Induced seismicity associated with hydraulic stimulation for the development of underground resources has been recognized as a risk factor in causing seismic hazards and is of public concern. To understand the simple physics behind induced seismicity, we analyzed the stress state of a fault plane where large seismicity was induced during hydraulic stimulation in the Cooper Basin, Australia and Basel, Switzerland. Using information regarding the stress magnitude and orientation, and the geometry of the fault plane where large events occurred, the stress state of these events was evaluated and the pore pressure necessary to cause shear slip was estimated. The fault plane of the large event in the Cooper Basin was close to being well oriented and only needed small increase in pore pressure (~10MPa) to induce shear slip. It was also discovered that the fault plane of the largest event at Basel required a moderate increase in pore pressure of around 20 MPa to induce a seismic event. Other large events occurring at different depths needed much lower pore pressures to induce shear slip. On the fault planes at Basel where these large events occurred, large shear stress was present, suggesting causality between shear stress and event magnitude.
Hydraulic stimulation is a necessary technology for the enhancement of formation permeability or improvement in system productivity in Enhanced/Engineered Geothermal System (EGS) projects. With the increasing demand for renewable energy, this technology has been used in many EGS projects to create economically feasible geothermal reservoirs. This method, also known as “fracking”, is used in the extraction of unconventional resources such as shale gas/oil. Fluid injection can induce shear slip on existing fractures or can initiate fractures leading to increased permeability. Acoustic energy released simultaneously with rock failure is often observed as induced seismicity, which is considered as the evidence relating these phenomena.
The growing number of large-magnitude, induced seismic events has recently been recognized as a serious problem associated with hydraulic stimulation in geothermal development (Majer et al., 2007). Although the magnitude of induced seismicity is typically less than 1.0, these large magnitude events have had moderate magnitudes (Mw 2~) and it is possible for them to be felt by local residents and also cause seismic hazards. Therefore, a quick development of methods to control seismic activity is necessary, and regulations or protocols based on scientific knowledge for sustainable and reliable geothermal development are required. However, many aspects of the physical mechanism related to large events remain unknown.
In modeling the propagation of injected fluid, as fluid flows through the flow paths in an existing fracture, pore pressure in the reservoir increases with pumping pressure. In an arbitrary existing fracture under tri-axial stress, this increase in pore pressure weakens the effective normal stress. When the shear stress working on a given fracture overcomes shear friction, the result is shear slip. This is known as the Coulomb failure criterion and is the principal behind induced seismicity. It can be described mathematically using equation (1),.
Yoon, Jeoung Seok (Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences) | Stephansson, Ove (Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences) | Zang, Arno (Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences) | Min, Ki-Bok (Seoul National University) | Lanaro, Flavio (SSM Swedish Radiation Safety Authority)
This study concerns dynamic simulations of earthquakes and the seismicity associated with induced slip of preexisting fractures in the rock mass for a repository for spent nuclear fuel planned to be constructed at Forsmark, Sweden. We use Particle Flow Code 2D (PFC2D) to model the repository volume that contains the faults and the pre-existing fracture system. The heterogeneous structure of the faults and the pre-existing fracture system are explicitly modelled in PFC2D using the smooth joint model. Earthquake at a fault is simulated by instantaneous release of the strain energy stored along the fault after build-up of the rock stresses. The release produces earthquakes and the seismic waves propagate and attenuate through the model. The earthquakes are simulated under different present day in situ stress conditions and under estimated future glacial cycles of Weichselian type. In particular, the time of ice cover and related forebulge, maximum thickness of the ice cover, and the retreat of the ice cover are considered. Modelling results demonstrate that the magnitudes and the stress drops of the induced seismic events associated with fracture slip tend to be the largest under stress condition of high anisotropy, in other words, where the ratio of the maximum and the minimum horizontal stresses is large. Among the seven tested in situ stress conditions, the occurrence of an earthquake under the stress condition at the time of forebulge in front of the ice cover is found to produce the largest induced moment magnitude (M -3).
Kumano, Yusuke (Tohoku University) | Asanuma, Hiroshi (Tohoku University) | Hotta, Akito (Tohoku University) | Niitsuma, Hiroaki (Tohoku University) | Schanz, Ulrich (Geothermal Explorers Ltd) | Häring, Markus (Geothermal Explorers Ltd)