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Fractures are ubiquitous in Earth's upper crust and strongly affect its physical properties. Their visibility for elastic waves hinges on their stiffness, a measure of the relation between changes in fracture aperture and applied stress, which itself significantly increases with increasing stress according to laboratory data. We constrained normal stiffness (and its stress dependence) of two fractures intersecting a borehole in the research mine 'Reiche Zeche' in Freiberg, Saxony, Germany, from pressure-recovery behaviour observed when the pressure in a double-packer interval enclosing them was briefly released during shut-in sequences. For the evaluation of the pressure recovery, we developed a generic model that accounts for the inherent convolution of mechanical closing of the fracture, fluid flow in the fracture, fluid exchange between fracture and the surrounding rock, here addressed as fracture leakage, and effects associated with finite borehole storage. We address the hydraulic system by considering three different 'storage containers', described by non-dimensionless storage parameters: the wellbore with a fixed storage capacity, the fracture with storage capacities related to fluid compressibility and fracture stiffness, and the surrounding rock constituting a container with unlimited storage capacity. To examine the model's characteristics, we investigated pressure transients predicted for specific cases, such as negligible fracture leakage, and subsequently numerically determined the sensitivity of the model predictions to the involved model parameters. The model's application to the field data yield estimates of properties of the fractures (length, aperture and stiffness) and of the surrounding rock mass (product of permeability and specific storage capacity). The parameters derived from the modelling compare well with independent constraints. An analysis of currently available fracture-stiffness data shows that the lack of knowledge of the stress dependence of stiffness severely hampers firm conclusions on scale dependence and the bias between methods. [ABSTRACT FROM AUTHOR]

The STIMTEC underground experiment was designed to investigate stimulation processes involved in hydrofrac and hydroshear activation aiming at a permeability enhancement in deep geothermal projects. We combined periodic pumping tests, high resolution seismic monitoring, structural analysis and drilling into stimulated volumes in an effort to improve near-real-time monitoring, phenomenological models of the hydrofrac/hydroshear process, and prognosis strategies. The ongoing experiment is located at the Reiche Zeche underground laboratory below Freiberg in Saxony/Germany at a depth of about 130 m below surface in metamorphic gneisses. A structural analysis of the test volume enclosed by tunnels preceded the experimental program. The massive gneiss was found to be strongly foliated and three steeply dipping shear zones were identified that cut across the test volume between the access tunnels. Subsequently, a combined seismic network consisted of 12 broad-band acoustic emission sensors (sensitivity 1-100 kHz) and three 1-component Wilcoxon accelerometers (sensitivity 50 Hz-25 kHz). These sensors were installed in boreholes drilled into the test volume, surrounding the stimulation site, and providing optimum spatial coverage. A stimulation borehole with 63 m length was drilled with 15° northward inclination. Analysis of the core material, and borehole logs were used to identify potential intervals for hydraulic tests and stimulation. With a maximum horizontal stress direction striking 10°W from N, as typical for Germany, stimulation-induced fractures were expected to form NNW-SSE striking tensile hydrofractures and NNE-SSW striking hydroshears. We performed a series of laboratory tests on core samples to estimate fracture toughness and elastic properties of the gneiss. A strong elastic wave anisotropy was found with fast and slow propagation parallel and perpendicular to the foliation, respectively. The laboratory measurements were found to be in good agreement with an extensive series of active ultrasound measurements performed in the mine to establish a velocity model of the test volume. In addition to a strong effect of metamorphic foliation on the elastic properties, the field measurements revealed significant local velocity variations possibly related to damage/fault zones. After an extended campaign characterizing the hydraulics of the test volume, a series of ten stimulation tests were performed in the injection borehole. Each stimulation stage consisted of a frac stage, several refracs, and a subsequent hydraulic testing period. A total of > 5000 high frequency events were induced. Currently, analyses of the induced microseismicity and the hydraulic measurements are performed. In Spring 2019, a series of boreholes will be drilled into the stimulated volume as identified by the spatial distribution of detected seismicity. These boreholes will be fully cored to retrieve samples for structural analysis and testing. A series of hydraulic tests using the mine-back boreholes will conclude the experiment. [ABSTRACT FROM AUTHOR]

A rock is characterized by two hydraulic properties, the ease with which fluids can flow through it and the ability to store liquids. Fluid which is injected into a saturated rock is partly transported through the connected pore space following the prevailing pore-pressure gradient. Part of the fluid is stored because of its compressibility and that of the pore space. Thus, to describe the complete hydraulic behavior of a material, it is necessary to determine the transport property, e.g., permeability, as well as the storage property, e.g., specific storage capacity.Hydraulic characteristics have been determined in laboratory tests for more than 50 years. Since the development of the oscillatory methods, it is possible to determine permeability and storage capacity reliably in a single experiment. In addition to the previously presented axial pore-pressure and pore-flow methods, a radial oscillatory pore-flow method is here used in laboratory experiments for the first time. The studied rock types Berea sandstone, Wilkeson sandstone and Westerly granite cover a range in connected porosity from <1 to 18 % and represent pore spaces from granular to micro-fracture dominated. The validity of the results was assessed by comparison with results from Darcy tests and physical limits.The hydraulic properties, determined with the three oscillatory methods, differ significantly partly in a qualitative (pressure and period dependence) but mainly in a quantitative (different absolute values and variability) way. We argue that information about the structure of the pore space can be obtained by combining the different oscillatory methods and varying the oscillation period. Penetration depth depends on the selection of period. Furthermore, the three methods examine different parts of a heterogeneous pore space. With the different oscillatory methods, a portfolio of methods for the determination of hydraulic properties in laboratory tests is now available whose employment in combination with systematic variations of the oscillation period provides an exceptional potential for improving the understanding of the relation between microstructure and hydraulic properties. [ABSTRACT FROM AUTHOR]