Search

Fracture injection/shut-in tests are commonly used to measure the state of stress in the subsurface. Injection creates a hydraulic fracture (or in some cases, opens a preexisting fracture), and then the pressure after shut-in is monitored to identify fracture closure. Different interpretation procedures have been proposed for estimating closure, and the procedures sometimes yield significantly different results. In this study, direct, in-situ strain measurements are used to observe fracture reopening and closure. The tests were performed as part of the EGS Collab project, a mesoscale project performed at 1.25 and 1.5 km depth at the Sanford Underground Research Facility. The tests were instrumented with the SIMFIP tool, a double-packer probe with a high-resolution three-dimensional borehole displacement sensor. The measurements provide a direct observation of the fracture closure signature, enabling a high-fidelity estimate of the fracture closure stress (ie, the normal stress on the fracture). In two of the four tests, injection created an opening mode fracture, and so the closure stress can be interpreted as the minimum principal stress. In the other two tests, injection probably opened preexisting natural fractures, and so the closure stress can be interpreted as the normal stress on the fractures. The strain measurements are compared against different proposed methods for estimating closure stress from pressure transients. The shut-in transients are analyzed with two techniques that are widely used in the field of petroleum engineering – the ‘tangent’ method and the ‘compliance’ method. In three of the four tests, the tangent method significantly underestimates the closure stress. The compliance method is reasonably accurate in all four tests. Closure stress is also interpreted using two other commonly-used methods – ‘first deviation from linearity’ and the method of (Hayashi and Haimson, 1991). In comparison with the SIMFIP data, these methods tend to overestimate the closure stress, evidently because they identify closure from early-time transient effects, such as near-wellbore tortuosity. In two of the tests, microseismic imaging provides an independent estimate of the size of the fracture created by injection. When combined with a simple mass balance calculation, the SIMFIP stress measurements yield predictions of fracture size that are reasonably consistent with the estimates from microseismic. The calculations imply an apparent fracture toughness 2-3x higher than typical laboratory-derived values.

The Teacher of the Year (TOY) program is the longest-standing teacher recognition program in the United States. The purpose of this study is to describe the characteristics of state and national TOY awardees and the schools in which they teach. To accomplish this aim, we develop a new data set including the characteristics of all TOY awardees and their schools from 1988 to 2019. Using descriptive and regression analysis, we find that TOY awardees are most likely to teach at the high school level, while the most common subjects taught were elementary education, English language arts, natural sciences, and, for National TOY awardees, social studies. They also have a greater probability of being selected from schools with a smaller fraction of students eligible for free or reduced-price lunch, and higher student enrollments. We discuss how these differences may impede the TOY program's efforts to amplify teachers' voice in education policymaking.

The EGS Collab project is performing well-monitored rock stimulation and flow tests at the 10-m scale in an underground research laboratory to inform challenges in implementing enhanced geothermal systems (EGS). This project, supported by the US Department of Energy, is gathering data and observations from the field tests and comparing these to simulation results to understand processes and to build confidence in numerical modeling of the processes. Experiment 1 (now complete) examined hydraulic fracturing in an underground test bed at the Sanford Underground Research Facility (SURF) in Lead, South Dakota, at a depth of approximately 1.5 km in a well-characterized phyllite. Geophysical monitoring instrumentation in six of eight sub-horizontal boreholes monitored stimulation events and flow tests. The other two boreholes were used to perform and carefully measure water injection and production. More than a dozen stimulations and nearly one year of flow tests in the testbed were performed. Detailed observations of processes occurring during stimulation and dynamic flow tests were collected and analyzed. Flow tests using ambient-temperature and chilled water were performed with intermittent tracer tests to examine system behavior. We achieved adaptive control of the tests using close monitoring of rapidly disseminated data and near-real-time simulation. Numerical simulation was critical in answering key experimental design questions, forecasting fracture behavior, and analyzing results. We were successful in performing many simulations in near-real-time in conjunction with the field experiments, with more detailed simulations performed later. The primary objective of Experiment 2 is to examine hydraulic shearing of natural fractures at a depth of 1.25 km in amphibolite at SURF. The stresses, rock type, and fracture conditions are different than in Experiment 1. The testbed consists of 9 boreholes, in addition to two earlier-drilled characterization boreholes. One borehole is used for injection, two fans of 2 monitoring wells have several geophysical measurement tools grouted in, and four open boreholes surrounding the injection hole are adaptively used for production and monitoring. We have encountered approximately five fracture set orientations in the testbed, and designed our testbed accordingly to maximize the potential for shear stimulation. Three stimulations have been performed to date from the injection borehole, each intersecting at least one production borehole. Different methods have been used for each stimulation, including a ramped flow, a high flow rate, and oscillating pressure.

The EGS Collab project, supported by the US Department of Energy, is addressing challenges in implementing enhanced geothermal systems (EGS). This includes improving understanding of the stimulation of crystalline rock to create appropriate flow pathways, and the ability to effectively simulate both the stimulation and the flow and transport processes in the resulting fracture network. The project is performing intensively monitored rock stimulation and flow tests at the 10-m scale in an underground research laboratory. Data and observations from the field test are compared to simulations to understand processes and to build confidence in numerical modeling of the processes. In Experiment 1, we examined hydraulic fracturing an underground test bed at the Sanford Underground Research Facility (SURF) in Lead, South Dakota, at a depth of approximately 1.5 km. We drilled eight sub-horizontal boreholes in a well-characterized phyllite. Six of the boreholes were instrumented with many sensor types to allow careful monitoring of stimulation events and flow tests, and the other two boreholes were used for water injection and production. We performed a number of stimulations and flow tests in the testbed. Our monitoring systems allowed detailed observations and collection of numerous data sets of processes occurring during stimulation and during dynamic flow tests. Long-term ambient temperature and chilled water flow tests were performed in addition to many tracer tests to examine system behavior. Data were rapidly analyzed, allowing adaptive control of the tests. Numerical simulation was used to answer key experimental design questions, to forecast fracture propagation trajectories and extents, and to analyze and evaluate results. Many simulations were performed in near-real-time in conjunction with the field experiments, with more detailed process study simulations performed on a longer timeframe. Experiment 2 will examine hydraulic shearing in a test bed being built at the SURF at a depth of about 1.25 km in amphibolite under a different set of stress and fracture conditions than Experiment 1. Five sets of fracture orientations were considered in design, and three orientations seem to be consistently observed.

Enhanced Geothermal Systems could provide a substantial contribution to the global energy demand if their implementation could overcome inherent challenges. Examples are insufficient created permeability, early thermal breakthrough, and unacceptable induced seismicity. Here we report on the seismic response of a mesoscale hydraulic fracturing experiment performed at 1.5-km depth at the Sanford Underground Research Facility. We have measured the seismic activity by utilizing a 100-kHz, continuous seismic monitoring system deployed in six 60-m length monitoring boreholes surrounding the experimental domain in 3-D. The achieved location uncertainty was on the order of 1 m and limited by the signal-to-noise ratio of detected events. These uncertainties were corroborated by detections of fracture intersections at the monitoring boreholes. Three intervals of the dedicated injection borehole were hydraulically stimulated by water injection at pressures up to 33 MPa and flow rates up to 5 L/min. We located 1,933 seismic events during several injection periods. The recorded seismicity delineates a complex fracture network comprised of multistrand hydraulic fractures and shear-reactivated, preexisting planes of weakness that grew unilaterally from the point of initiation. We find that heterogeneity of stress dictates the seismic outcome of hydraulic stimulations, even when relying on theoretically well-behaved hydraulic fractures. Once hydraulic fractures intersected boreholes, the boreholes acted as a pressure relief and fracture propagation ceased. In order to create an efficient subsurface heat exchanger, production boreholes should not be drilled before the end of hydraulic stimulations.

We investigate the influence of foliation orientation and fine-scale folding on the static and dynamic elastic properties and unconfined strength of the Poorman schist. Measurements from triaxial and uniaxial laboratory experiments reveal a significant amount of variability in the static and dynamic Young’s modulus depending on the sample orientation relative to the foliation plane. Dynamic P-wave modulus and S-wave modulus are stiffer in the direction parallel to the foliation plane as expected for transversely isotropic mediums with average Thomsen parameters values 0.133 and 0.119 for epsilon and gamma, respectively. Static Young’s modulus varies significantly between 21 and 117 GPa, and a peculiar trend is observed where some foliated sample groups show an anomalous decrease in the static Young’s modulus when the symmetry axis (x3-axis) is oriented obliquely to the direction of loading. Utilizing stress and strain relationships for transversely isotropic medium, we derive the analytical expression for Young’s modulus as a function of the elastic moduli E1, E3, ν31, and G13 and sample orientation to fit the static Young’s modulus measurements. Regression of the equation to the Young’s modulus data reveals that the decrease in static Young’s modulus at oblique symmetry axis orientations is directly influenced by a low shear modulus, G13, which we attribute to shear sliding along foliation planes during static deformation that occurs as soon as the foliation is subject to shear stress. We argue that such difference between dynamic and static anisotropy is a characteristic of near-zero porosity anisotropic rocks. The uniaxial compressive strength also shows significant variability ranging from 21.9 to 194.6 MPa across the five sample locations and is the lowest when the symmetry axis is oriented 45° or 60° from the direction of loading, also a result of shear sliding along foliation planes during static deformation.

How low clouds respond to warming constitutes a key uncertainty for climate projections. Here we observationally constrain low‐cloud feedback through a controlling factor analysis based on ridge regression. We find a moderately positive global low‐cloud feedback (0.45 W m−2 ${\mathrm{m}}^{-2}$K−1 ${\mathrm{K}}^{-1}$, 90% range 0.18–0.72 W m−2 ${\mathrm{m}}^{-2}$K−1 ${\mathrm{K}}^{-1}$), about twice the mean value (0.22 W m−2 ${\mathrm{m}}^{-2}$K−1 ${\mathrm{K}}^{-1}$) of 16 models from the Coupled Model Intercomparison Project. We link this discrepancy to a pervasive model mean‐state bias: models underestimate the low‐cloud response to warming because (a) they systematically underestimate present‐day tropical marine low‐cloud amount, and (b) the low‐cloud sensitivity to warming is proportional to this present‐day low‐cloud amount. Our results hence highlight the importance of reducing model biases in both the mean state of clouds and their sensitivity to environmental factors for accurate climate change projections. Plain Language Summary: Low clouds have a large impact on climate by reflecting a portion of incoming sunlight back to space. Hence any future changes in clouds under global warming could amplify or dampen climate change—a phenomenon known as "cloud feedback." Climate models however disagree on future low‐cloud changes, resulting in large uncertainty in future global‐warming projections. Here we perform a statistical analysis of global satellite observations of low clouds, using observed co‐variations between clouds and meteorology to constrain the feedback simulated by climate models. We find evidence of an amplifying feedback by low clouds, stronger than simulated by most climate models. We link this discrepancy to a low‐cloud deficit across wide swathes of the tropical oceans characterized by abundant low cloud cover in observations. Thus to reduce climate projection uncertainty, we propose it is important to understand and mitigate the low‐cloud deficit in climate models. Key Points: We implement a statistical learning‐based analysis to constrain low‐cloud feedback, with strong out‐of‐sample predictive skillThe observationally constrained global‐mean feedback is about double the mean value from an ensemble of 16 climate modelsThis discrepancy is consistent with a pervasive underestimate of mean‐state tropical marine low‐cloud amount in the models [ABSTRACT FROM AUTHOR]

We present laser-driven shock Hugoniot measurements of single-crystal (SC) 1,3,5-triamino-2,4,6-trinitrobenzene (TATB) between 15 and 83 GPa, spanning pressures below and well above the Chapman–Jouguet pressure of ∼ 28 GPa for TATB formulations (TATB grains mixed with plastic binders at 5–10 wt. %). The new SC data are generally ∼ 3% more compressible than previously published data on neat and formulated TATB measured in gas-gun and explosive-driven experiments. An exception is at compressions in the density of ∼ 1.5 (∼ 30–40 GPa), where our new SC data exhibit significantly lower pressures than previous results on overdriven TATB formulations, suggesting that our SC samples remain largely unreacted below 35 GPa over the short nanosecond-time scales inherent to our laser-driven experiments. These novel equation-of-state measurements are a critical step toward understanding TATB in its most fundamental form and improving predictive modeling of TATB-based explosives. [ABSTRACT FROM AUTHOR]