Thermal Overview of an Area NW of the Larderello Geothermal Field, Italy

Transcription

1 GRC Transactions, Vol. 37, 2013 Thermal Overview of an Area NW of the Larderello Geothermal Field, Italy Stefano Bellani and Fabrizio Gherardi CNR - Istituto di Geoscienze e Georisorse, Pisa Italy sbellani@igg.cnr.it f.gherardi@igg.cnr.it Keywords Numerical modeling, Larderello geothermal field, hydraulic parameters, thermal boundary conditions, Nusselt number, heat loss ABSTRACT A wide area enclosed between the ancient Etruscan to town of Volterra and the northern rim of the Larderello high enthalpy geothermal field (Tuscany, Italy) shows thermal features which suggest further investigations aimed at medium-high enthalpy geothermal energy exploitation. Thermal gradients are in the range C km -1, while surface heat flow spans between 100 and 200 mw m -2. Constrained by field data, unsteady forward simulations were performed to predict the spatial distribution of temperature and fluid circulation paths in a 6-7 km deep domain, under the assumption of impervious and isothermal top and bottom boundaries, lateral adiabatic faces and variable internal physical properties. Gridding specifications are based on industrial seismic profiles and deep exploratory geothermal wells along them, which allow temperature control down to about 3 km. The model area has been kept variable in a by 6-7 km range, depending on simulation time requests, in an effort to test possible border effects. The results indicate that the present temperature and pressure distribution of hot fluids with depth in the northern border area of the Larderello field allows to hypothesize a fruitful exploitation of the medium-high enthalpy geothermal resources. Further to this, total heat loss values for the area were calculated as a tool to evaluate the potential energy production, as well as Nusselt number values for the area, to ascertain the possibility of a natural fluid convection in the reservoir formations. Introduction Larderello-Travale, a vapor-dominated high enthalpy geothermal field (Fig.1), with an installed capacity of more than 720 MW (Buonasorte et al., 2007), is located on the inner side of the northern Apennines (Tuscany), where extensional tectonics has been active since early-middle Miocene. The extensional stage produced the thinning of the continental crust in the Tuscan area to a thickness of about km (Nicolich, 1989), with the emplacement of magmatic bodies at shallow crustal levels. In the post Pliocene the entire Tuscan thyrrenian belt underwent a widespread uplift (i.e m in the Larderello area: Marinelli et al., 1993). A further effect of the extensional tectonics was the emplacement of sedimentary basins (i.e. the Volterra Basin). The background regional heat flow (HF) of the whole coastal belt of Tuscany has average values of mw m -2. HF anomaly reaches values close to 1000 mw m -2 at Larderello (Fig. 2), and is still intense ( mw m -2 ) in the NW rim of the Larderello field (Volterra area, Figs. 1,2). Figure 1. Generalized geological map of the area - A: location map. Box 1: Area under study. Box 2: Larderello geothermal field. Geological formations: a) Neoautochtonous sediments (Miocene-Pliocene); b) Potential reservoir: Tuscan Nappe, Metamorphic Units (Paleozoic-Trias); c) Flysch facies units (Cretaceous-Eocene); d) Igneous rocks (Pliocene-Quaternary). 231

2 New shallow heat flow values in the area were calculated in this work by means of thermal experimental data (Table 1) from nine shallow geothermal gradient wells ( m) and two deep (1550 and 2950 m) geothermal exploratory wells, then contoured in the northern belt of the Larderello area to update the heat flow map of Baldi et al., 1995 (Fig. 2). Hydrocarbon and mining exploration were carried out since historical times in several sedimentary basins of Tuscany, as well as the geothermal exploration in the areas surrounding the Larderello field (still in progress), making available a wide set of geological and geophysical data. Part of these data were used to set up preliminary thermal 2-D and 3-D models, to be compared with the experimental borehole temperature data available for the area. Table 1. Geothermal exploratory wells in the area under study. Well name Latitude (North) Longitude (East) Heat flow (mw m -2 ) Depth (m) Although the start of the thermal activity of the nearby Larderello field has been estimated at 3.8 Ma (Gianelli and Laurenzi, 2001), the whole region, including the Volterra area, is still very far from thermal equilibrium. The proven existence of anomalous thermal conditions in the NW peripheral areas of the Larderello field supports the idea of further studies aimed at the development of geothermal applications. With the aim to assess this possibility, we focused on the fluid circulation as in the areas directly influenced by the normal faults, as in the adjacent zones. Results and Discussion Based on the industrial seismic profiles available (Pascucci et al., 1999; Fig. 3), we set up a simple 3-D thermal model, along with a 2-D model representing the axial section along the X axis of the 3-D model. Geology is represented by four layers, according to the stratigraphic sequences of the area: from the top down, the clayey and flysch covers, the potential reservoir made up of limestones and anhydrites, and the phyllites and quartzites of the metamorphic Basement s.l. (Fig.4). The general scheme of the model was based on a km grid: these dimensions were at times increased up to km to test possible border effects. The finite-difference grid considers a large number of cells, variable between 58.8 and cells in the 2-D and 3-D models, respectively. Maximum simulation times are comprised between 250 ka (2-D) and 50 ka (3-D), to reduce the computational load of the calculations. Two normal faults drawn in the NE part of the models account for the extensional tectonics affecting the area. The model is aimed at investigating the heat transfer mechanisms in the upper crust and the role of the hydraulic permeability k, using the SHEMAT 8.0 numerical code (Clauser et al., 2003). The regional conductive-convective models were realized by means of unsteady-state forward simulations, under the assumption of liquid-saturated conditions, impervious and isothermal top and bottom boundaries, lateral adiabatic faces and variable internal physical properties (Table 2). Thermal conductivities of the various lithologies may vary as a function of temperature according to Somerton (1992). Heat transfer mechanisms in the upper crust have been investigated by exploring the sensitivity on spatial variations of hydraulic and thermal properties of rocks. Most of the reference data used to calibrate the model come from in situ (thermal profiles) and laboratory (thermal conductivity of rocks on core samples) measurements. On the basis of drilling Bottom hole temperature ( C) Piancorboli SW Scornello SW Montegemoli SW La Sassa SW Gello SW S-1246 SW S-3395 SW S-CN5 SW Le Mandrie SW Montecatini 1 DW Villa Le Monache DW Figure 2. Surface heat flow (mw m -2 ) of the Larderello-Travale geothermal field and surrounding areas: SW 1-9: shallow exploratory wells ( m deep); DW1: 2950 m deep geothermal exploratory well; DW2: 1550 m deep geothermal exploratory well. (Contours after Baldi et al., 1995, updated and redrawn). DW1 and DW2 position coincides, respectively, with Monitoring Points 1 and 2 as in Fig.3. Table 2. Model conditions and properties (λ = thermal conductivity, W m -1 K -1 ; k = permeability, m 2 ). Boundaries Temperature Hydraulic Head Initial conditions Boundary conditions Other conditions Top 15 C Calculated, Bottom 380 to 420 C h MAX = 6100m Top Fixed Bottom Fixed No flow Lateral Adiabatic λ = λ(t) k subject to calibration No Heat Flow ρ WATER = 1050 kg m

3 Figure 3. Geological section of the area under study, obtained by merging line drawings of AGIP seismic profiles V4 and V6 as published in Pascucci et al. (1999), updated and redrawn. Geothermal exploratory wells/monitoring points as in Fig.2. Figure 4. 3-D scheme with lithology superimposed. A = Clayey cover; B = Flysch; C = Limestones and Anhydrites; D = Phyllites and Quartzites. A B Figures 5A-B. 2-D temperature patterns at 10 ka (box A) and 250 ka (box B), along with hydraulic flow field (vectors). results from literature (Barelli et al., 2000), we assumed the formations belonging to the Reservoir block to be characterized by the highest values of permeability and porosity. In contrast, rocks from the Basement layers are assumed to be less permeable and 233 porous. Due to the inherent uncertainty of many model parameters (e.g., subsurface geometry and properties of the host rocks) and of the initial and boundary conditions (e.g., bottom T), a wide set of models was run with bottom temperatures in the range C, and permeability contrast among the various layers up to 6 log units (Table 3). Figures 5A,B show models at selected calculation times as representative of the whole models suite. All models run have shown the sensitivity of the upwards heat transfer rate to the hydraulic resistance contrast between the deeper layers. This appears to be the major control factor of the thermal and flow fields: a major source of uncertainty pertains to the poor knowledge of the spatial variability of permeability. Numerical outputs show, instead, minor dependence on bottom temperature variability over the above mentioned temperature range, and are almost insensitive to porosity. The most striking geological feature in these models is the occurrence of two major normal faults, as seen in seismic profiles (Pascucci et al., 1999). These NW-trending, NE-dipping faults reach down to the Metamorphic Basement (Fig. 3) that underlies at depth the whole area. In the numerical model, these faults are characterized by highest permeabilities (Table 3), and are supposed to be preferential pathways for fluids circulation. This induces a general upwards perturbation of the isotherms, and possibly the onset of early (Fig. 5A) and persistent (Fig. 5B) convection in the central part of the modeled domain. Inspection of temperature distribution also reveals that, over short times (10 ka), heat conduction is the dominant heat transfer mechanism at the SW limit of the modelled domain (Fig. 5A), whereas, over larger time scales, water advection becomes the dominant heat transfer mechanism even far from the faults zone (Fig. 5B). This scenario strongly relies on the assumption that faults are prevalingly zones of water movement and not barriers to water movement, in agreement with the extensional tectonics of the investigated area (e.g. Cameli et al., 1993; Brogi et al., 2003). Further to this, a correlation between faults and fluid circulation in likely lithologies has been hypothesized on the basis of geophysical data (Bellani et al., 2004; Della Vedova et al., 2008) in the whole geothermal area of Larderello, a few

4 Table 3. Variability range of the modeled parameters. Model ID Parameter Cover Flysch Limestones & Anhydrites Quartzites & Phyllites kilometres south of the investigated area, whereas micro-seismic data (Batini et al., 1995) testify the existence of a kinematically active upper crust which favors the maintenance of open permeability along the faults. Simplified 3-D thermal models qualitatively confirm the predominance of convective heat transfer around the normal faults area (50 ka, Fig. 6). This thermal anomaly is reflected by a sharp uplift of the 200 C isotherm in correspondence of the faulted zone, and by 100 C isotherm roughly overlapping on the reservoir top (ca m depth), far from the faulted sector. The bottom of the reservoir lays instead on the 300 C isotherm. Experimental temperatures from two deep wells existing in the area were compared with the output at two monitoring points (Figs. 3 and 7), located in correspondence of the wells in the 2D model suite. This comparison gives a twofold image: down to a depth corresponding to the cover thickness the fit is almost perfect, while inside the reservoir, affected by the onset of convection in the model, the calculated temperatures turn out to be strongly affected by the position relative to the faulted zone, where convective effects prevail, as can be seen at monitoring point 2. Temperatures measured in deep well 1 are higher than those calculated at monitoring point 1, suggesting the existence of even more favourable conditions in the field than obtained by the numerical model. Information resulting from numerical modeling has been complemented with a simplified analysis of further characteristic thermal parameters. According to Richards and Blackwell (2002), total heat loss values (HL) for the area under study were obtained by calculating the areal extension encompassed by the local average surface heat flow contour (140 mw m -2 ), then converting it into Watts using the simple equation m 2 W m -2 = W. The calculated HL value for the area north of the Larderello field is W (15.6 MW). This figure is approximately one order of magnitude smaller 234 Faults 2-D Hydraulic D permeability (m 2 ) All models Thermal conductivity (W m -1 K -1 ) Figure 6. 3-D temperature patterns at 50 ka compared to the value of Wisian et al. (2001) for the Larderello geothermal field, and one half of the Travale one (same reference). This suggests the existence of a favorable geothermal potential in the area under study. Values of the Nusselt number, Nu, a dimensionless figure that gives the ratio of the total heat transfer to the heat transfer by conduction only, have also been calculated for the area as a further tool to evaluate the geothermal potential according to the equation (e.g. Haenel et al., 1988) Nu = (Q H)/(λ T A), Figure 7. Geotherms at selected monitoring points plotted against experimental thermal logs from exploratory geothermal wells DW1 and DW2 (see Fig. 2). Monitoring points data from the 2-D model suite. where Q is the total heat loss of the system, A the area and T the temperature difference (from top to bottom) in the conductive regime. We assumed this figure to be 435 C, with average surface temperature 15 C and temperature 450 C at the K-horizon. This is a discontinuous high amplitude seismic reflector, that underlies at depth the whole area of the geothermal fields of Tuscany (Larderello, Mt. Amiata). The K-horizon reaches minimum depths of about 3 to 6 km in correspondence of the geothermal areas, whereas it deepens to greater depths moving outwards: it is supposed to be isothermal. (Liotta and Ranalli, 1999; Bellani et al., 2004). λ is an average thermal conductivity value (2.5 W m K -1 ), and H is the depth to the K-horizon, evaluated around 7000 m in the area (Romagnoli et al., 2010). To have natural convection Nu has to be >1: the values obtained in this study range around 0.9, meaning that a natural

5 convection is not active, but the general thermal conditions of the area fall in a close vicinity of high enthalpy geothermal conditions. Final Considerations The geothermal potential of an unexploited area NW of the Larderello geothermal field has been tentatively assessed by developing simplified 2-D and 3-D numerical models based on geological and geophysical data. The models were run under the assumption of fully water-saturated conditions, accounting for conductive and convective heat transfer mechanisms. Numerical outputs have been checked by comparison with experimental temperatures from deep wells, and validated by calculating additional thermal parameters like HL and Nu. This thermal overview is aimed at the qualitative definition of underground temperatures, and at the evaluation of the feasibility of medium to high enthalpy applications. Major conclusions are as follows: The permeability contrast between the calcareous and anhydritic formations of the potential reservoir (layer C, Fig. 3) and the surrounding cover and basement units appears to be the most important parameter influencing the numerical temperature modeling. We noticed that major thermal-hydraulic perturbations extended to the Metamorphic Basement (layer D, Fig. 3) when the permeability contrast between these two layers was tentatively reduced. The normal faults highlighted by seismic profiles represent the major geological-structural controlling parameter, acting as preferential pathways for fluid movement, under a condition of advective heat transfer, rather than permeability barriers for underground fluids. The modeling foresees the conditions for the onset of a robust convection starting initially in the area affected by the faulting, extending westwards over longer simulation times (Figs. 5B and 6). The comparison between the experimental geotherms from the deep wells with those computed at the monitoring points gives a twofold image: down to a depth corresponding to the cover thickness the fit is almost perfect: inside the reservoir, the calculated temperatures appear strongly affected by the position relative to the faulted zone, where convective effects prevail (monitoring point 2). It is relevant to underline that temperatures measured in deep well 1 are higher than those calculated at monitoring point 1, suggesting even more favourable conditions in the field compared to the output of the numerical model. Integration of numerical data and analysis of HL and Nu suggests the area NW of the Larderello geothermal field as a possible target for medium to high enthalpy geothermal applications development. References Baldi P., S. Bellani, A. Ceccarelli, A. Fiordelisi, G. Rocchi, P. Squarci P. and L. Taffi, Geothermal anomalies and structural features of southern Tuscany (Italy). 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