Discovering geothermal supercritical fluids: a new frontier for seismic exploration

Abstract

Exploiting supercritical geothermal resources represents a frontier for the next generation of geothermal electrical power plant, as the heat capacity of supercritical fluids (SCF),which directly impacts on energy production, is much higher than that of fluids at subcritical conditions. Reconnaissance and location of intensively permeable and productive horizons at depth is the present limit for the development of SCF geothermal plants. We use, for the first time, teleseismic converted waves (i.e. receiver function) for discovering those horizons in the crust. Thanks to the capability of receiver function to map buried anisotropic materials, the SCF-bearing horizon is seen as the 4km-depth abrupt termination of a shallow, thick, ultra-high (>30%) anisotropic rock volume, in the center of the Larderello geothermal field. The SCF-bearing horizon develops within the granites of the geothermal field, bounding at depth the vapor-filled heavily-fractured rock matrix that hosts the shallow steam-dominated geothermal reservoirs. The sharp termination at depth of the anisotropic behavior of granites, coinciding with a 2 km-thick stripe of seismicity and diffuse fracturing, points out the sudden change in compressibility of the fluid filling the fractures and is a key-evidence of deep fluids that locally traversed the supercritical conditions. The presence of SCF and fracture permeability in nominally ductile granitic rocks open new scenarios for the understanding of magmatic systems and for geothermal exploitation.

Tectonic and Anthropogenic Microseismic Activity While Drilling Toward Supercritical Conditions in the Larderello‐Travale Geothermal Field, Italy

“…Using passive seismic data, Piana Agostinetti et al () showed that the continuous regional extent of the K‐horizon may be questioned. Piana Agostinetti et al () argued that such a reflector may be due to the occurrence of supercritical fluids above (young) cooling granitic intrusions as previously suggested by (Bertini et al, ). Besides the interpretation of the K‐horizon, seismic data show that this reflector occurs at 3,200 m in the southern part of the LTGF, in the so‐called Lago area (Bertani et al, ).…”

“…The model that we derive for the LTGF is summarized in Figure a. In agreement with previous studies (Bertini et al, ; Piana Agostinetti et al, ) we propose that the K‐horizon is a seismic reflector caused by a region where (supercritical) fluids may be found at near‐lithostatic pressures. The K‐horizon is to be conceived as an almost impermeable boundary isolating cooling granitoids from the overlying sedimentary cover.…”

“…Early studies interpreted the K‐horizon as a brittle to ductile transition following the °C isotherm (Batini et al, ; Liotta & Ranalli, ). The reflector was suggested to mimic the shape of the granitic intrusions at depth with no direct correlation with lithological discontinuities (Bertini et al, ; Gianelli et al, ; Gola et al, ; Piana Agostinetti et al, ). The drilling of the Venelle 2 reached 400 °C at 2,700 m depth (i.e., 200–300 m above the upper limit of the K‐horizon Bertani et al, ) pointing out that the isotherm marking the K‐horizon must hold higher values as suggested by the (suddenly) much higher temperatures encountered at 2,900 m depth (Bertani et al, ).…”

“…The drilling of the Venelle 2 reached 400 °C at 2,700 m depth (i.e., 200–300 m above the upper limit of the K‐horizon Bertani et al, ) pointing out that the isotherm marking the K‐horizon must hold higher values as suggested by the (suddenly) much higher temperatures encountered at 2,900 m depth (Bertani et al, ). Using passive seismic data, Piana Agostinetti et al () showed that the continuous regional extent of the K‐horizon may be questioned. Piana Agostinetti et al () argued that such a reflector may be due to the occurrence of supercritical fluids above (young) cooling granitic intrusions as previously suggested by (Bertini et al, ).…”

“…Early works suggest that it may represent the brittle to ductile transition (e.g., Brogi et al, ). More recent investigations, in agreement with previous studies (Bertini et al, ), suggest that the K‐horizon may be a seismic reflector caused by the occurrence of (supercritical) fluids at near‐lithostatic pressure (Piana Agostinetti et al, ).…”