--> Monitoring Of The Ground Surface Deformation Induced By Geothermal Production At Reykjanes, Iceland, Using Interferometric Analysis Of Synthetic Aperture Radar Images (Insar)

AAPG European Region, Geothermal Cross Over Technology Workshop, Part II

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Monitoring Of The Ground Surface Deformation Induced By Geothermal Production At Reykjanes, Iceland, Using Interferometric Analysis Of Synthetic Aperture Radar Images (Insar)

Abstract

The Reykjanes geothermal system is a high-temperature seawater system situated at the southwestern tip of the Reykjanes Peninsula, in Iceland. Sentinel-1 satellite data have been used to determine the cumulative ground surface deformation induced by geothermal utilization between April 2015 and October 2017, using Interferometric Synthetic Aperture Radar (InSAR) methods. Using SAR images acquired from ascending and descending satellite orbits, with different viewing geometries, we infer the approximate vertical and east displacement rates from coherent pixels in series of interferograms. They reveal a steady and linear subsidence within a sub-circular bowl centered on the well field, at a maximum rate of about 16 mm/yr in the line-of-sight (LOS) of the satellite. The characteristics of the deforming source were inverted using simple analytical models assuming the geothermal reservoir behaves as a pressure source embedded in an elastic half space. The results indicate a body at about 1 km depth contracting at a rate of -0.9 x 105 m3/yr over the two years as responsible for the deformation. Such geodetic results can be compared to various information, (e.g. production, pressure and temperature monitoring data, reservoir structure, rock properties), in order to get a better understanding of processes taking place in a harnessed geothermal system. In accordance with previous deformation results for the period 2009-2016, the present results confirm the decline in the subsidence rate in association with the decrease in production since the end of 2008. The maximum deformation rates were indeed observed within the two years following the commissioning of the 100MWe power plant (2006-2008). The deformation was initially elongated in the NE-SW direction and modelled as a 2.2km deep ellipsoidal source contracting at an average rate of -7.6 x 105 m3/yr. It appeared to correlate with a 35 bars pressure drop observed at 1.6 km depth along the NE-SW striking Reykjanes Fissure Swarm. Since reinjection started in 2009, a decrease in the rate of pressure decline has been observed at 1.6 km, followed by a slight increase in pressure since 2015. Extraction of steam from the steam cap formed above 1.1km depth as a result of the initial pressure drop also started in 2009. Since then, slight cooling has been suggested in the upper part of the system. We suggest a pressure drawdown within a steam cap near the top of the geothermal reservoir may explain the migration of the modelled source of deflation from 2.2 km to 1 km depth and the change in the deformation pattern since 2009. The increase in the pore fluid compressibility induced by the replacement of liquid water by steam together with thermal contraction of the upper-most volcano-sedimentary rocks caused by the re-condensation of steam or reinjection may explain the continuation of the subsidence, despite observed deep increase in pressure. InSAR is therefore a valuable tool to support the long-term monitoring of geothermal system, both to characterize the structure of the system and for revealing changes due to production.