The document summarizes research on the Los Humeros Geothermal System in Mexico. It discusses acid-rock interactions observed in some wells, extremely high boron concentrations found in the fluids, and variations in fluid chemistry over time. It presents the current conceptual model of the reservoir and examines some unresolved issues, including the sources of acidity, boron concentrations, and the nature and location of the heat source powering the system. The researchers estimate that only a small fraction of the stored heat could power a large electricity plant for decades, suggesting potential for future development as an enhanced geothermal system.

1. The Vapor-dominated Los Humeros Geothermal System, Mexico:
acid-rock interactions, high boron concentrations, and possible
implications for the future development
of the resource.
Wilfred A. Elders1, Georgina Izquierdo-Montalvo2, and Alfonso Aragón-Aguilar2
1Department of Earth Sciences, University of California, Riverside, USA.
2Gerencia de Geotermia, IIE, Cuernavaca, México.
Email: elders@ucr.edu
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LPC =
Los
Potreros
Collapse
(a sub-caldera)
The LHGS lies
entirely within the
LPC

4. Geologic cross section of the Los Humeros Caldera
2016-11-24 GEORG Reykjavik November 2016
Black = Post-caldera Volcanics
Pink = Caldera fill of Andesite Pyroclastics
Green = Mesozoic Marine sediments
Others = Pre-caldera Quaternary Volcanics (Basaltic Andesites)
Cross Section Courtesy Ernesto Camillo

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Geothermal well locations in
the Los Humeros Geothermal
System, within the Los Potreros
Collapse sub-caldera.
Various known and inferred
faults are also shown.
Blue Circle = Producing well.
Red Circle = Injection well.
White Circle = Non-producing
well.
High temperature
Low permeability
zone.

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0 50 100 150 200 250 300 350 400
Temperature (°C)
Circulation losses (m3
/h)
3000
2500
2000
1500
1000
500
0Depth(m)
With 12 hours standby
With 18 hours standby
With 30 hours standby
With 30 days standby
Circulation losses (m3/h)
H23
Lithologic
Units
1
2
3
4
5
6
9
Lithological units
1 = Pumice, basalt, basaltic
andesite, and rhyolite.
2 = lithic tuff,
3 =ignimbrite,
4 = andesite and ignimbrite,
5 = Quaternary augite andesite,
6 = altered vitreous tuff,
7 = Tertiary hornblende
andesite,
8 = Tertiary basalt,
9 = Basement, marble,
hornfels, and granite.
Temperature Profiles
measured in low
permeability well H 23

7. Pressure-enthalpy diagrams averaged for successive years of production from wells
H-3 and H-9 . The number at each red dot represents the last two numbers of the
year when the measurement was made.
(Compiled from data of CFE). Both are “blue” wells in the N.W. and W. sector.
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500 1000 1500 2000 2500 3000 3500
Enthalpy (kJ/kg)
10
100
Pressure(bar)
97
99
00
01
0203
04
05
07
0811
96
12
H 03 Directional wellx=0.4
x=0.6
x=0.2
x=0.8
Saturated
steam
Saturated
liquid
300°C
Two-phase
fluid
200°C isothermal line

8. Current Conceptual Model,
Bernard et al., 2011
• (1) At the top a shallow, water-dominated reservoir that
overlies a lithologic low permeability boundary.
• (2) A zone below this where partial condensation of steam
accompanying water-rock reaction and neutralization
occurs.
• (3) A deep, immature, acid brine boiling at ~ 350 C
producing a HCl-bearing steam with a high B content.
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But we have seen only very limited and very local evidence
for acid-rock reaction in the LHGS. Furthermore we have
seen no evidence for a fieldwide lithologic permeability
barrier.

9. SOME UNRESOLVED ISSUES AT THE LHGS
1. Sources of Acid Components in the Fluids
2. Extremely High Boron Contents
3. Large Variations in Fluid Chemistry with Time
4. Nature and Location of the Heat Source
5. Future Development of the Resource
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Well Cl HCO3 SO4 Na K Ca Mg SiO2 B Fe Mn
H-35/N 5 26.4 16.2 8.5 1.5 0.6 0.18 56 2051 1.9 0.02
H-37/N 425 448 4.3 373 38.2 0.5 0.007 776 258 0.2 0.006
H-19/C 230 119 168 138 20.9 191 0.01 1332 2708 0.6 0.008
H-45/C 10.1 24 1.4 0.9 0.7 0.4 0.01 16 308 1.8 0.04
H-6/S 55 78 74.7 76 14.5 0.13 0.02 730 389 0.2 0.006
H-39/S 79 342 7.5 136 29.5 0.21 0.003 1359 732 0.033 0.003
CHEMICAL CHARACTERISTICS OF THE LOS HUMEROS
GEOTHERMAL FLUIDS
Most of the wells in the LHGS produce high steam fraction with a small liquid
fraction and enthalpies greater than 2600 J/g). The chemical composition of
separated water indicates that they steam condensates.. Table 1 shows chemical
analysis of separated water from 6 wells at Los Humeros. Concentration is given in
mg/L, pH’s are in the range 3 to 5 at 25°C.
Collected in 2015 and analyzed by G. Izquierdo-Montalvo

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Temperature ranges of major hydrothermal alteration
minerals observed in various wells in the LHGS.
(Typical of an alkali-neutral water-dominated system)

12. Bleached and silicified ignimbrite in drill core 4 from well H-26 at 2000 - 2004.5 m
depth.
A: Core with relic pyroclastic lithic clasts. B: Cut surface showing relict eutaxitic and
pumiceous textures.
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A B
Acid alteration is lacking in the reservoir except for some local interaction
with acid fluids of limited extent seen in a few wells in the hotter, least
permeable, part of the reservoir.

13. Fluid chemistry data from the files of CFE, going back to 1993, reveal
variations in concentrations of some of the main components, by factors of
up to 5 or more, especially for boron (left Figure). The concentration of
Boron seems to be decoupled from that of Chlorine (right Figure).
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1980 1990 2000 2010 2020
Year
0
2000
4000
6000
8000
10000
B(ppm)
H35
0 20 40 60 80
Cl (ppm)
0
2000
4000
6000
8000
10000
B(ppm)
H35
B variations for different years and B/Cl ratios in fluids from well H 35

14. Boron concentrations (ppm) in samples of drill core samples
from the LHGS and nearby basement outcrops.
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Bernard et al. (2011) reported 11B values of four samples of separated
produced water from LHGS in the range of -1.7 + 0.3 with an average
of -0.8 which they suggest is a magmatic signature (Leeman et al.,
2005).

15. Boron concentrations* and 11B‰ isotopic ratios** of
separated water and steam from six wells in the LHGS.
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*Analyses by Georgina Izquierdo-M.
** Analyses by Terratech
We obtained for values in the range of + 0.2- 13.5 for 11B , which suggests that
the ultimate source of the boron is more likely to be the metamorphosed marine
sedimentary rocks of the basement.

16. • Sources of boron are not seen in the rocks drilled so far, and
we have not seen boron minerals the basement rocks.
• Presumably the boron is transported to the well head in a
vapor phase of H3BO3. The large amplitude fluctuations in
boron concentrations with time are not field wide events,
and do not correlate between different wells.
• It seems likely that there is a local mechanism that
concentrates boron and stores it at certain sites in the
reservoir.
• Then the boron is remobilized by from this secondary source,
releasing various amounts of boron to the produced fluids.
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17. SOME UNRESOLVED ISSUES AT THE LHGS
1. Sources of Acid Components in the Fluids
2. Extremely High Boron Contents
3. Large Variations in Fluid Chemistry with Time
4. Nature and Location of the Heat Source
5. Future Development of the Resource
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18. The Heat Source for the LHGS
An EGS Potential?
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Based on their interpretation of neotectonic and remote
sensing data, Norini et al. (2015) infer that the heat
source is a recently resurgent rhyolitic magma body
beneath the central eastern sector of the LHGS, north of
the Matabaya fault and east of the Las Vibradores fault.
Our preliminary volumetric estimate of the heat in
storage in the prism bounded by the non-producing
wells, H 23, H 26, and H 27 between the 200°C
isotherm and 3000 m depth, is about 300 GWh
(Aragón et al, 2014).
Only 1 % of this enthalpy could operate a >100 MWe
generating plant for least 30 years, and this is only a
very small part of the low permeability sector
believed to be underlain by the inferred magma body.
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