Geothermal Rocks Research Articles

Micro-scale investigation of the pore network of sandstone in the Pannonian Basin to improve geothermal energy development

This study focuses on the Pannonian Basin, specifically in Szentes, Hungary a region of significant geothermal potential, with particular emphasis on the Dunántúl Group; a collective name for the Zagyva and Újfalu formations, which consists of slightly consolidated delta-front sandstone sediments. This research is pivotal in understanding the challenges associated with clogging in geothermal wells, a problem that has led to the premature shutdown of injection wells in the region. Our approach integrates classical petrophysical and mineralogical methods with advanced techniques such as micro-Computed X-Ray Tomography imaging, 3D image analysis, and digital rock simulations. Our findings indicate that the target geothermal rock formations within the Dunántúl Group exhibit high porosity (27–31 %) and variable permeability (60–400 mD), dependent on the location and specific characteristics of the formation. Our micro-CT analyses further identified that the presence of fine-grained materials in smaller pores and generally weak cementation of grains substantially contributes to these challenges.

Numerical simulation of multi-field coupling in geothermal reservoir heat extraction of enhanced geothermal systems

The coupled analysis of multi-field heat and mass transfer in geothermal reservoirs is a pivotal concern within the realm of geothermal rock exploitation. It holds significant implications for the assessment of thermal energy capacity and the formulation of reservoir optimization strategies in the context of geothermal rock resources. Parameters governing production, along with fracture network characteristics (such as injection well temperature, injection well pressure, fracture width, and fracture network density), exert an influence on enhanced geothermal systems (EGS) heat production. In this study, aiming to comprehend the dynamic heat generation of EGS during prolonged exploitation, a coupling of various fields including permeation within the rock formations of geothermal reservoirs and the deformation of these rocks was achieved. In this study, we formulated the governing equations for the temperature field, stress field, and permeability field within the geothermal reservoir rock. Subsequently, we conducted numerical simulations to investigate the heat transfer process in an enhanced geothermal system. We analyzed the effects of injection well temperature, injection well pressure, primary fracture width, and secondary fracture density on the temperature distribution within the reservoir and the thermal power output of the production well. The research findings underscore that ill-conceived exploitation schemes markedly accelerate the thermal breakthrough rate of production wells, resulting in a diminished rate of geothermal resource extraction from the geothermal reservoir rock. Variations in influent well temperature and secondary fracture density exhibit an approximately linear impact on the output from production wells. Crucially, injection well pressure and primary fracture width emerge as pivotal factors influencing reservoir output response, with excessive widening of primary fractures leading to premature thermal breakthrough in production wells.

Capacity Allocation of Hybrid Power System with Hot Dry Rock Geothermal Energy, Thermal Storage, and PV Based on Game Approaches

This study utilizes hot dry rock (HDR) geothermal energy, which is not affected by climate, to address the capacity allocation of photovoltaic (PV) -storage hybrid power systems (HPSs) in frigid plateau regions. The study replaces the conventional electrochemical energy storage system with a stable HDR plant assisted by a flexible thermal storage (TS) plant. An HPS consisting of an HDR plant, a TS plant, and a PV plant is proposed. Game approaches are introduced to establish the game pattern model of the proposed HPS as the players. The annualized income of each player is used as the payoff function. Furthermore, non-cooperative game and cooperative game approaches for capacity allocation are proposed according to the interests of each player in the proposed HPS. Finally, the proposed model and approaches are validated by performing calculations for an HPS in the Gonghe Basin, Qinghai, China as a case study. The results show that in the proposed non-cooperative game approach, the players focus only on the individual payoff and neglect the overall system optimality. The proposed cooperative game approach for capacity allocation improves the flexibility of the HPS as well as the payoff of each game player. Thereby, the HPS can better satisfy the power fluctuation rate requirements of the grid and increase the equivalent firm capacity (EFC) of PV plants, which in turn indirectly guarantees the reliability of grid operation.

Heat mining from super-hot horizons of the Larderello geothermal field, Italy

Super-hot rock geothermal is an emerging source of renewable and carbon-free energy. This paper is the first attempt to explore fluid and heat flow dynamics in the reservoir-wellbore coupled system, to assess the power generation performance of a super-hot (>450 °C) enhanced geothermal system (EGS). We developed a high-performance code and built a 3-D wellbore-reservoir coupled model based on data from a recently completed deep-drilling project at Larderello, Italy. The general pattern of the super-hot EGS is characterized by a significant temperature plummet (>60 °C), after which the production fluid evolves from steam to a two-phase mixture till the end of the operation period. Reservoir pressure emerges as a key parameter to determine the temperature of the two-phase mixture. By realistically capturing phase transitions driven by coupled thermo-hydraulic processes during operations, our numerical model predicts a lower power generation efficiency compared to previous attempts based on ultra-simplified models. Although finalized at assessing the thermodynamic viability of a specific system, this modeling approach provides general information on fundamental thermo-hydraulic processes in the Earth crust that might be applied for the design of similar EGS projects elsewhere.

IDENTIFICATION OF ROCK CHARACTERISTICS USING XRF, XRD, AND SEM TESTS ON API ALAM IN PAMEKASAN

Research has been conducted into the nature of geothermal rocks in Pamekasan. This place is located in Tlanakan Pamekasan. This study aims to determine the composition of mineral compounds in the soil so that fire can burst out from the soil and the soil does not melt. Three steps can be done with simple refinement and filtering methods, namely X-Ray Fluorescence (XRF), X-ray diffraction (XRD), and Scanning Electron Microscope (SEM). The results of XRF analysis show the element of Api Alam 1 is Ca 66.2%, Api Alam 2 is Si 37.4%, and in hot springs, is Si 39.6%. The XRD results show that mineral qualitatively in Api Alam 1 was in the form of CaCO3 compound at 60.1% and SiO2 at 39.9%, while in Api Alam 2, hot water sources were in the form of SiO2 compound (>90%). SEM Results on Api Alam 1 show that the sample is almost homogeneous (the distribution of CaCO3 and SiO2 is uneven), the presence of slab-shaped clumps that identify the presence of varying grain size, and with high porosity, which indicates that the sample is partially amorphous in structure, in Api Alam 2. The Hot springs show irregular aggregations and poor homogeneity of the sample. The size of the three samples is ten μm or 10.000 nm, while PSA results showed an average size of Api Alam 1 was 509.7 nm, Api Alam 2 was 891.3 nm, and Hot Springs was 468.3 nm.

The Effects of PDC Cutter Geometries to the Drilling Dynamics in Various Geothermal Rocks: A Comprehensive Study Using Advanced Drilling Dynamics Simulation

Polycrystalline Diamond Compact (PDC) bit has been used widely in drilling operations since a long time ago. However, the utilization of this bit in Indonesia’s geothermal drilling has just recently started in 2016, thanks to the rapid development of PDC cutter grades and shapes in the last decade. Prior to that, the geothermal drilling operation relied on Tungsten Carbide Insert (TCI) bit to drill the hard formations. The crushing action of the insert bit is very durable to remove the rocks. However, the seal and bearing limit the life of these bit types, causing multiple bit runs in a long drilling section.PDC bit offers longer drilling periods if the cutters are still in good condition. However, the nature of geothermal formation usually consists of hard rocks with fractures that can reduce the durability of the conventional flat PDC cutters. To overcome this weakness and optimize drilling operation,. Instead of having a flat surface, new PDC cutters have different 3D shapes. Each has different benefits: improving cutter strength against impact, increasing aggressiveness and depth of cut, or enhancing wear resistance towards abrasive formations.Advance drilling dynamics simulation helps analyze which cutter types will be more suitable for different geothermal rocks characteristics. The simulation is based on 4D Finite Element Analysis, which models the interaction between the cutting structures and the drilled formation. The result shows the dynamic drilling response of each cutter on different rocks, including the generated vibration, the cutter’s depth of cut distribution on the bit face, and the potential wear pattern on the bit. This paper then discusses which cutter and PDC bit type will be more beneficial for geothermal drilling based on the bit aggressiveness and durability to optimize the drilling operation.

Evaluation of the Modification Potential for Carbonate Geothermal Rocks by considering Multiple Mechanics Properties and Acid Dissolution in the Jizhong Depression, China

Abstract Evaluation of geothermal reservoir modification potential is of great significance for improving the efficiency of geothermal exploitation and reducing costs. However, it is difficult to accurately evaluate the potential of geothermal reservoir modification given the limited number of parameters used in practical studies. Therefore, an evaluation method that considers the potential of geothermal development by applying multiple parameters is essential. In this study, five types of rock samples are collected in the field from the Fujunshan Formation, Gaoyuzhuang Formation, Majiagou Formation, and the second and fourth groups of the Wumishan Formation. The uniaxial compressive strength (UCS) and uniaxial tensile strength (UTS) of rock samples were obtained by uniaxial compression tests and Brazilian splitting tests, followed by the calculation of the brittleness index utilizing UCS and UTS. Then, the joint roughness coefficient and crack width were obtained by 3D scanning and postprocessing. In addition, the fracture extension degree of each lithology was obtained via acid dissolution experiments. Analytical hierarchy processes (AHP) associated with expert experience were adopted to evaluate the modification potential of rock samples under hydraulic stimulation and acid etching based on the above parameters. The results show that the carbonate rocks of the Majiagou Formation are prone to cracking, efficient for flow and heat exchange, sensitive to acid dissolution, and thus have the highest potential for modification. The proposed evaluation method and implemented case study can provide a practical reference and solid base for further applications.

Integration of gravity and magnetic inversion for geothermal system evaluation in Suli and Tulehu, Ambon, eastern Indonesia

The geothermal field of Suli and Tulehu is located in the eastern part of the Ambon Island, administratively situated in central Maluku regency, Maluku province, Indonesia. In the year 2017, the measurement of the Earth’s gravity and magnetic field has been done to evaluate the geothermal system in Suli and Tulehu. The application of gravity and magnetic methods was applied to determine the subsurface geological model describing geothermal sources, reservoirs, and faulting as a fluid conduit in the study area. Bouguer anomalies are positive in the range from 93 to 105 mGal, while magnetic anomalies are negative to positive in the range from − 656 to 310 nT, generally southwest to northeast. Geothermal manifestations in the form of hot springs associated with low residual Bouguer and magnetic anomalies positioned on the Banda-Hatuasa, Banda, and Huwe fault lines related to the alteration zone. Euler deconvolution analysis of the complete Bouguer anomaly data shows the maximum depth of fault detected at 1200 m. Gravity and magnetic inversion show high density and susceptibility contrast of rocks associated with Eriwakang and Simalopu pyroclastic rocks as geothermal sources. Low density and susceptibility contrast is estimated to be geothermal reservoirs due to interactions with geothermal source rocks below. A geological conceptual model based on the inversion modeling of gravity and magnetic data provides clear and comprehensive information on the geothermal systems in Suli and Tulehu, Ambon, the eastern part of Indonesia.

Analisa Produksi Panas Radiogenik, Densitas dan Kecepatan Seismik dari Singkapan Batu Granit Panas Bumi Nyelanding, Bangka Selatan

The purpose of this study was to determine the analysis of radiogenic heat production, density and seismic velocity of the outcrops of the South Bangka Nyelanding geothermal rock. The X-ray Fluorescence (XRF) method is applied to obtain heat-carrying radioactive elements in the form of Uranium, Thorium and Potassium and other oxides which are useful for studying seismic density and velocity. The main oxides used in this study were SiO2, TiO2, Al2O3, MgO, CaO, K2O and P2O5. The results showed that the density increased from the composition of the mineral felsic (acid) - mafic (base). Conclusion, as for the relationship with heat production, the SiO2 and P2O5 elements experienced a significant decrease compared to other oxides. As for seismic velocity, the results show that seismic velocity has a strong correlation with density.
 Keywords: Radiogenic Heat Production, Seismic Velocity, Density, Oxides

Cement–rock interaction in geothermal wells at high temperatures

The lifetime of a geothermal well is strongly dependent on the long-term performance of the cement used for its construction. The critical zones in well cements are the outermost layers, where the cement encounters either the steel casing or the rock formation. The current paper presents a study of the changes in the interfacial transition zone that forms between geothermal cements and volcanic rocks, after hydrothermal treatment at 290°C. Cement slurries were poured into cavities drilled into ignimbrite blocks and allowed to set at 90°C. The assemblages were autoclaved under typical geothermal conditions, both with and without a carbon dioxide overpressure at 290°C. The addition of silica to the cement dictates the phases that form in cement. When exposed to carbon dioxide, these calcium silicate hydrate phases carbonate, at distinct carbonation rates. The rate at which these phases form and/or carbonate affects the reaction extent of the cement with the rock and, eventually, the cement durability.

Investigation of 3D fractal dimension sensitivity on 3D fracture parameter

Box counting fractal dimension has been investigated as 3D Fracture parameters. Geothermal rock fractured and 3D fracture models are characterized its 3D fracture characteristics i.e. aperture, density and intensity. The investigation 3D Fractal dimension as aperture parameter, we use the rock samples and 3D Fracture models that have nearly the same intensity. We found that fractal dimension of 3D fracture has a strongly positive correlation with aperture both for 3D fracture in rock fractured samples and 3D fracture models. Based on this result we conclude that 3D fractal dimension of fracture can be used as parameter of fractures aperture and density. Meanwhile for fractal dimension of 3D fracture as intensity parameter, we use the rock samples and 3D Fracture models that have nearly the same aperture and density. We found that there’s no significant pattern of 3D fractal dimension on fracture intensity of rock samples. For further investigation we generate 3D fracture models. We found that the relation between 3D fractal dimension and 3D fracture intensity is very weak negative correlation. Based on this result, we are unable to conclude that the 3D fractal dimension of fracture can be used as parameter of fractures intensity.

Imaging Thermal Anomalies in Hot Dry Rock Geothermal Systems from Near-Surface Geophysical Modelling

Convective hydrothermal systems have been extensively studied using electrical and electromagnetic methods given the strong correlation between low conductivity anomalies associated with hydrothermal brines and high temperature areas. However, studies addressing the application of similar geophysical methods to hot dry rock geothermal systems are very limited in the literature. The Timanfaya volcanic area, located on Lanzarote Island (Canary Islands), comprises one of these hot dry rock systems, where ground temperatures ranging from 250 to 605 °C have been recorded in pyroclastic deposits at shallow (<70 m) depths. With the aim of characterizing the geophysical signature of the high ground temperature areas, three different geophysical techniques (ground penetrating radar, electromagnetic induction and magnetic prospecting) were applied in a well-known geothermal area located inside Timanfaya National Park. The area with the highest ground temperatures was correlated with the location that exhibited strong ground penetrating radar reflections, high resistivity values and low magnetic anomalies. Moreover, the high ground temperature imaging results depicted a shallow, bowl-shaped body that narrowed and deepened vertically to a depth greater than 45 m. The ground penetrating radar survey was repeated three years later and exhibited subtle variations of the signal reflection patterns, or signatures, suggesting a certain temporal variation of the ground temperature. By identifying similar areas with the same geophysical signature, up to four additional geothermal areas were revealed. We conclude that the combined use of ground penetrating radar, electromagnetic induction and magnetic methods constitutes a valuable tool to locate and study both the geometry at depth and seasonal variability of geothermal areas associated with hot dry rock systems.

Advanced three‐dimensional seismic imaging of deep supercritical geothermal rocks in Southern Tuscany

ABSTRACTThe seismic K‐Horizon is the key to gaining understanding on the deep supercritical geothermal rocks in Southern Tuscany. The K‐Horizon is hosted in metamorphic rocks, which cause strong seismic wavefield scattering resulting in a poor signal‐to‐noise ratio. Our study aims to reveal high‐resolution seismic images of the K‐Horizon below a geothermal field in Southern Tuscany, using an advanced three‐dimensional seismic depth imaging approach. The key seismic pre‐processing steps in the time domain include muting a large amount of persistent noise based on the statistical analysis of the seismic amplitudes, and tomostatics technique to correct for static effects. We carried out seismic depth imaging using Kirchhoff Pre‐Stack Depth Migration and Fresnel Volume Migration techniques. Each migration technique was tested with constant and heterogeneous three‐dimensional velocity models. Due to the difficulties in determining emergent angles for this low signal‐to‐noise ratio data set, the migration results with the heterogeneous three‐dimensional velocity model show less coherent reflections compared to the migration results using the constant velocity model. Both velocity models however lead to relatively the same structure and depth of the K‐Horizon, indicating the similarity of the average velocities along the wave propagation paths in both velocity models. With both velocity models Fresnel Volume Migration yields the K‐Horizon with better reflection coherency and higher signal‐to‐noise ratio than standard Kirchhoff Pre‐Stack Depth Migration. Nevertheless, both migration techniques have been able to reveal the K‐Horizon with relatively high resolution and provide a reliable basis for geothermal rock characterization as well as steering of the first geothermal well penetrating the K‐Horizon.

Effects of water saturation, fracture and salinity on electric and elastic properties of fractured geothermal rocks

Effects of water saturation, fracture and salinity on electric and elastic properties of fractured geothermal rocks

Rock mass strength and elastic modulus of the Buntsandstein: An important lithostratigraphic unit for geothermal exploitation in the Upper Rhine Graben

Geothermal exploitation in the Upper Rhine Graben increasingly targets the interface between the granitic basement and the overlying Buntsandstein unit. Results from deformation experiments are combined with structural assessments to provide reservoir-scale wet and dry strength and elastic modulus profiles for the Buntsandstein at Soultz-sous-Forêts (France). Our analysis finds five zones characterised by low strength and elastic modulus. The strength and elastic modulus of “massive” zones are lower when the rock is wet (i.e. water-saturated), highlighting the importance of performing wet deformation experiments for geothermal rock mass assessments. These data and methods can be used to provide assessments of other geothermal sites within the region to assist prospection, stimulation, and optimisation strategies.

Drilling Technical Difficulties and Solutions in Development of Hot Dry Rock Geothermal Energy

The exploration and development of hot dry rock resources, first of all, needs to address the drilling issues in deep, hot, hard and unstable formations. By studying geological features and storage conditions of hot dry rocks, the key technical difficulties of hot dry rock drilling are presented. The high-temperature resistance performance index of oil and gas drilling technologies at home and abroad are investigated. The applicability of high effective rock breaking tools, MWD instruments, drilling fluid systems, well cementing and completion technologies are analyzed, and feasibility analyses have been conducted on gas drilling, dry wellbore cementing and foam pressurized drilling techniques. On the basis of the above analyses, the developing directions and issues urgently to be addressed about domestic hot dry rock drilling technology are discussed so as to provide references for drilling program optimization and drilling technology research in the development of hot dry rock geothermal energy.

Preliminary Study of 2D Fracture Upscaling of Geothermal Rock Using IFS Fractal Model

Fractured rock plays important role in reservoir production. In larger scale, fractures are more likely to be heterogeneous and considered to be fractal in its nature. One of the characteristics of fractal structure is the scale independence. An investigation of fractal properties on natural fractured rock is therefore needed for modelling larger fracture. We have investigated the possibilities of fractal upscaling method to produce a larger geothermal fracture model based on smaller fracture data. We generate Iterated Function System (IFS) fractal model using parameters e.g. scale factor, angle between branch, initial line direction, and branch thickness. All the model parameters are obtained from smaller fracture data. We generate higher iteration model to be compared with larger geothermal fracture. The similarity between the IFS fractal model and natural fracture is measured by 2D box counting fractal dimension (D). The fractal dimension of first to fourth generation fractal model is (1.86 ± 0.02). The fractal dimension of the reference geothermal site is (1.86 ± 0.04). Besides of D, we found significant similarity of fracture parameters there are intensity and density between fracture model and natural fracture. Based on these result, we conclude that fractal upscaling using IFS fractal model is potential to model larger scale of 2D fracture.

The effect of drilling fluid and temperature on the cement/rock interaction in wells

The reduced lifetime of geothermal wells is one of the main issues facing the geothermal industry, with cement durability being one of the main causes for shortening of a well's life. While most wells are cemented with American Petroleum Institute (API) class cements, alternative cementing systems are currently being considered. In this paper, the interactions of two alternative cement formulations with a typical geothermal rock formation are compared with a class A cement. This allows understanding of the most critical zone in the geothermal cement, which is the outermost layer where the cement encounters and, consequently, interacts with the exterior harsh environment, including the volcanic formation and brine. The faster setting of white cement and high-alumina cement compared with the well Portland cement seems to reduce the reaction extent with the rock. For all the cements tested, both the increased curing temperature (from 150°C to 290°C) and the use of bentonite as drilling mud significantly decreased the bond between the cement and the rock.

Energy use – in terms of efficiency

In the recent decades it has become apparent that the human race can lead to a polluting and energy- wasting lifestyle and the depletion of natural resources and an ecological disaster as well.
 Energy efficiency is the realization of the chance to see a wider use of renewable energy. Renewable energy sources can be found in large quantities in Europe. A proper exploitation of these would be important because of the "traditional" energy sources’s sate is very critical in many ways.
 The utilization of renewable energy sources depends on many factors. The local natural conditions significantly determine each country's different renewable energy potentials. I find to be important the natural conditions, such as, solar radiation intensity , the number of sunny days per year, the wind conditions , the volume and their energy characteristics of the geothermal power resources , land features , soil and rock quality, the supply of fossil fuels or the nuclear possibilities of energy production. The economic environment is also a major influencing factor for the utilization of renewables. The conditions of price of fossil fuels (natural gas, oil and coal), the price of nuclear fuel production and other energy costs significantly influence the demand for renewables, as well as the level of subsidy and government tax policy. In addition, the international and national programs, objectives, strategies, subsidies and regulatory measures as well as technological factors can have a significant impact.
 In my paper I point out the opportunities of renewable energy should be given to live. Nowadays the positive effects of their use is undisputed. In addition to the environmentally friendly produced energy, we should strive for energy conservation and energy efficiency as well. These expectations appear in practice, which can be directly perceived by citizens, in fact we should live accordingly. Furthermore, the environmentally relevant regulations of living conditions should appear as environmental demands.

Temperature dependence of seismic properties in geothermal rocks at reservoir conditions

Petrophysical experiments on two Icelandic geothermal rock samples at simulated in situ reservoir conditions are analysed to delineate the effect of temperature on seismic velocity and attenuation. A goal of the present work is to predict the effect of the saturating pore fluid on seismic velocity using the Gassman equation, which has been modified for this purpose. To include the temperature effect in the equation, two assumptions are made: (1) the grain/mineral and dry bulk moduli are independent of temperature; and (2) the temperature dependence follows solely from the thermophysical characteristics of the saturating fluid through the fluid bulk modulus and fluid density. Laboratory measurements show that P-wave velocities decrease with increasing temperature. This change is related to the thermophysical characteristics of the saturating fluid; at higher temperatures bubbles and thermal microfractures are formed affecting seismic velocities. The measurements also show that at low temperatures seismic attenuation decreases with temperature due to the rapid decrease in the fluid viscosity. On the other hand, at higher temperatures the attenuation increases because of the generation of bubbles and thermal microfractures. Although having data from only two samples and that no measurements on dry samples were done, thus limiting the generality of the claims that can be made, the study presents a plausible approach to relate changes in seismic properties to geothermal system temperatures.