Petrophysical and mechanical rock property database of the Los Humeros and Acoculco geothermal fields (Mexico)
Authors: Weydt Leandra M. , Ramírez-Guzmán Ángel Andrés , Pola Antonio , Lepillier Baptiste , Kummerow Juliane , Mandrone Giuseppe , Comina Cesare , Deb Paromita , Norini Gianluca , Gonzalez-Partida Eduardo , Avellán Denis Ramón , Macías José Luis , Bär Kristian , Sass Ingo 
Published: February 2021

Abstract. Petrophysical and mechanical rock properties are key parameters for the
characterization of the deep subsurface in different disciplines such as
geothermal heat extraction, petroleum reservoir engineering or mining. They
are commonly used for the interpretation of geophysical data and the
parameterization of numerical models and thus are the basis for economic
reservoir assessment. However, detailed information regarding petrophysical
and mechanical rock properties for each relevant target horizon is often
scarce, inconsistent or distributed over multiple publications. Therefore,
subsurface models are often populated with generalized or assumed values
resulting in high uncertainties. Furthermore, diagenetic, metamorphic and
hydrothermal processes significantly affect the physiochemical and
mechanical properties often leading to high geological variability. A
sound understanding of the controlling factors is needed to identify
statistical and causal relationships between the properties as a basis for a
profound reservoir assessment and modeling. Within the scope of the GEMex project (EU H2020, grant agreement no. 727550), which aims
to develop new transferable exploration and exploitation approaches for
enhanced and super-hot unconventional geothermal systems, a new workflow was
applied to overcome the gap of knowledge of the reservoir properties. Two
caldera complexes located in the northeastern Trans-Mexican Volcanic Belt –
the Acoculco and Los Humeros caldera – were selected as demonstration sites. The workflow starts with outcrop analog and reservoir core sample studies
in order to define and characterize the properties of all key units from the
basement to the cap rock as well as their mineralogy and geochemistry. This
allows the identification of geological heterogeneities on different scales
(outcrop analysis, representative rock samples, thin sections and chemical
analysis) enabling a profound reservoir property prediction. More than 300 rock samples were taken from representative outcrops inside the Los Humeros and Acoculco calderas and the surrounding areas and from
exhumed “fossil systems” in Las Minas and Zacatlán. Additionally, 66
core samples from 16 wells of the Los Humeros geothermal field and 8 core
samples from well EAC1 of the Acoculco geothermal field were collected.
Samples were analyzed for particle and bulk density, porosity, permeability,
thermal conductivity, thermal diffusivity, and heat capacity, as well as
ultrasonic wave velocities, magnetic susceptibility and electric
resistivity. Afterwards, destructive rock mechanical tests (point load
tests, uniaxial and triaxial tests) were conducted to determine tensile
strength, uniaxial compressive strength, Young's modulus, Poisson's ratio, the
bulk modulus, the shear modulus, fracture toughness, cohesion and the friction
angle. In addition, X-ray diffraction (XRD) and X-ray fluorescence (XRF) analyses were performed on 137 samples to
provide information about the mineral assemblage, bulk geochemistry and the
intensity of hydrothermal alteration. An extensive rock property database was created (Weydt et al., 2020;
https://doi.org/10.25534/tudatalib-201.10), comprising 34 parameters
determined on more than 2160 plugs. More than 31 000 data entries were
compiled covering volcanic, sedimentary, metamorphic and igneous rocks from
different ages (Jurassic to Holocene), thus facilitating a wide field of
applications regarding resource assessment, modeling and statistical
analyses.