Modelling geothermally induced ground deformation: thermo-poro-elasticity model for the 2005 - 2008 period in the Reykjanes geothermal field, SW Iceland

Abstract

The onset of operation of the Reykjanes power plant in May 2006 was associated with pressure and temperature changes, as a result of the extraction of geothermal fluids. Change in pore pressure and temperature results in deformation of the rock matrix that causes vertical displacement. In the time period 16/06/2005 - 16/08/2008, analysis of Interferometric Synthetic Aperture Radar (InSAR) images revealed a NE-SW elongated displacement field with a maximum amount of subsidence of approximately 8 cm. Within this MS-study, analytical and numerical thermo-poro-elastic deformation models were created that use modelled temperature and pressure changes from an iTOUGH2 reservoir model (provided by HS Orka) to calculate predicted vertical displacements induced by pressure and temperature changes. The analytical model was built in Python and calculates surface displacement under the assumption of a homogeneous half-space. The homogeneous drained bulk modulus (Kd) was varied throughout different analytical model runs until the lowest RMS-value of residuals (difference between predicted model displacements and observations) was found for a value of Kd = 15 GPa. The modelled displacement field was able to reproduce about 73% of observed displacement field, but showed a more circular pattern compared to the elongated observed displacement pattern. The relative influence of pressure and temperature on the resulting displacement field was evaluated. The models suggest that deformation in the 2005-2008 period was mostly due to pressure change, that accounts for about 93% of the total displacement. A numerical finite-element model was built in COMSOL Multiphysics, allowing for heterogeneity of poro-elastic parameters in the model domain. A hetergeneous drained bulk modulus was integrated into models and the influence of laterally and depth dependent elastic moduli was evaluated resulting in more elongated subsidence.