Arno Zang

Fracture process zone in granite

In uniaxial compression tests performed on Aue granite cores (diameter 50 mm, length 100 mm), a steel loading plate was used to induce the formation of a discrete shear fracture. A zone of distributed microcracks surrounds the tip of the propagating fracture. This process zone is imaged by locating acoustic emission events using 12 piezoceramic sensors attached to the samples. Propagation velocity of the process zone is varied by using the rate of acoustic emissions to control the applied axial force. The resulting velocities range from 2 mm/s in displacement-controlled tests to 2 μm/s in tests controlled by acoustic emission rate. Wave velocities and amplitudes are monitored during fault formation. P waves transmitted through the approaching process zone show a drop in amplitude of 26 dB, and ultrasonic velocities are reduced by 10%. The width of the process zone is ∼9 times the grain diameter inferred from acoustic data but is only 2 times the grain size from optical crack inspection. The process zone of fast propagating fractures is wider than for slow ones. The density of microcracks and acoustic emissions increases approaching the main fracture. Shear displacement scales linearly with fracture length. Fault plane solutions from acoustic events show similar orientation of nodal planes on both sides of the shear fracture. The ratio of the process zone width to the fault length in Aue granite ranges from 0.01 to 0.1 inferred from crack data and acoustic emissions, respectively. The fracture surface energy is estimated from microstructure analysis to be ∼2 J. A lower bound estimate for the energy dissipated by acoustic events is 0.1 J.

Acoustic emission, microstructure, and damage model of dry and wet sandstone stressed to failure

Twenty-three uniaxial compression tests were performed on dry and wet Flechtingen sandstone from Germany. Compressive strength of wet core is 60% of the strength of dry core. Before fracture, the transverse P wave speed drops by 13% and the pulse amplitude by 22% for wet and 37% for dry cores. Accumulated strain energy doubles for dry core. Acoustic emissions (AE) are detected with 10 sensors for 19 cores. AE activity starts at 84% of the fracture strength of wet cores (55 MPa) and at 91% of the strength of dry cores (87 MPa). The ratio of located to recorded AE is 0.37 for dry and 0.13 for fully wet cores. AE hypocenter patterns document the development of two opposite fracture cones. The negative slope of cumulative AE-amplitude frequency distribution drops by 50% before failure in dry cores. The slope of the wet core drops and recovers. Energy discrimination of AE detected by a broadband sensor resolves different stages of damage and captures the onset of the dilatant throughgoing macrofracture. Using the analogy to visible light microfracturing events are separated into high-energy short pulses (blue AE) and low-energy pulses with long duration times (red AE). Blue AE are explained by intragranular grain breakage, red AE by multiple stick slip on crack planes or grain boundaries. Deformed cores show highly fractured calcite cement and mostly intact quartz grains. The stochastic damage model for brittle composites developed highlights that microfracturing of the sandstone is controlled by the amount and distribution of the weak mineral (calcite).

Numerical Investigation on Stress Shadowing in Fluid Injection-Induced Fracture Propagation in Naturally Fractured Geothermal Reservoirs

In low permeability shale reservoirs, multi-stage hydraulic fracturing is largely used to increase the productivity by enlarging the stimulated rock volume. Hydraulic fracture created alters the stress field around it, and affects the subsequent fractures by the change of the stress field, in particular, mostly increased minimum principal stress at the area of subsequent fracturing. This is called stress shadow which accumulates as the fracturing stages advance from toe to heel. Hydraulic fractures generated in such altered stress field are shorter and compact with orientation deviating significantly from the far-field maximum horizontal stress orientation. This paper presents 2D discrete element-based numerical modeling of multi-stage hydraulic fracturing in a naturally fractured reservoir and investigates stress shadowing. The stress shadowing is tested with two different injection scenarios: constant and cyclic rate injections. The results show that cyclic injection tends to lower the effect of stress shadow as well as mitigates the magnitude of the induced seismicity. Another modeling case is presented to show how the stress shadow can be utilized to optimize a hydraulic fracture network in application to Groß Schönebeck geothermal reservoir, rather than being mitigated. The modeling demonstrated that the stress shadow is successfully utilized for optimizing the geothermal heat exchanger by altering the initial in situ stress field from highly anisotropic to less or even to isotropic.

World Stress Map Database as a Resource for Rock Mechanics and Rock Engineering

Knowledge of the in situ stress state is of key importance for rock engineering. We inform the reader about the World Stress Map (WSM) database and its application to rock mechanics and rock engineering purpose, and in particular the orientation of maximum horizontal stress. We discuss the WSM and the quality ranking system of stress orientation data. We show one example of discrete-measured and computed-smoothed stress orientations from central and northern Europe with respect to relative plate velocity trajectories. We give first insights into ongoing development of a second, more Quantitative World Stress Map database which compiles globally rock-type specific stress magnitudes versus depth. We discuss the vertical stress component, and the lateral stress coefficient versus depth for different rock types. We display stress magnitudes in 2D and 3D stress space, and investigate stress ratios in relation to depth, lithology and tectonic faulting regime.

Hydro-Mechanical Coupled Discrete Element Modeling of Geothermal Reservoir Stimulation and Induced Seismicity

The injection of fluid underground, the use of CO2, water or waste for storage, or disposal purposes or both, results in a stress field change, the creation of fractures and the reactivation of pre-existing faults and joints. Sometimes these man-made processes are associated with seismicity, e.g. seismicity of a local magnitude 3.4 occurred in Basel Switzerland enhanced geothermal system (EGS) site. Such phenomena have lead to the development of numerical tools that are able to simulate fluid injection into underground reservoirs and predict induced seismicity. Appropriate measures for mitigating larger magnitude events (LME) can then be established after the reliability of the numerical tools is validated. In this context, this paper introduces hydro-mechanical coupled discrete element fracture network models developed for simulating hydraulic fracturing and induced seismicity. Particle Flow Code 2D is used in which the hydro-mechanical coupling routine is implemented, plus the seismicity computation routine. A fractured granitic reservoir with dimension of 2 km x 2 km is constructed using laboratory and field data from Soultz-sous-Forets European Hot Dry Rock project. For mechanical and hydraulic data of the pre-existing fractures, measured data from Forsmark Sweden was adopted for planning the construction of the final repository for spent nuclear fuels. Hydraulic fracturing of intact reservoirs (without fractures) and fractured reservoirs is performed by means of a fluid injection at the model centre under two different scenarios: 1) a one day injection with a monotonic bottom hole pressure (BHP) increase followed by 1.5 days of shut-in where BHP decays non-linearly, 2) a one day injection with a cyclic BHP increase and the same BHP decays. Simulation results are examined in terms of: 1) fracture propagation pattern, 2) magnitude of induced events, 3) potential of LME in post shut-in, 4) influence of different injection schemes on the fracturing pattern and magnitude of induced events. The final objective of this paper is to examine if the presented modeling approach is capable of capturing the field observations and providing a good understanding of the key issues in the development of EGS, in particular for soft stimulation strategies.

Mining-Induced Stress Transfer and Its Relation to a \text{M}_w 1.9 Seismic Event in an Ultra-deep South African Gold Mine

On 27 December 2007, a Mw\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}

Rock Fracture Criteria

\text{M}_w

Particle Based Discrete Element Modeling of Hydraulic Stimulation of Geothermal Reservoirs, Induced Seismicity and Fault Zone Deformation

\end{document} 1.9 seismic event occurred within a dyke in the deep-level Mponeng Gold Mine, South Africa. From the seismological network of the mine and the one from the Japanese–German Underground Acoustic Emission Research in South Africa (JAGUARS) group, the hypocentral depth (3,509 m), focal mechanism and aftershock location were estimated. Since no mining activity took place in the days before the event, dynamic triggering due to blasting can be ruled out as the cause. To investigate the hypothesis that stress transfer, due to excavation of the gold reef, induced the event, we set up a small-scale (450×300×310m3)\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}