Stress State Analysis of a Fault Plane with Large Induced Seismicity

Mukuhira, Y. (Tohoku University) | Ito, T. (Tohoku University) | Asanuma, H. (Fukushima Renewable Energy Institute AIST) | Häring, M. (Geo Explorers Ltd.)

OnePetro 

Abstract:

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.

Introduction

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),.