| Title | Characteristic of Pore Pressure Migration Clarified by Multidisciplinary Microseismic Analysis |
|---|---|
| Authors | Yusuke MUKUHIRA, Takatoshi ITO, Hriokazu MORIYA, Hiroshi ASANUMA, Markus HARING |
| Year | 2020 |
| Conference | World Geothermal Congress |
| Keywords | EGS, microseismicity, pore pressure, in-situ stress, stimulation, shut-in, fluid flow, Basel |
| Abstract | Understanding the pore pressure migration during the stimulation is an essential factor to realize the Enhanced Geothermal System (EGS) type development avoiding the risk of unexpected risk of large induced seismicity and enhancing the sustainability of the reservoir. The fluid flow occurred due to the injection can be tracked by monitoring microseismicity. Standard microseismic hypocenter provides information of pore pressure increase to a certain level but does not provide quantitative information. We fully utilized the in-situ stress information of stress magnitude and orientation, and geometry of existing fractures estimated by microseismic analysis. Then, we inverted the pore pressure increase required to generate shear slip on an existing fracture during stimulation. Spatiotemporal analysis of estimated pore pressure increase provided a lot of critical observations about pore pressure migration during and after the stimulation. We observed the pore pressure redistribution at the shut-in phase, and pore pressure gradient from injection point disappeared causing the pore pressure distribution nearly uniform. We interpreted that the higher pore pressure stayed in the vicinity of the injection well migrated to far field and that this relative uniformization of the pore pressure caused the large induced seismicity which often occurred at the edge of the previously stimulated zone. Another observation from the spatiotemporal distribution of pore pressure is that lower pore pressure migrates farther and faster. Meanwhile, higher pore pressure migrates more slowly. We were able to interpret these phenomena successfully with the relationship between fracture permeability and state of stress. Existing fracture in critical condition is more permeable than those in less critical condition. So, low pore pressure migrates via permeable and critical fractures causing the seismicity. Then, permeabilities of those fractures are enhanced furthermore, promoting farther migration of low pore pressure. When wellhead pressure reached a certain point, the migration characteristic has changed. The higher pore pressure stagnated near the injection point started migrating to the far field, which decreased higher migration rate of lower pore pressure. From these observations, we interpret that there is the most productive wellhead pressure to promote the migration of low pore pressure in a larger region according to each EGS reservoir characteristics. |