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 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.
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.
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.
Because of relatively recent decisions by the current administration and its renewed assessment of the nuclear life-cycle, the various deep geologic disposal medium options are once again open for consideration. This paper focuses on addressing the favorable creep properties and behavior of rock salt, from the computational modeling perspective, as it relates to its potential use as a disposal medium for a deep geologic repository. The various components that make up a computational modeling capability to address the thermo-mechanical behavior of rock salt over a wide range of time and space are presented here. Several example rock salt calculations are also presented to demonstrate the applicability and validity of the modeling capability described herein to address repository-scale problems. The evidence shown points to a mature computational capability that can generate results relevant to the design and assessment of a potential rock salt HLW repository. The computational capability described here can be used to help enable fuel cycle sustainability by appropriately vetting the use of geologic rock salt for use as a deep geologic disposal medium.
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.
Our current laboratory investigations quantified the local stress states on a laboratory fault which control the transition of sliding from stable (quasi-static) to unstable (dynamic), commonly referred to as rupture nucleation. A fault was experimentally modeled using two Poly(methyl methacrylate) samples in a direct shear configuration. A pressure-sensitive film was employed to localize and map the contact junctions (asperities) throughout the interface A portion of the fault, characterized by a high density distribution of larger asperities, experienced considerably less slow (aseismic) slip than neighboring regions. Foreshocks detected acoustically were observed to coalesce in this region moments before the interface rapidly slipped. A high definition video camera was focused on this region and light passing through asperities appeared brighter than the non-contacting regions. Foreshocks in this slip deprived region caused, sudden changes in light intensity passing through these asperities and were concomitant with recorded acoustic emission. Source radius of failing asperities measured from the camera (0.56 mm) were similar to that estimated from the Brune corner frequency model (0.81 mm). Dynamic triggering of smaller strong asperities directly adjacent to larger asperities (measured by the camera) likely increased the high-frequency content measured in smaller foreshocks.
Discontinuity sample size influences the experimental shearing parameters of rock joints. To study how peak shear strength evaluation depends on sample size several shear tests were conducted on artificial joints prepared with molds used for encapsulating rock joints. Discontinuity surfaces were shaped with two different saw-tooth profiles. For each one three joint surface sizes were considered: 64, 100 and 144 cm2. The same series of shear tests with dry joint surfaces was repeated with wetted discontinuity samples. Some reduction was observed in peak shear strength values. Tests with dry surfaces were simulated in numerical calculations that reproduced satisfactorily experimental results. Size effect seems to be more important when normal stress is high. With low normal stress imbrication cohesion is influenced by sample size. Some reflection is advisable to propose a transition from one situation to the other. Calibration of numerical models with more laboratory tests will validate them to be used in later developments.
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.
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.