QUANTIFYING CHANGES OF GEOPHYSICAL PROPERTIES IN SMALL REGIONS WITHIN EGS RESERVOIRS

摘要

Double-difference elastic-waveform inversion is a promising tool for quantitatively monitoring reservoir changes in enhanced geothermal systems (EGS). The traditional Tikhonov regularization used in waveform inversion tends to smooth reconstruction results and gives inaccurate values of changes of geophysical properties in small regions within EGS reservoirs. We develop a new double-difference elastic-waveform inversion method with a modified total-variation (TV) regularization to improve the quantification of reservoir changes within small regions. The double-difference elastic-waveform inversion method inverts for reservoir changes utilizing differences of time-lapse seismic data and the compressional- (P-) and shear-wave (S-wave) velocity models obtained using waveform inversion of the baseline data. The modified TV regularization improves not only the reconstruction of the baseline models, but also reservoir changes, particularly in small regions within EGS reservoirs. We use synthetic time-lapse seismic data for a Brady's EGS model to verify our new method. Our numerical results clearly demonstrate that our double-difference elastic-waveform inversion with a modified total-variation regularization scheme can accurately reconstruct the changes in small regions within an EGS reservoir.