ISRM-Suggested Method for Determining the Mode I Static Fracture Toughness Using Semi-Circular Bend Specimen

The International Society for Rock Mechanics has so far developed two standard methods for the determination of static fracture toughness of rock. They used three different core based specimens and tests were to be performed on a typical laboratory compression or tension load frame. Another method to determine the mode I fracture toughness of rock using semicircular bend specimen is herein presented. The specimen is semicircular in shape and made from typical cores taken from the rock with any relative material directions noted. The specimens are tested in three-point bending using a laboratory compression test instrument. The failure load along with its dimensions is used to determine the fracture toughness. Most sedimentary rocks which are layered in structure may exhibit fracture properties that depend on the orientation and therefore measurements in more than one material direction may be necessary. The fracture toughness measurements are expected to yield a size-independent material property if certain minimum specimen size requirements are satisfied

Numerical borehole breakout analysis using FRACOD2D

The borehole breakouts of a geothermal energy extraction well in the Northeast German basin (north of Berlin) are analysed using the fracture mechanics based software FRACOD2D. The objective of the study is to estimate the magnitude of the maximum horizontal stress around the vertical well at a depth of 4,100 m. The FRACOD2D is a fracture initiation and propagation code developed for rock fracture mechanics analysis. It is designed to predict the fracture propagation and interaction of fractures in a rock medium. The numerical model is set up for different magnitudes of the maximum horizontal stress and fluid pressure in the well. Data from a stimulation campaign estimate the minimum horizontal stress (50 MPa). The numerical results are analysed in terms of breakout angle. Comparison of the numerical results and field observations are discussed. The maximum horizontal stress is determined to be around 95 MPa at target depth. With the vertical stress being about 100 MPa this indicates a strike-slip faulting regime which is in perfect agreement with the structural geology data

Results of Stimulation Treatments at the Geothermal Research Wells in Groß Schönebeck, Germany

The aim of this work is to present results of previously performed fracture treatments in the geothermal research well GtGrSk4/05 at GroB Schonebeck. The fracture treatments included three hydraulic stimulations, two in the sandstone section of the Lower Permian and one in the volcanic section. In low permeable volcanic rocks we performed a cyclic waterfrac treatment over 6 days in conjunction with adding low sand concentrations. Flow rates of up to 150 1/s were realized with a total amount of injected water of 13170 m\ Monitoring water level in the offsetting well EGrSk3/90, which is 475 m apart at the final depth, showed a very rapidly water level increase due to the stimulation treatment. A possible explanation might be a fault zone in the volcanic rocks. It was known from previous treatments in the offsetting well that high permeable sandstones do not show a self propping effect, hence we performed two gel proppant treatments in these sandstones to maintain long-term access to the reservoir. A total amount of 100 to of high strength proppants with 500 m3 of cross-linked gel were injected during each treatment. The subsequent production test in conjunction with flowmeter logging showed the success of the treatments

Characterising and modelling the excavation damaged zone (EDZ) in crystalline rock in the context of radioactive waste disposal

This paper describes current knowledge about the nature of and potential for thermo-hydro-mechanical-chemical modelling of the Excavation Damaged Zone (EDZ) around the excavations for an underground radioactive waste repository. In the first part of the paper, the disturbances associated with excavation are explained, together with reviews of Workshops that have been held on the subject. In the second part of the paper, the results of a DECOVALEX research programme on modelling the EDZ are presented. Four research teams used four different models to simulate the complete stress-strain curve for Avro granite from the Swedish Aespoe Hard Rock Laboratory. Subsequent research extended the work to computer simulation of the evolution of the repository using a 'wall block model' and a 'near-field model'. This included assessing the evolution of stress, failure and permeability and time dependent effects during repository evolution. As discussed, all the computer models are well suited to sensitivity studies for evaluating the influence of their respective supporting parameters on the complete stress-strain curve for rock and for modelling the EDZ

SFA, TFA and a new thick frontier: graphical and analytical comparisons

This article compares OLS, the normal-half normal stochastic frontier approach (SFA) and the thick frontier approach (TFA) to an alternative thick frontier approach based on a mixture approach. Unlike the TFA approach, the new approach developed here uses all of the data, does not require grouping of the data into an arbitary number of size categories, does not require an arbitrarily chosen fraction (usually the lowest quartile) of lowest average cost firms upon which to base the frontier. The new thick frontier requires the estimation of only one more parameter than the SFA model, and is flexible enough to describe skewness in the data of almost any type. This article presents comparisons of the empirical relationships between these methods using a multiproduct cost function and data on US savings and loans in 1988. The new thick frontier method produces a much thinner 'thick' frontier characterizing a much greater fraction of the data than the old TFA approach.