Analysing induced seismicity in geothermal reservoirs: a modification of the Hall Plot
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The Habanero-1 well in the Cooper Basin, Australia and RRG-9 well in the Raft River geothermal field in the United State are two examples of an enhanced geothermal system. Both underwent hydraulic stimulations and experienced notable amounts of injection induced seismicity. Hall’s method plots the time integrated wellhead pressure versus the cumulative injection volume, with changes in the slope of the plot indicating changes in injectivity. In this paper, we analyse the induced seismicity using a modified version that plots cumulative earthquake count versus cumulative injection volume. This test provides a simple graphical interpretation of spatiotemporal features of the seismicity and any changes that occur during injection. The modified method is applied to datasets from the Cooper Basin and Raft River to analyse the seismicity and correlate this with changes in reservoir properties. In the case of Habanero-1, a linear trend is obtained indicating that the number of induced seismic events is directly proportional to the cumulative volume of water injected, however the high level of induced seismicity appears to be decoupled from any change in permeability. In contrast, at RRG-9, a piecewise linear slope was obtained indicating that, unlike Habanero-1, the productivity of seismic events with injected volume changed during the stimulation.