Relationship between pressure, density of induced earthquake hypocenters, and permeability in the 2011 Paralana EGS stimulation
Abstract
To create an Enhanced Geothermal System (EGS), high pressure injection is undertaken to reactivate pre-existing fractures and enhance their permeability. During the 2011 Paralana-2 EGS stimulation in South Australia, both injectivity, the ratio of the injection rate to wellhead pressure, and seismicity were recorded. An increase in injectivity indicates that permeability has been enhanced, although it does not constrain the location or magnitude of the change. Induced earthquakes, a spatiotemporal dataset, can confine the range of possible scenarios for permeability evolution. We consider a model in which the number of hypocenters recorded per unit of area of the injection plane (the hypocenter density) is proportional to fluid pressure increase. Then an inverse modelling approach is employed to recover the permeability enhancement distribution that is consistent with both the recorded changes in injectivity and seismicity. Our forward model is radial Darcy-flow with permeability a prescribed function of time and distance, i.e., k(r,t). Initially, we identify a range of permeability evolution scenarios that reproduce the observed injectivity increase with time. Thus, injectivity observations on their own are insufficient to constrain k(r,t). Then, we calibrate k(r,t) for a close match between the modelled pressure distribution and that inferred from the hypocenter density observations using a simple proportionality constant. The resulting permeability model is the one most likely to approximate permeability evolution during the Paralana stimulation.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2016
- Bibcode:
- 2016AGUFM.S31B2753R
- Keywords:
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- 7209 Earthquake dynamics;
- SEISMOLOGYDE: 7230 Seismicity and tectonics;
- SEISMOLOGYDE: 8168 Stresses: general;
- TECTONOPHYSICSDE: 8488 Volcanic hazards and risks;
- VOLCANOLOGY