We developed recently a new apparatus which allows laboratory fracturing experiments under tri-axial compression up to 15 MPa with pore water pressure up to 15 MPa. Silica sands with particle size of about 125μm are used as the simulated formation materials. In addition to the sand, some amount of kaolinite flour is mixed for adjusting permeability. The mixture is layered in a mold to form a cubical specimen of 200 × 200 × 200 mm3 with aid of a specially-designed press machine. A fracturing fluid with viscosity of 300 mPa s is injected into a specimen through a slit of a steel pipe buried in the specimen. After the tests, we excavate the specimen bit by bit and observe how the fracturing fluid has invaded into the specimen. In the present study, to examine the effect of pore water on the fracture formation, we carried out the tests for the specimens under various conditions of water saturation, pore pressure and confining stresses. Then we found that the fracturing pressure changes in proportion to the confining stress, and it is not influenced by water saturation and the initial value of pore pressure.
The laboratory testing described here can be seen as the first step in investigating potential thermal effects leading to the creation of a leakage pathway at or in the vicinity of a CO2 injection well. The occurrence of thermal stresses in metal casing, cement and formation can lead to either one or more of these materials developing cracks, or debonding between pairs of materials at their interface. A first investigation is thus concentrating on the rock immediately above the injection reservoir; this sealing rock is most often some variant of shale formation. Here we look at the required temperature contrast between the injected CO2 (or for that matter any other liquid) and the shale formation, in order to initiate tensile fracturing due to the development of tensile stresses exceeding the rock's tensile strength. Finite element simulations suggest that significant fracturing may occur for a temperature contrast of 80° C. An accompanying series of laboratory tests showed that for the chosen shale specimen, fracturing should only be of concern for much higher temperature contrasts.
In this paper, we present the results from laboratory and In Situ rock fracture experiments where calibrated Acoustic Emission (AE) sensors were implemented. By removing sensor distortions, we were able to study the size and magnitude of AE sources in two unique environments. The laboratory experiment was conducted on a cubic sample of Fontainbleau sandstone under true-triaxial stress conditions (σ1 > σ2 > σ3). The In Situ experiment was conducted on a 0.7 m x 0.7 m x 1.1 m rectangular prism of Lac du Bonnet granite located on the side of an underground tunnel during a large-scale excavation response test. For both sets of data, corner Frequency (fo) and moment magnitude (Mw) were found to be inside the ranges 190 kHz < fo < 750 kHz and -7.2 < Mw < -6.5, respectively and all source parameters appeared to obey scaling relationships derived for larger earthquakes.
A rock bolt represents the most important element in ensuring stability of rock mass excavations. This paper presents weaknesses and strengths of methods which deal with evaluation of rock bolts. Most widely used is pull-out test, conducted for capacity determination in destructive manner. To overcome ‘destructivity’, a method of acoustic emission shows that the force close to the failure point can be determined, so that the rock bolt will remain usable for strengthening purposes. Another NDT system, called GRANIT, is developed in order to determine load level inside rock bolt by analyzing its natural frequencies. When it comes to evaluation of grouting quality, efforts were made in order to find reliable solution, but existing methods still have weaknesses which limit their applicability. First developed and today still widely used is Boltometer which is based on emission of energy in rock bolt and on analyzing returned energy. However, dissipation of energy, due to karstic phenomena may not yield good results. Some efforts in improvement of weaknesses of Boltometer have been conducted and are presented in paper. A basic concept is given for novel method for NDT evaluation of grouting quality, which is in development at Faculty of Civil Engineering in Zagreb.
Understanding the geometry of a hydraulic fracture is key to predicting its behavior and performance. Physical measurement of field hydraulic fracture geometries beyond the borehole is difficult and typically cost prohibitive with the only published examples being mine-back studies and cores. Laboratory-scale hydraulic fracturing experiments can more accurately measure the fracture geometry due to smaller specimen size and improved monitoring capabilities. This paper presents laboratory work where hydraulic fracture treatments were performed using epoxy injection such that a propagating fracture could be stabilized and preserved at near-critical state. Constant backpressure was applied after hydraulic breakdown but before cessation of fracture extension to maintain near-critical state geometry. Preliminary results are presented giving measurement of fracture dimensions, including aperture, at the millimeter scale for a hydraulic fractured acrylic specimen. The pressure, flow rate, material strains, acoustic emissions, and video stills associated with this fracture are also presented and analyzed. A second experiment fracturing a 300×300×300 mm3 cubic foot granite block using epoxy is also discussed. Data regarding the interaction between shear and tensile dominated fractures is presented and discussed.
A robust understanding of the thermal stress development due to injection of cold fluids is crucial when developing the Åsgard field on the Norwegian Continental Shelf (NCS) offshore Norway. To get a better and more direct estimation of the stress reduction, a series of triaxial tests under uniaxial strain control were conducted with cooling on reservoir core samples. The purpose of the testing program was to find elastic properties, thermal expansion coefficients and change in confining stress due to temperature reduction. The results show that the cooling related stress effect is strongly stress path dependent. As the sample is subjected to more cooling the stress state tends to approach an elasto-plastic formulation leading to a more soft response of the material. As a consequence the measured stress effect is lower than the estimated which was based on elastic state assumptions.
Crushed salt is being considered as a backfilling material to place around nuclear waste within a salt repository environment. In-depth knowledge of salt thermal and mechanical properties as it reconsolidates is critical to thermal and mechanical modeling of the reconsolidation process.
An experimental study was completed to quantitatively evaluate the thermal conductivity of consolidated crushed salt as a function of porosity. Temperature dependence of this thermal conductivity was also determined. Porosities ranged from 1% to 40%, and temperatures ranged from ambient up to 300°C. This range of conditions is expected to more than cover those that might be encountered in a radioactive waste disposal facility. Two different experimental devices were used to measure these values.
The thermal conductivity of reconsolidated crushed salt decreases with increasing porosity or increasing temperature; conversely, salt thermal conductivity increases as the salt consolidates. Thermal conductivity of experimentally deformed bedded salt cores was shown to be related to fracture density, as a type of porosity. Crushed salt for this study came from the Waste Isolation Pilot Plant (WIPP). Salt was observed to dewater during heating, and the weight loss from dewatering was quantified.
A simple mixture theory model is presented to represent the data developed in this study.
This contribution will examine the design and capabilities of a new measuring system, specifically a high-pressure vessel, which not only enables ultrasonic sounding of rock samples by means of longitudinal P waves, but also by two perpendicularly polarized shear waves on spherical samples under hydrostatic pressures up to 100 MPa. The advantage of our approach is that it allows for the simultaneous measurement of P, S1 and S2 wave velocity propagation in 132 independent directions throughout the entire sphere (except the sphere poles). This new system was designed and constructed to enable the use of movable shear wave transducers (transmitter/receiver) in oil under confining stress. The same high-pressure measuring system enables the measurement of sample deformation in the points of P-wave ultrasonic sounding up to 400 MPa, what enables that mutual dependence between static and dynamic rock parameters to be studied. Data obtained will enable the calculation of many important seismic parameters like elastic anisotropy, crack presence and orientation, crack density tensor, their directional changes and closure under pressure, elastic wave attenuation and finally, full elastic stiffness tensor at different values of hydrostatic pressure. A laboratory approach based on this new high-pressure system enables the study of rock dynamic and static bulk moduli under different values of hydrostatic pressure.
The knowledge of the thermo-mechanical and hydraulically material behaviour of the in-situ rock salt is a main as-sumption to proof the static stability and tightness of salt caverns. The conduction of the material behaviour needed for the numeri-cal design is essentially determined by results, taken from lab tests which are performed at rock salt specimens. State of the art is to conduct temperature controlled uniaxial and triaxial short term test to analyze the strength of the rock salt, temperature controlled uniaxial and triaxial creep tests to analyze the rheological material behaviour and temperature controlled triaxial permeability tests to analyze the hydraulically material behaviour. Due to the demand of an increasing economical optimization of gas storage caverns as well as the installation of high performance storage caverns and compressed air energy storage caverns advanced lab tests are needed to characterize the material behaviour under extreme loading conditions. Regarding to an operation pattern with cyclic turnover, decreasing cavern inside pressure levels, increasing pressure rates and for example the “blow out” load case the aim of the present article is to explain improved lab tests, suitable to increase the performance and availability of storage caverns.
Blanco, Martín L. (Lawrence Berkeley National Laboratory) | Rutqvist, J. (Lawrence Berkeley National Laboratory) | Birkholzer, J.T. (Lawrence Berkeley National Laboratory) | Wolters, R. (Clausthal University of Technology) | Rutenberg, M. (Clausthal University of Technology) | Zhao, J. (Clausthal University of Technology) | Lux, K.-H. (Clausthal University of Technology)
The long-term thermal-hydraulic-mechanical response of a generic salt repository for high-level nuclear waste is investigated using the TOUGH-FLAC simulator, developed at Lawrence Berkeley National Laboratory, and the FLAC-TOUGH simulator, developed at Clausthal University of Technology. Although these sequential simulators rely on the same flow and geomechanics software, they are based on different numerical schemes. One of the aims of using two different approaches to model the same scenario is to gain reliability on the results obtained. The two simulators include state-of-the-art constitutive relationships and coupling functions. The generic scenario studied assumes in-drift emplacement of the waste packages and subsequent backfill of the drifts with crushed salt. The Lux/Wolters constitutive model for natural salt is used. The simulations are two-way coupled and include the stages of excavation, waste emplacement, backfilling and post-closure. This work has been performed within the framework of a collaboration effort between Lawrence Berkeley National Laboratory and Clausthal University of Technology. Although the predictions presented in this paper cover a post-closure period of 100 years, it is intended to continue the benchmark until 100,000 years. The results obtained so far provide confidence in the capabilities of the two simulators to evaluate the barriers integrity over the long-term.