Abstract
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 fluid
Highlights
Extraction of geothermal fluids can cause significant surface deformation, sometimes with magnitudes reaching the metre scale as observed at the Wairakei field in New Zealand (e.g. Hatton 1970; Allis 2000)
We focus on the relationship between fluid extraction and the volume change of the reservoir for cases where deformation is driven by changes in pore pressure
Considering how geothermal reservoirs evolve over time of exploitation and what the circumstances at the respective geothermal fields are, fluid conditions may play an important role in controlling the observed values of Vr/ Mf, t
Summary
Extraction of geothermal fluids can cause significant surface deformation, sometimes with magnitudes reaching the metre scale as observed at the Wairakei field in New Zealand (e.g. Hatton 1970; Allis 2000). We describe a simple methodology to help examine plausible parameter combinations and explain the magnitude of deformation for a given amount of extracted fluid To this end, we derive an equation that shows that the ratio of fluid-to-reservoir volume change depends on various reservoir properties, like the bulk compressibility of the reservoir rock and the compressibility of the pore fluid, as well as the fluid exchange with the surrounding formations (i.e. recharge in case of fluid extraction, leakage in case of fluid injection). Fialko & Simons 2000; Keiding et al 2010; Juncu et al 2017) This way, it seems that possible values for elastic rock properties can range over orders of magnitude, posing a challenge for deformation modelling in geothermal areas. Measurements of changes in pore pressure and the amount of extracted fluid can be used to better constrain reservoir conditions and estimate material properties, improving our understanding of reservoir processes
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