The Effect of Boiling on Seismic Properties of Water‐Saturated Fractured Rock

Abstract

Seismic campaigns for exploring geothermal systems aim at detecting permeable formations in the subsurface and evaluating the energy state of the pore fluids. High‐enthalpy geothermal resources are known to contain fluids ranging from liquid water up to liquid‐vapor mixtures in regions where boiling occurs and, ultimately, to vapor‐dominated fluids, for instance, if hot parts of the reservoir get depressurized during production. In this study, we implement the properties of single‐ and two‐phase fluids into a numerical poroelastic model to compute frequency‐dependent seismic velocities and attenuation factors of a fractured rock as a function of fluid state. Fluid properties are computed while considering that thermodynamic interaction between the fluid phases takes place. This leads to frequency‐dependent fluid properties and fluid internal attenuation. As shown in a first example, if the fluid contains very small amounts of vapor, fluid internal attenuation is of similar magnitude as attenuation in fractured rock due to other mechanisms. In a second example, seismic properties of a fractured geothermal reservoir with spatially varying fluid properties are calculated. Using the resulting seismic properties as an input model, the seismic response of the reservoir is then computed while the hydrothermal structure is assumed to vary over time. The resulting seismograms demonstrate that anomalies in the seismic response due to fluid state variability are small compared to variations caused by geological background heterogeneity. However, the hydrothermal structure in the reservoir can be delineated from amplitude anomalies when the variations due to geology can be ruled out such as in time‐lapse experiments.

Temperature Effect on the Velocity‐Porosity Relationship in Rocks

“…This effect on attenuation of the thermodynamic interaction between bubble and liquid has been described by Grab et al. (2017).…”

“…Jaya et al (2010) considered that even a small quantity of bubbles in the pore fluid (very low-salinity water at a temperature above 125°C and 50 bars pressure) may provide a more realistic explanation of the wave attenuation behavior. This effect on attenuation of the thermodynamic interaction between bubble and liquid has been described by Grab et al (2017).…”

A Review of Models for Bubble Clusters in Cavitating Flows

“…In different geophysical problems it is important to identify the influence of the presence of gas in large scale systems such as marine ecosystems (e.g. oceans), where sonnars and seismometers are usually used to inferre the properties of the sediments and crustal layers, volcanos [187,218,168] and petrophysical systems where models are crucial to interpret seismic data to detect the presence of gas in geological strata [185,198,88]. The interesting reflection and transmission properties of diluted bubbly systems are currently used to design new metamaterials [129] and to protect offshore structures in processes such as drilling and controlled underwater explosions [89].…”

Picking up the hydrothermal whisper at Ischia Island in the Covid-19 lockdown quiet

“…In a water–vapor mixture, the sound speed of the two-phase fluid as a function of pressure ( P ) and temperature ( T ) is 53 : where = is the vapor–liquid mass fraction; , with ρ ν and ρ l being the vapor and pure liquid densities, respectively, R = 8.32 J K −1 mol −1 is the universal gas constant, M = 18.02 g mol −1 is the water molecular weight, and γ = 1.31 is the isentropic exponent of steam; is the bulk modulus of the pure liquid phase.…”

“…Sound speed curves as a function of pressure, temperature and vapor mass fraction have been derived from Eq. ( 4 ) by a number of authors 53 , 54 . By assuming pressure and temperature in the range [1–50] bar and [100–264] °C, respectively, which are reasonable values for the Ischia aquifer up to 500 m depth, the sound speed c spans over the range [1400–1000] m/s, for low (10 –7 –10 –6 ) vapor mass fraction.…”