Transcript Slides

Gas geothermometers
Caprai A. (1) , Montalvo F.E. (2) , Tassi F. (3) , Vaselli O. (3)
(1) CNR - Institute of Geosciences and Earth Resources
(2) La Geo – El Salvador
(3) University of Florence - Department of Earth Sciences
[email protected] www.igg.cnr.it
Gas geothermometers are based on equilibrium chemical reactions between gaseous species. For each reaction considered a thermodynamic equilibrium constant may be
written, where the concentration of each species is represented by his partial pressure in vapor phase. The gas-gas equilibrium in geothermal fields with two phasecomponents should not reflect the real gas composition present in the reservoir. It depends from many factors like gas/steam ratio. It is assumed that there is no reequilibration of the chemical species from the source or sources to wellhead. The fluids analyzed are those collected at the well head. In geothermal fields the
concentrations (or ratios) of gases like CO2, H2S, H2, N2, NH3, and CH4 are controlled by temperature. Because of that, data from gas have been used to study a correlation
between the relative gas concentrations and the temperature of the reservoir using the D’Amore and Panichi (1980) geothermometer based on partial pressures of CO2,
H2S, CH4, H2, where CO2 is externally fixed (fig. 1). Hydrocarbon compounds in fumarolic gases result less abundant (up to one order of magnitude) with respect to those
measured in gases sampled from the productive wells. This compositional difference is likely to be caused by the partial dissolution into the superficial aquifer of
hydrocarbons which fed the fumaroles, since these compounds are characterized by a higher solubility with respect to that of the other inert gases (mainly due to their
higher molecular weight). On the contrary, productive-well fluids, directly derived from the geothermal reservoir, are not affected by this "scrubbing" process. Nevertheless,
light hydrocarbon compounds, such as methane, ethane, propane, propene, i-butane and i-butene, show very similar solubility, thus the equilibrium reactions among them,
depending on their reciprocal ratios and not on their absolute abundances, result almost independent from both phase transfer processes and the influence of superficial
aquifer. Therefore, it is reasonable to consider that the application of geothermometric techniques based on thermodynamic equilibrium of organic gases is a reliable tool to
evaluate the temperature of deep systems even by adopting the hydrocarbon composition of natural discharges.
Fig. 1
Bocca Grande
geot. D & P (°C)
310
290
270
150º 175º 200º
0.1
-16
225º
250º
275º
300º
-16
ott-06
225º
250º
275º
Pisciarelli basso-alto
0.01
0.01
0.005
-0.3
Green Sport
Agnano Volvo
-20
0.005
-22
0.001
-0.3
Bocca Grande
-0.1
-0.03
-26
-0.03
-24
0
-0.001
-0.1
FT
0.001
-24
325º
0.025
-18
-22
300º
0.05
Soffionissimo
-18
-20
gen-04
apr-01
lug-98
150º 175º 200º
0.1
325º
0.05
0.025
ott-95
gen-93
mag-90
ago-87
feb-82
nov-84
250
FT
The grid diagram by D’Amore & Truesdell (1985) shows that, using two
gas equilibrium equations expressed as gas concentrations versus water,
we can evaluate both the temperature of the reserves, and the “y” value
representing the steam fraction. This graphical method is based on the
Fischer-Tropsch [FT = 4log (H2/H20) – log (CH4/CO2)] and pyritemagnetite [HSH = 3log (H2S/H20) – log (H2/H20)] reactions.
The diagram shows that an increase in temperature and decrease in “y”
could indicate that there has been a contribution of hotter deeper fluids
with high saturation in the liquid phase. An increase in both parameters
(temp and steam fraction) could mean an apparent increase in
temperature caused by secondary vapour with zero saturation in the liquid
phase and a strong accumulation of local non-reactive gas in a pure gas
phase.
A decrease in calculated temperature and steam fraction “y” could be
caused by a local source of low temperature water crossed by the gas.
A change in temperature accompanied by an increase in the “y” value,
often is associated a decreasing in ratio (HSH/H2O).
330
0
-0.01
-0.001
-26
-0.01
-0.003
-0.003
rangoatde
variacion
200-225°C
Range
initial
conditions
200-225 °C
-28
range at initial conditions 235-250 °C
range at actual conditions 235 °C
-28
grid Pozzuoli.grf
-30
grid BG SF.grf
-30
Fig. 2
-12
-11
-10
-9
-8
-7
-12
-11
-10
-6
-9
-8
Fig. 3
-7
-6
HSH
HSH
1.0
eO
/2H 2
Fe
O/F
SO
HM
-2.0
-2.5
600
800
1000
T (K)
M
3.0
H
2.5
S= 1
SO 2/H 2 Fig. 4
hi
=0.01 Panic
S
H
/
&
SO 2 2
re
T
1.5
1400
H /Ar
2
G
log(i-C4H8/i-C4H10)
2.0
1200
D
o
'Am
1.0
0.5
0.0
Is reasonable to consider that the
application of geothermometric
techniques based on
thermodynamic equilibrium of
organic gases is a reliable tool to
evaluate the temperature of deep
systems even by adopting the
hydrocarbon composition of
natural discharges (figs. 4 and 5)
-0.5
-1.0
FM
Q
-1.5
-2.0
-2.5
-3.0
400
600
800
1000
T (K) H2/Ar
1200
1400
Fig. 5
Fig. 3. a) Calculated temperature (H 2/Ar geothermometer) vs. log(C 3H6/C3H8)
b) Calculated temperature (H2/Ar geothermometer) vs. log(i-C 4H8/i-C4H10)
References
D’Amore F., 1991. Gas geochemistry as a link between geothermal
exploration and exploitation. Edit. Application of geochemistry in
geothermal reservoir development. 93-117.
Caprai A., 2005. Volcanic and Geothermal gases and low–enthalpy Natural
Manifestations. Methods of Sampling and Analysis by Gas Chromatography.
Journal of Applied Sciences, Asian Network for Scientific Information, 5 (1),
85-92.
Ferrara G., Giuliani A., Magro G., 1981. La composizione isotopica
dell’argon nei gas fumarolici di Vulcano (Isole Eolie) e della Solfatara
(Campi Flegrei). Rend. Soc. Geol. It., 4, 221-224.
Giggenbach W.F. & Goguel R.L., 1989. Collection and analyses of
geothermal and volcanic water and gas discharges. Report N. CD 2401 (4th
ed.), Chemistry Division DSIR, Petone, New Zealand.
Caprai A., Leone G., Doveri M., Mussi M., Calvi E. Geochemical
surveillance of reactive gas from Pozzuoli Solfatara (Naples, Italy):
chronological evolution and local ground displacement.
This compositional difference is likely
caused by the partial dissolution into the
superficial aquifer of hydrocarbons which
fed the fumaroles, since these
compounds are characterized by a
higher solubility with respect to that of
the other inert gases (mainly due to their
higher molecular weight). On the
contrary, productive-well fluids, directly
derived from the geothermal reservoir,
are not affected by "scrubbing" process.
Nevertheless, light hydrocarbon
compounds, such as methane, ethane,
propane, propene, i-butane and i-butene,
show very similar solubilities, thus the
equilibrium reactions among them,
depending on their reciprocal ratios and
not on their absolute abundances, result
almost independent from both phase
transfer processes and the influence of
superficial aquifer (figs. 6 and 7).
The Giggenbach geothermometer (1991) (fig. 8), expresses the
LHA [log (XH2/XAr) vs the LCA (log X CO2/ XAr) values,
considering an equilibrium line on which are plotted all the
dissolved gases within a single liquid phase. The horizontal line
corresponds to the predicted composition for equilibrium in the
vapor phase. The other lines represent intermediate conditions;
the steam increase at equilibrium or the loss of Ar before
equilibrium is attained. The following figure shows the historical
data relative to the Bocca Grande and Soffionisssimo fumaroles
plotted against the data measured in Central American wells
and fumaroles. Note the similarity between the fumarole gas
values at Pozzuoli and for one well. The temperatures obtained
at Pozzuoli fall within the 300-350°C range, whereas inone well
the temp values are around 260°C, with the fumarole showing a
value of 300°C. The results for the other well indicate very low
temperature conditions.
Thanks to Calvi Enrico, Vaggi Marina, Bonomi Roberto for their support in interpretation of datas
0.9
i-C4H10/i-C4H8
-1.5
400
1.0
0.8
0.7
0.6
0.5
0.4
0.3
0.2
Fig. 6
0.1
25
C3H8/C3H6
FM
Q
-1.0
2
/H
-0.5
2
S=
0.
01
0.0
SO
log(C3H6/C3H8)
0.5
1.5
1
S=
20
15
10
5
Fig. 7
0
0
10
20
30
40
50
CH4/(C2-C9)
Fig. 2. CH4/(C2-C9) vs. C3H8/C3H6 and i-C4H10/i-C4H8) diagram
Fig. 8