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Reliable delivery of economic well performance of geothermal projects is affected to a high degree by uncertain reservoir quality. Construction of multilateral wells is well known as an effective concept to overcome the challenges of reservoir heterogeneity or low permeability by increasing the reservoir contact. However, the drilling costs for these structures are currently very high and multilateral well construction with standard rotary steerable systems is complex. Canopus’ directional steel shot drilling system (DSSD) has the potential to enable the construction of short-radius multilaterals at rates attractive for geothermal applications.As part of the European GEOTHERMICA project ‘DEPLOI the HEAT’, the operational performance and economic impact of Canopus’ DSSD system will be investigated. Full-scale tests at TNO’s Rijswijk Centre for Sustainable Geo-Energy (RCSG) and a field trial at the Hagerbach underground test facility (VSH) are planned for the first project year. The RCSG drilling rig enables full factory acceptance testing (FAT) of the drilling assembly before the trial at the VSH site is executed. The geology at the VSH site reflects conditions of typical mid-depth fractured limestone reservoirs in Switzerland and elsewhere. At VSH, a complete set of operational parameters and system longevity will be tested with a full-scale trial to prepare for live well deployment.Further, a techno-economic assessment of multilateral structures drilled with Canopus’ DSSD system will highlight the potential for increasing or safe-guarding well productivity and economically de-risking geothermal projects. Several operators are involved in this project and the techno-economic assessment of the drilling technology will be based on several site-specific data sets provided by them. Stochastic modeling approaches will be implemented to generate an ensemble of equally probable realizations of permeable structures in the subsurface. Then, the performance and costs of different well geometries and multilateral configurations in different subsurface model realizations will be evaluated and compared.The current status of this project will be presented with a focus on the factory acceptance testing at TNO, the VSH trial preparations and the workflow developed for the techno-economic assessment of this innovative multilateral drilling technology.

Advanced Geothermal Systems (AGS) generate electric power through a closed-loop circuit, after a working fluid extracts thermal energy from rocks at great depths via conductive heat transfer from the geologic formation to the working fluid through an impermeable wellbore wall. The slow conductive heat transfer rate present in AGS, compared to heat advection, makes AGS uneconomical to this date. To investigate what would be required to render AGS economical, we numerically model an example AGS using the genGEO simulator to obtain its electric power generation and its specific capital cost. Our numerical results show that using CO2 as the working fluid benefits AGS performance. Additionally, we find that there exists a working fluid mass flowrate, a lateral well length, and a wellbore diameter which minimize AGS costs. However, our results also show that AGS remain uneconomical with current, standard drilling technologies. Therefore, significant advancements in drilling technologies, that have the potential to reduce drilling costs by over 50%, are required to enable cost-competitive AGS implementations. Despite these challenges, the economic viability and societal acceptance potential of AGS are significantly raised when considering that negative externalities and their costs, so common for most other power plants, are practically non-existent with AGS., Renewable Energy, 186, ISSN:0960-1481, ISSN:1879-0682

It is well-known that the presence of geometry heterogeneity in porous media enhances solute mass mixing due to fluid velocity heterogeneity. However, laboratory measurements are still sparse on characterization of the role of high-permeability inclusions on solute transport, in particularly concerning fractured porous media. In this study, the transport of solutes is quantified after a pulse-like injection of soluble fluorescent dye into a 3D-printed fractured porous medium with distinct high-permeability (H-k) inclusions. The solute concentration and the pore-scale fluid velocity are determined using laser-induced fluorescence and particle image velocimetry techniques. The migration of solute is delineated with its breakthrough curve (BC), temporal and spatial moments, and mixing metrics (including the scalar dissipation rate, the volumetric dilution index, and the flux-related dilution index) in different regions of the medium. With the same H-k inclusions, compared to a H-k matrix, the low-permeability (L-k) matrix displays a higher peak in its BC, less solute mass retention, a higher peak solute velocity, a smaller peak dispersion coefficient, a lower mixing rate, and a smaller pore volume being occupied by the solute. The flux-related dilution index clearly captures the striated solute plume tails following the streamlines along dead-end fractures and along the interface between the H-k and L-k matrices. We propose a normalization of the scalar dissipation rate and the volumetric dilution index with respect to the maximum regional total solute mass, which offers a generalized examination of solute mixing for an open region with a varying total solute mass. Our study presents insights into the interplay between the geometric features of the fractured porous medium and the solute transport behaviors at the pore scale., Transport in Porous Media, 1 (283), ISSN:0169-3913, ISSN:1573-1634, 146

For many CO2-emitting industrial sectors, such as the cement and chemical industry, Carbon, Capture and Storage (CCS) will be necessary to reach any set climate target. CCS on its own is a very cost-intensive technology. Instead of considering CO2 as a waste to be disposed of, we propose to consider CO2 as a resource. The utilisation of CO2 in so-called CO2 Plume Geothermal (CPG) systems generates revenue by extracting geothermal energy, while permanently storing CO2 in the geological subsurface.To the best of our knowledge, this pioneer investigation is the first CCUS simulation feasibility study in Switzerland. Among others, we investigated the concept of injecting and circulating CO2 for geothermal power generation purposes from potential CO2 storage formations (saline reservoirs) in the Western part of the Swiss Molasse Basin ("Muschelkalk" and "Buntsandstein" formation).Old 2D-seismic data indicates a potential anticline structure in proximity of the Eclépens heat anomaly. Essentially, this conceptual study helps assessing it's potential CO2 storage capacity range and will be beneficial for future economical assessments. The interpretation of the intersected 2D seismic profiles reveals an apparent anticline structure that was integrated on a geological model with a footprint of 4.35 × 4.05 km2.For studying the dynamic reservoir behaviour during the CO2 circulation, we considered: (1) the petrophysical rock properties uncertainty range, (2) the injection and physics of a two-phase (CO2 and brine) fluid system, including the relative permeability characterisation, fluid model composition, the residual and solubility CO2 trapping, and (3) the thermophysical properties of resident-formation brine and the injected CO2 gas.Our study represents a first-order estimation of the expected CO2 storage capacity range at a possible anticline structure in two potential Triassic reservoir formations in the Western part of the Swiss Molasse Basin. Additionally, we assessed the effect of different well locations on CO2 injection operations.Our currently still-ongoing study will investigate production rates and resulting well flow regimes in a conceptual CO2 production well for geothermal energy production in the future. Nonetheless, our preliminary results indicate that, under ideal conditions, both reservoirs combined can store more than 8 Mt of CO2 over multiple decades of CCUS operation. From our results, we can clearly identify limiting factors on the overall storage capacity, such as for example the reservoir fluid pressure distribution and well operation constraints.

This paper presents the results of 124 pure Mode I fracture toughness tests on two types of anisotropic rocks, the metamorphic Grimsel Granite and the sedimentary Mont Terri Opalinus Clay. The results show that Mode I cracks in anisotropic rocks tend to kink towards the foliation or bedding planes that are weaker in strength. The experiment data for kink angle and apparent fracture toughness are compared to the predictions of three well-known crack growth criteria: the maximum tangential stress (MTS), the maximum energy release rate (MERR), and the minimum (maximum in anisotropic materials) strain energy density (MSED). All these criteria overestimate the kink angle, while underestimating the apparent fracture toughness. The MTS yields predictions that are closest to the experiment data, while the MSED predictions are the furthest. We also show that the inclusion of the T-stress into the MTS criterion improves the predictions of the fracture growth. This indicates that the T-stress can play a significant role in the Mode I fracture growth of anisotropic rocks.

During reactive transport modeling, the computing cost associated with chemical equilibrium calculations can be 10 to 10,000 times higher than that of fluid flow, heat transfer, and species transport computations. These calculations are performed at least once per mesh cell and once per time step, amounting to billions of them throughout the simulation employing high-resolution meshes. To radically reduce the computing cost of chemical equilibrium calculations (each requiring an iterative solution of a system of nonlinear equations), we consider an on-demand machine learning algorithm that enables quick and accurate prediction of new chemical equilibrium states using the results of previously solved chemical equilibrium problems within the same reactive transport simulation. The training operations occur on-demand, rather than before the start of the simulation when it is not clear how many training points are needed to accurately and reliably predict all possible chemical conditions that may occur during the simulation. Each on-demand training operation consists of fully solving the equilibrium problem and storing some key information about the just computed chemical equilibrium state (which is used subsequently to rapidly predict similar states whenever possible). We study the performance of the on-demand learning algorithm, which is mass conservative by construction, by applying it to a reactive transport modeling example and achieve a speed-up of one or two orders of magnitude (depending on the activity model used). The implementation and numerical tests are carried out in Reaktoro (reaktoro.org), a unified open-source framework for modeling chemically reactive systems.

Thermal spallation drilling is a contact-less means of borehole excavation that works by exposing a rock surface to a high-temperature jet flame. In this study, we investigate crucial factors for the success of such thermal drilling operations using numerical simulations of the thermomechanical processes leading to rock failure at the borehole surface. To that end, we integrate a model developed for spalling failure with our thermomechanical simulations. In particular, we consider the role of material heterogeneities, maximum jet-flame temperature and maximum jet-flame temperature rise time on the onset of inelastic deformation and subsequent damage. We further investigate differences in energy consumption for the studied system configurations. The simulations highlight the importance of material composition, as thermal spallation is favored in fine-grained material with strong material heterogeneity. The model is used to test the relationship between the jet-flame temperature and the onset of thermal spallation.

Mineral precipitation can drastically alter a reservoir’s ability to transmit mass and energy during various engineering/natural subsurface processes, such as geothermal energy extraction and geological carbon dioxide sequestration. However, it is still challenging to explain the relationships among permeability, porosity, and precipitation patterns in reservoirs, particularly in fracture-dominated reservoirs. Here, we investigate the pore-scale behavior of single-species mineral precipitation reactions in a fractured porous medium, using a phase field lattice-Boltzmann method. Parallel to the main flow direction, the medium is divided into two halves, one with a low-permeability matrix and one with a high-permeability matrix. Each matrix contains one flow-through and one dead-end fracture. A wide range of species diffusivity and reaction rates is explored to cover regimes from advection- to diffusion-dominated, and from transport- to reaction-limited. By employing the ratio of the Damkohler (Da) and the Peclet (Pe) number, four distinct precipitation patterns can be identified, namely (1) no precipitation (Da/Pe 100), (3) fracture isolation (1 1), and (4) diffusive precipitation (1 < Da/Pe

Geothermal electricity generation may play a role in reducing greenhouse gas emissions and addressing climate change in a cost-effective manner. Reservoir equations for pressure and temperature must be coupled to a power cycle model to calculate electricity generation from a geothermal power plant. This work focuses on sedimentary basin geothermal power production, which relies on flow through porous and permeable aquifers in sedimentary basins. Previous work has used numerical reservoir simulators, but we introduce analytical reservoir solutions for reservoir impedance, wellbore heat loss, and reservoir heat depletion in this work. The reservoir impedance and wellbore heat loss solutions are combined with a power cycle model to calculate electricity generation. The reservoir heat depletion solution provides insight into the reservoir lifetime because electricity generation decreases with reservoir temperature. We compare the analytical and numerical approaches and discuss their implications for geothermal electricity generation from sedimentary basins. Both approaches have merits, and the comparison herein can guide those who want to understand geothermal electricity production.

Journal of hydrology 610, 127793 (2022). doi:10.1016/j.jhydrol.2022.127793, Published by Elsevier, Amsterdam [u.a.]

CO2-Plume Geothermal (CPG) power plants can use geologically stored CO2 to generate electricity. In this study, a Flexible CO2 Plume Geothermal (CPG-F) facility is introduced, which can use geologically stored CO2 to provide dispatchable power, energy storage, or both dispatchable power and energy storage simultaneously—providing baseload power with dispatchable storage for demand response. It is found that a CPG-F facility can deliver more power than a CPG power plant, but with less daily energy production. For example, the CPG-F facility produces 7.2 MWe for 8 h (8 h-16 h duty cycle), which is 190% greater than power supplied from a CPG power plant, but the daily energy decreased by 61% from 60 MWe-h to 23 MWe-h. A CPG-F facility, designed for varying durations of energy storage, has a 70% higher capital cost than a CPG power plant, but costs 4% to 27% more than most CPG-F facilities, designed for a specific duration, while producing 90% to 310% more power than a CPG power plant. A CPG-F facility, designed to switch from providing 100% dispatchable power to 100% energy storage, only costs 3% more than a CPG-F facility, designed only for energy storage., Energy Conversion and Management, 253, ISSN:0196-8904, ISSN:1879-2227

Geothermal energy can be a limitless and CO2-free energy resource. However, moderate geothermal temperature gradients of ∼30 oC/km in most regions typically require employing so-called "Advanced" or "Enhanced" geothermal systems, called AGS and EGS, respectively, which require reservoirs with temperatures >150 oC. To access such high temperatures, we need to drill deeper than 5 km, i.e., in hard rock. The costs of drilling to such depths, using traditional rotary drilling, increase exponentially with depth and can be up to 80% of the total geothermal project investment. These high drilling costs can be reduced significantly with contactless drilling technologies (e.g., thermal spallation drilling, laser drilling, microwave drilling, and Plasma Pulse Geo-Drilling), as they avoid the lengthy tripping times associated with drill-bit damage. PPGD uses high-voltage pulses of a few microseconds duration to fracture the rock, thereby drilling without any mechanical abrasion. Future PPGD costs may be as low as 10% of mechanical rotary drilling costs (Schiegg et al., 2015). Our PPGD research addresses two outstanding questions: (1) Understand the fundamental physics of the electric breakdown inside the rock and associated rock fracturing processes, which we investigate numerically (Ezzat et al., 2022, 2021; Vogler et al., 2020; Walsh and Vogler, 2020). (2) Evaluate the PPGD performance under deep-wellbore conditions of ~5 km (i.e., high pore and lithostatic pressures, and high temperatures). Our ongoing numerical and experimental studies are expected to provide further insights into the applicability of PPGD for geothermal energy utilization. First, we numerically model the formation of a plasma in rock pores, which constitutes the onset of rock failure during the PPGD process. These numerical models show the significant effect of the pore characteristics on the PPGD process and give insight into how future PPGD operations should be designed. Second, we conduct PPGD physical experiments, where we employ lithostatic pressures of up to 1500 bar, pore pressures of up to 500 bar, temperatures of up to 80 oC, and voltages of up to 300 kV. Concluding these experiments with the associated challenges shall demonstrate whether PPGD is efficient at great depths of up to 5 km. Combining our numerical and experimental results allows optimizing future PPGD operations., EGUsphere

Transport in Porous Media, ISSN:0169-3913, ISSN:1573-1634

We evaluate the accuracy of three well-known fracture growth theories to predict crack trajectories in anisotropic rocks through comparison with new experimental data. The results of 99 fracture toughness tests on the metamorphic Grimsel Granite under four different ratios of mixed-mode I/II loadings are reported. For each ratio, the influence of the anisotropy orientation on the direction of fracture growth is also analyzed. Our results show that for certain loading configurations, the anisotropy offsets the loading influence in determining the direction of crack growth, whereas in other configurations, these factors reinforce each other. To evaluate the accuracy of the fracture growth theories, we compare the experiment results for the kink angle and the effective fracture toughness with the predictions of the maximum tangential stress (MTS), the maximum energy release rate (MERR), and the maximum strain energy density (MSED) criteria. The criteria are first assessed in their classical forms employed in the literature. It is demonstrated that the energy-based criteria in their classical formulation cannot yield good predictions. We then present modified forms of the ERR and SED functions in which the tensile and shear components are decomposed. These modified forms give significantly better predictions of fracture growth paths. The evaluation of the three criteria illustrates that the modified MSED criterion is the least accurate model even in the modified form, while the results predicted by MTS and modified MERR are well matched with the experimental results., International Journal of Solids and Structures, 241, ISSN:0020-7683, ISSN:1879-2146

Rough rock fractures have complex geometries which result in highly heterogeneous aperture fields. To accurately estimate the permeability of such fractures, heterogeneity of the aperture fields must be quantified. In this study heterogeneity of single rough rock fractures is for the first time parametrized by connectivity metrics, which quantify how connected the bounds of a heterogeneous field are. We use 3000 individual realizations of synthetic aperture fields with different statistical parameters and compute three connectivity metrics based on percolation theory for each realization. The sensitivity of the connectivity metrics with respect to the determining parameter, i.e the cutoff threshold, is studied and the correlation between permeability of the fractures and the computed connectivity metrics is presented. The results show that the Θ connectivity metric predicts the permeability with higher accuracy. All three studied connectivity metrics provide better permeability estimations when a larger aperture value is chosen as the cutoff threshold. Overall, this study elucidates that using connectivity metrics provides a less expensive alternative to fluid flow simulations when an estimation of fracture permeability is desired., Advances in Water Resources, 161, ISSN:0309-1708, ISSN:1872-9657

CO2-Plume Geothermal (CPG) power plants can produce heat and/or electric power. One of the most important parameters for the design of a CPG system is the CO2 mass flowrate. Firstly, the flowrate determines the power generated. Secondly, the flowrate has a significant effect on the fluid pressure drawdown in the geologic reservoir at the production well inlet. This pressure drawdown is important because it can lead to water flow in the reservoir towards and into the borehole. Thirdly, the CO2 flowrate directly affects the two-phase (CO2 and water) flow regime within the production well. An annular flow regime, dominated by the flow of the CO2 phase in the well, is favorable to increase CPG efficiency. Thus, flowrate optimizations of CPG systems need to honor all of the above processes. We investigate the effects of various operational parameters (maximum flowrate, admissible reservoir-pressure drawdown, borehole diameter) and reservoir parameters (permeability anisotropy and relative permeability curves) on the CO2 and water flow regime in the production well and on the power generation of a CPG system. We use a numerical modeling approach that couples the reservoir processes with the well and power plant systems. Our results show that water accumulation in the CPG vertical production well can occur. However, with proper CPG system design, it is possible to prevent such water accumulation in the production well and to maximize CPG electric power output. ISSN:0375-6505

Drilling costs can be 80% of geothermal project investment, so decreasing these deep drilling costs substantially reduces overall project costs, contributing to less expensive geothermal electricity or heat generation. Plasma Pulse Geo Drilling (PPGD) is a contactless drilling technique that uses high-voltage pulses to fracture the rock without mechanical abrasion, which may reduce drilling costs by up to 90% of conventional mechanical rotary drilling costs. However, further development of PPGD requires a better understanding of the underlying fundamental physics, specifically the dielectric breakdown of rocks with pore fluids subjected to high-voltage pulses. This paper presents a numerical model to investigate the effects of the pore characteristics (i.e., pore fluid, shape, size, and pressure) on the occurrence of the local electric breakdown (i.e., plasma formation in the pore fluid) inside the granite pores and thus on PPGD efficiency. Investigated are: (i) two pore fluids, consisting of air (gas) or liquid water; (ii) three pore shapes, i.e., ellipses, circles, and squares; (iii) pore sizes ranging from 10 to 150 µm; (iv) pore pressures ranging from 0.1 to 2.5 MPa. The study shows how the investigated pore characteristics affect the local electric breakdown and, consequently, the PPGD process., Energies, 15 (1), ISSN:1996-1073

This paper introduces a new methodology to measure the elastic constants of transversely isotropic rocks from a single uniaxial compression test. We first give the mathematical proof that a uniaxial compression test provides only four independent strain equations. As a result, the exact determination of all five independent elastic constants from only one test is not possible. An approximate determination of the Young’s moduli and the Poisson’s ratios is however practical and efficient when adding the Saint-Venant relation as the fifth equation. Explicit formulae are then developed to calculate both secant and tangent definitions of the five elastic constants from a minimum of four strain measurements. The results of this new methodology applied on three granitic samples demonstrate a significant stress-induced nonlinear behavior, where the tangent moduli increase by a factor of three to four when the rock is loaded up to 20 MPa. The static elastic constants obtained from the uniaxial compression test are also found to be significantly smaller than the dynamic ones obtained from the ultrasonic measurements., Journal of Rock Mechanics and Geotechnical Engineering, 11 (6), ISSN:1674-7755, ISSN:2589-0417

In order to assess the thermo-hydraulic modelling capabilities of various geothermal simulators, a comparative test suite was created, consisting of a set of cases designed with conditions relevant to the low-enthalpy range of geothermal operations within the European HEATSTORE research project. In an effort to increase confidence in the usage of each simulator, the suite was used as a benchmark by a set of 10 simulators of diverse origin, formulation, and licensing characteristics: COMSOL, MARTHE, ComPASS, Nexus-CSMP++, MOOSE, SEAWATv4, CODE_BRIGHT, Tough3, PFLOTRAN, and Eclipse 100. The synthetic test cases (TCs) consist of a transient pressure test verification (TC1), a well-test comparison (TC2), a thermal transport experiment validation (TC3), and a convection onset comparison (TC4), chosen to represent well-defined subsets of the coupled physical processes acting in subsurface geothermal operations. The results from the four test cases were compared among the participants, to known analytical solutions, and to experimental measurements where applicable, to establish them as reference expectations for future studies. A basic description, problem specification, and corresponding results are presented and discussed. Most participating simulators were able to perform most tests reliably at a level of accuracy that is considered sufficient for application to modelling tasks in real geothermal projects. Significant relative deviations from the reference solutions occurred where strong, sudden (e.g. initial) gradients affected the accuracy of the numerical discretization, but also due to sub-optimal model setup caused by simulator limitations (e.g. providing an equation of state for water properties)., Geothermics, 96, ISSN:0375-6505

The increased interest in subsurface development (e.g., unconventional hydrocarbon, deep geothermal, waste disposal) and the associated (triggered or induced) seismicity calls for a better understanding of the hydro-seismo-mechanical coupling in fractured rock masses. Being able to bridge the knowledge gap between laboratory and reservoir scales, controllable meso-scale in situ experiments are deemed indispensable. In an effort to access and instrument rock masses of hectometer size, the Bedretto Underground Laboratory for Geosciences and Geoenergies (‘Bedretto Lab’) was established in 2018 in the existing Bedretto Tunnel (Ticino, Switzerland), with an average overburden of 1000 m. In this paper, we introduce the Bedretto Lab, its general setting and current status. Combined geological, geomechanical and geophysical methods were employed in a hectometer-scale rock mass explored by several boreholes to characterize the in situ conditions and internal structures of the rock volume. The rock volume features three distinct units, with the middle fault zone sandwiched by two relatively intact units. The middle fault zone unit appears to be a representative feature of the site, as similar structures repeat every several hundreds of meters along the tunnel. The lithological variations across the characterization boreholes manifest the complexity and heterogeneity of the rock volume, and are accompanied by compartmentalized hydrostructures and significant stress rotations. With this complexity, the characterized rock volume is considered characteristic of the heterogeneity that is typically encountered in subsurface exploration and development. The Bedretto Lab can adequately serve as a test-bed that allows for in-depth study of the hydro-seismo-mechanical response of fractured crystalline rock masses., Solid Earth Discussions, ISSN:1869-9537

Plasma Pulse Geo Drilling (PPGD) is a contact-less drilling technique, where an electric discharge across a rock sample causes the rock to fracture. Experimental results have shown PPGD drilling operations are successful if certain electrode spacings, pulse voltages, and pulse rise times are given. However, the underlying physics of the electric breakdown within the rock, which cause damage in the process, are still poorly understood. This study presents a novel methodology to numerically study plasma generation for electric pulses between 200 and 500 kV in rock pores with a width between 10 and 100 μm. We further investigate whether the pressure increase, induced by the plasma generation, is sufficient to cause rock fracturing, which is indicative of the onset of drilling success. We find that rock fracturing occurs in simulations with a 100 μm pore size and an imposed pulse voltage of approximately 400 kV. Furthermore, pulses with voltages lower than 400 kV induce damage near the electrodes, which expands from pulse to pulse, and eventually, rock fracturing occurs. Additionally, we find that the likelihood for fracturing increases with increasing pore voltage drop, which increases with pore size, electric pulse voltage, and rock effective relative permittivity while being inversely proportional to the rock porosity and pulse rise time.

CO2 Plume Geothermal (CPG) energy systems circulate geologically stored CO2 to extract geothermal heat from naturally permeable sedimentary basins. CPG systems can generate more electricity than brine systems in geologic reservoirs with moderate temperature and permeability. Here, we numerically simulate the temperature depletion of a sedimentary basin and find the corresponding CPG electricity generation variation over time. We find that for a given reservoir depth, temperature, thickness, permeability, and well configuration, an optimal well spacing provides the largest average electric generation over the reservoir lifetime. If wells are spaced closer than optimal, higher peak electricity is generated, but the reservoir heat depletes more quickly. If wells are spaced greater than optimal, reservoirs maintain heat longer but have higher resistance to flow and thus lower peak electricity is generated. Additionally, spacing the wells 10% greater than optimal affects electricity generation less than spacing wells 10% closer than optimal. Our simulations also show that for a 300 m thick reservoir, a 707 m well spacing provides consistent electricity over 50 years, whereas a 300 m well spacing yields large heat and electricity reductions over time. Finally, increasing injection or production well pipe diameters does not necessarily increase average electric generation., Renewable Energy, 172, ISSN:0960-1481, ISSN:1879-0682

The conventional semi-circular bend (SCB) test of anisotropic rocks, with symmetric loading, generates a Mixed-Mode I / II crack tip loading when the crack is not aligned with one of the principal material directions. This paper presents a modified SCB test for anisotropic rocks to ensure a pure Mode I crack tip loading. It is demonstrated that the stress intensity factor (SIF) solution of an anisotropic SCB specimen depends on two dimensionless parameters, the anisotropy ratios of the Young’s modulus and apparent shear modulus, as well as the anisotropy orientation. These two dimensionless parameters are selected since they have physical meaning, and are generally bounded for rock materials. Based on these anisotropy parameters, the semi-circular test is modified to configure pure Mode I , or tensile stress at the crack tip, by using an asymmetric three-point bend configuration. Extensive finite element analyses are performed to obtain the span ratio and normalized SIF of the SCB specimen with different anisotropy ratios and orientations.

The implementation of bulk energy storage (BES) technologies can help to achieve higher penetration and utilization of variable renewable energy technologies (e.g., wind and solar), but it can also alter the dispatch order in regional electricity systems in other ways. These changes to the dispatch order affect the total amount of carbon dioxide (CO2) that is emitted to the atmosphere and the amount of total water that is required by the electricity generating facilities. In a case study of the Electricity Reliability Council of Texas system, we separately investigated the value that three BES technologies (CO2-Geothermal Bulk Energy Storage, Compressed Air Energy Storage, Pumped Hydro Energy Storage) could have for reducing system-wide CO2 emissions and water requirements. In addition to increasing the utilization of wind power capacity, the dispatch of BES also led to an increase in the utilization of natural gas power capacity and of coal power capacity, and a decrease in the utilization of nuclear power capacity, depending on the character of the net load, the CO2 price, the water price, and the BES technology. These changes to the dispatch order provided positive value (e.g., increase in natural gas generally reduced CO2 emissions; decrease in nuclear utilization always decreased water requirements) or negative value (e.g., increase in coal sometimes increased CO2 emissions; increase in natural gas sometimes increased water requirements) to the regional electricity system. We also found that these values to the system can be greater than the cost of operating the BES facility. At present, there are mechanisms to compensate BES facilities for ancillary grid services, and our results suggest that similar mechanisms could be enacted to compensate BES facilities for their contribution to the environmental sustainability of the system.