Abstract
Hot dry rock (HDR) geothermal energy has many advantages, such as being renewable, clean, widely distributed, and without time and weather limitations. Hydraulic fracturing is usually needed for the exploitation of HDR geothermal energy. It has many hidden faults in the reservoir/caprock sequences. Injecting fluid into underground formations during hydraulic fracturing often induces fault slip and leads to earthquakes. Therefore, to well understand the induced fault slip and earthquakes is important for the applications and development of HDR geothermal exploitation. In this study, we investigated the hazardous injection area of the induced earthquakes during hydraulic fracturing. The study was based on a hydraulic fracturing test in Qiabuqia geothermal field in China. According to the field, a fault-surrounding rock-fracturing region system was developed to study the influences of fluid injection on the stability of the specific fault. A total of 60 hydraulic fracturing regions and 180 numerical experiments were designed. The results revealed that the hazardous injection regions that threaten the fault’s stability were near to the fault and concentrated on the following four areas: (a) above the top of the fault in underlying strata; (b) above the top of the hanging wall of the fault in underlying strata; (c) near to the fault planes in both footwall and hanging wall; (d) at the bottom of the footwall of the fault in underlying strata. The hazardous injection area can be controlled effectively by adjusting the injection pressure.
Highlights
Geothermal energy has great potential to simultaneously solve the energy crisis and climate problems due to its significant advantages
Note that region TML1 was on the left of TM and region TMR1 was on the right of TM. e results may reveal that injecting fluid above the hanging wall of the fault may induce a larger earthquake than injecting fluid above the footwall of the fault
When the fault is connected to the injection area, the main cause of fault reactivation and slip is the reduction of effective stress and the weakening of the physical and mechanical properties of the fault induced by fluid injection
Summary
Geothermal energy has great potential to simultaneously solve the energy crisis and climate problems due to its significant advantages. Massive fluid injection during hydraulic fracturing and the subsequent heat mining often induce seismicity, which may delay or hinder the acceptance of EGS. E massive fluid injected into underground formations can often induce fault reactivation and slip. The induced fault slip can occur when the fault does not exist within the subsurface targeted zone (no longer the problem of effective stress) [23]. It revealed that induced earthquakes may extend far away from the hydraulically connected region Both pore-pressure increase (direct pressure effect) and stress perturbation (indirect pressure effect) due to fluid injection can cause fault slip and induce earthquakes. (1) e influence of the spatial distribution of injection zone on induced fault slip and magnitude of seismicity. For a given fault in the site, it is necessary to find out which injection zone is more likely to lead the fault to slip and induce seismicity. (2) e influence of the change of distance between the injection zone and fault on fault slip. (3) e dangerous injection region significantly affects the fault. (4) How to reduce the EGS-induced fault slip and the magnitude of seismicity
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