Coupled poroelastic stressing and pore-pressure accumulation along pre-existing faults in deep basement contribute to recent occurrence of seismic events at subsurface energy exploration sites. Our coupled fluid-flow and geomechanical model describes the physical processes inducing seismicity corresponding to the sequential stimulation operations in Pohang, South Korea. Simulation results show that prolonged accumulation of poroelastic energy and pore pressure along a fault can nucleate seismic events larger than Mw3 even after terminating well operations. In particular the possibility of large seismic events can be increased by multiple-well operations with alternate injection and extraction that can enhance the degree of pore-pressure diffusion and subsequent stress transfer through a rigid and low-permeability rock to the fault. This study demonstrates that the proper mechanistic model and optimal well operations need to be accounted for to mitigate unexpected seismic hazards in the presence of the site-specific uncertainty such as hidden/undetected faults and stress regime.
Measuring the size and orientation of borehole breakouts is one of the primary methods for determining the orientation and magnitudes of the in situ stresses in the subsurface. To better understand the effects of anisotropy on borehole breakouts, experiments were conducted on Mancos Shale, a finely laminated mudrock. A novel testing configuration was developed to conduct borehole breakout experiments in a standard triaxial vessel and load frame. Samples were prepared at three different orientations and deformed under 6.9 to 20.7 MPa confining pressure. The results show a variation of peak strength and breakout geometry depending on the lamination orientation. Samples deformed parallel to laminations failed at a higher maximum compressive stress than samples deformed perpendicular to laminations, which were stronger than inclined samples. These relationships are quantified by a cosine-based failure envelope. Observed breakout shapes in perpendicular samples are V-shaped and symmetric around the borehole, which advance as a series of fractures of increasing size into the sidewalls. In inclined samples, fractures form along weaker laminations planes and grow in an en echelon pattern towards the axial stress direction. In parallel samples, long fractures grow from the wellbore towards the axial stress direction. The observed geometries highlight potential sources of error in calculating in situ stresses from borehole breakouts.
The Nevada National Security Site (NNSS) serves as the geologic setting for a Source Physics Experiment (SPE) program. The SPE provides ground truth data to create and improve strong ground motion and seismic S-wave generation and propagation models. The NNSS was chosen as the test bed because it provides a variety of geologic settings ranging from relatively simple to very complex. Each series of SPE testing will comprise the setting and firing of explosive charges (source) placed in a central borehole at varying depths and recording ground motions in instrumented boreholes located in two rings around the source, positioned at different radii. Modeling using advanced simulation codes will be performed both before and after each test to predict ground response and to improve models based on acquired field data, respectively. A key component in the predictive capability and ultimate validation of the models is the full understanding of the intervening geology between the source and the instrumented boreholes including the geomechanical behavior of the site's rock/structural features. This report summarizes unconfined compression testing (UCS) from coreholes U-15n#12 and U-15n#13 and compares those datasets to UCS results from coreholes U-15n and U-15n#10. U-15n#12 corehole was drilled at -60° to the horizontal and U-15n#13 was drilled vertically in granitic rock (quartz monzonite) after the third SPE shot. Figure 1 illustrates at the surface, U 15n#12 and U-15n#13 coreholes were approximately 30 meters and 10 meters from the central SPE borehole (U-15n) respectively. Corehole U-15n#12 intersects the central SPE borehole (U 15n) at a core depth of 174 feet (approximately 150 feet vertical depth). The location of U 15n#12 and U-15n#13 is the site of the first, second and third SPE's, in Area 15 of the NNSS.
Dynamic Brazilian tension (DBR) tests from core hole U-15n are part of a larger material characterization effort for the Source Physics Experiment (SPE) project. This larger effort encompasses characterizing Climax Stock granite rock from the Nevada National Security Site (NNSS) both before and after each SPE shot. The current test series includes DBR tests on dry intact granite and fault material at depths of -85 and -150 ft.
Triaxial compression tests from core hole U-15n are part of a larger material characterization effort for the Source Physics Experiment (SPE) project. This larger effort encompasses characterizing Climax Stock granite rock from the Nevada National Security Site (NNSS) both before and after each SPE shot. The current test series includes triaxial compression tests on dry and saturated intact granite and fault material at 100, 200, 300, and 400 MPa confining pressure.
The Nevada National Security Site (NNSS) serves as the geologic setting for a Source Physics Experiment (SPE) program. The SPE provides ground truth data to create and improve strong ground motion and seismic S-wave generation and propagation models. The NNSS was chosen as the test bed because it provides a variety of geologic settings ranging from relatively simple to very complex. Each series of SPE testing will comprise the setting and firing of explosive charges (source) placed in a central borehole at varying depths and recording ground motions in instrumented boreholes located in two rings around the source, positioned at different radii. Modeling using advanced simulation codes will be performed both before and after each test to predict ground response and to improve models based on acquired field data, respectively. A key component in the predictive capability and ultimate validation of the models is the full understanding of the intervening geology between the source and the instrumented boreholes including the geomechanical behavior of the site's rock/structural features. This memorandum reports on an initial phase of unconfined compression testing from corehole U-15n#10. Specimens tested came from the U-15n#10 core hole, which was drilled at -60° to the horizontal in granitic rock (quartz monzonite) after the second SPE shot (SPE-2). Figure 1 illustrates at the surface, the core hole was approximately 90 feet from the central SPE borehole. Corehole U 15n#10 intersects the central SPE borehole (U-15n) at a core depth of 170 feet (approximately 150 feet vertical depth) which is within the highly damaged zone of SPE-2. The U-15n#10 location is the site of the first, second and third SPE's, in Area 15 of the NNSS.
In recent years, seismicity rates in the US have dramatically risen due to increased activity in onshore oil and gas production. This project attempts to tie observations about induced seismicity to dehydration reactions in laumontite, a common mineral found in fault gouge in crystalline basement formations. It is the hypothesis of this study that in addition to pressurerelated changes in the in situ stress state, the injection of wastewater pushes new fluids into crystalline fault fracture networks that are not in chemical equilibrium with the mineral assemblages, particularly laumontite in fault gouge. Experiments were conducted under hydrothermal conditions where samples of laumontite were exposed to NaC1 brines at different pH values. After exposure to different fluid chemistries for 8 weeks at 90° C, we did not observe substantial alteration of laumontite. In hydrostatic compaction experiments, all samples deformed similarly in the presence of different fluids. Pore pressure decreases were observed at the start of a 1 week hold at 85° C in a 1M NaC1 pH 3 solution, suggesting that acidic fluids might stabilize pore pressures in basement fault networks. Friction experiments on laumontite and kaolinite powders showed both materials have similar coefficients of friction. Mixtures with partial kaolinite content showed a slight decrease in the coefficient of friction, which could be sufficient to trigger slip on critically stressed basement faults.
The state of stress in the earth is complicated and it is difficult to determine all three components and directions of the stress. However, the state of stress affects all activities which take place in the earth, from causing earthquakes on critically stressed faults, to affecting production from hydraulically fractured shale reservoirs, to determining closure rates around a subterranean nuclear waste repository. Current state of the art methods commonly have errors in magnitude and direction of up to 40%. This is especially true for the intermediate principal stress. This project seeks to better understand the means which are used to determine the state of stress in the earth and improve upon current methods to decrease the uncertainty in the measurement. This is achieved by a multipronged experimental investigation which is closely coupled with advanced constitutive and numeric modeling.
The study described in this report involves heated and unheated pressurized slot testing to determine thermo-mechanical properties of the Tptpll (Tertiary, Paintbrush, Topopah Spring Tuff Formation, crystal poor, lower lithophysal) and Tptpul (upper lithophysal) lithostratigraphic units at Yucca Mountain, Nevada. A large volume fraction of the proposed repository at Yucca Mountain may reside in the Tptpll lithostratigraphic unit. This unit is characterized by voids, or lithophysae, which range in size from centimeters to meters, making a field program an effective method of measuring bulk thermal-mechanical rock properties (thermal expansion, rock mass modulus, compressive strength, time-dependent deformation) over a range of temperature and rock conditions. The field tests outlined in this report provide data for the determination of thermo-mechanical properties of this unit. Rock-mass response data collected during this field test will reduce the uncertainty in key thermal-mechanical modeling parameters (rock-mass modulus, strength and thermal expansion) for the Tptpll lithostratigraphic unit, and provide a basis for understanding thermal-mechanical behavior of this unit. The measurements will be used to evaluate numerical models of the thermal-mechanical response of the repository. These numerical models are then used to predict pre- and post-closure repository response. ACKNOWLEDGEMENTS The authors would like to thank David Bronowski, Ronnie Taylor, Ray E. Finley, Cliff Howard, Michael Schuhen (all SNL) and Fred Homuth (LANL) for their work in the planning and implementation of the tests described in this report. This is a reprint of SAND2004-2703, which was originally printed in July 2004. At that time, it was printed for a restricted audience. It has now been approved for unlimited release.
A novel experimental geometry is combined with acoustic emission monitoring capability to measure crack growth and damage accumulation during laboratory simulations of borehole breakout. Three different experiments are conducted in this study using Sierra White Granite. In the first experiment, the sample is deformed at a constant 17.2 MPa confining pressure without pore fluids; in the second experiment, the sample is held at a constant effective pressure of 17.2 MPa with a constant pore pressure; and in the third experiment, pore pressure is modified to induce failure at otherwise constant stress. The results demonstrate that effective pressure and stress path have controlling influence on breakout initiation and damage accumulation in laboratory simulations of wellbore behavior. Excellent agreement between the dry test and constant pore pressure test verify the application of the effective pressure law to borehole deformation. Located AE events coincide with post-test observations of damage and fracture locations. Comparison of AE behavior between the experiments with pore pressure show that breakouts develop prior to peak stress, and continued loading drives damage further into the formation and generates shear fractures.