Hydraulic Properties Of Rocks Research Articles

Rock mechanical modelling of the Bentonite Rock Interaction Experiment, Äspö Hard Rock Laboratory, Sweden

The Bentonite Rock Interaction Experiment (BRIE) was performed in a tunnel at a depth of 420 m at the Äspö Hard Rock Laboratory in Sweden. The experiment focused on the hydraulic properties of rock and bentonite aiming at investigating the exchange of water across a bentonite-rock interface. The hypothesis for the mechanical modelling presented here was that changes in flow (observed in rock and on bentonite parcels) were due to local mechanical deformation. Induced stresses related to the construction (and experimental) stages for the BRIE site such as excavation of tunnels, drilling and over-coring of two vertical, tunnel-floor boreholes and, finally, installation and swelling of bentonite, were expected to be the main causes of these deformations. We assumed that this could be investigated using a step-wise rock mechanical modelling approach (with a relevant modelling sequence) and validated by using a transdisciplinary approach including field structural geological mapping (geometric, kinematic and dynamic interpretation of the exposed fracture sets) and hydrogeological investigations.For key fractures intersecting the boreholes, the modelled fracture normal and shear displacements were found to be local, small, and in line with field observations and measurements for BRIE. Results point at an agreement between the spatial locations of changes in flow identified from the bentonite parcels and the locations of inelastic deformation indicated by mechanical modelling for a reverse stress regime. Besides providing information about the key fractures, the structural mapping allowed to establish solid relationships between brittle structural features in the tunnel and in the cores, which were used as, or compared to, the main fracture input to the rock mechanical modelling. The identified fracture sets were found to be structurally reconcilable with the larger-scale tectonic picture of the area.

Preliminary studies for an integrated assessment of the hydrothermal potential of the Pechelbronn Group in the northern Upper Rhine Graben

Abstract. The northern Upper Rhine Graben is due to its tectonic setting and the positive geothermal anomaly a key region for geothermal heat and power production in Europe. In this area the Upper Eocene to Lower Oligocene Pechelbronn Group reaches depths of up to 2800 m with temperatures of locally more than 130 ∘C. In order to assess the hydrothermal potential of the Pechelbronn Group a large dataset is compiled and evaluated. Petrophysical parameters are measured on core samples of eight boreholes (courtesy of Exxon Mobil). Additionally, 15 gamma-ray logs, 99 lithology logs as well as more than 2500 porosity and permeability measurements on cores of some of these boreholes are available. The Lower Pechelbronn Beds are composed of fluvial to lacustrine sediments, the Middle Pechelbronn Beds were deposited in a brackish to marine environment and the Upper Pechelbronn Beds consist of fluvial/alluvial to marine deposits. In between the western and eastern masterfaults of the Upper Rhine Graben several fault blocks exist, with fault orientation being sub-parallel to the graben shoulders. During the syntectonic deposition of the Pechelbronn Group these fault blocks acted as isolated depocenters, resulting in considerable thickness and depositional facies variations on the regional and local scale (few tens to several hundreds of meters). Laboratory measurements of sonic wave velocity, density, porosity, permeability, thermal conductivity and diffusivity are conducted on the core samples that are classified into lithofacies groups. Statistically evaluated petrophysical parameters are assigned to each group. The gamma-ray logs serve to verify the lithological classification and can further be used for correlation analysis or joint inversion with the petrophysical data. Well data, seismic sections, isolines and geological profiles are used to construct a geological 3-D model. It is planned to use the petrophysical, thermal and hydraulic rock properties at a later stage to parametrize the model unit and to determine, together with the temperature and thickness of the model unit, the expected flow rates and reservoir temperatures and thus the hydrothermal potential.

Experimental investigation on deformation characteristics and permeability evolution of rock under confining pressure unloading conditions

Deformation behavior and hydraulic properties of rock are the two main factors that influence safety of excavation and use of rock engineering due to in situ stress release. The primary objective of this study is to explore deformation characteristics and permeability properties and provide some parameters to character the rock under unloading conditions. A series of triaxial tests with permeability and acoustic emission signal measurement were conducted under the path of confining pressure unloading prior to the peak stress. Deformation behavior and permeability evolution in the whole stress—strain process based on these experimental results were analyzed in detail. Results demonstrate that, under the confining pressure unloading conditions, a good correspondence relationship among the stress—axial strain curve, permeability—axial strain curve and acoustic emission activity pattern was obtained. After the confining pressure was unloaded, the radial strain grew much faster than the axial strain, which induced the volumetric strain growing rapidly. All failures under confining pressure unloading conditions featured brittle shear failure with a single macro shear rupture surface. With the decrease in deformation modulus during the confining pressure unloading process, the damage variable gradually increases, indicating that confining pressure unloading was a process of damage accumulation and strength degradation. From the entire loading and unloading process, there was a certain positive correlation between the permeability and volumetric strain.

Physical and hydraulic properties of modern sinter deposits: El Tatio, Atacama

Sinters are siliceous, sedimentary deposits that form in geothermal areas. Formation occurs in two steps. Hot water circulates in the subsurface and dissolves silica from the host rock, usually rhyolites. Silica then precipitates after hot water is discharged and cools. Extensive sinter formations are linked to up-flow areas of fluids originating from high temperature (>175°C) deep reservoirs. Fluid geochemistry, microbial communities, and environmental conditions of deposition determine the texture of sinter and pore framework. Porosity strongly influences physical and hydraulic properties of rocks. To better understand the properties controlling the transport of fluids, and interpret geophysical observations in geothermal systems, we studied 17 samples of modern geyserite sinter deposits (<10ka) from the active El Tatio geothermal field in northern Chile. We measured the physical properties (hydraulic, seismic, and electrical), and internal microstructure (using μX-Ray computed tomography). We find that the pore structure, and thus hydraulic and physical properties, is controlled by the distribution of microbial matter. Based on velocity-porosity relationships, permeability-porosity scaling, and image analysis of the 3D pore structure; we find that the physical and hydraulic properties of sinter more closely resemble those of vesicular volcanic rocks and other material formed by precipitation in geothermal settings (i.e., travertine) than clastic sedimentary rocks.

Representative elementary volumes for evaluating effective seismic properties of heterogeneous poroelastic media

Understanding and quantifying seismic energy dissipation in fluid-saturated porous rocks is of considerable interest because it offers the perspective of extracting information with regard to the elastic and hydraulic rock properties. An important, if not dominant, attenuation mechanism prevailing in the seismic frequency band is wave-induced fluid pressure diffusion in response to the contrasts in elastic stiffness in the mesoscopic-scale range. An effective way to estimate seismic velocity dispersion and attenuation related to this phenomenon is through the application of numerical upscaling procedures to synthetic rock samples of interest. However, the estimated seismic properties are meaningful only if the underlying sample volume is at least of the size of a representative elementary volume (REV). In the given context, the definition of an REV and the corresponding implications for the estimation of the effective seismic properties remain largely unexplored. To alleviate this problem, we have studied the characteristics of REVs for a set of idealized rock samples sharing high levels of velocity dispersion and attenuation. For periodically heterogeneous poroelastic media, the REV size was driven by boundary condition effects. Our results determined that boundary condition effects were absent for layered media and negligible in the presence of patchy saturation. Conversely, strong boundary condition effects arose in the presence of a periodic distribution of finite-length fractures, thus leading to large REV sizes. The results thus point to the importance of carefully determining the REV sizes of heterogeneous porous rocks for computing effective seismic properties, especially in the presence of strong dry frame stiffness contrasts.

Brittle versus ductile deformation as the main control of the deep fluid circulation in oceanic crust

AbstractThe brittle to ductile transition may strongly influence hydraulic properties of rocks at the depth and temperature ranges that hydrothermal fluids circulate. To examine this transition in the context of the oceanic crust, we conducted a series of deformation experiments on a natural basalt sample at in situ oceanic crust conditions. Dilatancy was measured during deformation. The method consisted in monitoring the volume of pore fluid that flows into or out of the sample at constant pore pressure. Mechanical and microstructural observations at experimental constant strain rate of 10−5 s−1 indicated that the basalt was brittle and dilatant up to 800°C. At higher temperature, the deformation mode became macroscopically ductile and samples compacted. These observations have important implications on heat transfer and fluid migration in oceanic crust.

An analytical study of seismoelectric signals produced by 1-D mesoscopic heterogeneities

The presence of mesoscopic heterogeneities in fluid-saturated porous rocks can produce measurable seismoelectric signals due to wave-induced fluid flow between regions of differing compressibility. The dependence of these signals on the petrophysical and structural characteristics of the probed rock mass remains largely unexplored. In this work, we derive an analytical solution to describe the seismoelectric response of a rock sample, containing a horizontal layer at its center, that is subjected to an oscillatory compressibility test. We then adapt this general solution to compute the seismoelectric signature of a particular case related to a sample that is permeated by a horizontal fracture located at its center. Analyses of the general and particular solutions are performed to study the impact of different petrophysical and structural parameters on the seismoelectric response. We find that the amplitude of the seismoelectric signal is directly proportional to the applied stress, to the Skempton coefficient contrast between the host rock and the layer, and to a weighted average of the effective excess charge of the two materials. Our results also demonstrate that the frequency at which the maximum electrical potential amplitude prevails does not depend on the applied stress or the Skempton coefficient contrast. In presence of strong permeability variations, this frequency is rather controlled by the permeability and thickness of the less permeable material. The results of this study thus indicate that seismoelectric measurements can potentially be used to estimate key mechanical and hydraulic rock properties of mesoscopic heterogeneities, such as compressibility, permeability, and fracture compliance.

Sedimentary stylolite networks and connectivity in limestone: Large-scale field observations and implications for structure evolution

Stylolites are rough surfaces, formed by localized rock dissolution, and prevalent in carbonates and other sedimentary rocks. Their impact on porosity and permeability, and capacity to accommodate compactive strain, are well documented. This paper presents a meso-scale field study on sedimentary stylolites in carbonates, characterizing large-scale distributions of stylolites, including measurements conducted on longer than kilometer-long stylolites. Our field study suggests that on large scales connections between stylolites become important. Since connectivity, and also lack of connectivity, are expected to play a significant role in strain accommodation and hydraulic rock properties, we suggest that large-scale analysis may require a new characterization scheme for “stylolite populations”, based on their connectivity. We therefore divide sedimentary stylolite populations into three end-member types, which are correlated with the three possibilities for percolation of such systems: isolated stylolites (with zero percolation/connectivity), long-parallel stylolites (with 2-dimensional percolation/connectivity), and interconnected stylolite networks (with 3-dimensional percolation/connectivity). New statistical parameters and measures are devised and used to quantitatively characterize the different population types. Schematic mechanistic models are then offered to explain the evolution of the three end-member connectivity-classes. In addition we discuss the effect on fluid flow of the different population types.

Laboratory Measurements of Fluid Transport Properties on Tight Gas Sandstones and Applications

Deep gas reservoirs are of great interest for the E&P industry. Large areas of such reservoirs have permeabilities below 1 mD. The reservoir rocks in these areas show a strong stress sensitivity of the fluid transport properties and a considerable productivity decline due to changing stress conditions during the production process. For correct modeling and simulation of Tight Gas reservoirs it is important to know the behavior of the fluid transport properties under the changing stress condition the reservoir experiences. In several measurement series the effects of changing overburden and pore pressure on Rotliegend sandstone samples from north German Tight Gas reservoirs have been quantified and used to set up correlation functions. With the correlation functions from the own measurements and additional data and correlations from literature a Rock Data Catalog has been developed as tool to help reservoir engineers with modeling and simulation of such reservoirs. The Rock Data Catalog consists of the Rock Database and the Correlation Module. The Rock Database contains general and petrophysical rock data. The Correlation Module uses this data to generate secondary data of e.g. in-situ capillary and hydraulic rock properties with appropriate correlation functions.Viability of the economic gas production from Tight Gas Reservoirs strongly depends on reservoir quality. Therefore identification of high quality reservoir parts or so called Sweet Spots for placing production wells and planning hydraulic fracturing stimulation, is one of key issues of the tight gas reservoir characterization and evaluation. The data and correlation functions collected in the Rock Data Catalog could also be used to identify Sweet Spots in Tight Gas reservoirs. Several rock parameters and properties, which affect the fluid flow in a reservoir (like lithology, clay content, water saturation, permeability, pore size distribution) can be identified and used to set up a Sweet Spot Index as a measure for the reservoir quality.

Inference of permeability distribution from injection‐induced discrete microseismic events with kernel density estimation and ensemble Kalman filter

Hydraulic stimulation of subsurface rocks is performed in developing geothermal and hydrocarbon reservoirs to create permeable zones and enhance flow and transport in low‐permeability formations. Borehole fluid injection often induces measurable microearthquakes (MEQs). While the nature and source of the processes that lead to triggering of these events is yet to be fully understood, a major hypothesis has linked these events to an increase in pore pressure that decreases the effective compressional stress and causes sliding along preexisting cracks. Based on this hypothesis, the distribution of the resulting microseismicity clouds can be viewed as monitoring data that carry important information about the spatial distribution of hydraulic rock properties. However, integration of fluid‐induced microseismicity events into prior rock permeability distributions is complicated by the discrete nature of the MEQ events, which is not amenable to well‐established inversion methods. We use kernel density estimation to first interpret the MEQ data events as continuous seismicity density measurements and, subsequently, assimilate them to estimate rock permeability distribution. We apply the ensemble Kalman filter (EnKF) for microseimic data integration where we update a prior ensemble of permeability distributions to obtain a new set of calibrated models for prediction. The EnKF offers several advantages for this application, including the ensemble formulation for uncertainty assessment, convenient gradient‐free implementation, and the flexibility to incorporate various failure mechanisms and additional data types. Using several numerical experiments, we illustrate the suitability of the proposed approach for characterization of reservoir hydraulic properties from discrete MEQ monitoring measurements.

Microseismic estimates of hydraulic diffusivity in case of non-linear fluid-rock interaction

SUMMARY As a rule, rock stimulation by fluid injection induces microearthquakes. Assuming that pore-fluid pressure diffusion is responsible for this phenomenon, one can use seismicity to estimate the hydraulic properties of rocks. Previously, for the estimation of hydraulic transport properties from the triggering front, it has been assumed that they are independent of time and pressure. However, fluid injections can strongly change permeability of rocks (e.g. hydraulic fracturing). In this paper, we investigate what kind of diffusivity estimates are provided by the triggering front in cases where hydraulic transport properties are functions of pore-fluid pressure (i.e. they are changing during the injection). In this case, the pressure relaxation results in a non-linear pore-fluid pressure diffusion associated with heterogeneously and time-dependent distributed permeability. We consider numerically two models of pressure-dependent hydraulic diffusivity, a power law and an exponential law. We generate synthetic microseismicity by solving corresponding 1-, 2- and 3-D non-linear diffusion equations. Our results show that the triggering front provides reasonable estimates of the effective diffusivity approximately corresponding to the hydraulic diffusivity resulting from the stimulation (including hydraulic fracturing) of rocks and thus constitute a significant conceptual update of the seismicity-based reservoir characterization approach.

3D-Modellierung der tiefengeothermischen Potenziale von Hessen - Eingangsdaten und Potenzialausweisung

Within the scope of the research project “3D-modelling of the deep geothermal potentials of Hesse” the deep geothermal potential of the Federal State of Hesse was quantifi ed and assessed in a qualitative analysis. The quantifi cation of the heat stored under ground and the qualitative analysis was done for different geothermal systems. These are hydrothermal and petrothermal systems, as well as fault zones and deep borehole heat exchangers. For the assessment of the deep geothermal potential, knowledge of the geological structure and the geothermal properties of the potential reservoir rocks are indispensable. Therefore, a 3D model of the deep geothermal potential of the Federal State of Hesse (Germany) has been developed (cf. Arndt et al. 2011). Systematic measurements of thermophysical and hydraulic rock properties such as thermal conductivity, heat capacity and permeability of relevant geologic formations have been combined with in-situ temperature measurements, hydrothermal upwelling zones, characteristics of geological faults and were added to the 3D geological structural model. Using a multiple criteria approach, the various rock and reservoir properties were assessed incorporating their relevance for the different geothermal systems to allow the qualitative analysis. Therefore, threshold values for each parameter were defi ned specifying whether the potential is very high, high, medium, low or very low. This method was tested for the one-dimensional case (virtual drilling) and the two-dimensional case (geological-geothermical cross-sections) before being applied to the 3D model. The resulting geothermal model, which incorporates the quantifi cation and the qualitative analysis, is an important tool, which can be used at an early stage of the planning phase for the design of geothermal power plants. Furthermore, it allows quantifi cation of the deep geothermal potential and is intended to be an instrument for public information. Schlusselworter: Geothermie, geothermisches Potenzial, thermophysikalische Gesteinseigenschaften, hydraulische Kennwerte, geologisches 3D-Modell, Oberrheingraben, Hessen, Deutschland, GOCAD

Wavelet Reconstruction of Geologic Facies From Nonlinear Dynamic Flow Measurements

The discrete wavelet transform (DWT) that is widely used in compressing natural images is considered for an effective representation of the geological facies in subsurface flow and transport inverse modeling problems. The inference of the heterogeneous hydraulic rock properties from the scattered dynamic measurements of the flow rates and pressures is a frequently encountered ill-posed inverse problem in subsurface characterization. To better pose this inverse problem, the original grid-based description of the spatial facies maps is replaced with a small number of DWT coefficients that are estimated from indirect nonlinear dynamic measurements. The compressed description of the facies in the wavelet domain after removing the unresolvable high-frequency components leads to an inverse problem with fewer parameters to resolve and improved geologic facies continuity. The main difficulty in the application of the DWT to inverse problems is the lack of sufficient data to resolve higher frequency detail coefficients. Prior information and sensitivity of the flow response to variation in the DWT coefficients are used to infer the location and value of the significant DWT coefficients. The results suggest that the large-scale geologic facies description that control the global flow pattern can be successfully inferred from the dynamic measurements in a reduced wavelet domain. While the flow data may contain information about significant DWT coefficients, a limited observability in ill-posed inverse problems may not allow the identification of these coefficients and the corresponding local spatial features. Therefore, an effective exploitation of the space-frequency localization advantage of the wavelets over the Fourier bases may not be available in solving ill-posed inverse problems.

Effective solution of nonlinear subsurface flow inverse problems in sparse bases

Identification of spatially variable hydraulic rock properties such as permeability and porosity is necessary for accurate prediction of fluid flow displacement in subsurface environments. The estimation of these properties from dynamic flow data usually involves solving a highly underdetermined nonlinear inverse problem in which a limited set of measurements is combined with prior knowledge to estimate the unknown parameters. The overwhelming number of unknowns, relative to available data, leads to many parameter combinations that explain the data equally well, but fail to predict the flow behavior in the reservoir. To improve solution non-uniqueness and numerical stability, additional information is typically incorporated into the solution procedure. One way to regularize underdetermined inverse problems is to demand certain structural properties from the solution that are usually derived from the physics of the problem. In this paper, we exploit the compact representation of spatially correlated geologic formations in sparse bases to facilitate the reconstruction of rock hydraulic property distributions from flow measurements that are nonlinearly related to unknown parameters. By formulating the solution in a compressive basis such as the wavelet or Fourier, we show that minimizing a data misfit cost function augmented with an additive or multiplicative regularization term that promotes sparse solutions, the reconstruction results can be improved. Convergence to a relevant sparse solution is adaptively carried out through an iteratively reweighting algorithm in the transform domain. We evaluate the performance of our inversion algorithm using a set of two-phase waterflooding experiments in an oil reservoir where nonlinear dynamic flow data are integrated to infer the spatial distribution of rock permeability.

Mine Water Utilization for Geothermal Purposes in Freiberg, Germany: Determination of Hydrogeological and Thermophysical Rock Parameters

Flooded underground mines provide an innovative opportunity to extract low-grade geothermal energy. To evaluate these geothermal systems, in situ rock sampling and the investigation of thermal (heat conduction) and hydraulic rock properties (thermal convection) is essential. This paper discusses how these geothermal parameters were determined using Freiberg grey gneiss rock samples from the Alter Tiefer Furstenstolln gallery, where a geothermal mine water project has been successfully carried out in Spring, 2009. Unsteady autoclaving with the unsteady-state two-box method allowed us to measure the permeability of the nearly impermeable gneiss. As expected, samples drilled perpendicular to the rock foliation plane had lower permeabilities than those drilled parallel. Furthermore, it was demonstrated that the permeability varied due to the existence of fissures and the amount of the mantle pressure. The thermal conductivity was also anisotropic, and was higher in samples drilled parallel to the foliation than in those drilled perpendicular to it. Since gneiss is nearly nonporous, the effect of waterlogged pores on the thermal conductivity is minimal. The radiogenic heat production of the sampled Freiberg grey gneiss was found to lie in the lower half of the range of upper crustal metamorphic rocks.

Stochastic evaluation of fluvial to marginal marine sealing facies

Hydrocarbon top seal analyses are particularly difficult in fluvial and marginal marine settings. Although these settings often contain beds comprised of a low-permeability, clay-rich lithology associated with high quality seals, their presence does not guarantee an effective seal because individual beds within the stratal unit may be laterally discontinuous. Lateral facies changes, truncation of fine-grained beds, or the presence of crevasse splays may provide cross-stratal migration pathways, reducing the effective sealing capacity to a magnitude less than the closure height. To evaluate probable bed seal capacities in such a setting, we have calculated 75 statistically equivalent geologic models of a heterogeneous bed seal based upon facies interpretations derived from measured data. Measured capillary and hydraulic properties of rocks in each facies were then fit to Gaussian distributions. We populated the geologic models with flow properties by randomly selecting from these distributions. Two realizations of the rock property distributions were performed for each of the 75 object-based geologic models. Gas charge was simulated in each of the rock property model trials using a percolation-based hydrocarbon migration software program. The results of the simulations return a probability distribution of bed seal capacities, measured in equilibrium gas column heights, from which cumulative frequency diagrams and percentile rankings may be derived. Channel amalgamation by incision juxtaposing levee overbank and/or channel facies formed the most common type of cross-stratal leak point. A factor of 10 difference in the P50 seal capacity was observed by varying the net volume fraction of channels from 10% to 30%. Higher seal capacity realizations were found to have a low density of channels near the structural crest, a variable which outweighs all other stochastic parameters. These results highlight the importance of a risking-based methodology to account for variability in bed seal stratigraphy in fluvial to marginal marine settings.

On the relationship between stress and elastic strain for porous and fractured rock

Modeling the mechanical deformations of porous and fractured rocks requires a stress–strain relationship. Experience with inherently heterogeneous earth materials suggests that different varieties of Hooke's law should be applied within regions of the rock having significantly different stress–strain behavior. We apply this idea by dividing a rock body conceptually into two distinct parts. The natural strain (volume change divided by rock volume at the current stress state), rather than the engineering strain (volume change divided by the unstressed rock volume), should be used in Hooke's law for accurate modeling of the elastic deformation of that part of the pore volume subject to a relatively large degree of relative deformation (i.e., cracks or fractures). This approach permits the derivation of constitutive relations between stress and a variety of mechanical and/or hydraulic rock properties. We show that the theoretical predictions of this method are generally consistent with empirical expressions (from field data) and also laboratory rock experimental data.

Rapid natural acid weathering, physical erosion, and debris-flow hazards in scar areas developed on hydrothermally-altered rocks along the Red River Valley near Questa, New Mexico, USA

In southern Rocky Mountains, catchments characterized by acidic, metalliferous waters that are relatively unaffected by human activity usually occur within areas that have active or historical mining activity. The US Geological Survey has utilized these mineralized but unmined catchments to constrain geochemical processes that control the surfaceand ground-water chemistry associated with near surface acid weathering as well as to estimate premining conditions. Study areas include the upper Animas River watershed, Lake City, Mt. Emmons, and Montezuma in Colorado and Questa in New Mexico. Although host-rock lithologies range from Precambrian gneisses to Cretaceous sedimentary units to Tertiary volcanic complexes, mineralization is Tertiary in age and associated with intermediate to felsic composition, porphyritic plutons. Pyrite is ubiquitous. Variability of metal concentrations in water is caused by two main factors: mineralogy and hydrology. Parameters that potentially affect water chemistry include: host-rock lithology, intensity of hydrothermal alteration, sulfide mineralogy and chemistry, gangue mineralogy, length of flow path, precipitation, evaporation, and redox conditions. Springs and headwater streams have pH values as low as 2.5, sulfate up to 3700 mg/L and high dissolved metal concentrations (for example: A1 up to 170 rag/L; Fe up to 250 mg/L; Cu up to 3.5 mg/L and Zn up to 14 mg/L). With the exception of evaporative waters, the lowest pH values and highest Fe and A1 concentrations occur in water draining the most intense hydrothermally altered areas consisting of the mineral assemblage quartz-sericite-pyrite. Stream beds tend to be coated with iron floc, and some reaches are underlain by ferricrete. When iron-rich ground water interacts with oxygenated waters in the stream or hyporheic zone, ferrous iron is oxidized to ferric iron, which is less soluble, leading to the precipitation of iron oxyhydroxides. Ground-water wells have been drilled and sampled in two unmined, alpine catchments to characterize constituent concentrations, to identify hydrogeochemical processes controlling constituent concentrations, to determine rock hydraulic properties, and to delineate flow paths. By using an integrated approach to investigating surface and ground waters in these acidic catchments a more complete understanding of the hydrogeologic framework is gained.

Fluid-Induced Seismicity: Theory, Modeling, and Applications

Operations including borehole fluid injections are typical for exploration and development of hydrocarbon or geothermic reservoirs. Microseismicity occurring during such operations has a large potential for understanding physics of the seismogenic process as well as in obtaining detailed information about reservoirs at locations as far as several kilometers from boreholes. We propose that the phenomenon of microseismicity triggering by borehole fluid injections is related to the process of the Frenkel—Biot slow wave propagation. In the low-frequency range (hours or days of fluid injection durations) this process reduces to the pore-pressure diffusion. We search for diffusion-related features of induced microseismicity. Two types of such signatures are considered. The first one is related to the geometry of microseismic clouds. Another type of signature is related to the probability of microearthquakes. On this basis we introduce a concept for interpretation of microseismic data which provides a possibility to infer information about hydraulic properties of rocks. Such information can be of significant importance for industrial applications and for understanding physical properties of geological structures.

Characterization of hydraulic properties of rocks using probability of fluid-induced microearthquakes

The use of borehole fluid injections is typical for exploration and development of hydrocarbon or geothermal reservoirs. Such injections often induce small-magnitude earthquakes. The nature of processes leading to triggering of such microseismicity is still not completely understood. Here, we consider induced microseismicity, using as examples two case studies of geothermal reservoirs in crystalline rocks and one case study of a tight-gas sandstone reservoir. In all three cases, we found that the probability of induced earthquakes occurring is very well described by the relaxation law of pressure perturbation in fluids filling the pore space in rocks. This strongly supports the hypothesis of seismicity triggered by pore pressure. Moreover, this opens additional possibilities of using passive seismic monitoring to characterize hydraulic properties of rocks on the reservoir scale with high precision.