The effect of fluid compressibility and elastic rock properties on deformation of geothermal reservoirs

A geothermal reservoir deforms when the extraction of pore fluid exceeds reservoir recharge, causing a decrease in pore pressure. The magnitude of this deformation is related to the amount of pore fluid that is extracted. Assuming compressible material properties in a homogeneous reservoir, we derive an expression for the ratio of reservoir volume change per extracted fluid mass. We show that this ratio depends on a number of parameters, notably the compressibilities of reservoir rock and pore fluid. We apply the obtained relationship to three different geothermal areas (Hellisheidi, Reykjanes and The Geysers) to illustrate under which circumstances the relation between reservoir deformation and the amount of extracted fluid is able to help us learn more about reservoir conditions. We find that the fluid compressibility, depending on whether the system is single-phase or two-phase, may explain large differences in estimates of reservoir volume changes per mass of extracted flui

Injection-induced surface deformation and seismicity at the Hellisheidi geothermal field, Iceland

Induced seismicity is often associated with fluid injection but only rarely linked to surface deformation. At the Hellisheidi geothermal power plant in south-west Iceland we observe up to 2 cm of surface displacements during 2011–2012, indicating expansion of the crust. The displacements occurred at the same time as a strong increase in seismicity was detected and coincide with the initial phase of geothermal wastewater reinjection at Hellisheidi. Reinjection started on September 1, 2011 with a flow rate of around 500 kg/s. Micro-seismicity increased immediately in the area north of the injection sites, with the largest seismic events in the sequence being two M4 earthquakes on October 15, 2011. Semi-continuous GPS sites installed on October 15 and 17, and on November 2, 2011 reveal a transient signal which indicates that most of the deformation occurred in the first months after the start of the injection. The surface deformation is evident in ascending TerraSAR-X data covering June 2011 to May 2012 as well. We use an inverse modeling approach and simulate both the InSAR and GPS data to find the most plausible cause of the deformation signal, investigating how surface deformation, seismicity and fluid injection may be connected to each other. We argue that fluid injection caused an increase in pore pressure which resulted in increased seismicity and fault slip. Both pore pressure increase and fault slip contribute to the surface deformation