- Email: [email protected]
Helium in the larderello geothermal fluid
Geothermic& Vol. 13, No. 3+ pp. 227 Printed in Great Britain.
HELIUM
IN THE
239, 1984.
0375
LARDERELLO
GEOTHERMAL
6505/84 $3.00 + 0.00 Pergamon Press Ltd. 1984 CNR.
FLUID*
F. D ' A M O R E [ and A. TRUESDELL:]: +lnternattonal Institute f o r Geothermal Research, CNR, Via del Buongu.sto 1, Pi,sa, Italy and +U.S. Geological Survey, Menlo Park, C+al~fornia, U.5+A. ( Received April 1983 ; accepted/or publication July 1983) AbslracI--A recent survey of helium concentrations in l.arderello steam showed that lhere has.been a strong decrease since earlier surveys and that the helium is concentrated in central zones of steam upflov,. The He decline and present distribution are interpreted as being due to the release of stored He produced by production-induced microfracturing.
INTRODUCTION In February 1977 an extensive survey was made of helium, gas/steam ratio, boric acid and major gas constituents of the fluid from the Larderello geothermal field ( D ' A m o r e et al., 1977; D'Amore, 1977). This survey covered 90 wells distributed over more than 200 km ~, and was conducted by the International Institute for Geothermal Research, Pisa, Italy, in collaboration with the French organization S T E P A M - C E A , who provided a portable spectrometer for measuring helium. The objective of the survey was to confirm the existence of an abnormally high helium content noted in geothermal areas (Roberts et al., 1975). The fact that helium is chemically inert enabled us to distinguish the effects of physical processes (which would affect only the helium) and chemical processes affecting the other gases. This paper only deals with the observed concentrations of He~; for an interpretation of the elementary and isotopic abundances of all the noble gases in the Larderello fluids, see the paper by Nuti (1984). P R E S E N T A T I O N OF RESULTS The results of the survey are given in Table 1, which shows the chemical parameters measured in 90 wells in the Larderello geothermal area. The gas/steam ratio is expressed in STP litres of gas per kg of steam. Boric acid is expressed in ppm (by weight) in the condensate, helium in ppm (by volume) in the non-condensable gas, and the gas species in volume %. The gas compositions were provided by ENEL. Figure 1 shows the distribution of the helium content in the non-condensable (dry) gas ( D ' A m o r e et al., 1977). The helium content measured in the geothermal field exhibits a distinctive areal distribution within the basin. The highest values (above 20 ppm) occur in the classical zone of the Larderello area, in Gabbri zone, in Monterotondo and Lustignano zones and the part of the Val di Cornia lying between the latter two zones. The absolute maximum, however, was measured in the well Sesta 1 (37 ppm), whose fluid consists of pressurized cold gas. This well lies on the eastern margin of the field, in an area of strong condensation not shown in Fig. 1. Helium concentrations decrease from north to south along the Gabbri - Castelnuovo line to reach a minima of 1 ppm at Castelnuovo. The southern-most part of the field (Lago and Lagoni Rossi) was not covered by the survey, so the present-day helium trend cannot be evaluated for *Work conducted as part of the ENEt.
CNR collaboration agreemem.
227
M
g
~
e
~
~
~
~ ~
~
~
r
~ d 2 ~ d ~
J
~
. . . .
~
.
.
d
.
.
d
.
~
~
d
~
~
N
2
~
~
d
~
d
~
N
~
d
.
~
.
.
d
.
m
~
~
. . . . .
~
~ d d d d d d
-
•
Helium in the Larderello Geothermal Fluid
229
Table I. Continued
Well 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90
G a s - steam ratio (I STP/kg)
H,BO, (ppm)
He (ppm)
CO:
17.7 40.1 24.2 19.0 28.4 110.5 20.7 20.3 -22.4 23.3 17.4 20.8 -15.4 15.4 14.4 21.2 16.0 22.5 20.8 -9.0 6.7 -16.3 -20.6 15.6 17.1 20.6
213 -164 225 439 -155 --156 419 324 114 --105 249 62 95 35 312 -420 484 -310 -178 204 328 440
13.6 19.6 9.6 18.1 17.1 22.9 10.5 8.7 2.4 12.2 23.7 14.9 I 1.2 8.4 9.5 11.5 15.7 0.8 15.9 1.3 17.0 30.8 28.4 33.4 20.2 25.4 23.9 13.3 13.4 18.2 24.8
H:S
CH~
N:
H:
0.53 0.53 0.16 0.54 0.68 0.86 0.35 0.31 0.10 0.44 0.83 0.50 0.42 0.40 0.30 0.35 0.46 0.03 0.55 0.03 0.71 0.57 1.03 1.25 0.72 0.47 0.70 0.45 0.56 0.63 0.64
2.68 3.40 2.90 3.13 2.95 3.34 2.11 2.11 1.45 3.06 3.26 3.86 2.60 2.24 2.16 2.46 2.83 1.99 2.75 1.85 2.07 3.88 3.56 3.97 3.65 3.95 3.30 3.21 3.52 3.78 3.89
(,,olume %) 92.97 91.58 93.99 92.25 91.90 90.26 94.93 95.25 96.63 92.80 91.08 91.15 93.81 95.15 94.56 93.93 92,86 97.21 92.91 97.28 93.09 87.67 88.79 87.46 87.47 90.97 90.65 91.22 91.83 91.04 90.36
2.57 2.81 1.97 2.27 2.62 3.99 1.44 1.26 1.36 2.10 3.17 3.19 1.71 1.33 1.82 2.16 2.21 0.64 2.18 0.71 2.06 4.49 2.51 3.02 5.35 3.06 2.62 2.82 2.35 2.95 3.31
1.26 1.67 0.98 1.81 1.86 1.55 1.17 1.08 0.47 1.60 1.66 1.31 1.47 0.88 1.17 2.09 1.64 0.12 1.61 0.14 2.07 3.39 3.97 4.29 2.82 2.56 2.73 2.30 1.74 1.60 1.81
this area. He/steam ratio, considered by Mazor (1978/1979) to be the significant factor in a restricted area o f the field (seven wells), shows no clear distribution within the basin in the present survey (90 wells). The linear correlations between He and the other species studied lead to the following values for the correlation coefficient r: x CH,
+
CH, N2 H2S H2 H3BO~
N2
y
r
He
0.93
He He He He He
0.90 0.81 0.66 0.61 0.82
There is obviously a strong correlation between He and CH4, N~ and H3BO~. Figures 2 and 3 show the He values versus the data on H,BO, and CH4 + N2. DISCUSSION
Helium at Larderello is of non-atmospheric origin The helium present in the geothermal fluids studied during this survey is not of atmospheric
230
F. D ' A m o r e a n d A . Truesdell 0
km
2
o_
Gc]bbrI
4.4.i
o ,,.. \ \
1 •
/
I J.19
~3, \ \
.67
~z6
\Lustignano
e61
~,'r "50 CasteLnuo:o 49
\ / r , ~O"
69 o
",
3.-,...... 4 4,' ",
~ o/'~-'LardereLLo ~, ~;~;38.23 .32~7
X O ~ n 0 75 F ~ / ; 9 ~ °SB "71 ~ ( ('77 64 "62 k o65X
°42
0 o74
sg. %.63
.78 .75
C,
~o"~ o87 / - . . 86J 89o /"-&O / •88 / 90 q /
I
o76
SQsso "80
"85 /
Lagonl Rossi ~ ' ~ - - " Logo
i
i
___20 -- -
~',181 ,
e82
o83
'l Monterotondo
/I
Fig. 1. Distribution of He content (in ppm) in the non-condensable gas in [he Larderello field (1977 data). Numbers refer to the wells listed in T a b l e I.
origin as (a) the He/At ratio in the fluid (0.5 -2.0) is much higher than that in the atmosphere (5.7 x 10-'), despite the greater solubility of Ar relative to He in water at recharge temperatures; (b) the helium content is several orders of magnitude higher than atmospheric helium expected to accompany the atmospheric noble gases Ne, Kr and Xe (Mazor, 1978/1979); (c) many helium and radiogenic argon analyses in natural gases in America and New Zealand have shown that the He/radiogenic Ar ratio is a f u n c t i o n of the concentration of uranium, thorium and potassium in the deep igneous and sedimentary rocks, with the usual observed ratio of He/radiogenic Ar between 2 and 6 (Zartman el al., 1961 ; Wasserburg et al., 1963; Ferrara et al., 1963). A correlation was found between helium content and radiogenic argon in the fluid of six wells sited between Larderello and Castelnuovo (Mazor, 1978/1979). This indicates that radiogenic argon and helium both come from the same rocks; and (d) helium is strongly correlated with methane and nitrogen, neither of which is of atmospheric origin at Larderello. The non-atmospheric origin of the nitrogen is shown by the N,/Ar ratios observed of 400- 900 (Table 3), compared with 3 5 - 8 0 for air-saturated water. Methane is a trace constituent in air (2 ppm), but is a major constituent of the geothermal gas (average 1.2070, Tables 1 and 2). The high ratios of CH, and N~ to Ar suggest the decay of organic compounds containing C and N derived from the largely sedimentary rocks of the geothermal reservoir (Sborgi, 1934; Ellis, 1967; Krauskopf, 1959; Hulston and McCabe, 1962; D'Amore and Nuti, 1977).
Helium in the Larderello Geothermal Fluid
23 I
50O I - I
5 •
•
O" m
•
o'~
• •
Z
I
-r-
E
: •
Ill
•
• ~"
•
°o~ ...
• •
• •
5o
•
"~- e •o
. ~,~..
J
...... ."
•
~....
•
L
J
5
1~
15
2O
25
ppm He
3O
35
o
I
5
~o
1
I
15
I
20
Fig. 2
I
25
1
30
He ppm Fig. 3
Fig. 2. Correlation between He content in non-condensable gas and H , B O , in the condensate, both in ppm (19"77 data). Fig. 3. Correlation between He content (ppm) and (CH4 + N2) (volume %) ]n the non-condensable ga', (1977 data).
Storage of helium in the Larderello system The non-atmospheric origin of the helium (demonstrated above) resutts from the radioactive decay of uranium and thorium (and their daughters), which produce alpha particles to form He atoms. This process must result in an accumulation with time in rocks containing U and Th. At Larderello much of this He must be released into the geothermal fluids circulating in the reservoir before exploitation ( D ' A m o r e and Truesdell, 1979) and some must be stored in rock minerals and more or less released with exploitation-induced pressure decreases. The amount of stored He (in fluids and rocks) is very large indeed. The calculated production of He in the crust and mantle is about 1000 x 1030 atoms per year, of which only 7 x 10 '° atoms escape from the atmosphere (Damon and Kulp, 1958; Reimer, 1976). He in the atmosphere represents only 2 million years' production (MacDonald, 1963), with mos! of the remaining He stored in rocks and underground fluids. Some exploited natural gases contain commercially extractable concentrations of He. Release of stored He and other constituents The rate of production of He from Larderello found in the recent survey (10" atoms/s) is many orders of magnitude higher than would be expected from any reasonable volume oi reservoir rocks. The He flux is equal to the normal flow from l0 ~ km ~ of the earth's surface, but issues from only the 10 s krn ~ area of the field. (Because only part of the field wa~ surveyed the total flux was calculated assuming that the observed average He concentration would he the same for the remaining wells.) The obvious explanation for the relatively high flow of He from Larderello is that He, Like other constituents of the geothermal fluid including heat and gases, is being removed from
F. D ' A m o r e
232
and A.
Truesdell
Table 2. Some chemical parameters of the fluid in the period 1934 to 1951 Well*
Year
Het
CO:{
H:S{
CH,{
N2{
H,{
H ,BO,,~
36 38 45 46 a b
1938 1948 1943 1939 1948 1948 1934 1938 1940 1941 1940 1941 1939 1934 1934 1940 1940 1939 1940 1940 1939 1939 1939 1950 1951 1951 1939 1939 1939 1939 1939
18.9 18.0 4.9 17.5 16.2 19.4 25.0 17.3 16.6 20.3 19.3 4.8 18.5 23.0 21.9 3.3 5.2 10.2 14.0 13.0 12.6 20.6 15.5 6.7 4.7 7.7 19.7 20.6 19.2 29.4 28.5
92.25 93.70 95.40 93 36 93.50 93.50 93.35 93.34 94.13 92.91 93.32 95.53 93.42 93.10 93.06 95.84 97.07 94.30 93.57
2.95 2.70 1.60 2.64 2.70 2.50 2.45 2.96 2.27 279 2.48 1.67 2.38 2.40 2.54 1.96 0.91 2.50 2.43
1.51 1.15 0.30 1.36 1.25 1.45 1.67 1.34 1.26 1.38 1.36 0.36 1.60 1.78 1.47 0.22 0.25 1.00 1.48 t.34 1.26 1.69 1.55 0.65 0.29 0.61 1.19 1.22 1.71 2.04 1.70
0.74 0.81 0.21 0.92 (I.84 0.85 0.95 0.76 (t.27 (I.84 0.84 0.18 0.85 1,01 0.79 0.11 0.12 0.65 0.74 0.87 0.58 0.93 0.77 0.31 0.19 0.45 0.63 0.89 0.61 0.50 0.85
2.55
400 41(I 320 500 320 460 650 610 500 380 34(1 500 470 500 50(1 520 440 620 400 670 684 920 800 260 250 210 350 300 45(1 400 300
LA LA LA LA LA LA
c LA d e f g h i j k I m n o p q r s 4 t 9 u v w y z
LA LA LA LA LA LA LA LA CA CA CA CA CA CA CA CA LA LA LA SE SE SA LG LR
--
-
95.28 94.30 94.61 94.90 95.82 93.90 91.90 92.00 92,70 87.36 88.85
1.72 1.77 1.89 2.30 2.18 2.90 3.00 3.00 2.50 3.34 2.95
1.64 2.49 1.72 1.71 1.70 1.58 1,60 1.5 r 2.21 2,00 2.26 1.85 1.71 2.14 1.88 1.62 1.54 1.78 1.78 1.16 1.31 1.18 1.84 1.52 2.14 3.28 2.89 2.48 6.76 5.65
*Well number is the same as in Table l, the letters refer to other wells. ?ppm in the non-condensable gas. {Volume %. §ppm in the condensate. LA; Larderello (central zone). CA; Castelnuovo. SE; Serrazzano. LG; Lago.
LR; Lagoni Rossi.
Table 3. Concentrations of argon in the steam from some wells at Larderello (classical zone)*
Well
Gas/steam ratio?
HzS
CO2
CH4
N,,
H2
Ar
I II 111 IV V VI VII VIII
22.8 25.2 26.0 36.3 60.0 61.8 85.3 126.0
2.80 1.93 2.05 1.97 1.02 1.32 1.07 0.35
93.01 93.75 94.16 94.42 97.03 95.24 95.94 97.27
1.32 1.52 1.12 1.22 0.61 1.10 1.10 0.96
0.60 0.87 0.53 0.67 0.38 1.24 0.87 0.84
2.27 1.94 2.12 1.67 0.97 1.10 1.01 0.57
0.00095 0.00124 0.00117 0.00127 0.00100 0.00139 0.001 I 0 0.00092
*Expressed in moles of a r g o n / m o l e s ot steam (CA,). t G a s / s t e a m ratio is expressed in litres of gas S T P / k g of steam. Gas composition is expressed in volume %.
CA,
× 10' 1.74 2.52 2.44 3.71 4.80 6.89 7.57 9.36
Helium in the Larderello Geothermal Fluid
233
storage at rates much greater than their natural flux. The release of stored He is demonstrated by the decrease of He concentrations with time. Analyses made by Sborgi (1934) and by the Larderello company before 1948 (given in Table 2) show that there has been a decline of 35 to 75°7o. Early He values in the central part of the Larderello field average (13 wells) 20 _+ 3 ppm, compared to the present average of about 13 ppm for the same area. In the Castelnuovo zone the early He values average 12 _+ 6 p p m (eight wells), compared to the present average of 3 _+ 2 ppm. One well sited halfway between these two zones has a value of 5 ppm, compared to the 2 ppm found recently. A good correlation between He and volume 070 of (CH, + N2) can also be noted in these older data (Fig. 4). 3
Z +
••
t/ 1
I
10
1
15
I
20
I
25
I
30
He (pprn) Fig. 4. Correlation between Fie content (ppm) and (CFi, + N~) (volume %) in the non-condensable gas (1934 to 195l data).
The He content retains higher values wherever the C H , and N~ values are higher. However, the decrease in He content with time is usually not accompanied by a corresponding decrease in CH, and N2. It is possible that the elevated temperatures of the Larderello field may have caused an accelerated rate of release of trapped He. Experimentally the diffusivity of He in silicate glass increases 103 times for a temperature change from 20°C to 500°C. Tectonic activity related to the heat concentration may also provide more pathways to the surface. These mechanisms could be partial explanations for the occurrence of high concentrations of He in fluids from the high temperature zones (compare Figs 1 and 5), but do not explain the correlation with CH, and N2, and we do not consider them primary reasons for the high He in these zones. The correlation of He with high temperatures and fracturing is more likely a coincidence, as will be discussed later.
Correlation of He with boric acid A strong correlation can also be noted between helium concentration in the gas and the boric acid steam ratio with r -- 0.82 in the recent survey. Comparison of the old values for the boric acid content in the well fluids with 1977 data further highlights the correlation between boric acid and He. Let us also compare the oldest H3BO3 data in Table 2 referring to the central, so-called classical, area of the Larderello field with the 1977 data on H,BO~, and, for the same area, the He values for both periods. During the period of the old helium measurements, the boric acid concentration ranged between 380 and 600 ppm; it has now decreased to its present average of about 200 ppm. The
234
F. D ' A m o r e and A. Truesdell 0
2
4 kin
'q3
a
L ANDENELLO
~,
,.---50
MON~EROTON
~
i
Fig. 5. Temperature map (°C) of the top of the reservoir in the Larderello field. Vertical lines: ophiolite outcrops. Squared areas: limestone outcrops. Blank areas: shales.
boric acid content in the well fluid has thus, like helium content, decreased with time, but at a faster rate. These parallel declines in He and H3BO3 might imply a genetic connection between these substances. However, their chemical behavlour is extremely different and it seems most likely that they were found in the central zone.of Larderello for different reasons. Boric acid is found close to the upflow o f steam because it is highly soluble in liquid water and accumulates in the first condensate formed as steam flowing laterally from the centre cools conductively ( D ' A m o r e and Truesdell, 1979). This condensate was near the surface and formed a large part of early production but is now less important. Helium release was also related to heavy exploitation of the near surface central zones, but this release was due to the pressure decrease discussed below. Thus the correlation o f He with boric acid, high temperature and fracturing results from the shallow depth of the reservoirs and heavy early exploitation in the central zones of steam upflow, rather than from any genetic connection.
Helium in the Larderello Geothermal Fluid
235
Comparison with the convection model of D'Amore and Truesdell The occurrence of the highest He concentrations in the central high temperature zones, and the good correlation with H3BO3, are inconsistent with the behaviour of He as a water-insoluble gas predicted by the convection model of D ' A m o r e and Truesdell (1979). From this model water-insoluble gases should be swept from the central upflow zones to accumulate in the peripheral zones of condensation. In contrast H3BO3 should occur (as it does) in the highest concentrations in zones of upflow before being partially removed in liquid formed during steam condensation. The relationship of He concentrations to zones of upflow and condensation was examined through the study of the relations between helium data and the corresponding gas/steam ratio. Isolating the L a r d e r e l l o - Castelnuovo basin from the rest of the field, and plotting the helium data (in ppm) of the non-condensable gas versus the gas/steam ratios (STP litres of gas/kg of steam) (Fig. 6), two different trends can be noted (1 and 2 in Fig. 6) that are related to well locations in the field (Fig. 7).
i
I
IIl
~o-
~
i
--, OAr • o
•
:
J
•
• / ,'
0 0 O0 jO ~ •
0~
•
I
. I
I'Ij~o
c
o
•
E
~/--/
•/~A
'~
•
•
4o
~Z oo
0
0
i
o
_ _ 6o
I litre$
STP/kg
~
h
loo
~2o
__
Fig, 6. Correlation between He content (ppm) in the non-condensable gas and gas/steam ratio (I S T P / k g steam) for the basin NE of the Larderello field (towns of Larderello and Castelnuovo).
The gas/steam ratio in the central area lies within a rather narrow range, whereas the present He content will depe~,d on the age of the well and, possibly, on local temperatures. The He contents are, in fact, higher in the wells drilled most recently. The zone further south of Castelnuovo is affected by dilution of steam derived from shallow recharge waters and has lower reservoir temperatures than in the central zone. Moreover, its wells are usually very old. As expected, low He contents are found in this area. In the central and marginal zones (1 and 2) the factors influencing the gas/steam ratio and He concentration in the gas are those summarized in Fig. 8. The horizontal arrows indicate the effect of these factors on gas/steam ratios, and the vertical arrows the effects on the He contents. Condensation may tend to increase the CO2 content (main gas) with respect to steam, but it also selectively increases the He/CO2 ratio, as He is about five times less soluble than CO2 at 2 3 0 - 250°C. Exhaustion of the He accumulation, which cannot be renewed in the short-term, will tend to lower the He/CO2 ratio. A temperature decrease in the reservoir caused by mixing with low enthalpy fluids would also have the same effect. The wells of group B (Fig. 6) are much younger than those of group D or E (Castelnuovo). Figure 7 shows a B - E alignment
F. D'Amore and A. Truesdell
236
/ /
I, o
.,
Z/ \ \
•
A
•
• ~~-°~'~
1/
•
•
%o
•
~
D
'
•
~
'
•
•
A
• •
•
• E
\
Fig. 7. Map of the zone corresponding to the points of Fig. 6, This zone is divided into five areas corresponding to five groups of wells classified in Fig. 6. ZONE I
ZONE 2
DILUTION
DILUTION
CONDENSATION
Zx ~.~.:I:
>m ZX
O~
o-r
mO
~A no
gz
mE
z
"Q~ z
;t iNCREASE
gas/steam
Fig. 8. Schematic representation of the factors influencing gas/steam ratio and He concentration in the gas. Horizontal
arrows refer to the effect of these factors on gas/steam ratio; vertical arrows refer to their effect on He content. along a N - S axis. O n e isolated point o f group E in Fig. 6, with high H e content and low g a s / s t e a m ratio, represents a well f r o m the C a s t e l n u o v o z o n e producing from a z o n e o f the reservoir affected by dilution. It is deeper than the other wells and thus hotter and less affected by exploitation.
H e l i u m in the Larderello G e o t h e r m a l Fluid
237
The slope of trend No. 2 can be calculated by assuming that condensation is the main phenomenon, together with a generalized exhaustion of He accumulation. In the model of D ' A m o r e and Truesdell (1979) an equation was developed to explain the distribution of the CO2, NH3, H3BO3, HCI and 1sO concentrations in the steam related to the amount of condensation by a Raleigh process, and applied to Larderello. Let K, be the distribution coefficient of the species i, that is, the ratio between its concentration in the steam and in the liquid. Let m be a residual steam mass and m ° the initial mass of steam betore any condensation process; m / m ° is the fraction of residual steam after a given amount of condensation. The mass of steam m contains a concentration C, of the species i, while C? represents the concentration in the mass m °. For an isothermal condensation, we have: C,/CT~ = ( m / m ° ) ~l/~'i
i~
The slope of straight line 2 between 0 and 130 l S T P / k g and 0 - 15 ppm of He can be reproduced approximately at 230°C by assuming a m a x i m u m condensation of 80% ( m / m ° = 0.2), and initial He and CO2 concentrations of 5 p p m and 35 1 S T P / k g respectively. If He were predominantly influenced by the original convective circulation in the reservoir described by D ' A m o r e and Truesdell (1979), then its concentration should increase with increasing condensation away from zones of steam upflow. In this respect it should behave similarily to Ar (also chemically inert), which shows the expected increase with steam condensation ( D ' A m o r e et al., 1980). Table 3 shows that the argon concentration is directly proportional to gas/steam ratio. Helium is, however, principally concentrated in the zones of upflow which correspond with the zones of highest production. This observation and the strong correlation with CH, and N2 leads us to propose the following theory. A m o d e l f o r He release in the central zones o f Larderello
In the original (pre-exploitation) condition, He in the Larderello reservoir was contained in two states: (1) dissolved in the geothermal fluid which circulated in the manner described by D ' A m o r e and Truesdell (1979). This He would have had the expected increase with condensation and its distribution in the fluid was similar to that o f Ar as measured recently and (2) contained in blind cavities, fluid inclusions and grain boundaries of the reservoir rock. This He was immobilized and did not take part in the fluid circulation. When exploitation of the field began the pressures in the reservoir were greatly reduced, causing vaporization of liquid water and the production of steam. The reduction of pressure also vaporized water in cavities (and, possibly, inclusions), with release of existing stress and breaking of the rock, as indicated by high microseismicity (Bufe and Shearer, 1981; Batini et al., 1981a,b). This breaking of the rocks not only released water as steam but also released previously immobilized gases including He, CH4 and N2. The evolution of CH4 and N2 may have taken place over a long period as exposed organic matter continued to decompose, but the He release would have been over a shorter time since its storage was only in existing trapped fluids and on grain boundaries. Thus, while the release of He would depend on the rate of rock breaking induced by pressure decrease (and indicated by the rate of seismicity), and would decrease abruptly after each break, the release of C H , and N~ would be more gradual. This would explain the continued high contents of CH, and N2 even when He contents have declined. Applying this theory to the geographical distribution of He shown in Figs l, 6 and 7, we can see that the high He (and CH~ and N2) in the central Larderello area is related to the great decrease in pressure due to intensive exploitation in this zone. High He in a fex~. ~,ells (C in Figs
238
F. D ' A m o r e a n d A . Truesdell
6 a n d 7) m a y also result f r o m the g r e a t e r d e p t h o f drilling a n d greater r o c k v o l u m e t a p p e d . T h e seismicity o f the central L a r d e r e l l o zone is not high at p r e s e n t (Batini et al., 1981a,b), but pressures in this zone h a d a l r e a d y d e c r e a s e d greatly a n d were relatively low a n d c o n s t a n t ( c o n t r o l l e d by m i n i m u m p o w e r p l a n t inlet pressures) d u r i n g the p e r i o d o f the seismic survey. T h e s t r o n g r e l a t i o n between m i c r o e a r t h q u a k e s a n d p r o d u c t i o n d e m o n s t r a t e d at T h e G e y s e r s (Bufe a n d S h e a r e r , 1981) suggests that the central L a r d e r e l l o zone m u s t have been seismically active d u r i n g the p e r i o d o f m a j o r pressure lowering d u r i n g h e a v y drilling.
CONCLUSIONS C o n c e n t r a t i o n s o f H e in L a r d e r e l l o steam have been s h o w n to result f r o m the release o f H e c o n t a i n e d in circulating fluids (evident m o s t l y in p e r i p h e r a l zones) a n d o f H e c o n t a i n e d in sealed cavities a n d on g r a i n b o u n d a r i e s . T h e u n e x p e c t e d l y high c o n c e n t r a t i o n s o f H e in the central zones o f s t e a m u p f l o w result f r o m the relatively great l o w e r i n g o f p r e s s u r e a n d resulting m i c r o f r a c t u r i n g o f r o c k in these zones c a u s e d by h e a v y e x p l o i t a t i o n o f shallow steam, T h e c o r r e l a t i o n o f H e with C H , a n d N2 is genetic b e c a u s e all o f these gases are f o r m e d in rock m a t e r i a l s ( f r o m U, Th a n d o r g a n i c m a t t e r ) a n d released d u r i n g m i c r o f r a c t u r i n g . T h e c o r r e l a t i o n with b o r i c acid, high t e m p e r a t u r e s a n d m a c r o f r a c t u r e s is a c o i n c i d e n t a l result o f the shallow reservoir d e p t h in steam u p f l o w zones, which lead to early, s t r o n g e x p l o i t a t i o n a n d pressure r e d u c t i o n with H e release a n d the presence o f high t e m p e r a t u r e s , tectonic activity a n d s t r o n g s e p a r a t i o n o f b o r i c acid into c o n d e n s a t e c h a r a c t e r i s t i c o f these zones. He, as N~, does n o t f o l l o w the c o n d e n s a t i o n m o d e l o f D ' A m o r e a n d Truesdell (1979), as o p p o s e d to CO2, NH3, H3BO3, HCI, A r a n d the '~O o f the s t e a m . Acknowledgement--The authors wish to thank ENEL for providing data on gas composition.
REFERENCES Batini, F., Bufe, C., Cameli, G. M,, Console, R. and Fiordelisi, A. (1981a) Seismic monitoring.in Italian geothermal areas: I. Seismic activity in the Larderello- Travale region. Proc. 2nd DOE-ENEL Workshop Cooperative Res. Geoth. Energy, Lawrence Berkeley Laboratory, Berkeley, 20-23 October 1980, pp. 20-47. Batini, F., Cameli, G. M., Carabelli, E. and Fiordelisi, A. (1981b) Seismic monitoring in Italian geothermal areas: 1I. Seismic activity in the geothermal fields during exploration. ,Proc. 2nd DOE-ENEL Workshop Cooperative Res. Geoth. Energy, Lawrence Berkeley Laboratory, Berkeley, 2 0 - 23 October 1980, pp. 48- 85. Bufe, C. G. and Shearer, P. M. (1981) Geothermal induced seismicity--bane or blessing? Proc. 2nd DOE-ENEL Workshop Cooperative Res. Geoth. Energy, Lawrence Berkeley Laboratory, Berkeley, 20-23 October 1980, pp. 86- 97. Damon, P. E. and Kulp, J. L. (1958) Inert gases and the evolution of the atmosphere. Geochim. cosmochim. Acta 13, 280- 292. D'Amore, F. (1977) Nuove considerazioni relative ai tenori di elio e radon dei fluidi del campo geotermico di Larderello. Internal Report, International Institute for Geothermal Research, CNR, Pisa. D'Amore, F. and Nuti, S. (1977) Notes on the chemistry of geothermal gases. Geothermics 6, 39-65. D'Amore, F. and Truesdell, A. H. (1979) Models for steam chemistry at Larderello and The Geysers. Proc. 5th Workshop Geoth. Reserv. Engn., Stanford, 12-14 December 1979, pp. 283-297. D'Amore, F., Caprai, A., Del Chicca, F. and Del Marmol, M. A. (1980) Studio del tenore del gas argon nei fluidi dei campi geotermici della Toscana. Internal Report, International Institute for Geothermal Research, CNR, Pisa. D'Amore, F., Ferrara, G. C., Noto, P. and Nuti, S. (1977) Tenori di clio nei fluidi del campo geotermieo di Larderello. Prime considerazioni sulle relazioni tra clio e altri componenti del fluido. Internal Report, International Institute for Geothermal Research, CNR, Pisa. Ellis, A. J. (1967) Chemistry of some explored geothermal systems, in Geochemistry of Hydrothermal Ore Deposits (Edited by H. L. Barnes) pp. 465-511. Holt, Rinehart & Winston, New York. Ferrara, G., Gonfiantini, R. and Pistoia, P. (1963) Isotopic composition of argon from steam jets of Tuscany, in Nuclear Geology of Geothermal Areas (Edited by E. Tongiorgi) pp. 267 - 275. Spoleto. Hulston, J. R. and McCabe, W. J. (1962) Mass spectrometer measurements in the thermal areas of New Zealand. Part 1. Carbon dioxide and residual gas analyses. Geochim. cosmochim. Acta 26, 383 - 387. Krauskopf, K. B. (1959) The use of equilibrium calculations in finding the composition of a magmatic gas phase, in Researches in Geochemistry (Edited by P. H. Abelson) pp. 260 - 278. John Wiley.
H e l i u m in the Larderello G e o t h e r m a l Fluid
239
MacDonald, G. J. F. (1963) The escape of helium from the earth's atmosphere. Rev. Geophys. I, 305. Mazor, E. (1978/1979) Noble gases in a section across th.e vapour-dominated geothermal field of Larderelto, ha[y. Geofis. pura appl. !17, 2 6 2 - 2 7 5 . Nuti, S+ (1984) Elementary and isotopic compositions of noble gases in geothermal fluids of Tuscany, haly. Geothermics 13, 215 - 226. Reimer, G. M. (1976) Helium detection as a guide for uranium exploration. U.S. Geol. Surv. Open File Report 76 - 240. Roberts, A. A., Friedman, I., Donovan, T. J. and Denton, E. H. (1975) Helium survey, a possible technique for locating geothermal reservoirs. Geophys. Res. Lett. 2, 2 0 9 - 210. Sborgi, U. (1934) Studi e ricerche sui gas dei soffioni boraciferi con particolare riguardo al loro contenuto in clio ed altri gas nobili. Reale Accademia d'ltalia, Memorie della classe di Scienze fisiche, matematiche e naturali, V, pp. 667 - 713. Wasserburg, G. J., Mazor, E. and Zartman, R. E. (1963) Isotopic and chemical composition of some terrestrial natural gases, in Earth Science and Meteoritics (Edited by J. Geiss and E. D. Goldberg) pp. 219 240. NorthHolland, Amsterdam. Zartman, R. E., Wasserburg, G.. J. and Reynolds, J. H. (1961) Helium, argon and carbon in some natural gases. J. Geophys. Res. 66, 2 7 7 - 306.
HELIUM
IN THE
239, 1984.
0375
LARDERELLO
GEOTHERMAL
6505/84 $3.00 + 0.00 Pergamon Press Ltd. 1984 CNR.
FLUID*
F. D ' A M O R E [ and A. TRUESDELL:]: +lnternattonal Institute f o r Geothermal Research, CNR, Via del Buongu.sto 1, Pi,sa, Italy and +U.S. Geological Survey, Menlo Park, C+al~fornia, U.5+A. ( Received April 1983 ; accepted/or publication July 1983) AbslracI--A recent survey of helium concentrations in l.arderello steam showed that lhere has.been a strong decrease since earlier surveys and that the helium is concentrated in central zones of steam upflov,. The He decline and present distribution are interpreted as being due to the release of stored He produced by production-induced microfracturing.
INTRODUCTION In February 1977 an extensive survey was made of helium, gas/steam ratio, boric acid and major gas constituents of the fluid from the Larderello geothermal field ( D ' A m o r e et al., 1977; D'Amore, 1977). This survey covered 90 wells distributed over more than 200 km ~, and was conducted by the International Institute for Geothermal Research, Pisa, Italy, in collaboration with the French organization S T E P A M - C E A , who provided a portable spectrometer for measuring helium. The objective of the survey was to confirm the existence of an abnormally high helium content noted in geothermal areas (Roberts et al., 1975). The fact that helium is chemically inert enabled us to distinguish the effects of physical processes (which would affect only the helium) and chemical processes affecting the other gases. This paper only deals with the observed concentrations of He~; for an interpretation of the elementary and isotopic abundances of all the noble gases in the Larderello fluids, see the paper by Nuti (1984). P R E S E N T A T I O N OF RESULTS The results of the survey are given in Table 1, which shows the chemical parameters measured in 90 wells in the Larderello geothermal area. The gas/steam ratio is expressed in STP litres of gas per kg of steam. Boric acid is expressed in ppm (by weight) in the condensate, helium in ppm (by volume) in the non-condensable gas, and the gas species in volume %. The gas compositions were provided by ENEL. Figure 1 shows the distribution of the helium content in the non-condensable (dry) gas ( D ' A m o r e et al., 1977). The helium content measured in the geothermal field exhibits a distinctive areal distribution within the basin. The highest values (above 20 ppm) occur in the classical zone of the Larderello area, in Gabbri zone, in Monterotondo and Lustignano zones and the part of the Val di Cornia lying between the latter two zones. The absolute maximum, however, was measured in the well Sesta 1 (37 ppm), whose fluid consists of pressurized cold gas. This well lies on the eastern margin of the field, in an area of strong condensation not shown in Fig. 1. Helium concentrations decrease from north to south along the Gabbri - Castelnuovo line to reach a minima of 1 ppm at Castelnuovo. The southern-most part of the field (Lago and Lagoni Rossi) was not covered by the survey, so the present-day helium trend cannot be evaluated for *Work conducted as part of the ENEt.
CNR collaboration agreemem.
227
M
g
~
e
~
~
~
~ ~
~
~
r
~ d 2 ~ d ~
J
~
. . . .
~
.
.
d
.
.
d
.
~
~
d
~
~
N
2
~
~
d
~
d
~
N
~
d
.
~
.
.
d
.
m
~
~
. . . . .
~
~ d d d d d d
-
•
Helium in the Larderello Geothermal Fluid
229
Table I. Continued
Well 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90
G a s - steam ratio (I STP/kg)
H,BO, (ppm)
He (ppm)
CO:
17.7 40.1 24.2 19.0 28.4 110.5 20.7 20.3 -22.4 23.3 17.4 20.8 -15.4 15.4 14.4 21.2 16.0 22.5 20.8 -9.0 6.7 -16.3 -20.6 15.6 17.1 20.6
213 -164 225 439 -155 --156 419 324 114 --105 249 62 95 35 312 -420 484 -310 -178 204 328 440
13.6 19.6 9.6 18.1 17.1 22.9 10.5 8.7 2.4 12.2 23.7 14.9 I 1.2 8.4 9.5 11.5 15.7 0.8 15.9 1.3 17.0 30.8 28.4 33.4 20.2 25.4 23.9 13.3 13.4 18.2 24.8
H:S
CH~
N:
H:
0.53 0.53 0.16 0.54 0.68 0.86 0.35 0.31 0.10 0.44 0.83 0.50 0.42 0.40 0.30 0.35 0.46 0.03 0.55 0.03 0.71 0.57 1.03 1.25 0.72 0.47 0.70 0.45 0.56 0.63 0.64
2.68 3.40 2.90 3.13 2.95 3.34 2.11 2.11 1.45 3.06 3.26 3.86 2.60 2.24 2.16 2.46 2.83 1.99 2.75 1.85 2.07 3.88 3.56 3.97 3.65 3.95 3.30 3.21 3.52 3.78 3.89
(,,olume %) 92.97 91.58 93.99 92.25 91.90 90.26 94.93 95.25 96.63 92.80 91.08 91.15 93.81 95.15 94.56 93.93 92,86 97.21 92.91 97.28 93.09 87.67 88.79 87.46 87.47 90.97 90.65 91.22 91.83 91.04 90.36
2.57 2.81 1.97 2.27 2.62 3.99 1.44 1.26 1.36 2.10 3.17 3.19 1.71 1.33 1.82 2.16 2.21 0.64 2.18 0.71 2.06 4.49 2.51 3.02 5.35 3.06 2.62 2.82 2.35 2.95 3.31
1.26 1.67 0.98 1.81 1.86 1.55 1.17 1.08 0.47 1.60 1.66 1.31 1.47 0.88 1.17 2.09 1.64 0.12 1.61 0.14 2.07 3.39 3.97 4.29 2.82 2.56 2.73 2.30 1.74 1.60 1.81
this area. He/steam ratio, considered by Mazor (1978/1979) to be the significant factor in a restricted area o f the field (seven wells), shows no clear distribution within the basin in the present survey (90 wells). The linear correlations between He and the other species studied lead to the following values for the correlation coefficient r: x CH,
+
CH, N2 H2S H2 H3BO~
N2
y
r
He
0.93
He He He He He
0.90 0.81 0.66 0.61 0.82
There is obviously a strong correlation between He and CH4, N~ and H3BO~. Figures 2 and 3 show the He values versus the data on H,BO, and CH4 + N2. DISCUSSION
Helium at Larderello is of non-atmospheric origin The helium present in the geothermal fluids studied during this survey is not of atmospheric
230
F. D ' A m o r e a n d A . Truesdell 0
km
2
o_
Gc]bbrI
4.4.i
o ,,.. \ \
1 •
/
I J.19
~3, \ \
.67
~z6
\Lustignano
e61
~,'r "50 CasteLnuo:o 49
\ / r , ~O"
69 o
",
3.-,...... 4 4,' ",
~ o/'~-'LardereLLo ~, ~;~;38.23 .32~7
X O ~ n 0 75 F ~ / ; 9 ~ °SB "71 ~ ( ('77 64 "62 k o65X
°42
0 o74
sg. %.63
.78 .75
C,
~o"~ o87 / - . . 86J 89o /"-&O / •88 / 90 q /
I
o76
SQsso "80
"85 /
Lagonl Rossi ~ ' ~ - - " Logo
i
i
___20 -- -
~',181 ,
e82
o83
'l Monterotondo
/I
Fig. 1. Distribution of He content (in ppm) in the non-condensable gas in [he Larderello field (1977 data). Numbers refer to the wells listed in T a b l e I.
origin as (a) the He/At ratio in the fluid (0.5 -2.0) is much higher than that in the atmosphere (5.7 x 10-'), despite the greater solubility of Ar relative to He in water at recharge temperatures; (b) the helium content is several orders of magnitude higher than atmospheric helium expected to accompany the atmospheric noble gases Ne, Kr and Xe (Mazor, 1978/1979); (c) many helium and radiogenic argon analyses in natural gases in America and New Zealand have shown that the He/radiogenic Ar ratio is a f u n c t i o n of the concentration of uranium, thorium and potassium in the deep igneous and sedimentary rocks, with the usual observed ratio of He/radiogenic Ar between 2 and 6 (Zartman el al., 1961 ; Wasserburg et al., 1963; Ferrara et al., 1963). A correlation was found between helium content and radiogenic argon in the fluid of six wells sited between Larderello and Castelnuovo (Mazor, 1978/1979). This indicates that radiogenic argon and helium both come from the same rocks; and (d) helium is strongly correlated with methane and nitrogen, neither of which is of atmospheric origin at Larderello. The non-atmospheric origin of the nitrogen is shown by the N,/Ar ratios observed of 400- 900 (Table 3), compared with 3 5 - 8 0 for air-saturated water. Methane is a trace constituent in air (2 ppm), but is a major constituent of the geothermal gas (average 1.2070, Tables 1 and 2). The high ratios of CH, and N~ to Ar suggest the decay of organic compounds containing C and N derived from the largely sedimentary rocks of the geothermal reservoir (Sborgi, 1934; Ellis, 1967; Krauskopf, 1959; Hulston and McCabe, 1962; D'Amore and Nuti, 1977).
Helium in the Larderello Geothermal Fluid
23 I
50O I - I
5 •
•
O" m
•
o'~
• •
Z
I
-r-
E
: •
Ill
•
• ~"
•
°o~ ...
• •
• •
5o
•
"~- e •o
. ~,~..
J
...... ."
•
~....
•
L
J
5
1~
15
2O
25
ppm He
3O
35
o
I
5
~o
1
I
15
I
20
Fig. 2
I
25
1
30
He ppm Fig. 3
Fig. 2. Correlation between He content in non-condensable gas and H , B O , in the condensate, both in ppm (19"77 data). Fig. 3. Correlation between He content (ppm) and (CH4 + N2) (volume %) ]n the non-condensable ga', (1977 data).
Storage of helium in the Larderello system The non-atmospheric origin of the helium (demonstrated above) resutts from the radioactive decay of uranium and thorium (and their daughters), which produce alpha particles to form He atoms. This process must result in an accumulation with time in rocks containing U and Th. At Larderello much of this He must be released into the geothermal fluids circulating in the reservoir before exploitation ( D ' A m o r e and Truesdell, 1979) and some must be stored in rock minerals and more or less released with exploitation-induced pressure decreases. The amount of stored He (in fluids and rocks) is very large indeed. The calculated production of He in the crust and mantle is about 1000 x 1030 atoms per year, of which only 7 x 10 '° atoms escape from the atmosphere (Damon and Kulp, 1958; Reimer, 1976). He in the atmosphere represents only 2 million years' production (MacDonald, 1963), with mos! of the remaining He stored in rocks and underground fluids. Some exploited natural gases contain commercially extractable concentrations of He. Release of stored He and other constituents The rate of production of He from Larderello found in the recent survey (10" atoms/s) is many orders of magnitude higher than would be expected from any reasonable volume oi reservoir rocks. The He flux is equal to the normal flow from l0 ~ km ~ of the earth's surface, but issues from only the 10 s krn ~ area of the field. (Because only part of the field wa~ surveyed the total flux was calculated assuming that the observed average He concentration would he the same for the remaining wells.) The obvious explanation for the relatively high flow of He from Larderello is that He, Like other constituents of the geothermal fluid including heat and gases, is being removed from
F. D ' A m o r e
232
and A.
Truesdell
Table 2. Some chemical parameters of the fluid in the period 1934 to 1951 Well*
Year
Het
CO:{
H:S{
CH,{
N2{
H,{
H ,BO,,~
36 38 45 46 a b
1938 1948 1943 1939 1948 1948 1934 1938 1940 1941 1940 1941 1939 1934 1934 1940 1940 1939 1940 1940 1939 1939 1939 1950 1951 1951 1939 1939 1939 1939 1939
18.9 18.0 4.9 17.5 16.2 19.4 25.0 17.3 16.6 20.3 19.3 4.8 18.5 23.0 21.9 3.3 5.2 10.2 14.0 13.0 12.6 20.6 15.5 6.7 4.7 7.7 19.7 20.6 19.2 29.4 28.5
92.25 93.70 95.40 93 36 93.50 93.50 93.35 93.34 94.13 92.91 93.32 95.53 93.42 93.10 93.06 95.84 97.07 94.30 93.57
2.95 2.70 1.60 2.64 2.70 2.50 2.45 2.96 2.27 279 2.48 1.67 2.38 2.40 2.54 1.96 0.91 2.50 2.43
1.51 1.15 0.30 1.36 1.25 1.45 1.67 1.34 1.26 1.38 1.36 0.36 1.60 1.78 1.47 0.22 0.25 1.00 1.48 t.34 1.26 1.69 1.55 0.65 0.29 0.61 1.19 1.22 1.71 2.04 1.70
0.74 0.81 0.21 0.92 (I.84 0.85 0.95 0.76 (t.27 (I.84 0.84 0.18 0.85 1,01 0.79 0.11 0.12 0.65 0.74 0.87 0.58 0.93 0.77 0.31 0.19 0.45 0.63 0.89 0.61 0.50 0.85
2.55
400 41(I 320 500 320 460 650 610 500 380 34(1 500 470 500 50(1 520 440 620 400 670 684 920 800 260 250 210 350 300 45(1 400 300
LA LA LA LA LA LA
c LA d e f g h i j k I m n o p q r s 4 t 9 u v w y z
LA LA LA LA LA LA LA LA CA CA CA CA CA CA CA CA LA LA LA SE SE SA LG LR
--
-
95.28 94.30 94.61 94.90 95.82 93.90 91.90 92.00 92,70 87.36 88.85
1.72 1.77 1.89 2.30 2.18 2.90 3.00 3.00 2.50 3.34 2.95
1.64 2.49 1.72 1.71 1.70 1.58 1,60 1.5 r 2.21 2,00 2.26 1.85 1.71 2.14 1.88 1.62 1.54 1.78 1.78 1.16 1.31 1.18 1.84 1.52 2.14 3.28 2.89 2.48 6.76 5.65
*Well number is the same as in Table l, the letters refer to other wells. ?ppm in the non-condensable gas. {Volume %. §ppm in the condensate. LA; Larderello (central zone). CA; Castelnuovo. SE; Serrazzano. LG; Lago.
LR; Lagoni Rossi.
Table 3. Concentrations of argon in the steam from some wells at Larderello (classical zone)*
Well
Gas/steam ratio?
HzS
CO2
CH4
N,,
H2
Ar
I II 111 IV V VI VII VIII
22.8 25.2 26.0 36.3 60.0 61.8 85.3 126.0
2.80 1.93 2.05 1.97 1.02 1.32 1.07 0.35
93.01 93.75 94.16 94.42 97.03 95.24 95.94 97.27
1.32 1.52 1.12 1.22 0.61 1.10 1.10 0.96
0.60 0.87 0.53 0.67 0.38 1.24 0.87 0.84
2.27 1.94 2.12 1.67 0.97 1.10 1.01 0.57
0.00095 0.00124 0.00117 0.00127 0.00100 0.00139 0.001 I 0 0.00092
*Expressed in moles of a r g o n / m o l e s ot steam (CA,). t G a s / s t e a m ratio is expressed in litres of gas S T P / k g of steam. Gas composition is expressed in volume %.
CA,
× 10' 1.74 2.52 2.44 3.71 4.80 6.89 7.57 9.36
Helium in the Larderello Geothermal Fluid
233
storage at rates much greater than their natural flux. The release of stored He is demonstrated by the decrease of He concentrations with time. Analyses made by Sborgi (1934) and by the Larderello company before 1948 (given in Table 2) show that there has been a decline of 35 to 75°7o. Early He values in the central part of the Larderello field average (13 wells) 20 _+ 3 ppm, compared to the present average of about 13 ppm for the same area. In the Castelnuovo zone the early He values average 12 _+ 6 p p m (eight wells), compared to the present average of 3 _+ 2 ppm. One well sited halfway between these two zones has a value of 5 ppm, compared to the 2 ppm found recently. A good correlation between He and volume 070 of (CH, + N2) can also be noted in these older data (Fig. 4). 3
Z +
••
t/ 1
I
10
1
15
I
20
I
25
I
30
He (pprn) Fig. 4. Correlation between Fie content (ppm) and (CFi, + N~) (volume %) in the non-condensable gas (1934 to 195l data).
The He content retains higher values wherever the C H , and N~ values are higher. However, the decrease in He content with time is usually not accompanied by a corresponding decrease in CH, and N2. It is possible that the elevated temperatures of the Larderello field may have caused an accelerated rate of release of trapped He. Experimentally the diffusivity of He in silicate glass increases 103 times for a temperature change from 20°C to 500°C. Tectonic activity related to the heat concentration may also provide more pathways to the surface. These mechanisms could be partial explanations for the occurrence of high concentrations of He in fluids from the high temperature zones (compare Figs 1 and 5), but do not explain the correlation with CH, and N2, and we do not consider them primary reasons for the high He in these zones. The correlation of He with high temperatures and fracturing is more likely a coincidence, as will be discussed later.
Correlation of He with boric acid A strong correlation can also be noted between helium concentration in the gas and the boric acid steam ratio with r -- 0.82 in the recent survey. Comparison of the old values for the boric acid content in the well fluids with 1977 data further highlights the correlation between boric acid and He. Let us also compare the oldest H3BO3 data in Table 2 referring to the central, so-called classical, area of the Larderello field with the 1977 data on H,BO~, and, for the same area, the He values for both periods. During the period of the old helium measurements, the boric acid concentration ranged between 380 and 600 ppm; it has now decreased to its present average of about 200 ppm. The
234
F. D ' A m o r e and A. Truesdell 0
2
4 kin
'q3
a
L ANDENELLO
~,
,.---50
MON~EROTON
~
i
Fig. 5. Temperature map (°C) of the top of the reservoir in the Larderello field. Vertical lines: ophiolite outcrops. Squared areas: limestone outcrops. Blank areas: shales.
boric acid content in the well fluid has thus, like helium content, decreased with time, but at a faster rate. These parallel declines in He and H3BO3 might imply a genetic connection between these substances. However, their chemical behavlour is extremely different and it seems most likely that they were found in the central zone.of Larderello for different reasons. Boric acid is found close to the upflow o f steam because it is highly soluble in liquid water and accumulates in the first condensate formed as steam flowing laterally from the centre cools conductively ( D ' A m o r e and Truesdell, 1979). This condensate was near the surface and formed a large part of early production but is now less important. Helium release was also related to heavy exploitation of the near surface central zones, but this release was due to the pressure decrease discussed below. Thus the correlation o f He with boric acid, high temperature and fracturing results from the shallow depth of the reservoirs and heavy early exploitation in the central zones of steam upflow, rather than from any genetic connection.
Helium in the Larderello Geothermal Fluid
235
Comparison with the convection model of D'Amore and Truesdell The occurrence of the highest He concentrations in the central high temperature zones, and the good correlation with H3BO3, are inconsistent with the behaviour of He as a water-insoluble gas predicted by the convection model of D ' A m o r e and Truesdell (1979). From this model water-insoluble gases should be swept from the central upflow zones to accumulate in the peripheral zones of condensation. In contrast H3BO3 should occur (as it does) in the highest concentrations in zones of upflow before being partially removed in liquid formed during steam condensation. The relationship of He concentrations to zones of upflow and condensation was examined through the study of the relations between helium data and the corresponding gas/steam ratio. Isolating the L a r d e r e l l o - Castelnuovo basin from the rest of the field, and plotting the helium data (in ppm) of the non-condensable gas versus the gas/steam ratios (STP litres of gas/kg of steam) (Fig. 6), two different trends can be noted (1 and 2 in Fig. 6) that are related to well locations in the field (Fig. 7).
i
I
IIl
~o-
~
i
--, OAr • o
•
:
J
•
• / ,'
0 0 O0 jO ~ •
0~
•
I
. I
I'Ij~o
c
o
•
E
~/--/
•/~A
'~
•
•
4o
~Z oo
0
0
i
o
_ _ 6o
I litre$
STP/kg
~
h
loo
~2o
__
Fig, 6. Correlation between He content (ppm) in the non-condensable gas and gas/steam ratio (I S T P / k g steam) for the basin NE of the Larderello field (towns of Larderello and Castelnuovo).
The gas/steam ratio in the central area lies within a rather narrow range, whereas the present He content will depe~,d on the age of the well and, possibly, on local temperatures. The He contents are, in fact, higher in the wells drilled most recently. The zone further south of Castelnuovo is affected by dilution of steam derived from shallow recharge waters and has lower reservoir temperatures than in the central zone. Moreover, its wells are usually very old. As expected, low He contents are found in this area. In the central and marginal zones (1 and 2) the factors influencing the gas/steam ratio and He concentration in the gas are those summarized in Fig. 8. The horizontal arrows indicate the effect of these factors on gas/steam ratios, and the vertical arrows the effects on the He contents. Condensation may tend to increase the CO2 content (main gas) with respect to steam, but it also selectively increases the He/CO2 ratio, as He is about five times less soluble than CO2 at 2 3 0 - 250°C. Exhaustion of the He accumulation, which cannot be renewed in the short-term, will tend to lower the He/CO2 ratio. A temperature decrease in the reservoir caused by mixing with low enthalpy fluids would also have the same effect. The wells of group B (Fig. 6) are much younger than those of group D or E (Castelnuovo). Figure 7 shows a B - E alignment
F. D'Amore and A. Truesdell
236
/ /
I, o
.,
Z/ \ \
•
A
•
• ~~-°~'~
1/
•
•
%o
•
~
D
'
•
~
'
•
•
A
• •
•
• E
\
Fig. 7. Map of the zone corresponding to the points of Fig. 6, This zone is divided into five areas corresponding to five groups of wells classified in Fig. 6. ZONE I
ZONE 2
DILUTION
DILUTION
CONDENSATION
Zx ~.~.:I:
>m ZX
O~
o-r
mO
~A no
gz
mE
z
"Q~ z
;t iNCREASE
gas/steam
Fig. 8. Schematic representation of the factors influencing gas/steam ratio and He concentration in the gas. Horizontal
arrows refer to the effect of these factors on gas/steam ratio; vertical arrows refer to their effect on He content. along a N - S axis. O n e isolated point o f group E in Fig. 6, with high H e content and low g a s / s t e a m ratio, represents a well f r o m the C a s t e l n u o v o z o n e producing from a z o n e o f the reservoir affected by dilution. It is deeper than the other wells and thus hotter and less affected by exploitation.
H e l i u m in the Larderello G e o t h e r m a l Fluid
237
The slope of trend No. 2 can be calculated by assuming that condensation is the main phenomenon, together with a generalized exhaustion of He accumulation. In the model of D ' A m o r e and Truesdell (1979) an equation was developed to explain the distribution of the CO2, NH3, H3BO3, HCI and 1sO concentrations in the steam related to the amount of condensation by a Raleigh process, and applied to Larderello. Let K, be the distribution coefficient of the species i, that is, the ratio between its concentration in the steam and in the liquid. Let m be a residual steam mass and m ° the initial mass of steam betore any condensation process; m / m ° is the fraction of residual steam after a given amount of condensation. The mass of steam m contains a concentration C, of the species i, while C? represents the concentration in the mass m °. For an isothermal condensation, we have: C,/CT~ = ( m / m ° ) ~l/~'i
i~
The slope of straight line 2 between 0 and 130 l S T P / k g and 0 - 15 ppm of He can be reproduced approximately at 230°C by assuming a m a x i m u m condensation of 80% ( m / m ° = 0.2), and initial He and CO2 concentrations of 5 p p m and 35 1 S T P / k g respectively. If He were predominantly influenced by the original convective circulation in the reservoir described by D ' A m o r e and Truesdell (1979), then its concentration should increase with increasing condensation away from zones of steam upflow. In this respect it should behave similarily to Ar (also chemically inert), which shows the expected increase with steam condensation ( D ' A m o r e et al., 1980). Table 3 shows that the argon concentration is directly proportional to gas/steam ratio. Helium is, however, principally concentrated in the zones of upflow which correspond with the zones of highest production. This observation and the strong correlation with CH, and N2 leads us to propose the following theory. A m o d e l f o r He release in the central zones o f Larderello
In the original (pre-exploitation) condition, He in the Larderello reservoir was contained in two states: (1) dissolved in the geothermal fluid which circulated in the manner described by D ' A m o r e and Truesdell (1979). This He would have had the expected increase with condensation and its distribution in the fluid was similar to that o f Ar as measured recently and (2) contained in blind cavities, fluid inclusions and grain boundaries of the reservoir rock. This He was immobilized and did not take part in the fluid circulation. When exploitation of the field began the pressures in the reservoir were greatly reduced, causing vaporization of liquid water and the production of steam. The reduction of pressure also vaporized water in cavities (and, possibly, inclusions), with release of existing stress and breaking of the rock, as indicated by high microseismicity (Bufe and Shearer, 1981; Batini et al., 1981a,b). This breaking of the rocks not only released water as steam but also released previously immobilized gases including He, CH4 and N2. The evolution of CH4 and N2 may have taken place over a long period as exposed organic matter continued to decompose, but the He release would have been over a shorter time since its storage was only in existing trapped fluids and on grain boundaries. Thus, while the release of He would depend on the rate of rock breaking induced by pressure decrease (and indicated by the rate of seismicity), and would decrease abruptly after each break, the release of C H , and N~ would be more gradual. This would explain the continued high contents of CH, and N2 even when He contents have declined. Applying this theory to the geographical distribution of He shown in Figs l, 6 and 7, we can see that the high He (and CH~ and N2) in the central Larderello area is related to the great decrease in pressure due to intensive exploitation in this zone. High He in a fex~. ~,ells (C in Figs
238
F. D ' A m o r e a n d A . Truesdell
6 a n d 7) m a y also result f r o m the g r e a t e r d e p t h o f drilling a n d greater r o c k v o l u m e t a p p e d . T h e seismicity o f the central L a r d e r e l l o zone is not high at p r e s e n t (Batini et al., 1981a,b), but pressures in this zone h a d a l r e a d y d e c r e a s e d greatly a n d were relatively low a n d c o n s t a n t ( c o n t r o l l e d by m i n i m u m p o w e r p l a n t inlet pressures) d u r i n g the p e r i o d o f the seismic survey. T h e s t r o n g r e l a t i o n between m i c r o e a r t h q u a k e s a n d p r o d u c t i o n d e m o n s t r a t e d at T h e G e y s e r s (Bufe a n d S h e a r e r , 1981) suggests that the central L a r d e r e l l o zone m u s t have been seismically active d u r i n g the p e r i o d o f m a j o r pressure lowering d u r i n g h e a v y drilling.
CONCLUSIONS C o n c e n t r a t i o n s o f H e in L a r d e r e l l o steam have been s h o w n to result f r o m the release o f H e c o n t a i n e d in circulating fluids (evident m o s t l y in p e r i p h e r a l zones) a n d o f H e c o n t a i n e d in sealed cavities a n d on g r a i n b o u n d a r i e s . T h e u n e x p e c t e d l y high c o n c e n t r a t i o n s o f H e in the central zones o f s t e a m u p f l o w result f r o m the relatively great l o w e r i n g o f p r e s s u r e a n d resulting m i c r o f r a c t u r i n g o f r o c k in these zones c a u s e d by h e a v y e x p l o i t a t i o n o f shallow steam, T h e c o r r e l a t i o n o f H e with C H , a n d N2 is genetic b e c a u s e all o f these gases are f o r m e d in rock m a t e r i a l s ( f r o m U, Th a n d o r g a n i c m a t t e r ) a n d released d u r i n g m i c r o f r a c t u r i n g . T h e c o r r e l a t i o n with b o r i c acid, high t e m p e r a t u r e s a n d m a c r o f r a c t u r e s is a c o i n c i d e n t a l result o f the shallow reservoir d e p t h in steam u p f l o w zones, which lead to early, s t r o n g e x p l o i t a t i o n a n d pressure r e d u c t i o n with H e release a n d the presence o f high t e m p e r a t u r e s , tectonic activity a n d s t r o n g s e p a r a t i o n o f b o r i c acid into c o n d e n s a t e c h a r a c t e r i s t i c o f these zones. He, as N~, does n o t f o l l o w the c o n d e n s a t i o n m o d e l o f D ' A m o r e a n d Truesdell (1979), as o p p o s e d to CO2, NH3, H3BO3, HCI, A r a n d the '~O o f the s t e a m . Acknowledgement--The authors wish to thank ENEL for providing data on gas composition.
REFERENCES Batini, F., Bufe, C., Cameli, G. M,, Console, R. and Fiordelisi, A. (1981a) Seismic monitoring.in Italian geothermal areas: I. Seismic activity in the Larderello- Travale region. Proc. 2nd DOE-ENEL Workshop Cooperative Res. Geoth. Energy, Lawrence Berkeley Laboratory, Berkeley, 20-23 October 1980, pp. 20-47. Batini, F., Cameli, G. M., Carabelli, E. and Fiordelisi, A. (1981b) Seismic monitoring in Italian geothermal areas: 1I. Seismic activity in the geothermal fields during exploration. ,Proc. 2nd DOE-ENEL Workshop Cooperative Res. Geoth. Energy, Lawrence Berkeley Laboratory, Berkeley, 2 0 - 23 October 1980, pp. 48- 85. Bufe, C. G. and Shearer, P. M. (1981) Geothermal induced seismicity--bane or blessing? Proc. 2nd DOE-ENEL Workshop Cooperative Res. Geoth. Energy, Lawrence Berkeley Laboratory, Berkeley, 20-23 October 1980, pp. 86- 97. Damon, P. E. and Kulp, J. L. (1958) Inert gases and the evolution of the atmosphere. Geochim. cosmochim. Acta 13, 280- 292. D'Amore, F. (1977) Nuove considerazioni relative ai tenori di elio e radon dei fluidi del campo geotermico di Larderello. Internal Report, International Institute for Geothermal Research, CNR, Pisa. D'Amore, F. and Nuti, S. (1977) Notes on the chemistry of geothermal gases. Geothermics 6, 39-65. D'Amore, F. and Truesdell, A. H. (1979) Models for steam chemistry at Larderello and The Geysers. Proc. 5th Workshop Geoth. Reserv. Engn., Stanford, 12-14 December 1979, pp. 283-297. D'Amore, F., Caprai, A., Del Chicca, F. and Del Marmol, M. A. (1980) Studio del tenore del gas argon nei fluidi dei campi geotermici della Toscana. Internal Report, International Institute for Geothermal Research, CNR, Pisa. D'Amore, F., Ferrara, G. C., Noto, P. and Nuti, S. (1977) Tenori di clio nei fluidi del campo geotermieo di Larderello. Prime considerazioni sulle relazioni tra clio e altri componenti del fluido. Internal Report, International Institute for Geothermal Research, CNR, Pisa. Ellis, A. J. (1967) Chemistry of some explored geothermal systems, in Geochemistry of Hydrothermal Ore Deposits (Edited by H. L. Barnes) pp. 465-511. Holt, Rinehart & Winston, New York. Ferrara, G., Gonfiantini, R. and Pistoia, P. (1963) Isotopic composition of argon from steam jets of Tuscany, in Nuclear Geology of Geothermal Areas (Edited by E. Tongiorgi) pp. 267 - 275. Spoleto. Hulston, J. R. and McCabe, W. J. (1962) Mass spectrometer measurements in the thermal areas of New Zealand. Part 1. Carbon dioxide and residual gas analyses. Geochim. cosmochim. Acta 26, 383 - 387. Krauskopf, K. B. (1959) The use of equilibrium calculations in finding the composition of a magmatic gas phase, in Researches in Geochemistry (Edited by P. H. Abelson) pp. 260 - 278. John Wiley.
H e l i u m in the Larderello G e o t h e r m a l Fluid
239
MacDonald, G. J. F. (1963) The escape of helium from the earth's atmosphere. Rev. Geophys. I, 305. Mazor, E. (1978/1979) Noble gases in a section across th.e vapour-dominated geothermal field of Larderelto, ha[y. Geofis. pura appl. !17, 2 6 2 - 2 7 5 . Nuti, S+ (1984) Elementary and isotopic compositions of noble gases in geothermal fluids of Tuscany, haly. Geothermics 13, 215 - 226. Reimer, G. M. (1976) Helium detection as a guide for uranium exploration. U.S. Geol. Surv. Open File Report 76 - 240. Roberts, A. A., Friedman, I., Donovan, T. J. and Denton, E. H. (1975) Helium survey, a possible technique for locating geothermal reservoirs. Geophys. Res. Lett. 2, 2 0 9 - 210. Sborgi, U. (1934) Studi e ricerche sui gas dei soffioni boraciferi con particolare riguardo al loro contenuto in clio ed altri gas nobili. Reale Accademia d'ltalia, Memorie della classe di Scienze fisiche, matematiche e naturali, V, pp. 667 - 713. Wasserburg, G. J., Mazor, E. and Zartman, R. E. (1963) Isotopic and chemical composition of some terrestrial natural gases, in Earth Science and Meteoritics (Edited by J. Geiss and E. D. Goldberg) pp. 219 240. NorthHolland, Amsterdam. Zartman, R. E., Wasserburg, G.. J. and Reynolds, J. H. (1961) Helium, argon and carbon in some natural gases. J. Geophys. Res. 66, 2 7 7 - 306.