VOLATILITY OF HCI AND THE THERMODYNAMICS OF BRINES
DURING BRINE DRYOUT
J. M. Simonson and Donald A. Palmer
Chemical and Analytical Sciences Division
Oak Ridge National Laboratory
ABSTRACT
Laboratory
measurements of liquid-vapor
partitioning (volatility) of chlorides from brines
to steam can be used to indicate the potential for
corrosion problems in geothermal systems.
Measurements of volatilities of solutes in
chloride brines have established a possible
mechanism for the production of high-chloride
steam from slightly acidic high temperature
brines. Questions concerning the fate of NaCl
in the steam production process have been
addressed through extensive measurements of
its volatility from brines ranging in
concentration from dilute solutions to halite
saturation. Recent measurements of chloride
partitioning to steam over brines in contact with
Geysers rock samples are consistent with our
concept of the process for production of highchloride steam.
INTRODUCTION
The production of corrosive steam is a problem
which affects the development and operation of
geothermal resources, including The Geysers.
High concentrations of chloride (to -100 ppm)
have been noted in some wells, particularly in
the higher-temperature reservoir characteristic
of the Northwest Geysers. The source of this
corrosive, high-chloride steam is of significant
interest, particularly as an understanding of the
production of this steam could lead to the
development or optimization of mitigation
methods for addressing corrosion problems. To
address these questions we have carried out a
number of laboratory studies of the partitioning
of relatively nonvolatile solutes between brines
and steam, including experiments in which
brines in contact with rock samples from The
Geysers are evaporated to dryness with
continuous sampling of steam condensate and
analysis of the composition of these samples as
functions
of
temperature
and
brine
concentration. In this project, a part of the
ORNL program 'Fundamental Thermodynamics
of Geothermal Systems,' we have determined
the thermodynamic partitioning constants for
HCI (Simonson and Palmer, 1993) and NaCl
(Simonson et al., 1994).
The assumption underlying the application of
liquid-vapor equilibrium measurements to the
problem of high-chloride steam production from
a vapor-dominated reservoir (e.g., The Geysers)
is that a brine source exists in equilibrium with
steam. In cases where the steam pressures are
too low for equilibrium with a coexisting brine
at the reservoir temperature, application of the
equilibrium volatility results to questions of
steam composition requires that either: ( 1 ) any
reaction between undersaturated steam and (dry)
rock does not change the chloride composition
of steam compared with the brine-equilibrated
values, or (2) that the properties (Le., solute
volatilities) of brines adsorbed on reservoir
rocks are essentially those of the bulk brine,
after adjusting for the lowering of the activity of
water. These requirements appear at first to
limit severely the application of an assumed
brine-steam equilibrium mechanism for highchloride steam formation to problems
encountered in vapor-dominated reservoirs.
However, significant problems with highchloride steam have been noted in some wells in
the 'high temperature' region of the Northwest
Geysers. Shook (1995) has developed a
conceptual model for a vapor-dominated
reservoir with a high-temperature 'feature'
which indicates significant liquid content (as
high-salinity brine) in the high temperature
zone, and seismic data support the presence of
liquid (Romero et al., 1995). Regardless of the
source of chloride in steam, mitigation of high
chloride by desuperheating to partial
"The submitted rnanuscriDt has been authored
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contract No. DE-AC05-960R22464. According1
the U.S. Government retains a nonexclusive,
royally-free license to publish or reproducethe
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Portions of this document may be illegible
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1
condensation requires knowledge of the liquidvapor partitioning of solutes in order to optimize
Thus an
the extent of desuperheating.
understanding of brine thermodynamics and
solute volatility over wide ranges of temperature
and composition relates directly to problems
arising from the presence of corrosive solutes in
geothermal steam.
From our initial study of HCI volatility
(Simonson and Palmer, 1993) it was clear that
partitioning of significant levels of chloride to
steam as HCI would require that the brine pH at
temperature be too low to be consistent with
rocks which did not show alteration by strongly
acidic fluids. High-temperature pH values
below 3 are required to volatilize HCI to the
extent of 100 ppm chloride in steam at 350"C,
and this pH requirement is essentially
independent of brine salinity due to the effect of
additional NaCl on the activity coefficient of
HCl(aq) (Simonson and Palmer, 1993;
Simonson and Palmer, unpublished results).
Our experiments have also shown that addition
of the hydrolyzable ions Mg2+ and Ca2+ at
temperatures to 350°C does not lower the brine
pH suficiently to give chloride concentrations
in steam at levels to 100 ppm.
Some wells in the Northwest Geysers show high
concentrations of ammonia as noncondensible
gas (M. Walters, pers. comm.). We have shown
that consideration of the volatility of ammonium
chloride (Palmer and Simonson, 1993) gives a
mechanism for the production of high-chloride
steam from high-salinity brine of near-neutral
pH at 350" C (Simonson and Palmer, 1995).
However, the volatility of NaCl raises important
questions for an assumed high-temperature
equilibrium between highly saline brine and
steam, in that only very low levels of sodium
have been found in wellhead condensate
samples. Measurements of the volatility of
NaCl(aq) can give important information on the
possibility of chloride transport to steam from
equilibration with brines in geothermal systems,
in that a relatively high volatility of NaCl
implies either that the steam was not in
equilibrium with a saline brine at high
temperature in the reservoir, or that some
mechanism
(e.g.,
precipitation
on
depressurization) exists to effectively 'strip'
NaCl from steam on production.
Here we report the results of recent experiments
on the volatility of chlorides over NaCl(aq)
brines at temperatures to 350°C and at
concentrations ranging to halite saturation.
Comparisons of these new results with available
and extrapolated literature measurements are
made, and implications for a conceptual
reservoir model including equilibration of steam
with saline brines are discussed. Finally, we
describe some very recent experiments and
preliminary results obtained from equilibration
of NaCl(aq) with rock samples from the hightemperature reservoir of the Northwest Geysers.
EXPERIMENTAL
The apparatus and techniques used in our
laboratory measurements of solute partitioning
from brines to steam at high temperatures have
been described in detail previously (Simonson
and Palmer, 1993; Palmer and Simonson, 1993).
Coexisting brine and steam are equilibrated at
temperatures to 350°C in a platinum-lined
autoclave of -600 cm3 internal volume, and
samples of the two phases (liquid and steam
condensate) are withdrawn for subsequent
analysis through platinum sample lines.
Samples are analyzed for both chloride and
counterion (e.g., Na+) concentrations primarily
by ion chromatography, with other techniques
used as appropriate. No recharge of the liquid
phase occurs through a given series of vaporphase sample acquisition, and the experimental
series can be extended through solid-brine
saturation (e.g., halite precipitation) to the
(solid 3. vapor) two-phase condition as desired.
A highly simplified schematic of the hightemperature lined autoclave is shown in Figure
1.
NaCI Partitioning
It has proven to be remarkably difficult to
obtain reliable values for the partitioning of
NaCl(aq) to three-phase saturation with our
DDaratus and techniques.
/
The very low
1
Sample Lines
(Platinum Tubing)
L
-3
'
Vessel Liner
(Platinum)
i
t
-4.
-E
E'
-5
.
Y
-
cn
0
-6'
Steam
Brlne
Solid
Pressure Vessel
(Stainless Steel)
Figure 1. High-temperature lined autoclave for
liquid-vapor equilibration and sampling.
volatility of NaCl requires complete separation
of vapor from entrained brine within the
apparatus.
For example, contamination of
steam condensate samples at 250°C with
approximately 100 ppb of entrained brine
droplets results in an apparent concentration in
steam which is too large by an order of
magnitude. In addition, superheating of lines to
prevent solution reflux on sampling may lead to
precipitation of NaCl(cr) within the lines, and
anomalous low values of the condensate
concentration. The very large uncertainties in
the experimental results are reflected in Figure
2, where our recent data are compared with
values along the three-phase line given by
Bischoff and Pitzer (1989), and with those
extrapolated from halite solubilities in dry steam
as measured by Annellini and Tester (1993).
-7
0
.
Bitchaff cnd Piker (1989)
ArmelliiisndTeser(1993)
a
-a
200
250
a ORNLM.
300
350
400
450
500
550
tPC
Figure 2. Ratios of vapor and brine molalities of
NaCl(aq) from 250 to 500°C.
It is clear that a definitive selection between the
curves shown on Figure 2 for the NaCI
concentration in steam in the three-phase system
cannot be made based on the scattered results
obtained in our new measurements. Further, the
curve reported by Bischoff and Pitzer (1989) is
based in the subcritical region on direct
experimental measurements on the three-phase
assemblage at temperatures to 325"C, while
below 4OOOC the curve of Armellini and Tester
(1993) represents an extrapolation in both
temperature and density of their two-phase
(halite + steam) results. However, our results
for NaCl(aq) volatility at lower brine
concentrations do not extrapolate smoothly to
the values for the three-phase assemblage given
by Bischoff and Pitzer; a pronounced minimum
in the ratio mv/ml appears in the plots of this
ratio against brine molality. The lower-molality
volatility data obtained in this program
extrapolate relatively smoothly to the curve
taken from the work of Armellini and Tester, as
shown in Figure 3.
0
2
I
4
,
6
I
0
1
I
O
I
l
Z
m/(mol.kg*')
Figure 3.
Partitioning of NaCl(aq) and
extrapolation to halite saturation. Different
symbols represent distinct series of runs.
The experimental data for NaCl(aq) partitioning
cited by Bischoff and Pitzer (1989) are from
Bischoff, Rosenbauer, and Pitzer (1986). Their
experiments were carried out using fresh,
unbuffered solutions at each temperature, with
excess NaCl(cr) added to insure halite saturation
at temperature. Steam condensate samples were
analyzed for chloride ion only, with sample pH
checked to determine whether excess HCI had
volatilized in the experiments. At 300"C, just
below the temperature range of the experimental
values, it is possible to estimate the amount of
chloride which should be present in a vapor
sample obtained from fresh, unbuffered
NaCl(aq) brine by combining the hydrogen ion
molality in solution calculated from the
equilibrium quotient for the ionization of water
in NaCI(aq) media (Busey and Mesmer, 1978)
with the equilibrium constant for HCI
partitioning (Simonson and Palmer, 1993). This
calculation indicates that a chloride molality of
about 6x10-5 should be present from HCl
partitioning, giving a pH near 4 in the
condensate sample and corresponding to about 3
ppm if the chloride is assumed to be NaCl in the
sample. This compares with a value of 2.4 ppm
NaCl in steam at three-phase saturation as given
by Bischoff and Pitzer (1989). It is not
reasonable to extend this calculation to
significantly higher temperatures, and Bischoff,
Rosenbauer and Pitzer did not note low pH in
any of their samples. However, it appears that
there remains some question concerning the
volatility of NaCl at high molalities, including
the halite-saturated condition, which are
unfortunately not resolved by our recent
experimental results. The partitioning of NaCl
to steam from halite-saturated brine at 350°C
may be as high as about 20 ppm (Bischoff and
Pitzer, 1989); the lower estimate extrapolated
from Armellini and Tester is about 5 ppm.
Either of these levels would require some
mechanism for stripping NaCl from steam
equilibrated with brine at 35OoC, as steam
condensate samples generally contain less than
1 ppm Na. It may be possible to determine
partitioning from saturated solutions more
reliably from measurements of halite solubility
in superheated steam as a function of
temperature and density, and we expect to
attempt these measurements using a packedcolumn equilibration apparatus.
Chloride partitioning over rock samples
It is clear from the lack of sodium in wellhead
condensate samples that partitioning of NaCl to
steam is not a significant contributor to the total
chloride causing problems in the Northwest
Geysers. We have measured the concentration
of solutes in steam condensate samples obtained
from the multiphase systems (steam + brine +
rock samples +/- halite) to address the
possibility that equilibration of brines with
reservoir rocks could lead to high chloride
levels in steam.
Measurements have been carried out at 300°C
on NaCl(aq) brines in equilibrium with samples
from well MLM-3 and at 245°C with a sample
from L'esperance-2. The MLM-3 sample used
(provided by J. Hulen, EGI) has been
characterized as caprock while the L'esperance2 sample (provided by T. Anderson, UNOCAL
Geothermal) is attributed to the hightemperature reservoir of the Northwest Geysers
(Moore and Gunderson, 1995). In both cases
the rock samples were ball milled and sieved
(150-200 mesh) and treated with dilute HCl(aq)
.
at 25°C to remove any freshly exposed
carbonates. The samples were then rinsed
repeatedly with deionized water and decanted to
remove fines, then dried in a vacuum oven at
1 10°C to remove any residual HCI.
Approximately 70 g of the MLM-3 sample was
placed in the platinum liner of the volatility
apparatus as indicated in Figure 1, and -200 g
of NaCl(aq) unbuffered brine, initially 1
mol-kg-1 added to the system. Several days
were allowed for initial equilibration at 300°C
prior to withdrawal of any steam samples. All
steam condensate samples were analyzed by ion
chromatography.
'
Samples from this run indicated essentially no
chloride in excess of that expected from the
volatility of NaCI. However, high levels of both
cations and anions other than sodium and
chloride were found, and ammonia was present
in excess. The additional cation was tentatively
identified as ammonium ion, although the
presence of excess ammonia prevented
quantitative determination of the cation
concentration in the condensate samples. The
anion was identified as thiosulfate by its
retention time on the ion chromatography
column. It was not possible to quantify with
good precision the levels of thiosulfate present
in the samples due to the extremely long
retention time (strong binding) of this ion on the
column. Noting the apparent lack of any
enhancement of chloride partitioning over this
sample and the significant difficulties in sample
analysis introduced by the ammonia and
thiosulfate, further equilibrations of the MLM-3
sample beyond the initial series at 300°C were
not carried out.
After several equilibration series in the volatility
apparatus of NaCl(aq) with no rock samples
present, and subsequent thorough cleaning of
the platinum liner, the rock sample from
L'esperance-2 was loaded in the apparatus with
NaCl(aq). Some difficulties with the apparatus
were encountered on initial heating of the
system to 300"C, and after repairs a series of
' steam condensate samples were obtained at
245°C. A strong odor of sulfides noted on the
initial heating of the system to 300°C was found
to be significantly lessened in the runs at 245°C;
it remains to be seen whether the strong
evolution of sulfides (apparently H2S) from the
L'esperance-2 sample will recur at higher
temperature or whether volatile sulfides were
effectively stripped from this sample on initial
heating.
-'m
A
If
-3
-4
E"
% -8
Y
-8
-6
-3
z2
r"
E"
L
'E
-8
-4
Y
-5
-6
0
2
4
6
8
10
12
Sample Number
14
16
18
Figure 4.
Molalities in steam condensate
samples from equilibrations over {NaCl(aq) +
L'esperance-2 rock} at 245°C.
'Analyses of solutes in steam condensate
samples taken from equilibrations over
{NaCl(aq) + L'esperance-2 rock} at 245°C are
shown in Figure 4. The average pressure during
sampling is shown for each sample in the upper
plot; the range of nearly constant pressure
indicates halite saturation. A blockage in the
liquid-phase sample line precluded sampling of
the brine during this run. The total sodium ion
molalities in the condensate samples after
Sample #3 are too high to be consistent with
partitioning of NaCI, and we tentatively
attribute the observed high sodium ion
concentrations to entrainment of liquid in the
vapor samples. It should be noted that as the
brine concentration was of the order of 10
mol-kg-1, entrained moisture (brine) in steam at
.
,
I
the level of 10 ppm would be sufficient to give
the high sodium levels noted in the condensate
samples.
If the high sodium concentrations in steam
condensate are attributed to brine carryover, the
extent of transport of chloride as a true vapor
species should be indicated by the difference in
chloride and sodium molalities in the
condensate samples. These values, and the
molalities of ammonium ion in the condensate
samples as determined by the magnitude and
retention time of ion chromatograph peaks, are
shown in the lower plot of Figure 4. There is a
clear enhancement of chloride in steam over that
associated with NaCl, to the level of about 1
ppm C1 in steam. The ammonium molalities in
the condensate samples are consistently higher
than the chloride concentrations.
These
ammonium ion levels cannot be attributed to
brine carryover, as no ammonium ion was added
to the initial brine and it is unlikely that
ammonium ion at levels higher than 1 mobkg-1
could have been leached from the rock sample.
The concentrations of ammonium ion,
ammonia, and hydrogen ion in a high-chloride
brine which are consistent with the chloride and
ammonium concentrations shown in the lower
frame of Figure 4 can be calculated from the
partitioning constants for NH3, NH4Cl and HCI.
This calculation has been described in detail
previously (Simonson and Palmer, 1995). For
the present case the results indicate a brine pH
at temperature of about 5.0, with -3xlO-5
molekg-1 ammonia and - 5 ~ 1 0 - 5 mol-kg-1
ammonium ion in solution.
It is clear that the apparent enhancement of
chloride partitioning shown in these samples
warrants further investigation, and additional
experiments on these samples are in progress.
Particular areas for attention in these studies
include obtaining both liquid and condensedvapor samples from the equilibrium
assemblages, and in addressing the role of H2S
in establishing brine pH at low and high
temperatures. In analyzing these condensedvapor samples it was found that the sample pH
remained remarkably constant at pH = 5.0. The
chloride and ammonia concentrations appear to
preclude an amrnonidammoniurn buffer as a
control for the pH at room temperature. We
have noted that the samples contain HzS, but
have not as yet been able to quantify the
concentrations in the samples. In addition,
analyses by I C P h a s s spectrometry indicate the
presence of significant levels of iron,
magnesium, and boron in the condensate
samples, including those which are assumed not
to be contaminated by entrained brine. It is
expected that further detailed study of the
volatilities of components from brines
equilibrated with these rock samples will
provide significant new information on the
distribution of solutes to steam in the hightemperature reservoir of the Northwest Geysers.
ACKNOWLEDGEMENTS
We are grateful to Jeff Hulen of the Energy and
Geoscimces Institute and to Timothy Anderson
of Unocal Geothermal and Power Operations for
supplying the Geysers rock samples from well
MLM-3 and L'esperance-2, respectively. We
thank Drs. R. W. Carter and D. B. Joyce for
their contributions to experimental aspects of
this work, and Shelby Morton for her efforts in
analyzing the composition of the vapor samples.
This research was supported by the Geothermal
Division, Office of Energy Efficiency and
Renewable Energy, and by the Division of
Chemical Sciences, Office of Basic Energy
Sciences, U.S. Department of Energy, under
contract DE-AC05-960R22464 with Lockheed
Martin Energy Research Corporation.
REFERENCES
Armellini, F. J. and J. W. Tester, "Solubility of
sodium chloride and sulfate in sub- and
supercritical water vapor from 450-550" C and
100-250 bar", Fluid Phase Equilibria, 84, 123142, 1993.
Bischoff, J. L., R. J. Rosenbauer, and K. S .
Pitzer, "The system NaCLH20: Relations of
vapor-liquid near the critical temperature of
C
I
.
,I
water and of vapor-liquid-halite from 300” to
5OO0C”, Geochim. Cosmochim. Acta, 50, 14371444,1986.
Bischoff, J. L. and K. S . Pitzer, “Liquid-vapor
relations for the system NaCLH20: Summary
of the P-T-x surface from 300” to 500”C”, Am.
J. Science, 289,2 17-248, 1989.
Busey, R. H., and R. E. Mesmer,
“Thermodynamic quantities for the ionization of
water in sodium chioride media to 3OO0C,” J.
Chem. Eng. Data, 23, 175-176, 1978.
Moore, J.
inclusion
evolving
Geochim.
1995.
N. and R. P. Gunderson, “Fluid
and isotopic systematics of an
magmatic-hydrothermal system,”
Cosmochim. Acta, 59, 3887-3907,
Palmer, D. A. and J. M. Simonson, “Volatility
of ammonium chloride over aqueous solutions
to high temperatures,” J. Chem. Eng. Data, 38,
465-474, 1993.
Romero, Jr., A. E., T. V. McEvilly, E. L. Majer
and D. Vasco, “Characterization of the
geothermal system beneath the Northwest
Geysers steam fieid, California, from seismicity
and velocity patterns,” Geothermics, 24, 47 1487, 1995.
Shook, G. M., “Development of a vapordominated reservoir with a “high-temperature”
component,” Geothermics, 24,489-505, 1995.
Simonson, J. M. and D. A. Palmer, “Liquidvapor partitioning of HCl(aq) to 35OoC”,
Geochim. Cosmochim. Acta, 57, 1-7, 1993.
Simonson, J. M., D. A. Palmer, and R. W.
Carter, “Liquid-vapor partitioning of NaCl(aq)
from concentrated brines at temperatures to
350”C”, Proceedings of the Nineteenth
Workshop
on
Geothermal
Reservoir
Engineering, Stanford, CA, pp. 245-25 1, 1994.
Simonson, J. M. and D. A. Palmer, “Liquidvapor partitioning in the system Na-H-NH4NH3-OH-CI-H20 to 35OoC,” Proceedings of
the World Geothermal Congress, Florence,
Italy, pp. 969-974, 1995.