Thermal Properties Of Rocks Research Articles

Thermal Properties of Rocks and Environmental Sustainability: A Review

Thermal Properties of Rocks and Environmental Sustainability: A Review

A Study on the Thermal Physical Property Changes in Formation Rocks during Rapid Preheating of SAGD

The incorporation and application of SAGD rapid preheating technology effectively solve the problem of the long preheating cycle in the SAGD steam cycle. The thermal properties of reservoir rocks are an important factor affecting the heat transfer law governing their formation during the rapid preheating process of SAGD. During the rapid preheating process of SAGD, the expansion of the reservoir and the steam cycle process will cause changes in the pore permeability, oil-water saturation, and temperature of the reservoir rocks, which will inevitably lead to differences in the changes that occur in the thermal properties of the reservoir rocks, compared to those under the influence of a single factor. In this study, experiments were conducted to determine the thermal properties of reservoir rocks under the combined influence of pore permeability, oil-water saturation, and temperature, quantitatively characterizing the changes in the thermal properties of reservoir rocks. Using the orthogonal method to design and carry out experiments for determining the thermal properties of reservoir rocks, the main factors affecting the thermal properties of reservoir rocks and the significance of each factor’s impact on the thermal properties of reservoir rocks were determined through intuitive analysis and variance analysis of the experimental results. Finally, a regression equation that can characterize changes in the thermal properties of reservoir rocks under the influence of multiple factors was obtained through multiple nonlinear regressions of the experimental results.

A coupled thermo-mechanical model for investigating cracking and failure of composite interbedded rock

Composite interbedded rock is a special geological phenomenon that occurs when layers of rock (beds) of a given lithology lie between or alternate with other layers of a different lithology. The thermal and mechanical properties of the composite interbedded rocks exhibit strong anisotropy. Studying the thermomechanical coupling properties of natural composite interbedded rocks using existing experimental techniques is challenging. A coupled thermo-mechanical composite interbedded rock model was developed based on Fourier's heat-conduction law and Newton's second law. The thermodynamic responses and biaxial mechanical properties of composite interbedded rocks with different interlayer angles under the coupling of initial stress and thermal loading were studied for the first time. The results indicate that the thermal stresses within the granite interlayers are higher than those within the sandstone interlayers. Throughout the thermal loading process, thermal cracking occurred primarily within the (stiffer) granite interlayers. The number of thermal cracks generated during the cooling process far exceeds that during heating; at 400 °C, the number of thermal cracks during cooling is more than 35 times that during heating. When the interlayer dip angle approaches 90° and is aligned with the major principal stress direction, the phenomenon of the “double peaks” and layered failure becomes more pronounced. Within the range of variables considered, the relative impact of the three variables on peak stress in composite interbedded rock is as follows, from greatest to least: interlayer angle, thermal shock temperature, and model size. The results and developed modeling methods will contribute to the management and optimization of future engineering geology projects.

Thermal spalling failure mechanism of heterogeneous granite under moving heat source

The conventional mechanical methods employed for rock breaking encounter substantial challenges in deep, hard formation drilling, characterized by low penetration rates and exorbitant drilling costs. Consequently, there exists an urgent necessity to explore novel drilling methods and technologies. Thermal spallation drilling, as a non-contact rock breaking technique, offers high efficiency in rock fragmentation and substantially reduces drilling costs. However, the mechanism underlying rock fragmentation induced by thermal spallation remains unclear. Therefore, this study establishes a thermo-mechanical coupling model based on heterogeneous granite subjected to a high-temperature moving heat source. The model comprehensively considers the geometric shape, contact, and strength heterogeneity of rock minerals, and analyzes the effects of heating source speed, radius, mineral composition, heterogeneity index, and grain size on the thermal spallation mechanism of rock. The research findings indicate that lower heating source speeds and larger radii result in higher surface temperatures and greater damage volumes, whereas increasing speed and reducing radius enhance fragmentation efficiency, with speed exerting a more significant influence. Thermal spallation primarily results in tensile damage to rocks, with the surface damage and depth of damage being respectively influenced by the heating source speed and radius. Particle size exhibits minimal impact on thermal spallation, while increased quartz content enhances the rock's thermal properties, resulting in significant thermal spallation effects. Additionally, the location of rock damage is closely correlated with thermal parameters. These findings significantly contribute to a better understanding of the thermal spallation fracture mechanism in granite and facilitate the development of technical solutions for thermal spallation drilling methods.

Estimation of unfrozen water content of saturated sandstones using nuclear magnetic resonance, mercury intrusion porosimetry, and ultrasonic tests

The unfrozen water content (UWC) is a crucial parameter that affects the strength and thermal properties of rocks in relation to engineering construction and geological disasters in cold regions. In this study, three different methods were employed to test and estimate the UWC of saturated sandstones, including nuclear magnetic resonance (NMR), mercury intrusion porosimetry (MIP), and ultrasonic methods. The NMR method enabled the direct measurement of the UWC of sandstones using the free induction decay (FID). The MIP method was used to analyze the pore structures of sandstones, with the UWC subsequently calculated based on pore ice crystallization. Therefore, the MIP test constituted an indirect measurement method. Furthermore, a correlation was established between the P-wave velocity and the UWC of these sandstones based on the mixture theory, which could be employed to estimate the UWC as an empirical method. All methods demonstrated that the UWC initially exhibited a rapid decrease from 0 °C to −5 °C and then generally became constant beyond −20 °C. However, these test methods had different characteristics. The NMR method was used to directly and accurately calculate the UWC in the laboratory. However, the cost and complexity of NMR equipment have precluded its use in the field. The UWC can be effectively estimated by the MIP test, but the estimation accuracy is influenced by the ice crystallization process and the pore size distribution. The P-wave velocity has been demonstrated to be a straightforward and practical empirical parameter and was utilized to estimate the UWC based on the mixture theory. This method may be more suitable in the field. All methods confirmed the existence of a hysteresis phenomenon in the freezing-thawing process. The average hysteresis coefficient was approximately 0.538, thus validating the Gibbs–Thomson equation. This study not only presents alternative methodologies for estimating the UWC of saturated sandstones but also contribute to our understanding of the freezing-thawing process of pore water.

Investigation of thermal cooling effect of long-term circulation in Raft River enhanced geothermal system

The Raft River Enhanced Geothermal System (EGS) demonstration project incorporated three hydraulic stimulations in well RRG-9 ST1 to improve geothermal production. This study investigated the injectivity index (injection rate divided by injection pressure) increase during long-term injection into RRG-9-ST1 after the stimulations from the perspective of thermoelasticity. We have used a one-dimensional approximate solution to estimate the injectivity/aperture change due to thermal cooling. The inferred aperture change vs. square root of time falls along a linear trend. The thermal coefficient (a combination of rock thermal properties, rock initial temperature, and injected fluid temperature) back-calculated by fitting the field injection data is in the same order of magnitude as the value directly calculated from these parameters. We have conducted fully-coupled thermal-hydro-mechanical modeling to simulate long-term circulation (cold water injection). The simulated injectivity index increased by 1.7 × 10−8 m3/(Pa·s) after 272 days of cold-water injection, similar to the field data, which increased by 2.5 × 10−8 m3/(Pa·s). Both the trend and the magnitude of the simulated injectivity change match with the field data recorded at Raft River. The injectivity index increased faster at the beginning and then the rate of increase became smaller with time. The injectivity index increase is hypothesized to be the result of fracture aperture increase due to cooling effects. The simulated fracture aperture at the injection well increased by 0.64 mm, and the simulated maximum fracture aperture increased by 1.93 mm. The trend of the simulated aperture change also matches the value interpreted from the field data using the 1D approximate solutions. The study suggests that cooling is the major reason for the injectivity increase at Raft River EGS during the long-term injection. More broadly, cold water injection could serve as a method to increase the injectivity in other settings after high-volume and high rate/pressure hydraulic stimulation

Thermal and Physical Properties of Barrovian Metamorphic Sequence Rocks in the Ailao Shan‐Red River Shear Zone, and Implications for Crustal Channel Flow

AbstractThe collisional history between Greater India and the Eurasian plate has been well constrained by the study of exhumed Barrovian metamorphic sequence (BMS) rocks in the Himalayan Range. However, in the southeastern Tibetan Plateau, the collisional records have been obscured by intense, regional‐scale strike‐slip overprinting and recrystallization. Here, in BMS rocks from the Ailao Shan–Red River shear zone (ARSZ), we report the first discovery of a >250 km long, high‐pressure (high‐P) granulite belt (>1.0 GPa), identified by the presence of relict kyanite and associated decompression reaction textures. Petrological phase equilibrium modeling showed that exposed micaschists in the region represent exhumed middle crust (20–25 km, 600–670°C), while the high‐P granulite rocks are remnants of thickened lower crust (45–55 km, 800–850°C). This indicates that the northeast edge of the ARSZ experienced an additional ∼25 km of uplift and exhumation compared to the southwest side, facilitated by brittle thrusting/imbrication along the Ailao Shan fault (micaschists) and ductile extrusion along the Red River fault (granulite). Geochronological study shows that the upper portion of the BMS preserves older metamorphic ages (52–34 Ma) than the lower portion (32–29 Ma), which was attributed to spatial variation in cooling rates. Using calculated P–T–t–d paths, we also examined variation in density and seismic wave speeds for BMS in the ARSZ. Our data correlate with fieldwork conducted elsewhere within the Himalayan Range indicating that the kyanite to sillimanite transition zone may serve as a “cap” for the horizontal migration of melt within the lower crust.

Analysis of thermal properties of main sedimentary rocks in the Beijing area

The Beijing area is abundant in geothermal resources, yet there has been limited research on the thermal properties of rocks and their influencing factors. This paper focuses on the thermal properties of sedimentary rocks in the region, conducting experimental analysis to investigate these properties and their influencing factors. The experiment involved collecting primary sedimentary rock outcrop samples from around Beijing, testing the thermophysical parameters of 48 samples using a Hot Disk thermal constant analyzer. By combining this data with the standard stratum profile and historical information about Beijing, the thermal conductivity of the formation was calculated using the harmonic mean method, allowing for an analysis of the thermal properties of primary sedimentary rocks in the study area. The results indicate that overall distribution of thermal conductivity for sedimentary rocks in the Beijing area ranges from 1.48 to 6.55 W/(m·K), while thermal diffusivity ranges from 0.76 × 10−6 to 4.04 × 10−6 m2/s, and specific heat distribution ranges from 0.57 to 2.52MJ/(m3·K). Furthermore, according to harmonic mean calculations, it was found that Jixian formation exhibits the highest thermal conductivity value, whereas Triassic formation displays the lowest. This study on the thermal properties of sedimentary rocks provides valuable insights for the geothermal field research in the Beijing area.

Numerical simulation of hydrothermal flow in the North China Plain: A case study of Henan Province

The distribution and flow of geothermal energy in the North China region are influenced by geological structures and rock thermal properties. However, the interactions between geothermal energy, groundwater movement, and hydrogeochemical processes are not well understood. In this study, a numerical model was developed to simulate these interactions under different geological conditions. The results showed that the stress field of geological structures affects the flow path and velocity distribution of geothermal fluid, leading to distortion and uneven temperature distribution. The distribution of high-mineralized zones of geothermal water indicated the main recharge area in the northern part of the North China Basin (Henan). Hydrochemistry analysis revealed a dynamic balance between centrifugal and centripetal flows in sandstone geothermal water. The study provides important insights for the exploration and development of underground geothermal water, as well as the assessment and optimization of geothermal resources in large basins.

Technique and results of determination of vertical variations in rock thermal properties, temperature gradient and heat flow

The paper describes the results of experimental geothermal studies of the formation surrounding Savitskaya-300 well (depth 3555 m) drilled in the Volga-Ural oil and gas basin (Orenburg region, Russia). The work was aimed, firstly, at obtaining geothermal data for basin modeling and, secondly, at improving the instrumental and methodological foundations of experimental studies. Continuous core profiling of the rock thermal conductivity, volumetric heat capacity, and thermal anisotropy was performed on all 2886 recovered core samples with the total length of 329.1 m. Additional measurements were conducted at fluid saturation and elevated temperatures. For non-cored intervals, the rock thermal properties were determined from well-logging data and via measurements on rock cuttings. The studied formation is characterized by high thermal heterogeneity at both the level of strata (horizons) and individual samples. Significant thermal anisotropy up to 1.50–1.66 in the lower part of the well has been observed. To obtain data on vertical variations of equilibrium temperature and its gradient, temperature logging was performed 7 times within the 2 to 186 days of well shut-in. New data on the recovery of temperature and its gradient after drilling are important for the practice of applied and fundamental geothermal surveys. A significant increase in heat flow with depth was established based on the results of temperature gradient and rock thermal conductivity determination for 34 depth intervals. The observed general vertical variation in the whole depth range of the well up to ∼3.3 km depth can be attributed to paleoclimatic effect in the Late Pleistocene-Holocene, while the sharp increase in heat flow at ∼3.3 km within Fransian sediments is possibly due to advective heat transfer by fluid filtration. The apparent heat flow was established to be 84.3 mW⋅m−2 in the lower section of the well, below 3.3 km. The value may be close to the steady-state heat flow value, which is 84% higher than the previous published average data for the studied area.

Electromagnetic prediction of rock thermal properties beyond boreholes: Soultz-sous-Forets (France) case study

Estimation of deep temperature, thermo-hydro-dynamic and basin simulation, heat exchange and geothermal resource assessment require knowledge of the rock thermal properties beyond boreholes. At the absence of suitable methods and instruments for such estimates we propose a new approach to prediction of thermal conductivity, specific heat capacity and heat flow density based on results of electromagnetic sounding of the study area. Artificial neural network used to this end is taught by correspondence between laboratory measurements on cores from the drilled boreholes and adjacent electrical conductivity profiles determined by inversion of the electromagnetic sounding data collected in their vicinity. This approach is used to estimate rock thermal properties along NW-SE magnetotelluric profile crossing Soultz-sous-Forêts geothermal area, Alsace, France. We demonstrate that accuracy of such prediction both in depths below the boreholes and in the interwell space is comparable with that usually achievable in the borehole scale. In particular, it is shown that relative errors of thermal conductivity prediction at depths twice as large as the borehole depth are as small as 2%, and the accuracy of cross-well thermal conductivity prediction is 6% and 10% for dry and wet rocks, respectively. On the other hand, the prediction accuracy for matrix thermal conductivity of a rock from the known profile of dry thermal conductivity is about 8%. Based on the results of our studies, we constructed dry and wet thermal conductivity sections from the electrical conductivity model determined earlier by 2-D inversion of magnetotelluric data. The temperature model of the study area built along the same profile by means of electromagnetic geothermometer and the obtained thermal conductivity sections enabled estimating the profiles of the vertical component of heat flow density at the surface from dry and wet thermal conductivity sections. They could be considered as minimum and maximum constraints, respectively, bounding the area of its possible values, which, in turn, depend on in situ thermal conductivity of fluids filling the rock pores. The range of possible vertical heat flow density variations (100–150 ± 10% mW·m− 2) agrees well with heat flow estimates in the Upper Rhine graben and, in particular, in Soultz-sous-Forêts geothermal area roughly estimated earlier from borehole measurements. Using the artificial neural network trained on the correspondence between the specific heat capacity determined in the borehole and adjacent electrical conductivity profile, we built the model of specific heat capacity. Against the background, a zone of enhanced specific heat capacity ranging between 2.05 and 2.15 MJ·m− 3·K− 1 was observed in the domain located at the base of the sedimentary cover and upper part of the granitic basement.

Overview of the thermal properties of rocks and sediments in Slovenia

The use of geothermal energy, which comes from both deep geothermal systems and the shallow underground, has been developing rapidly in the last few decades. The purpose of the paper is to present the results of measurements of the thermal properties of all rock samples and sediments that were available from boreholes, two tunnels and numerous surface locations in Slovenia in the period from 1982 to the end of 2022. In relation to the shallow geothermal potential, a special effort is needed to characterize the thermal properties of the rocks and sediments and to implement thermal energy transfer technology. In this sense, knowledge of the thermal conductivity of rocks and sediments is required to assess the possibility of low-enthalpy heat exchange in a given local area. The largest number of measurements was taken to determine thermal conductivity. Determinations of thermal diffusivity were carried out on a much smaller number of rock and sediment samples, as well as determinations of radiogenic heat production in rocks. The results of thermal conductivity measurements on 430 samples from 119 wells, 20 samples from two tunnels and 156 samples from surface locations are shown. The highest thermal conductivities are shown by samples of dolomite, quartz conglomerate and conglomerate, phyllonite, quartz phyllite and gneiss, while the lowest are measured in sediments such as clay, lignite with clay, peat and dry sand. The determined radioactive heat generation is the lowest for milonitized dolomite and highest for dark grey sandstone with shale clasts. Our results are comparable to those already published worldwide, and they could be the basis for the possible future Slovenian standard for the thermal properties of measured rocks and sediments.

Numerical study of surrounding rock heat dissipation and wind temperature prediction in high geotemperature coal face

The heat damage in a coal face of a high-temperature mine is a significant mine disaster owing to various heat sources. Understanding the influence of various heat sources on the air temperature distribution in the coal face can help us to determine reasonable working conditions. Therefore, it is necessary to take into account the coupling relationship of multiple heat sources and obtain accurate prediction results. In this paper, the finite volume calculation method is used to establish a numerical model of heat dissipation of surrounding rock considering the difference in thermal and physical properties of coal and rock in the coal face. And then, a high-temperature coal mine in the northeast of China is taken as an example to explore the airflow temperature distribution law of the coal face under the influence of the absolute heat source and the relative heat source via the finite difference method. Furthermore, an orthogonal test was performed to investigate the influence degree of major factors on heat exchange of the coal face through range analysis. This work supplements the existing numerical model and research, and provides a theoretical basis and more accurate practical methods for predicting the thermal environment of coal face, especially in high mining depth mines.

Basic analysis of rock mechanical and thermal properties in South Korea

This study analyzed the mechanical and thermal properties of various rock types found in South Korea. The results showed that both igneous and metamorphic rocks possess higher strength compared to sedimentary rocks. The Young's modulus of rocks is dependent on the extent of weathering they have undergone. The average cohesion of granites was found to be relatively higher compared to other rock types, and their friction angle also exhibited a relatively high value with a considerable variance. The results of uniaxial compression strength testing with respect to depth revealed that rock strength generally increased with depth, however, there was a large variance in strength distribution in each depth interval. Based on these findings, it can be concluded that South Korea can also secure HLW disposal sites and facilities in terms of rock mechanics by using crystalline rocks, similar to countries such as Sweden and Finland where disposal facilities are being built.Regarding the thermal properties of rocks, they are influenced by the distribution of the parent rock. The thermal conductivity is highly concentrated in the southwest and central regions of South Korea, while the geothermal gradient is high in the northeast, west, and some parts of the southeast regions. The southeast region of Korea has a high geothermal heat flow, and some central northern regions also exhibit relatively high geothermal heat flow. In light of these distributional characteristics, it is crucial to continue conducting precise studies on the mechanical and thermal properties of rocks in the future disposal depths of spent nuclear fuel.

Effect of mineralogical variations on physico-mechanical and thermal properties of granitic rocks

The present study aims to explain the geochemical and mineralogical details of the granitic rock types in Gabal EL-Faliq area, South Eastern Desert of Egypt, in relation to geotechnical engineering and their suitability as dimension stones. The objective of the current research was achieved through two steps; the first step involved geological studies such as the petrographic, geochemical, and mineralogical investigations. The second and applicable step involved the geotechnical assessment of the studied rocks by measuring their engineering properties such as physical, mechanical, and thermal expansion properties. The petrographic investigation revealed that the studied granitic rocks are divided into two main classes: (1) gneissose granites (Biotite–Perthite) of medium to fine-grained size and (2) alkali-feldspar granites of coarse to medium-grained size. Mineralogically, the studied rocks are composed mainly of albite, orthoclase and quartz in varying proportions, along with some accessory minerals such as apatite and rutile in addition to some minor quantities of iron-group minerals such as hematite and ilmenite. The engineering properties showed that the maximum water absorption and apparent porosity values are 0.34% and 0.77%, respectively, while the minimum bulk density is 2604.03 kg/m3. The compressive strength ranges from 999.68 to 2469.10 kg/cm2, while the abrasion resistance varies from 29.67 to 54.64 Ha. The increase in albite content led to an increase in water absorption while a decrease in bulk density and compressive strength. The increase in the grain size led to an increase in apparent porosity and a decrease in mechanical properties. A Great variation in the expansion coefficient as well as the length change, occurs under changes in temperature, mineral composition, and physical properties. The increase in heating temperatures caused an insignificant increase in linear thermal expansion with a maximum value of 0.0385% at 100 °C. These results indicated the suitability of the studied granites as dimension stones for use in indoor and outdoor decorative purposes (cladding/paving) under variable temperature conditions.

Characteristics of geothermal field and evaluation of geothermal resource potential in the Yingjiang Basin

Geothermal resource, a green and sustainable energy resource, plays an important role in achieving ‘emission peak’ and ‘carbon neutrality’ targets. The Yingjiang Basin is located in the eastern branch of the Mediterranean-Himalayan high-temperature geothermal belt and exhibits considerable potential for geothermal resources. However, current investigations into the distribution of deep geothermal resources in this region are somewhat limited. In this paper, the transient plane source (TPS) method is used to measure the thermal conductivity parameters of 31 rock samples within the study area. Additionally, the one-dimensional steady-state heat conduction equation is employed to calculate the deep geothermal field, considering the constraints of rock thermal properties and terrestrial heat flow in the study area. Furthermore, the “stripping method” is used to determine the contribution rate of sedimentary layer to terrestrial heat flow, while the volume method is applied to estimate the geothermal resources at burial depths of 3000–5000 m. The results show that (1) The heat generation rate of granite is the highest with an average value of 4.52 μW/m3, followed by gneiss with an average value in the range of 2.0–3.5 W/(m·K), mudstone and sandstone being the lowest with an average value between 1.0 and 2.0 W/(m·K). (2) The main contributor of terrestrial heat flow in the study area is mantle heat flow, and the contribution of sedimentary layers to terrestrial heat flow only accounts for about 2%. (3) The geothermal resources in Yingjiang Basin within the depth range of 3000–5000 m is 93.6 × 1015 kJ, or 3.2 × 109 tonnes standard coal equivalent (SCE).

Thermal properties of rocks from deep boreholes in Poland in terms of obtaining geothermal energy from enhanced geothermal systems

In the case of unconventional geothermal systems, the thermal conditions are decisive, i.e. heat flux and temperature at a certain depth, and the physical properties of rocks, their susceptibility to fracturing, etc., which should be determined on a local scale. The work carried out was aimed at determining the basic thermal parameters of rocks, i.e.: effusivity, thermal conductivity and diffusivity, basing on tests of samples taken from boreholes, representing selected geothermal structures in Poland. Both high-temperature structures (100-135 °C) recognized up to a depth of about 3,800 m within the Fore-Sudetic Monocline and up to about 3,000 m in the Szczecin Synclinorium and the Leba Elevation, as well as medium-temperature structures (60-90 °C), occurring at depth 2,000- 2,500 m (Warszawa Synclinorium, Lublin Synclinorium), were analyzed. Some samples representing low temperature structures (with temperatures below 60 °C), such as Lublin synclinorium and Podlasie-Lublin elevation from depths from about 1,000 to 1,500 m, were also analyzed. The tests of thermal parameters of rocks coupled with simulations showed, that the formations with the highest mean diffusivity and thermal conductivity values are characterized by the largest thermal penetration depth and smallest temperature drop. The research allowed to conclude that among the examined rocks, the Cambrian sandstones of the Leba Elevation and the Zechstein dolomites of the Fore-Sudetic monocline are characterized by the most appropriate parameters from the point of view of obtaining geothermal energy from the enhanced geothermal systems in Poland. Cited as: Labus, K., Labus, M., Leśniak, G. Thermal properties of rocks from deep boreholes in Poland in terms of obtaining geothermal energy from enhanced geothermal systems. Advances in Geo-Energy Research, 2023, 8(2): 76-88. https://doi.org/10.46690/ager.2023.05.02

The risk of fluid-injection-induced fault reactivation in carbonate reservoirs: an investigation of a geothermal system in the Ruhr region (Germany)

In this study, we investigate the potential for fluid-injection-induced fault reactivation and induced seismicity risk during simultaneous injection-extraction operation in a theoretical geothermal doublet system in a carbonate reservoir in the Ruhr region. Using a coupled three-dimensional thermo-hydro-mechanical approach, we investigate the probability of injection-induced fault rupture. We perform a sensitivity study assuming variability of the fault and matrix permeability, injection/production flow rates, well placement options, rock thermal properties, and evaluate the influence of thermally induced stresses. The ruptured fault areas were calculated based on a Coulomb friction law and a notion that the shear slip is controlled by the ratio of shear to effective normal stresses acting on a pre-existing plane of weakness in the in situ stress field configuration. Ruptured fault areas in the intrinsically not critically-stressed environment, using location-specific empirical correlations, were used to compute local moment magnitudes of potential earthquakes. Based on this study, we conclude that, in the long-term, thermally-induced stresses play a dominant role during fault reactivation and greatly increase the likelihood for induced seismicity. We, therefore, propose that a minimum safe distance between an injection well and a fault should be based primarily on the radius of a thermal plume generated during the expected lifetime of a geothermal system. Results from this study provide valuable insights for the development of future deep geothermal systems in the Ruhr region and other geothermal reservoirs worldwide.

Petrological, petrophysical and petrothermal study of a folded sedimentary succession: the Oliana anticline (Southern Pyrenees), outcrop analogue of a geothermal reservoir

The Oliana anticline (Southern Pyrenees) has been characterized as an outcrop analogue of a geothermal reservoir using field data (stratigraphy and fracturing) and petrological, petrophysical and petrothermal analyses. Five lithofacies were established including conglomerates, hybrid arenites, lithic arenites, carbonates and evaporites. Petrophysical measurements indicate widely dispersed values of bulk density, connected porosity, permeability and velocity of compressional acoustic waves. Connected porosity is the factor that mostly influences bulk density, compressional wave velocity and permeability. In turn, diagenetic processes (such as dissolution and cementation) and fracturing, coupled with petrological features such as mineral composition, matrix content and grain size, are the most critical factors controlling rock porosity along the Oliana anticline. Thermal conductivity measures reveal a compositional control on the thermal properties of rocks. Thermal characterization of the structure reveals a low conductive area that matches the carbonate and evaporite succession of the anticline core and a highly conductive zone associated with the detrital succession of the fold limbs. The Oliana anticline has been classified as a petrothermal system due to the low permeability values of the studied sedimentary succession. Despite such classification, this contribution provides a useful exploration tool for future studies of non-conventional geothermal and CO2 storage sites located in folded sedimentary successions in the proximal domain of foreland basins.

Texture-Dependent Thermal Properties of Sandstone Rocks Examined by Scanning Electron Microscopy for Thermal Energy Storage Applications

Abstract Sedimentary rocks are widely used as geological reservoirs and as host rocks for geothermal energy systems. The thermal properties of sedimentary rocks, such as thermal conductivity, thermal diffusivity, and volumetric specific heat, play a critical role in their suitability for these applications. This study examined the thermal properties of 30 different sandstone rock samples using scanning electron microscopy (SEM) analysis. The SEM images of rock samples with different thermal properties were compared to analyze how textural properties influence thermal properties. Our results suggest that the thermal properties of sedimentary rocks are highly dependent on their texture. Specifically, we found that rocks with a higher degree of roughness, tend to exhibit lower thermal conductivity and thermal diffusivity. The presence of pores and cracks impacted the thermal properties of the sandstone rocks examined. The average surface roughness extracted from images showed a strong negative correlation with thermal conductivity and diffusivity (−0.59 and −0.6, respectively) obtained experimentally, while pore, cracks, and voids area have a less apparent negative correlation (−0.18 and −0.17) likely due to their complex effect on heat transfer. The size, shape, and distribution of voids affect heat transfer, with interconnected voids providing networks for heat flow, and smaller voids trapping heat more effectively. The texture of sedimentary rocks plays a critical role in determining their thermal properties. This knowledge can be used to optimize the understanding of the potential of sandstone reservoirs in applications, such as geothermal energy or thermal energy storage.