Simulation of dynamic piezomagnetic response during gas injection and extraction process in gas storage

The seepage of pore fluids in subsurface rocks under a pressure gradient can alter the stress state of the strata and induce magnetic variation through the piezomagnetic effect. This is a possible reason for the generation of geomagnetic field anomaly. Previous studies on the piezomagnetic effect mainly focused on the steady magnetic field changes caused by static stress, while the dynamic piezomagnetic response associated with fluid seepage processes has not been adequately studied. In this paper, we employ the finite element method to simulate the dynamic piezomagnetic response during gas injection and extraction processes, and investigate the characteristics of the piezomagnetic field. First, using a homogeneous model, we simulate the piezomagnetic response during the short-term rapid gas injection and the long-term continuous gas injection and extraction, respectively. Second, we investigate the effects of initial intensity and declination of the magnetization in local surface regions on the distribution of the surface magnetic field. Finally, we simulate the variations in the surface magnetic field induced by piezomagnetic effects during the gas injection and extraction processes in the Hutubi underground gas storage. The results indicate that: (1) For short-term rapid gas injection, the magnetic field reaches its peak at the end of the injection and subsequently decays gradually over time. The peak amplitude of the magnetic field depends on the rock permeability, while the peak time is nearly unaffected by the permeability. In contrast, for long-term continuous gas injection and extraction, the magnetic field reaches its peak several days after the injection rate attains its maximum, followed by a gradual decay. In this case, both the peak amplitude and peak time are controlled by the permeability. (2) The total intensity of the surface magnetic field is jointly regulated by the initial intensity and declination of the magnetization of the rocks, whereas the polarity distribution is primarily determined by the magnetization direction. (3) In the Hutubi area, the simulated surface magnetic field intensity exhibits an initial increase followed by a decrease during the injection-extraction process, which shows good consistency with the magnetic field changes observed during the injection period. The results of the present study provide a theoretical basis for future applications of geomagnetic anomalies in the inversion of the subsurface fluid migration, and also offer valuable insights into the mechanisms of the geomagnetic anomalies observed prior to earthquakes.