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Authors | Guanhong Feng Tianfu Xu Yue'an Zhao Fabrizio Gherardi

Title | Heat mining from super-hot horizons of the Larderello geothermal field, Italy

Source | Renewable Energy

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Geothermal energy from ultra-high temperature rocks is an emerging renewable and carbon-free energy source. In this paper, we make a first attempt to explore the fluid and heat flow dynamics in a coupled reservoir-wellbore system to evaluate the power generation performance of an ultra-high temperature (>450°C) enhanced geothermal system (EGS). We developed a high-performance code and constructed a 3D wellbore-reservoir coupling model based on data from a recently completed deep drilling project in Ladrero, Italy. The general model of an ultra-high temperature EGS exhibits a significant temperature drop (>60°C), after which the production fluid evolves from steam to a two-phase mixture until the end of the operating period. The reservoir pressure becomes a key parameter in determining the temperature of the two-phase mixture. By realistically capturing the phase change driven by coupled thermohydraulic processes during operation, our numerical model predicts lower power generation efficiencies than previous attempts based on an oversimplified model. Although this modeling approach was ultimately used to assess the thermodynamic viability of a specific system, it provides general information about the underlying thermohydraulic processes in the crust that may be applicable to the design of similar EGS projects in other regions.

In this paper, we explore the feasibility of an ultra-high temperature EGS at depth in the Radrero geothermal field, Italy, through thermohydraulic modeling. The dual objectives of the numerical simulations are (i) to gain insights into the fundamental processes of fluid and heat transport associated with the operation of deep geothermal twin wells in the mid-crust and (ii) to provide a preliminary assessment of the thermodynamic feasibility of similar projects for long-term use in the exploitation of conventional hydrothermal resources. This technical challenge responds to the need to extend the life of the Ladrero geothermal field and potentially increase its productivity in a sustainable manner.

Despite current approximations, numerical simulations reveal the fundamental thermohydraulic processes expected under supercritical geothermal conditions. This information is applicable not only to Ladrero, but also to other similar environments at mid-crustal depths, in order to define the most suitable technology to guarantee the sustainable operation of the power plant and to adapt the ground equipment/technology to the possible decrease in productivity of the geothermal twin wells. The main results are presented below:

  • Under the conditions considered in this study, the geothermal twin wells produce steam phases for approximately ten years. At an operating mass flow rate of 12.5 kg/s, up to 6.5 MWe of electricity can be generated, with a corresponding generation efficiency of 0.52 MWe × s × kg-1, which is not as high as previously calculated by the oversimplified model.
  • A sharp drop in outflow temperature greater than 60°C is predicted at the onset of the two-phase condition, caused primarily by the difference between the saturation temperature and the original reservoir temperature. After that, the outflow presents as a two-phase mixture for the remainder of the operating period, with its temperature determined by the reservoir pressure;
  • Reservoir pressure becomes a key factor in controlling the thermohydraulic evolution of the system because pressure can determine the timing of the phase transition and the outflow temperature. In general, higher reservoir pressures are beneficial in offsetting temperature drops and improving fluid flow;
  • Ultra-high temperature EGS projects require compensatory injection schemes. Injection and production flow rates need to be carefully optimized to reduce friction losses that may adversely affect wellhead pressures and temperatures, thereby affecting the overall energy conversion process.



Fig. 1 . Location map of the Larderello geothermal field (a) and temperature vs depth diagram of temperature values measured along the vertical profile of the Venelle 2 well (empty squares; b). Curves for temperature (T, red), pressure (P, blue), and steam saturation (S steam , green) calculated values are also provided (more details in the (more details in the Supplementary Materials).

Fig. 2 . 3-D and X-Z 2-D view of the conceptual model of fractured reservoir.

Fig. 3 . Pressure, temperature, and steam saturation evolution at two observational points located at the top and the bottom of the fractured reservoir, along (a) Patterns over 100 years production. (b) Detail of first 17 years production.

Fig. 4 . Temperature, pressure and steam saturation along the line connecting the bottom hole of the injection (point "a") and production (point "d") wells after two selected times (3.7 and 6.5 years) close to the second temperature drop. Temperature, pressure and steam saturation along the line connecting the bottom hole of the injection (point "a") and production (point "d") wells after two selected times (3.7 and 6.5 years) close to the second temperature drop. Points marked "b" (x coordinate -120 m) and "c" (x coordinate +20 m) locate at intermediate The two wells are located at intermediate distances between the two wells.

Fig. 5 . Temperature and steam saturation distribution along the X-Z cross-section connecting the injection and production wells after 30 years.

Fig. 6 . (a) Pressure, temperature, and steam saturation evolution at the production wellhead; (b) production flow rate and recovery percentage of injected water.

Fig. 7 . Pressure (a), temperature (b), steam saturation (c), and mixture density (d) evolution in the production well.

Fig. 8 . Fluid specific enthalpy (kJ/kg) range of currently operating geothermal power plants (modified after [ 57 ]).

Fig. 9 . Temporal evolution of heat flow (a), thermal and power generation, and conversion efficiency (b).

Fig. 10 . Temporal evolution of heat flow (a), thermal and power generation, and conversion efficiency (b) for different permeability cases.

Fig. 11 . Pressure (a) and temperature (b) patterns at the production wellhead under different conditions of fracture permeability.

Fig. 12 . Contribution of different mechanisms to the pressure change for different permeability values after 3.5 and 20 years.

Fig. 13 . Phase mobility contours under different pressure and temperature conditions.

Disclaimer: This article is for academic communication and dissemination only, and does not constitute investment advice



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