| Title | Migration of Shut-in Pressure and Its Effect to Occurrence of the Large Events at Basel Hydraulic Stimulation |
|---|---|
| Authors | Yusuke MUKUHIRA, Hiroshi ASANUMA, Markus HÄRING |
| Year | 2016 |
| Conference | Stanford Geothermal Workshop |
| Keywords | induced seismicity, large magnitude, shut-in, Basel |
| Abstract | The occurrence of induced/triggered seismicity is recognized as a serious environmental burden associated with hydraulic stimulation at Enhanced Geothermal System (EGS) development. We have conducted physics based study for understanding the physical process of the large induced events. Our fundamental analysis on Basel microseismic data set investigated the characteristics of the large events and we have revealed that the several large events occurred just after the shut-in phase. Large events at shut-in phase has become interest of many scientists to understand behind the physics. We took geomechanical approach for this research topic and implemented research to understand the reason of occurrence of the large events at the shut-in phase. We analysed microseismicity at shat-in phase and revealed fundamental characteristics of shut-in events. Seismic cloud extended around 100 m even after shut-in phase and many events occurred at periphery of previously stimulated zone. Simultaneously, few seismic events occurred within the previously seismically activated region. We estimated pore pressure increase necessary for shear slip on the fault planes. The stress orientation and magnitude at Basel has been investigated and Fault Plane Solutions (FPS) of the large events were estimated by other research group. We compiled this information and calculated shear/normal stress working on the fault plane. Eventually we computed pore pressure increase using Coulomb failure criterion with constant friction coefficient of 0.85. Estimated pore pressure had acceptable value comparing well head pressure. Under the assumption that all seismic events were triggered only by pore pressure increase, pore pressure increase was migrated on hypocenters of the events in time series and space. We found pore pressure at the edge of seismic cloud increased higher than during stimulation and pressure gradient from injection well to outside of reservoir no longer existed after shut-in. We interpreted that this caused migration of shut-in pressure. Therefore, there should be causality between migration of pore pressure and occurrence of the large events. We, at the present, interpreted that the shut-in pressure penetrated most part of the fault plane of the larger events and large part of existing fracture of the larger events reached critical state. Finally, this caused large scale of shear slip, which was not able to occur during the stimulation due to pressure gradient. |