A3 Wilfred A. Elders The vapor-dominated Los Humeros geothermal system, Mexico: acid-rock interactions, high boron concentrations, and possible implimentation

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: [email protected]

2016-11-24 GEORG Reykjavik November 2016

GEORG Reykjavik November 20162016-11-24


LPC =Los PotrerosCollapse(a sub-caldera)

The LHGS lies entirely within the LPC

GEORG Reykjavik November 2016

Geologic cross section of the Los Humeros Caldera

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Black = Post-caldera VolcanicsPink = Caldera fill of Andesite PyroclasticsGreen = Mesozoic Marine sediments Others = Pre-caldera Quaternary Volcanics (Basaltic Andesites)

Cross Section Courtesy Ernesto Camillo


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 temperatureLow permeability zone.


0 50 100 150 200 250 300 350 400 T em p era tu re (°C )C ircu la tion lo sses (m 3/h )

3000

2500

2000

1500

1000

500

0D

epth

(m

)W ith 1 2 h o u rs s tand b yW ith 1 8 h o u rs s tand b yW ith 3 0 h o u rs s tand b yW ith 3 0 d ay s s tan d b yC ircu la tio n lo sse s (m 3/h )

H 2 3

L ith o lo g ic U n its

1

2

3

4

5

69

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


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 3500Enthalpy (kJ/kg)

10

100

Pres

sure

(bar

)

97

99

00 01

020304

05

070811

96

12

H 03 D irectional w ell

x=0 .8

Saturatedsteam

Saturatedliquid

300°C

Two-phasefluid

200°C isothermal line


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.


SOME UNRESOLVED ISSUES AT THE LHGS

1. Sources of Acid Components in the Fluids2. Extremely High Boron Contents 3. Large Variations in Fluid Chemistry with Time4. Nature and Location of the Heat Source5. 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


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


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.


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 2020Y ear

0

2000

4000

6000

8000

10000

B (p

pm)

H 3 5

0 20 40 60 80C l (p p m )

0

2000

4000

6000

8000

10000

B (p

pm)

H 3 5

B variations for different years and B/Cl ratios in fluids from well H 35


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).


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.


• 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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SOME UNRESOLVED ISSUES AT THE LHGS

1. Sources of Acid Components in the Fluids2. Extremely High Boron Contents 3. Large Variations in Fluid Chemistry with Time4. Nature and Location of the Heat Source5. Future Development of the Resource

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The Heat Source for the LHGSAn 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.

• ÞAKKA PÉR KÆLEGA FYRIR• MUCHAS GRACIAS• THANK YOU VERY MUCH

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A3 Wilfred A. Elders The vapor-dominated Los Humeros geothermal system, Mexico: acid-rock interactions, high boron concentrations, and possible implimentation