2019 年 15 巻 1 号 p. 13-16
Knowing how pore pressure migrates at the fluid injection is essential to understand various phenomena in the reservoir, such as induced seismicity and fluid flow behavior. We took the privilege of our study, Basel Switzerland, where principal stress magnitude and orientation are available. We estimated the pore pressure required to cause shear slip using Coulomb failure criteria from stress information, the geometry of the fault planes of microseismic events, and a constant coefficient of friction. We performed a time series analysis of pore pressure distribution, and the results indicated that lower pore pressure migrated faster and further during the stimulation. Increasing injection pressure change the pressure gradient from the injection well to the edge of the stimulated zone. Higher pore pressure stared migration according to the increase of injection pressure, which slowed the migration of lower pore pressure. Our observations suggest the best sufficient injection pressure for each reservoir, and it can stimulate the existing fractures effectively and safely. We also found pore pressure migrated to the far field even after shut-in and redistribution of pore pressure at shut-in brought sufficient pore pressure increase to induce seismicity in the peripheral region. After the shut-in, the pore pressure gradient away from the well lessened, and eventually pressure became almost uniform. These observations suggest that pore pressure redistribution due to shut-in caused uniformly distributed pore pressure at the edge of the seismic zone. Shut-in pressure destabilized a large part of the fault existed in those zones, resulting in the outbreak of larger induced seismicity at the shut-in phase.