conducted on the silty clay containing tetrahydrofuran hydrate (SCTH) similar to that of South China Sea. The stress-strain curves and Mohr circles are obtained at different degree of hydrate saturations and confining pressures. The silty clay containing tetrahydrofuran hydrate shows typical ductile behavior, the shear strength increases linearly with the increase of hydrate saturation and confining pressure. The secant modulus increases with the increase of hydrate saturation. The shear strength is analyzed based on the Drucker-Prager criterion, Mohr-Coulomb criterion, Lade-Duncan criterion, while the secant modulus is analyzed using the mixed law of composite materials. It is shown that the strength can be well described by the Drucker-Prager criterion and Mohr-Coulomb criterion, and the secant modulus is close to the harmonic average of the modulus of soil skeleton and hydrate in SCTH.
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