Petrophysical and mechanical rock property database of the Los Humeros and Acoculco geothermal fields (Mexico)

Abstract

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.

Database of petrophysical properties of the Mid-German Crystalline Rise

“…As follows, all applied methods are briefly described. For a more detailed methodological description, please refer to Weydt et al (2021).…”

“…It is structured in two superentities, namely "metadata" and "petrophysical properties", and further hierarchical structured into logical subdivisions. (after Hirschmann, 1995;Voges et al, 1993;Klügel, 1997;Kroner et al, 2008) of the Mid-German Crystalline Rise outcrops: (a) Odenwald (after Stein, 2001;Will and Schmädicke, 2001;McCann et al, 2008), (b) Spessart (after Okrusch, 1983;Dombrowski et al, 1995;McCann et al, 2008) and (c) the Ruhla Mountains (Zeh et al, 2003;McCann et al, 2008).…”

“…Despite the importance of petrophysical properties, as well as the importance of crystalline basement rocks in deep geothermal energy, to which 85 % to 90 % of the German geothermal potential is accredited (Deutscher Bundestag, 2003), such data are often either unpublished, only published for confined areas (e.g. Mielke et al, 2016;Aretz et al, 2016;Weydt et al, 2021;Weinert et al, 2020a) or published without important meta information. The search for suitable petrophysical properties can therefore be very time-consuming, and often only widely averaged properties can be found and are commonly used neglecting local heterogeneities, vertical and lateral variability and anisotropic behaviour of the rocks.…”

Geophysical characterization of geothermal fields: Inversion of temperature data and ground motions

“…The calculated moment magnitude from all three slip models based on the InSAR data is approximately 4.35, using a shear modulus of 8 GPa (Table 6.1). We estimated the shear modulus of the volcano-sedimentary units of the LHVC based on the mechanical rock property database of Los Humeros (Weydt et al, 2018;Weydt et al, 2021). The estimated moment magnitude of 4.35 is larger than the Mw=4.2 value reported by Lermo Samaniego et al (2016), corresponding to an approximately 1.7 times larger seismic moment (3.8 x 10 15 Nm in case of model 3, compared to 2.2 x 10 15 Nm of the seismological solution).…”

“…For all models we approximated the subsurface with an elastic half-space with a Poisson's ratio ν = 0.25. This value is considered as a lower bound for the Poisson's ratio of the geological units of the area, calculated based on the mechanical rock property database of Los Humeros (Weydt et al, 2018;Weydt et al, 2021). We approximate the Poisson's ratio of the LHVC units (subdivided into a post-caldera unit, caldera unit, and pre-caldera unit) between 0.265-0.27, with and uncertainty of ±0.02.…”