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T h e r m o - H y d r o - M e c h a n i c al Coupling in F r a c t u r ed R o ck Edited by Hans-Joachim Kumpel Springer Basel AG Reprint from Pure and Applied Geophysics (PAGEOPH), Volume 160 (2003), No. 5-6 Editor(s): Hans-Joachim Kumpel Leibniz-Institute for Applied Geosciences (GGA) Stilleweg 2 D-30655 Hannover Germany e-mail: [email protected] A CIP catalogue record for this book is available from the Library of Congress, Washington D.C., USA Bibliographic information published by Die Deutsche Bibliothek Die Deutsche Bibliothek lists this publication in the Deutsche Nationalbibliographie; Detailed bibliographic data is available in the Internet at http://dnb.ddb.de ISBN 978-3-7643-0253-5 ISBN 978-3-0348-8083-1 (eBook) DOI 10.1007/978-3-0348-8083-1 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, re-use of illustrations, recitation, broadcasting, reproduction on microfilms or in other ways, and storage in data banks. For any kind of use, permission of the copyright owner must be obtained. © 2003 Springer Basel AG Originally published by Birkhäuser Verlag in 2003 Printed on acid-free paper produced from chlorine-free pulp 9 8 7 6 5 4 3 2 1 www.birkhauser-science.ch Contents 809 Introduction H.-J. Kiimpel 813 Borehole Breakouts in Berea Sandstone Reveal a New Fracture Mechanism B. C. Haimson 833 A Model for the Mechanical Behaviour of Bentheim Sandstone in the Brittle Regime E. Klein, T. Reusch!e 851 Failure Model and Spatial Distribution of Damage in Rothbach Sandstone in the Brittle-ductile Transition P. Besuelle, P. Baud, T. Wong 869 Velocity Measurements and Crack Density Determination During Wet Triaxial Experiments on Oshima and Toki Granites A. Schubnel, o. Nishizawa, K. Masuda, X. J. Lei, Z. Xue, Y. Gueguen 889 Permeability of Triaxially Compressed Sandstone: Influence of Deformation and Strain-rate on Permeability J. Heiland 909 Comparison of Measured and Modelled Hydraulic Conductivities of Factured Sandstone Cores S. Baraka-Lokmane, R. Liedl, G. Teutsch 929 An Integrated Study on Physical Properties of a KTB Gneiss Sample and Marble from Portugal: Pressure Dependence of the Permeability and Frequency Dependence of the Complex Electrical Impedance S. Heikamp, G. Nover 937 Permeability-porosity Relationships in Rocks Subjected to Various Evolution Processes Y. Bernabe, U. Mok, B. Evans 961 Slow Crack Propagation and Slip Correlations J. Schmittbuhl, A. Delaplace, K. J. Maloy, H. Perfettini, J. P. Vi/otte 977 Pressure Oscillation Effects on the Saffman-Taylor Instability N. Gland, D. Pisarenko 989 A Unified Model for Characterisation and Mechanical Behaviour of Rock Fractures F. Lanaro, O. Stephansson 999 Analytical Model for Permeability Evolution in Microcracking Rock G. D. H. Simpson, Y. Gueguen, F. Schneider 1009 A Simple Model for Deviations from the Cubic Law for a Fracture Undergoing Dilation or Closure S. Sisavath, A. Al-Yaarubi, C. C. Pain, R. W. Zimmerman 1023 Scale Effects Related to Flow in Rough Fractures Y. Meheust, J. Schmittbuhl 1051 Triggering of Seismicity by Pore-pressure Perturbations: Permeability-related Signatures of the Phenomenon S. A. Shapiro, R. Patzig, E. Rothert, J. Rindschwentner 1067 Scale Dependence of Hydraulic and Structural Parameters in the Crystalline Rock of the KTB G. Zimmermann, H. Burkhardt, L. Engelhard 1087 Mechanisms of Pore Pressure-stress Coupling which Can Adversely Affect Stress Measurements Conducted in Deep Tunnels K. Evans, T. Dahlo, J.-A. Roti 1103 Role of Stress-controlled Flow Pathways in HDR Geothermal Reservoirs T. Ito, K. Hayashi 1125 Porosity and Thermal Conductivity of the Soultz-sous-Forets Granite F. Surma, Y. Geraud 1137 Interrelations Between Thermal Conductivity and Other Physical Properties of Rocks: Experimental Data Y. Popov, V. Tertychnyi, R. Romushkevich, D. Korobkov, J. Pohl © Birkhauser Verlag, Basel, 2003 Pure appl. geophys. 160 (2003) 809-812 I Pure and Applied Geophysics 0033-4553/03/060809-4 Special issue 'Thermo-Hydro-Mechanical Coupling in Fractured Rock' Introduction The supply and protection of groundwater, the production of hydrocarbon reservoirs, land subsidence in coastal areas, exploitation of geothermal energy and the long-term disposal of critical wastes or of C02 at depth are all issues of obviously high socio-economic relevance. They all have in common to be closely related to fluid flow in porous and/or fractured rock. In many cases the conditions of fluid flow depend on the rheological behavior of rocks. Accordingly, mechanical coupling between the liquid phase and the rock matrix can generally not be neglected. The subject is receiving increasing interest from many researchers and reservoir engineers. Comprehensive insight into coupled processes of rocks and fluids is often hampered due to the fact that rocks and rock formations are enormously complex. Each rock sample consists of a myriad of mineral particles, forming its matrix, and of fluid molecules residing in voids. Any two rock samples differ in their geochemical constituents, size, and shape of grains, structure of pore space, fracture networks, etc. Dealing with real rock material often means studying a system of a virtually infinite number of unknowns. How do geologists, petrophysicists, reservoir engineers handle such material? A series of three Euroconferences on themes related to rock physics, rock mechanics and fluid flow in rocks, all supported by the European Union, took place in the years 1998, 1999, and 2000 (BOUTECA and GUEGUEN, 1999; COUPLES, MAIN, and MEREDITH, 2000; KUMPEL, 2001). For the last event, approximately eighty researchers met in November 2000 at Bad Honnefnear Bonn, Germany, to exchange their latest knowledge and findings regarding thermo-hydro-mechanical coupling in fractured rock. Thematic sessions during this meeting were - Experiments and models of induced fracturing - Fracture networks and transport properties - Coupling between thermal, mechanical, and hydraulic properties. The participants agreed that new insights are generally based on both, experiments and numerical modeling, from the micro- to the macro- scale. For instance, the development of fractures around boreholes and the formation of H.-J. Kümpel (ed.), Thermo-Hydro-Mechanical Coupling in Fractured Rock © Springer Basel AG 2003 810 Hans-Joachim KiimpeJ Pure app!. geophys., compaction bands in sandstones as a function of porosity and grain size was reported to occur in a surprisingly variable manner. Laboratory tests have confirmed that deformation can be extremely localized, even for seemingly homogeneous rocks. Coupling between rock stress and fluid flow although affected by various petrophysical quantities has been shown to be strongly dependent on the geometry of fracture networks. Observations in a gallery have revealed that all permeable fractures in crystalline rock were critically stressed so that they can be thought as having formed under stress, whereas many non-permeable fractures were found to be critically stressed as well, testifying the transient character of the status quo. Also, seismicity that has been induced by massive hydraulic tests in many boreholes worldwide appears to indicate a low mechanical stability of water saturated crustal formations even down to depths of nine kilometers. More general conclusions of the Bad Honnef Conference concern the suitability of standard methods and of widely used parameters in describing coupled phenomena in rocks. Models of a compact rock matrix with connected voids and those of bonded aggregates of individual grains are two opposing, though valid approaches to simulate transport of liquids through a porous medium. Application of stochastic models and effective media concepts are important means to bridge spatial gaps ranging from millimeters to kilometers, and the dynamics of processes on time scales from seconds to hundreds of years. However, a fundamental effective parameter like permeability, which by definition is a measure of average transport of fluid volume through a hydraulically conductive medium per time increment, may not always be a useful quantity. Large scale field experiments have shown that fluid flow may be highly channelized and that, depending on the specific site conditions, traditional upscaling can give erroneous results. Ultimately, better understanding of the various physical processes in fractured rock helps to improve predictions on how fluids can best be pumped out of or injected into subsurface reservoirs, in which way toxic fluids are absorbed or released by geologic formations, or to what extent rocks compact when pore fluids are extracted. Twenty articles, most of which were presented at the Bad Honnef Conference, are contained in this special issue. Only a few of them deal with fully thermo-hydro mechanical coupling. Rather, the individual investigations present contributions to processes where such coupling is relevant. The special issue in whole covers aspects of coupling from various points of view. Recent progresses made in understanding of coupled phenomena in rock physics/rock mechanics is well documented in this volume. The first four articles describe findings from laboratory experiments that are related to different types of sandstones and granites. They include processes like the formation of compaction bands (contribution no. 1), fracture mechanics in the brittle deformation regime (2), the spatial distribution of damage zones in stressed rock samples (3), and the damage behavior of wet granites submitted to deviatoric stress Vol. 160,2003 Introduction 811 (4). The next three papers extend these views by taking a closer look on parameters that govern hydraulic diffusivity in sandstones and other types of rocks. Specific targets addressed are the influence of differential stress on permeability (5), imaging of the fracture geometry (6), and pressure induced variations in the pore geometry (7). Contributions no. 8 to 10 cover investigations of permeability-porosity relationships during rock evolution (8), of the formation, propagation, and roughness of fractures in a plexi-glass block (9), and pressure oscillation effects of two-phase flow under controlled conditions (10). The subsequent four articles focus on diverse modeling approaches. Issues considered are how the geometry and the mechanical behavior of fractures can be characterized by mathematical expressions (11), how the evolution of permeability in a microcracking rock can be expressed by an analytical model (12), deviations from the cubic law for a fracture of varying aperture (13), and the numerical simulation of scale effects in flow through fractures (14). Three further papers refer to in situ observations, being related to topics as the assessment of in situ permeability from the spatio temporal distribution of an aftershock sequence (15), to the scale dependence of hydraulic pathways in crystalline rock (16), and to the significance of pore pressure - stress coupling in deep tunnels and galleries (17). The last three contributions deal with thermal aspects, namely, stress controlled pathways of fluid flow in Hot-Dry-Rock reservoirs (18), relationships between porosity and thermal conductivity for granite (19), and measurements of thermal conductivity through optical scanning (20). Many problems in rock physics and rock mechanics are still unsolved. Future research should continue to focus on scaling rules - in both directions, i.e., on upscaling and downscaling. Particularly poor knowledge seems to exist on interactions between petrophysical and geochemical processes. What are the key parameters in such type of coupling, and how can they be obtained? Findings from experimental studies is and will remain an invaluable source for future progress in understanding hydro-thermo-mechanical coupling in fractured rock. Nonetheless, special attention must be paid to the question how site specific observations in the field are. Given the complexity of the target material, an ever broader collection of successful predictions and pitfalls forms our imagination of appropriate models, and what aspects will continue to sharpen our insight into the multi-scale phenomena we are encountering. The papers collected in this volume provide some idea of the exciting meeting at Bad Honnef. I would like to thank all the authors for contributing their original articles to this special issue. The following reviewers are acknowledged for their assistance: S. Baraka-Lokmane, B. Bourbiaux, M. Bouteca, D.S. Daev, C. David, K. Evans, Y. Gueguen, B.C. Haimson, S. Heikamp, J. Heiland, T. Ito, E. Klein, F. Lanaro, B. Maillot, Y. Meheust, G. Nover, D. Pisarenko, Y.A. Popov, T. Popp, T. Reuschle, E.H. Saenger, J. Schmittbuhl, A. Schubnel, S.A. Shapiro, G.D.H. Simpson, O. Stephansson, F. Surma, R.W. Zimmerman, G. Zimmermann. 812 Hans-Joachim Kiimpel Pure appl. geophys., REFERENCES BOUTECA, M., and GUEGUEN, Y. (1999), Rock Conference Looks at Pore Pressure, Scale Effects, and Deformation, EOS (Transactions American Geophysical Union), April 20, 188. COUPLES, G., MAIN, I., and MEREDITH, Ph. (2000), Relationships between Damage and Localization: A Eurocoriference on Rock Mechanics and Rock Physics, Newsletter Physical Properties of Earth Materials, October 2000, 3-8. KDMPEL, H.-J. (2001), Conference Addresses Thermo-hydro-mechanical Coupling in Fractured Rock, EOS (Transactions American Geophysical Union), May 29, 248. Hans-Joachim Kiimpel Leibniz-Institute for Applied Geosciences (GGA) Stilleweg 2 D-30655 Hannover Germany E-mail: [email protected] © Birkhauser Verlag, Basel, 2003 Pure appl. geophys. 160 (2003) 813-831 I Pure and Applied Geophysics 0033 -4553/03/060813-19 Borehole Breakouts in Berea Sandstone Reveal a New Fracture Mechanism BEZALEL C. HAIMSON1 Abstract-Vertical drilling experiments in high-porosity (22% and 25%) Berea sandstone subjected to critical true triaxial far-field stresses, in which (lH (maximum horizontal stress) > (lv (vertical stress) > (lh (least horizontal stress), revealed a new and non-dilatant failure mechanism that results in thin and very long tabular borehole breakouts that have the appearance of fractures, and which counterintuitively develop orthogonally to (lH. These breakouts are fundamentally different from those induced in crystalline rocks, as well as limestones and medium-porosity Berea sandstone. Breakouts in these rocks are typically dog-eared in shape, a result of dilatant multi-cracking tangential to the hole and subparallel to the maximum far-field horizontal stress (lH, followed by progressive buckling and shearing of detached rock flakes created by the cracks. In the high-porosity sandstone a narrow layer of grains compacted normal to (lH is observed just ahead of the breakout tip. This layer is nearly identical to "compaction bands" observed in the field. It is suggested that when a critical tangential stress concentration is reached along the (lh spring line at the borehole wall, grain bonding breaks down and a compaction band is formed normal to (lH. Debonded loose grains are expelled into the borehole, assisted by the circulating drilling fluid. As the breakout tip advances, the stress concentration ahead of it persists or may even increase, extending the compaction band, which in turn leads to breakout lengthening. Key words: Rock mechanics, fractures, in situ stress, borehole breakouts, sandstone, compaction band. Introduction Stress-induced instability around boreholes drilled vertically into the earth's crust often results in 'breakouts' or zones of cross section elongation due to brittle fracture at the borehole wall. Extensive field evidence and laboratory experiments suggest that breakout orientation along the perimeter of such boreholes is typically aligned with the direction of the minimum horizontal in situ stress (Jh (BELL and GOUGH, 1979; SHAMIR and ZOBACK, 1992; HAIMSON and HERRICK, 1986; LEE and HAIMSON, 1993). The compressive stress concentration there, generated by the prevailing crustal stresses, is at its highest. Breakouts are at once both a cause for concern regarding the borehole structural integrity, and potentially a most I Department of Materials Science and Engineering, and the Geological Engr. Program, University of Wisconsin, 1509 University Avenue, Madison, WI 53706-1595, U.S.A. E-mail: [email protected] H.-J. Kümpel (ed.), Thermo-Hydro-Mechanical Coupling in Fractured Rock © Springer Basel AG 2003

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