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 are 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 a 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, GA Nr. 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 analogue 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 of the Los Humeros and Acoculco calderas, 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, heat capacity, as well as ultra-sonic 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, bulk modulus, shear modulus, fracture toughness, cohesion and friction angle. In addition, XRD and 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, http://dx.doi.org/10.25534/tudatalib-201.2), comprising 34 parameters determined on more than 2,160 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.

The mechanical behaviour and failure modes of volcanic rocks: a review

“…Bakker et al 2019;Parisio et al 2019Parisio et al , 2020. The strength and mechanical properties of volcanic rocks are also of important in the context of geothermal energy exploitation (Siratovich et al 2016;Eggertsson et al 2020;Weaver et al 2020;Heap et al 2020c;Weydt et al 2020). Finally, the failure mode of volcanic rocks also controls how their physical properties evolve.…”

The impact of hydrothermal alteration on the physiochemical characteristics of reservoir rocks: the case of the Los Humeros geothermal field (Mexico)

“…Various geological (Ferriz and Mahood 1984;Carrasco-Núñez et al 2017a, b), geochemical (Prol-Ledesma and Browne 1989;Martıńez-Serrano 1993Izquíerdo et al 2011Izquíerdo et al , 2015, geophysical (Lermo et al 2008;Arzate et al 2018), and hydrological studies (Tello 2005) have been performed in the past and most recently within the framework of the GEMex project (EU-H2020, GA Nr. 727,550;Jolie et al 2018;Weydt et al 2018Weydt et al , 2021bLucci et al 2020;Urbani et al 2020Urbani et al , 2021 to improve reservoir understanding and to create conceptual (Cedillo 2000;) and 3D geological models (Calcagno et al 2020;Deb et al 2019). However, even after more than 40 years of exploration, information on rock properties of the different geological units in the study area were scarce or not available.…”

The thermal conductivity of unlithified granular volcanic materials: The influence of hydrothermal alteration and degree of water saturation

“…Experimental studies have provided values for the thermal properties of volcanic rocks for modelling (Horai et al, 1970;Fujii and Osako, 1973;Robertson and Peck, 1974;Bagdassarov and Dingwell, 1994;Whittington et al, 2009;Romine et al, 2012;Mielke et al, 2015Mielke et al, , 2016Mielke et al, , 2017Vélez et al, 2018;Heap et al, 2020;Weydt et al, 2021;Heap et al, 2022a). These studies, and others, have shown that porosity plays a first-order role in dictating thermal properties of volcanic rocks.…”