Assessment of geothermal potential of Campanian ...

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subsequently, until 1985, by AGIP-ENEL. Companies joint venture (Fig. 1) [1]. Fig. 1. Productive test during geothermal exploration at Campi. Flegrei, from [1].

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Assessment of geothermal potential of Campanian volcanoes (Southern Italy): from drilling exploration to numerical simulation. STEFANO CARLINO, RENATO SOMMA, ANTONIO TROIANO, MARIA GIULIA DI GIUSEPPE, CLAUDIA TROISE and GIUSEPPE DE NATALE Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Napoli “Osservatorio Vesuviano” Via Diocleziano, 328, 80124 Naples (Italy) [email protected] Abstract: - We analyze here the data related to geothermal exploration of Campanian volcanoes (Campi Flegrei caldera and Ischia Island), in order to assess the geothermal potential of these areas and the sustainability of the geothermal resource for electric production. A numerical simulation is also performed, to evaluate the possible disturbance on temperature and pressure filed, in the shallow geothermal reservoir of Campi Flegrei and Ischia Island, related to heat withdrawal for electric production of small geothermal power plant size (5MWe). Such analysis is performed by using numerical simulations based on a well-known thermofluid-dynamical code (TOUGH2®). Obtained results show that such geothermal exploitation generates a perturbation of temperature and pressure field which, however, is confined in a relative small volume around the well. At shallow level (0100m) the exploitation does not produce any appreciable disturbance, and can be made compatible with local spa industry. The results are crucial in the assessment of geothermal exploitation sustainability of the area. Key-Words: - geothermal exploitation, volcanic areas, energy production, power plants. consolidate energy production of Larderello geothermal field (Tuscany) and by the oil cries during the mid-seventies [1,2].

1 Introduction Since 1939 a drillings campaign, finalized to geothermal energy exploitation, have been performed in the volcanic district of Campania region (Campi Flegrei, Ischia Island and Vesuvius, Southern Italy), by SAFEN Company and subsequently, until 1985, by AGIP-ENEL Companies joint venture (Fig. 1) [1].

Fig. 2. Heat flow isolines of Campania Region (Southern Italy). The highest heat flow is recorded around the active volcano district formed by Ischia Island and Campi Flerei caldera [2]. Fig. 1. Productive test geothermal exploration at Flegrei, from [1]

during Campi

The results of drillings were particularly interesting at Campi Flegrei and Ischia, where the high heat flux and temperature (> 150°C) (Fig. 2) recorded at shallow depth (500 to 2000 m), highlighted the possibility of geothermal exploitation also in the

The attention to the geothermal energy exploration in Southern Italy was drawn by either the

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Plan during that time, was not attained, and, except for Tuscany region [1], the project of geothermal exploitation was discarded at the end of 1980 years. Since present time, the geothermal resource at Ischia and Campi Flegrei has been used just for baths and wellness, and in some cases for house heating. In recent time new exploitation leases have been submitted to the Italian Minister for Economic Development, to develop innovative projects of zero emission geothermal power plants (pilot power plants with maximum size of 5MWe). In particular, the INGV (Istituto Nazionale di Geofisica e Vulcanologia), is involved in providing technical and scientific data, to assess the feasibility of two geothermal power plants projects, which will be located in the central sector of Campi Fegrei caldera and in the western sector of Ischia Island, respectively (see Fig. 3 and 4). To carry on these projects, we have to take into account few important environmental and technical features, such as: the sustainability of power plants in highly urbanized areas; the choice of efficient machines for heat exchange between geothermal and working fluid; the stability of the process to avoid precipitation and incrustation of high salinity fluid; the behavior of geothermal reservoir during the long term fluid withdrawal. Our study in mainly focused on the latter point.

high enthalpy field [2]. The project was abandoned at the middle of 1980’s, mainly due to the decrease of oil price and to the nuclear choice of the Italian Government (suddenly stopped in 1986 after Chernobiyl disaster). Nowadays, the advancements in geothermal exploitation technologies, the high costs of fossil fuels and the increasing value of sustainable and clean energy sources make geothermal energy in these areas largely appealing. The data obtained by drillings (down to 3 km of depth) and investigation between 1939 and 1985 allowed us to asses, by using the volume method, the geothermal potential for Campi Flegrei and Ischia, which correspond to a thermal energy of about 6 and 11 GWy respectively [2]. Such high values, together with the nowadays green energy policies, point out the invaluable interest of geothermal assessment and exploitation in Campanian volcanic district. We show here a summary of main results related to geothermal exploration since 1939 at Campi Flegrei caldera and Ischia Island and, based on these results, we show a numerical simulations, which is aimed to understand the thermal and pressure response of volcanic geothermal systems to withdrawal and reinjection of fluids, in order to produce a net power of 5MWe (for each plant). Here we also present two projects for installation of 5MWe pilot power plants (ORC) at Ischia Island and Campi Flegrei caldera respectively. This study is developed in the framework of the new Italian regulation, which incentives the zero emission, and small size, geothermal power plants (5MWe). These projects represent a new starting point for the exploitation of geothermal resources in Southern Italy, where many areas are characterized by the presence of deep to shallow magma bodies, associated with large geothermal potential.

2 The pilot power plant projects A first attempt of binary cycle power plant installation was developed, in the western sector of Ischia Island, in 1950. This plant, the first one of this kind worldwide, produced a net power of 250 kWe (500kW nominal) [3], using a fluid with temperature of 130°C, extracted at a depth of ~200m. The endeavor was abandoned few years later due to technical problems related to well incrustation and corrosion, which the adopted technology at that time was not able to solve. Otherwise, the tentative to improve the technology of such plants was also abandoned in view of the nationalization of private electric companies. In fact, the increase of geothermal energy production in Italy, as forecasted by the Italian Energy National

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Fig. 3. The Campi Flegrei caldera with indication of: caldera boundary (dotted line); isoline of geothermal gradient (°Ckm-1) (red lines); shallow geothermal drillings (200) (red circles); location of 5MWe planned geothermal power plant.


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popular thermal baths of Pozzuoli and Baia impracticable. The attention for the extensive geothermal fields at Ischia and Campi Flegrei, turns from a merely spa use to a geothermal exploitation during the early XX century. An extensive campaign for geothermal investigation started in 1939 with many drillings performed at Ischia and Campi Flegrei, by the SAFEN Company (Fig. 3,4). The researches continued later, in the framework of the Joint Venture AGIP-ENEL companies, and were focused in the Campi Flegrei area, starting from 1979. Several wells reached few kilometers of depth, with a maximum depth of 3 km. The geothermal researches involved also the Vesuvius area, starting from 1952, but the temperature gradient measured in the 2km depth well (~30°Ckm-1), southernmost to the volcano, was not of interest for geothermal exploitation. On the contrary an high heat flow of the hottest areas at Ischia and Campi Flegrei, was inferred from temperature in the boreholes, which ranges between 200 mWm-2 and 500mWm-2, (from thermal conductivity between 1.5 Wm-1K-1 to 2.7 Wm-1K-1) [1,2,5]. The geothermal gradient varies from 150°Ckm-1 to 220°Ckm-1, with the highest values in the southwestern sector of Ischia Island and western sector of Campi Flegrei caldera

Fig. 4. The island of Ischia with indication of: isoline of geothermal gradient (°Ckm-1) (red lines); shallow geothermal drillings (200) (red circles); location of 5MWe planned geothermal power plant

3 Geological features and geothermal drillings Ischia Island and Campi Flegrei caldera are two active volcanic areas, located west to the city of Naples. These areas are characterized by elevated geothermal gradient and heat flow as consequence of upward migration of magmatic sources and occurrence of vigorous hydrothermal circulation (Fig. 2) [2]. The ancient volcanic activity is thought to be related to the dynamic of large magmatic chambers, which produced caldera-forming eruptions such as the Campania Ignimbrite (39ky B.P.) and Neapolitan Yellow Tuff (15ky B.P.) at Campi Flegrei and Mount Epomeo Green Tuff (55Ky B.P.) at Ischia. The amount of emitted magma during these events ranges from few tens to more than 200 Km3 of dense rocks equivalent (D.R.E). The location of both residual shallow magma chambers (2-4 km) and deeper magmatic system (7-8 km) have been inferred from seismic tomography, gravity and geochemical data [4]. Advection processes due to hot fluids transport, enriched of magmatic gases, produced large surface manifestations at Campi Flegrei and Ischia, with fumaroles fields and thermal springs whose surface temperature varies from 20°C to 100°C. These features were an appeal for people living in Southern Italy, since Roman Empire age. The first systematic study of hot springs was performed at Ischia by Giulio Iasolino (1588), who introduced them into medical practice and boost Ischia’s fame. Otherwise, the interest in Ischia also grown due to the eruption of the nearby Campi Flegrei, in 1538 A.D., which made the more

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4 Geothermal reservoirs Different aquifers, with temperature between 150°C and 250°C, were identified at Ischia Island and Campi Flegrei caldera, generally localized at shallow depth (500–1000 m, 1800–2000 m), within the tuffs and volcano-sedimentary formations. These reservoirs were indentified during the productive tests performed by SAFEN, AGIP and ENEL companies since 1939 [1]. The aquifers were characterized by a flow rate between 20ls−1 55ls−1, with content in weight of vapor between, 40% and 50%. The characteristics of the wells and productive reservoirs were established through both short (2–3 days) and long (3–4 months) pumping tests. The temperatures vs. depth profiles are reported in Fig. 5 and 6. During the drillings, hydraulic conductivity tests were performed, to infer the permeability at different depths [1,2]. The average permeability values at different depth, for Campi Flegrei, are reported in table 1.


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Organic Rankine Cycle (ORC) plant. For an ORC power plant the enthalpy drop (∆H) per unit time across the turbine during expansion can roughly calculated as: ∆H=m·Cp·∆T, where m is the flow rate, Cp is the constant pressure heat capacity of organic fluid and ∆T is its temperature drop. For a commonly used liquid (isopentane) heat capacity of 2.29kJkg-1K-1 and ∆T= 50°C, the potential production for each well, in the investigated areas, ranges from ~2290 kJs-1 (2.3MWe) to ~6298kJs-1 (6.3MWe). The variability of the output power depends mainly on the flow rate and wellhead temperature. Depth range (m) Permeability (m2)* 0-500 10-15 500-1000 (Aquifer 1) 10-14 1000-1400 10-18 1400-1800 10-17 1800-2000 (Aquifer 2) 10-15 2000-3000 10-18 Table 1. Values of permeability, at different depth, inferred from borehole hydraulic tests at Campi Flegrei.

Fig. 5. Measured temperature profile vs. depth, for different wells at Campi Flegrei. The blue thicker lines represent the vertical extension of the productive aquifers identified during the productive tests.

For the efficiency of a turbine of 85% the maximum possible nominal output power becomes 1.96MWe and 5.3MWe, respectively. In a first approximation, we can use these values to plan the number of wells for electric production. The evaluations reported in the previous sections allow us to obtain sufficient constrains to estimate the effects of fluids withdrawal from geothermal reservoir and their reinjection, for a net production of 5MWe. To this aim the numerical code TOUGH2 ® has been used for numerical evaluations of Temperature and Pressure changes due to geothermal exploitation, via a finite Volumes resolution of Mass and Energy balance equations in a region of space, discretized in a mesh grid. The reservoir has been modeled as a porous medium with average porosity φ =0.2. A saturated twophases mixture of water and vapor flows in the reservoir, due to the fixed porosity, with fluid maintaining thermal equilibrium with surrounding rocks matrix. Fluid advection is described by Darcy's law[6] : ρ F = −K (∇P − ρg ) µ where F is the fluid mass flow rate per unit of cross sectional area, K is the absolute permeability, μ is viscosity, ρ is density, ∇P is the pressure gradient and g is the gravity acceleration. Darcy's law is written for the case when the pore space contains a single fluid phase, for example a liquid (aqueous)

Fig. 6. Measured temperature profile vs depth, for different wells at Ischia Island. The blue thicker lines represent the vertical extension of the productive aquifers.

4 Simulation of geothermal exploitation and sustainability of resource Taking into account the above flow rate data and temperatures for different aquifers, it is possible to estimate the effective producible power for an

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phase. When liquid and gas phases coexist in the pore space of the medium, a multiphase version of Darcy’s law is applied. The simulation takes into account the fluid density variation with temperature and the effect of thermal expansion. An example of the geometry of the problem and the boundary conditions, for Campi Flegrei caldera, is reported in Fig. 6.

Fig 7. Simulation of thermal perturbation, along a vertical section, due to withdrawal and total reinjection of 55ls-1 of geothermal fluid for 30 years (Campi Flegrei).

Fig 6. Example of geometry and boundary condition for the modeling of Campi Flegrei geothermal reservoirs.

An example of the results of simulation, for Campi Flegrei, is reported in Fig. 7 to 10.This is conducted to assess the response of the geothermal system for 2 productive wells and 2 reinjection wells (with depth of 1km), for 30 years of exploitation. The using of 2 reinjection wells (instead of one) could practically mitigate the excess of localized thermal and pressure variations during reinjection (this can mitigate the problem of possible induced seismicity). The adopted geometrical configuration of the wells is a square 1000m side. For this simulation the thermal perturbation occurred into four separate volumes (Fig. 7 and 8). Along the productive wells the decreasing of temperature does not exceed 20°C. The maximum temperature variation is confined in a small volume, after 30 years of simulation. Along the reinjections wells, the drop of temperature ranges from 120°C, close to the well axis, to 25°C at a distance of 600m (Fig. 7 and 8). A more complex pressure field variation is obtained for this wells configuration (Fig. 9 and 10). The maximum absolute value of pressure variation along the productive and reinjection wells is 10bar. The perturbed pressure field is partially propagated to the surface, above the reinjection zone, with value from +2,5 to + 5 bar. This slight increment occurred also at depth range of 1300-1600m. In the deeper part of the system, down to 2400m, a slight decreasing of pressure of few bar is also noted.

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Fig 8. Same simulation of fig 8, along an horizontal section (depth 1km) (Campi Flegrei).

Fig 9. Simulation of pressure perturbation, along a vertical section, due to withdrawal and total reinjection of 55ls-1 of geothermal fluid for 30 years (Campi Flegrei).


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Settore Esplor e Ric Geoterm-Metodol per l’Esplor Geotermica, San Donato Milanese Italy 1987(1–2):3X2. [2] Carlino S, Somma R, Troise C, De Natale G (2012) The geothermal exploration of Campanian volcanoes: Historical review and future development. Renew Sustain Energy Rev 16(1):1004–1030 [3] Carlino S, Somma R, Troiano A, Di Giuseppe MG, Troise C, De Natale G (2014) The geothermal system of Ischia Island (southern Italy): critical review and sustainability analysis of geothermal resource for electricity generation. Renew Energy 62:177–196 [4] Carlino S, Somma R (2010) Eruptive versus noneriptive behaviour of large calderas: the example of Campi Flegrei caldera (southern Italy). Bull Volcanol 2010. doi:10.1007/s00445-010-0370-y [5] Della Vedova B, Bellani S, Pellis G, Squarci P (2001) Deep temperatures and surface heat flow distribution. In: Vai GB, Martini IP (eds) Anatomy of an orogen: the apennines and adjacent mediterranean basins. Kluwer Academic Publishers, Dordrecht, pp 65–76 [6] Darcy H. Les fontaines publiques de la ville de Dijon: exposition et application des principes à suivre et des formules à employer dans les questions de distribution d'eau (Paris: V. Dalmont), 1856.

Fig 10. Same simulation of fig 9, along an horizontal section (depth 1km) (Campi Flegrei).

4 Conclusions This study shows the main results related to geothermal exploration of Campanian volcanoes, from which is possible to infer an high geothermal potential for electrical energy generation. We also performed a simulation to understand the response of geothermal reservoirs, in terms of thermal and pressure perturbation, after 30 years of exploitation. The simulation is based on the production of 5MWe for each power plant. Thermal variations, after 30 years of fluid withdrawal, are sustainable for electric production (5MWe). Otherwise, pressure variations are significant, and they have to be considered in terms of possible induced seismicity. At shallower level (0-100m) the exploitation does not produce any appreciable thermal disturbance, and can be made compatible with local spa industry. The results of this study are useful to understand the behavior of geothermal systems of active volcanoes during exploitation, and to provide new data in the planning new zero emission and small geothermal power plants, in high urbanized and high risk volcanic areas, such as Campi Flegrei caldera and Ischia Island. The latter problem is of particular interest because, with the introduction of new Italian regulations which favor and incentivize innovative pilot power plant, many geothermal project and investigations, applied to volcanic districts, have started in the last 4 years, while a numbers of exploration leases have been submitted to the Ministry for Economic Development of Italy. References: [1] AGIP (1987) Geologia e geofisica del sistema geotermico dei Campi Flegrei, Technical report.

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