Crystalline Rock Mass Research Articles

Formation of the Near-Fault Damage Zone during Dynamic Rupture in a Crystalline Rock Mass

Formation of the Near-Fault Damage Zone during Dynamic Rupture in a Crystalline Rock Mass

AREHS: effects of changing boundary conditions on the development of hydrogeological systems: numerical long-term modelling considering thermal–hydraulic–mechanical (–chemical) coupled effects

Abstract. The objective of the AREHS project, funded by BASE (FKZ 4719F10402), is to model the effects of changing external boundary conditions on the hydrogeologically relevant parameters and effects (e.g. hydraulic permeability, porosity, migration pathways, fluid availability, hydraulic gradients) of a generic geological repository in Germany in all three potential host rocks (clay, salt, and crystalline rocks) in its hydrogeological setting. Special attention is paid to the cyclic mechanical loading due to glaciation events. This results in stress changes (M – mechanical processes) as well as induced temperature effects (T – thermal processes) due to permafrost and warm periods. Since the thermal, hydraulic (H – hydraulic processes), and mechanical processes are strongly coupled, they have to be covered by state-of-the-art coupled thermal–hydraulic–mechanical (THM) (C) modelling. The presentation consists of a (a) presentation of important findings for all three host rock types and a (b) presentation of the overall workflow. Complex, time-varying boundary conditions have been formulated for modelling glacial cycles. Investigating the effects of these boundary conditions with THM simulation reveals a variety of coupling effects. To conduct the complex modelling workflow for claystone and salt rock effectively, an automated workflow was developed and tested. It handles the transformation from a simulator-independent geological model to a numerical model specific to the simulator. This contains a suitable finite-element mesh and a parameterization of the fully coupled or optionally isolated thermal–hydraulic–mechanical processes, which are implemented and executed in the OpenGeoSys simulator. Simulation results are presented for selected physical quantities at characteristic local and temporal points with respect to the position of a migrating glacier. The investigation results are reproducible through full automation, container deployment, and a continuous integration process running on a GitLab instance (see Fig. 1a). To simulate the response of a fractured crystalline rock mass to THM impacts, the explicit integration of the fracture network in numerical models is necessary. The workflow couples the distinct element method (DEM) software “3DEC” and the discrete fracture network (DFN) software “DFN.Lab” automatically (Fig. 1b).

Mineralogy and Chemistry of Lakshanhatti Dolomite of Proterozoic Kaladgi Basin, Karnataka, India

Abstract The dolomites of Lakshanhatti Dolomite Member of Simikeri Subgroup of the Proterozoic Kaladgi Basin of Karnataka are medium to fine grained, compact and massive crystalline rocks, and are characterized by stromatolitic and cherty laminations. They are composed mainly of dolomite mineral with traces of calcite and quartz. The MgO content of these dolomites is almost uniform, and varies between 19.57 and 21.35%, while their CaO content varies widely from 30.24 to 38.39%. They are characterized by low SiO2 (av. 0.47%) and Al2O3 contents, indicating negligible amount of detrital quartz and clay fractions in dolomites. The higher contents of Fe (av. 5225 ppm) and Mn (av. 179 ppm) suggest for their substitution in Mg/Ca cations during dolomitization. The values of MgO (av. 20.47%), CaO (av. 35.07%) and insolubles like SiO2, Al2O3 and Fe2O3 (av. 1.31%) categorize these dolomites as refractory grade. It is inferred that the dolomites with stromatolitic structures were formed as primary precipitates due to microbial activity and minor evaporation in a tidal flat shallow marine environment, while the secondary dolomites were formed by the replacement of limestone during burial and diagenesis.

Seismic reflectivity, fracturing and stress field data from the FFC-1 exploratory geothermal project in SW Skåne, Sweden

Enhanced geothermal systems (EGS) are a potential heat source in many parts of the world, even in locations where the temperature gradient is relatively low. We present here an integrated study of reflection seismic data, borehole logs and seismicity analysis performed in conjunction with a geothermal exploratory project operated by E.ON in Malmö, Sweden. In 2020, the pre-existing 2.1 km deep FFC-1 borehole through the sedimentary cover was deepened into the crystalline basement to about 3.1 km vertical depth. Combined interpretation of the reflection seismic data and geophysical wireline logs show that most of the reflectivity in the Precambrian basement is likely generated by lenses of mafic amphibolite embedded in a felsic gneissic matrix. The general structural bedding and foliation is gently dipping to sub-horizontal, similar to other locations in southwest Sweden. Fracture frequency is relatively high in the crystalline rock mass, with heavy fracturing in the uppermost part of the crystalline basement, obscuring a clear reflection from the top of the Precambrian. Highly fractured and hydraulically conductive intervals are also found between 2,562 and 2,695 m based on a temperature drop and the interpretation of the geophysical data. Open fractures, both natural and induced, have a clear N–S orientation, contrasting with the expected NW–SE direction based on the orientation of the Sorgenfrei-Tornquist Zone and earthquake fault plane solutions to the north. This difference may be partly explained by local variations in the stress field near the FFC-1 borehole and vairations in the stress field with depth. Despite this, the data from the FFC-1 well provide novel and unique information on the complex physical state of the crystalline basement on the margin of the Fennoscandian Shield, which further addresses the need for obtaining in-situ stress data to fully understand the local stress field prior to any stimulation. A temperature of 84°C measured at 3 km depth indicates that a desired EGS temperature of 120–140°C may be reached at 5–6 km depth, assuming a temperature gradient of about 20°C. If the relatively high fracture frequency and occurrence of fracture zones down to 3.1 km are also present at these target depths, then the FFC-1 location may be suitable for heat extraction if the rock mass is properly characterized before stimulation.

Multi-disciplinary characterizations of the BedrettoLab – a new underground geoscience research facility

Abstract. The increased interest in subsurface development (e.g., unconventional hydrocarbon, engineered geothermal systems (EGSs), 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 (“BedrettoLab”) was established in 2018 in the existing Bedretto Tunnel (Ticino, Switzerland), with an average overburden of 1000 m. In this paper, we introduce the BedrettoLab, 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 BedrettoLab can adequately serve as a test-bed that allows for in-depth study of the hydro-seismo-mechanical response of fractured crystalline rock masses.

Estimating fractured rock effective permeability using discrete fracture networks constrained by electrical resistivity data

Although the permeability of fractured rock mass is a fundamentally important property for the safe construction of civil and mining engineering structures such as tunnels, in situ characterization of permeability without resorting to hydraulic tests is difficult. For rapid, wide-area estimation, a method that can be conducted at a field-scale using geological and geophysical investigation data is proposed. The method is not based on conventional hydraulic test results. Instead, it combines the stochastic generation of fracture networks with the crack tensor theory. The most important parameter for this method is the fracture length distribution. Although the distribution parameters in the DFN model are assigned through sampling, a bias is generally experienced because of the limited sampling area. To improve the estimation of such parameters, in-situ electrical resistivity data and a symmetric self-consistent method are used to constrain the fracture length distribution. The proposed method is applied to the fractured crystalline rock mass of the Mizunami Underground Research Laboratory (URL) in the Tono area of central Japan. Its effectiveness and correctness are demonstrated through good correspondence of the derived effective permeability with the in-situ measured permeability.

Effect of capillarity in a fractured medium on water-sealed properties: a theoretical and experimental investigation

Underground water-sealed oil storage caverns are typically built in crystalline rock masses of good quality, and the water-sealed effect is realised by filling the fractures with water. Theoretical equations for the water-sealed properties of a fractured medium were established in this study considering the capillary force. The capillarity in the fractured medium and its effect on the water-sealed properties were investigated using a single-fracture model test method. The dynamic variation in the water–oil displacement process in a fracture was described by varying the fracture width and the liquid height in the two tanks. An index was introduced to characterise the degree of water–oil displacement quantitatively. The results show that the capillarity in the fracture thus improves the water-sealed properties of underground caverns. The current water-sealed criterion and empirical formula are relatively conservative. The water–oil displacement process in the fracture is controlled by the capillary fracture water height and the oil height in the liquid tank. The variation in the capillary fracture water height corresponding to a unit of oil injection (N) can be used to characterise the water–oil displacement process quantitatively. The results of this research can provide a reference for studying the effect of the capillarity of a fractured medium on the water-sealed properties of underground oil storage.

About this title - Sweden: Lithotectonic Framework, Tectonic Evolution and Mineral Resources

The solid rock mass of Sweden forms a natural field laboratory revealing insight into the westward growth and reworking of one of the planet's ancient continental nuclei. Three major geological units are exposed in different parts of the country: the western part of the Fennoscandian Shield, mainly sedimentary rocks deposited on this crystalline rock mass and the Caledonide orogen. This volume synthesizes the tectonic evolution of Sweden over more than 2500 million years from the Neoarchean to the Neogene. Following an introduction describing the lithotectonic framework of the country and the organization of the volume, the tectonic evolution is addressed essentially chronologically. Different phases of intracratonic rifting, accretionary orogeny, continent–continent collisional orogeny and platformal sedimentation are identified. Sweden is one of Europe's major suppliers of metals, and the country's mineral resources are also presented in the context of the lithotectonic framework. Sweden: Lithotectonic Framework, Tectonic Evolution and Mineral Resources has been designed to interest a professional geoscientific audience and advanced students of Earth Sciences.

Nonlinear Suppression of High‐Frequency S Waves by the Near‐Field Velocity Pulse With Reference to the 2002 Denali Earthquake

The near‐field velocity pulse of large strike‐slip earthquakes brings near‐fault stiff crystalline rock into failure. The uppermost ~2 km beneath Pump Station 10 (PS10) likely failed nonlinearly during the 2002 Denali earthquake. High‐frequency S waves traversed this region during and immediately after failure with only weak waves reaching the surface. In contrast, high‐frequency S waves were briefly weak at station LUC only during the strong near‐field velocity pulse of the 1992 Landers earthquake. The observed horizontal spectra during the near‐field velocity pulse at PS10 decreased exponentially with frequency with a low apparently constant Q of ~20. This observation is incompatible with simple elastic‐plastic and nonlinear viscoelastic rheologies that do not preferentially attenuate high frequencies. Candidate rheologies involve heterogeneous crystalline rock masses where nonlinear domains act in parallel. Maxwell (spring and dash pot) elements in parallel produce apparently constant Q over a range of frequencies. This rheology may arise from pseudolinear inelastic interaction of weak stresses from high‐frequency S waves with strong low‐frequency stresses from the near‐field velocity pulse. Alternatively, inelastic deformation associated with the high‐frequency waves may interact nonlinearly with low‐frequency deformation associated with healing of damage associated with the near‐field velocity pulse. The latter process is attractive for the PS10 signal which remained weak after the near‐field velocity pulse had passed. We unsuccessfully examined aftershock records for healing of damage within the uppermost crystalline rock.

Hydraulic fracture propagation in a heterogeneous stress field in a crystalline rock mass

Abstract. As part of the In-situ Stimulation and Circulation (ISC) experiment, hydraulic fracturing (HF) tests were conducted in a moderately fractured crystalline rock mass at the Grimsel Test Site (GTS), Switzerland. The aim of these injection tests was to improve our understanding of processes associated with high-pressure fluid injection. A total of six HF experiments were performed in two inclined boreholes; the surrounding rock mass was accessed with 12 observation boreholes, which allows for the high-resolution monitoring of fracture fluid pressure, strain, and microseismicity in an exceptionally well-characterized rock mass. A similar injection protocol was used for all six experiments to investigate the complexity of the fracture propagation processes. At the borehole scale, these processes involved newly created tensile fractures intersecting the injection interval, while at the cross-hole scale, the natural network of fractures dominated the propagation process. The six HF experiments can be divided into two groups based on their injection location (i.e., south or north to a brittle–ductile shear zone), their similarity of injection pressures, and their response to deformation and pressure propagation. The injection tests performed in the south connect upon propagation to the brittle–ductile shear zone. Thus, the shear zone acts as a dominant drain and a constant pressure boundary. The experiments executed north of the shear zone show smaller injection pressures and larger backflow during bleed-off phases. From a seismic perspective, the injection tests show high variability in seismic response independently of the location of injection. For two injection experiments, we observe reorientation of the seismic cloud as the fracture propagated away from the wellbore. In both cases, the main propagation direction is normal to the minimum principal stress direction. The reorientation during propagation is interpreted to be related to a strong stress heterogeneity and the intersection of natural fractures striking differently than the propagating hydraulic fracture. The seismic activity was limited to about 10 m of radial distance from the injection point. In contrast, strain and pressure signals reach further into the rock mass, indicating that the process zone around the injection point is larger than the zone illuminated by seismic signals. Furthermore, strain signals indicate not just single fracture openings but also the propagation of multiple fractures. Transmissivities of injection intervals increase about 2–4 orders of magnitudes.

The engineering of safe hydraulic stimulations for EGS development in hot crystalline rock masses

Two types of hydraulic stimulation may be considered for the development of Enhanced Geothermal Systems (EGS), namely hydraulic fracturing and shear induced dilatancy. Only shear induced dilatancy has been found efficient for the shallow (750 m deep) granite test site at Le Mayet de Montagne.On this site, integration of focal mechanisms of induced seismic activity together with results from Hydraulic Tests on Pre-existing Fractures has provided a well constrained characterization of the complete regional stress field. Combination of this regional stress field characterization with focal mechanisms help map the pore pressure and its heterogeneity. This integrated inversion demonstrates that a Representative Elementary Volume (REV) of the order of 30x30x30 m3 may be defined for the absolute stress characterization, but cannot be used for the definition of effective stresses in this shallow depth environment.But the development by shear induced dilatancy of two EGS reservoirs at Soultz-sous-forêts have demonstrated the validity of contemplating large VER for characterizing effective stresses for depths greater than 2.8 km in this impervious granite massif. Stress measurements conducted on this site together with the stabilization of injection pressure at a given plateau, independent of the flow rate, have been found to be consistent with the development of fresh shear zones characterized by the Hoek and Brown criterion.Shear induced dilatancy has generated aseismic slips at Le Mayet de Montagne as well as for both the reservoirs developed at Soultz. This opens the way to developing safe shear induced dilatancy through appropriate injection protocols.

A case study on optimum identification of water curtain hydraulic pressure using three-phase flow numerical simulation

Groundwater-sealed liquid propane storage cavern is usually built in crystalline rock mass. Water curtain system is an indispensable facility to ensure the containment efficiency of the cavern due to the strong heterogeneity of fractured rock mass and the special properties of stored product. In this study, a propane cavern in Yantai, Shandong province is selected as a study case to investigate the reasonable water curtain pressure. Stochastic mathematical method was applied to calculate the permeability coefficient of the study area. A vertical 2D three-phase flow model based on THOUGH2 is established to study the migration characteristics of the stored product under different water curtain pressure and the influence of water curtain pressure on the gas containment of the cavern. It was found that the leakage of stored product can be restrained effectively when the water curtain pressure is greater than or equal to 0.8 MPa. When the water curtain is greater than or equal to 1.2 MPa, part of water in the water curtain is discharged from the simulation area, which will reduce the utilization rate of water resources and increase the operating cost. Therefore, the reasonable range of horizontal water curtain pressure is 0.8~1.1 MPa under this case or similar condition. The geological conditions and the migration characteristics of stored product considered in this paper, and the reasonable range of water curtain pressure obtained as well, can provide a new reference and methodology for the optimum determination of water curtain pressure in groundwater-sealed cavern engineering to increase the efficiency and reduce the cost of the work.

The seismo-hydromechanical behavior during deep geothermal reservoir stimulations: open questions tackled in a decameter-scale in situ stimulation experiment

Abstract. In this contribution, we present a review of scientific research results that address seismo-hydromechanically coupled processes relevant for the development of a sustainable heat exchanger in low-permeability crystalline rock and introduce the design of the In situ Stimulation and Circulation (ISC) experiment at the Grimsel Test Site dedicated to studying such processes under controlled conditions. The review shows that research on reservoir stimulation for deep geothermal energy exploitation has been largely based on laboratory observations, large-scale projects and numerical models. Observations of full-scale reservoir stimulations have yielded important results. However, the limited access to the reservoir and limitations in the control on the experimental conditions during deep reservoir stimulations is insufficient to resolve the details of the hydromechanical processes that would enhance process understanding in a way that aids future stimulation design. Small-scale laboratory experiments provide fundamental insights into various processes relevant for enhanced geothermal energy, but suffer from (1) difficulties and uncertainties in upscaling the results to the field scale and (2) relatively homogeneous material and stress conditions that lead to an oversimplistic fracture flow and/or hydraulic fracture propagation behavior that is not representative of a heterogeneous reservoir. Thus, there is a need for intermediate-scale hydraulic stimulation experiments with high experimental control that bridge the various scales and for which access to the target rock mass with a comprehensive monitoring system is possible. The ISC experiment is designed to address open research questions in a naturally fractured and faulted crystalline rock mass at the Grimsel Test Site (Switzerland). Two hydraulic injection phases were executed to enhance the permeability of the rock mass. During the injection phases the rock mass deformation across fractures and within intact rock, the pore pressure distribution and propagation, and the microseismic response were monitored at a high spatial and temporal resolution.

Keynote: Fatigue Hydraulic Fracturing

In this article, the concept of Fatigue Hydraulic Fracturing (FHF) is described, and its geothermal application is discussed. The basic idea behind fatigue fracturing is to vary the effective stress magnitudes at the fracture tip to optimize fracture initiation and growth. The optimization process can include lowering seismic radiated energy and/or generating fracture networks with various geometry and permeability. Historically, we start referring to results from mechanical laboratory core testing, discrete element simulation of fluid-induced seismicity, and application of cyclic water-fracs at the enhanced geothermal system site Groß-Schönebeck, Germany. Then, an in situ experiment at Äspö Hard Rock Laboratory is summarized to bridge the gap between laboratory core testing and wellbore-size hydraulic fracture treatments in hard rock. Three different fluid injection schemes (continuous, progressive and pulse injection) are tested underground in naturally fractured, crystalline rock mass in terms of associated induced seismicity and permeability performance. Under controlled conditions, hydraulic fractures are extended to about 20–40 m2 in size from a 28 m long, horizontal borehole drilled from a tunnel at 410 m depth. The facture process is mapped by an extensive array of acoustic emission and micro-seismic monitoring instruments. Results from three water-injection tests in Ävrö granodiorite indicate that the fracture breakdown pressure in tendency becomes lower and the number of fluid-induced seismic events becomes less when continuous, conventional fluid injection is replaced by progressive fluid-injection with several phases of depressurization simulating the fatigue treatment. One reason for this may be that in the dynamic, fatigue treatment a larger fracture process zone is generated compared to the size of the fluid pressurized zone developing during the injection phases into crystalline rock. We see mine-scale tests with hybrid sensor arrays of importance to identify and understand the actual hydraulic fracture mechanisms in hard rock. In addition, the mesoscale data obtained underground allow downscaling to laboratory core results, and upscaling to borehole reservoir stimulation results.

Preliminary geological, geochemical and numerical study on the first EGS project in Turkey

Geothermal energy is attracting more and more attention due to its large capacity and lack of dependency on the weather. Currently, many countries have planned enhanced geothermal system (EGS) projects. In this paper the first EGS project in Turkey, which is being implemented at the license area of SDS Energy Inc., in Dikili of the Izmir province, is introduced. Extensive geological, paleostress (279 fault-slip data from 33 locations), geophysical (magnetotelluric and vertical electrical sounding at 80 and 129 locations, respectively) and geochemical studies as well as paleostress measurements have been conducted in this area within the scope of this project. In the light of all these studies, it has been determined that the Dikili region is remarkable in terms of its high thermal gradient of about 7 °C/100 m. The geothermal reservoir formation “the Kozak granodiorite” is a homogeneous, crystalline volcanic rock mass with high radiogenic heat production, and suitable for an EGS application. The analysis shows that the dominating fault system is normal, and the corresponding primary stress regime is extensional. Based on the geological, geophysical surveys and the estimated in situ stresses, numerical studies were carried out to assess the results of the hydraulic fracturing and geothermal energy production using the numerical codes FLAC3Dplus and TOUGH2MP, respectively, in the area A of the Dikili site. The simulation results show that the stimulated reservoir volume and area could reach 44.5 million m3 and 1 km2, respectively, with an injection volume of 122,931 m3. Assuming the fractured zone has a height of 1000 m and a half-length of 1200 m (the distance between injection and production wells being 1000 m), an average overall geothermal capacity of 83.7 MWth in 20 years could be reached with an injection rate of 250 l/s. The injection strategy and design parameters of the reservoir stimulation and geothermal production will be further optimized with the project running.

Application of Vertical Electrical Soundings (VES) for Delineating Sub Surface Lithology for Foundations. (CVG Link Canal A Case Study)

In today’s world, field investigation is a necessity to get a detailed overview of any area for civil engineering construction purpose. Precise determination of engineering geological properties is essential to plan for a proper design and successful construction for any civil engineering structure. The traditionally practiced conventional methods for the same are invasive, costly and time consuming. Electrical Resistivity Survey is an attractive tool for delineating subsurface geology without soil disturbance. Reliable correlation between electrical resistivity values and other field geological parameters can help in successful interpretation of the engineering properties and behavior of soil in evaluation of difficult terrains e.g. for obtaining hard rock position, obtaining continuity of rock strata and for knowing the position of various sub – stratifications. This has led to develop and put in practice the geophysical method of subsurface investigation for a more precise, economical and fast assessment of large areas like the present study area of Cauvery-Vaigai link canal project. The present paper presents the results of the use of Vertical Electrical Sounding coupled with Hydrogeological studies of the Cauvery-Vaigai-Gundar (CVG) link canal area. On the basis of generalized Vertical Electrical Soundings (VES) log it is seen that the aquifers in tertiary rocks are deep – seated, the depth varying from 80 to 300 m from the ground level. It was observed that the open wells are tapping unconfined aquifer system is highly weathered rock, moderately weathered/ fractured hard rock and highly fractured hard rock. The bore wells are tapping the confined aquifer system 40 m Below Ground Level (BGL) in highly fractured hard rock. The aquifers in the bore wells were reported to be in between 40 to 120 m BGL. Most of the bore wells are dry at present due to depletion of aquifer owing to very little precipitation and over exploitation. From resistivity survey it is inferred that where ever highly fractured hard rock are expected along the link, abundant quantity of ground water may be present there in that fractured zones. The result of resistivity values of different rock types of the area show resistivity of less than 20 ohm-m in the weathered soil, while highly fractured rocks show its range between 100 to 250 ohm-m, moderately fractured rocks shows range between 250 and 500 ohm-m and the massive crystalline rocks exhibit the range of more than 500 ohm-m. The water table shows periodical variations. During the excavation of the canal, the study of water table condition will give an idea about the chances of striking ground water to cut and cross drainage portions as ground water poses a serious hazard during the excavation works.

Development of connected permeability in massive crystalline rocks through hydraulic fracture propagation and shearing accompanying fluid injection

AbstractThe ability to generate deep flow in massive crystalline rocks is governed by the interconnectivity of the fracture network and its permeability, which in turn is largely dependent on the in situ stress field. The increase of stress with depth reduces fracture aperture, leading to a decrease in rock mass permeability. The frequency of natural fractures also decreases with depth, resulting in less connectivity. The permeability of crystalline rocks is typically reduced to about 10−17–10−15 m2 at targeted depths for enhanced geothermal systems (EGS) applications, that is, >3 km. Therefore, fluid injection methods are required to hydraulically fracture the rock and increase its permeability. In the mining sector, fluid injection methods are being investigated to increase rock fragmentation and mitigate high‐stress hazards due to operations moving to unprecedented depths. Here as well, detailed understanding of permeability and its enhancement is required. This paper reports findings from a series of hydromechanically coupled distinct‐element models developed in support of a hydraulic fracture experiment testing hypotheses related to enhanced permeability, increased fragmentation, and modified stress fields. Two principal injection designs are tested as follows: injection of a high flow rate through a narrow‐packed interval and injection of a low flow rate across a wider packed interval. Results show that the development of connected permeability is almost exclusively orthogonal to the minimum principal stress, leading to strongly anisotropic flow. This is because of the stress transfer associated with opening of tensile fractures, which increases the confining stress acting across neighboring natural fractures. This limits the hydraulic response of fractures and the capacity to create symmetric isotropic permeability relative to the injection wellbore. These findings suggest that the development of permeability at depth can be improved by targeting a set of fluid injections through smaller packed intervals instead of a single longer injection in open boreholes.

Endolithic pedogenesis and rock varnish on massive crystalline rocks in East Antarctica

Desert varnish and endolithic organisms are two widespread phenomena that have been studied in detail separately; their interaction and their genetic relationships have virtually escaped the attention of researchers. Both phenomena are of indubitable interest for pedology: endolithic organisms as an agent of soil formation and rock varnish as a probable product of pedogenesis. It is argued that the system of endolithic organisms, their functioning products, and the rock has all the features inherent to soils: the rock layer subjected to the influence of external abiogenic factors and living organisms dwelling in the rock and synthesizing and decomposing organic substances. The action of biogenic and abiogenic agents leads to the in situ transformation of the rock with the accumulation and removal of the products of this transformation and with the development of vertical heterogeneity in the form of microhorizons composing the soil microprofile. Instrumental measurements indicate that the carbon content in the endolithic horizons developed by biota in granitoid rocks of the Larsemann Hills oasis varies from 0.2 to 3.3%, the nitrogen content in these horizons varies from 0.02 to 0.47%, and the radiocarbon age of their organic matter reaches 480 ± 25 yrs. The products of the pedogenesis are represented by fine earth materials and by abundant and often multilayered films and coatings on the rock surface and on the lower sides of the desquamation (spalling) plates. Scanning electron microscopy with X-ray microprobe analysis indicates that the major elements composing these films are O, C, Si, Al, Fe, Ca, Mg, and S. It is shown that the films of the rock varnish and the organomineral films in the fissured zone of the rock under the plate with endolithic communities have certain similarity in their morphology and composition: the films of the rock varnish also contain biota (dead cells or cells in the dormant state), and their botryoidal structure is similar to the structure of the biofilms inside the endolithic system. In both types of films, amorphous aluminum and silicon compounds are present, and the accumulation of Fe, Ca, Mg, S, Cl, and some other elements takes place. It is argued that some varieties of rock varnish are the products of endolithic pedogenesis; in essence, they represent the horizons of micropaleosols exposed to the surface in the course of spalling and then transformed by the external environmental agents.

A survey of the induced seismic responses to fluid injection in geothermal and CO 2 reservoirs in Europe

The paper documents 41 European case histories that describe the seismogenic response of crystalline and sedimentary rocks to fluid injection. It is part of an on-going study to identify factors that have a bearing on the seismic hazard associated with fluid injection. The data generally support the view that injection in sedimentary rocks tends to be less seismogenic than in crystalline rocks. In both cases, the presence of faults near the wells that allow pressures to penetrate significant distances vertically and laterally can be expected to increase the risk of producing felt events. All cases of injection into crystalline rocks produce seismic events, albeit usually of non-damaging magnitudes, and all crystalline rock masses were found to be critically stressed, regardless of the strength of their seismogenic responses to injection. Thus, these data suggest that criticality of stress, whilst a necessary condition for producing earthquakes that would disturb (or be felt by) the local population, is not a sufficient condition. The data considered here are not fully consistent with the concept that injection into deeper crystalline formations tends to produce larger magnitude events. The data are too few to evaluate the combined effect of depth and injected fluid volume on the size of the largest events. Injection at sites with low natural seismicity, defined by the expectation that the local peak ground acceleration has less than a 10% chance of exceeding 0.07 g in 50 years, has not produced felt events. Although the database is limited, this suggests that low natural seismicity, corresponding to hazard levels at or below 0.07 g, may be a useful indicator of a low propensity for fluid injection to produce felt or damaging events. However, higher values do not necessarily imply a high propensity.

The relevance of vein texture in understanding the past hydraulic behaviour of a crystalline rock mass: reconstruction of the palaeohydrology of the Mecsekalja Zone, south Hungary

This study reconstructs the palaeohydrogeologic evolution of the shallow-to-moderate Mesozoic subsidence history for the Mecsekalja Zone (MZ), a narrow metamorphic belt in the eastern Mecsek Mountains, Hungary. Brittle deformation of the MZ produced a vein system with a cement history consisting of five sequential carbonate generations and one quartz phase. Vein textures suggest different fluid-flow mechanisms for the parent fluids of subsequent cement generations. Combined microthermometric and stable-isotope measurements permit reconstruction of the character of subsequent fluid generations with different flow types, as defined by vein textures, yielding new information regarding the hydraulic behaviour of a metamorphic crystalline complex. Textural observations and geochemical data suggest that fracture-controlled flow pathways and externally derived fluids were typical of some flow events, while percolation through the rock matrix and the relationship to the Cretaceous volcanism and dyke emplacement were typical of others. The difference in the mode of calcite deposition from pervasive fluids (i.e. pervasive carbonatisation along grain boundaries versus deposition in antitaxial veins) between two calcite generations related to the volcanism inspired a stress-dependent model of antitaxial vein growth. Textural and isotope variations in a vein generation produced by the same parent fluid indicate rock-dependent hydraulic behaviour for different rock types, distinct action of the contemporaneous fracture systems and different extents of fluid–rock interaction. Cathodoluminescence microscopy and fluid-inclusion microthermometry shed light on the possible role of hydraulic fracturing in the formation of massive calcite. The time of formation was estimated from the isotope composition of the oldest calcite generation and its presumptive relationship with the sedimentary sequences to the north, whereas microthermometry permitted conciliation of the reconstructed flow sequence with the Mesozoic subsidence history of the Moragy Block (including the MZ).