Chemical and isotopic variations of the gas components

Figure 8 shows the compositional variations between H²O-CO²-N² and H²O-CO²-H² species of the gas discharged by four wells located at different distances from reinjection sites.

H2O/100 ^H2O/1OO

%CO2

CO2

%CO2

0.9

N2x30 CO2 1.8 3.0

H2x30

Figure 8. Relative concentrations of H2O,CO2,N2 and H2 (mmol/mol of total fluid) observed in three selected wells before (full symbols) and after (open symbols).

N2/100

air

groundwater

Ar

Figure 9. Relative concentrations (mmol/mol on water free basis) of N2, He and Ar components before (full symbols) and after (empty symbols) reinjection.

Composition crustal and magmatic fluids together with N2/Ar ratios for the atmosphere and air

saturated ground-waters are reported for comparison ,

The variations observed for CH⁴ and H²S components are similar to those observed for N² and H2 respectively. The comparison between samples collected before and after-reinjection (full and empty symbols) indicates that only well No. 137 is strongly affected by the injected water.

The distribution of the data (Fig. 8) can be explained by a mixing between a deep component with a CO2 content of about 1.8 % and injected gas-depleted water. In the case of the other three wells the composition of H2 hardly changed.

H² variations imply that dilution effects of reinjected water are still evident even if the gas/steam ratios are not sensibly affected. On the other hand, the distribution of N2 suggests that reinjected water contributes largely to define the content of N2 in the geothermal fluid.

Well No. 137 trends, in fact, towards the point where the N² / H²O ratio corresponds to that of the air saturated groundwater at 20°C.

The contribution of meteoric gases to the fluids discharged after reinjection is well documented by Figure 9, where the relative contents of N2, He and Ar components are reported for gas mixtures collected before and after-reinjection.

Injected and/or infiltrated waters previously equilibrated with the atmosphere introduce in the deep reservoir 2.5xl04 mmol/mol of Ar. This amount is very similar to the average content of this component in the fluid produced during the period 1930-1965 (l.oxlO"4).

However the two endmembers differ in the N2 contents. The N2/Ar ratios are between 38 and 84 (values for the air and for the saturated underground-water, respectively), while in the original fluid this ratio is about 800. The radiogenic He is present in the deep fluid with a concentration of about 4X10"4 mmol/mol, but is completely absent in the secondary steam produced by reinjected waters.

The observed trend in Figure 9 seems to be the result of a simple mixing between deep components and the meteoric ones, introduced by waste fluids. Empty circles refer to gas emerging everywhere in the Larderello geothermal system, while full circles represent the situation recorded in the inner part of the field. In general, we can see that pre-reinjection data are able to define a mixing process between an almost pure magmatic component and a crustal one. Post-reinjection data reveal Ar as a very sensitive gas for evaluating the amount of the reinjected water in a single well. The usefulness of this tracer will be of the same order as the isotopic composition of the steam and the gas/steam ratio. Because the distribution coefficients of gases are high in the liquid/vapor transitions, gas concentrations in the produced steam are highly sensitive to the fraction of the injected water which evaporates nearby the productive zone.

Distribution and characterization of geothermal sub-units

derived from gas chemical

composition (all data up to 1989)

I main reinjection sites 1 unusual deep steam

| "I original deep steam Qow CO2) original deep steam (high CO2)

liquid water invasion mixed steam

Figure 10. Zonatlon of the Larderello geothermal field derived from chemical analyses of the fluids delivered by the productive wells after reinjection. Reinjection sites used in the period from 1979 to 1989 are indicated.

106

Distribution and characterization of geothermal sub-units

derived from isotopic data

(survey 1992)

». main reinjection sites

2 Km

original deep fluids

liquid water from natural infiltration liquid water from reinjection

Figure 1 1. Zonatlon of the Larderello geothermal field derived from the isotopic analyses of the steam produced during reinjection.

In evaluating recovery coefficients (i.e. of the amount of the reinjected water which is recovered from the productive wells near the injection sites), probably the relative concentrations of Ar, He and N² (or CO² ) may represent a tool which reflect 'immediately' the variations occurring in the reinjection sites. On the other hand, the use of stable isotopic composition of the steam delivered at the surface may give 'smoothed' results because the secondary vapour (from injected waters) consists of an admixture of the new evaporation fronts and steam from residual liquid plumes.

A picture of the state of the field in 1988 was obtained from the trends in the H² O-CO²-H² and H²O-CO²-CH⁴ diagrams for all the wells for which analyses were performed after the start of reinjection. The reference cores, and the vapour-rich zones of the field induced by the natural inflow or artificial injection of water are distinguishable in Figure 10. These

107

two different contributions are generally recognisable, but cannot be discriminated on the basis of the gas chemistry. For both of them the variation consists of a substantial dilution of the original gaseous mixtures.

In August 1992, an isotopic survey was carried-out in the field to distinguish the contribution of the natural fluid relative to the injected ones. Considering the range of the isotopic variations of the vapor produced before injection (6D = -40 %o = constant, and 6¹⁸O ranging between -6 %o and 0 %o), the natural recharge (6D ~ -40 %o 5¹⁸O = -7 %o) tends to modify only the ¹⁸O content of the steam, while the share of injected water (oD and 5¹⁸O ~ +5

%o) determine the variations in both isotopes.

The congruence of 6¹⁸O and 8²H distribution over the entire field (Fig. 11) with the chemical data is very good. The slight differences are changes in injection wells and the different response of the two methodologies for drawing up these maps. In addition, man-made and natural recharges are well distinguished.