ADGEOAdvances in GeosciencesADGEOAdv. Geosci.1680-7359Copernicus PublicationsGöttingen, Germany10.5194/adgeo-45-281-2018Outcrop analogue study to determine reservoir properties of the Los Humeros and Acoculco geothermal fields, MexicoOutcrop analogue study to determine reservoir properties of the Los HumerosWeydtLeandra M.weydt@geo.tu-darmstadt.deBärKristianhttps://orcid.org/0000-0003-4039-7148ColomberoChiaraCominaCesareDebParomitaLepillierBaptisteMandroneGiuseppehttps://orcid.org/0000-0002-5397-9377MilschHaraldRochelleChristopher A.VagnonFedericoSassIngoDepartment of Geothermal Science and Technology, Technische Universitädt Darmstadt, Schnittspahnstraße 9, 64287 Darmstadt, GermanyDepartment of Earth Sciences, University of Torino, Via Valperga Caluso 35, 10125 Torino, ItalyInstitute for Applied Geophysics and Geothermal Energy, EON Energy Research Center, RWTH Aachen, Mathieustraße 10, 52074 Aachen, GermanyFaculty of Civil Engineering and Geosciences, Delft University of Technology, Stevinweg 1, 2628CD Delft, the NetherlandsHelmholtz Centre Potsdam – GFZ Research Centre for Geosciences, Section 6.2 – Geothermal Energy Systems, Telegrafenberg, 14473 Potsdam, GermanyBritish Geological Survey, Keyworth, NG125GG, Nottingham, England, UKDarmstadt Graduate School of Excellence Energy Science and Engineering, Jovanka-Bontschits Straße 2, 64287 Darmstadt, GermanyLeandra M. Weydt (weydt@geo.tu-darmstadt.de)11September20184528128731May201817August201823August2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://adgeo.copernicus.org/articles/45/281/2018/adgeo-45-281-2018.htmlThe full text article is available as a PDF file from https://adgeo.copernicus.org/articles/45/281/2018/adgeo-45-281-2018.pdf
The Los Humeros geothermal system is steam dominated and
currently under exploration with 65 wells (23 producing). Having
temperatures above 380 ∘C, the system is characterized as a super
hot geothermal system (SHGS). The development of such systems is still
challenging due to the high temperatures and aggressive reservoir fluids
which lead to corrosion and scaling problems. The geothermal system in
Acoculco (Puebla, Mexico; so far only explored via two exploration wells) is
characterized by temperatures of approximately 300 ∘C at a depth
of about 2 km. In both wells no geothermal fluids were found, even though a
well-developed fracture network exists. Therefore, it is planned to develop
an enhanced geothermal system (EGS).
For better reservoir understanding and prospective modeling, extensive
geological, geochemical, geophysical and technical investigations are
performed within the scope of the GEMex project. Outcrop analogue studies
have been carried out in order to identify the main fracture pattern,
geometry and distribution of geological units in the area and to
characterize all key units from the basement to the cap rock regarding
petro- and thermo-physical rock properties and mineralogy. Ongoing
investigations aim to identify geological and structural heterogeneities on
different scales to enable a more reliable prediction of reservoir
properties. Beside geological investigations, physical properties of the
reservoir fluids are determined to improve the understanding of the
hydrochemical processes in the reservoir and the fluid-rock interactions,
which affect the reservoir rock properties.
Simplified work flow within the GEMex project.
Introduction
Unconventional geothermal systems like EGS or high temperature geothermal
systems (>350∘C) have the worldwide largest
potential for deep geothermal energy utilization (Huenges, 2010) and have
raised the interest of the industry and scientific community in the last
decades. Several issues like corrosion and scaling effects have been
encountered in the past while trying to exploit and operate unconventional
or hot geothermal reservoirs with supercritical conditions (Reinsch et al.,
2017). Comprehensive and detailed exploration is needed to improve reservoir
understanding and to enable a better reservoir modelling, which helps to
handle the supercritical fluid conditions in the reservoir. Therefore, the
GEMex project (EU-H2020, GA Nr. 727550) focuses on the development of (hot)
EGS and the investigation of high-temperature geothermal fields
(>350∘C) on two sites in the northeastern part of
the Trans-Mexican Volcanic Belt (TMVB), the Acoculco and Los Humeros caldera
complexes (Puebla).
The project comprises a multidisciplinary approach in order to find new
transferable exploration approaches and technologies based on three
milestones which are (1) resource assessment, (2) reservoir characterization
and (3) concepts for site development (Jolie et al., 2018, Fig. 1). Outcrop
analogue studies were conducted to investigate fracture patterns, geometry
and dimensions , as well as to characterize the key lithologies with regard
to thermo- and petrophysical rock properties and mineralogy at different
scales (outcrop, rock sample, thin section). Furthermore, the project
comprises a complex work flow for the compilation and integration of
extensive data sets from different scientific disciplines. The resulting
data and models of all work groups will be combined in integrated reservoir
models at a local, regional and superregional scale.
This paper presents first results of the `reservoir characterization' work
group as part of milestone 2 and gives an overview of the project's work
flow and methods applied in the field and laboratory as well as the
subsequent processing of the data sets generated. It should serve as an
example for upcoming projects and help to develop concepts for the
characterization of high-temperature unconventional geothermal fields.
Geological setting
The Acoculco and Los Humeros caldera complexes are located in the
northeastern part of the TMVB, 125 and 180 km east of Mexico City,
respectively. The Acoculco caldera complex has a 20×18 km asymmetric
structure (Avellán et al., 2017) and lies within the older and
larger Tulancingo caldera (Pfeiffer et al., 2014). The Acocuclo caldera
comprises Pliocene and Pleistocene calcalkaline volcanic rocks resulting
from three main volcanic periods in the area (López-Hernández et al., 2009).
The first and second period are related to the collapse of the Tulancingo
caldera (3.0–2.7 Ma) and the Acoculco caldera (1.7–0.24 Ma) producing mainly
dacitic to rhyodacitic lavas and rhyolitic domes and andesitic ingnimbrites,
respectively. A third episode (1.8–0.2 Ma) is related to monogenetic
volcanism without a caldera collapse (López-Hernández et al., 2009).
Schematic work flow within the “Reservoir characterization” work
group using the example of the El Dorado mine in Las Minas (d) with
view on the footwall of the present fault (photo from Maximilian Bech). The
quarry exposes exoskarn in many variations. A lot of calcium-rich minerals were
found which indicates the close proximity to the host rock (limestone). Outcrop
analysis included detailed investigation of kinematic indicators, mineralogy (a)
and the main fracture pattern (e, f). Representative rock samples were
taken for petrophysical rock measurements (b), geochemical and thin
section analysis. (c) shows skarn with clinopyroxene, quartz,
plagioclase and anortite (photo from Caterina Bianco).
The younger Los Humeros caldera comprises Pleistocene to Holocene basaltic
andesite-rhyolite volcanic rocks and has a 21×15 km irregular circular
shape (Carrasco-Núñez et al., 2018). The emplacement of the Los
Humeros caldera was associated with two main caldera-forming eruptions and
multiple voluminous plinian eruptions as well as alternating episodes of
dacitic and rhyodacitic dome-forming eruptions (Carrasco-Núñez et
al., 2017a). Thick sequences of hornblende andesites and basaltic lava flows
(10.5 Ma, Cuyoaco and Alseseca andesites), Toba Humeros vitric tuffs and
Teziutlán augite andesites (5–1.55 Ma; Norini et al., 2015 and references
herein) form the geothermal reservoir in the subsurface of the Los Humeros
geothermal system (Carrasco-Núñez et al., 2018). Ignimbrites
(Xáltipan ignimbrite at ∼160 ka and Zaragoza ignimbrite
at ∼69 ka; Carrasco-Núñez et al., 2017b), tuffs, ash
fall deposits and diverse pyroclastic flows form the cap rock of the
reservoir, which is overlain by Holocene to recent basalt lava flows.
The volcanic rocks of the Acoculco and Los Humeros geothermal fields were
emplaced on intensively folded Mesozoic sedimentary rocks belonging to the
Sierra Madre Oriental (López-Hernández et al., 2009) comprising Jurassic
sandstones, shales and hydrocarbon-rich limestones and dolomites overlain by
Cretaceous limestones and shales. Granitic and syenitic plutons of Cenozoic
age and basaltic dykes intruded into the sedimentary sequences and led to
local metamorphism of marble, hornfels and skarn (Ferriz and Mahood, 1984).
The Los Humeros geothermal system is steam dominated and exploited and
operated by CFE since 1990. About 65 wells have been drilled so far (depths
between 1500 and 3100 m), while 23 are still productive (Romo-Jones et
al., 2017). Produced power is about 68 MW with a possible total capacity of
about 94 MW. Temperatures around 380 ∘C were encountered at
depths below 2 km in the northern parts of the caldera complex (Pinti et al., 2017).
Up to now, two exploration wells have been drilled in the Acoculco
geothermal field, which encountered temperatures of approximately
300 ∘C at a depth of about 2 km (Canet et al., 2015). In both
wells no geothermal fluids were found (López-Hernández et al., 2009), even
though it was thought that a well-developed fracture network exists in the
area. Therefore, it is planned to develop a deep EGS and enhance connections
between networks of fractures in order to connect the existing wells to
proximal fluid bearing fracture zones (Jolie et al., 2018).
Field work and reservoir characterization
Outcrop analogue studies offer a cost-effective opportunity to investigate
and correlate facies, diagenetic processes and petrophysical properties from
outcrops to the reservoir. The key lithologies and their properties were
analyzed on different scales (Fig. 2). Several joint field trips with
project partners were conducted. Whenever possible, different groups joined
forces to enhance the comparability of results.
Petrophysical properties measured at dry conditions. The Pre-caldera
and Caldera group were merged for simplification. For the number of analysed
samples please see Table S1.
In addition to analysis of outcropping rocks in the vicinity of the active
geothermal areas, particular attention was paid to equivalent exhumed
systems at Zacatlán (east of Acoculco) and Las Minas (east of Los Humeros),
exposing all units from the cap rock to the basement. These
“fossil systems” serve as proxies and help to understand fluid flow and
mineralization processes in the reservoirs.
Relatively little is known about the petro- and thermo-physical rock
properties in the study area. Further data are needed for processing and
interpreting the geophysical data and for parameterizing reservoir models.
In order to create a comprehensive data base, which covers all relevant
units from the basement to the cap rock, rock samples were taken from
outcrops inside the calderas, in the surrounding areas and from exhumed
systems. This is necessary for a large enough data base to upscale and
correlate within the reservoir model and identify heterogeneities. Samples
are being analyzed for petrophysical (e.g. density, porosity, permeability)
and thermophysical properties (thermal conductivity, thermal diffusivity,
heat capacity) as well as ultra-sonic wave velocities, electric resistivity
and magnetic susceptibility by different partners subsequently to compare
and validate different methods. First results of the petrophysical
measurements are shown in Fig. 3 and Table S1 (Supplement).
Complementary to the laboratory measurements, further ultra sonic wave
velocity and electric resistivity measurements were performed directly in
the field on sample material of selected outcrops (shallow geophysical
surveys, see chapters “Complementary field work” and “Planned investigations
on the sample material” in the Supplement).
Material and methods
More than 250 rock samples were taken for rock property measurements from
about 125 outcrops covering all key lithologies (Fig. S1 in the Supplement).
Plugs with diameters from 25 to 64 mm were drilled from the
samples and subsequently dried at 105 ∘C for at least 24 h
until constant mass was achieved.
Samples were analyzed for density, porosity, permeability,thermal
conductivity and diffusivity, ultra sonic wave velocity, electrical
resistivity and magnetic susceptibility. Whenever possible, all parameters
were measured on each plug or rock sample for direct correlation. The rock
samples were classified after Norini et al. (2015) and Avellán et al. (2017)
into (1) Post-caldera volcanism, (2) Caldera volcanism, (3) Pre-caldera volcanism
and (4) Basement and Intrusive rocks.
Grain- and bulk density measurements were performed with an AccuPyc 1330 gas
pycnometer and a GeoPyc 1360 powder pycnometer (Micromeritics, 1997,
1998). Afterwards porosities were calculated. Density and porosity of large
samples which did not fit into the pycnometers were determined using
saturation and caliper techniques as per ISRM (1979).
Matrix permeability was determined with a column permeameter after Hornung
and Aigner (2004). The plugs were measured using at least five air pressure
levels ranging from 1 to 5 bar. Measurement accuracy varies from 5 % for
high permeable rocks (K>10-14 m2) to 400 % for impermeable rocks
(K<10-16 m2) (Bär, 2012).
Ultra sonic wave velocity was measured with pulse generators (UKS-D from
Geotron-Elektronik, 2011 and Pundit Lab, Proceq, ASTM D2845-08, 2008)
comprising point-source transmitter-receiver transducers. Polarized pulses
at high voltage in a frequency range from 20 kHz to 1 MHz for the UKS-D and
54 and 250 kHz for the Pundit Lab were generated. Additional field
measurements were also performed on irregular shaped outcrop samples for
P-wave velocities. P-wave measurements were performed using the same Pundit
Lab. Proceq device along different directions on the sample surfaces in
order to identify anisotropy and the effect of fractures. The combined
measurement of both P-and S-wave velocity offers the opportunity to retrieve
elastic mechanical parameters like Poisson ratio, Young's modulus and G modulus
(Mielke et al., 2017).
Electric resistivity measurements were carried out with a purpose built
square quadrupole after Clement et al. (2011). Electrical resistivity
measurements were performed with current injection between two subsequent
electrodes and the measurement of the resulting electric potential between
the remaining pair of electrodes. Concerning the field samples, the
electrical resistivity was estimated from electrical resistivity
tomographies performed in the sampling areas.
For the determination of thermal conductivity and thermal diffusivity a
thermal conductivity scanner was used after Popov et al. (1999, 2016). The
measurement accuracy is 3 % (Lippman and Rauen, 2009).
Furthermore, a Multi-Sensor Core Logger (MSCL) from GeoTEK (2000) was used
for measurements of gamma density, P-wave velocity, magnetic susceptibility
and electrical resistivity on whole core samples with a diameter of 60–64 mm.
Matrix density was calculated based on attenuation of gamma rays emitted
from Cesium-137, while porosity was calculated from the density
measurements. The P-wave velocity was measured by calculating the travel
time through the core of an ultra-sonic pulse generated and received by. The
accuracy is set about 0.2 % depending on core condition (GeoTEK, 2018).
Magnetic susceptibility was determined using a Bartington loop sensor with a
5 % calibration accuracy.
Preliminary Results
The field campaigns have shown the geological complexity of the two
reservoirs. Composition, extension and distribution of the volcanic
sequences are very variable within both sites. Dark grey, massive to
slightly porous, aphanetic (basaltic) andesites (Teziutlán andesite)
were found north of the Los Humeros caldera, while andesites exposed east
and west of the Los Humeros caldera (Cuyoaco and Alseseca andesite) have
shown a medium grained, porphyric texture comprising plagioclase and
hornblende. The carbonatic basement is intensively folded and faulted by the
Laramide orogeny. The most frequently encountered basement facies in the
study area are Middle and Upper Cretaceous units (e.g. Orizaba formation)
comprising 15 to 60 cm thick beds of massive, mudstones with marl and
chert layers (or nodules; both up to 30 cm thick) as well as grey to
greenish shales and massive grey limestones without chert (bed thickness
∼1 m and more). Jurassic units were found only in
Zacatlán as well as north and northwest of the Los Humeros caldera. We
sampled red, altered sandstones and massive, hydrocarbon-rich limestones.
Marble/skarn quarries in Las Minas indicate complex fluid-rock reactions in
the carbonatic rocks, which led to a range of dissolution/precipitation
processes and fracturing. One reason for the complexity of these systems is
the overprinting of initial high temperature alteration by lower temperature
alteration as the system cooled.
Hydrothermal alteration of different intensities can be observed in dykes
and fault zones in the outcrops cutting through all key lithologies, which
is one of the key processes affecting the petrophysical properties (Fig. S2).
Likewise, several different intrusive bodies
intruded into the sedimentary basement spread over the Los Humeros area
showing intensive alteration (and weathering). Following studies will focus
on quantifying the impact of hydrothermal alteration on the aforementioned rock properties.
The first petrophysical property results (Fig. 3, Table S1) enable the
classification of lithofacies types with distinct properties. The data
indicate that Cretaceous units are relatively homogenous throughout the
study area with very low matrix permeability (<10-16 m2) and
porosity (<5 %); thus geothermal fluid
movement must be fracture controlled. Andesite samples are more variable and
indicate slight regional trends, which confirm field observations. The
skarns show the highest variability in rock properties, explained by their
variable mineralogical composition. This was caused by the complex
metamorphic processes in the contact zones as well as the type of protolith:
carbonate (exoskarn) or igneous rock (endoskarn). The ignimbrites and ash
falls can be clearly distinguished from the other units. Results of P- and
S-wave and electric resistivity field measurements are in good agreement
with lab measurements (Fig. S3).
Conclusions and outlook
The field campaigns revealed the complexity of the two reservoirs. Detailed
outcrop analysis is paramount to characterize and discover heterogeneities
within the geological units. Extensive structural analysis serve to
characterize the main fracture patterns and to identify fluid pathways.
Shallow geophysical measurements help to characterize the field scale rock
mechanical behaviour of main fault zones. The results of petrophysical
measurements already enable the classification of different lithofacies
types with distinct properties. Future investigations will focus on core
samples from wells of the Los Humeros geothermal field for direct comparison
of outcrop analogues and the reservoir.
Hydrothermal alteration of different intensities can be observed in outcrops
in the vicinity of dykes and fault zones. Likewise, intensive hydrothermal
alteration and leaching processes have been observed on borehole core
samples of Los Humeros. The presented multidisciplinary approach will
improve the understanding of complex processes within hot unconventional
reservoirs with supercritical conditions and will enable more precise
reservoir models and serve as an example for upcoming projects.
No data sets were used in this article.
The supplement related to this article is available online at: https://doi.org/10.5194/adgeo-45-281-2018-supplement.
The authors declare that they have no conflict of interest.
This article is part of the special issue “European Geosciences
Union General Assembly 2018, EGU Division Energy, Resources & Environment (ERE)”.
It is a result of the EGU General Assembly 2018, Vienna, Austria, 8–13 April 2018.
Acknowledgements
We thank CFE for providing us access to the field and core storage and for
their support during fieldwork and sampling. Furthermore, we thank Caterina Bianco,
Thomas Kramer and Maximilian Bech for their contribution to this paper.
This project has received funding from the European Union's Horizon 2020
research and innovation programme under grant agreement No. 727550. Christopher A. Rochelle
publishes with the permission of the Executive Director of the British
Geological Survey.
We would like to thank Viktor Bruckman, Christopher Juhlin and one anonymous
referee for their constructive comments.
Edited by: Viktor Bruckman
Reviewed by: Christopher Juhlin and one anonymous referee
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