The microstructure and mineralogy of volcanic rocks is varied and complex, and their mechanical behaviour is similarly varied and complex. This review summarises recent developments in our understanding of the mechanical behaviour and failure modes of volcanic rocks. Compiled data show that, although porosity exerts a first-order influence on the uniaxial compressive strength of volcanic rocks, parameters such as the partitioning of the void space (pores and microcracks), pore and crystal size and shape, and alteration also play a role. The presence of water, strain rate, and temperature can also influence uniaxial compressive strength. We also discuss the merits of micromechanical models in understanding the mechanical behaviour of volcanic rocks (which includes a review of the available fracture toughness data). Compiled data show that the effective pressure required for the onset of hydrostatic inelastic compaction in volcanic rocks decreases as a function of increasing porosity, and represents the pressure required for cataclastic pore collapse. Differences between brittle and ductile mechanical behaviour (stress-strain curves and the evolution of porosity and acoustic emission activity) from triaxial deformation experiments are outlined. Brittle behaviour is typically characterised by shear fracture formation, and an increase in porosity and permeability. Ductile deformation can either be distributed (cataclastic pore collapse) or localised (compaction bands) and is characterised by a decrease in porosity and permeability. The available data show that tuffs deform by delocalised cataclasis and extrusive volcanic rocks develop compaction bands (planes of collapsed pores connected by microcracks). Brittle failure envelopes and compactive yield caps for volcanic rocks are compared, highlighting that porosity exerts a first-order control on the stresses required for the brittle-ductile transition and shear-enhanced compaction. However, these data cannot be explained by porosity alone and other microstructural parameters, such as pore size, must also play a role. Compactive yield caps for tuffs are elliptical, similar to data for sedimentary rocks, but are linear for extrusive volcanic rocks. Linear yield caps are considered to be a result of a high pre-existing microcrack density and/or a heterogeneous distribution of porosity. However, it is still unclear, with the available data, why compaction bands develop in some volcanic rocks but not others, which microstructural attributes influence the stresses required for the brittle-ductile transition and shear-enhanced compaction, and why the compactive yield caps of extrusive volcanic rocks are linear. We also review the Young’s modulus, tensile strength, and frictional properties of volcanic rocks. Finally, we review how laboratory data have and can be used to improve our understanding of volcanic systems and highlight directions for future research. A deep understanding of the mechanical behaviour and failure modes of volcanic rock can help refine and develop tools to routinely monitor the hazards posed by active volcanoes.

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

Machine learning in subsurface geothermal energy: Two decades in review

An approach to reconstruct the thermal history in active magmatic systems: Implications for the Los Humeros volcanic complex, Mexico

. Petrophysical properties are a key element for reservoir
characterization but also for interpreting the results of various
geophysical exploration methods or geophysical well logs. Furthermore,
petrophysical properties are commonly used to populate numerical models and
are often critically governing the model results. Despite the common need for
detailed petrophysical properties, data are still very scarce and often not
available for the area of interest. Furthermore, both the online research
for published property measurements or compilations, as well as dedicated
measurement campaigns of the selected properties, which require
comprehensive laboratory equipment, can be very time-consuming and costly.
To date, most published research results are often focused on a limited
selection of parameters only, and hence researching various petrophysical
properties, needed to account for the thermal–hydraulic–mechanical behaviour
of selected rock types or reservoir settings, can be very laborious. Since for deep geothermal energy in central Europe, the majority of the
geothermal potential or resource is assigned to the crystalline basement, a
comprehensive database of petrophysical properties comprising rock
densities, porosity, rock matrix permeability, thermal properties (thermal
conductivity and diffusivity, specific heat capacity) as well as rock
mechanical properties as compressional and shear wave velocities, unconfined
compressive strength, Young's modulus, Poisson's ratio, tensile strength and
triaxial shear strength was compiled from measurements conducted at the
HydroThermikum lab facilities of the Technical University of Darmstadt. Analysed samples were mostly derived from abandoned or active quarries and
natural or artificial outcrops such as road cuts, riverbanks or steep hillslopes. Furthermore, samples of the cored deep wells Worms 3 (samples from
2175–2195 m), Stockstadt 33R (samples from 2245–2267 m), Weiterstadt 1
(samples from 2502–2504 m), Tiefbohrung Gros-Umstadt/Heubach, B/89–B02
and the cored shallow wells (Forschungsbohrung Messel GA 1 and 2) as well
as GWM17 Zwingenberg, GWM1A Zwingenberg, Langenthal BK2/05, EWS267/1
Heubach, and archive samples of the Institut fur Steinkonservierung e.V. in Mainz originating from a comprehensive large-scale sampling campaign
in 2007 were investigated. The database (Weinert et al., 2020b; https://doi.org/10.25534/tudatalib-278 )
aims to provide easily accessible petrophysical properties of the Mid-German
Crystalline Rise, measured on 224 locations in Bavaria, Hessen,
Rhineland-Palatinate and Thuringia and comprising 26 951 single data points.
Each data point is addressed with the respective metadata such as the sample
identifier, sampling location, petrography and, if applicable, stratigraphy
and sampling depth (in the case of well samples).

/pdf/database-of-petrophysical-properties-of-the-mid-german-2921narkl8.pdf

Database of petrophysical properties of the Mid-German Crystalline Rise

http://manuscript.elsevier.com/S037702732200107X/pdf/S037702732200107X.pdf

The tensile strength of hydrothermally altered volcanic rocks

. 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 Zacatlan. 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.

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Ángel Andrés Ramírez-Guzmán

Papers

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