Definition of the "reference cores"

Figure 2 shows Xn²o - Xco² - XH^Z and Xn²o - Xco² - Xcn⁴ diagrams for all the fluids sampled before 1979. Points are very scattered and occupy over 60% of the plots with a common distribution between two straight lines. These lines define an internal triangle containing the H2O and CO2 corners. The limits of the chemical variations are represented by the values log Xn²o / X H²= log X H²o / Xcn⁴ =3.2 and log X co²/ X H² = log X co² / X c H⁴= 18.

H 2 0 / I O C

all available data from 1940 to 1979

SO 30 20 (C02/H2)

30H2 C02

50 30 20 (C02/CH4)

30CH4

Figure2. Relative concentrations of H20-C02-H2 and H20-C02-CH4 components expressed in mmol/mol on the total fluid. Samples derived from different sub-units of the field are distinguishable. The number of records used is reported in brackets.

Larderello

Central area :

• 825 records

1-11930-1949), 341 record»

2-U950-1959); 6 records 3-11960-1969); 204 records 4-U970-1979): 274 records

H2O/10O

Figure 3. •

Relative concentrations of H20-C02-H2 in the fluid from selected wells located

in a restricted area of th sub-unit of Larderello. The variations 1n time are shown in left corner.

30H2

2OO 100 50 (C02/H2)

Considering the trends in time of the individual wells in greater detail, the variations in the CO²/CH⁴ ratios for some of them are over 10 times. While the percentage of CO² over 25 times were observed. Very little scattering around an average value was found for the others. For example, data of the 'central' Larderello area show oscillations lower than 50% around the average values of the two parameters, namely 2.0±1.0 and 50±25 for the CO² and CO²/H²O ratios respectively (Fig. 3).

Because H2S, CH4, N2, NH3 and H3BO3 show the same pattern, the wells which maintained very similar chemical characteristics in the course of their production history were then isolated in the different sub-units of the geothermal field.

The calculated average chemical compositions of fluid components for the 'central' areas of each sub-unit are reported in Table 1.

Table 1. Fluid compositions of the main sub-units of the Larderello geothermal field for the period 1940-1979. The values, expressed in mmol/mol of total fluid, are calculated as an average of the analyses of the fluids delivered by wells characterised by 'stable' chemical composition.

Molar

' Sborgf et al. data (1932)

r average values of central areas (1940-1979)

<LD>

O^Castelnuovo(CN) (SS) Monterotondo <MW

onl Roosl <LGn>

û(|Lago (LC) 4g Surf ace

manifestations

30H2 C02

200 100 50 30 20 (C02/H2)

so 30 20 (C02/CH4)

30CH4

Figure 4. Relative concentrations of H2O-CO2-H2 and H20-C02-CH4 components related to the 'reference cores', surface manifestations existing before drilling, and first shallow drillholes (i.e. 'initial conditions').

Using the data from Table 1, the relative variations of H² and C⁴ with respect to H²O and CO² are plotted in the triangular diagrams (Fig. 4). The average composition calculated for each area (open symbols) occupy distinct positions in the plots, confirming the existence of compositional differences in the chemistry of the main gas components of the fluids emerging in different areas of the Larderello field. These areas will be indicated as 'reference core' of the different sub-units.

It is important to emphasize that most of the selected wells correspond to those drilled (between 1930 and 1940) near the old surface natural manifestations, which have now almost disappeared. Chemical analyses of gas samples collected both from wells and surface manifestations during and before 1930 are available from Sborgi, et al. [12]. These data are plotted in Fig. 4 (black symbols) and compared with the average values of the sub-units. A clear overlap between calculated and observed values both for H² and CH⁴ are observed.

Over half of Sborgi data came from wells with average depths of about 200 m in the early 1930s. Other data included shallow wells dating back to the century or even to manifestations existing in the pre-industrial period. Some of these represent steam jets sampled in 1895 and 1911 which died down in the late 20s due to interference with the wells later drilled in the area. They were considered as precious initial reference, more so since, in addition to the main constituents of the gas mixtures, their He and Ar contents were determined.

It should be stressed that the steam production of the whole field in the 1920s do not exceed the few hundreds tons/hour, most of it from natural manifestations. With the drilling, in the early 1930s, the steam production at Larderello area alone exceeded 700 t/h (380 of which produced by the wells 'Soff.l' and 'Soff.2'), while for the whole field it was around 1150 t/h. This very sizeable increase in production led to the radical depletion of the natural manifestations, which practically no longer existed by the end of the decade.

From the foregoing, it is evident that, though based on limited data, the Sborgi data may be considered the 'initial conditions' of the field. The relative position of the different productive areas of the boraciferous region (Fig. 4) constitutes a well-documented 'reference core' for each sub-units.

Two relevant exceptions are shown by the graphs of Fig. 4 where a depletion of the H² and CH⁴ contents in the gas mixtures emerge during the initial conditions in the Lago-Lagoni Rossi and Larderello-Castelnuovo sub-units.

This fact suggests the existence of different behaviours of the fluids in the northern areas of the field with respect to the southern ones. It also represents a little uncertainty on the definition of the 'reference cores'. So, figures from Table 1 are fundamental for the analyses of Figure 3, that is, for the interpretation of the temporal and geographical evolution of the fluids of the whole field.

One important consequence of the nearly perfect overlap between 'initial conditions' and the 'reference cores' is that the fluid from wells in the vicinity of the manifestations remained practically stable during the exploitation history of the field. The incompleteness of certain historical series of data, such those in the southern areas, is however not an irremediable inconvenience.

3.2. Variation of the gas chemistry as function of the expantion of the exploited areas