Title:

Why Geothermal Energy Research Needs Statistical Seismology

Authors:

C.E. BACHMANN, S. WIEMER, B.P. GOERTZ-ALLMANN, B. MENA, F. CATALLI and J. WOESSNER

Key Words:

induced seismicity, Basel, statistical seismology

Geo Location:

Basel, Switzerland

Conference:

Stanford Geothermal Workshop

Year:

2012

Session:

Enhanced Geothermal Systems

Language:

English

Paper Number:

Bachmann

File Size:

671 KB

View File:

Abstract:

Hydraulic fracturing is an increasingly utilized technology to enhance the extraction of hot water or gas from a subsurface reservoir. Fluids are pressed into the reservoir formation from a treatment well at high pressures to open fractures and hence increase the permeability of the reservoir. Monitoring the seismic emission associated with the fluid injection allows to estimate the stimulated reservoir volume, and hence the effectiveness of the treatment. However, oftentimes little is known about the mechanical properties of the reservoir rocks, making it difficult to predict the response of the medium to the fluid injection. On the one hand, one would like to ensure that the fluid injection operation alters the medium sufficiently to make the reservoir economic, and on the other hand it needs to be insured that the magnitude of induced seismic events does not exceed values where shaking can affect surface infrastructure. A proper estimation of the in-situ mechanical properties of the reservoir is therefore necessary for an assessment of both the economics of reservoir treatment as well as the associated seismic hazard at the surface. We introduce ISHA – a probabilistic hazard assessment for induced seismicity, which combines statistical and physics-based models to determine the seismic hazard during the different stages of an EGS experiment. Statistical models have the advantage that they can easily be adapted to the particular conditions of the induced seismicity and that many have been previously tested for other seismic sequences. These models can then be enriched by first order physical models, for example of the pore pressure distribution or the total flow rate. We show examples from an EGS project in Basel, Switzerland, where 3,500 events were located within a volume of approximately 2 km³ at a depth of 4-5 km. We then investigate the spatio-temporal variability of the earthquake size distribution, characterized by the b-value, and find significant variations ranging from high values close to the injection point to lower values further away. Additionally, the b-value changes from high values during the fluid injection to lower values later on. A model, simulating the pore pressure diffusion and relating the event-sizes to the differential stress via an inverse relationship established for tectonic events, aims to evaluate this observation of the b-value distribution. The model implies that high pore pressures lead to high b- values as preferably events with smaller sizes are induced. Moderate pressures lead to values of b similar to the regional average. Since pore pressures decline as a function of distance to the injection point, the probability of observing a large magnitude event thus increases with distance. We are therefore able to establish a link between the seismological observables and the geomechanical properties of the source region and thus a reservoir. Understanding the geomechanical properties is essential for estimating the probability of exceeding a certain magnitude value in the induced seismicity and hence the associated seismic hazard of the operation.


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