A seismic-properties and wave-propagation analysis for the long-term monitoring of supercritical geothermal systems

This study is focused on the modeling of the seismic response in a supercritical geothermal system subjected to a long-term cold and isothermal re-injection during 20 years of exploitation. The modeling aims to assess the usability of seismic-methods for monitoring supercritical geothermal reservoir. We use a synthetic data set for the analysis of the seismic properties of the reservoir: pressure and temperature evolution is derived from a previous numerical study of a two-dimensional supercritical reservoir with pure water as the pore fluid. We calculate the related variations in time of the seismic properties of the reservoir saturated-rock formation using the Burgers-Gassmann mechanical model to evaluate the effects of high temperature on the rock frame, and to account for fluid-properties variations. We observe that, at the analyzed pressure and temperature conditions, the seismic properties of the reservoir formation are mainly determined by the thermophysical characteristics of the pore fluid. During the long-term cold re-injection we observe a decrease in the compressional seismic velocity in the cooled area near the injection well, with a radius that increases in time. We use a 2D full waveform propagation algorithm based on the Burgers-Gassmann model to calculate synthetic signals in a single-well acquisition layout, with a time step of half year. We observe that in the cool injection scenario the changes in the anomalous area can be seismically monitored, while in the case of isothermal injection, changes due to the re-injection are not seismically appreciable.