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2 DEVELOPMENT OF DEEP EXPLORATION IN THE GEOTHERMAL AREAS OF TUSCANY, ITALY Guido Cappetti', Romano Celati2, Ugo Cigni', Paolo Squarci2, Giancarlo Stefani' and Learco Taffi2 ENEL, Unita Nazionale Geotermica P.zza Bartolo de Sassoferato Pisa, Italy 'Istituto lnternazionale per le RiceJche Geotermiche CNR Via del ~ u~izg~~to Pisa, Italy 3ENEL, Uizita Nazionale Geotermica Lirrderello (PI), Italy ABSTRACT The first deep well (2703 m) in the Larderello field was drilled in 1961, with the target of improving knowledge on the structural setting of the formations underlying the shallow exploited reservoir, their permeability and the physicalchemical characteristics of the fluid encountered. This borehole proved the existence of deep fractures and of increases in temperature and pressure with depth. Production from deep wells was, however, considered uneconomic until the oil crisis of the 1370~~ when the deep drilling program was given renewed impetus. Serious drilling problems were encountered in the central part of the Larderello field, where closely spaced fractures and corrosive fluids are encountered for very long depth intervals. These difficulties have so far prevented the development of a systematic productionoriented deep drilling program in this area. Deep drilling gave commercially interesting results in the peripheral areas of Larderello and in the Travale and Mt. Amiata areas, where great thicknesses of impermeable formations are found before reaching the deep productive horizon. INTR~~UCTIO~ The structural and hydrogeological model of the Tuscan geothermal fields (Larderello, Travale and Mt. Amiata) has been continually revised and improved as new information, based mainly on drilling data, is made available. A more accurate picture has thus been attained utilizing new data on the stratigraphic, structural and hydrogeological characteristics of the rocks and the physical and chemical characteristics of the fluids present in the underground. Prior to deep exploration and the seismic reflection prospectings, our knowledge was limited to the first 700 to 800 m b.g.1.' The structuralhydrogeological model developed for that depth interval in the three Tuscan geothermal fields includes: A a practically impermeable cover of Neogene clastic formations (upper MiocenePliocene) and all~hthonous flysch (Ligurian Nappes: Cretaceous Eocene) (Figure 1, units 2 and 3); B a reservoir of Mesozoic evaporitic and carbonate formations overlain locally by an Oligocene arenaceous series (Tuscan Nappe) (Figure 1, units 4 and 5). Permeability in this complex is tied mainly to tectonic movements, and preferentially occurs close to the planes separating different tectonic units, such as the contact between units 3 and 4 or 3 and 5 (Figure 1); C a TriassicPaleozoic basement of terrigenous (phyllite and quartzite) formations of varying grades of metamorphism and generally low permeability. The first step in the direction of deep exploration, directed at the deeper levels of the TriassicPaleozoic 303

3 ~ Deep Exploration in Tuscany, Italy W E I A a I.rt.:l Rql ~ I"J mj b A' 4000 Figure 1. Geological cross section through the ~rdetellogeothermal field. 1. Mr. Amiata rhyodacitic volcanites ( Ma). 2. Neogene sediments (upper Miocene Pliocene). 3. Fl ysc h nappes: shal y ma rl ya renaceous formations (Cretaceous Eocene). 4. Tuscan Nappe: asa nds tone, b pol ych rome shale and calcarenite. 5. Tuscan Nappe: limestone, magnesian limestone and breccia, dolostone and anhydrite (Upper TriasLower Cretaceous). 6. Tectonic slices complex: quartzite, phyllite,anhydrite and dolostone (Upper Trias) with metamorphic basement rocks (Paleozoic). 7. Filladi Inferiori Groups: phyllite and quartzite (OrdovicianSilurian) (Larderello basement). 8. Boccheggiano Formation: carbonatic quartzite, phyllite, metagreywacke, basic metatuffite with dolomitic marble and anhydrite (~voniansiiurian)(travale basement). 9. ~etagreywac~e and metapelite (~rboniferousdevonian(?)) (Mt. Amiata basement). 10. Marble (Paleozoic) (Mt. Amiata basement). 11. Micaschis~,gneiss and amphibolite (Lower PaleozoicPrecambrian(?)). 12. Deep reflecting horizon. 13. Isotherm. For location see Figure 3. basement, was taken as far back as 1951; assuming that the geothermal fluids were of magmatic origin, a drilling program was drawn up for a 3000 m well aimed at recovering a higher temperature and higher pressure steam. This project was postponed for financial reasons until the early 1960s, when a 2703 m well was drilled in the Larderello zone. This well crossed several fractured horizons within the basement, whose temperatures and pressures increased with depth. The well, however, proved noncommercial. The unsatisfactory results in terms of production, drilling problems and the low cost of the conventional energy sources caused the initial project to be abandoned and replaced by one with more limited objectives. The wells programmed from 1968 on were directed at exploring the layers of the reservoir immediately below those already exploited, in densely drilled zones whose reservoir pressures were close to the rating value of the power plant. Even a small increase in pressure would have improved conversion efficiency. The first of these wells, Sperimentale 1, was drilled at Larderello in and reached a depth of 1097 m, cro'ssing about 750 m of the 'basement' format~ons. The well revealed the existence of three productive levels at 300 m, 370 m and 750 m, with pressures increasing from 4.8 to 6.9 bar (Ferrara, Panichi and Stefani, 1970). The oil crisis of the early 1970s brought a renewal of interest in the geothermal resource and offered new prospects for deep exploration. Hitherto exploitation of the geothermal fields had been considered a two dimensional problem, i.e. determining the areal extension of a welldefined productive horizon, but at this point a third dimension, depth, attained equal importance. The geological studies of the basement formations, either outcropping or crossed by wells, led co their stratigraphic reconstruction and to an interpretation of the structural setting (Gianelli, Puxeddu and Squarci, 1978). The structural and stratigraphic data suggested that exploitable fractured horizons could exist at depth. This hypothesis was also corroborated by the results of a seismic survey at Larderello, which revealed the existence of a series of deep reflecting horizons, one of which extends over the entire region at depths between 3000 and 5000 m (Batini and others, 1978). This horizon is also present in the Travale and Mt. Amiata areas, but at greater depths. On the basis of the available data, well Sasso 22 was drilled in the period 1978 to 1980, reaching a final depth of 4092 m. This is the deepest geothermal borehole in Italy, and the first to encounter temperatures in the order of 400" C. During this same period deep exploration also began in the Mt. Amiata geothermal region, with one well drilled in the Bagnore field and another in the ~iancastagnaio field, in Both wells discovered a waterdominated reservoir within the basement, 2000 m below the already exploited horizon, with a pressure of about 200 bar and temperatures above 300 C. Deep exploration began in the Travale field in The first two wells were drilled in the 'old' field, which is no longer exploited. They encountered a vapordominated 304

4 Cappetti and others S.Pompeo 2 S s ' , B Figure 2. Geological cross section through the Larderello geothermal field. (For symbols see Figure 1. For location see Figure 3.) Figure 3. Temperature distribution at the top of the geothermal reservoir (A) and at 3000 m depth (B) in the LarderelloTravale geothermal region productive horizon at a depth of about 1800 m, separated from the shallower waterdominated reservoir by about 1300 m of practically impermeable terrains. DEEP EXPLORATION IN THE LARDERELLO AREA The structural situation beneath tectonic units 3,4 and 5 (Ligurian and Tuscan Nappes) is complicated (Figures 1 and 2) by tectonic repetitions of series, consisting of Triassic detrital me tasediments (phyllite, quartzite), anhydrites, dolostones, with rocks of the Paleozoic metamorphic basement. This complex (6) consists of "tectonic slices." Beneath it, and separated by a tectonic surface, is the top of the metamorphic basement, consisting predominant~y of phyllites (7) and of micaschist and gneiss (11). These formations are presumed to belong to the Mid PaleozoicPrecambrian. According to petrologic and radiometeric data, the basement rocks have undergone a Hercynian polyphased metamorphism, followed by late Hercynian and Alpine thermal events (Puxeddu, 1984). The thickness of this unit varies from 1.5 to 34 km (Figures 1 and 2). 305

5 Deep Exploration in Tuscany, Italy The first deep well, Sasso 22 (Figure 3), provided information on permeability distribution and on the thermodynamic characteristics of the fluids within the basement (Bertini and others, 1780). Some of the relevant characteristics of the well are shown in Figure 4. A sequence of permeable horizons were detected within the basement; permeability was found in the 1500 to 1700 m interval in the Filladi Inferiori formation and in the 2400 to 3800 m interval in the micaschist and gneiss. Reliable values of reservoir pressure were obtained by transient test analysis at 1200 m (25 bar) and 3000 m (55 bar). Reservoir temperatures were obtained by temperature transient analysis at bottomhole, in impermeable intervals. Temperature and pressure data reveal that steam is the dominant phase in the reservoir, at least to depths of almost 300 m. ABSORPTION PROFlL E RESERVOIR TEMPERATURE ( C ) (% OF FLOW) P I I I 4 I!.. Figure 4. Stratigraphy and completion of well Sasso 22, and results of flowmeter log, run with bottomhole at 2960 m and undisturbed reservoir temperature (estimated from temperature buildup). (For geological symbols see Figure 1.) Due to the results of Sasso 22, and particularly to the difficulties encountered in drilling densely fractured formations, the deep exploration programs were continued along two different lines: research activity in the central part of the field, aimed at improving reservoir knowledge and drilling technology; exploitation activity, directed at recovering fluids of higher temperature and pressure in the marginal areas of the field,.where low permeability had been detected in the upper parts of the potential reservoir formations. As part of the research program, well San Pompeo 2 was drilled about 5 km s,outhwest of Sasso 22, where the highest temperatures were recorded in the upper part of the reservoir (>300 C at 1200 m), and where the main reflecting horizon detected by seismic prospecting was nearest the surface (3000 m)(batini and others, 1983). Dense fracturing in the basement was also encountered by this well down to a depth of 2300 m, in correspondence to the Filladi Inferiori formation, with reservoir pressures of about 30 bar. Beneath this zone the well crossed impermeable phyllites and micaschists to 2730 m, at which depth it encountered a fractured horizon. The high temperatures and pressures of the fluid in the latter impeded further drilling and measurements. Reservoir pressure and temperature could not be measured, as each time the bit reached this horizon violent explosions caused the formation to cave in and hundreds of meters of debris filled the borehole. An indirect evaluation of pressure can, however, be made for the deep horizon if we consider that: the well blew out during drilling with a return circulation of water; after the last violent blowout 2560 m of the well were accessible; beyond that depth it was filled with debris. The well was then shutin in order to record pressure build up. When wellhead pressure reached 150 bar, it was 212 bar at 2560 m. Extrapolation to 2730 m gave about 240 bar. No further pressure build up could be recorded at wellhead due to leakages in the surface equipment and in the casing. Shortly afterwards pressure and temperature logs were run while the well was open and emanating only small quantities of gas and steam: there was no liquid in the well and a temperature of 334 C was measured at 2560 m. On the basis of these data, we can infer that fluids with pressures above 240 bar and temperatures above 400 C are present in the fractured horizon encountered at 2730 m. It would appear that the deep fractured horizon is isolated from the overlying productive horizons. Deep exploration in the marginal areas began in the northwest zone of the field and revealed productive horizons at depths between 2000 and 3000 m, in the micaschist formation; pressures of about 70 bar and temperatures of 280 to 330 C were recorded. These wells produce dry steam at rates of 25 to 30 t/h. Exploration will also be extended to the southeast margins of the field, near the absorption areas of meteoric waters. The upper part of the reservoir in these areas is affected by cold water circulation but high temperature gradients have been found in the basement, resulting in temperatues of about 300 C at a depth of 2000 m. Deep exploration confirms that the areas with high temperature in the upper parts of the reservoir are mainly limited in size by circulation of the meteoric waters. The thermal anomaly at 3 km depth is much more widespread, as shown in Figure 3 and in the cross sections in Figures 1 and 2; the maximum on the southern margin of the geothermal field corresponds to the peak of the deep reflecting horizon. DEEP EXPLORATION IN THE TRAVALE AREA In the Travale area the complex of tectonic slices is missing and the Tuscan Nappe lies directly over the basement. The latter is characterized by a thick Paleozoic series of alternations of carbonate quartzites, phyllites, metagreywackes, graphitic phyllites and basic metatuffites with recrystallized dolostones and anhydrites (Castellucci, Minissale and Puxeddu, 1983) ( Boccheggiano Formation ) (Figure 5). Beneath the Boccheggiano 306

6 Cappetti and others w 500 s.i I E Figure 5. Geological cross section through the Travale geothermal field. (For symbols see Figure 1. For location see Figure 3.) v v v v v, v w v V N v v v v Figure 6. Geological sketch map of the Mt. Amiata geothermal region. 1. Mt. Amiata rhyodacitic volcanites ( Ma) and Radicofani trachybasalts (0.97 Ma). 2. Neogene mainlyclayey sediments (Pliocene). 3. Flysch nappe: shalymarlyarenaceous formations (CretaceousEocene). 4. Tuscan Nappe: polychrome marl, shale and calcarenite (Upper CretaceousEocene). 5. Tuscan Nappe: limestone, magnesian limestone and breccia, dolostone and anhydrite (Upper TriasLower Cretaceous). 6. Location of the geological cross section. 7. Geothermal field. 8. Area of deep exploitation. 9. Deep geothermal well. Formation there are probably micaschists identical to those of the Larderello zone, as found in a deep well drilled between Castelnuovo and Travale. They could, therefore, form a continuous horizon below both areas. Deep exploration began in the Travale field in the 1980s and so far only two 2000 m wells have been drilled in the old field zone, near the outcrops of the reservoir formations (Burgassi and others, 1975). In the upper levels of the reservoir in this area complex interactions between the vapor~ominated system and recent meteoric waters (Cataldi and others, 1970; Celati and others, 1977) led to the abandonment of exploitation of the shallow reservoir in the 1960s. The deep wells encountered fractured horizons at 1700 to 1800 m depth, with temperatures of 280 to 290 C and pressures of 65 to 75 bar. The deep horizon seems to be connected with the now exploited vapordominated system, whose initial pressures were 60 to 70 bar. The isotherms drawn in Figure 5 show a rise in the carbonateevaporitic shallow reservoir in correspondence to the uplifted sector of the structure. This rise was caused by a flow of steam that, ascending through the fracture system on the western margin of the graben, spread into the shallow fractured horizon. The temperature inversion beneath the carbonateevaporitic reservoir can be ascribed to low permeability. The deep thermal anomaly also has a maximum in correspondence to the uplifted structure on the western margin of the graben. w Bagnore Piancastagnaio E A?? km L Figrrre 7. Geological cross section through the Mt. Amiata geothermal field (For symbols see Figure 1.) 307

7 Deep Exploration in Tuscany, Italy DEEP EXPLORATION IN THE MT. AMIATA AREA In this area (Figure 6), as in Travale, the complex of tectonic slices is missing (Figure 7). Beneath the evaporitic formation of complex 5 is a thick sequence of metapelite and metagreywacke of probable Carboniferous Devonian(?) age (Bagnoli and others, 1980). There are also lithotypes, in complex tectonic structures, that seem to belong to the Boccheggiano Formation. Underlying the CarboniferousDevonian(?) phyllitic series is a predominantly carbonate metamorphic formation that has been crossed by a few wells for about 500 m. The Bagnore and Piancastagnaio fields (Figure 6) were discovered in the late 50s and early 60s in the Mt. Amiata area (Calamai and others, 1970) and are still being exploited. Production in these fields was obtained from the upper part of the Triassic carbonateevaporitic formation underlying the cover. The wells drilled here led to the areal delimitation of the shallow thermal anomalies, so that the policy was essentially to exploit the identified resource. At the end of the 1970s, in an attempt at recovering fluids of higher temperature at greater depths, two exploratory wells were drilled, one at Bagnore and the other at Piancastagnaio, at the same time as deep exploration was beginning at Larderello. These and later wells revealed the existence of a mainly phyllitic formation, practically impermeable to a depth of 2.6 to 3.5 km beneath the carbonateevaporitic formation. At these depths a waterdominated reservoir was found with temperatures between 300 and 350 C and pressures of 200 to 260 bar. Despite the limited transmissibility of the productive layer (0.1 to 0.5 darcy.m) and great depth, the wells are commercial producers because of high reservoir temperatures and pressures. Flashing occurs in the formation after a short production period. Until now only one of the Mt. Amiata deep wells (PC 26) has produced for a long period (2 years),giving about 40 t/h of steam with 5 percent noncondensable gas and 5 m3/h of liquid water with 20 g/1 TDS (predominantly boron, sodium chloride and silica). So far 10 deep wells have been drilled. All the wells drilled at Piancastagnaio are productive. The relatively low gas content (about 5 percent) permits their utilization in condensing power plants. The noncommerical wells are located in the Bagnore area, Poggio Nibbio area and in the graben east of Piancastagnaio, where reservoir iemperature is lower than at Piancastagnaio. All these wells did reach the deep permeable horizon, so it presumably extends beneath the entire area of Bagnore, Poggio Nibbio and Piancastagnaio. The cooling at depth in the Poggio Nibbio and Bagnore areas seems to be caused by the presence, in the vicinity, of important absorption areas of meteoric waters connected with the deep reservoir. The well drilled in the graben east of Piancastagnaio also encountered the permeable horizon at about 2600 m depth, but the temperature there was below 250 C. Temperatures of 350 C were found at about 4 km depth, but in impermeable formations. The development program for the Piancastagnaio area based on these results includes the drilling of another 47 wells over an area of about 30 km2, at depths between 2500 and 3500 m; these should permit the installation of seven 20 MW power plants, totaling 140 MW. TECHNOLOGICAL PROBLEMS OF DEEP DRILLING The deep wells drilled in the peripheral areas of the Larderello field, and in the Travale and Me. Amiata areas, presented no serious technological problems as, once the shallower reservoir had been isolated by the casing, the deeper basement formations were usually compact enough for drilling to proceed to the deep productive horizon with mud or water circulation. Deep drilling in the central area of the Larderello field, on the contrary, met with a series of problems caused mainly by the intensely fractured reservoir rocks, their high temperatures and corrosive fluids. The presence in the basement of rocks of very different drilling coefficients and complex schistosity patterns made it very difficult to maintain the well verticality required to reach the deep layers. In these conditions, a specially stabilized drill string had to be used regularly so as to control deviation of the well axis and avoid friction, wear and fatigue on the drill string. This methodology, commonly used in drilling, is particularly hazardous in geothermal wells when drilling without return circulation. In these circumstances, drilling long stretches of open hole is a risky endeavour. Consequently, the fractured zones have to be isolated by casings and the problem arises of drilling to great depths with industrially acceptable diameters. The presence of fractures also creates problems in achieving a successful cementation of the casings. Corrosion becomes a serious problem when drilling without return circulation, as there is no possibility of controlling the chemistry of the fluid in the well. Generalized corrosion and pitting on drillpipes were observed in many wells when crossing highly fractured formations; it is thought to be caused by a waterrock interaction in a hightemperature environment (leaching) with dissolution of the most soluble salts, and by the presence of reservoir fluids that generally contain gases such as C02 and H2S and such notoriously aggressive ions as chloride and ammonium. As these corrosion phenomena usually develop at a rate comparable to ordinary mechanical wear, they can be kept under control. Any extensively damaged material can be replaced whenever necessary. Stress corrosion (SCC), on the other hand, is a far more serious problem as, in this case, it is impossible to predict failure of the drill string. The consequences to the well are at times disastrous. Stress corrosion was particularly severe in the tool joints and in the slip area of the drillpipes. These phenomena were particularly aggressive in well San Pompeo 2. Generalized corrosion was noted throughout the entire thickness of the drill pipes and decarburization of the casing steel caused by atomic hydrogen attack. All these phenomena are due to high temperatures (>400 C) and extremely aggressive reservoir fluids. Metallographic analysis of some fragments of casing showed that carbon had dropped from 0.35 to 0.08 percent and the original ferrite + perlite structure of the material had been transformed to ferrite only. Research is now 308

8 Capetti and others directed at solving these corrosion problems, at Batini, F., Bertini, G., Bottai, A., Burgassi, P.D., Cappetti, G., Gianelli, G. constructing special instrumentation and equipment, and and Puxeddu, M., 1983, European Geothermal Update: 3rd Intern. at developing muds and cements for use in deep drilling. Seminar on Results of EC Research and Demonstration Projects in the Field of Geothermal Energy, Munich, 29 Nov.1Dec. 1983, p. CONCLUSIONS (1) Deep exploration on the inside of the Larderello field revealed the existence of a series of fractured horizons in the basement to the investigated depth of about 4000 m, and the presence of the vapordominated system down to about 3000 m. Well San Pompeo 2 reached the lower layers of the metamorphic basement and encountered within it a fractured zone containing abnormally high temperature and high pressure fluids, separated from the overlying fracture systems. Technological problems ensuing from drilling into intensely fractured formations (Le. without return circulation), as well as corrosive fluids, have so far prevented actuating a systematic productionoriented drilling program in this zone. (2) Deep drilling gave commerically interesting results in the marginal areas of the Larderello field, at Travale and at Monte Amiata. Great thicknesses of impermeable formations were encountered beneath the shallow exploited reservoir in these areas, so that deep productive horizons could be reached without particular d r i lli ng problems. In the Mt. Amiata area, where the exploitable areas of the shallow reservoir were defined, deep drilling revealed the existence of a very wide productive horizon; the development program is expected to increase the present installed capacity tenfold. (3) Deep exploration revealed the presence of much wider productive horizons than the fields exploited at present, so that exploitation programs can also be drawn up for zones that were formerly noncommercial. REFERENCES Bagnoli, G., Gianelli, G., Puxeddu, M., Rau, A., Squarci, P. and Tongiorgi, M., 1980, Segnalazione di una potente successione clastica di eta probabilmente carbonifera ne1 basamento della Toscana meridionale: Mem. SOC. Geol. It., v. 21, p Batini, F., Burgassi, P.D., Cameli, G.M., Nicolich, R. and Squarci, P., 1978, Contribution to the study of the deep lithospheric profiles: deep reflecting horizons in the LarderelloTravale geothermal field: Mem. SOC. Geol. It., v. 9, p Bertini, G., Giovannoni, A., Stefani, G.C., Gianelli, G., Puxeddu, M. and Squarci, P., 1980, Deep exploration in the Larderello field: Sass0 22 drilling venture: Advances in European Geothermal Research, Proc. 2nd International Seminar on Results of EC Geothermal Energy Research, Strasbourg, 46 March 1980, Riedel, Dordrecht, p Burgassi, P.D., Stefani, G.C., Cataldi, R., Rossi, A.,Squarci, P. and Taffi, L., 1975, Recent developments of geothermal exploration in the TravaleRad~condoli area: Proceedings Second U.N. Sym~sium on the Development and Use of Geothermal Resources, San Francisco, May 1975, v. 3, p Calamai, A., Cataldi, R., Squarci, P. and Taffi, L , Geology,geophysics and hydrogeology of the Monte Amiata geothermal fields: Geothermics, special issue 1, p. 19. Castellucci, P., Minissale, A. and Puxeddu, M., 1983, Nature and tectonic setting of TravaleRadicondoli basement in the Larderello geothermal field (Italy): Mem. SOC. Geol. *It., v. 25, p Cataldi, R., ROSSI, A., Squarci, P., Stefani, G.C. and Taffi, L., 1970, Contribution to the knowledge of Larderello geothermal region: remarks on the Travale field: Geothermics, spec. issue 2, v. 2, pt. 1, p Celati, R., Squarci, P., Stefani,G.C. and Taffi, L., 1977,Studyof water levels in Larderello region geothermal wells for reconstruction of reservoir pressure trend: Geothermics, v. 6, p Ferrara, G.C., Panichi, C. and Stefani, G.C., 1970, Remarks on the geothermal phenomenon in an intensively exploited field. Results of an experimental well: Geothermics, special issue 2, v, 2, pt. 1, p Gianelli. G,, Puxeddu, M. and Squarci, P., 1978, Structural setting of the LarderelloTravale geothermal region: Mem. SOC. Geol. It., v. 19, p Puxeddu, M., 1984, Structure and Late Cenozoic evolution of the upper lithosphere in southwest Tuscany (Italy): Tectonophysics, v. 101, p