511 results on '"Dresen, Georg"'
Search Results
202. Deformation mechanism maps for feldspar rocks
- Author
-
Rybacki, Erik, primary and Dresen, Georg, additional
- Published
- 2004
- Full Text
- View/download PDF
203. Rheology of calcite–quartz aggregates deformed to large strain in torsion
- Author
-
Rybacki, Erik, primary, Paterson, Mervyn S., additional, Wirth, Richard, additional, and Dresen, Georg, additional
- Published
- 2003
- Full Text
- View/download PDF
204. Upscaling: Effective Medium Theory, Numerical Methods and the Fractal Dream.
- Author
-
Dresen, Georg, Zang, Arno, Stephansson, Ove, Guéguen, Y., Ravalec, M., and Ricard, L.
- Abstract
Upscaling is a major issue regarding mechanical and transport properties of rocks. This paper examines three issues relative to upscaling. The first one is a brief overview of Effective Medium Theory (EMT), which is a key tool to predict average rock properties at a macroscopic scale in the case of a statistically homogeneous medium. EMT is of particular interest in the calculation of elastic properties. As discussed in this paper, EMT can thus provide a possible way to perform upscaling, although it is by no means the only one, and in particular it is irrelevant if the medium does not adhere to statistical homogeneity. This last circumstance is examined in part two of the paper. We focus on the example of constructing a hydrocarbon reservoir model. Such a construction is a required step in the process of making reasonable predictions for oil production. Taking into account rock permeability, lithological units and various structural discontinuities at different scales is part of this construction. The result is that stochastic reservoir models are built that rely on various numerical upscaling methods. These methods are reviewed. They provide techniques which make it possible to deal with upscaling on a general basis. Finally, a last case in which upscaling is trivial is considered in the third part of the paper. This is the fractal case. Fractal models have become popular precisely because they are free of the assumption of statistical homogeneity and yet do not involve numerical methods. It is suggested that using a physical criterion as a means to discriminate whether fractality is a dream or reality would be more satisfactory than relying on a limited data set alone. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
205. Comparison of Numerical and Physical Models for Understanding Shear Fracture Processes.
- Author
-
Dresen, Georg, Zang, Arno, Stephansson, Ove, Napier, John, and Backers, Tobias
- Abstract
An understanding of the formation of shear fractures is important in many rock engineering design problems. Laboratory experiments have been performed to determine the Mode II fracture toughness of Mizunami granite rock samples using a cylindrical ‘punch-through' testing device. In this paper we attempt to understand and interpret the experimental results by numerical simulation of the fundamental shear fracture initiation and coalescence processes, using a random array of displacement discontinuity crack elements. It is found that qualitative agreement between the experimental and numerical results can be established, provided that shear-like micro-scale failure processes can be accommodated by the failure initiation rules that are used in the numerical simulations. In particular, it is found that the use of an exclusively tension-driven failure initiation rule does not allow the formation of macro-shear structures. It is apparent, also, that further investigation is required to determine how consistent rules can be established to link micro-failure criteria to equivalent macro-strength and toughness properties for a macro-shear slip surface. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
206. Discrete Element Modeling of Stress and Strain Evolution Within and Outside a Depleting Reservoir.
- Author
-
Dresen, Georg, Zang, Arno, Stephansson, Ove, Alassi, Haitham T. I., Liming Li, and Holt, Rune M.
- Abstract
Stress changes within and around a depleting petroleum reservoir can lead to reservoir compaction and surface subsidence, affect drilling and productivity of oil wells, and influence seismic waves used for monitoring of reservoir performance. Currently modeling efforts are split into more or less coupled geomechanical (normally linearly elastic), fluid flow, and geophysical simulations. There is evidence (from e.g. induced seismicity) that faults may be triggered or generated as a result of reservoir depletion. The numerical technique that most adequately incorporates fracture formation is the DEM (Discrete Element Method). This paper demonstrates the feasibility of the DEM (here PFC; Particle Flow Code) to handle this problem. Using an element size of 20 m, 2-D and 3-D simulations have been performed of stress and strain evolution within and around a depleting reservoir. Within limits of elasticity, the simulations largely reproduce analytical predictions; the accuracy is however limited by the element size. When the elastic limit is exceeded, faulting is predicted, particularly near the edge of the reservoir. Simulations have also been performed to study the activation of a pre-existing fault near a depleting reservoir. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
207. True Triaxial Stresses and the Brittle Fracture of Rock.
- Author
-
Dresen, Georg, Zang, Arno, Stephansson, Ove, and Haimson, Bezalel
- Abstract
This paper reviews the efforts made in the last 100 years to characterize the effect of the intermediate principal stress σ2 on brittle fracture of rocks, and on their strength criteria. The most common theories of failure in geomechanics, such as those of Coulomb, and Mohr, disregard σ2 and are typically based on triaxial testing of cylindrical rock samples subjected to equal minimum and intermediate principal stresses (σ3=σ2). However, as early as 1915 Böker conducted conventional triaxial extension tests (σ1=σ2) on the same Carrara marble tested earlier in conventional triaxial compression by von Kármán that showed a different strength behavior. Efforts to incorporate the effect of σ2 on rock strength continued in the second half of the last century through the work of Nadai, Drucker and Prager, Murrell, Handin, Wiebols and Cook, and others. In 1971 Mogi designed a high-capacity true triaxial testing machine, and was the first to obtain complete true triaxial strength criteria for several rocks based on experimental data. Following his pioneering work, several other laboratories developed equipment and conducted true triaxial tests revealing the extent of σ2 effect on rock strength (e.g., Takahashi and Koide, Michelis, Smart, Wawersik). Testing equipment emulating Mogi's but considerably more compact was developed at the University of Wisconsin and used for true triaxial testing of some very strong crystalline rocks. Test results revealed three distinct compressive failure mechanisms, depending on loading mode and rock type: shear faulting resulting from extensile microcrack localization, multiple splitting along the σ1 axis, and nondilatant shear failure. The true triaxial strength criterion for the KTB amphibolite derived from such tests was used in conjunction with logged breakout dimensions to estimate the maximum horizontal in situ stress in the KTB ultra deep scientific hole. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
208. A Method for Testing Dynamic Tensile Strength and Elastic Modulus of Rock Materials Using SHPB.
- Author
-
Dresen, Georg, Zang, Arno, Stephansson, Ove, Wang, Q. Z., Li, W., and Song, X. L.
- Abstract
An experimental procedure for testing dynamic tensile strength and elastic modulus of rock materials at high strain rate loading is presented in this paper. In our test the split Hopkinson pressure bar (SHPB) was used to diametrally impact the Brazilian disc (BD) and flattened Brazilian disc (FBD) specimens of marble. A tensile strain rate of about 45 1/s was achieved at the center of the specimen. In order to improve the accuracy of the analysis, the initiation time difference between the strain waves acting on the two flat ends of the FBD specimen was treated properly. Typical failure modes corresponding to different loading conditions were observed. It was verified with a finite-element simulation that the equilibrium condition was established in the specimen before its failure. This numerical simulation validates the experimental procedure and also proves the suitability of formulation for the basic equations. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
209. Fracture Toughness Evaluation Based on Tension-softening Model and its Application to Hydraulic Fracturing.
- Author
-
Dresen, Georg, Zang, Arno, Stephansson, Ove, Sato, Kazushi, and Hashida, Toshiyuki
- Abstract
This paper discusses the applicability of the tension-softening model in the determination of the fracture toughness of rocks, where the fracture toughness evaluated based on the tension-softening model is compared with the crack growth resistance deduced from laboratory-scale hydraulic fracturing tests. It is generally accepted that the fracture process is dominated by the growth of a fracture process zone for most types of rocks. In this study, the J-integral based technique is employed to determine the fracture toughness of Iidate granite on the basis of the tension-softening model, where compact tension specimens of different dimensions were tested in order to examine the specimen size effect on the measured fracture toughness. It was shown that the tension-softening relation deduced from the J-integral based technique allowed us to determine the specimen size independent fracture toughness Kc of Iidate granite.Laboratory-scale hydraulic fracturing tests were performed on cubic specimens (up to a 10 m sized specimen), where cyclic pressurization was conducted using a rubber-made straddle packer to observe the extent of the hydraulically induced crack. The experimental results of pressure and crack length were then used to construct the crack growth resistance curve based on the stress intensity factor K. The crack growth resistance obtained from the hydraulic fracturing tests was observed to initially increase and then level off, giving a constant K value for a long crack extension stage. The plateau K value in the crack growth resistance curve was found to be in reasonable agreement with the fracture toughness Kc deduced from the tension-softening relation. It was demonstrated that the tension-softening model provides a useful tool to determine the appropriate fracture toughness of rocks, which may be applicable for the analysis of the process of large-scale crack extension in rock masses. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
210. Cohesive Crack Analysis of Toughness Increase Due to Confining Pressure.
- Author
-
Dresen, Georg, Zang, Arno, Stephansson, Ove, Sato, Kazushi, and Hashida, Toshiyuki
- Abstract
Apparent fracture toughness in Mode I of microcracking materials such as rocks under confining pressure is analyzed based on a cohesive crack model. In rocks, the apparent fracture toughness for crack propagation varies with the confining pressure. This study provides analytical solutions for the apparent fracture toughness using a cohesive crack model, which is a model for the fracture process zone. The problem analyzed in this study is a fluid-driven fracture of a two-dimensional crack with a cohesive zone under confining pressure. The size of the cohesive zone is assumed to be negligibly small in comparison to the crack length. The analyses are performed for two types of cohesive stress distribution, namely the constant cohesive stress (Dugdale model) and the linearly decreasing cohesive stress. Furthermore, the problem for a more general cohesive stress distribution is analyzed based on the fracture energy concept. The analytical solutions are confirmed by comparing them with the results of numerical computations performed using the body force method. The analytical solution suggests a substantial increase in the apparent fracture toughness due to increased confining pressures, even if the size of the fracture process zone is small. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
211. An Update on the Fracture Toughness Testing Methods Related to the Cracked Chevron-notched Brazilian Disk (CCNBD) Specimen.
- Author
-
Dresen, Georg, Zang, Arno, Stephansson, Ove, Fowell, R. J., Xu, C., and Dowd, P. A.
- Abstract
This paper reviews the use of the cracked Chevron-notched Brazilian disc (CCNBD) for fracture toughness testing. Theoretical and experimental backgrounds of the method are described. Some issues regarding the current development (i.e., recalibration) of the specimen geometry are presented and discussed. A number of geometries related to the CCNBD proposed recently for fracture toughness testing of rock are then introduced and commented on. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
212. Can Damage Mechanics Explain Temporal Scaling Laws in Brittle Fracture and Seismicity?
- Author
-
Dresen, Georg, Zang, Arno, Stephansson, Ove, Turcote, Donald L., and Shcherbakov, Robert
- Abstract
Time delays associated with processes leading to a failure or stress relaxation in materials and earthquakes are studied in terms of continuum damage mechanics. Damage mechanics is a quasiempirical approach that describes inelastic irreversible phenomena in the deformation of solids. When a rock sample is loaded, there is generally a time delay before the rock fails. This period is characterized by the occurrence and coalescence of microcracks which radiate acoustic signals of broad amplitudes. These acoustic emission events have been shown to exhibit power-law scaling as they increase in intensity prior to a rupture. In case of seismogenic processes in the Earth's brittle crust, all earthquakes are followed by an aftershock sequence. A universal feature of aftershocks is that their rate decays in time according to the modified Omori's law, a power-law decay. In this paper a model of continuum damage mechanics in which damage (microcracking) starts to develop when the applied stress exceeds a prescribed yield stress (a material parameter) is introduced to explain both laboratory experiments and systematic temporal variations in seismicity. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
213. Stress Sensitivity of Seismic and Electric Rock Properties of the Upper Continental Crust at the KTB.
- Author
-
Dresen, Georg, Zang, Arno, Stephansson, Ove, Kaselow, Axel, Becker, Katharina, and Shapiro, Serge A.
- Abstract
We test the hypothesis that the general trend of P-wave and S-wave sonic log velocities and resistivity with depth in the pilot hole of the KTB site Germany, can be explained by the progressive closure of the compliant porosity with increasingly effective pressure. We introduce a quantity θc characterizing the stress sensitivity of the mentioned properties. An analysis of the downhole measurements showed that estimates of the quantitiy θc for seismic velocities and electrical formation factor of the in situ formation coincide. Moreover, this quantity is 3.5 to 4.5 times larger than the averaged stress sensitivity obtained from core samples. We conclude that the hypothesis mentioned above is consistent with both data sets. Moreover, since θc corresponds approximately to the inverse of the effective crack aspect ratio, larger in situ estimates of θc might reflect the influence of fractures and faults on the stress sensitivity of the crystalline formation in contrast to the stress sensitivity of the nearly intact core samples. Finally, because the stress sensitivity is directly related to the elastic nonlinearity we conclude that the elastic nonlinearity (i.e., deviation from linear stress-strain relationship i.e., Hooke's law) of the KTB rocks is significantly larger in situ than in the laboratory. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
214. Fracture in Westerly Granite under AE Feedback and Constant Strain Rate Loading: Nucleation, Quasi-static Propagation, and the Transition to Unstable Fracture Propagation.
- Author
-
Dresen, Georg, Zang, Arno, Stephansson, Ove, Thompson, Ben D., Young, R. Paul, and Lockner, David A.
- Abstract
New observations of fracture nucleation are presented from three triaxial compression experiments on intact samples of Westerly granite, using Acoustic Emission (AE) monitoring. By conducting the tests under different loading conditions, the fracture process is demonstrated for quasistatic fracture (under AE Feedback load), a slowly developing unstable fracture (loaded at a ‘slow' constant strain rate of 2.5 × 10)−6 /s) and an unstable fracture that develops near instantaneously (loaded at a ‘fast' constant strain rate of 5 × 10)−5 /s). By recording a continuous ultrasonic waveform during the critical period of fracture, the entire AE catalogue can be captured and the exact time of fracture defined. Under constant strain loading, three stages are observed: (1) An initial nucleation or stable growth phase at a rate of ∼ 1.3 mm/s, (2) a sudden increase to a constant or slowly accelerating propagation speed of ∼ 18 mm/s, and (3) unstable, accelerating propagation. In the ∼ 100 ms before rupture, the high level of AE activity (as seen on the continuous record) prevented the location of discrete AE events. A lower bound estimate of the average propagation velocity (using the time-to-rupture and the existing fracture length) suggests values of a few m/s. However from a low gain acoustic record, we infer that in the final few ms, the fracture propagation speed increased to 175 m/s. These results demonstrate similarities between fracture nucleation in intact rock and the nucleation of dynamic instabilities in stick slip experiments. It is suggested that the ability to constrain the size of an evolving fracture provides a crucial tool in further understanding the controls on fracture nucleation. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
215. Ultrasonic Velocities, Acoustic Emission Characteristics and Crack Damage of Basalt and Granite.
- Author
-
Zang, Arno, Stephansson, Ove, Stanchits, Sergei, Vinciguerra, Sergio, and Dresen, Georg
- Abstract
Acoustic emissions (AE), compressional (P), shear (S) wave velocities, and volumetric strain of Etna basalt and Aue granite were measured simultaneously during triaxial compression tests. Deformation-induced AE activity and velocity changes were monitored using twelve P-wave sensors and eight orthogonally polarized S-wave piezoelectric sensors; volumetric strain was measured using two pairs of orthogonal strain gages glued directly to the rock surface. P-wave velocity in basalt is about 3 km/s at atmospheric pressure, but increases by > 50% when the hydrostatic pressure is increased to 120 MPa. In granite samples initial P-wave velocity is 5 km/s and increases with pressure by < 20%. The pressure-induced changes of elastic wave speed indicate dominantly compliant low-aspect ratio pores in both materials, in addition Etna basalt also contains high-aspect ratio voids. In triaxial loading, stress-induced anisotropy of P-wave velocities was significantly higher for basalt than for granite, with vertical velocity components being faster than horizontal velocities. However, with increasing axial load, horizontal velocities show a small increase for basalt but a significant decrease for granite. Using first motion polarity we determined AE source types generated during triaxial loading of the samples. With increasing differential stress AE activity in granite and basalt increased with a significant contribution of tensile events. Close to failure the relative contribution of tensile events and horizontal wave velocities decreased significantly. A concomitant increase of double-couple events indicating shear, suggests shear cracks linking previously formed tensile cracks. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
216. Quantifying Damage, Saturation and Anisotropy in Cracked Rocks by Inverting Elastic Wave Velocities.
- Author
-
Dresen, Georg, Zang, Arno, Stephansson, Ove, Schubnel, Alexandre, Benson, Philip M., Thompson, Ben D., Hazzard, Jim F., and Young, R. Paul
- Abstract
Crack damage results in a decrease of elastic wave velocities and in the development of anisotropy. Using non-interactive crack effective medium theory as a fundamental tool, we calculate dry and wet elastic properties of cracked rocks in terms of a crack density tensor, average crack aspect ratio and mean crack fabric orientation from the solid grains and fluid elastic properties. Using this same tool, we show that both the anisotropy and shear-wave splitting of elastic waves can be derived. Two simple crack distributions are considered for which the predicted anisotropy depends strongly on the saturation, reaching up to 60% in the dry case. Comparison with experimental data on two granites, a basalt and a marble, shows that the range of validity of the non-interactive effective medium theory model extends to a total crack density of approximately 0.5, considering symmetries up to orthorhombic. In the isotropic case, Kachanov's (1994) non-interactive effective medium model was used in order to invert elastic wave velocities and infer both crack density and aspect ratio evolutions. Inversions are stable and give coherent results in terms of crack density and aperture evolution. Crack density variations can be interpreted in terms of crack growth and/or changes of the crack surface contact areas as cracks are being closed or opened respectively. More importantly, the recovered evolution of aspect ratio shows an exponentially decreasing aspect ratio (and therefore aperture) with pressure, which has broader geophysical implications, in particular on fluid flow. The recovered evolution of aspect ratio is also consistent with current mechanical theories of crack closure. In the anisotropic cases—both transverse isotropic and orthorhombic symmetries were considered—anisotropy and saturation patterns were well reproduced by the modelling, and mean crack fabric orientations we recovered are consistent with in situ geophysical imaging. Our results point out that: (1) It is possible to predict damage, anisotropy and saturation in terms of a crack density tensor and mean crack aspect ratio and orientation; (2) using well constrained wave velocity data, it is possible to extrapolate the contemporaneous evolution of crack density, anisotropy and saturation using wave velocity inversion as a tool; 3) using such an inversion tool opens the door in linking elastic properties, variations to permeability. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
217. Fracture Toughness Measurements and Acoustic Emission Activity in Brittle Rocks.
- Author
-
Dresen, Georg, Zang, Arno, Stephansson, Ove, Nasseri, M. H. B., Mohanty, B., and Young, R. P.
- Abstract
Fracture toughness measurements under static loading conditions have been carried out in Barre and Lac du Bonnet granites. An advanced AE technique has been adopted to monitor real-time crack initiation and propagation around the principal crack in these tests to understand the processes of brittle failure under tension and related characteristics of the resulting fracture process zone. The anisotropy of Mode I fracture toughness has been investigated along specific directions. Microcrack density and orientation analysis from thin section studies have shown these characteristics to be the primary cause of the observed variation in fracture toughness, which is seen to vary between 1.14 MPa.(m)1/2 and 1.89 MPa.(m)1/2 in Barre granite. The latter value represents the case in which the crack is propagated at right angles to the main set of microcracks. The creation of a significant fracture process zone surrounding the propagating main crack has been confirmed. Real-time imaging of the fracture process and formation of fracture process zone by AE techniques yielded results in very good agreement with those obtained by direct optical analysis. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
218. Permeability Evolution During Non-linear Viscous Creep of Calcite Rocks.
- Author
-
Zang, Arno, Stephansson, Ove, Dresen, Georg, Xiao, Xiaohui, Evans, Brian, and Bernabé, Yves
- 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. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
219. Spatial Scaling of Effective Modulus and Correlation of Deformation Near the Critical Point of Fracturing.
- Author
-
Zang, Arno, Stephansson, Ove, Dresen, Georg, Heffer, Kes, and King, Peter
- 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, Ldµ, 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. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
220. Understanding the Seismic Velocity Structure of Campi Flegrei Caldera (Italy): From the Laboratory to the Field Scale.
- Author
-
Zang, Arno, Stephansson, Ove, Dresen, Georg, Vinciguerra, S., Trovato, C., Meredith, P.G., Benson, P.M., Troise, C., and De Natale, G.
- 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. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
221. Monitoring Fracture Propagation in a Soft Rock (Neapolitan Tuff) Using Acoustic Emissions and Digital Images.
- Author
-
Zang, Arno, Stephansson, Ove, Dresen, Georg, Hall, Stephen A., de Sanctis, Fiorenza, and Viggiani, Gioacchino
- 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. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
222. Characterisation of Tensile Damage in Rock Samples Induced by Different Stress Paths.
- Author
-
Zang, Arno, Stephansson, Ove, Dresen, Georg, Ganne, Patrick, and Vervoort, André
- 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. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
223. Analysis of Temporal Structures of Seismic Events on Different Scale Levels.
- Author
-
Zang, Arno, Stephansson, Ove, Dresen, Georg, and German, V.I.
- 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. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
224. Experimental Investigation into the Scale Dependence of Fluid Transport in Heterogeneous Rocks.
- Author
-
Zang, Arno, Stephansson, Ove, Dresen, Georg, Song, Insun, and Renner, Jörg
- 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. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
225. Short-Timescale Chemo-Mechanical Effects and their Influence on the Transport Properties of Fractured Rock.
- Author
-
Zang, Arno, Stephansson, Ove, Dresen, Georg, Elsworth, Derek, and Yusuhara, Hideaki
- 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. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
226. Fluid Pressure Variation in a Sedimentary Geothermal Reservoir in the North German Basin: Case Study Groß Schönebeck.
- Author
-
Zang, Arno, Stephansson, Ove, Dresen, Georg, Huenges, Ernst, Trautwein, Ute, Legarth, Björn, and Zimmermann, Günter
- 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. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
227. Determination of Porosity and Permeability out of Dispersion Analyses of Borehole Wall Displacements.
- Author
-
Zang, Arno, Stephansson, Ove, Dresen, Georg, Albrecht, Michael, and Mansurov, Vladimir
- 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. [ABSTRACT FROM AUTHOR]
- Published
- 2006
- Full Text
- View/download PDF
228. Dislocation microstructure and phase distribution in a lower crustal shear zone – an example from the Ivrea-Zone, Italy
- Author
-
Kenkmann, Thomas, primary and Dresen, Georg, additional
- Published
- 2001
- Full Text
- View/download PDF
229. Fracture process zone in granite
- Author
-
Zang, Arno, primary, Wagner, F. Christian, additional, Stanchits, Sergei, additional, Janssen, Christoph, additional, and Dresen, Georg, additional
- Published
- 2000
- Full Text
- View/download PDF
230. Geophysik: Scher-Zonen und Erdbeben: Die Bewertung von Erdbebengefahren setzt das Verständnis von Scher-Zonen voraus
- Author
-
Weber, Michael, primary, Dresen, Georg, additional, Haak, Volker, additional, and Kaufmann, Hermann, additional
- Published
- 2000
- Full Text
- View/download PDF
231. Hydrogen-enhanced electrical conductivity of diopside crystals
- Author
-
Wang, Zichao, primary, Ji, Shaocheng, additional, and Dresen, Georg, additional
- Published
- 1999
- Full Text
- View/download PDF
232. Source analysis of acoustic emissions in Aue granite cores under symmetric and asymmetric compressive loads
- Author
-
Zang, Arno, primary, Christian Wagner, F., additional, Stanchits, Sergei, additional, Dresen, Georg, additional, Andresen, Reimer, additional, and Haidekker, Mark A., additional
- Published
- 1998
- Full Text
- View/download PDF
233. Effect of quartz inclusions on plastic flow in marble
- Author
-
Dresen, Georg, primary, Evans, Brian, additional, and Olgaard, David L., additional
- Published
- 1998
- Full Text
- View/download PDF
234. Quartz dislocation microstructure between 7000 m and 9100 m depth from the Continental Deep Drilling Program KTB
- Author
-
Dresen, Georg, primary, Duyster, Johannes, additional, Stöckhert, Bernhard, additional, Wirth, Richard, additional, and Zulauf, Gernold, additional
- Published
- 1997
- Full Text
- View/download PDF
235. Effect of semibrittle deformation on transport properties of calcite rocks
- Author
-
Siddiqi, Gunter, primary, Evans, Brian, additional, Dresen, Georg, additional, and Freund, Dieter, additional
- Published
- 1997
- Full Text
- View/download PDF
236. Acoustic emission, microstructure, and damage model of dry and wet sandstone stressed to failure
- Author
-
Zang, Arno, primary, Wagner, Christian F., additional, and Dresen, Georg, additional
- Published
- 1996
- Full Text
- View/download PDF
237. Brittle failure and fracture reactivation in sandstone by fluid injection
- Author
-
Charalampidou, Elli-Maria, Stanchits, Sergei, Kwiatek, Grzegorz, and Dresen, Georg
- Abstract
We performed laboratory experiments on sandstone specimens to study brittle failure and the reactivation of an experimentally produced failure plane induced by pore-pressure perturbations using constant force control in high compressive stress states. Here, we focus on the shear failure of a dry sample and the later on induced fracture plane reactivation due to water injection. Acoustic Emission (AE) monitoring has been used during both experiments. We also used ultrasonic wave velocities to monitor pore fluid migration through the initially dry specimen. To characterise AE source mechanisms, we analysed first motion polarities and performed full moment tensor inversion at all stages of the experiments. For the case of water injection on the dry specimen that previously failed in shear, AE activity during formation of new fractures is dominated by tensile and shear sources as opposed to the fracture plane reactivation, when compressive and shear sources are most frequent. Furthermore, during the reactivation of the latter, compressive sources involve higher compressive components compared to the shear failure case. The polarity method and the moment tensor inversion reveal similar source mechanisms but the latter provides more information on the source components.
- Published
- 2015
- Full Text
- View/download PDF
238. Nanoseismicity and picoseismicity rate changes from static stress triggering caused by a Mw2.2 earthquake in Mponeng gold mine, South Africa
- Author
-
Kozłowska, Maria, Orlecka‐Sikora, Beata, Kwiatek, Grzegorz, Boettcher, Margaret S., and Dresen, Georg
- Abstract
Static stress changes following large earthquakes are known to affect the rate and distribution of aftershocks, yet this process has not been thoroughly investigated for nanoseismicity and picoseismicity at centimeter length scales. Here we utilize a unique data set of M≥ −3.4 earthquakes following a Mw2.2 earthquake in Mponeng gold mine, South Africa, that was recorded during a quiet interval in the mine to investigate if rate‐ and state‐based modeling is valid for shallow, mining‐induced seismicity. We use Dieterich's (1994) rate‐ and state‐dependent formulation for earthquake productivity, which requires estimation of four parameters: (1) Coulomb stress changes due to the main shock, (2) the reference seismicity rate, (3) frictional resistance parameter, and (4) the duration of aftershock relaxation time. Comparisons of the modeled spatiotemporal patterns of seismicity based on two different source models with the observed distribution show that while the spatial patterns match well, the rate of modeled aftershocks is lower than the observed rate. To test our model, we used three metrics of the goodness‐of‐fit evaluation. The null hypothesis, of no significant difference between modeled and observed seismicity rates, was only rejected in the depth interval containing the main shock. Results show that mining‐induced earthquakes may be followed by a stress relaxation expressed through aftershocks located on the rupture plane and in regions of positive Coulomb stress change. Furthermore, we demonstrate that the main features of the temporal and spatial distributions of very small, mining‐induced earthquakes can be successfully determined using rate‐ and state‐based stress modeling. Mining earthquakes can be followed by stress relaxation in a form of aftershocksRate‐ and state‐based aftershock modeling is valid for shallow, induced earthquakes
- Published
- 2015
- Full Text
- View/download PDF
239. Mechanically Induced Fracture-Face Skin--Insights From Laboratory Testing and Modeling Approaches.
- Author
-
Reinicke, Andreas, Blöcher, Guido, Zimmermann, Günter, Huenges, Ernst, Dresen, Georg, Legarth, Bjorn, and Makurat, Axel
- Subjects
MICROMECHANICS ,STRAINS & stresses (Mechanics) ,QUARTZ ,STRESS concentration ,POROSITY - Abstract
In the context of this work, a new formation-damage mechanism is proposed--the mechanically induced fracture-face skin (FFS). This new mechanism results from mechanical interactions between the proppants and the reservoir rock caused by the increasing stress on the rock/proppant system during production. Proppant embedment into the fracture face and proppant crushing lead to fines production and may impair the fracture performance. To achieve sustainable, long-term productivity from a reservoir, it is indispensable to understand the hydraulic and mechanical interactions in rock/proppant systems. In this study, permeability measurements on sandstones with propped fractures under stress using various flow cells were performed, allowing localization and quantification of the mechanical damage at the fracture face. The laboratory experiments identified a permeability reduction at the fracture face of up to 90%. The mechanical damage at the rock/proppant interface began immediately with loading of the rock/proppant system and for fracture-closure stresses less than 35 MPa; the damage was localized at the fracture face. Microstructure analysis identified quartz-grain crushing, fines production, and pore-space blocking at the fracture face, causing the observed mechanically induced FFS. At higher stresses, damage and embedment of the ceramic proppants reduce the fracture permeability further. Numerical modeling of the rock/proppant system identified highly inhomogeneous stress distributions in the granular system of grains and proppants. High tensile-stress concentrations beneath the area of contact between quartz grains and proppants were observed, even at small differential stress applied to the rock/ proppant system. These high-stress concentrations were responsible for the early onset of damage at the fracture face. Therefore, even low differential stresses, which are expected under in-situ conditions, may affect the productivity of a hydraulically fractured well. [ABSTRACT FROM AUTHOR]
- Published
- 2013
- Full Text
- View/download PDF
240. Deformation of dry and wet sandstone targets during hypervelocity impact experiments, as revealed from the MEMIN Program.
- Author
-
BUHL, Elmar, POELCHAU, Michael H., DRESEN, Georg, and KENKMANN, Thomas
- Subjects
DEFORMATIONS (Mechanics) ,SANDSTONE ,HYPERVELOCITY ,IMPACT craters ,EXPERIMENTS ,QUARTZ ,POROUS materials - Abstract
- Hypervelocity impact experiments on dry and water-saturated targets of fine-grained quartz sandstone, performed within the MEMIN project, have been investigated to determine the effects of porosity and pore space saturation on deformation mechanisms in the crater's subsurface. A dry sandstone cube and a 90% water-saturated sandstone cube (Seeberger Sandstein, 20 cm side length, about 23% porosity) were impacted at the Fraunhofer EMI acceleration facilities by 2.5 mm diameter steel spheres at 4.8 and 5.3 km s
−1 , respectively. Microstructural postimpact analyses of the bisected craters revealed differences in the subsurface deformation for the dry and the wet target experiments. Enhanced grain comminution and compaction in the dry experiment and a wider extent of localized deformation in the saturated experiment suggest a direct influence of pore water on deformation mechanisms. We suggest that the pore water reduces the shock impedance mismatch between grains and pore space, and thus reduces the peak stresses at grain-grain contacts. This effect inhibits profound grain comminution and effective compaction, but allows for reduced shock wave attenuation and a more effective transport of energy into the target. The reduced shock wave attenuation is supposed to be responsible for the enhanced crater growth and the development of 'near surface' fractures in the wet target. [ABSTRACT FROM AUTHOR]- Published
- 2013
- Full Text
- View/download PDF
241. Brittle and semibrittle deformation of synthetic marbles composed of two phases
- Author
-
Dresen, Georg, primary and Evans, Brian, additional
- Published
- 1993
- Full Text
- View/download PDF
242. Depressurized Cavities within High-strain Shear Zones: their Role in the Segregation and Flow of SiO2-rich Melt in Feldspar-dominated Rocks.
- Author
-
Spiess, Richard, Dibona, Raffaella, Rybacki, Erik, Wirth, Richard, and Dresen, Georg
- Subjects
SHEAR zones ,SILICA ,FELDSPAR ,PRESSURE ,DIFFUSION ,ROCK deformation ,ANORTHITE - Abstract
We observe void growth and coalescence into cavity-bearing shear bands during deformation of wet synthetic anorthite aggregates containing <3 vol. % silica-enriched melt. Samples were deformed in the Newtonian creep regime to high strain during torsion experiments at 1100°C and 400 MPa confining pressure. Localized cavity-bearing shear bands show an S–C'-geometry: the bands (C') are oriented at about 30° to the compression direction of the imposed simple shear and the internal foliation (S) of the bands is rotated towards the horizontal external shear plane. Cavity-bearing shear bands started to nucleate in the sample periphery above a shear strain threshold of ≈2. Quartz crystallized from the water-saturated SiO2-rich melt within large cavities inside these bands, which requires that the melt is decompressed by >200 MPa during their formation. The dynamically evolving cavities are sites of locally reduced pressure that collect the melt distributed in the adjacent matrix. Therefore, cavitation damage under ductile conditions may result in the development of an efficient melt channelling system controlling SiO2-rich melt flow in the lower crust. Electron backscatter diffraction analysis shows that the quartz inside the cavity bands has a crystallographic preferred orientation (CPO). The development of the CPO is explained by the preferred dissolution of crystals oriented with the rhombohedra and trigonal dipyramids orthogonal to the compression direction and by preferential growth of crystals aligned with their <0001> axis in the extension direction of the externally applied simple shear deformation. [ABSTRACT FROM AUTHOR]
- Published
- 2012
- Full Text
- View/download PDF
243. Microseismicity induced during fluid-injection: A case study from the geothermal site at Groß Schönebeck, North German Basin.
- Author
-
Kwiatek, Grzegorz, Bohnhoff, Marco, Dresen, Georg, Schulze, Ali, Schulte, Thomas, Zimmermann, Günter, and Huenges, Ernst
- Abstract
The technical feasibility of geothermal power production in a low enthalpy environment will be investigated in the geothermal site at Groß Schönebeck, North German Basin, where a borehole doublet was completed in 2007. In order to complete the Enhanced Geothermal System, three massive hydraulic stimulations were performed. A seismic network was deployed including a single 3-component downhole seismic sensor at only 500 m distance to the injection point. Injection rates reached up to 9 m/min and the maximum injection well-head pressure was as high as ∼60 MPa. A total of 80 very small (−1.8 < M < −1.0) induced seismic events were detected. The hypocenters were determined for 29 events. The events show a strong spatial and temporal clustering and a maximum seismicity rate of 22 events per day. Spectral parameters were estimated from the downhole seismometer and related to those from other types of induced seismicity. The majority of events occurred towards the end of stimulation phases indicating a similar behavior as observed at similar treatments in crystalline environments but in our case at a smaller level of seismic activity and at lower magnitudes. [ABSTRACT FROM AUTHOR]
- Published
- 2010
- Full Text
- View/download PDF
244. Hydraulic fracturing stimulation techniques and formation damage mechanisms—Implications from laboratory testing of tight sandstone–proppant systems.
- Author
-
Reinicke, Andreas, Rybacki, Erik, Stanchits, Sergei, Huenges, Ernst, and Dresen, Georg
- Subjects
HYDRAULIC fracturing ,FORMATION damage (Petroleum engineering) ,SANDSTONE ,RESERVOIRS ,PERMEABILITY ,QUARTZ - Abstract
Abstract: Reservoir formation damage may seriously affect the productivity of a reservoir during various phases of fluid recovery from the subsurface. Hydraulic fracturing technology is one tool to overcome inflow impairments due to formation damage and to increase the productivity of reservoirs. However, the increase in productivity by hydraulic fracturing operations can be limited by permeability alterations adjacent to the newly created fracture face. Such an impairment of the inflow to the fracture is commonly referred to as fracture face skin (FFS). Here, we focus on mechanically induced fracture face skin, which may result from stress-induced mechanical interactions between proppants and reservoir rock during production. In order to achieve sustainable, long-term productivity from a reservoir, it is indispensable to understand the hydraulic and mechanical interactions in rock–proppant systems. We performed permeability measurements on tight sandstones with propped fractures under stress using two different flow cells, allowing to localise and quantify the mechanical damage at the fracture face. The laboratory experiments revealed a permeability reduction of this rock–proppant system down to 77% of initial rock permeability at 50MPa differential stress leading to a permeability reduction in the fracture face skin zone up to a factor of 6. Considerable mechanical damage at the rock–proppant interface was already observed for stresses of about 5MPa. Microstructure analysis identified quartz grain crushing, fines production, and pore space blocking at the fracture face causing the observed mechanically induced FFS. At higher stresses, damage and embedment of the ceramic proppants further reduces the fracture permeability. Therefore, even low differential stresses, which are expected under in-situ conditions, may considerably affect the productivity of hydraulic proppant fracturing stimulation campaigns, in particular in unconventional reservoirs where the fracture face is considerably larger compared to conventional hydraulic stimulations. [Copyright &y& Elsevier]
- Published
- 2010
- Full Text
- View/download PDF
245. Reaction rim growth in the system MgO-Al2O3-SiO2 under uniaxial stress.
- Author
-
Götze, Lutz Christoph, Abart, Rainer, Rybacki, Erik, Keller, Lukas M., Petrishcheva, Elena, and Dresen, Georg
- Subjects
THERMODYNAMICS ,ROCK creep ,POLYCRYSTALS ,CORUNDUM ,ENSTATITE - Abstract
We synthesize reaction rims between thermodynamically incompatible phases in the system MgO-Al
2 O3 -SiO2 applying uniaxial load using a creep apparatus. Synthesis experiments are done in the MgO-SiO2 and in the MgO-Al2 O3 subsystems at temperatures ranging from 1150 to 1350 °C imposing vertical stresses of 1.2 to 29 MPa at ambient pressure and under a constant flow of dry argon. Single crystals of synthetic and natural quartz and forsterite, synthetic periclase and synthetic corundum polycrystals are used as starting materials. We produce enstatite rims at forsterite-quartz contacts, enstatite-forsterite double rims at periclase-quartz contacts and spinel rims at periclase-corundum contacts. We find that rim growth under the “dry” conditions of our experiments is sluggish compared to what has been found previously in nominally “dry” piston cylinder experiments. We further observe that the nature of starting material, synthetic or natural, has a major influence on rim growth rates, where natural samples are more reactive than synthetic ones. At a given temperature the effect of stress variation is larger than what is anticipated from the modification of the thermodynamic driving force for reaction due to the storage of elastic strain energy in the reactant phases. We speculate that this may be due to modification of the physical properties of the polycrystals that constitute the reaction rims or by deformation under the imposed load. In our experiments rim growth is very sluggish at forsterite-quartz interfaces. Rim growth is more rapid at periclase-quartz contacts. The spinel rims that are produced at periclase-corundum interfaces show parabolic growth indicating that reaction rim growth is essentially diffusion controlled. From the analysis of time series done in the MgO-Al2 O3 subsystem we derive effective diffusivities for the Al2 O3 and the MgO components in a spinel polycrystal as ${\rm D}_{MgO} = 1.4 \pm 0.2 \cdot 10^{-15}$ m2 /s and ${\rm D}_{Al_2O_3} = 3.7 \pm 0.6 \cdot 10^{-16}$ m2 /s for T = 1350 °C and a vertical stress of 2.9 MPa. [ABSTRACT FROM AUTHOR]- Published
- 2010
- Full Text
- View/download PDF
246. Borehole breakout evolution through acoustic emission location analysis
- Author
-
Dresen, Georg, Stanchits, Sergei, and Rybacki, Erik
- Subjects
- *
MICROMECHANICS , *ACOUSTIC emission , *BOREHOLE mining , *NUCLEATION , *PENETRATION mechanics , *MICROSTRUCTURE , *SANDSTONE , *TESTING - Abstract
Abstract: A series of thick-wall cylinder tests were performed on Bentheim sandstone to investigate the nucleation and propagation of borehole breakouts. Isotropic compression tests showed a significant decrease of the critical pressure required to nucleate breakouts with increasing borehole diameter. Advanced analysis of acoustic emission radiation and optical microstructures indicate a three stage process of breakout nucleation and growth. Relatively few acoustic emissions are observed during the elastic loading stage of the specimens. Once breakout nucleation has occurred at the borehole wall a drastic increase of acoustic emission activity is observed. A close spatial correspondence between located acoustic events and the breakout indicates formation of two symmetric cusp-shaped breakouts on opposite sides of the borehole. With increasing isotropic pressure the breakouts grow forming parallel-sided slots that are surrounded by a process zone revealing grain crushing and pore collapse as typically observed in experimentally produced compaction bands. We apply a simple fracture mechanics approach to predict the observed size effect of the critical pressure required to initiate breakouts. [Copyright &y& Elsevier]
- Published
- 2010
- Full Text
- View/download PDF
247. Zur Raumlage und Form von Scherbrüchen — Betrachtungen am Mohrschen Spannungskreis
- Author
-
Dresen, Georg, primary
- Published
- 1991
- Full Text
- View/download PDF
248. Initiation and Propagation of Compaction Bands in Dry and Wet Bentheim Sandstone.
- Author
-
Stanchits, Sergei, Fortin, Jerome, Gueguen, Yves, and Dresen, Georg
- Subjects
ACOUSTIC emission ,COMPACTING ,POROSITY ,SANDSTONE ,STRESS corrosion - Abstract
We investigated initiation and propagation of compaction bands (CB) in six wet and four dry Bentheim sandstone samples deformed in axial compression tests with strain rates ranging from 3.2 × 10
−8 s−1 to 3.2 × 10−4 s−1 . Circumferential notches with 0.8-mm width and 5-mm depth served to initiate CB at mid-sample length. Wet samples were saturated with distilled water and deformed at 195 MPa confining pressure and 10 MPa pore pressure. Dry samples were deformed at 185 MPa confining pressure. Twelve P-wave sensors, eight S-wave sensors and two pairs of orthogonally oriented strain-gages were glued to the sample surface to monitor acoustic emission (AE), velocities and local strain during the loading process. Nucleation of compaction bands is indicated by AE clusters close to the notch tips. With progressive loading, AE activity increased and AE hypocenters indicated propagation of a single CB normal to the sample axis. CB propagation from the sample periphery towards the centre was monitored. Microstructural analysis of deformed samples shows excellent agreement between location of AE clusters and CBs. In both dry and wet samples the lateral propagation of CBs was about 100 times faster than axial shortening rates. At the slowest displacement rate, AE activity during band propagation was reduced and CB nucleation in wet samples occurred at 20% lower stresses. This may indicate an increasing contribution of stress corrosion processes to the formation of the compaction bands. In dry and wet samples inelastic compaction energy per area ranged between 16 and 80 kJ m−2 . This is in good agreement with previous estimates from laboratory and field studies. [ABSTRACT FROM AUTHOR]- Published
- 2009
- Full Text
- View/download PDF
249. Deformation of Earth Materials: Six Easy Pieces
- Author
-
EVANS, BRIAN, primary and DRESEN, GEORG, additional
- Published
- 1991
- Full Text
- View/download PDF
250. Rheology of the Lower Crust and Upper Mantle: Evidence from Rock Mechanics, Geodesy, and Field Observations.
- Author
-
Bürgmann, Roland and Dresen, Georg
- Subjects
- *
RHEOLOGY , *CRUST of the earth , *EARTH'S mantle , *ROCK mechanics , *GEODETIC observations , *DEFORMATIONS (Mechanics) - Abstract
Rock-mechanics experiments, geodetic observations of postloading strain transients, and micro- and macrostructural studies of exhumed ductile shear zones provide complementary views of the style and rheology of deformation deep in Earth's crust and upper mantle. Overall, results obtained in small-scale laboratory experiments provide robust constraints on deformation mechanisms and viscosities at the natural laboratory conditions. Geodetic inferences of the viscous strength of the upper mantle are consistent with flow of mantle rocks at temperatures and water contents determined from surface heat-flow, seismic, and mantle xenolith studies. Laboratory results show that deformation mechanisms and rheology strongly vary as a function of stress, grain size, and fluids. Field studies reveal a strong tendency for deformation in the lower crust and uppermost mantle in and adjacent to fault zones to localize into systems of discrete shear zones with strongly reduced grain size and strength. Deformation mechanisms and rheology may vary over short spatial (shear zone) and temporal (earthquake cycle) scales. [ABSTRACT FROM AUTHOR]
- Published
- 2008
- Full Text
- View/download PDF
Catalog
Discovery Service for Jio Institute Digital Library
For full access to our library's resources, please sign in.