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2006 | Book

Rock Damage and Fluid Transport, Part II Read chapter Read first chapter

Rock Damage and Fluid Transport, Part II

Editors: Arno Zang, Ove Stephansson, Georg Dresen

Publisher: Birkhäuser Basel

Book Series : Pageoph Topical Volumes

Part of: Springer Professional "Wirtschaft+Technik" , Springer Professional "Technik"

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About this book

Mechanical properties and fluid transport in rocks are intimately linked as deformation of a solid rock matrix immediately affects the pore space and permeability. The coupling of fluid circulation and deformation processes in crustal rocks results in significant complexity of the mechanical and fluid transport behavior. This often poses severe technical and economic problems for reservoir and geotechnical engineering projects involved in oil and gas production, CO2 sequestration, mining and underground waste disposal. The volume results from the 5th Euroconference on Rock Physics and Geomechanics, which was held in Potsdam, Germany in September 2004.

Part I of the topical volume mainly contains contributions investigating the nucleation and evolution of crack damage in rocks, new or modified techniques to measure rock fracture toughness and a discussion of upscaling techniques relating mechanical and fluid transport behavior in rocks at different spatial scales.

Part II contains contributions discussing fluid flow and transport in rocks as observed on the laboratory scale and in boreholes. The evolution of rock damage pertinent to the stability of underground excavations is studied and scaling relations of elastic properties and seismic events are discussed.

Table of Contents

Frontmatter
Rock Damage and Fluid Transport, Part II
Abstract
This volume forms the second part of a topical issue generated from contributions presented at the Euroconference on Rock Physics and Geomechanics that was held at the GeoForschungsZentrum Potsdam, Germany in September 2004. Part I of this volume (Pure and Applied Geophysics, Volume 163, No. 5) mainly contains papers related to problems of rock damage, fracture mechanics, and stresses in rock. Part II contains contributions on transport properties of fractured rocks which were studied with advanced techniques at the laboratory scale and the field scale. Elsworth and Yasuhara conduct circulation experiments on fractures in novaculite and limestone, intermittently imaged by X-ray computer tomography. Xiao et al. measure permeability, storage capacity, volumetric strain and creep during the deformation of hot-pressed calcite samples with different volume fractions of quartz. Song and Renner investigate the influence of heterogeneities on hydraulic properties of Fontainebleau sandstone applying harmonic pore pressure testing. On a larger scale, Albrecht and Mansurov estimate porosity and permeability analysing the velocity dispersion of elastic waves near boreholes. Huenges et al. monitor fluid pressure variations in a borehole sunk in geothermal reservoir rocks of the Rotliegend formation in NE-Germany.
Arno Zang, Ove Stephansson, Georg Dresen
Short-Timescale Chemo-Mechanical Effects and their Influence on the Transport Properties of Fractured Rock
Abstract
Anomalous changes in permeability are reported in fractures circulated by fluids undersaturated with respect to the mineral host. Under net dissolution and net removal of mineral mass, fractures may alternately gape or seal, depending on the prevailing mechanical and chemical conditions. The influence on transport properties is observed to be large, rapid, and irreversible: Permeabilities may change by two orders of magnitude in a month, and the direction of permeability change may switch spontaneously, for no apparent change in environmental forcing. These behaviors are apparent in continuous circulation experiments conducted on fractures in novaculite and limestone, intermittently imaged by X-ray CT. In novaculite, permeability reduces by two orders of magnitude as silica is net removed from the sample. Surprisingly, these changes can occur at modest temperatures (∼80°C) and stresses (∼3.5 MPa), where compaction progresses as temperatures are incremented. Isothermal (∼20°C) circulation tests in limestone show similar compaction driven by pressure solution. Where circulation remains undersaturated in Ca, the change in permeability spontaneously switches from net reduction to net increase as a wormhole forms. The surprising magnitude and rapidity of these changes are investigated in the context of the competition between stress- and chemistry-mediated effects.
Derek Elsworth, Hideaki Yusuhara
Permeability Evolution During Non-linear Viscous Creep of Calcite Rocks
Abstract
Permeability, storage capacity and volumetric strain were measured in situ during deformation of hot-pressed calcite aggregates containing 10, 20, and 30 wt% quartz. Both isostatic and conventional triaxial loading conditions were used. The tests were performed at confining pressure of 300 MPa, pore pressures between 50 to 290 MPa, temperatures from 673 to 873 K and strain rates of 3 × 10−5 s−1. Argon gas was used as the pore fluid. The initial porosities of the starting samples varied from 5% to 9%, with higher porosity correlated to higher quartz content. Microstructural observations after the experiment indicate two kinds of pores are present: 1) Angular, crack-like pores along boundaries between quartz grains or between quartz and calcite grains and 2) equant and tubular voids within the calcite matrix. Under isostatic loading conditions, the compaction rate covaries with porosity and increases with increasing effective pressure. Most of the permeability reduction induced during compaction is irreversible and probably owes to plastic processes. As has been found in previous studies on hot-pressed calcite aggregates, permeability, k, is nonlinearly related to porosity, ø. Over small changes in porosity, the two parameters are approximately related as \( k{\mathbf{ }}\alpha {\mathbf{ }}\phi ^n \) . The exponent n strongly increases as porosity decreases to a finite value (from about 4 to 6% depending on quartz content), suggesting a porosity percolation threshold. When subjected to triaxial deformation, the calcite-quartz aggregates exhibit shearenhanced compaction, but permeability does not decrease as rapidly as it does under isostatic conditions. During triaxial compaction the exponent n only varies between 2 and 3. Non-isostatic deformation seems to reduce the percolation threshold, and, in fact, enhances the permeability relative to that at the same porosity during isostatic compaction. Our data provide constraints on the governing parameters of the compaction theory which describes fluid flow through a viscous matrix, and may have important implications for expulsion of sedimentary fluids, for fluid flow during deformation and metamorphism, and melt extraction from partially molten rocks.
Xiaohui Xiao, Brian Evans, Yves Bernabé
Experimental Investigation into the Scale Dependence of Fluid Transport in Heterogeneous Rocks
Abstract
We investigated the dependence of hydraulic properties on the spatial scale of intrinsic and artificial heterogeneity, applying harmonic pore pressure testing to two varieties of Fontainebleau sandstone at various periods and effective pressures. Blocks with porosities of about 5 and 8% were chosen exhibiting a permeability of about 2·10−19 and 1·10−13 m2, respectively. The permeability of the less permeable variety strongly depends on sample size. Artificial heterogeneous rock samples were prepared by stacking pieces of the two sandstone varieties perpendicular and parallel to the main flow direction. The perturbation of the fluid flow owing to the interfaces between pieces of the same variety is controlled by the orientation of and subordinately by the effective stress on the interfaces. Constraints on conduit geometry derived from the effect of interfaces indicate that interconnectivity is more important than pore radius at the lower porosity. The effective permeability of alternating stacks of the two varieties differs tremendously for the two interface orientations; arithmetic and harmonic averages coincide with the effective properties parallel and perpendicular to the main flow, respectively. When the oscillation period is varied two regimes are observed, one with constant permeability at long periods and a second with decreasing permeability for decreasing period at short periods. Order of magnitude considerations of penetration depth suggest that this period dependence may be related to heterogeneity.
Insun Song, Jörg Renner
Determination of Porosity and Permeability out of Dispersion Analyses of Borehole Wall Displacements
Abstract
Waves carried in the borehole can be measured by the use of controlled sources inside the borehole and receivers located in the same borehole as well. This article gives the theoretical background and develops a new method to determine rock-physical parameters out of such controlled measurements of waves carried inside the borehole. Theoretical dispersion curves of the group velocities are matched to dispersion analyses of registrations obtained in the borehole. This inversion process intends to determine the in situ porosity and permeability of the rock.
Michael Albrecht, Vladimir Mansurov
Fluid Pressure Variation in a Sedimentary Geothermal Reservoir in the North German Basin: Case Study Groß Schönebeck
Abstract
The Rotliegend of the North German basin is the target reservoir of an interdisciplinary investigation program to develop a technology for the generation of geothermal electricity from low-enthalpy reservoirs. An in situ downhole laboratory was established in the 4.3 km deep well Groß Schönebeck with the purpose of developing appropriate stimulation methods to increase permeability of deep aquifers by enhancing or creating secondary porosity and flow paths. The goal is to learn how to enhance the inflow performance of a well from a variety of rock type in low permeable geothermal reservoirs. A change in effective stress due to fluid pressure was observed to be one of the key parameters influencing flow properties both downhole, and in laboratory experiments on reservoir rocks. Fluid pressure variation was induced using proppant-gel-frac techniques as well as waterfrac techniques in several different new experiments in the borehole. A pressure step test indicates generation and extension of multiple fractures with closure pressures between 6 and 8.4 MPa above formation pressure. In a 24-hour production test 859 m3 water was produced from depth indicating an increase of productivity in comparison with former tests. Different depth sections and transmissibility values were observed in the borehole depending on fluid pressure. In addition, laboratory experiments were performed on core samples from the sandstone reservoir under uniaxial strain conditions, i.e., no lateral strain, constant axial load. The experiments on the borehole and the laboratory scale were realized on the same rock types under comparable stress conditions with similar pore pressure variations. Nevertheless, stress dependences of permeability are not easy to compare from scale to scale. Laboratory investigations reflect permeability variations due to microstructural heterogeneities and the behavior in the borehole is dominated by the generation of connections to large-scale structural patterns.
Ernst Huenges, Ute Trautwein, Björn Legarth, Günter Zimmermann
Characterisation of Tensile Damage in Rock Samples Induced by Different Stress Paths
Abstract
Cracks, induced by different stress states, are studied systematically on thin slices of crinoidal limestone samples. Samples damaged by tension are compared to samples in which first damage is induced by compressive stresses, followed by tensile stresses. To quantify the effect of the compressive stresses, samples only damaged by compression are also studied. The tensile stresses cause the occurrence of an intergranular crack. When the sample is first damaged by compressive stresses, former induced damage influences the growth of this crack. It results in an intergranular crack with a zigzag profile. The average orientation of the intergranular crack is, however, the same as in the samples damaged by tension only.
Patrick Ganne, André Vervoort
Monitoring Fracture Propagation in a Soft Rock (Neapolitan Tuff) Using Acoustic Emissions and Digital Images
Abstract
Sudden and unexpected collapses of underground cavities below the city of Naples (Italy) represent a serious safety hazard. The collapses occur due to the detachment of large blocks from the cavity roofs, walls and pillars, often a long time after the original quarry excavation has been completed. It is recognised that existing discontinuities, e.g., fractures, play an important role in the failure process by inducing local stress-concentrations and reducing the overall material strength. The larger fractures, which ultimately lead to collapse occur through interaction, propagation and coalescence of these discontinuities. This paper presents recent results of experiments carried out on natural, dry specimens of Neapolitan finegrained tuff tuff to investigate the mechanisms involved in sample failure. A better understanding of fracture development and rock bridge behaviour is gained through a combination of AE and photographic monitoring in an experimental program considering samples with artificial pre-existing heterogeneities, which simulate the in situ discontinuities. For a range of rock bridge geometries the mechanisms and timing of different stages of the failure process are identified and characterised. The results show that, in general, a classical description of failure, for samples without artificial flaws or with only a single flaw, is followed: (1) crack closure; (2) linear stress-strain response and crack initiation with stable crack growth; (3) crack damage and unstable crack growth leading to failure. For samples with two artificial pre-existing flaws the third phase is split into two parts and failure of the sample occurs only after both the unstable propagation of external wing cracks and coalescence of the internal cracks in the bridge. In terms of the timing and duration of each phase, it is seen that phases 1 and 2 have little dependence on the flaw configuration but phase 3 seems to depend directly on this. In particular the angle in rock bridge between the inner tips of the pre-existing flaws, β, plays a key role: phase 3 is shorter for β=120° than for β=105°. These differences by bridge rotation, which appears to take longer than the simpler mode of failure for β=120°. It has only been possible to determine the time ranges of interest using the AE signatures, whilst the photographs allow the fracture geometry evolution to be described. Additionally the frequency character of AE events is investigated and shown to have significant potential for characterisation of AE source types and thus failure processes.
Stephen A. Hall, Fiorenza de Sanctis, Gioacchino Viggiani
Understanding the Seismic Velocity Structure of Campi Flegrei Caldera (Italy): From the Laboratory to the Field Scale
Abstract
We report laboratory measurements of P- and S-wave velocities on samples of tuff from Campi Flegrei (Italy), and a new tomographic velocity map of the Campi Flegrei caldera. Laboratory measurements were made in a hydrostatic pressure vessel during both increasing and decreasing effective pressure cycles. Selected samples were also thermally stressed at temperatures up to 600°C to induce thermal crack damage. Acoustic emission output was recorded throughout each thermal stressing experiment, and velocities were measured after thermal stressing. Laboratory P- and S-wave velocities are initially low for the tuff, which has an initial porosity of ∼45%, but both increase by between 25 and 50% over the effective pressure range of 5 to 80 MPa, corresponding to a decrease of porosity of ∼70%. Marked velocity hysteresis, due to inelastic damage processes, is also observed in samples subjected to a pressurization-depressurization cycle. Tomographic seismic velocity distributions obtained from field recordings are in general agreement with the laboratory measurements. Integration of the laboratory ultrasonic and seismic tomography data indicates that the tuffs of the Campi Flegrei caldera can be water or gas saturated, and shows that inelastic pore collapse and cracking produced by mechanical and thermal stress can significantly change the velocity properties of Campi Flegrei tuffs at depth. These changes need to be taken into account in accurately interpreting the crustal structure from tomographic data.
S. Vinciguerra, C. Trovato, P.G. Meredith, P.M. Benson, C. Troise, G. De Natale
Spatial Scaling of Effective Modulus and Correlation of Deformation Near the Critical Point of Fracturing
Abstract
Many observations point to the lithosphere being metastable and close to a critical mechanical point. Exercises in modelling deformation, past or present, across subsurface reservoirs need to take account of this criticality in an efficient way. Using a renormalization technique, the spatial scaling of effective elastic modulus is derived for 2-D and 3-D bodies close to the critical point of through-going fracturing. The resulting exponent, dµ, of spatial scaling of effective modulus with size, L d µ, takes the values ~−2.5 and−4.2 in two- and three-dimensional space, respectively. The exponents are compatible with those for scaling of effective modulus with fracture density near the percolation threshold determined by other workers from numerical experiments; the high absolute values are also approximately consistent with empirical data from a) fluctuations in depth of a seismic surface; b)-‘1/k’ scaling of heterogeneities observed in one-dimensional well-log samples; c) spatial correlation of slip displacements induced by water injection. The effective modulus scaling modifies the spatial correlation of components of displacement or strain for a domain close to the critical point of fracturing. This correlation function has been used to geostatistically interpolate components of the strain tensor across subsurface reservoirs with the prime purpose of predicting fracture densities between drilled wells. Simulations of strain distributions appear realistic and can be conditioned to surface depths and observations at wells of fracture-related information such as densities and orientations, welltest permeabilities, changes in well-test permeabilities, etc.
Kes Heffer, Peter King
Analysis of Temporal Structures of Seismic Events on Different Scale Levels
Abstract
A statistical model for describing the energy scaling of the distribution of inter-event times is described. By considering the diverse region seismicity (natural and induced) on different scale (energy/ magnitude) levels the self-similarity of the distribution has been determined. A comparison between the distribution of inter-event times on different scale levels and the most popular distributions of reliability theory has been carried out. The distribution of inter-event times for different scale levels is well approximated by the Weibull distribution. The Weibull distribution, with parameters which obey the scaling model and the Gutenberg-Richter law, has been tested.
V.I. German
Metadata
Title
Rock Damage and Fluid Transport, Part II
Editors
Arno Zang
Ove Stephansson
Georg Dresen
Copyright Year
2006
Publisher
Birkhäuser Basel
Electronic ISBN
978-3-7643-8124-0
Print ISBN
978-3-7643-7993-3
DOI
https://doi.org/10.1007/978-3-7643-8124-0