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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.

The mechanical behavior of frozen soils plays a central role in the design, construction, and maintenance of buildings and infrastructure in cold regions. We present and discuss the main results of an experimental campaign focused on the mechanical behavior of frozen soils from Alaska. The data set is unique because it involves natural frozen soils and covers a wide range of temperatures (i.e., from −6 °C up to −26 °C) and confinements (i.e., from unconfined conditions up to a cell pressure of 54.6 MPa). An elastoplastic model for frozen soils is presented and validated based on the test results. The elastoplastic model accounts for the effects of temperature and cryogenic suction on key properties associated with frozen soil behavior, namely, increase of preconsolidation pressure, stiffness, and strength with the decrease of temperature. Other aspects such as the volumetric behavior of frozen soils at different temperatures and the smooth transition between elastic and plastic states are also qualitatively well captured by the model. We implemented the constitutive model in a fully coupled thermo-hydro-mechanical simulator and we analyzed the behavior of a foundation in a thawing ground. The proposed approach is able to capture the main feature of frozen soil behavior.

A helium leakage detection system was modified to measure gas permeability on extracted cores of nearly impermeable rock. The Helium-Mass-Spectrometry-Permeameter (HMSP) is duplicating the classic Darcy’s experiment with a constant pressure differential and steady-state flow through a sample using helium gas. Under triaxial stress condition, the newly developed HMSP can measure hydraulic permeability of rocks and geomaterials down to the nanoDarcy scale (10−21 m2). The extension of measuring the lower end of the permeability scale may help answer important questions regarding the permeability of rock at great depth where fractures may close completely under high lithostatic stress.

As a part of the U.S. Department of Energy (DOE) SubTER (Subsurface Technology and Engineering Research, Development and Demonstration) initiative, University of Wisconsin- Madison, Sandia National Laboratories, and Lawrence Berkeley National Laboratory conducted the Permeability (k) and Induced Seismicity Management for Energy Technologies (kISMET) project. The objectives of the project are to define the in situ status of stress in the Sanford Underground Research Facility (SURF) in Lead, South Dakota and to establish the relations between in situ stress and induced fracture through hydraulically stimulating the fracture. (SURF) in Lead, South Dakota. In situ tests are conducted in a 7.6 cm diameter and 100 long vertical borehole located in the 4850 Level West Access Drift near Davies Campus of SURF (Figure 1). The borehole is located in the zone of Precambrian Metamorphic Schist.

There are four basic thermal materials used on the space shuttle Columbia, Reinforced-Carbon-Carbon (RCC), Low- and High Temperature Reusable Surface Insulation tiles (LRSI and HRSI, respectively), and Felt Reusable Surface Insulation (FRSI) blanket 1 . The purpose of this work was to obtain the mechanical behavior of the materials involved in the modeling of the foam impact scenario for the shuttle accident investigation. Experiments were designed to provide the modeling parameters of HRSI tiles and RCC as well as to give insight into the failure phenomena under different loading conditions. For each material the results are presented as a function of orientation and applied strain rate. The effect of aging on the RCC material properties is also discussed.

Simulation results of near-wellbore failure phenomena are presented from a joint experimental-numerical study directed at developing a robust numerical simulation capability for the exploration and prediction of near-wellbore mechanics. An experimental procedure was developed for the laboratory simulation of slurry injection. A true-triaxial vessel, which applied realistic, three-dimensional stress conditions, was used to perform slurry injection into Berea sandstone. Under anisotropic horizontal stress conditions, vertical hydraulic fractures initiated and propagated in the direction of the maximum horizontal stress. Under isotropic horizontal stress conditions, multiple vertical fractures were induced and propagated in random orientation. A computationally efficient numerical model based on the discrete element method (DEM) is described and applied to simulate various wellbore phenomena. Radially graded, two-dimensional DEM models of the near-wellbore region were created of bonded disk elements. Source ...

We have conducted five hydraulic fracturing stress measurement campaigns in Korea, involving 13 test holes ranging in depth from 30 to 250 m, at locations from North Seoul to the southern coast of the peninsula. The measurements reveal consistent crustal stress magnitudes and directions that suggest persistence throughout western and southern Korea. The maximum horizontal stress σ H is oriented between ENE–WSW and E–W, in accord with plate movement and deformation, and with directions indicated by both focal mechanism solutions from earthquakes inland and offshore as well as borehole breakouts in mainland China close to its eastern coast. With respect to magnitudes, the vertical stress is the overall minimum stress at all tested locations, suggesting a thrust faulting regime within the relatively shallow depths reached by our tests. Typically, such a stress regime becomes one favoring strike-slip at greater depths, as is also indicated by the focal mechanism solutions around Korea.

A set of 10 complete hydraulic fracturing stress measurements in an anorthosite was conducted as part of the site investigation of the SM3 hydroelectric project, located some 800 km northeast of Montreal, Quebec. These were the first such measurements in that region of Canada. The vertical test hole was N-size, 260 m deep and adjacent to the future powerhouse. Within the relatively short depth range tested two apparent stress regimes were detected. In the 160–220 m range all three principal in situ stresses approach being lithostatic, suggesting stress relief probably induced by natural fracturing in the rock as evidence by considerable jointing the extracted core. At depths comparable to those of the planned powerhouse (220–255 m) the least horizontal stress is quite consistent and averages 11.0 (± 1.4) MPa, the maximum horizontal stress averages 17.7 (± 3.1) MPa and acts in a NE-SW direction, and the vertical stress is 7.5 (± 0.4) MPa. Thus, ratios of 1.6:1 between the two horizontal principal stresses and of 2.5:1 between the maximum horizontal and vertical stresses, can be expected, and should be carefully considered in design, as should the fact that both horizontal stresses are substantially larger than the vertical stress. The northeastern direction of the maximum horizontal stress, as well as the compressive nature of the measured stresses are in accord with most measurements in eastern North America.

The determination of material parameters for rocks which display anisotropic behaviour has become more important in light of the development of powerful and economical analytical modeling techniques. The study presented herein is based on a testing framework aimed at determining the material parameters required to describe the behaviour of a transverse isotropic solid under axisymmetric loading conditions.

We conducted 9 complete hydraulic fracturing in situ stress measurements in vertical borehole HF1 at the 420 Level of AECL's Underground Research Laboratory (URL), near Pinawa, Manitoba. The tests did not result in vertical fractures. Thus, a generalized least-squares criterion was employed to compute the estimated principal horizontal in situ stresses. The large number of redundant test results enabled us to estimate the stress condition with confidence, even though the hydraulic fractures were only gently inclined. The estimated in situ stress regime within the range between 25 and 95 m below the 420 Level (445–515 m below the surface) is S v = 12–14 MPA ; S h = 36(±16) MPA ; S H = 54(±13) MPa at 120°(±32°). The results are consistent with other independently conducted measurements at the URL.

We subjected boreholes drilled vertically in the center of cubical specimens of Lac du Bonnet granite to constant vertical (S v ) and least horizontal (S h ) farfield stresses, and increased the maximum horizontal stress (S H ) until breakouts occurred. A thin section study revealed that extensile cracking behind the borehole in two zones aligned with S h preceded the development of breakouts. Progressive detachment of thin long flakes bounded by these densely spaced subparallel cracks led to ‘dog ear’ shaped breakouts. Breakout mechanism and appearance match those found in the field. Results also suggest the potential for using breakout occurrence to estimate S H if S h is known or vice versa.

Three dimensional finite element analyses were performed to evaluate the structural integrity of the caverns located at the Bayou Choctaw (BC) site which is considered a candidate for expansion. Fifteen active and nine abandoned caverns exist at BC, with a total cavern volume of some 164 MMB. A 3D model allowing control of each cavern individually was constructed because the location and depth of caverns and the date of excavation are irregular. The total cavern volume has practical interest, as this void space affects total creep closure in the BC salt mass. Operations including both cavern workover, where wellhead pressures are temporarily reduced to atmospheric, and cavern enlargement due to leaching during oil drawdowns that use water to displace the oil from the caverns, were modeled to account for as many as the five future oil drawdowns in the six SPR caverns. The impacts on cavern stability, underground creep closure, surface subsidence, infrastructure, and well integrity were quantified.

3-D finite element analyses were performed to evaluate the structural integrity of caverns located at the Strategic Petroleum Reserve's Big Hill site. State-of-art analyses simulated the current site configuration and considered additional caverns. The addition of 5 caverns to account for a full site and a full dome containing 31 caverns were modeled. Operations including both normal and cavern workover pressures and cavern enlargement due to leaching were modeled to account for as many as 5 future oil drawdowns. Under the modeled conditions, caverns were placed very close to the edge of the salt dome. The web of salt separating the caverns and the web of salt between the caverns and edge of the salt dome were reduced due to leaching. The impacts on cavern stability, underground creep closure, surface subsidence and infrastructure, and well integrity were quantified. The analyses included recently derived damage criterion obtained from testing of Big Hill salt cores. The results show that from a structural view point, many additional caverns can be safely added to Big Hill.

To establish mechanical properties and failure criteria of silicon carbide (SiC-N) ceramics, a series of quasi-static compression tests has been completed using a high-pressure vessel and a unique sample alignment jig. This report summarizes the test methods, set-up, relevant observations, and results from the constitutive experimental efforts. Results from the uniaxial and triaxial compression tests established the failure threshold for the SiC-N ceramics in terms of stress invariants (I{sub 1} and J{sub 2}) over the range 1246 < I{sub 1} < 2405. In this range, results are fitted to the following limit function (Fossum and Brannon, 2004) {radical}J{sub 2}(MPa) = a{sub 1} - a{sub 3}e -a{sub 2}(I{sub 1}/3) + a{sub 4} I{sub 1}/3, where a{sub 1} = 10181 MPa, a{sub 2} = 4.2 x 10{sup -4}, a{sub 3} = 11372 MPa, and a{sub 4} = 1.046. Combining these quasistatic triaxial compression strength measurements with existing data at higher pressures naturally results in different values for the least-squares fit to this function, appropriate over a broader pressure range. These triaxial compression tests are significant because they constitute the first successful measurements of SiC-N compressive strength under quasistatic conditions. Having an unconfined compressive strength of {approx}3800 MPa, SiC-N has been heretofore tested more » only under dynamic conditions to achieve a sufficiently large load to induce failure. Obtaining reliable quasi-static strength measurements has required design of a special alignment jig and load-spreader assembly, as well as redundant gages to ensure alignment. When considered in combination with existing dynamic strength measurements, these data significantly advance the characterization of pressure-dependence of strength, which is important for penetration simulations where failed regions are often at lower pressures than intact regions. « less

To establish strength criteria of Big Hill salt, a series of quasi-static triaxial compression tests have been completed. This report summarizes the test methods, set-up, relevant observations, and results. The triaxial compression tests established dilatant damage criteria for Big Hill salt in terms of stress invariants (I{sub 1} and J{sub 2}) and principal stresses ({sigma}{sub a,d} and {sigma}{sub 3}), respectively: {radical}J{sub 2}(psi) = 1746-1320.5 exp{sup -0.00034I{sub 1}(psi)}; {sigma}{sub a,d}(psi) = 2248 + 1.25 {sigma}{sub 3} (psi). For the confining pressure of 1,000 psi, the dilatant damage strength of Big Hill salt is identical to the typical salt strength ({radical}J{sub 2} = 0.27 I{sub 1}). However, for higher confining pressure, the typical strength criterion overestimates the damage strength of Big Hill salt.

This document is the final report for the Compressed Air Energy Storage Monitoring to Support Refrigerated-Mined Rock Cavern Technology (CAES Monitoring to Support RMRCT) (DE-FC26-01NT40868) project to have been conducted by CAES Development Co., along with Sandia National Laboratories. This document provides a final report covering tasks 1.0 and subtasks 2.1, 2.2, and 2.5 of task 2.0 of the Statement of Project Objectives and constitutes the final project deliverable. The proposed work was to have provided physical measurements and analyses of large-scale rock mass response to pressure cycling. The goal was to develop proof-of-concept data for a previously developed and DOE sponsored technology (RMRCT or Refrigerated-Mined Rock Cavern Technology). In the RMRCT concept, a room and pillar mine developed in rock serves as a pressure vessel. That vessel will need to contain pressure of about 1370 psi (and cycle down to 300 psi). The measurements gathered in this study would have provided a means to determine directly rock mass response during cyclic loading on the same scale, under similar pressure conditions. The CAES project has been delayed due to national economic unrest in the energy sector.

A suite of laboratory triaxial compression and triaxial steady-state creep tests provide quasi-static elastic constants and damage criteria for bedded rock salt and dolomite extracted from Cavern Well No.1 of the Tioga field in northern Pennsylvania. The elastic constants, quasi-static damage criteria, and creep parameters of host rocks provides information for evaluating a proposed cavern field for gas storage near Tioga, Pennsylvania. The Young's modulus of the dolomite was determined to be 6.4 ({+-}1.0) x 10{sup 6} psi, with a Poisson's ratio of 0.26 ({+-}0.04). The elastic Young's modulus was obtained from the slope of the unloading-reloading portion of the stress-strain plots as 7.8 ({+-}0.9) x 10{sup 6} psi. The damage criterion of the dolomite based on the peak load was determined to be J{sub 2}{sup 0.5} (psi) = 3113 + 0.34 I{sub 1} (psi) where I{sub 1} and J{sub 2} are first and second invariants respectively. Using the dilation limit as a threshold level for damage, the damage criterion was conservatively estimated as J{sub 2}{sup 0.5} (psi) = 2614 + 0.30 I{sub 1} (psi). The Young's modulus of the rock salt, which will host the storage cavern, was determined to be 2.4 ({+-}0.65) x 10{sup 6} psi, with a Poisson's ratio of 0.24 ({+-}0.07). The elastic Young's modulus was determined to be 5.0 ({+-}0.46) x 10{sup 6} psi. Unlike the dolomite specimens under triaxial compression, rock salt specimens did not show shear failure with peak axial load. Instead, most specimens showed distinct dilatancy as an indication of internal damage. Based on dilation limit, the damage criterion for the rock salt was estimated as J{sub 2}{sup 0.5} (psi) = 704 + 0.17 I{sub 1} (psi). In order to determine the time dependent deformation of the rock salt, we conducted five triaxial creep tests. The creep deformation of the Tioga rock salt was modeled based on the following three-parameter power law as {var_epsilon}{sub s} = 1.2 x 10{sup -17} {sigma}{sup 4.75} exp(-6161/T), where {var_epsilon}{sub s} is the steady state strain rate in s{sup -1}, {sigma} is the applied axial stress difference in psi, and T is the temperature in Kelvin.

To obtain a well-determined constitutive database for the Alaskan frozen soil at confining pressures up to 100 MPa and temperatures down to –25°C, a series of laboratory tests was conducted using a unique high-pressure, low-temperature apparatus and the split Hopkinson pressure bar (SHPB). Quasi-static compression tests and indirect tension (or Brazilian) tests, are required to constrain the variabilities of material properties of frozen soil. The SHPB tests are required to obtain dynamic compression properties and the strain rate dependency of the frozen soil. The results from laboratory material testing showed that Alaskan frozen soil exhibits pressure and temperature dependence, rate sensitivity, anisotropy, brittle and ductile behavior, volumetric compaction, and dilation. The rate-sensitive and anisotropic version of a plasticity model, being developed by Fossum and Fredrich (2000), was able to represent the deformation behavior of such a complex material very well. This model includes high strain-rate sensitivity and anisotropy in both the elastic and plastic regimes. The model is defined through a continuous yield and loading surface for unified dilation and compaction phenomena. It is envisioned that this model will be used to predict the deformation and failure of frozen soil under the dynamic loading conditions resulting from projectile penetration into frozen soil targets.

Data from two series of hydraulic fracturing experiments at the Carwynnen test site were used to estimate the state of in situ stress in the Cornwall granite. We discuss several techniques of pressure record interpretation, and alternate methods of in situ stress calculation.

Statistical analysis of hydraulic fracturing field data enhances the objectivity of determining shut-in and fracture reopening pressures and fracture orientations. Using nonlinear regression analysis (NLRA), we isolate the closed-fracture segment of the pressure-time curve by fitting it to an exponential decay model. The shut-in pressure (Ps) is expected to be in the range between Psepd−1 (the onset of the post fracture-closure segment) and Psepd−u (the pressure level of the best-fitting exponential curve extrapolated to the time of pump shut-off). We employ bilinear regression analysis to improve the objectivity of selecting Ps from the pressure-decay rate vs pressure plot. The same analysis helps to identify Ps in the pressure vs flowrate record obtained during steprate pressurization of the hydrofractured interval. We identify unambiguously the fracture reopening pressure (Pr) by superposing the ascending portion of the pressure-time curves in the fracture-inducing cycle and a subsequent cycle. Pr is the onset of deviation between the two curves, defined as the point where the difference in pressures is larger than a statistical “reference threshold”, provided the difference continues to increase from there on. Circular statistics and a sinusoidal curve-fitting method provide the means of objectively delineating the complete trace of induced hydrofractures from fragmented or splaying traces on the impression packer or on the borehole televiewer photograph. Examples of field hydraulic fracturing data and statistically determined pressures and fracture orientations are presented. A startling discovery was made in the course of determining Pr, with our new technique: it appears to be very nearly equal to the estimated Ps. A review of the literature shows that in a great number of cases Pr and Ps are indeed close to each other, which raises the question whether we are in fact reading the correct fracture reopening pressure.