Seismic signatures of partial steam saturation in fractured geothermal reservoirs: Insights from poroelasticity

Abstract

Detecting the presence of gaseous formation fluids, estimatingthe respective volumes, and characterizing their spatial distributionare important for a wide range of applications, notably forgeothermal energy production. The ability to obtain such informationfrom remote geophysical measurements constitutes afundamental challenge, which needs to be overcome to addressa wide range of problems, such as the estimation of the reservoirtemperature and pressure conditions. With these motivations, wecompute the body wave velocities of a fractured granitic geothermalreservoir formation with varying quantities of steam toanalyze the seismic signatures in a partial saturation context.We use a poroelastic upscaling approach that accounts for mesoscalefluid pressure diffusion (FPD) effects induced by the seismicstrain field, and, thus, describes the governing physicalprocessesmore accurately than standard representations. Changesin seismic velocities due to steam saturation are compared with changes associated with fracture density variations, as both areplausible results of pressure changes in geothermal reservoirs.We find that steam saturation has a significant impact on P-wavevelocities while affecting S-wave velocities to a significantlylesser extent. This contrasting behavior allows us to discriminatebetween fracture density and steam saturation changes by meansof P- and S-wave velocity ratio analyses. To evaluate the potentialof seismic methods to provide this information, a canonical geothermalreservoir model is used to compute the Rayleigh wavevelocity dispersion and seismic reflection amplitude variationwith angle (AVA) curves. These studies reveal that AVA analysesallow differentiating changes in fracture density from changesin steam saturation. We also note that the Rayleigh-wave-basedtechniques are much less sensitive to steam content changes thanto fracture density changes. Comparisons with elastic approachesindicate that including FPD effects through the use of a poroelasticmodel is crucial for the reliable detection and characterizationof steam in fractured geothermal reservoirs.