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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

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.

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

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.

This paper introduces a methodology for the direct determination of the shear moduli in transversely isotropic rocks, using a single test, where a cylindrical specimen is subjected to uniaxial compression. A method is also developed to determine the orientation of the isotropy plane as well as the dynamic elastic constants using ultrasonic measurements on a single cylindrical specimen. Explicit formulae are developed to calculate the shear moduli from strain gauge measurements at different polar angles. The calculation of shear moduli from these formulae requires no knowledge about Young's moduli or Poisson's ratios and depends only on the orientation of the isotropy plane. Several strain gauge setups are designed to obtain the shear moduli from different numbers and arrangements of strain gauges. We demonstrate, that the shear moduli can be determined accurately and efficiently with only three strain gauge measurements. The orientation of the isotropy plane is measured with different methods, including ultrasonic measurements. The results show, that the isotropy plane of the tested granitic samples slightly deviates from the foliation plane. However, the foliation plane can still determine the orientation of the isotropy plane with a good approximation.

To enable fast uncertainty quantification of fluid flow in a discrete fracture network (DFN), we present two approaches to quickly compute fluid flow in DFNs using combinatorial optimization algorithms. Specifically, the presented Hanan Shortest Path Maxflow (HSPM) and Intersection Shortest Path Maxflow (ISPM) methods translate DFN geometries and properties to a graph on which a max flow algorithm computes a combinatorial flow, from which an overall fluid flow rate is estimated using a shortest path decomposition of this flow. The two approaches are assessed by comparing their predictions with results from explicit numerical simulations of simple test cases as well as stochastic DFN realizations covering a range of fracture densities. Both methods have a high accuracy and very low computational cost, which can facilitate much-needed in-depth analyses of the propagation of uncertainty in fracture and fracture-network properties to fluid flow rates.

The creation of deep reservoirs for geothermal energy or oil and gas extraction is impeded by insufficient stimulation. Direction and extension of the created fractures are complex to control and, therefore, large stimulated and interconnected fracture networks are difficult to create. This lack of control and efficiency poses an inherent risk of uneconomic reservoirs, due to insufficient heat-sweep surfaces or hydraulic shortcuts. Therefore, we present a technique, which locally increases the cross section of a borehole by applying a thermal spallation process to the sidewalls of the borehole. By controlled and local enlargement of the well bore diameter, initial fracture sources are created, potentially reducing the injection pressure during hydraulic stimulation, initiating fracture growth, optimizing fracture propagation and increasing the number of accessible preexisting fractures. Consequently, local thermal borehole enlargement reduces project failure risks by providing better control on subsequent stimulation processes. In order to demonstrate the applicability of the suggested technique, we conducted a shallow field test in an underground rock laboratory. Two types of borehole enlargements were created in a 14.5 m deep borehole, indicating the feasibility of the technology to improve the productivity of geothermal, oil and gas reservoirs.

We investigate how subsurface fluids of different compositions affect the electricity generation of geothermal power plants. First, we outline a numerical model capable of accounting for the thermophysical properties of geothermal fluids of arbitrary composition within simulations of geothermal power production. The behavior of brines with varying compositions from geothermal sites around the globe are then examined using the model. The effect of each brine on an idealized binary geothermal power plant is simulated, and their performances compared by calculating the amount of heat exchanged from the fluid to the plant's secondary cycle. Our simulations combine (1) a newly developed Non-linear Equation System Solver (NESS), for simulating individual geothermal power plant components, (2) the advanced geochemical speciation solver, Reaktoro, used for calculation of thermodynamic fluid properties, and (3) compositional models for the calculation of fluid-dynamical properties (e.g., viscosity as a function of temperature and brine composition). The accuracy of the model is verified by comparing its predictions with experimental data from single-salt, binary-salt, and multiple-salt solutions. The geothermal power plant simulations show that the brines considered in this study can be divided into three main categories: (1) those of largely meteoric origin with low salinity for which the effect of salt concentration is negligible; (2) moderate-depth brines with high concentrations of Na + and K + ions, whose performance is well approximated by pure NaCl solutions of equivalent salinity; and (3) deeper, high-salinity brines that require a more detailed consideration of their composition for accurate simulation of plant operations.

Injecting supercritical CO 2 (scCO 2 ) into brine-filled fracture-dominated reservoirs causes brine displacement and possibly evaporite precipitations that alter the fracture space. Here, we report on isothermal laboratory experiments on scCO 2 -induced flow-through drying in a naturally fractured granodiorite specimen under effective normal stresses of 5-10 MPa, where two drying regimes are identified. A novel approach is developed to delineate the evolution of brine saturation and relative permeability from fluid production and differential pressure measurements. Under higher compressive stresses, the derived relative permeability curves indicate lower mobility of brine and higher mobility of the scCO 2 phase. The derived fractional flow curves also suggest an increase in channelling and a decrease in brine sweep efficiencies under higher compressive stresses. Finally, lowering compressive stresses seems to promote less water evaporation. Our experimental results assist in understanding the injectivity of single fractures and eventually of fracture networks during subsurface applications that involve scCO 2 injection into saline formations.

Aquifer thermal energy storage (ATES) is a time-shifting thermal energy storage technology where waste heat is stored in an aquifer for weeks or months until it may be used at the surface. It can reduce carbon emissions and HVAC costs. Low-temperature ( 25 °C) aquifer thermal energy storage (LT-ATES) is already widely-deployed in central and northern Europe, and there is renewed interest in high-temperature ( > 50 °C) aquifer thermal energy storage (HT-ATES). However, it is unclear if LT-ATES guidelines for well spacing, reservoir depth, and transmissivity will apply to HT-ATES. We develop a thermo-hydro-mechanical-economic (THM$) analytical framework to balance three reservoir-engineering and economic constraints for an HT-ATES doublet connected to a district heating network. We find the optimal well spacing and flow rate are defined by the “reservoir constraints” at shallow depth and low permeability and are defined by the “economic constraints” at great depth and high permeability. We find the optimal well spacing is 1.8 times the thermal radius. We find that the levelized cost of heat is minimized at an intermediate depth. The minimum economically-viable transmissivity (MEVT) is the transmissivity below which HT-ATES is sure to be economically unattractive. We find the MEVT is relatively insensitive to depth, reservoir thickness, and faulting regime. Therefore, it can be approximated as 5 ⋅ 1 0 − 13 m 3 . The MEVT is useful for HT-ATES pre-assessment and can facilitate global estimates of HT-ATES potential.

Geophysical and petrological probes are key to understanding the structure and the thermochemical state of active magmatic systems. Recent advances in laboratory analyses, field investigations and numerical methods have allowed increasingly complex data-constraint models with new insights into magma plumbing systems and melt evolution. However, there is still a need for methods to quantitatively link geophysical and petrological observables for a more consistent description of magmatic processes at both micro- and macro-scales. Whilst modern geophysical studies provide detailed 3-D subsurface images that help to characterize magma reservoirs by relating state variables with physical material properties, constraints from on-site petrological analyses and thermodynamic modelling of melt evolution are at best incorporated qualitatively. Here, we combine modelling of phase equilibria in cooling magma and laboratory measurements of electrical properties of melt to derive the evolution of electrical conductivity in a crystallizing silicic magmatic system. We apply this framework to 3-D electrical conductivity images from magnetotelluric studies of two volcanoes in the Ethiopian Rift. The presented approach enables us to constrain key variables such as melt content, temperature and magmatic volatile abundance at depth. Our study shows that accounting for magmatic volatiles as an independent phase is crucial for understanding electrical conductivity structures in magma reservoirs at an advanced state of crystallization. Furthermore, our results deepen the understanding of the mechanisms behind volcanic unrest and help assess the long-term potential of hydrothermal reservoirs for geothermal energy production.

High-voltage direct current (HVDC) transmission infrastructure can transmit electricity from regions with high-quality variable wind and solar resources to those with high electricity demand. In these situations, bulk energy storage (BES) could beneficially increase the utilization of HVDC transmission capacity. Here, we investigate that benefit for an emerging BES approach that uses geologically stored CO2 and sedimentary basin geothermal resources to time-shift variable electricity production. For a realistic case study of a 1 GW wind farm in Eastern Wyoming selling electricity to Los Angeles, California (U.S.A.), our results suggest that a generic CO2-BES design can increase the utilization of the HVDC transmission capacity, thereby increasing total revenue across combinations of electricity prices, wind conditions, and geothermal heat depletion. The CO2-BES facility could extract geothermal heat, dispatch geothermally generated electricity, and time-shift wind-generated electricity. With CO2-BES, total revenue always increases and the optimal HVDC transmission capacity increases in some combinations. To be profitable, the facility needs a modest $7.78/tCO2 to $10.20/tCO2, because its cost exceeds the increase in revenue. This last result highlights the need for further research to understand how to design a CO2-BES facility that is tailored to the geologic setting and its intended role in the energy system.

A direct CO2-Plume Geothermal (CPG) system is a novel technology that uses captured and geologically stored CO2 as the subsurface working fluid in sedimentary basin reservoirs to extract geothermal energy. In such a CPG system, the CO2 that enters the production well is likely saturated with H2O from the geothermal reservoir. However, direct CPG models thus far have only considered energy production via pure (i.e. dry) CO2 in the production well and its direct conversion in power generation equipment. Therefore, we analyze here, how the wellhead fluid pressure, temperature, liquid water fraction, and the resultant CPG turbine power output are impacted by the production of CO2 saturated with H2O for reservoir depths ranging from 2.5 km to 5.0 km and geothermal temperature gradients between 20 °C/km and 50 °C/km. We demonstrate that the H2O in solution is exothermically exsolved in the vertical well, increasing the fluid temperature relative to dry CO2, resulting in the production of liquid H2O at the wellhead. The increased wellhead fluid temperature increases the turbine power output on average by 15% to 25% and up to a maximum of 41%, when the water enthalpy of exsolution is considered and the water is (conservatively) removed before the turbine, which decreases the fluid mass flow rate through the turbine and thus power output. We show that the enthalpy of exsolution and the CO2-H2O solution density are fundamental components in the calculation of CPG power generation and thus should not be neglected or substituted with the properties of dry CO2.

The geochemical literature provides numerous thermodynamic databases compiled from equilibrium constants of reactions. These databases are typically used in speciation calculations based on the law of mass action (LMA) approach. Unfortunately, such LMA databases cannot be directly used in equilibrium speciation methods based on the Gibbs energy minimization (GEM) approach because of their lack of standard chemical potentials of species. Therefore, we present in this work a simple conversion approach that calculates apparent standard chemical potentials of species from equilibrium constants of reactions. We assess the consistency and accuracy of the use of apparent standard chemical potentials in GEM algorithms by benchmarking equilibrium speciation calculations using GEM and LMA methods with the same LMA database. In all cases, we use PHREEQC to perform the LMA calculations, and we use its LMA databases to calculate the equilibrium constants of reactions. GEM calculations are performed using a Gibbs energy minimization method of Reaktoro — a unified open-source framework for numerical modeling of chemically reactive systems. By comparing the GEM and LMA results, we show that the use of apparent standard chemical potentials in GEM methods produces consistent and accurate equilibrium speciation results, thus validating our new, practical conversion technique that enables GEM algorithms to take advantage of many existing LMA databases, consequently extending and diversifying their range of applicability.

We present an extended law of mass-action (xLMA) method for multiphase equilibrium calculations and apply it in the context of reactive transport modeling. This extended LMA formulation differs from its conventional counterpart in that (i) it is directly derived from the Gibbs energy minimization (GEM) problem (i.e., the fundamental problem that describes the state of equilibrium of a chemical system under constant temperature and pressure); and (ii) it extends the conventional mass-action equations with Lagrange multipliers from the Gibbs energy minimization problem, which can be interpreted as stability indices of the chemical species. Accounting for these multipliers enables the method to determine all stable phases without presuming their types (e.g., aqueous, gaseous) or their presence in the equilibrium state. Therefore, the here proposed xLMA method inherits traits of Gibbs energy minimization algorithms that allow it to naturally detect the phases present in equilibrium, which can be single-component phases (e.g., pure solids or liquids) or non-ideal multi-component phases (e.g., aqueous, melts, gaseous, solid solutions, adsorption, or ion exchange). Moreover, our xLMA method requires no technique that tentatively adds or removes reactions based on phase stability indices (e.g., saturation indices for minerals), since the extended mass-action equations are valid even when their corresponding reactions involve unstable species. We successfully apply the proposed method to a reactive transport modeling problem in which we use PHREEQC and GEMS as alternative backends for the calculation of thermodynamic properties such as equilibrium constants of reactions, standard chemical potentials of species, and activity coefficients. Our tests show that our algorithm is efficient and robust for demanding applications, such as reactive transport modeling, where it converges within 1–3 iterations in most cases. The proposed xLMA method is implemented in Reaktoro, a unified open-source framework for modeling chemically reactive systems.

The mechanical behavior of fractures in solids, such as rocks, has strong implications for reservoir engineering applications. Deformations, and the corresponding change in solid contact area and aperture field, impact rock fracture stiffness and permeability thus altering the reservoir properties significantly. Simulating contact between fractures is numerically difficult as the non-penetration constraints lead to a nonlinear problem and the surface meshes of the solid bodies on the opposing fracture sides may be non-matching. Furthermore, the challenging geometry of the arising constraints requires to solve the problem in several iterations, adjusting the constraints in each one. Here we present a novel discrete implementation of a dual mortar method and a non-smooth SQP solver, suitable for parallel computing, and apply it to a two body contact problem consisting of realistic rock fracture geometries from the Grimsel underground laboratory in Switzerland. The contributions of this article are: 1) a novel, parallel implementation of a dual mortar method and non-smooth SQP method, 2) realistic rock geometries with rough surfaces, and 3) numerical examples, which prove that the dual mortar method is capable of replicating the nonlinear closure behavior of fractures, observed in laboratory experiments., preprint

Tracer tests were conducted as part of decameter-scale in-situ hydraulic stimulation experiments at the Grimsel Test Site to investigate the hydraulic properties of a stimulated crystalline rock volume and to study the stimulation-induced hydrodynamic changes. Temporal moment analysis yielded an increase in tracer swept pore volume with prominent flow channeling. Post-stimulation tomographic inversion of the hydraulic conductivity, K, distribution indicated an increase in the geometric mean of logK and a decrease in the Dykstra–Parsons heterogeneity index. These results indicate that new flow path connections were created by the stimulation programs, enabling the tracers to sweep larger volumes, while accessing flow paths with larger hydraulic conductivities.

We investigate the potential for extracting heat from produced natural gas and utilizing supercritical carbon dioxide (CO2) as a working fluid for the dual purpose of enhancing gas recovery (EGR) and extracting geothermal energy (CO2-Plume Geothermal – CPG) from deep natural gas reservoirs for electric power generation, while ultimately storing all of the subsurface-injected CO2. Thus, the approach constitutes a CO2 capture double-utilization and storage (CCUUS) system. The synergies achieved by the above combinations include shared infrastructure and subsurface working fluid. We integrate the reservoir processes with the wellbore and surface power-generation systems such that the combined system’s power output can be optimized. Using the subsurface fluid flow and heat transport simulation code TOUGH2, coupled to a wellbore heat-transfer model, we set up an anticlinal natural gas reservoir model and assess the technical feasibility of the proposed system. The simulations show that the injection of CO2 for natural gas recovery and for the establishment of a CO2 plume (necessary for CPG) can be conveniently combined. During the CPG stage, following EGR, a CO2-circulation mass flowrate of 110 kg/s results in a maximum net power output of 2 MWe for this initial, conceptual, small system, which is scalable. After a decade, the net power decreases when thermal breakthrough occurs at the production wells. The results confirm that the combined system can improve the gas field’s overall energy production, enable CO2 sequestration, and extend the useful lifetime of the gas field. Hence, deep (partially depleted) natural gas reservoirs appear to constitute ideal sites for the deployment of not only geologic CO2 storage but also CPG.

Accessing hydrocarbons, geothermal energy and mineral resources requires more and more drilling to great depths and into hard rocks, as many shallow resources in soft rocks have been mined already. Drilling into hard rock to great depths, however, requires reducing the effort (i.e., energy), time (i.e., increasing the rate of penetration) and cost associated with such operations. Thus, a Combined Thermo-Mechanical Drilling (CTMD) technology is proposed, which employs a heat source (e.g., a flame jet) and includes two main drilling modes: (I) Thermal spallation drilling, investigated here as a field test and (II) Flame-assisted rotary drilling, investigated as a field test in the companion paper. The CTMD technology is expected to reduce drilling costs, especially in hard rocks, by enhancing the rock penetration rate and increasing the bit lifetime. ModeI of the CTMD technology (thermal spallation drilling) is investigated here by implementing the concept on a full-scale drilling rig to investigate its feasibility and performance under realistic field conditions. During the test, the successful thermal spallation process is monitored, employing a novel acoustic emission system. The effects of thermal spallation in the granite rock are analyzed to provide conclusions regarding the rock removal performance and the application potential of the technology. The field test shows that thermal spallation of the granitic rock can be successfully achieved even when a liquid (water) is used as the drilling fluid, as long as the heat source is appropriately shielded by compressed-air jets. Thermal damage of the surrounding rock is investigated after the spallation test, employing micro-computer tomography imaging and modeling the stability of the cracks, generated by the spallation field test. This study shows that thermally induced damage is mainly confined within a narrow region close to the rock surface, suggesting that thermal spallation only marginally affects the overall mechanical stability of the borehole. Thus, this confirms that, as part of the Combined Thermo-Mechanical Drilling (CTMD) technology, thermal spallation drilling is a promising mode that has a high potential of facilitating the drilling of deep boreholes in hard rocks.

To enhance the drilling performance in deep hard rocks and reduce overall drilling efforts, this work proposes a Combined Thermo-Mechanical Drilling (CTMD) technology. This technology employs a heat source (e.g., a flame jet) and includes two main drilling modes: (I) thermal spallation drilling, investigated in the companion paper and (II) flame-assisted rotary drilling, investigated here as a field test. The CTMD technology is expected to reduce drilling efforts, especially in hard rocks, enhancing the rock penetration rate and increasing the bit lifetime, all of which reduces the drilling costs. The present work investigates Mode II (flame-assisted rotary drilling) of the CTMD technology by implementing the concept in an existing drilling rig and testing the technology under relevant process conditions. This contribution studies the underlying rock removal mechanism of CTMD and demonstrates its drilling performance, compared to conventional rotary drilling methods. Acoustic emission monitoring, and analysis of the collected drill cuttings provide multiple evidences for thermal-cracking-enhanced rock removal during the flame-assisted rotary drilling. This removal mechanism appears to represent an optimal compromise to minimize rock fragmentation and cutting transport efforts during drilling, compared to a less efficient mechanical scraping of the hard granite rock, observed during the standalone-mechanical drill test. The drilling performance, in terms of removal and wear rates, are evaluated for the flame-assisted rotary drilling. This shows that the proposed drilling approach is capable of enhancing the removal process in hard granite rock, by a factor of 2.5, compared to standalone-mechanical drilling. The implementation of this drilling approach into a conventional drilling system shows that integration of thermal assistance to conventional rotary drilling requires marginal technical efforts. Additionally, this technology can profit from established knowledge in conventional mechanical drilling, facilitating its implementation to improve drilling performance in hard rocks. Hence, this study demonstrates that the Combined Thermo-Mechanical Drilling method is feasible and concludes that this technology constitutes a promising approach to improve the drilling process, thereby increasing the viability of accessing deep geo-resources in hard rocks.

This paper presents a theoretical and experimental analysis of the directional variations of different measures of Mode I fracture toughness in anisotropic rocks and possibly other types of solids. We report the theoretical basis for the directional dependence of three measures of fracture toughness: the critical stress intensity factor, the critical energy release rate and the critical strain energy density. The equivalency of these three measures in anisotropic materials is discussed. We then provide a full set of experimental results on the fracture toughness variation in an anisotropic rock that exhibits transverse isotropy. The results give supporting evidence that the critical Mode I stress intensity factor in fact varies with direction based on a sinusoidal function. This indicates that there exist two principal values of the fracture toughness along with the principal material directions within the plane. Once these two principal values are determined, all three measures of the fracture toughness can be predicted in any direction, provided that the elastic constants of the material are known, and that the symmetry condition employed in this analysis is fulfilled.

Combined thermo-mechanical drilling is a novel technology to enhance drilling performance in deep hard rock formations. In this work, we demonstrate this technology in the field by implementing the concept on a full-scale drilling rig, and we show its feasibility under realistic process conditions. We provide evidence that the novel drilling method can increase the removal performance in hard rocks by up to a factor of three, compared to conventional drilling methods. From the findings of this work, we conclude that integration of thermal assistance to conventional rotary drilling constitutes an interesting approach to facilitate the drilling process, and therefore increase the access viability to deep georesources in hard rocks.

Carbonate minerals and CO 2 are both considerably more soluble at low temperatures than they are at elevated temperatures. This inverse solubility has led a number of researchers to hypothesize that injecting low-temperature (i.e., less than the background reservoir temperature) CO 2 into deep, saline reservoirs for CO 2 Capture, Utilization, and Storage (CCUS) will dissolve CO 2 and carbonate minerals near the injection well and subsequently exsolve and re-precipitate these phases as the fluids flow into the geothermally warm portion of the reservoir. In this study, we utilize high performance computing to examine the coupled effects of cool CO 2 injection and background hydraulic head gradients on reservoir-scale mineral volume changes. We employ the fully coupled reactive transport simulator PFLOTRAN with calculations distributed over up to 800 processors to test 21 scenarios designed to represent a range of reservoir depths, hydraulic head gradients, and CO 2 injection rates and temperatures. In the default simulations, 50°C CO 2 is injected at a rate of 50kg/s into a 200bar, 100°C calcite or dolomite reservoir. By comparing these simulations with others run at varying conditions, we show that the effect of cool CO 2 injection on reservoir-scale mineral volume changes tends to be relatively minor. We conclude that the low heat capacity of CO 2 effectively prevents low-temperature CO 2 injection from decreasing the temperature across large portions of the simulated carbonate reservoirs. This small thermal perturbation, combined with the low relative permeability of brine within the supercritical CO 2 plume, yields limited dissolution and precipitation effects directly attributable to cool CO 2 injection. Finally, we calculate that relatively high water-to-rock ratios, which may occur over much longer CCUS reservoir lifetimes or in materials with sufficiently high brine relative permeability within the supercritical CO 2 plume, would be required to substantially affect injectivity through thermally-induced mineral dissolution and precipitation. Importantly, this study shows the utility of reservoir scale-reactive transport simulators for testing hypotheses and placing laboratory-scale observations into a CCUS reservoir-scale context.

To investigate CO 2 Capture, Utilization, and Storage (CCUS) in sandstones, we performed three 150 °C flow-through experiments on K-feldspar-rich cores from the Eau Claire formation. By characterizing fluid and solid samples from these experiments using a suite of analytical techniques, we explored the coupled evolution of fluid chemistry, mineral reaction rates, and hydrogeochemical properties during CO 2 sequestration in feldspar-rich sandstone. Overall, our results confirm predictions that the heightened acidity resulting from supercritical CO 2 injection into feldspar-rich sandstone will dissolve primary feldspars and precipitate secondary aluminum minerals. A core through which CO 2 -rich deionized water was recycled for 52 days decreased in bulk permeability, exhibited generally low porosity associated with high surface area in post-experiment core sub-samples, and produced an Al hydroxide secondary mineral, such as boehmite. However, two samples subjected to ∼3 day single-pass experiments run with CO 2 -rich, 0.94 mol/kg NaCl brines decreased in bulk permeability, showed generally elevated porosity associated with elevated surface area in post-experiment core sub-samples, and produced a phase with kaolinite-like stoichiometry. CO 2 -induced metal mobilization during the experiments was relatively minor and likely related to Ca mineral dissolution. Based on the relatively rapid approach to equilibrium, the relatively slow near-equilibrium reaction rates, and the minor magnitudes of permeability changes in these experiments, we conclude that CCUS systems with projected lifetimes of several decades are geochemically feasible in the feldspar-rich sandstone end-member examined here. Additionally, the observation that K-feldspar dissolution rates calculated from our whole-rock experiments are in good agreement with literature parameterizations suggests that the latter can be utilized to model CCUS in K-feldspar-rich sandstone. Finally, by performing a number of reactive transport modeling experiments to explore processes occurring during the flow-through experiments, we have found that the overall progress of feldspar hydrolysis is negligibly affected by quartz dissolution, but significantly impacted by the rates of secondary mineral precipitation and their effect on feldspar saturation state. The observations produced here are critical to the development of models of CCUS operations, yet more work, particularly in the quantification of coupled dissolution and precipitation processes, will be required in order to produce models that can accurately predict the behavior of these systems.

Several studies suggest that CO 2 -based geothermal energy systems may be operated economically when added to ongoing geologic CO 2 sequestration. Alternatively, we demonstrate here that CO 2 -Plume Geothermal (CPG) systems may be operated long-term with a finite amount of CO 2 . We analyze the performance of such CO 2 -limited CPG systems as a function of various geologic and operational parameters. We find that the amount of CO 2 required increases with reservoir depth, permeability, and well spacing and decreases with larger geothermal gradients. Furthermore, the onset of reservoir heat depletion decreases for increasing geothermal gradients and for both particularly shallow and deep reservoirs.

Aqueous species equilibrium constants and activity models form the foundation of the complex speciation codes used to model the geochemistry of geothermal energy production, extremophilic ecosystems, ore deposition, and a variety of other processes. Researchers have shown that a simple three species model (i.e., Na+, Cl−, and NaCl(aq)) can accurately describe conductivity measurements of concentrated NaCl and KCl solutions at elevated temperatures and pressures (Sharygin et al., 2002). In this model, activity coefficients of the charged species (e.g., Na+, K+, Cl−) become sufficiently low that the complexes must redisocciate with increasing salt concentration in order to meet equilibrium constant constraints. Redissociation decreases the proportion of the elements bound up as neutral complexes, and thereby increases the true ionic strength of the solution. In this contribution, we explore the consequences of the redissociation phenomenon in albite–paragonite–quartz (APQ) buffered systems. We focus on the implications of the redissociation phenomenon for mineral solubilities, particularly the observation that, at certain temperatures and pressures, calculated activities of charged ions in solution remain practically constant even as element concentrations increase from

In contrast to conventional hydrothermal systems or enhanced geothermal systems, CO 2 Plume Geothermal (CPG) systems generate electricity by using CO 2 that has been geothermally heated due to sequestration in a sedimentary basin. Four CPG and two brine-based geothermal systems are modeled to estimate their power production for sedimentary basin reservoir depths between 1 and 5 km, geothermal temperature gradients from 20 to 50 °C km −1 , reservoir permeabilities from 1 × 10 −15 to 1 × 10 −12 m 2 and well casing inner diameters from 0.14 m to 0.41 m. Results show that CPG direct-type systems produce more electricity than brine-based geothermal systems at depths between 2 and 3 km, and at permeabilities between 10 −14 and 10 −13 m 2 , often by a factor of two. This better performance of CPG is due to the low kinematic viscosity of CO 2 , relative to brine at those depths, and the strong thermosiphon effect generated by CO 2 . When CO 2 is used instead of R245fa as the secondary working fluid in an organic Rankine cycle (ORC), the power production of both the CPG and the brine–reservoir system increases substantially; for example, by 22% and 20% for subsurface brine and CO 2 systems, respectively, with a 35 °C km −1 thermal gradient, 0.27 m production and 0.41 m injection well diameters, and 5 × 10 −14 m 2 reservoir permeability.

Hydrothermal flow experiments of single-pass injection of CO2-charged brine were conducted on nine dolomite cores to examine fluid–rock reactions in dolomite reservoirs under geologic carbon sequestration conditions. Post-experimental X-ray computed tomography (XRCT) analysis illustrates a range of dissolution patterns, and significant increases in core bulk permeability were measured as the dolomite dissolved. Outflow fluids were below dolomite saturation, and cation concentrations decreased with time due to reductions in reactive surface area with reaction progress. To determine changes in reactive surface area, we employ a power-law relationship between reactive surface area and porosity (Luquot and Gouze, 2009). The exponent in this relationship is interpreted to be a geometrical parameter that controls the degree of surface area change per change in core porosity. Combined with XRCT reconstructions of dissolution patterns, we demonstrate that this exponent is inversely related to both the flow path diameter and tortuosity of the dissolution channel. Even though XRCT reconstructions illustrate dissolution at selected regions within each core, relatively high Ba and Mn recoveries in fluid samples suggest that dissolution occurred along the core's entire length and width. Analysis of porosity–permeability data indicates an increase in the rate of permeability enhancement per increase in porosity with reaction progress as dissolution channels lengthen along the core. Finally, we incorporate the surface area–porosity model of Luquot and Gouze (2009) with our experimentally fit parameters into TOUGHREACT to simulate experimental observations.

Internal consistency of thermodynamic data has long been considered vital for confident calculations of aqueous geochemical processes. However, an internally consistent mineral thermodynamic data set is not necessarily consistent with calculations of aqueous species thermodynamic properties due, potentially, to improper or inconsistent constraints used in the derivation process. In this study, we attempt to accommodate the need for a mineral thermodynamic data set that is internally consistent with respect to aqueous species thermodynamic properties by adapting the least squares optimization methods of Powell and Holland (1985). This adapted method allows for both the derivation of mineral thermodynamic properties from fluid chemistry measurements of solutions in equilibrium with mineral assemblages, as well as estimates of the uncertainty on the derived results. Using a large number of phase equilibria, solubility, and calorimetric measurements, we have developed a thermodynamic data set of 12 key aluminum-bearing mineral phases. These data are derived to be consistent with Na+ and K+ speciation data presented by Shock and Helgeson (1988), H4SiO4(aq) data presented by Stefansson (2001), and the Al speciation data set presented by Tagirov and Schott (2001). Many of the constraining phase equilibrium measurements are exactly the same as those used to develop other thermodynamic data, yet our derived values tend to be quite different than some of the others’ due to our choices of reference data. The differing values of mineral thermodynamic properties have implications for calculations of Al mineral solubilities; specifically, kaolinite solubilities calculated with the developed data set are as much as 6.75 times lower and 73% greater than those calculated with Helgeson et al. (1978) and Holland and Powell (2011) data, respectively. Where possible, calculations and experimental data are compared at low T, and the disagreement between the two sources reiterates the common assertion that low-T measurements of phase equilibria and mineral solubilities in the aluminum system rarely represent equilibrium between water and well-crystallized, aluminum-bearing minerals. As an ancillary benefit of the derived data, we show that it may be combined with high precision measurements of aqueous complex association constants to derive neutral species activity coefficients in supercritical fluids. Although this contribution is specific to the aluminum system, the methods and concepts developed here can help to improve the calculation of water–rock interactions in a broad range of earth systems.

CPG (CO2 Plume Geothermal) energy systems use CO2 to extract thermal energy from naturally permeable geologic formations at depth. CO2 has advantages over brine: high mobility, low solubility of amorphous silica, and higher density sensitivity to temperature. The density of CO2 changes substantially between geothermal reservoir and surface plant, resulting in a buoyancy-driven convective current – a thermosiphon – that reduces or eliminates pumping requirements. We estimated and compared the strength of this thermosiphon for CO2 and for 20 weight percent NaCl brine for reservoir depths up to 5 km and geothermal gradients of 20, 35, and 50 °C/km. We found that through the reservoir, CO2 has a pressure drop approximately 3–12 times less than brine at the same mass flowrate, making the CO2 thermosiphon sufficient to produce power using reservoirs as shallow as 0.5 km. At 2.5 km depth with a 35 °C/km gradient – the approximate western U.S. continental mean – the CO2 thermosiphon converted approximately 10% of the energy extracted from the reservoir to fluid circulation, compared to less than 1% with brine, where additional mechanical pumping is necessary. We found CO2 is a particularly advantageous working fluid at depths between 0.5 km and 3 km.

We present an approach that uses the huge fluid and thermal storage capacity of the subsurface, together with geologic CO 2 storage, to harvest, store, and dispatch energy from subsurface (geothermal) and surface (solar, nuclear, fossil) thermal resources, as well as energy from electrical grids. Captured CO 2 is injected into saline aquifers to store pressure, generate artesian flow of brine, and provide an additional working fluid for efficient heat extraction and power conversion. Concentric rings of injection and production wells are used to create a hydraulic divide to store pressure, CO 2 , and thermal energy. Such storage can take excess power from the grid and excess/waste thermal energy, and dispatch that energy when it is demanded, enabling increased penetration of variable renewables. Stored CO 2 functions as a cushion gas to provide enormous pressure-storage capacity and displaces large quantities of brine, which can be desalinated and/or treated for a variety of beneficial uses. Geothermal power and energy-storage applications may generate enough revenues to justify CO 2 capture costs.

Permanence and security of carbon dioxide (CO 2 ) in geologic formations requires dissolution of CO 2 into brine, which slightly increases the brine density. Previous studies have shown that this small increase in brine density induces convective currents, which greatly enhances the mixing efficiency and thus CO 2 storage capacity and rate in the brine. Density-driven convection, in turn, is known to be largely dominated by permeability heterogeneity. This study explores the relationship between the process of density-driven convection and the permeability heterogeneity of an aquifer during CO 2 dissolution storage, using high-resolution numerical simulations. While the porosity is kept constant, the heterogeneity of the aquifer is introduced through a spatially varying permeability field, characterized by the Dykstra-Parsons coefficient and the correlation length. Depending on the concentration profile of dissolved CO 2 , we classify the convective finger patterns as dispersive, preferential, and unbiased fingering. Our results indicate that the transition between unbiased and both preferential and dispersive fingering is mainly governed by the Dykstra-Parsons coefficient, whereas the transition between preferential and dispersive fingering is controlled by the permeability correlation length. Furthermore, we find that the CO 2 dissolution flux at the top boundary will reach a time-independent steady state. Although this flux strongly correlates with permeability distribution, it generally increases with the permeability heterogeneity when the correlation length is less than the system size.

Recent geotechnical research shows that geothermal heat can be efficiently mined by circulating carbon dioxide through naturally permeable rock formations -- a method called CO 2 Plume Geothermal -- the same geologic reservoirs that are suitable for deep saline aquifer CO 2 sequestration or enhanced oil recovery. This paper describes the effect of thermal drawdown on reservoir pressure buildup during sequestration operations, revealing that geothermal heat mining can decrease overpressurization by 10% or more.

Summary The configuration of glacier hydrological systems is often inferred from proxy data, such as hydrographs, that are collected in proglacial streams. Seasonal changes in the peakedness of hydrographs are thought to reflect changes in the configuration of the subglacial drainage system. However, the amount of information that proglacial hydrographs contain about drainage system configurations depends critically on the degree to which the drainage systems modify recharge hydrographs. If the drainage system does not modify recharge hydrographs, then proglacial hydrographs primarily reflect the recharge conditions produced by supraglacial inputs. Here, we develop a theoretical framework to determine the circumstances under which glacier drainage systems can modify recharge hydrographs and the circumstances under which recharge pulses pass through glaciers unchanged. We address the capability of single conduits, simple arborescent conduit networks, and linked cavity systems to modify diurnal recharge pulses. Simulations of discharge through large sets of such systems demonstrate that, unless large reservoirs or significant constrictions are present, the discharge hydrographs of simple glacial conduit systems are nearly identical to their recharge hydrographs. Conduit systems tend not to modify hydrographs because the changes in storage within englacial and subglacial conduit networks on short time scales are typically small compared to their ability to transmit water. This finding suggests that proglacial hydrographs reflect a variety of factors, including surface melt rate, surface water transfer, and subglacial water transfer. In many cases the influence of suglacial processes may be relatively minor. As a result, the evolution of proglacial hydrographs cannot be used unambiguously to infer changes in the structure or efficiency of englacial or subglacial hydrological systems, without accurate knowledge of the nature of the recharge hydrograph driving the flow.

article i nfo Annual layer analysis in two stalagmites collected from Spring Valley Caverns, southeastern Minnesota, reveals hydrological response of the cave to extreme rainfall events in the Midwest, USA. Cave-flooding events are identified within the two samples by the presence of detrital layers composed of clay sized particles. Comparison with instrumental records of precipitation demonstrates a strong correlation between these cave-flood events and extreme rainfall observed in the Upper Mississippi Valley. A simple model is developed to assess the nature of rainfall capable of flooding the cave. The model is first calibrated to the last 50-yr (1950-1998 A.D.) instrumental record of daily precipitation data for the town of Spring Valley and verified with the first 50 yr of record from 1900 to 1949 A.D. Frequency analysis shows that these extreme flood events have increased from the last half of the nineteenth century. Comparison with other paleohydrological records shows increased occurrence of extreme rain events during periods of higher moisture availability. Our study implies that increased moisture availability in the Midwestern region, due to rise in temperature from global warming could lead to an increase in the occurrence of extreme rainfall events.

Many complex natural systems studied in the geosciences are characterized by simple local-scale interactions that result in complex emergent behavior. Simulations of these systems, often implemented in parallel using standard central processing unit (CPU) clusters, may be better suited to parallel processing environments with large numbers of simple processors. Such an environment is found in graphics processing units (GPUs) on graphics cards. This paper discusses GPU implementations of three example applications from computational fluid dynamics, seismic wave propagation, and rock magnetism. These candidate applications involve important numerical modeling techniques, widely employed in physical system simulations, that are themselves examples of distinct computing classes identified as fundamental to scientific and engineering computing. The presented numerical methods (and respective computing classes they belong to) are: (1) a lattice-Boltzmann code for geofluid dynamics (structured grid class); (2) a spectral-finite-element code for seismic wave propagation simulations (sparse linear algebra class); and (3) a least-squares minimization code for interpreting magnetic force microscopy data (dense linear algebra class). Significant performance increases (between 10x and 30x in most cases) are seen in all three applications, demonstrating the power of GPU implementations for these types of simulations and, more generally, their associated computing classes.

Partial-bounceback lattice-Boltzmann methods employ a probabilistic meso-scale model that varies individual lattice node properties to reflect a material's local permeability. These types of models have great potential in a range of geofluid, and other science and engineering, simulations of complex fluid flow. However, there are several different possible approaches for formulating partial-bounceback algorithms. This paper introduces a new partial-bounceback algorithm and compares it to two pre-existing partial-bounceback models. Unlike the two other partial-bounceback methods, the new approach conserves mass in heterogeneous media and shows improvements in simulating buoyancy-driven flow as well as diffusive processes. Further, the new model is better-suited for parallel processing implementations, resulting in faster simulations. Finally, we derive an analytical expression for calculating the permeability in all three models; a critical component for accurately matching simulation parameters to physical permeabilities.

Magmas often contain multiple interacting phases of embedded solid and gas inclusions. Multiphase percolation theory provides a means of modeling assemblies of these different classes of magmatic inclusions in a simple, yet powerful way. Like its single phase counterpart, multiphase percolation theory describes the connectivity of discrete inclusion assemblies as a function of phase topology. In addition, multiphase percolation employs basic laws to distinguish separate classes of objects and is characterized by its dependency on the order in which the different phases appear. This paper examines two applications of multiphase percolation theory: the first considers how the presence of bubble inclusions influences yield stress onset and growth in a magma's crystal network; the second examines the effect of bi-modal bubble-size distributions on magma permeability. We find that the presence of bubbles induces crystal clustering, thereby 1) reducing the percolation threshold, or critical crystal volume fraction, ϕ c , at which the crystals form a space-spanning network providing a minimum yield stress, and 2) resulting in a larger yield stress for a given crystal volume fraction above ϕ c . This increase in the yield stress of the crystal network may also occur when crystal clusters are formed due to processes other than bubble formation, such as heterogeneous crystallization, synneusis, and heterogeneity due to deformation or flow. Further, we find that bimodal bubble size distributions can significantly affect the permeability of the system beyond the percolation threshold. This study thus demonstrates that larger-scale structures and topologies, as well as the order in which different phases appear, can have significant effects on macroscopic properties in multiphase materials.

Magmas and other suspensions that develop sample-spanning crystal networks undergo a change in rheology from Newtonian to Bingham flow due to the onset of a finite yield stress in the crystal network. Although percolation theory provides a prediction of the crystal volume fraction at which this transition occurs, the manner in which yield stress grows with increasing crystal number densities is less-well understood. This paper discusses a simple numerical approach that models yield stress in magmatic crystalline assemblies. In this approach, the crystal network is represented by an assembly of soft-core interpenetrating cuboid (rectangular prism) particles, whose mechanical properties are simulated in a network model. The model is used to investigate the influence of particle shape and alignment anisotropy on the yield stress of crystal networks with particle volume fractions above the percolation threshold. In keeping with previous studies, the simulation predicts a local minimum in the onset of yield stress for assemblies of cubic particles, compared to those with more anisotropic shapes. The new model also predicts the growth of yield stress above (and close to) the percolation threshold. The predictions of the model are compared with results obtained from a critical path analysis. Good agreement is found between a characteristic stiffness obtained from critical path analysis, the growth in assembly stiffness predicted by the model (both of which have approximately cubic power-law exponents) and, to a lesser extent, the growth in yield stress (with a power-law exponent of 3.5). The effect of preferred particle alignment on yield stress is also investigated and found to obey similar power-law behavior.

We examine 20-yr data sets of seismic activity from 10 volcanic areas in the western United States for annual periodic signals (seasonality), focusing on large calderas (Long Valley caldera and Yellowstone) and stratovolcanoes (Cascade Range). We apply several statistical methods to test for seasonality in the seismic catalogs. In 4 of the 10 regions, statistically significant seasonal modulation of seismicity (N90% probability) occurs, such that there is an increase in the monthly seismicity during a given portion of the year. In five regions, seasonal seismicity is significant in the upper 3 km of the crust. Peak seismicity occurs in the summer and autumn in Mt. St. Helens, Hebgen Lake/Madison Valley, Yellowstone Lake, and Mammoth Mountain. In the eastern south moat of Long Valley caldera (LVC) peak seismicity occurs in the winter and spring. We quantify the possible external forcing mechanisms that could modulate seasonal seismicity. Both snow unloading and groundwater recharge can generate large stress changes of N5 kPa at seismogenic depths and may thus contribute to seasonality. D 2005 Elsevier B.V. All rights reserved.

SUPCRT92 is a widely used software package for calculating the standard thermodynamic properties of minerals, gases, aqueous species, and reactions. However, it is labor-intensive and error-prone to use it directly to produce databases for geochemical modeling programs such as EQ3/6, the Geochemist's Workbench, and TOUGHREACT. DBCreate is a SUPCRT92-based software program written in FORTRAN90/95 and was developed in order to produce the required databases for these programs in a rapid and convenient way. This paper describes the overall structure of the program and provides detailed usage instructions.

Groundwater recharge at Mt. Hood, Oregon, is dominated by spring snow melt which provides a natural largeamplitude and narrow-width pore-fluid pressure signal. Time delays between this seasonal groundwater recharge and seismicity triggered by groundwater recharge can thus be used to estimate large-scale hydraulic diffusivities and the state of stress in the crust. We approximate seasonal variations in groundwater recharge with discharge in runoffdominated streams at high elevations. We interpolate the time series ofnumber ofearthquakes, N, seismic moment, Mo, and stream discharge, Q, and determine cross-correlation coefficients at equivalent frequency bands between Q and both N and Mo. We find statistically significant correlation coefficients at a mean time lag of about 151 days. This time lag and a mean earthquake depth of about 4.5 km are used in the solution to the pressure diffusion equation, under periodic (1 year) boundary conditions, to estimate a hydraulic diffusivity of UW10 31 m 2 /s, a hydraulic conductivity ofabout KhW10 37 m/s, and a permeability ofabout kW10 315 m 2 . Periodic boundary conditions also allow us to determine a critical pore-fluid pressure fraction, PP/P0W0.1, ofthe applied near-surf ace pore-fluid pressure perturbation, P0W0.1 MPa, that has to be reached at the mean earthquake depth to cause hydroseismicity. The low magnitude of PPW0.01 MPa is consistent with other studies that propose 0.019 PP9 0.1 MPa and suggests that the state of stress in the crust near Mt. Hood could be near critical for failure. Therefore, we conclude that, while earthquakes occur throughout the year at Mt. Hood, elevated seismicity levels along pre-existing faults south of Mt. Hood during summer months are hydrologically induced by a reduction in effective stress.

The formation of a continuous crystal network in magmas and lavas can provide finite yield strength, dy, and can thus cause a change from Newtonian to Bingham rheology. The rheology of crystal^melt suspensions affects geological processes, such as ascent of magma through volcanic conduits, flow of lava across the Earth’s surface, melt extraction from crystal mushes under compression, convection in magmatic bodies, and shear wave propagation through partial melting zones. Here, three-dimensional numerical models are used to investigate the onset of ‘static’ yield strength in a zero-shear environment. Crystals are positioned randomly in space and can be approximated as convex polyhedra of any shape, size and orientation. We determine the critical crystal volume fraction, Pc, at which a crystal network first forms. The value of Pc is a function of object shape and orientation distribution, and decreases with increasing randomness in object orientation and increasing shape anisotropy. For example, while parallel-aligned convex objects yield Pc = 0.29, randomly oriented cubes exhibit a maximum Pc of 0.22. Approximations of plagioclase crystals as randomly oriented elongated and flattened prisms (tablets) with aspect ratios between 1:4:16 and 1:1:2 yield 0.086Pc 6 0.20, respectively. The dependence of Pc on particle orientation implies that the flow regime and resulting particle ordering may affect the onset of yield strength. Pc in zero-shear environments is a lower bound for Pc. Finally the average total excluded volume is used, within its limitation of being a ‘quasi-invariant’, to develop a scaling relation between dy and P for suspensions of different particle shapes. fl 2001 Elsevier Science B.V. All rights reserved.