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THE USE OF GAS CHEMISTRY AND ISOTOPE GEOTHERMOMETRY TO STUDY THE EFFECTS OF EXPLOITATION AND REINJECTION
3. CHEMISTRY OF GAS MIXTURES
3.2. Variation of the gas chemistry as function of the expantion of the exploited areas Starting from the values of the 'reference core' for each sub-units, we are going to
analyse the variations observed in the gas compositions of the new wells until the pre-reinjection period (1979).
Figures 5 and 6 show the relative concentrations of H2O-CO2-H2-CH4 of gases sampled from wells drilled in the outlying zones of Larderello. The diagrams display the areas where homogeneous sets of points assume a quite different position from those belonging to the reference cores, and areas showing a more stable gas contents.
The variations occur both for CH4 and H2 of the same sub-units; Larderello , Castelnuovo and Lago show the largest ones, while Monterotondo, Serrazzano and Sasso appear to be unstable in time. Lagoni Rossi describes a particular trend.
Two main processes seem to be responsible for the distribution of data points in Figs.
5 and 6 : i) an addition of gas (mainly CO2 ) to the system may result to a shift toward the CO2 corner starting from the CO2 / H2 and CO2 / CH4 ratios of about 40 and 80 which are the values assumed for the reference core. The values reach a maximum of about 200 and 800 respectively; ii) an addition of water to the system may account for the trend which describe the strong depletion of % CO2 in the gas mixture with respect to the reference gas of the area.
In the case of CH4 , the decrease in CO2 is often coupled with a strong decrease in CH4, thereby reaching lowest values of 900 in the CO2 / CH4 ratio.
3.2.1 Larderello sub-unit
The above processes occur in different zones of the Larderello area. Wells located northeast and northwest of the reference area are affected by the addition of CO2, while wells drilled in southern and south-eastern area are affected by the addition of water.
99
H20/100 H2O/10O
\
LaldexcDo 1932 data '•« ™40-1979 data
300 100 50 30
(CO2/H2J
Bpfarth-Weatem u QNath-Eaatera or Q South-Weatera •
,3OH2
H2O/100
Monterotondo
30H2
300 ÏOO 50 30 (CO2/H2)
£ Central 4re*
QNorthern Q North-Butem QSouOiem
30H2
MO 100 50 30 1CO2/H2)
H2O/100
(268)
LAGO
C02 30H2
(CO2/H2)
H20/100 H2O/100
\
Senrazzano
/
MO 105 Sff 30 ÏO 10 8
(CO2/H2)
1940-1979 daU
^v Cenbalueft South-Bojitein a Northern area
30H2 C02
200 100 50 30 [CO2/H2]
30H2
H2O/100
CO2
MO 100 W 30 20 10 5
(CO2/H2)
1960-1979 data Notth-Eaatem tt
Southern ana
30H2
Figure 5.
Relative concentrations of H20-C02-H2 components for the fluids emerging in the seven sub-ts of the Larderello gethermal field.
CO2
LardereDo 1932 data 1940-1979 data mNtcth-Weatera area QN<«Ua.BM(Kn ftTMk
QSouth-Westem area
~s8ö ÜÄ 5? 30*" 20 ÎÔ S~
CO2/CH4
30CH4 C02
CO2
Monterotondo
^ 1932 data 3 j5 1960-1979 data
Southern are*
aoo loo so 30 C02/CH4
30CH4
H20/100
Seimzano
CO2 [ S i O f 3 0
CO2/CH4
Serrazzano
«r 1939 data 1940-1979 data
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^7 Noithem area
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CO2
North-&o*tero are«
aoo 100 BO CO2/CH4
30CH4
CutelnuOTo 1932 data 1940-1979 data
~MO Ï5S S? ^0 (C02/CH4)
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f 1932 data a Central area
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H2O/1OO
300 100 SO 30 (C02/CH4)
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Figure 6.
Relative concentrations of H20-C02-CH4 components for the fluids emerging in the seven sub-units of the Larderello geothermal field
The NW and NE zones are represented in the Figures 5 and 6 with different symbols even if they follow the same pattern when the H2 component is concerned. However, some differences appear to be relevant when CH4 is considered.
Hence, the Larderello area can be represented as a complex of three to four zones.
The northernmost zones display a stable H2 and CH4 content, but more CO2 (up to 5 times) than in the central, reference core zone. The other zones, including the wells drilled to the south, is basically characterised by a decrease in the gas/steam ratio.
3.2.2 Castelnuovo sub-unit
There has been a more complex history in Castelnuovo than in Larderello. Until the mid-1960s, all the old wells, also drilled in a limited area near the manifestations, maintained a nearly constant composition. After this date, substantial modifications were observed in the wells drilled in the southernmost zone. At the same time, new wells, generally located in the north north-eastern areas with respect to the reference core, produce fluids with gas components different from those of the central area.
The observed trends are reported in Figures 5 and 6 both for their H2 and CH4 contents as compared to the H2O and CO2 concentrations. The variations are practically identical to those described for gases from the Larderello sub-unit area.
To sum up, four zones are recognisable in the Castelnuovo area. The first, which is considered as a reference core, is the central zone. It was once larger, to include, before 1960, the wells further south, which from that time on can be differentiated as separated units.
The third zone is made up by the wells to the northeast, and the fourth by those located to the north. The fluids produced here can be considered a mixture between those present in the central core and those of the north-eastern zone.
3.2.3 Lago sub-unit
Triangular diagrams again show the H2 and CH4 variations with respect to the major components (Figs. 5 and 6). Three sets of data have been identified. The first corresponds to an internal zone defined as the reference core. The other two outlying zones are located further south. For the whole field, the H2O-CO2-H2 plot shows the greatest separation between the position of the wells belonging to the reference cores and the historical data of
1932, the latter being characterised by a higher H2 content.
The possible hypotheses to explain this difference are the following:
The historical data for Lago refer to wells with a limited flow rates ( at the time, less than 7 t/h) located in the southern zone which, in 1963, had undergone a considerable evolution. The period the variations began cannot be documented due to lack of previous data.
The wells which, owing to their unchanged characteristics from 1963 till the present (reference core) are located further north and were drilled at least 25 years after the wells for which historical data were collected.
In conclusion, despite these limitations, the only process affecting the reference core and two peripheral zones in the Lago area is that of H2O incursion. While the CO2 / H2 ratio remain approximately constant (R = 25), the relative concentration of CH4 with respect to CO2 increases from R = 30 to R = 500.
Similar results have also been found for the areas of Monterotondo, Lagoni Rossi and, to a lesser degree, Sasso and Serrazzano.