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

Petrophysical and rock mechanical properties are key parameters for the characterization of the deep subsurface in different disciplines such as geothermal heat extraction, petroleum reservoir engineering or mining, are commonly used for the interpretation of geophysical data and the parameterization of numerical models and as thus are the basis for economic reservoir assessment. However, detailed information regarding these properties for each target horizon are often scarce, inconsistent or spread over multiple publications. Thus, subsurface models are often populated with generalized or assumed values resulting in high uncertainty. Furthermore, diagenetic, metamorphic and hydrothermal processes significantly affect the physiochemical and mechanical properties often leading to high variability. A sound understanding of the controlling factors is needed to identify statistical and causal relationships between the properties as basis for a profound reservoir assessment and modelling. Within the scope of the GEMex project (EU-H2020, GA Nr. 727550), which aims to develop new transferable exploration and exploitation approaches for 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 Los Humeros geothermal system is steam dominated and has been exploited since the 1990’s with 65 wellbores (28 still producing). With temperatures above 380 °C, the system is characterized as a super-hot geothermal system. The geothermal system in Acoculco (presently consisting of two exploration wells) is characterized by temperatures of approximately 300 °C at a depth of about 2 km. It contains almost no fluids, even though a well-developed fracture network exists in the study area. Therefore, the system serves as a demonstration site for the development of an enhanced geothermal system. 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 340 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 were obtained. 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 ratio, bulk modulus, shear modulus, fracture toughness, cohesion and friction angle. In addition, XRD and XRF analyses were performed on 131 samples to provide information about the mineral assemblage, bulk geochemistry and the intensity of hydrothermal alteration. An extensive rock property database was created comprising 34 parameters determined on more than 2160 plugs. More than 31,000 data points 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.V1.

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

Petrophysical and rock mechanical properties are key parameters for the characterization of the deep subsurface in different disciplines such as geothermal heat extraction, petroleum reservoir engineering or mining, are commonly used for the interpretation of geophysical data and the parameterization of numerical models and as thus are the basis for economic reservoir assessment. However, detailed information regarding these properties for each target horizon are often scarce, inconsistent or spread over multiple publications. Thus, subsurface models are often populated with generalized or assumed values resulting in high uncertainty. Furthermore, diagenetic, metamorphic and hydrothermal processes significantly affect the physiochemical and mechanical properties often leading to high variability. A sound understanding of the controlling factors is needed to identify statistical and causal relationships between the properties as basis for a profound reservoir assessment and modelling. Within the scope of the GEMex project (EU-H2020, GA Nr. 727550), which aims to develop new transferable exploration and exploitation approaches for 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 Los Humeros geothermal system is steam dominated and has been exploited since the 1990’s with 65 wellbores (28 still producing). With temperatures above 380 °C, the system is characterized as a super-hot geothermal system. The geothermal system in Acoculco (presently consisting of two exploration wells) is characterized by temperatures of approximately 300 °C at a depth of about 2 km. It contains almost no fluids, even though a well-developed fracture network exists in the study area. Therefore, the system serves as a demonstration site for the development of an enhanced geothermal system. 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 340 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 were obtained. 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 ratio, bulk modulus, shear modulus, fracture toughness, cohesion and friction angle. In addition, XRD and XRF analyses were performed on 131 samples to provide information about the mineral assemblage, bulk geochemistry and the intensity of hydrothermal alteration. An extensive rock property database was created comprising 34 parameters determined on more than 2160 plugs. More than 31,000 data points 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.V1.

Der Einfluß von intergranularem, überkritischen Wasser auf die elastischen Gesteinseigenschaften

Titelblatt und Inhaltsverzeichnis 1\. INTRODUCTION 2\. THE DEVELOPMENT OF AN EXPERIMENTAL DEVICE 3\. DATA AQUISITION AND PROCESSING 4\. THE SAMPLES 5\. RESULTS 6\. DISCUSSION Bibliography Methodes of Sample Characterisation DanksagungWater is known to play a key role in processes linked to active tectonic settings. In particular, the concept of earthquake-triggering by dehydration embrittlement due to fluid release during mineral dehydration and an increasing pore fluid pressure is discussed intensively. Nonetheless, little is known about the petrophysical signature of mineral reactions. The present study depicts a first approach to monitor the influence of supercritical fluids on the elastic properties of rocks. For this a high pressure/ high temperature ultrasonic set-up was developed, capable of investigating relatively large cores (d = 30 mm; l = 25 mm) of low porous amphibolite and serpentinite at undrained conditions, where water remains in the system. P and S wave velocities were deduced in an internally heated gas-pressure vessel at a maximum confining pressure of 1 GPa and temperatures up to 750 °C. Based on numerous previous laboratory experiments, it is established that at drained conditions an increase of temperature leads to a slight linear decrease of elastic wave velocities, which is generally attributed to the intrinsic change of elastic properties of the rock matrix. At undrained conditions (this study), the serpentinite shows a similar behaviour: Up to 600 °C temperature derivatives (ðv/ðT)Pc of -0.60 ×10−3 and -0.46 ×10−3 [km s−1 °C−1] were observed for P and S waves, respectively, which are assumed to represent the intrinsic temperature dependency of the elastic properties of the rock-forming minerals. The onset of antigorite decomposition is displayed by a sharp decrease of velocities above 600 °C and an increase of porosity. In contrast, the seismic signature of the undrained amphibolite is characterised by an alternation of considerably decreasing and increasing P wave velocities up to 700 °C. This observation indicates pore-fluid pressure induced changes in the porosity and permeability of the rock. A significant reduction of P wave velocities was observed already at temperatures < 400 °C, accompanied by an increasing porosity. This reveals the dominance of grain boundary effects and the minor influence of the elastic moduli of the rock-constituent minerals on the T dependency even at high confining pressure. The differences in the seismic signatures obtained from measurements on serpentinite and amphibolite in this study, is attributed to variable strength behaviours of both rock samples. This, in turn, seem to be controlled by complex interactions of microstructural particularities, pore fluid volume and the fluid pressure. Following the interpretation of thermally induced microcracking, it is assumed that the medium grained, euhedral crystals of the amphibolite favour the loss of grain contacts. This effect is additionally amplified by accessory mica and chlorite on the grain boundaries, as in combination with adsorbed water they form effective lubricants and may enhance sliding at grain boundaries. The experimental results show that in this case already small amounts of supercritical water lead to an almost complete loosening of grain contacts and thus effect the elastic properties of the rock dramatically. The serpentinite, in contrast, might have a higher tensile strength due to its fine grained matrix of interlocked minerals. Thus, no significant velocity reduction was observed until larger quantities of water were released.Wasser wird gemeinhin eine Schlüsselrolle in Prozessen zugewiesen, die in tektonisch aktiven Gebieten ablaufen. Im besonderen die Idee der Erdbeben- Triggerung durch dehydration embrittlement als Folge von Mineralentwässerungen und einem damit einhergehenden Anstieg des Porenfluiddrucks wird intensiv diskutiert. Nichtsdestotrotz ist bisher wenig über die petrophysikalische Signatur von Mineralreaktionen bekannt. Die vorliegende Studie stellt einen ersten Ansatz zur Beobachtung des Einflusses überkritischer Fluide auf die elastischen Eigenschaften von Gesteinen dar. Dazu wurde ein Hochdruck-/ Hochtemperatur Ultraschallaufbau entwickelt, der die Untersuchung relativ großer Amphibolit- und Serpentinitproben unter undrainierten Bedingungen erlaubt. Die P- und S-Wellengeschwindigkeiten (vp, vs) wurden in einem innenbeheizten Gasdruckautoklaven bis maximal 1 GPa und 750 °C bestimmt. Frühere Laborexperimente zeigen, daß unter drainierten Bedingungen eine Temperaturerhöhung zur schwachen linearen Abnahme von vp und vs führt, was generell auf die intrinsische Änderung der elastischen Eigenschaften der Gesteinsmatrix zurückgeführt wird. Unter undrainierten Bedingungen (diese Studie) zeigt der Serpentinit ein ähnliches Verhalten: Bis 600 °C wurden (ðv/ðT)Pc von -0.60 ×10−3 und -0.46 ×10−3 [km s−1 °C−1] für P- bzw. S-Wellen beobachtet. Es wird angenommen, daß diese die intrinsische Temperaturabhängigkeit der elastischen Eigenschaften der gesteinsbildenden Minerale darstellen. Der Beginn der Antigoritdekomposition wird durch eine scharfe Abnahme der Geschwindigkeiten oberhalb von 600 °C und eine Zunahme der Porosität angezeigt. Im Gegensatz dazu wird die seismische Signatur des undrainierten Amphibolites bis 700 °C durch einen Wechsel beträchtlicher Zu- und Abnahmen von vp bestimmt. Diese Beobachtung zeigt durch Porenfluiddruck induzierte Änderungen der Porosität und Permeabilität des Gesteins an. Schon bei Temperaturen < 400 °C wurde eine signifikante Abnahme von vp unter Zunahme der Porosität beobachtet. Dies offenbart die Dominanz von Korngrenzeneffekten und den untergeordneten Einfluß der elastischen Moduli der gesteinsbildenden Minerale auf die Temperaturabhängigkeit auch unter hohem Umschließungsdruck. Die Unterschiede der seismischen Signaturen des in dieser Studie untersuchten Amphibolites und Serpentinites wird den unterschiedlichen Festigkeiten beider Gesteinsproben zugeordnet. Diese scheint wiederum durch ein komplexes Zusammenspiel von mikrotexturellen Besonderheiten, Porenfluidvolumen und dem Porendruck kontrolliert zu werden. In Anlehnung an frühere Interpretationen zur thermisch induzierten Rißbildung wird hier angenommen, daß die mittelkörnigen, euhedralen Kristalle des Amphibolites eine Auflockerung des Kornverbandes entlang der Korngrenzen begünstigen. Zusätzlich wird dieser Effekt durch die Akzessorien Glimmer und Chlorit verstärkt, die sich auf den Korngrenzen finden und im Zusammenwirken mit Wasser ein effektives Schmiermittel bilden, das das Gleiten entlang von Korngrenzen verbessert. Die experimentellen Ergebnisse zeigen, daß in diesem Falle schon geringe Mengen überkritischen Wassers zu einer fast vollständigen Auflockerung des Kornverbandes führen und somit die elastischen Eigenschaften des Gesteins dramatisch beeinflussen. Im Gegensatz dazu hat der Serpentinit aufgrund seiner feinkörnigen Matrix aus miteinander verzahnten Mineralen möglicherweise eine höhere Zugfestigkeit. Daher wurde hier keine signifikante Geschwindigkeitsabnahme beobachtet, bis größere Mengen Wasser freigesetzt wurden

A fracture flow permeability and stress dependency simulation applied to multi-reservoirs, multi-production scenarios analysis

The use of the subsurface and the exploitation of subsurface resources require prior knowledge of fluid flow through fracture networks. For nuclear waste disposal, for the enhancement of hydrocarbon recovery from a field, or the development of an enhanced geothermal system (EGS), it is fundamental to constrain the fractures and the fracture network. This study is part of the GEMex project, an international collaboration of two consortia, one from Europe and one from Mexico. The research is based on exploration, characterization and assessment of two geothermal systems located in the Trans-Mexican volcanic belt, Los Humeros and Acoculco. In Acoculco, two wells reached very high temperatures, but did not find any fluids. For that reason, the Acoculco Caldera is foreseen as an EGS development site, hoping to connect existing wells to a productive zone. This implies that the fluid flow through the geothermal reservoir would be mainly fracture dominated. This study investigates the dependency of fracture permeability, constrained by fracture lengths and apertures, with stress field conditions. Simulations are computed in 2D, using COMSOL Multiphysics® Finite Elements Method Software, populated with mechanical data obtained in the rock physics laboratory and with dense discrete fracture networks generated from 1D scanline surveys measured in Las Minas analogue outcrops for Acoculco reservoir. The method offers a prediction for multiple scenarios of the reservoir flow characteristics which could be a major improvement in the development of the EGS technology.</p

A fracture flow permeability and stress dependency simulation applied to multi-reservoirs, multi-production scenarios analysis

The use of the subsurface and the exploitation of subsurface resources require prior knowledge of fluid flow through fracture networks. For nuclear waste disposal, for the enhancement of hydrocarbon recovery from a field, or the development of an enhanced geothermal system (EGS), it is fundamental to constrain the fractures and the fracture network. This study is part of the GEMex project, an international collaboration of two consortia, one from Europe and one from Mexico. The research is based on exploration, characterization and assessment of two geothermal systems located in the Trans-Mexican volcanic belt, Los Humeros and Acoculco. In Acoculco, two wells reached very high temperatures, but did not find any fluids. For that reason, the Acoculco Caldera is foreseen as an EGS development site, hoping to connect existing wells to a productive zone. This implies that the fluid flow through the geothermal reservoir would be mainly fracture dominated. This study investigates the dependency of fracture permeability, constrained by fracture lengths and apertures, with stress field conditions. Simulations are computed in 2D, using COMSOL Multiphysics® Finite Elements Method Software, populated with mechanical data obtained in the rock physics laboratory and with dense discrete fracture networks generated from 1D scanline surveys measured in Las Minas analogue outcrops for Acoculco reservoir. The method offers a prediction for multiple scenarios of the reservoir flow characteristics which could be a major improvement in the development of the EGS technology.Petroleum EngineeringApplied Geolog

Seismic modeling of the AVO/AVA response to CO2 injection at the Ketzin site, Germany

Over 64 kilotons of CO2 have been injected (May, 2013) into a heterogeneous sandstone reservoir (saline aquifer) at 630-650 m depth. 4D seismics have been applied to monitor CO2 at the Ketzin site. However, the obtained time-lapse seismic signals have been so far interpreted as being caused by fluid saturation changes only. Modeling of the AVO/AVA response allows us to study two kinds of effects: CO2-saturation- and pore-pressure-related effects. Our results indicate that it is rather infeasible to discriminate between both these effects at the Ketzin site dealing with the real seismic data with limited signal/noise ratios. © 2013 The Authors. Published by Elsevier Ltd

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

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&lt;span idCombining double low line"page572"/&gt; 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; &lt;a hrefCombining double low line"https://doi.org/10.25534/tudatalib-201.10"&gt;https://doi.org/10.25534/tudatalib-201.10&lt;/a&gt;), 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..Reservoir Engineerin