LAUSR

Detecting the presence of gaseous formation fluids, estimating the respective volumes, and characterizing their spatial distribution is important for a wide range of applications, notably for geothermal energy production. The ability to obtain such information from remote geophysical measurements constitutes a fundamental challenge, which needs to be overcome to address a wide range of problems, such as the estimation of the reservoir temperature and pressure conditions. With these motivations, we compute the body wave velocities of a fractured granitic geothermal reservoir formation with varying quantities of steam to analyze the seismic signatures in a partial saturation context. We employ a poroelastic upscaling approach that accounts for mesoscale fluid pressure diffusion (FPD) effects induced by the seismic strain field, and, thus, describes the governing physical processes more accurately than standard representations. Changes in seismic velocities due to steam saturation are compared with changes associated with fracture density variations, as both are plausible results of pressure changes in geothermal reservoirs. We find that steam saturation has a significant impact on P-wave velocities while affecting S-wave velocities to a significantly lesser extent. This contrasting behavior allows to discriminate between fracture density and steam saturation changes by means of P- and S-wave velocity ratio analyses. To evaluate the potential of seismic methods to provide this information, a canonical geothermal reservoir model is employed to compute Rayleigh wave velocity dispersion and seismic reflection amplitude vs angle (AVA) curves. These studies reveal that AVA analyses allow to differentiate changes in fracture density from changes in steam saturation. We also note that Rayleigh-wave-based techniques are much less sensitive to steam content changes than to fracture density changes. Comparisons with elastic approaches show that including FPD effects through the use of a poroelastic model is crucial for the reliable detection and characterization of steam in fractured geothermal reservoirs.